Questions on organisation of the nervous system

Question 1

A person treads on a sharp stone, pulls the foot away without thinking, and then notices that their heart is beating faster. Explain which divisions of the nervous system bring about each of these two responses, and explain why only the first of them could also have been performed on purpose.

Accepted Answer

B1 the withdrawal is a reflex carried by the peripheral nervous system: a sensory neurone carries impulses from the receptor into the spinal cord, a relay neurone in the CNS connects the pathway, and a motor neurone carries impulses out to the effector B1 the effector is skeletal muscle, which is supplied by the somatic nervous system, and the same motor pathways can be driven voluntarily from the cerebrum, which initiates movement, so the action could also have been performed on purpose B1 the faster heartbeat is the work of the autonomic nervous system, whose sympathetic division prepares the body for activity, acting on instruction from the cardiovascular centre in the medulla oblongata B1 cardiac muscle is myogenic, so the beat itself originates at the sinoatrial node and the sympathetic impulses only raise its rate, with parasympathetic impulses lowering it again once the danger has passed B1 the autonomic system supplies glands, smooth muscle and cardiac muscle, and its pathways are not driven from the conscious cortex, so the change in heart rate happens whether or not the person notices it and cannot be decided on

Question 2

Describe the effects of the sympathetic and the parasympathetic systems on the heart, the pupils, the gut and the bronchioles.

Accepted Answer

B1 the sympathetic system speeds the heart up and the parasympathetic system slows it down B1 the sympathetic system dilates the pupils and the parasympathetic system constricts them B1 the sympathetic system inhibits movement and secretion in the gut, whereas the parasympathetic system stimulates them B1 the sympathetic system dilates the bronchioles and the parasympathetic system constricts them; overall the first prepares the body for activity and the second returns it to rest

Question 3

Compare skeletal muscle, involuntary muscle and cardiac muscle as tissues.

Accepted Answer

B1 skeletal muscle is made of long multinucleate striated fibres, involuntary muscle of small spindle-shaped cells with one nucleus and no striations, and cardiac muscle of short branched striated cells joined end to end at intercalated discs so the tissue contracts as a connected sheet B1 skeletal muscle takes its orders from the somatic system and involuntary muscle from the autonomic system, whereas cardiac muscle is myogenic: the beat begins in the muscle itself at the sinoatrial node B1 the autonomic system only adjusts the heart's rate, sympathetic impulses raising it and parasympathetic impulses lowering it on instruction from the medulla, rather than causing the beat as it causes contraction in the other two tissues B1 skeletal muscle contracts rapidly and powerfully and fatigues, whereas involuntary muscle contracts slowly and steadily and cardiac muscle rhythmically and continuously, and neither of these fatigues

Question 4

After a head injury a patient's heart rate and breathing become irregular, while their speech, memory and balance are unaffected. Suggest which region of the brain has been damaged, and suggest how the other regions can be ruled out.

Accepted Answer

B1 the medulla oblongata, which holds the cardiovascular centre that sets heart rate and blood vessel diameter and the ventilation centre that sets breathing rhythm B1 the cerebrum can be ruled out because the conscious functions of its cortex, language and memory, are intact B1 the cerebellum can be ruled out because balance and the coordination of movement, which it handles, are unaffected B1 the hypothalamus and pituitary govern thermoregulation, osmoregulation and hormone release rather than heart rate and breathing, and nothing in the symptoms involves them

Question 5

Explain how the pupil of the eye becomes smaller when a person walks from a dark room into bright sunlight.

Accepted Answer

B1 the increase in light is detected by the rods and cones of the retina, and impulses pass along the optic nerve to the brain B1 impulses are sent along parasympathetic motor neurones to the iris B1 the circular muscles of the iris contract B1 the radial muscles relax, so the pupil constricts and less light enters the eye, protecting the retina

Question 6

Describe the structure of the spinal cord as it appears in a cross-section.

Accepted Answer

B1 a central region of grey matter, roughly H-shaped or butterfly-shaped, containing cell bodies, dendrites, relay neurones and synapses B1 a narrow central canal running through the middle of the grey matter B1 an outer region of white matter made of myelinated axons running up and down the cord B1 a dorsal root carrying sensory neurones in, with their cell bodies in the dorsal root ganglion, and a ventral root carrying motor neurones out, joining to form a spinal nerve

Question 7

Compare the somatic nervous system with the autonomic nervous system, referring to what each supplies, whether its responses are voluntary, and an effector each system controls.

Accepted Answer

B1 the somatic system supplies skeletal muscle, whereas the autonomic system supplies glands, smooth muscle and cardiac muscle B1 somatic responses are voluntary or reflex, whereas autonomic responses are not under conscious control B1 an effector of the somatic system is a skeletal muscle such as the biceps, whereas an effector of the autonomic system is cardiac muscle or the smooth muscle of the gut B1 both are motor divisions of the peripheral nervous system, carrying impulses out from the central nervous system to their own effectors

Question 8

Name the two lobes of the pituitary gland, and name what each one does with the hormones of the hypothalamus.

Accepted Answer

B1 the posterior lobe of the pituitary B1 stores and releases hormones made by the hypothalamus B1 the anterior lobe of the pituitary B1 secretes its own hormones, under the control of the hypothalamus

Question 9

A patient loses the ability to regulate body temperature and water balance after a head injury, but their movement, speech and heart rate are normal. Suggest which region of the brain has been damaged, and suggest how the other regions can be ruled out.

Accepted Answer

B1 the hypothalamus, because it holds the osmoreceptors that monitor water potential and the centre that regulates temperature B1 the cerebrum can be ruled out because the conscious functions of its cortex, speech and voluntary movement, are unaffected B1 the medulla oblongata can be ruled out because heart rate and breathing rhythm, which its centres control, are normal B1 the cerebellum can be ruled out because balance and the coordination of movement, which it handles, are unaffected

Question 10

Name the region of the brain that sets heart rate and breathing rhythm, the region that coordinates balance and the timing of movement, and the region that monitors the water potential of the blood.

Accepted Answer

B1 the medulla oblongata contains the cardiovascular centre and the ventilation centre B1 the cerebellum coordinates balance and the timing and strength of contractions B1 the hypothalamus holds the osmoreceptors that monitor the water potential of the blood

Question 11

Explain how a sensory neurone can be recognised in an unlabelled drawing, and explain why its long fibre is called a dendron rather than an axon.

Accepted Answer

B1 its cell body lies neither at the start nor at the end of the fibre but on a short side branch part-way along it, housed in a ganglion just outside the spinal cord B1 a dendron carries impulses towards a cell body and an axon carries them away from it B1 the impulse in the long fibre is travelling from the receptor towards the cell body, so that fibre is a dendron, and only the short fibre running on into the CNS is an axon

Question 12

Some eye drops used before an eye examination block the acetylcholine receptors of the circular muscles of the iris. Suggest why the pupils stay dilated even when a bright light is shone into the eye.

Accepted Answer

B1 the parasympathetic neurones that supply the circular muscles release acetylcholine, which cannot now bind, so the circular muscles cannot contract B1 the radial muscles are not opposed, so the pupil is held wide B1 bright light still produces impulses that reach the iris, but the effector cannot respond, so the pupil reflex does not occur

Question 13

An injury damages the ventral roots that serve the muscles of one leg, leaving the dorsal roots intact. Suggest the effect on the person's ability to feel a touch on that leg and to move it, and explain your answer.

Accepted Answer

B1 the touch is still felt, because sensory neurones enter the cord by the intact dorsal roots and impulses can travel up the white matter to the brain B1 the leg cannot be moved, either voluntarily or by reflex B1 the axons of the motor neurones leave the cord by the ventral roots, so impulses cannot reach the leg muscles

Question 14

Describe the structure of a relay neurone, and describe its position within a reflex arc.

Accepted Answer

B1 a relay neurone has a cell body with many short processes and no long fibre, because nothing it connects to is far away B1 it lies entirely within the central nervous system B1 within a reflex arc it links the sensory neurone to the motor neurone

Question 15

Explain why both the sympathetic and parasympathetic systems are usually active in an organ at the same time, rather than only one being switched on.

Accepted Answer

B1 the two systems act antagonistically on the same organs, one increasing and the other decreasing its activity B1 the state of the organ at any moment reflects the balance between the two opposing sets of impulses arriving at it, rather than either system being fully off B1 this lets the balance shift smoothly in either direction, for example by raising sympathetic activity, lowering parasympathetic activity, or both together, which a simple on/off switch could not achieve

Question 16

Explain why damage to the medulla oblongata is more rapidly life-threatening than similar damage to the cerebellum.

Accepted Answer

B1 the medulla oblongata contains the cardiovascular centre, which sets heart rate and blood vessel diameter, and the ventilation centre, which sets the rhythm of breathing B1 these centres are not duplicated anywhere else in the nervous system, so damage to them can stop the heart or breathing directly B1 the cerebellum coordinates the timing and strength of movement and balance, so damage there causes clumsy, poorly coordinated movement rather than removing a function needed to stay alive from moment to moment

Question 17

State which parts of the body make up the central nervous system, and state what the somatic nervous system supplies.

Accepted Answer

B1 the central nervous system is the brain and the spinal cord B1 the somatic nervous system carries impulses to skeletal muscle, and its responses are voluntary or reflex

Question 18

State what a nerve is, and state how it differs from a single neurone.

Accepted Answer

B1 a nerve is a bundle of the axons and dendrons of many neurones, wrapped together in connective tissue B1 a neurone is a single cell, so a nerve is not a neurone but contains the fibres of many of them

Question 19

Name the term for the swelling just outside the spinal cord, on a reflex pathway, that contains the cell bodies of many neurones, and name the type of neurone whose cell body is found there.

Accepted Answer

B1 a ganglion, outside the spinal cord B1 a sensory neurone