Questions on receptors: transduction and the reflex arc

Question 1

A student covers one half of a choice chamber with black card and releases 50 woodlice at the centre, in five groups of ten. After ten minutes, 33 are in the dark half and 17 in the light half. The critical value of chi-squared at p = 0.05 with 1 degree of freedom is 3.84. Calculate chi-squared and use it to reach a conclusion. In a control run with no card, 29 of 50 woodlice were found in the half that the card had covered. Explain what this control suggests about the conclusion, and how the method should be changed.

Accepted Answer

M1 expected number on each side = 50 ÷ 2 = 25 M1 (33 − 25)² ÷ 25 = 2.56 and (17 − 25)² ÷ 25 = 2.56 A1 chi-squared = 2.56 + 2.56 = 5.12 B1 5.12 is greater than 3.84 at p = 0.05 with 1 degree of freedom, so the null hypothesis is rejected: significantly more woodlice were in the dark half than chance would give B1 the control shows a lean towards that half with no difference in light, for example from a window or draught, so part of the excess in the dark may be side bias rather than a response to light B1 swap the card to the other half between runs, so any side bias falls equally on the light and dark halves

Question 2

Describe how a firm pressure applied to a Pacinian corpuscle leads to an action potential in the sensory neurone it surrounds.

Accepted Answer

B1 the pressure deforms the lamellae of connective tissue, which stretches the membrane of the nerve ending inside them B1 stretch-mediated sodium channels in that membrane are pulled open B1 sodium ions diffuse into the ending down their electrochemical gradient, so the inside becomes less negative B1 this depolarisation is the generator potential, and it is graded, so a firmer pressure opens more channels and gives a larger one A1 the generator potential is large enough to bring the membrane to the threshold of about −55 mV, so an action potential is triggered in the sensory neurone

Question 3

State the five stages, in order, through which a reflex response passes, from the detection of a stimulus to the response itself.

Accepted Answer

B1 stimulus, the detectable change B1 receptor, which detects it B1 coordinator, the central nervous system B1 effector, the muscle or gland B1 response, the resulting action

Question 4

A person eats salted food. Describe how the sodium chloride dissolved in the saliva leads to an action potential in the sensory neurone at the base of a taste bud receptor cell.

Accepted Answer

B1 sodium ions from the dissolved salt diffuse through channels in the membrane of the microvilli, which project through the pore of the taste bud into the saliva B1 the positive charge entering depolarises the receptor cell, which is the transduction step: the chemical stimulus has become a change in membrane potential B1 when the depolarisation reaches threshold, voltage-gated calcium channels in the receptor cell open and calcium ions enter A1 the calcium makes vesicles of neurotransmitter fuse with the basal membrane, and the transmitter released across the synapse triggers the action potential in the sensory neurone, because the receptor cell is an epithelial cell and carries no impulse of its own

Question 5

Explain why a withdrawal reflex is faster than a conscious response, and explain why the statement that the brain is not involved in a reflex would not be credited.

Accepted Answer

B1 the pathway is short: the impulses enter the spinal cord and leave it again without travelling up to the brain first B1 there are only two synapses to cross, and each synapse adds a delay of about half a millisecond B1 the pathway is fixed, so no conscious decision has to be made before the effector is stimulated A1 impulses do reach the brain along a branch of the pathway, which is how the pain is felt a moment later; the point is that the response does not wait for a decision from it

Question 6

Compare rod cells with cone cells in the human retina, referring to their pigment, the light each needs, their distribution and the image each produces.

Accepted Answer

B1 rods contain rhodopsin, which is broken down by dim light, whereas cones contain iodopsin, which needs much brighter light to break down B1 rods are found in the peripheral retina and are absent from the fovea, whereas cones are concentrated at the fovea and thin out towards the edges B1 rods respond to all wavelengths together and give a monochrome image, whereas cones come in three types with different pigments, so comparing them gives colour B1 there are about 120 million rods and only about 6 million cones, and several rods share one bipolar neurone whereas each cone has one to itself

Question 7

Explain how the wiring of the retina makes rods more sensitive than cones in dim light but gives them lower visual acuity.

Accepted Answer

B1 several rods converge on a single bipolar neurone, whereas each cone has a bipolar neurone to itself B1 in dim light one rod produces only a small generator potential, but the contributions of several rods are added together by spatial summation B1 the total is enough to reach threshold and an impulse is sent, while a cone must reach threshold on its own and so needs far more light A1 two points of light falling on two rods that share a neurone send one impulse along one fibre and are seen as one point, whereas two cones send impulses along separate fibres and the points are resolved

Question 8

A choice chamber shows that woodlice gather on the humid side. Describe how a student could find out whether this response to humidity is a kinesis or a taxis.

Accepted Answer

B1 follow individual woodlice, by tracing each path on a gridded arena or filming from above B1 measure the speed of each woodlouse, as distance moved per unit time, in dry air and in humid air B1 count the rate of turning, as turns per minute, in dry air and in humid air B1 a kinesis if speed or rate of turning is greater in dry air but the paths have no direction; a taxis if the paths lead directly towards the humid side

Question 9

Describe the structure of the human retina, in the order in which light entering the eye passes through it.

Accepted Answer

B1 light passes first through a layer of ganglion cells B1 it then passes through a layer of bipolar neurones, which connect the rods and cones to the ganglion cells B1 it then reaches the rods and cones, the light-sensitive receptors, at the back of the retina, with a pigmented layer behind them that absorbs light B1 the axons of the ganglion cells run across the inner surface of the retina and leave it as the optic nerve at the blind spot, where there are no rods or cones (the fovea, packed with cones and with no rods, also accepted)

Question 10

A Pacinian corpuscle is the ending of a sensory neurone, wrapped in layers of connective tissue, and pressure opens stretch-mediated sodium channels in the neurone's own membrane. A taste receptor cell is an epithelial cell, not a neurone: sodium ions in saliva enter it through channels in its microvilli, and it passes the signal across a synapse to a sensory neurone. Using this information, explain why the depolarisation in the Pacinian corpuscle can itself start an action potential in the sensory neurone while the depolarisation of the taste receptor cell cannot, and explain one consequence of this difference for the signal.

Accepted Answer

B1 in both, sodium ions entering through the opened channels depolarise the membrane, giving a generator potential whose size depends on the strength of the stimulus B1 in the Pacinian corpuscle the depolarised membrane belongs to the sensory neurone itself, so once it reaches threshold, voltage-gated sodium channels open and the action potential starts in that same neurone B1 the taste receptor cell is a separate cell, so its depolarisation can affect the sensory neurone only by causing transmitter release, and the transmitter must cross the synapse and bring the neurone to threshold B1 any one consequence: the synapse adds a delay; the signal can pass only from receptor cell to neurone; the frequency of impulses in the neurone depends on how much transmitter the receptor cell releases

Question 11

Explain what is meant by saying that a receptor is a transducer, and explain why a Pacinian corpuscle would not respond to light shone directly onto it.

Accepted Answer

B1 a transducer is something that converts energy from one form into another, and a receptor converts the energy of its stimulus into a change in membrane potential B1 the Pacinian corpuscle contains stretch-mediated sodium channels, which open only when the membrane they sit in is physically deformed B1 light falling on the corpuscle does not deform the lamellae or stretch the membrane of the nerve ending inside them, so those channels are not opened B1 without the channels opening, no sodium ions enter and no generator potential is produced, so light alone cannot trigger an action potential in this receptor

Question 12

In a rare condition the stretch-mediated sodium channels in a person's Pacinian corpuscles are locked permanently open. Suggest what this does to the impulses sent along the sensory neurone when no pressure is applied, and suggest why the person would find it hard to judge how firmly they are being pressed.

Accepted Answer

B1 sodium ions would diffuse into the nerve ending continuously even with no pressure applied, so the membrane would be permanently depolarised B1 the depolarisation would already be at or beyond threshold, so action potentials would be sent to the central nervous system when nothing was touching the skin B1 the generator potential could no longer be graded by the pressure, because the channels cannot open any further, so impulse frequency would no longer report how firm the pressure was

Question 13

Compare a taxis with a kinesis as responses that keep a mobile organism in a favourable environment.

Accepted Answer

B1 a taxis is a directional response, in which the organism moves towards or away from the stimulus, whereas a kinesis is non-directional B1 in a taxis the direction of movement is set by the direction of the stimulus, whereas in a kinesis the speed of movement or the rate of turning changes with the intensity of the stimulus B1 both keep the organism in favourable conditions: a taxis by moving it towards them, such as woodlice moving away from light into the dark, and a kinesis by making it leave unfavourable areas quickly and stay longer in favourable ones

Question 14

Twenty woodlice are released in the middle of a choice chamber with dry air on one side and humid air on the other, and no difference in light. After ten minutes most of them are on the humid side, although none was seen to head straight for it. Explain how a kinesis could produce this result.

Accepted Answer

B1 a kinesis is non-directional: the woodlice do not move towards the humid air, but their speed of movement or rate of turning depends on how dry the air is B1 woodlice in the dry air move faster, so they spend little time there and soon leave B1 woodlice in the humid air slow down, so they remain there longer and the number on the humid side builds up, which keeps them where they lose less water

Question 15

Explain why light falling on one small region of the retina, the blind spot, is not detected.

Accepted Answer

B1 the axons of the ganglion cells run across the surface of the retina and leave the eye together at this point as the optic nerve B1 there are no rods or cones at the blind spot, because the nerve fibres pass through all the layers of the retina there B1 with no receptors, the light is not absorbed by any pigment, no generator potential is produced and no action potentials are sent to the brain from that point

Question 16

Explain why the generator potential produced by a receptor is described as graded, while the action potential it may trigger is described as all-or-nothing.

Accepted Answer

B1 the generator potential varies in size with the strength of the stimulus, because a firmer stimulus opens more of the receptor's channels and admits more ions B1 the action potential it triggers is always the same size, reaching the same peak of about +40 mV whatever the size of the generator potential that set it off B1 once threshold is reached the action potential is produced by positive feedback that runs to completion on its own, so nothing about the stimulus can make it any larger

Question 17

In one Pacinian corpuscle, each arbitrary unit of pressure produces 4 mV of depolarisation, and the relationship is proportional. The resting potential is −70 mV and the threshold is −55 mV. Calculate the minimum whole number of pressure units needed to trigger an action potential.

Accepted Answer

M1 the depolarisation needed to reach threshold is −55 − (−70) = 15 mV M1 dividing by the depolarisation per unit gives 15 ÷ 4 = 3.75 units A1 4 units, because 3 units gives only 12 mV of depolarisation, which does not reach threshold

Question 18

A genetic condition reduces the number of sodium channels in the microvilli of a person's taste bud receptor cells. Suggest the effect this has on the depolarisation produced by a given concentration of dissolved salt, and suggest what it does to how salty the food tastes.

Accepted Answer

B1 with fewer channels available, less sodium enters the receptor cell for a given salt concentration, so the depolarisation produced is smaller B1 a smaller depolarisation is less likely to reach the threshold at which voltage-gated calcium channels open and neurotransmitter is released onto the sensory neurone B1 fewer action potentials are triggered in the sensory neurone at a given salt concentration, and since intensity is coded by frequency, the food would taste less salty than it would to someone with a normal number of channels

Question 19

Describe how the arrangement of neurones in the knee-jerk stretch reflex differs from the arrangement in a withdrawal reflex.

Accepted Answer

B1 a withdrawal reflex arc has three neurones, sensory, relay and motor, connected by two synapses B1 the knee-jerk stretch reflex arc has only two neurones, sensory and motor, with no relay neurone at all B1 with one fewer synapse to cross, the knee-jerk reflex is faster than a reflex arc that includes a relay neurone

Question 20

Name the type of neurone that carries impulses from a receptor into the spinal cord in a reflex arc, and name the type that carries impulses out of the spinal cord to the effector.

Accepted Answer

B1 the sensory neurone carries impulses from the receptor into the spinal cord B1 the motor neurone carries impulses from the spinal cord to the effector

Question 21

State what is meant by a stimulus, and state what is meant by an effector.

Accepted Answer

B1 a stimulus is a detectable change in the internal or external environment B1 an effector is a muscle or a gland that brings about a response