Solved Examples

Part 1 - The Neural System and Its Divisions

Start where the chapter starts. Every later question in this chapter assumes you can say, without hesitating, which structure belongs to which division and which way an impulse is travelling.

Question 1

Q. This chapter is built out of pairs. Give both members of each of the following pairs: the two systems that jointly coordinate the body; the two parts of the human neural system; the two kinds of nerve fibre in the peripheral neural system; the two divisions of the peripheral neural system; the two divisions of the autonomic neural system; the two types of synapse; the two types of axon.

Answer.

Learn them as pairs and you have most of the one-mark questions in the chapter already answered.

Question 2

Q. Set out the whole human neural system as a branching chart, from the neural system at the top down to the two divisions of the autonomic neural system, and say where the visceral nervous system fits on that chart.

Answer. The chart branches twice on the peripheral side and once more after that.

1. Human neural system
2. splits into central neural system (CNS) and peripheral neural system (PNS).
3. CNS = brain + spinal cord. It is the site of information processing and control. It does not branch any further in this chapter.
4. PNS = all the nerves of the body associated with the CNS. Its fibres are of two kinds - afferent, carrying impulses from the tissues and organs to the CNS, and efferent, carrying regulatory impulses from the CNS to the peripheral tissues and organs.
5. The PNS divides into the somatic neural system, which relays impulses from the CNS to the skeletal muscles, and the autonomic neural system, which transmits impulses from the CNS to the involuntary organs and the smooth muscles of the body.
6. The autonomic neural system is further classified into the sympathetic neural system and the parasympathetic neural system.

The visceral nervous system is not a fourth branch. It is the part of the PNS that comprises the whole complex of nerves, fibres, ganglia and plexuses by which impulses travel from the CNS to the viscera and from the viscera to the CNS. Draw it hanging off the PNS box, not off the top box.

Question 3

Q. Classify each of the following as belonging to the CNS or to the PNS. Where it is PNS, say whether it is somatic or autonomic: the brain; the spinal cord; the cranial nerves; the spinal nerves; the nerve running to the biceps muscle of the arm; the nerve supplying the smooth muscle of the gut; the ganglia and plexuses supplying the viscera.

Answer.

The trap in this list is the pair of nerves. A nerve that arises from the CNS is still a nerve, and the PNS comprises all the nerves of the body associated with the CNS. Only the brain and the spinal cord are CNS.

Question 4

Q. Work out the total number of pairs of nerves that connect the central neural system with the rest of the body, and the number of individual nerves that comes to. Then say which of those nerves belong to the CNS.

Answer. Add the two sets.

12 plus 31 is 43 pairs. A pair is two nerves, one on each side, so 43 pairs is 86 individual nerves.

None of them belongs to the CNS. The CNS is the brain and the spinal cord only. Every one of these 86 nerves is part of the PNS, because the PNS comprises all the nerves of the body associated with the CNS. The word arise tells you where a nerve starts, not which system owns it.

Question 5

Q. Two students argue about the visceral nervous system. One calls it a third division of the neural system, standing alongside the CNS and the PNS. The other says it is a part of the PNS. Settle the argument, and state what the visceral nervous system is made of and in which directions its impulses travel.

Answer. The second student is right. The visceral nervous system is the part of the peripheral neural system, not a system of its own. The human neural system has two parts only - the CNS and the PNS.

What it is made of: the whole complex of nerves, fibres, ganglia and plexuses - four kinds of structure, and all four are named in the definition.

Which way its impulses travel: both ways. Impulses are carried from the CNS to the viscera and from the viscera to the CNS. That two-way traffic is the reason it needs both efferent and afferent fibres, and it is the half of the definition students leave out.

Question 6

Q. For each of the following, name the division of the PNS at work, and say whether the action is voluntary or involuntary: lifting a cup; the heart beating faster during a race; the gut wall churning food; kicking a football; the pupil narrowing in bright light.

Answer.

The one test that sorts every row: ask what kind of muscle or organ is on the receiving end. A skeletal muscle means somatic. An involuntary organ or a smooth muscle means autonomic.

Question 7

Q. A person suffers a complete injury to the spinal cord in the neck. Below the level of the injury all sensation is lost, and the brain can no longer move the legs, even though the nerves of the legs themselves are undamaged. Explain both losses, and name the kind of fibre that fails in each case.

Answer. The spinal cord is part of the CNS, and the CNS is the site of information processing and control. It is also the road every message from the legs to the brain, and from the brain to the legs, must travel on. Cut that road and both directions of traffic stop.

1. Loss of sensation. Signals from the receptors in the legs are carried by afferent nerve fibres, which carry impulses from the tissues and organs to the CNS. Those impulses now reach the spinal cord below the injury and can go no further, so they never arrive at the brain and nothing is felt.
2. Loss of movement. Commands for the leg muscles leave the brain along the CNS and pass out to the muscles along efferent nerve fibres of the somatic neural system, which relay impulses from the CNS to the skeletal muscles. The command cannot get past the injury, so the muscle is never told to contract.

The nerves of the legs are healthy but disconnected. The failure is in the CNS, not in the PNS - which is exactly why the question is worth asking.

[NEET Important] The three classification facts in this run are asked over and over. CNS is the brain and the spinal cord, nothing else. Every nerve is PNS, including the cranial nerves. Afferent goes IN to the CNS and efferent goes OUT to the tissues. The commonest distractor swaps afferent and efferent, and the second commonest puts the spinal nerves in the CNS because they arise from the spinal cord.

Part 2 - The Neuron and the Impulse

The neuron half of the chapter is one structure and one mechanism. The structure is lost by putting a part in the wrong place; the mechanism is lost by naming the wrong ion.

Question 8

Q. Complete a table of the three major parts of a neuron, giving for each one whether it holds Nissl's granules, the direction in which it carries an impulse, and the structure that identifies it.

Answer. A neuron is composed of three major parts - the cell body, the dendrites and the axon.

The one line that gets the mark: dendrites transmit impulses TOWARDS the cell body and the axon transmits impulses AWAY from the cell body, delivering them to a synapse or to a neuro-muscular junction.

Question 9

Q. Three neurons are examined under a microscope. Neuron A has one axon and four dendrites. Neuron B has one axon and one dendrite. Neuron C has a cell body with one axon and no dendrite at all. Name each type and give the place in the body where the chapter says each is found.

Answer. Neurons are divided into these types on the basis of the number of axons and dendrites.

Every one of the three has exactly one axon. The count that separates them is the number of dendrites, and the distractor that catches students is a "unipolar neuron" described as having a single dendrite. It has a single axon and no dendrite.

Question 10

Q. Myelinated fibres are found in the spinal and cranial nerves, while unmyelinated fibres are commonly found in the autonomous and the somatic neural systems. Explain what the myelin sheath does for a fibre, and why the two kinds are found where they are.

Answer. Both kinds of axon are wrapped by Schwann cells; only one kind gets a sheath out of it.

• In a myelinated nerve fibre the Schwann cells form a myelin sheath around the axon. The sheath is not continuous - the gaps between two adjacent myelin sheaths are called nodes of Ranvier.
• In an unmyelinated nerve fibre the axon is enclosed by a Schwann cell that does not form a myelin sheath.

What the sheath buys the fibre. In a myelinated fibre the impulse jumps from one node of Ranvier to the next, instead of being regenerated at every point along the membrane. So conduction is much faster and costs the fibre much less energy.

Why they sit where they do. The spinal and cranial nerves are the long-distance cables of the body, carrying impulses between the CNS and distant organs, and speed there is worth the extra insulation. The fibres of the autonomous and somatic neural systems that the chapter calls unmyelinated manage with step-by-step conduction along the whole length of the membrane.

Question 11

Q. The axoplasm of a resting neuron holds a high concentration of $\mathrm{K^+}$, and potassium ions carry a positive charge. Explain why the inner surface of the membrane is nevertheless negative.

Answer. Because the potassium ions are not the only thing inside. The axoplasm holds a high concentration of $\mathrm{K^+}$ AND a high concentration of negatively charged proteins, along with a low concentration of $\mathrm{Na^+}$. The fluid outside is the mirror image - a low concentration of $\mathrm{K^+}$ and a high concentration of $\mathrm{Na^+}$.

Two facts about the membrane then decide the charge:

1. The membrane is impermeable to the negatively charged proteins in the axoplasm. Those negative charges are trapped inside and can never balance themselves out against the fluid outside.
2. The resting membrane is comparatively more permeable to $\mathrm{K^+}$ and nearly impermeable to $\mathrm{Na^+}$, and the sodium-potassium pump works constantly, moving 3 $\mathrm{Na^+}$ out for every 2 $\mathrm{K^+}$ in.

The result is that the outer surface of the axonal membrane possesses a positive charge while the inner surface is negatively charged, and is therefore polarised. The electrical potential difference across the resting plasma membrane is called the resting potential, taken in examinations as about -70 millivolts.

So the answer is the protein, not the potassium. Counting only the positive ions is what makes this question look impossible.

Question 12

Q. The sodium-potassium pump transports 3 $\mathrm{Na^+}$ out for every 2 $\mathrm{K^+}$ in. Work out the net charge moved across the membrane in one cycle of the pump and say what that does to the two surfaces. Then predict what would happen to the resting potential if the pump moved 2 ions out for 2 ions in.

Answer. Count the charges each way. Three positive charges leave the cell and two positive charges enter it. Three out minus two in leaves a net of one positive charge moved outwards per cycle.

That single charge is what the pump contributes to the polarised state: it makes the outer surface a little more positive and the inner surface a little more negative, on top of the negatively charged proteins already trapped in the axoplasm. The pump does this by active transport, which means it spends energy to move each ion against its concentration gradient - that is how the concentration gradient of the two ions is maintained at rest.

If the pump moved 2 out for 2 in, it would move no net charge at all. The gradients would still be pushed, but the pump would no longer add anything to the separation of charge, so the membrane would be less strongly polarised and the resting potential would be smaller. Push the idea further - stop the pump altogether and the gradients themselves run down, the polarised state is lost, and the neuron stops being an excitable cell.

Question 13

Q. Complete this table of the charge on the outer and the inner surface of the axonal membrane at each stage of a nerve impulse, and name the ion movement responsible for each stage.

Answer.

The potential difference across the membrane at the depolarised site is the action potential, which is in fact termed a nerve impulse. After repolarisation the fibre becomes once more responsive to further stimulation.

Question 14

Q. An action potential is generated at site 'A' and passes to site 'B', which lies immediately ahead of it. Explain why the impulse goes on forwards and does not immediately travel back into site 'A'.

Answer. Take the two sites in turn.

1. At site 'A' the membrane has become freely permeable to $\mathrm{Na^+}$. The rapid influx of $\mathrm{Na^+}$ reverses the polarity there, so the outer surface becomes negative and the inner surface positive, and site 'A' is depolarised.
2. At site 'B', immediately ahead, the membrane is still positive on the outer surface and negative on the inner surface. That difference is the whole reason a current can flow: current flows on the inner surface from site 'A' to site 'B' and on the outer surface from site 'B' to site 'A', completing the circuit, and the polarity at site 'B' is reversed, so an action potential is generated at site 'B'.
3. Now look back at site 'A'. The stimulus-induced rise in permeability to $\mathrm{Na^+}$ is extremely short-lived and is quickly followed by a rise in permeability to $\mathrm{K^+}$. Within a fraction of a second $\mathrm{K^+}$ diffuses outside the membrane and restores the resting potential at site 'A'.

So by the time site 'B' is carrying the action potential, site 'A' has already been repolarised and is positive outside once more. There is no longer a reversed patch behind for the current to flow into, so the only place the circuit can be completed is the next unexcited point ahead. The sequence is repeated along the length of the axon and the impulse is conducted in one direction.

Question 15

Q. In a disease the myelin sheath around the fibres of a cranial nerve is destroyed, although the axons themselves survive. Predict what happens to the speed of conduction and to the energy the fibre spends, and explain both.

Answer. Conduction becomes much slower, and the fibre spends much more energy.

Why. The myelin sheath is what creates the nodes of Ranvier - the gaps between two adjacent myelin sheaths. In a healthy myelinated fibre the impulse jumps from one node of Ranvier to the next, so the membrane has to generate an action potential only at the nodes.

Destroy the sheath and the nodes go with it. The fibre is left conducting step by step along the whole length of its membrane, the way an unmyelinated fibre does - the action potential must be regenerated at every point in turn. Every one of those regenerations means another rapid influx of $\mathrm{Na^+}$ that the sodium-potassium pump must afterwards undo by active transport, and that is where the extra energy goes.

[NEET Important] Three facts from this run are asked almost every year. The resting membrane is more permeable to $\mathrm{K^+}$ and nearly impermeable to $\mathrm{Na^+}$; the pump moves 3 $\mathrm{Na^+}$ out for 2 $\mathrm{K^+}$ in; the impulse is a rapid influx of $\mathrm{Na^+}$ followed by a rise in permeability to $\mathrm{K^+}$. The distractors reverse the two ions in any one of the three. Read which ion is named before you read what it is doing.

Part 3 - The Synapse

A synapse is the one place in the chapter where an electrical signal is handed over as a chemical one and picked up as an electrical signal again. Learn the handover as a numbered sequence, because that is how it is marked.

Question 16

Q. Give a brief account of the mechanism of synaptic transmission. This is one of the chapter-end exercises.

Answer. A nerve impulse is passed from one neuron to another through a junction called a synapse. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft. There are two types of synapses, namely electrical synapses and chemical synapses.

At an electrical synapse the membranes of the pre-synaptic and post-synaptic neurons are in very close proximity. Electrical current can flow directly from one neuron into the other across these synapses. Transmission of an impulse across an electrical synapse is very similar to impulse conduction along a single axon. Impulse transmission across an electrical synapse is always faster than that across a chemical synapse, but electrical synapses are rare in our system.

At a chemical synapse the membranes of the pre-synaptic and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft, and chemicals called neurotransmitters are involved in the transmission of impulses. The sequence runs like this:

1. The axon terminals of the pre-synaptic neuron contain vesicles filled with neurotransmitters.
2. When an impulse - an action potential - arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane.
3. The vesicles fuse with the plasma membrane of the axon terminal.
4. The neurotransmitters are released into the synaptic cleft, the fluid-filled space between the two neurons.
5. The released neurotransmitters cross the cleft and bind to their specific receptors, present on the post-synaptic membrane.
6. This binding opens ion channels, allowing the entry of ions.
7. The entry of ions can generate a new potential in the post-synaptic neuron.
8. The new potential developed may be either excitatory or inhibitory - the post-synaptic neuron is either pushed towards firing an impulse of its own or held back from it.

The one line that gets the mark: at a chemical synapse the impulse is carried across by a neurotransmitter released into the synaptic cleft, which binds to specific receptors on the post-synaptic membrane, opens ion channels, and generates a new potential that may be excitatory or inhibitory.

Question 17

Q. An impulse is recorded crossing two junctions in the body. At the first junction there is no measurable delay between the arrival of the impulse on one side and its appearance on the other. At the second there is a short but definite delay. Identify the type of synapse at each junction and account for the delay.

Answer. The first is an electrical synapse and the second is a chemical synapse.

At the electrical synapse the two membranes are in very close proximity, and electrical current flows directly from one neuron into the other. Nothing has to be made, released or bound, so transmission is very similar to impulse conduction along a single axon, and transmission across an electrical synapse is always faster than across a chemical synapse.

At the chemical synapse the current cannot cross, because the membranes are separated by a fluid-filled synaptic cleft. The signal has to be handed over in stages - vesicles move to the membrane, fuse with it, release neurotransmitters into the cleft, the neurotransmitters bind to receptors on the post-synaptic membrane, ion channels open, and only then does a new potential appear. Each of those stages takes time, and their sum is the delay that was measured.

Do not conclude that the faster kind is the commoner kind. Electrical synapses are rare in our system; almost all of the transmission in the human body is chemical.

Question 18

Q. The same neurotransmitter is released at two different synapses. At one of them the post-synaptic neuron is pushed towards firing an impulse; at the other it is not. Explain how the chapter accounts for this.

Answer. The neurotransmitter does not carry the instruction; the post-synaptic membrane decides what the instruction means.

The chapter says that the released neurotransmitters bind to their SPECIFIC receptors present on the post-synaptic membrane, that this binding opens ion channels, and that the entry of ions generates a new potential in the post-synaptic neuron. Then it adds the sentence this question turns on: the new potential developed may be either excitatory or inhibitory.

So the outcome depends on which receptors the post-synaptic membrane carries and which ion channels their binding opens. If the ions that enter drive the membrane towards depolarisation, the new potential is excitatory and the post-synaptic neuron is pushed towards generating an impulse of its own. If they hold the membrane away from that, the new potential is inhibitory and the neuron is held back.

The same chemical, two different receiving membranes, two different answers. That is why a synapse is a control point and not just a gap to be crossed.

Question 19

Q. A drug prevents the synaptic vesicles from fusing with the plasma membrane of the axon terminal. Number the steps of chemical synaptic transmission, say at which step transmission now fails, and state what a recording on each side of the synapse would show.

Answer. The steps, numbered: (1) the impulse arrives at the axon terminal; (2) it stimulates the movement of the synaptic vesicles towards the membrane; (3) the vesicles fuse with the plasma membrane; (4) neurotransmitters are released into the synaptic cleft; (5) they bind to their specific receptors on the post-synaptic membrane; (6) the binding opens ion channels; (7) the entry of ions generates a new potential in the post-synaptic neuron.

Transmission fails at step 3. Steps 1 and 2 still happen - the impulse arrives and the vesicles move towards the membrane - but with fusion blocked, nothing is released, and steps 4 to 7 cannot follow.

The lesson the failure teaches: the pre-synaptic impulse is not what crosses the synapse. The neurotransmitter is the messenger, and a synapse with no release is a dead end however healthy the axon behind it.

Question 20

Q. The definition of a synapse says the two membranes "may or may not be separated by a gap called the synaptic cleft". Why is the definition worded so cautiously?

Answer. Because one definition has to cover both types of synapse, and only one of them has a gap.

• At a chemical synapse the membranes of the pre-synaptic and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft. Here the gap is present, and it is essential - the neurotransmitters are released into it.
• At an electrical synapse the membranes of the pre-synaptic and post-synaptic neurons are in very close proximity, close enough that electrical current flows directly from one neuron into the other. There is no fluid-filled cleft to speak of.

A definition that insisted on a synaptic cleft would quietly throw the electrical synapse out of the category. The cautious wording keeps both kinds inside one definition, and it tells you in advance that the presence or absence of the cleft is exactly what separates the two types.

Question 21

Q. Compare the conduction of an impulse along an axon with its transmission across a chemical synapse.

Answer.

The one line that gets the mark: along an axon the impulse travels as an electrical event in the membrane of one neuron; across a chemical synapse it is handed over as a chemical messenger between two neurons, and only then becomes electrical again.

[NEET Important] The sequence at a chemical synapse is asked as a jumbled-order item. Fix the order by asking what has to exist before the next step can happen - the vesicle must move before it can fuse, it must fuse before anything is released, and the neurotransmitter must bind before any channel can open. The other favourite is the comparison of the two synapse types: the electrical synapse is always the faster one, and it is the rare one.

Part 4 - The Brain

The brain half of the chapter is an address book. Every question is some version of "which part, and what does it do", so the fastest way to revise it is to keep the map in front of you as a map.

Question 22

Q. Match each part of the brain in Column A with the function it controls in Column B.

Answer.

The answer key: (i) - (d); (ii) - (b); (iii) - (f); (iv) - (c); (v) - (g); (vi) - (a); (vii) - (e).

The two rows that get swapped are (iv) and (v). The thalamus coordinates sensory and motor signalling; the midbrain receives and integrates the visual, tactile and auditory inputs. Both sentences contain the word "sensory" in spirit, and that is what the examiner is counting on.

Question 23

Q. A surgeon has to reach the tissue of the brain from the outside. Name, in order, every protective covering that must be passed, with the identifying feature of each.

Answer. The brain is well protected by the skull. Inside the skull it is covered by the cranial meninges, and there are three layers of them. Working from the outside inwards:

Two things go wrong in this answer. Students put the pia mater on the outside because its name sounds soft, and they forget the skull, which the chapter names first. The order to memorise is skull, dura mater, arachnoid, pia mater - hard to soft, outside to in.

Question 24

Q. The cerebral cortex is thrown into prominent folds, and the cerebellum has a very convoluted surface. What problem are both of them solving, and what does the chapter say each one gains by it?

Answer. Both are solving the same problem - there is only so much room inside the skull, and both parts need to hold more neurons than a smooth surface could carry.

• Of the cerebellum the chapter says it plainly: it has a very convoluted surface in order to provide the additional space for many more neurons.
• Of the cerebrum, the layer of cells which covers the cerebral hemisphere is called the cerebral cortex and is thrown into prominent folds. That cortex is the grey matter, and it is grey because of the concentration of neuron cell bodies in it. Folding the sheet packs more of that cell-body-rich layer into the same volume.

So folding buys surface, and surface is where the cell bodies sit. Underneath the folded grey sheet lie the fibres of the tracts, covered with the myelin sheath, which make the inner part of the cerebral hemisphere and give it an opaque white appearance - the white matter.

Question 25

Q. Four patients are described. (a) One staggers and cannot keep his balance, though his muscles are strong. (b) One has irregular breathing and cardiovascular reflexes that are no longer adjusted. (c) One can no longer regulate body temperature and has lost the normal urge to eat and drink. (d) One moves and feels normally, but the expression of emotion, motivation and sexual behaviour is deranged. Name the part of the brain damaged in each case and the division of the brain it belongs to.

Answer.

Patient (a) is the one that gets mislabelled. Staggering looks like a muscle problem or a cerebral one, but strong muscles with poor balance point at the cerebellum every time.

Question 26

Q. Set out the counts this chapter expects you to know about the brain and its nerves, and name the count that is most often got wrong.

Answer.

The count that goes wrong is the brain stem. It has three regions - midbrain, pons and medulla oblongata - and the cerebellum is not one of them, even though the cerebellum is a hindbrain part sitting right beside two of the three. The brain stem forms the connections between the brain and the spinal cord; the cerebellum sits to one side of that road rather than on it.

Question 27

Q. Start at the outer surface of a cerebral hemisphere and travel inwards. Name what you pass through in order, give the colour of each layer with the reason for that colour, and name the structures you meet at the centre.

Answer. In order:

1. The cerebral cortex. The layer of cells which covers the cerebral hemisphere, thrown into prominent folds. It is called the grey matter because of its greyish appearance, which is due to the concentration of neuron cell bodies there. It carries the motor areas, sensory areas and the association areas, the last of which are neither clearly sensory nor motor and are responsible for intersensory associations, memory and communication.
2. The white matter. Below the cortex lie the fibres of the tracts, which are covered with the myelin sheath. These make up the inner part of the cerebral hemisphere and give it an opaque white appearance.
3. The corpus callosum. Travelling across the midline you meet the tract of nerve fibres which connects the two cerebral hemispheres, separated as they are by a deep cleft running longitudinally.
4. The thalamus. The cerebrum wraps around it. It is a major coordinating centre for sensory and motor signalling.
5. The hypothalamus. It lies at the base of the thalamus, and contains centres which control body temperature and the urge for eating and drinking, along with groups of neurosecretory cells which secrete hypothalamic hormones.

The colour rule in one line: cell bodies make grey matter and lie on the outside; myelinated fibres make white matter and lie on the inside. Reverse them and you lose the mark.

Question 28

Q. In an accident the cerebrum is severely damaged while the brain stem is left intact. Which functions would be lost and which would continue? Name the part responsible in each case.

Answer. The brain stem is the midbrain, the pons and the medulla oblongata, and it forms the connections between the brain and the spinal cord. The cerebrum is none of those three, so the damage and the survival divide cleanly.

So the body goes on running while the person is unable to think, speak or move to order. That division is exactly what the chapter means when it separates the command and control work of the cerebrum from the vital involuntary work of the medulla.

[NEET Important] Almost every brain question is a which-part question, and the wrong options are always the neighbouring real parts. Thalamus - sensory and motor coordination. Hypothalamus - temperature, hunger, thirst, the 24-hour rhythm. Midbrain - visual, tactile and auditory input. Cerebellum - balance. Medulla - respiration, cardiovascular reflexes, gastric secretions. Pons - fibre tracts between regions of the brain. Learn those six lines and the address questions stop being guesswork.

Part 5 - Mixed and Harder Problems

These last items cross the boundaries between the parts of the chapter. They are the ones that separate a student who has learnt the lists from a student who can use them.

Question 29

Q. Say whether each of these statements is true or false, and correct every false one. (i) The spinal cord is a part of the peripheral neural system. (ii) At rest the axonal membrane is more permeable to sodium ions than to potassium ions. (iii) An electrical synapse conducts an impulse faster than a chemical synapse. (iv) The cerebellum is one of the three regions of the brain stem. (v) Afferent fibres carry regulatory impulses from the CNS to the peripheral tissues and organs.

Answer.

Four of the five are false, and every one of them is false in the same way - a real term has been put next to a real description that belongs to its partner. Read the pairing, not the words.

Question 30

Q. The passage below contains five errors. Find them all and write the corrected version. "When a stimulus is applied at site 'A', the membrane there becomes freely permeable to potassium ions. The rapid influx makes the outer surface positive and the inner surface negative. The potential difference so produced is called the resting potential. The rise in permeability lasts a long time, and the resting state is finally restored by sodium ions moving out of the axon."

Answer. The five errors, in order.

The corrected passage. When a stimulus is applied at site 'A', the membrane there becomes freely permeable to $\mathrm{Na^+}$. The rapid influx of $\mathrm{Na^+}$ reverses the polarity, so the outer surface becomes negative and the inner surface positive, and the site is depolarised. The potential difference so produced is the action potential, which is in fact termed a nerve impulse. The rise in permeability to $\mathrm{Na^+}$ is extremely short-lived and is quickly followed by a rise in permeability to $\mathrm{K^+}$; within a fraction of a second $\mathrm{K^+}$ diffuses outside the membrane and the resting potential is restored, and the fibre becomes once more responsive to further stimulation.

Question 31

Q. Match each structure in Column A with the system or part of the body it belongs to in Column B.

Answer.

The answer key: (i) - (d); (ii) - (f); (iii) - (a); (iv) - (b); (v) - (c); (vi) - (g); (vii) - (e).

Two rows deserve a second look. Nissl's granules are in the cell body AND the dendrites, not in the axon - the axon's identifying structure is the synaptic knob at the end of each branch, holding the synaptic vesicles containing neurotransmitters. And the synaptic cleft belongs to a chemical synapse only; at an electrical synapse the membranes are in very close proximity with no such gap.

Question 32

Q. For each of the following structures, state what would fail if it stopped working: the sodium-potassium pump; the myelin sheath; the synaptic vesicles; the receptors on the post-synaptic membrane; the medulla.

Answer.

Read the table as a chain. The first two rows break the impulse within a neuron, the next two break the handover between two neurons, and the last breaks the control centre the whole chain was reporting to.

Question 33

Q. Pick the odd one out of each group and justify your choice. (a) dura mater, arachnoid, pia mater, corpus callosum. (b) cerebrum, thalamus, hypothalamus, cerebellum. (c) midbrain, pons, medulla, cerebellum. (d) multipolar, bipolar, unipolar, myelinated. (e) sympathetic, parasympathetic, autonomic, somatic.

Answer.

The cerebellum is the odd one out twice, for two different reasons. In (b) it is odd because of the division it belongs to; in (c) because of the list it is missing from. Read what the group has in common before you pick.

Question 34

Q. Write a full account of what happens between the decision to lift a hand, taken in the brain, and the contraction of the muscle that lifts it. Name every part of the neural system involved, in order.

Answer. Follow the command down the road it actually takes.

1. The command is formed in the cerebrum. The cerebrum forms the major part of the human brain and controls voluntary movements. The order is generated in the motor areas of the cerebral cortex, the grey matter whose greyish colour comes from the concentration of neuron cell bodies. The neurons doing the work there are multipolar, the type found in the cerebral cortex.
2. It travels through the white matter. The fibres of the tracts, covered with the myelin sheath, make up the inner part of the cerebral hemisphere - the white matter - and carry the command away from the cortex.
3. It passes down the brain stem. The midbrain, the pons and the medulla oblongata together form the connections between the brain and the spinal cord, and the pons in particular consists of fibre tracts that interconnect different regions of the brain.
4. It reaches the spinal cord, which with the brain makes up the CNS, the site of information processing and control.
5. It leaves the CNS along an efferent fibre. Efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues or organs, and this one runs in a spinal nerve - one of the 31 pairs that arise from the spinal cord.
6. The somatic neural system takes it to the muscle, because the somatic division is the one that relays impulses from the CNS to the skeletal muscles.
7. Along each axon the impulse moves as a wave. A stimulus makes the membrane freely permeable to $\mathrm{Na^+}$ at one site, the rapid influx reverses the polarity and an action potential appears; the site immediately ahead is depolarised in turn, while the site behind is repolarised by $\mathrm{K^+}$ diffusing outside. In a myelinated fibre the impulse jumps from one node of Ranvier to the next, which is why the arm moves at once and not in a second or two.
8. Every junction on the way is crossed chemically. At each chemical synapse the impulse stimulates the movement of the synaptic vesicles towards the membrane; they fuse and release neurotransmitters into the synaptic cleft; these bind to specific receptors on the post-synaptic membrane, open ion channels, and generate a new potential in the next neuron.
9. The axon ends at a neuro-muscular junction - the chapter names this, along with the synapse, as the destination an axon delivers to - and the muscle contracts, lifting the hand.
10. Two more parts are working while it happens. The thalamus, which the cerebrum wraps around, is the major coordinating centre for sensory and motor signalling, and the cerebellum is keeping the balance of the body so that lifting the hand does not topple you.

The single sentence that holds the whole answer together: the command is generated in the cerebrum, carried down through the brain stem and the spinal cord of the CNS, and delivered by an efferent fibre of the somatic neural system to a skeletal muscle, travelling as an electrical wave along each axon and as a chemical messenger across each synapse.