AQA(B) A2 Module 4

Energy, Control And Continuity

Specification

Metabolism

Respiration

Photosynthesis

Human Nervous system

Nerve Cells

The Reflex Arc

The Nerve Impulse

Synapses

Drugs

The eye

The Brain

Muscles

Excretion

Excretion

The Kidney

Hormones

Homeostasis

Temperature Homeostasis

Blood Sugar Homeostasis

Blood water Homeostasis

Classical Genetics

Meiosis

Fertilisation

Monohybrid cross

Sex Determination

Sex-Linkage

Multiple Alleles

Dihybrid Crosses

Population Genetics

Variation

Natural Selection

Speciation

Classification

 

Module 4 Specification

Metabolism

The relationship between photosynthesis and respiration. The synthesis of ATP from ADP and inorganic phosphate, and its role as the immediate source of energy for biological processes.

Cellular Respiration

Respiration as the process by which energy in organic molecules is made available for other processes within an organism. The structure and role of mitochondria in respiration. The biochemistry of aerobic respiration only in sufficient detail to show that:

Photosynthesis

Photosynthesis as a process in which light energy is used in the synthesis of organic molecules. The structure and role of chloroplasts in relation to photosynthesis.

Control and Coordination

Organisms increase their chances of survival by responding to changes in their environment.

The Human Nervous System

Information is transferred in the nervous system through detection of stimuli by receptors and the initiation of a nerve impulse, leading to an associated response by effectors by means of a coordinator. A simple reflex arc involving three neurones.

The general role of the sympathetic and parasympathetic components of the autonomic nervous system. The specific effects of the autonomic nervous system on controlling:

Neurones and the Action Potential

Synapses and Drugs

The Eye

The Brain

The principal functions of the cerebral hemispheres:

Muscles

Muscles are effectors that enable movement to be carried out

Excretion

Waste products of metabolism are frequently toxic and must be removed from the body. Deamination of excess amino acids and the production of urea. (Details of the ornithine cycle not required.)

The Kidney

The processes involved in the formation of urine in the kidney, including ultrafiltration in the renal capsule and selective reabsorption in the proximal convoluted tubule. The role of the loop of Henle in maintaining a gradient of ions across the medulla.

Human Endocrine System

Information is transferred by hormones released by endocrine glands and affecting the physiological activities of target cells.

Homeostasis

Physiological control systems operate in mammals to maintain a constant internal environment – this is homeostasis. The principle of negative feedback and its role in restoring systems to their original levels.

Genetics

Meiosis and Fertilisation

The principal events associated with meiosis, to include: pairing by homologous chromosomes; formation of bivalents; chiasma formation and exchange between chromatids; separation of chromatids; production of haploid cells. (Details and names of individual stages of meiosis are not required.)

Candidates should be able to explain the behaviour of alleles and homologous chromosomes during meiosis and fertilisation, i.e.

Classical Genetics

The genotype is the genetic constitution of an organism. The expression of this genetic constitution and its interaction with the environment is the phenotype.

A gene can exist in different forms called alleles which are positioned in the same relative position (locus) on homologous chromosomes. The alleles at a specific locus may be either homozygous or heterozygous. Alleles may be dominant, recessive or codominant.

Candidates should be able to apply the above principles to interpret and use fully annotated genetic diagrams to predict the results of:

Population Genetics

Individuals within a species may show a wide range of variation. Similarities and differences between individuals within a species may be the result of genetic factors, differences in environmental factors, or a combination of both. Variation between individuals may be either continuous or discontinuous.

Candidates should be able to interpret data to determine the relative effects of genetic and environmental factors involved in continuous and discontinuous variation. Candidates should be able to explain how crossing over, independent assortment of chromosomes, random fusion of gametes and mutation contribute to genetic variation.

Natural Selection and Evolution

Predation, disease and competition result in differential survival and reproduction. Those organisms with a selective advantage are more likely to survive, reproduce and pass on their genes to the next generation.

Use specific examples to explain how natural selection produces changes within a species. Interpret data and use unfamiliar information to explain how natural selection produces change within a population. Evolutionary change over a long period of time has resulted in a great diversity of forms among living organisms.

The concept of the species in terms of production of fertile offspring. Natural selection and isolation may result in changes in the allele and phenotype frequency and lead to the formation of a new species.

Classification

A classification system comprises a hierarchy in which groups are contained within larger composite groups with no overlap. The phylogenetic groupings are based on patterns of evolutionary history. The principles and importance of taxonomy.

One hierarchy comprises Kingdom, Phylum, Class, Order, Family, Genus, Species. The distinguishing features of the five kingdoms – prokaryotes, protoctists, fungi, plants and animals.

 

Metabolism

Metabolism refers to all the chemical reactions taking place in a cell. There are thousands of these in a typical cell, and to make them easier to understand, biochemists arrange them into metabolic pathways. The intermediates in these metabolic pathways are called metabolites.

Photosynthesis and respiration are the reverse of each other, and you couldn’t have one without the other. The net result of all the photosynthesis and respiration by living organisms is the conversion of light energy to heat energy.

 

Cellular Respiration

The equation for cellular respiration is usually simplified to:

glucose + oxygen Õ carbon dioxide + water (+ energy)

But in fact respiration is a complex metabolic pathway, comprising at least 30 separate steps. To understand respiration in detail we can break it up into 3 stages:

Before we look at these stages in detail, there are a few points from this summary.

Mitochondria

Much of respiration takes place in the mitochondria. Mitochondria have a double membrane: the outer membrane contains many protein channels called porins, which let almost any small molecule through; while the inner membrane is more normal and is impermeable to most materials. The inner membrane is highly folded into folds called christae, giving a larger surface area. The electron microscope reveals blobs on the inner membrane, which were originally called stalked particles. These have now been identified as the enzyme complex that synthesises ATP, are is more correctly called ATP synthase (more later). the space inside the inner membrane is called the matrix, and is where the Krebs cycle takes place. The matrix also contains DNA, tRNA and ribosomes, and some genes are replicated and expressed here.

Details of Respiration

1. Glucose enters cells from the tissue fluid by passive transport using a specific glucose carrier. This carrier can be controlled (gated) by hormones such as insulin, so that uptake of glucose can be regulated.

2. The first step is the phosphorylation of glucose to form glucose phosphate, using phosphate from ATP. Glucose phosphate no longer fits the membrane carrier, so it can’t leave the cell. This ensures that pure glucose is kept at a very low concentration inside the cell, so it will always diffuse down its concentration gradient from the tissue fluid into the cell. Glucose phosphate is also the starting material for the synthesis of pentose sugars (and therefore nucleotides) and for glycogen.

3. Glucose is phosphorylated again (using another ATP) and split into two triose phosphate (3 carbon) sugars. From now on everything happens twice per original glucose molecule.

4. The triose sugar is changed over several steps to form pyruvate, a 3-carbon compound. In these steps some energy is released to form ATP (the only ATP formed in glycolysis), and a hydrogen atom is also released. This hydrogen atom is very important as it stores energy, which is later used by the respiratory chain to make more ATP. The hydrogen atom is taken up and carried to the respiratory chain by the coenzyme NAD, which becomes reduced in the process.

(oxidised form Õ) NAD + H Õ NADH (← reduced form)

Pyruvate marks the end of glycolysis, the first stage of respiration. In the presence of oxygen pyruvate enters the mitochondrial matrix to proceed with aerobic respiration, but in the absence of oxygen it is converted into lactate (in animals and bacteria) or ethanol (in plants and fungi). These are both examples of anaerobic respiration. Pyruvate can also be turned back into glucose by reversing glycolysis, and this is called gluconeogenesis.

5. Once pyruvate has entered the inside of the mitochondria (the matrix), it is converted to a compound called acetyl coA. Since this step is between glycolysis and the Krebs Cycle, it is referred to as the link reaction. In this reaction pyruvate loses a CO2 and a hydrogen to form a 2-carbon acetyl compound, which is temporarily attached to another coenzyme called coenzyme A (or just coA), so the product is called acetyl coA. The CO2 diffuses through the mitochondrial and cell membranes by lipid diffusion, out into the tissue fluid and into the blood, where it is carried to the lungs for removal. The hydrogen is taken up by NAD again.

6. The acetyl CoA then enters the Krebs Cycle, named after Sir Hans Krebs, who discovered it in the 1940s at Leeds University. It is one of several cyclic metabolic pathways, and is also known as the citric acid cycle or the tricarboxylic acid cycle. The 2-carbon acetyl is transferred from acetyl coA to the 4-carbon oxaloacetate to form the 6-carbon citrate. Citrate is then gradually broken down in several steps to re-form oxaloacetate, producing carbon dioxide and hydrogen in the process. As before, the CO2 diffuses out the cell and the hydrogen is taken up by NAD, or by an alternative hydrogen carrier called FAD. These hydrogens are carried to the inner mitochondrial membrane for the final part of respiration.

The Respiratory Chain

The respiratory chain (or electron transport chain) is an unusual metabolic pathway in that it takes place within the inner mitochondrial membrane, using integral membrane proteins. These proteins form four huge trans-membrane complexes called complexes I, II, II and IV. The complexes each contain up to 40 individual polypeptide chains, which perform many different functions including enzymes and trans-membrane pumps. In the respiratory chain the hydrogen atoms from NADH gradually release all their energy to form ATP, and are finally combined with oxygen to form water.

1. NADH molecules bind to Complex I and release their hydrogen atoms as protons (H+) and electrons (e-). The NAD molecules then returns to the Krebs Cycle to collect more hydrogen. FADH binds to complex II rather than complex I to release its hydrogen.

2. The electrons are passed down the chain of proteins complexes from I to IV, each complex binding electrons more tightly than the previous one. In complexes I, II and IV the electrons give up some of their energy, which is then used to pump protons across the inner mitochondrial membrane by active transport through the complexes. Altogether 10 protons are pumped across the membrane for every hydrogen from NADH (or 6 protons for FADH).

3. In complex IV the electrons are combined with protons and molecular oxygen to form water, the final end-product of respiration. The oxygen diffused in from the tissue fluid, crossing the cell and mitochondrial membranes by lipid diffusion. Oxygen is only involved at the very last stage of respiration as the final electron acceptor, but without the whole respiratory chain stops.

4. The energy of the electrons is now stored in the form of a proton gradient across the inner mitochondrial membrane. It’s a bit like using energy to pump water uphill into a high reservoir, where it is stored as potential energy. And just as the potential energy in the water can be used to generate electricity in a hydroelectric power station, so the energy in the proton gradient can be used to generate ATP in the ATP synthase enzyme. The ATP synthase enzyme has a proton channel through it, and as the protons "fall down" this channel their energy is used to make ATP, spinning the globular head as they go. It takes 4 protons to synthesise 1 ATP molecule.

This method of storing energy by creating a protons gradient across a membrane is called chemiosmosis, and was discovered by Peter Mitchell in the 1960s, for which work he got a Nobel prize in 1978. Some poisons act by making proton channels in mitochondrial membranes, so giving an alternative route for protons and stopping the synthesis of ATP. This also happens naturally in the brown fat tissue of new-born babies and hibernating mammals: respiration takes place, but no ATP is made, with the energy being turned into heat instead.

 

How Much ATP is Made in Respiration?

We can now summarise respiration and see how much ATP is made from each glucose molecule. ATP is made in two different ways:

The table below is an "ATP account" for aerobic respiration, and shows that 32 molecules of ATP are made for each molecule of glucose used in aerobic respiration. This is the maximum possible yield; often less ATP is made, depending on the circumstances. Note that anaerobic respiration (glycolysis) only produces 2 molecules of ATP.

Stage

molecules produced per glucose

Final ATP yield

(old method)

Final ATP yield

(new method)

Glycolysis

2 ATP used

-2

-2

4 ATP produced (2 per triose phosphate)

4

4

2 NADH produced (1 per triose phosphate)

6

5

Link Reaction

2 NADH produced (1 per pyruvate)

6

5

Krebs Cycle

2 ATP produced (1 per acetyl coA)

2

2

6 NADH produced (3 per acetyl coA)

18

15

2 FADH produced (1 per acetyl coA)

4

3

Total

38

32

Other substances can also be used to make ATP. Triglycerides are broken down to fatty acids and glycerol, both of which enter the Krebs Cycle. A typical triglyceride might make 50 acetyl CoA molecules, yielding 500 ATP molecules. Fats are a very good energy store, yielding 2.5 times as much ATP per g dry mass as carbohydrates. Proteins are not normally used to make ATP, but in times of starvation they can be broken down and used in respiration. They are first broken down to amino acids, which are converted into pyruvate and Krebs Cycle metabolites and then used to make ATP.

Photosynthesis

Photosynthesis is essentially the reverse of respiration. It is usually simplified to:

carbon dioxide + water (+ light energy) Õ glucose + oxygen

But again this simplification hides numerous separate steps. To understand photosynthesis in detail we can break it up into 2 stages:

We shall see that there are many similarities between photosynthesis and respiration, and even the same enzymes are used in some steps.

Chloroplasts

Photosynthesis takes place entirely within chloroplasts. Like mitochondria, chloroplasts have a double membrane, but in addition chloroplasts have a third membrane called the thylakoid membrane. This is folded into thin vesicles (the thylakoids), enclosing small spaces called the thylakoid lumen. The thylakoid vesicles are often layered in stacks called grana. The thylakoid membrane contains the same ATP synthase particles found in mitochondria. Chloroplasts also contain DNA, tRNA and ribososomes, and they often store the products of photosynthesis as starch grains and lipid droplets.

 

Chlorophyll

Chloroplasts contain two different kinds of chlorophyll, called chlorophyll a and b, together with a number of other light-absorbing accessory pigments, such as the carotenoids and luteins (or xanthophylls). These different pigments absorb light at different wavelengths, so having several different pigments allows more of the visible spectrum to be used. The absorption spectra of pure samples of some of these pigments are shown in the graph on the left. A low absorption means that those wavelengths are not absorbed and used, but instead are reflected or transmitted. Different species of plant have different combinations of photosynthetic pigments, giving rise to different coloured leaves. In addition, plants adapted to shady conditions tend to have a higher concentration of chlorophyll and so have dark green leaves, while those adapted to bright conditions need less chlorophyll and have pale green leaves.

By measuring the rate of photosynthesis using different wavelengths of light, an action spectrum is obtained. The action spectrum can be well explained by the absorption spectra above, showing that these pigments are responsible for photosynthesis.

 

 

Chlorophyll is a fairly small molecule (not a protein) with a structure similar to haem, but with a magnesium atom instead of iron. Chlorophyll and the other pigments are arranged in complexes with proteins, called photosystems. Each photosystem contains some 200 chlorophyll molecules and 50 molecules of accessory pigments, together with several protein molecules (including enzymes) and lipids. These photosystems are located in the thylakoid membranes and they hold the light-absorbing pigments in the best position to maximise the absorbance of photons of light. The chloroplasts of green plants have two kinds of photosystem called photosystem I (PSI) and photosystem II (PSII). These absorb light at different wavelengths and have slightly different jobs in the light dependent reactions of photosynthesis.

The Light-Dependent Reactions

The light-dependent reactions take place on the thylakoid membranes using four membrane-bound protein complexes called photosystem I (PSI), photosystem II (PSII), cytochrome complex (C) and ferredoxin complex (FD). In these reactions light energy is used to split water, oxygen is given off, and ATP and hydrogen are produced.

1. Chlorophyll molecules in PSII absorb photons of light, exciting chlorophyll electrons to a higher energy level and causing a charge separation within PSII. This charge separation drives the splitting (or photolysis) of water molecules to make oxygen (O2), protons (H+) and electrons (e-):

2H2O O2 + 4H+ + 4e-

Water is a very stable molecule and it requires the energy from 4 photons of light to split 1 water molecule. The oxygen produced diffuses out of the chloroplast and eventually into the air; the protons build up in the thylakoid lumen causing a proton gradient; and the electrons from water replace the excited electrons that have been ejected from chlorophyll.

2. The excited, high-energy electrons are passed along a chain of protein complexes in the membrane, similar to the respiratory chain. They are passed from PSII to C, where the energy is used to pump 4 protons from stroma to lumen; then to PSI, where more light energy is absorbed by the chlorophyll molecules and the electrons are given more energy; and finally to FD.

3. In the ferredoxin complex each electron is recombined with a proton to form a hydrogen atom, which is taken up by the hydrogen carrier NADP. Note that while respiration uses NAD to carry hydrogen, photosynthesis always uses its close relative, NADP.

4. The combination of the water splitting and the proton pumping by the cytochrome complex cause protons to build up inside the thylakoid lumen. This generates a proton gradient across the thylakoid membrane. This gradient is used to make ATP using the ATP synthase enzyme in exactly the same way as respiration. This synthesis of ATP is called photophosphorylation because it uses light energy to phosphorylate ADP.

The Light-Independent Reactions

The light-independent, or carbon-fixing reactions, of photosynthesis take place in the stroma of the chloroplasts and comprise another cyclic pathway, called the Calvin Cycle, after the American scientist who discovered it.

1. Carbon dioxide binds to the 5-carbon sugar ribulose bisphosphate (RuBP) to form 2 molecules of the 3-carbon compound glycerate phosphate. This carbon-fixing reaction is catalysed by the enzyme ribulose bisphosphate carboxylase, always known as rubisco. It is a very slow and inefficient enzyme, so large amounts of it are needed (recall that increasing enzyme concentration increases reaction rate), and it comprises about 50% of the mass of chloroplasts, making the most abundant protein in nature. Rubisco is synthesised in chloroplasts, using chloroplast (not nuclear) DNA.

2. Glycerate phosphate is an acid, not a carbohydrate, so it is reduced and activated to form triose phosphate, the same 3-carbon sugar as that found in glycolysis. The ATP and NADPH from the light-dependent reactions provide the energy for this step. The ADP and NADP return to the thylakoid membrane for recycling.

3. Triose phosphate is a branching point. Most of the triose phosphate continues through a complex series of reactions to regenerate the RuBP and complete the cycle. 5 triose phosphate molecules (15 carbons) combine to form 3 RuBP molecules (15 carbons).

4. Every 3 turns of the Calvin Cycle 3 CO2 molecules are fixed to make 1 new triose phosphate molecule. This leaves the cycle, and two of these triose phosphate molecules combine to form one glucose molecule using the glycolysis enzymes in reverse. The glucose can then be used to make other material that the plant needs.

 

The Human Nervous System

Humans, like all living organisms, can respond to their environment. Humans have two complimentary control systems to do this: the nervous system and the endocrine (hormonal) system. We’ll look at the endocrine system later, but first we’ll look at the nervous system. The human nervous system controls everything from breathing and producing digestive enzymes, to memory and intelligence.

Nerve Cells

The nervous system composed of nerve cells, or neurones:

A neurone has a cell body with extensions leading off it. Numerous dendrons and dendrites provide a large surface area for connecting with other neurones, and carry nerve impulses towards the cell body. A single long axon carries the nerve impulse away from the cell body. The axon is only 10µm in diameter but can be up to 4m in length in a large animal (a piece of spaghetti the same shape would be 400m long)! Most neurones have many companion cells called Schwann cells, which wrap their cell membrane around the axon many times in a spiral to form a thick insulating lipid layer called the myelin sheath. Nerve impulse can be passed from the axon of one neurone to the dendron of another at a synapse. A nerve is a discrete bundle of several thousand neurone axons.

Humans have three types of neurone:

The Reflex Arc

The three types of neurones are arranged in circuits and networks, the simplest of which is the reflex arc.

In a simple reflex arc, such as the knee jerk, a stimulus is detected by a receptor cell, which synapses with a sensory neurone. The sensory neurone carries the impulse from site of the stimulus to the central nervous system (the brain or spinal cord), where it synapses with an interneurone. The interneurone synapses with a motor neurone, which carries the nerve impulse out to an effector, such as a muscle, which responds by contracting.

Reflex arc can also be represented by a simple flow diagram:

 

 

The Organisation Of The Human Nervous System

The human nervous system is far more complex than a simple reflex arc, although the same stages still apply. The organisation of the human nervous system is shown in this diagram:

It is easy to forget that much of the human nervous system is concerned with routine, involuntary jobs, such as homeostasis, digestion, posture, breathing, etc. This is the job of the autonomic nervous system, and its motor functions are split into two divisions, with anatomically distinct neurones. Most body organs are innervated by two separate sets of motor neurones; one from the sympathetic system and one from the parasympathetic system. These neurones have opposite (or antagonistic) effects. In general the sympathetic system stimulates the "fight or flight" responses to threatening situations, while the parasympathetic system relaxes the body. The details are listed in this table:

 

Organ

Sympathetic System

Parasympathetic System

 

Eye

Tear glands

Salivary glands

Lungs

Heart

Gut

Liver

Bladder

Dilates pupil

No effect

Inhibits saliva production

Dilates bronchi

Speeds up heart rate

Inhibits peristalsis

Stimulates glucose production

Inhibits urination

Constricts pupil

Stimulates tear secretion

Stimulates saliva production

Constricts bronchi

Slows down heart rate

Stimulates peristalsis

Stimulates bile production

Stimulates urination

 

The Nerve Impulse

Neurones and muscle cells are electrically excitable cells, which means that they can transmit electrical nerve impulses. These impulses are due to events in the cell membrane, so to understand the nerve impulse we need to revise some properties of cell membranes.

The Membrane Potential

All animal cell membranes contain a protein pump called the Na+K+ATPase. This uses the energy from ATP splitting to simultaneously pump 3 sodium ions out of the cell and 2 potassium ions in. If this was to continue unchecked there would be no sodium or potassium ions left to pump, but there are also sodium and potassium ion channels in the membrane. These channels are normally closed, but even when closed, they "leak", allowing sodium ions to leak in and potassium ions to leak out, down their respective concentration gradients.

The combination of the Na+K+ATPase pump and the leak channels cause a stable imbalance of Na+ and K+ ions across the membrane. This imbalance causes a potential difference across all animal cell membranes, called the membrane potential. The membrane potential is always negative inside the cell, and varies in size from –20 to –200 mV in different cells and species. The Na+K+ATPase is thought to have evolved as an osmoregulator to keep the internal water potential high and so stop water entering animal cells and bursting them. Plant cells don’t need this as they have strong cells walls to prevent bursting.

The Action Potential

In nerve and muscle cells the membranes are electrically excitable, which means that they can change their membrane potential, and this is the basis of the nerve impulse. The sodium and potassium channels in these cells are voltage-gated, which means that they can open and close depending on the voltage across the membrane.

The nature of the nerve impulse was discovered by Hodgkin, Huxley and Katz in Plymouth in the 1940s, for which work they received a Nobel prize in 1963. They used squid giant neurones, whose axons are almost 1mm in diameter, big enough to insert wire electrodes so that they could measure the potential difference across the cell membrane. In a typical experiment they would apply an electrical pulse at one end of an axon and measure the voltage changes at the other end, using an oscilloscope:

The normal membrane potential of these nerve cells is –70mV (inside the axon), and since this potential can change in nerve cells it is called the resting potential. When a stimulating pulse was applied a brief reversal of the membrane potential, lasting about a millisecond, was recorded. This brief reversal is called the action potential:

The action potential has 2 phases called depolarisation and repolarisation.

1. Depolarisation. The stimulating electrodes cause the membrane potential to change a little. The voltage-gated ion channels can detect this change, and when the potential reaches –30mV the sodium channels open for 0.5ms. The causes sodium ions to rush in, making the inside of the cell more positive. This phase is referred to as a depolarisation since the normal voltage polarity (negative inside) is reversed (becomes positive inside).

2. Repolarisation. When the membrane potential reaches 0V, the potassium channels open for 0.5ms, causing potassium ions to rush out, making the inside more negative again. Since this restores the original polarity, it is called repolarisation.

The Na+K+ATPase pump runs continuously, restoring the resting concentrations of sodium and potassium ions.

How do Nerve Impulses Start?

In the squid experiments the action potential was initiated by the stimulating electrodes. In living cells they are started by receptor cells. These all contain special sodium channels that are not voltage-gated, but instead are gated by the appropriate stimulus (directly or indirectly). For example chemical-gated sodium channels in tongue taste receptor cells open when a certain chemical in food binds to them; mechanically-gated ion channels in the hair cells of the inner ear open when they are distorted by sound vibrations; and so on. In each case the correct stimulus causes the sodium channel to open; which causes sodium ions to flow into the cell; which causes a depolarisation of the membrane potential, which affects the voltage-gated sodium channels nearby and starts an action potential.

How are Nerve Impulses Propagated?

Once an action potential has started it is moved (propagated) along an axon automatically. The local reversal of the membrane potential is detected by the surrounding voltage-gated ion channels, which open when the potential changes enough.

The ion channels have two other features that help the nerve impulse work effectively:

How Fast are Nerve Impulses?

Action potentials can travel along axons at speeds of 0.1-100 m/s. This means that nerve impulses can get from one part of a body to another in a few milliseconds, which allows for fast responses to stimuli. (Impulses are much slower than electrical currents in wires, which travel at close to the speed of light, 3x108 m/s.) The speed is affected by 3 factors:

 

Synapses

The junction between two neurones is called a synapse. An action potential cannot cross the synaptic cleft between neurones, and instead the nerve impulse is carried by chemicals called neurotransmitters. These chemicals are made by the cell that is sending the impulse (the pre-synaptic neurone) and stored in synaptic vesicles at the end of the axon. The cell that is receiving the nerve impulse (the post-synaptic neurone) has chemical-gated ion channels in its membrane, called neuroreceptors. These have specific binding sites for the neurotransmitters.

1. At the end of the pre-synaptic neurone there are voltage-gated calcium channels. When an action potential reaches the synapse these channels open, causing calcium ions to flow into the cell.

2. These calcium ions cause the synaptic vesicles to fuse with the cell membrane, releasing their contents (the neurotransmitter chemicals) by exocytosis.

3. The neurotransmitters diffuse across the synaptic cleft.

4. The neurotransmitter binds to the neuroreceptors in the post-synaptic membrane, causing the channels to open. In the example shown these are sodium channels, so sodium ions flow in.

5. This causes a depolarisation of the post-synaptic cell membrane, which may initiate an action potential.

6. The neurotransmitter is broken down by a specific enzyme in the synaptic cleft; for example the enzyme acetylcholinesterase breaks down the neurotransmitter acetylcholine. The breakdown products are absorbed by the pre-synaptic neurone by endocytosis and used to re-synthesise more neurotransmitter, using energy from the mitochondria. This stops the synapse being permanently on.

Different Types of Synapse

The human nervous system uses a number of different neurotransmitter and neuroreceptors, and they don’t all work in the same way. We can group synapses into 5 types:

1. Excitatory Ion Channel Synapses.

These synapses have neuroreceptors that are sodium channels. When the channels open, positive ions flow in, causing a local depolarisation and making an action potential more likely. This was the kind of synapse described above. Typical neurotransmitters are acetylcholine, glutamate or aspartate.

2. Inhibitory Ion Channel Synapses.

These synapses have neuroreceptors that are chloride channels. When the channels open, negative ions flow in causing a local hyperpolarisation and making an action potential less likely. So with these synapses an impulse in one neurone can inhibit an impulse in the next. Typical neurotransmitters are glycine or GABA.

3. Non Channel Synapses.

These synapses have neuroreceptors that are not channels at all, but instead are membrane-bound enzymes. When activated by the neurotransmitter, they catalyse the production of a "messenger chemical" inside the cell, which in turn can affect many aspects of the cell’s metabolism. In particular they can alter the number and sensitivity of the ion channel receptors in the same cell. These synapses are involved in slow and long-lasting responses like learning and memory. Typical neurotransmitters are adrenaline, noradrenaline (NB adrenaline is called epinephrine in America), dopamine, serotonin, endorphin, angiotensin, and acetylcholine.

4. Neuromuscular Junctions.

These are the synapses formed between motor neurones and muscle cells. They always use the neurotransmitter acetylcholine, and are always excitatory. We shall look at these when we do muscles. Motor neurones also form specialised synapses with secretory cells.

5. Electrical Synapses.

In these synapses the membranes of the two cells actually touch, and they share proteins. This allows the action potential to pass directly from one membrane to the next. They are very fast, but are quite rare, found only in the heart and the eye.

Summation

One neurone can have thousands of synapses on its body and dendrons. So it has many inputs, but only one output. The output through the axon is called the Grand Postsynaptic Potential (GPP) and is the sum of all the excitatory and inhibitory potentials from all that cell’s synapses. If there are more excitatory potentials than inhibitory ones then there will be a GPP, and the neurone will "fire", but if there are more inhibitory potentials than excitatory ones then there will not be a GPP and the neurone will not fire.

This summation is the basis of the processing power in the nervous system. Neurones (especially interneurones) are a bit like logic gates in a computer, where the output depends on the state of one or more inputs. By connecting enough logic gates together you can make a computer, and by connecting enough neurones together to can make a nervous system, including a human brain.

 

Drugs and the Nervous System

Almost all drugs taken by humans (medicinal and recreational) affect the nervous system. From our understanding of the human nervous system we can understand how many common drugs work. Drugs can affect the nervous system in various ways, shown in this table:

Drug action

Effect

Examples

Mimic a neurotransmitter

Stimulate the release of a neurotransmitter

Open a neuroreceptor channel

Block a neuroreceptor channel

Inhibit the breakdown enzyme

Inhibit the Na+K+ATPase pump

Block the Na+ or K+ channels

Switch on a synapse

Switch on a synapse

Switch on a synapse

Switch off a synapse

Switch on a synapse

Stop action potentials

Stop action potentials

levodopa

cocaine, caffeine

atropine, curare, opioids, atropine

neostigmine, DDT

tetrodoxin, anaesthetics

Drugs that stimulate a nervous system are called agonists, and those that inhibit a system are called antagonists. By designing drugs to affect specific neurotransmitters or neuroreceptors, drugs can be targeted at different parts of the nervous system. The following paragraph describe the action of some common drugs. You do not need to know any of this, but you should be able to understand how they work.

1. Drugs acting on the central nervous system

In the reticular activating system (RAS) in the brain stem noradrenaline receptors are excitatory and cause wakefulness, while GABA receptors are inhibitory and cause drowsiness. Caffeine (in coffee, cocoa and cola), theophylline (in tea), amphetamines, ecstasy (MDMA) and cocaine all promote the release of noradrenaline in RAS, so are stimulants. Antidepressant drugs, such as the tricyclics, inhibit the breakdown and absorption of noradrenaline, so extending its effect. Alcohol, benzodiazepines (e.g. mogadon, valium, librium), barbiturates, and marijuana all activate GABA receptors, causing more inhibition of RAS and so are tranquillisers, sedatives and depressants. The narcotics or opioid group of drugs, which include morphine, codeine, opium, methadone and diamorphine (heroin), all block opiate receptors, blocking transmission of pain signals in the brain and spinal chord. The brain’s natural endorphins appear to have a similar action.

The brain neurotransmitter dopamine has a number of roles, including muscle control, pain inhibition and general stimulation. Some psychosis disorders such as schizophrenia and manic depression are caused by an excess of dopamine, and antipsychotic drugs are used to block the dopamine receptors and so reduce its effects. Parkinson’s disease (shaking of head and limbs) is caused by too little dopamine compared to acetylcholine production in the midbrain. The balance can be restored with levodopa, which mimics dopamine, or with anticholinergic drugs (such as procyclidine), which block the muscarinic acetylcholine receptors.

Tetrodotoxin (from the Japanese puffer fish) blocks voltage-gated sodium channels, while tetraethylamonium blocks the voltage-gated potassium channel. Both are powerful nerve poisons. General anaesthetics temporarily inhibit the sodium channels. Strychnine blocks glycine receptors in the brain, causing muscle convulsions and death.

2. Drugs acting on the somatic nervous system

Curare and abungarotoxin (both snake venoms) block the nicotinic acetylcholine receptors in the somatic nervous system, and so relax skeletal muscle. Myasthenia gravis (a weakening of the muscles in the face and throat caused by inactive nicotinic acetylcholine receptors) is treated by the drug neostigmine, which inhibits acetylcholinesterase, so increasing the amount of acetylcholine at the neuromuscular junction. Nerve gas and organophosphate insecticides (DDT) inhibit acetylcholinesterase, so nicotinic acetylcholine receptors are always active, causing muscle spasms and death. Damaged tissues release prostaglandins, which stimulate pain neurones (amongst other things). The non-narcotic analgesics such as aspirin, paracetamol and ibuprofen block prostaglandin production at source of pain, while paracetamol has a similar effect in the brain. Local anaesthetics such as procaine block all sensory and motor synapses at the site of application.

 

3. Drugs acting on the autonomic nervous system

Sympathetic agonists like salbutamol and isoprenaline, activate the adrenergic receptors in the sympathetic system, encouraging smooth muscle relaxation, and are used as bronchodilators in the treatment of asthma. Sympathetic antagonists like the beta blockers block the noradrenaline receptors in the sympathetic nervous system. They cause dilation of blood vessels in the treatment of high blood pressure and migraines, and reduce heartbeat rate in the treatment of angina and abnormal heart rhythms. Parasympathetic antagonists like atropine (from the deadly nightshade belladonna) inhibit the muscarinic acetylcholine receptors in parasympathetic system, and are used as eye drops to relax the ciliary muscles in the eye.

 

The Eye

The Sclera

The strong outer layer that hold the eye together. It is soft connective tissue, and the spherical shape of the eye is maintained by the pressure of the liquid inside.

The Choroid

This layer contains the blood vessels that feed every cell of the eye. It also contains the pigmented cells that make the retina appear black.

The Retina

This contains the light-sensitive photoreceptor cells and their associated neurones.

The Cornea

This is a specialised part of the cornea at the front of the eye. It is made of aligned collagen fibres and is transparent and tough.

The Iris

This is made of pigmented cells, which give eye colour, and muscle cells, which control the amount of light entering the eye.

The Lens

This is a transparent, rubbery tissue made of crystallin proteins, which crystallise to form a glass-like lens. The cells are laid down in layers as the eye develops to form the lens shape, but then die, so the lens does not need a blood supply. The proteins in different parts of the lens have slightly different refractive indices, which correct for chromatic aberrations.

The Ciliary body

This supports the lens. It comprises circular muscles and radial elastic fibres called suspensory ligaments. Together theses control the shape of the lens, as described below.

The Humours

These are the quaint names for the fluids inside the eye. They are secreted by the cells of the choroid. The vitreous humour behind the lens is more viscous than the aqueous humour in front of the lens.

The Retina

The retina contains the photoreceptor cells and their associated interneurones and sensory neurones. They are arranged as shown in this diagram:

A surprising feature of the retina is that it is back-to-front (inverted). The photoreceptor cells are at the back of the retina, and the light has to pass through several layers of neurones to reach them. This is due to the evolutionary history of the eye, and in fact doesn’t matter very much as the neurones are small and transparent. There are two kinds of photoreceptor cells in human eyes: rods and cones, and we shall look at the difference between these shortly. These rods and cones form synapses with special interneurones called bipolar neurones, which in turn synapse with sensory neurones called ganglion cells. The axons of these ganglion cells cover the inner surface of the retina and eventually form the optic nerve (containing about a million axons) that leads to the brain.

Visual Transduction

Visual transduction is the process by which light initiates a nerve impulse. The structure of a rod cell is:

The detection of light is carried out on the membrane disks in the outer segment. These disks contain thousands of molecules of rhodopsin, the photoreceptor molecule. Rhodopsin consists of a membrane-bound protein called opsin and a covalently-bound prosthetic group called retinal. Retinal is made from vitamin A, and a dietary deficiency in this vitamin causes night-blindness (poor vision in dim light). Retinal is the light-sensitive part, and it can exists in 2 forms: a cis form and a trans form:

In the dark retinal is in the cis form, but when it absorbs a photon of light it quickly switches to the trans form. This changes its shape and therefore the shape of the opsin protein as well. This process is called bleaching. The reverse reaction (trans to cis retinal) requires an enzyme reaction and is very slow, taking a few minutes. This explains why you are initially blind when you walk from sunlight to a dark room: in the light almost all your retinal was in the trans form, and it takes some time to form enough cis retinal to respond to the light indoors.

The final result of the bleaching of the rhodopsin in a rod cell is a nerve impulse through a sensory neurone in the optic nerve to the brain. However the details of the process are complicated and unexpected. Rod cell membranes contain a special sodium channel that is controlled by rhodopsin. Rhodopsin with cis retinal opens it and rhodopsin with trans retinal closes it. This means in the dark the channel is open, allowing sodium ions to flow in and causing the rod cell to be depolarised. This in turn means that rod cells release neurotransmitter in the dark. However the synapse with the bipolar cell is an inhibitory synapse, so the neurotransmitter stops the bipolar cell making a nerve impulse. In the light everything is reversed, and the bipolar cell is depolarised and forms a nerve impulse, which is passed to the ganglion cell and to the brain. Fortunately you don’t have to remember this, but you should be able to understand it.

Rods and Cones

Why are there two types of photoreceptor cell? The rods and cones serve two different functions as shown in this table:

Rods

Cones

  • Outer segment is rod shaped

  • Outer segment is cone shaped
  • 109 cells per eye, distributed throughout the retina, so used for peripheral vision.
  • 106 cells per eye, found mainly in the fovea, so can only detect images in centre of retina.
  • Good sensitivity – can detect a single photon of light, so are used for night vision.
  • Poor sensitivity – need bright light, so only work in the day.
  • Only 1 type, so only monochromatic vision.
  • 3 types (red green and blue), so are responsible for colour vision.
  • Many rods usually connected to one bipolar cell, so poor acuity (i.e. rods are not good at resolving fine detail).
  • Each cone usually connected to one bipolar cell, so good acuity (i.e. cones are used for resolving fine detail such as reading).
  • Although there are far more rods than cones, we use cones most of the time because they have fine discrimination and can resolve colours. To do this we constantly move our eyes so that images are focused on the small area of the retina called the fovea. You can only read one word of a book at a time, but your eyes move so quickly that it appears that you can see much more. the more densely-packed the cone cells, the better the visual acuity. In the fovea of human eyes there are 160 000 cones per mm2, while hawks have 1 million cones per mm2, so they really do have far better acuity.

    Colour Vision

    There are three different kinds of cone cell, each with a different form of opsin (they have the same retinal). These three forms of rhodopsin are sensitive to different parts of the spectrum, so there are red cones (10%), green cones (45%) and blue cones (45%). Coloured light will stimulate these three cells differently, so by comparing the nerve impulses from the three kinds of cone, the brain can detect any colour. For example:

    This is called the trichromatic theory of colour vision. The role of the brain in processing visual information is complex and not well understood, but our ability to detect colours depends on lighting conditions and other features of the image.

    The red, green and blue opsin proteins are made by three different genes. The green and red genes are on the X chromosome, which means that males have only one copy of these genes (i.e. they’re haploid for these genes). About 8% of males have a defect in one or other of these genes, leading to red-green colour blindness. Other forms of colour blindness are also possible, but are much rarer.

    Accommodation

    Accommodation refers to the ability of the eye to alter its focus so that clear images of both close and distant objects can be formed on the retina. Cameras do this by altering the distance between the lens and film, but eyes do it by altering the shape and therefore the focal length of the lens. Remember that most of the focusing is actually done by the cornea and the job of the lens to mainly to adjust the focus. The shape of the lens is controlled by the suspensory ligaments and the ciliary muscles.

    The suspensory ligaments are purely passive, but the ciliary muscles are innervated with motor neurones from the autonomic nervous system, and accommodation is controlled automatically by the brain.

    The Iris

    The retina is extremely sensitive to light, and can be damaged by too much light. The iris constantly regulates the amount of light entering the eye so that there is enough light to stimulate the cones, but not enough to damage them. The iris is composed of two sets of muscles: circular and radial, which have opposite effects (i.e. they’re antagonistic). By contracting and relaxing these muscles the pupil can be constricted and dilated:

    The iris is under the control of the autonomic nervous system and is innervated by two nerves: one from the sympathetic system and one from the parasympathetic system. Impulses from the sympathetic nerve cause pupil dilation and impulses from the parasympathetic nerve causes pupil constriction. The drug atropine inhibits the parasympathetic nerve, causing the pupil to dilate. This is useful in eye operations.

    The iris is a good example of a reflex arc.

    The Brain

    The human brain is the site of the major coordination in the nervous system. It contains around 1010 neurones, each making thousands of connections to others, so the number of pathways through the brain is vast. Different regions of the brain can be identified by their appearance, and it turns out that each region has a different role.

    These regions of the brain are all involved in involuntary functions, and are connected to the autonomic nervous system. A large part of the brain’s processing concerns these routine processes that keep the body working. By contrast, the upper half of the brain, the cerebrum, is responsible for all voluntary activities, and is connected to the somatic nervous system. The cerebrum is divided down the middle by a deep cleft into two cerebral hemispheres. The two halves are quite separate except for the corpus callosum, a bundle of 200 million neurones which run between the two halves. The inside contains fluid and only the outer few mm of the cerebral hemispheres contains neurones, and this is called the cerebral cortex (or just cortex). The cortex is highly folded and so has a large surface area. The cortex is the most complicated, fascinating and least-understood part of the brain.

    The Cerebral Cortex

    Various techniques have been used to investigate the functions of different parts of the brain. Patients with injuries to specific parts of the brain (such as strike victims) can be studies to see which functions are altered. The brain itself has no pain receptors, so during an operation on the brain, it can be studied while the patient is alert. Different parts of the brain can be stimulated electrically to see which muscles in the body respond, or conversely different parts of the body can be stimulated to see which regions of the brain show electrical activity. More recently, the non-invasive technique of magnetic resonance imaging (MRI) has been used to study brain activity of a subject without an operation.

    Studies like these have shown that the various functions of the cortex are localised into discrete areas. These areas can be split into three groups:

    Some of these areas are shown on this map of the surface of the cerebral cortex.

    Motor and Sensory Areas

    The main motor area controls the main skeletal muscles of the body, and the main sensory area receives input from the various skin receptors all over the body. These two areas are duplicated on the two cerebral hemispheres, but they control the opposite side of the body. So the main sensory and motor areas of the left cerebral hemisphere are linked to the right side of the body, and those of the right cerebral hemisphere are linked to the left side of the body.

    These two areas have been studied in great detail, and diagrams can be drawn mapping the part of the cortex to the corresponding part of the body. Such a map (also called a homunculus or "little man") can be drawn for the main sensory and motor areas:

    The sensory and motor maps are similar, though not identical, and they show that regions of the body with many sensory (or motor) neurones have correspondingly large areas of the cortex linked to them. So the lips occupy a larger region of the sensory cortex than the shoulder, because they have many more sensory neurones. Similarly, the tongue occupies a larger region of the motor cortex than the trunk because it has more motor neurones controlling its muscles.

     

    Association Areas

    While the jobs of the sensory and motor areas are reasonably well defined, the jobs of the association areas are far less clear. The association areas contain multiple copies of the sensory maps and they change as the sensory maps change. These copies are used to compare (or associate) sensory input with previous experiences, and so make decisions. They are therefore involved in advanced skills such as visual recognition, language understanding (aural and read), speech, writing and memory retrieval. The frontal lobes are particularly large in humans, and thought to be responsible for such higher functions as abstract thought, personality and emotion. We’ll look briefly at two examples of advanced processing: comprehension and visual processing.

    Comprehension

    This flow diagram shows how different areas of the cortex work together during a school lesson when a student has to understand the teacher’s written and spoken word, write notes, and answer questions.

    Unlike the sensory and motor areas, the association areas are not duplicated in the two hemispheres. Association areas in the two hemispheres seem to supervise different skills.

    These distributions apply to most right-handers, and are often reversed for left-handers. However, even this generalisation is often not true. For example, Broca’s area, the speech association areas is quite well-defined and well studied. 95% of right-handers have Broca’s area in their left hemisphere while 5% have it in their right. 70% of left-handers have Broca’s area in their left hemisphere, 15 in their right, and 15% in both hemispheres! Any reference to "right brain skills" or "left brain skills" should be taken with a large dose of scepticism.

    Visual Processing.

    The visual sensory area is at the back of the brain and receives sensory input from the optic nerves. Some of the neurones from each optic nerve cross over in the optic chiasma in the middle of the brain, so that neurones from the left half of the retinas of both eyes go to the visual sensory area in the left hemisphere and neurones from the right half of the retinas of both eyes go to the visual sensory area in the right hemisphere. Thus the two hemispheres see slightly different images from opposite side of the visual field, and the differences can be used to help judge distance.

    The mechanism of visual processing is complex and not well understood, but it is clear so far that the brain definitely does not work like a digital camera, by forming an image of pixels. Instead it seems to recognise shapes. The neurones in the visual cortex are arranged in 6 layers, each with a different hierarchical function in processing the visual information. The first layer recognises sloping lines, the second recognises complete shapes, the third recognises moving lines, and so on.

     

     

    Muscle

     

    ENGINE FOR SALE

    Powerful (100W/kg)

    Large Force (200kN/m2)

    Very Efficient (>50%)

    Silent Operation

    Non-Polluting

    Doesn’t Overheat (38°C)

    Uses a Variety of Fuels

    Lasts a Lifetime

    Good to Eat

    Ł10-00 per kg at your Supermarket

     

    Muscle is indeed a remarkable tissue. In engineering terms it far superior to anything we have been able to invent, and it is responsible for almost all the movements in animals. There are three types of muscle:

    This is always attached to the skeleton, and is under voluntary control via the motor neurones of the somatic nervous system. It is the most abundant & best understood type of muscle. It can be subdivided into red (slow) muscle and white (fast) muscle (see module 3).

    This is special type of red skeletal muscle. It looks and works much like skeletal muscle, but is not attached to skeleton, and is not under voluntary control (see module 3 for details).

    This is found in internal body organs such as the wall of the gut, the uterus, blood arteries, the iris, and glandular ducts. It is under involuntary control via the autonomic nervous system or hormones. Smooth muscle usually forms a ring, which tightens when it contracts, so there is no need of a skeleton to pull against.

    Unless mentioned otherwise, the rest of this section is about skeletal muscle.

     

    Muscles and the Skeleton

    Skeletal muscles cause the skeleton to move (or articulate) at joints. They are attached to the skeleton by tendons, which transmit the muscle force to the bone and can also change the direction of the force. Tendons are made of collagen fibres and are very strong and stiff (i.e. not elastic). The non-moving attachment point (nearest to the trunk) is called the origin, and moving end (furthest from the trunk) is called the insertion. The skeleton provides leverage, magnifying either the movement or the force.

    Muscles are either relaxed or contracted. In the relaxed state muscle is compliant (can be stretched), while in the contracted state muscle exerts a pulling force, causing it to shorten or generate force. Since muscles can only pull (not push), they work in pairs called antagonistic muscles. The muscle that bends (flexes) the joint is called the flexor muscle, and the muscle that straightens (extends) the joint is called the extensor muscle. The best-known example of antagonistic muscles are the biceps and triceps muscles, which articulate the elbow joint:

     

    The "relaxed" muscle is actually never completely relaxed. It is always slightly contracted to provide resistance to the antagonistic muscle and so cause a smoother movement. This slightly contracted condition is called tonus, or muscle tone. Most movements also involve many muscles working together, e.g. to bend a finger or to smile. These groups of muscles are called synergistic muscles.

    Muscle Structure

    A single muscle (such as the biceps) contains around 1000 muscle fibres running the whole length of the muscle and joined together at the tendons.

    Each muscle fibre is actually a single muscle cell about 100µm in diameter and a few cm long. These giant cells have many nuclei, as they were formed from the fusion of many smaller cells. Their cytoplasm is packed full of myofibrils, bundles of proteins filaments that cause contraction, and mitochondria to provide energy for contraction.

    The electron microscope shows that each myofibril is made up of repeating dark and light bands. In the middle of the dark band is a line called the M line and in the middle of the light band is a line called the Z line. The repeating unit from one Z line to the next is called a sarcomere.

    A very high resolution electron micrograph shows that each myofibril is made of parallel filaments. There are two kinds of alternating filaments, called the thick and thin filaments. These two filaments are linked at intervals by blobs called cross bridges, which actually stick out from the thick filaments.

    The thick filament is made of a protein called myosin. A myosin molecule is shaped a bit like a golf club, but with two heads. Many of these molecules stick together to form the thick filament, with the "handles" lying together to form the backbone and the "heads" sticking out in all directions to form the cross bridges.

    The thin filament is made of a protein called actin. Actin is a globular molecule, but it polymerises to form a long double helix chain. The thin filament also contains troponin and tropomyosin, two proteins involved in the control of muscle contraction.

     

    The thick and thin filaments are arranged in a precise lattice to form a sarcomere. The thick filaments are joined together at the M line, and the thin filaments are joined together at the Z line, but the two kinds of filaments are not joined to each other. The position of the filaments in the sarcomere explains the banding pattern seen by the electron microscope:

    Mechanism Of Muscle Contraction- the Sliding Filament Theory

    Knowing the structure of the sarcomere enables us to understand what happens when a muscle contracts. The mechanism of muscle contraction can be deduced by comparing electron micrographs of relaxed and contracted muscle:

    These show that each sarcomere gets shorter when the muscle contracts, so the whole muscle gets shorter. But the dark band, which represents the thick filament, does not change in length. This shows that the filaments don’t contract themselves, but instead they slide past each other. This sliding filament theory was first proposed by Huxley and Hanson in 1954, and has been confirmed by many experiments since.

     

    The Cross Bridge Cycle

    What makes the filaments slide past each other? Energy is provided by the splitting of ATP, and the ATPase that does this splitting is located in the myosin cross bridge head. These cross bridges can also attach to actin, so they are able to cause the filament sliding by "walking" along the thin filament. This cross bridge walking is called the cross bridge cycle, and it has 4 steps. One step actually causes the sliding, while the other 3 simply reset the cross bridge back to its starting state. It is analogous to the 4 steps involved in rowing a boat:

    1. The cross bridge swings out from the thick filament and attaches to the thin filament. [Put oars in water.]

    2. The cross bridge changes shape and rotates through 45°, causing the filaments to slide. The energy from ATP splitting is used for this "power stroke" step, and the products (ADP + Pi) are released. [Pull oars to drive boat through water.]

    3. A new ATP molecule binds to myosin and the cross bridge detaches from the thin filament. [push oars out of water.]

    4. The cross bridge changes back to its original shape, while detached (so as not to push the filaments back again). It is now ready to start a new cycle, but further along the thin filament. [push oars into starting position.]

    One ATP molecule is split by each cross bridge in each cycle, which takes a few milliseconds. During a contraction, thousands of cross bridges in each sarcomere go through this cycle thousands of times, like a millipede running along the ground. Fortunately the cross bridges are all out of synch, so there are always many cross bridges attached at any time to maintain the force.

     

     

     

    Control Of Muscle Contraction

    How is the cross bridge cycle switched off in a relaxed muscle? This is where the regulatory proteins on the thin filament, troponin and tropomyosin, are involved. Tropomyosin is a long thin molecule, and it can change its position on the thin filament. In a relaxed muscle is it on the outside of the filament, covering the actin molecules so that myosin cross bridges can’t attach. This is why relaxed muscle is compliant: there are no connections between the thick and thin filaments. In a contracting muscle the tropomyosin has moved into the groove of the double helix, revealing the actin molecules and allowing the cross bridges to attach.

    Contraction of skeletal muscle is initiated by a nerve impulse, and we can now look at the sequence of events from impulse to contraction (sometimes called excitation contraction coupling).

    1. An action potential arrives at the end of a motor neurone, at the neuromuscular junction.

    2. This causes the release of the neurotransmitter acetylcholine.

    3 This initiates an action potential in the muscle cell membrane.

    4. This action potential is carried quickly throughout the large muscle cell by invaginations in the cell membrane called T-tubules.

    5. The action potential causes the sarcoplasmic reticulum (large membrane vesicles) to release its store of calcium into the myofibrils.

    6. The calcium binds to troponin on the thin filament, which changes shape, moving tropomyosin into the groove in the process.

    7. Myosin cross bridges can now attach and the cross bridge cycle can take place.

    Relaxation is the reverse of these steps. This process may seem complicated, but it allows for very fast responses so that we can escape from predators and play the piano.