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Receptors: turning a stimulus into something a neurone can carry

Heat, pressure and light are three different kinds of energy, and none of them travels along a nerve. A receptor converts whichever one it is built for into a change in membrane potential, and from that point on the nervous system deals in a single currency.

Before this Membrane structure and channel proteins · Active transport and the use of ATP

Before you start

Your fingertip detects heat and sends the heat up your arm; your eye detects colour and sends colour along the optic nerve. Most people start with some version of that, and it quietly wrecks the next two lessons. Nothing about the stimulus travels anywhere. A receptor is a transducer: it converts the energy of one particular stimulus into a change in membrane potential, and after that the nervous system handles pressure, light and sound in exactly the same way. You can tell them apart because of which neurone the impulses arrive along and how often they arrive, not because the impulses themselves differ.

What you should be able to do

From stimulus to response

Coordination in an animal always runs through the same five stages, and questions are written against them so precisely that it is worth being able to recite the sequence: stimulus, receptor, coordinator, effector, response. The coordinator is the central nervous system, and the two neurones on either side of it are the sensory and the motor.

A reflex is what you get when that pathway takes the shortest route available. Touch something hot and your hand is moving before you have registered pain. The impulses go from the receptor along a sensory neurone into the spinal cord, across a relay neurone, straight back out along a motor neurone to the biceps, and the arm flexes. Two synapses, and nothing else in the way.

Follow the arrows round. The pathway enters the cord and leaves it again without going up to the brain first, which is what makes it fast; the branch heading upwards is why you feel the pain a moment later.

Be careful how you phrase the brain's part in this. Impulses do reach it — that is how you know your hand moved and how you learn not to touch the pan again — but the response does not wait for them. An answer saying 'the brain is not involved' will usually be marked down, while 'the response does not require a decision from the brain' scores.

Speed is the whole point, and it is bought in three ways: a short pathway, only two synapses to cross, and a route through the spinal cord rather than the brain. Since every synapse costs about half a millisecond, the arithmetic of keeping the number small is not trivial. The protection is obvious once you list what reflexes actually do — withdraw from heat, blink at a moving object, cough on an inhaled crumb, narrow the pupil in bright light. Each one limits damage in the fraction of a second before conscious thought could arrive.

Stimulus
A detectable change in the internal or external environment.
Receptor
A cell or structure that detects one kind of stimulus and converts it into a change in membrane potential.
Effector
A muscle or gland that brings about a response.
Reflex
A rapid, automatic response to a stimulus, involving a fixed pathway of neurones and no conscious decision.
Reflex arc
The pathway of neurones taken by a reflex: receptor, sensory neurone, relay neurone, motor neurone, effector.

What a receptor actually does

Two properties define every receptor you will meet. It is specific — a rod cell responds to light and not to pressure, however hard you press on your closed eye — and it is a transducer, converting the energy of that stimulus into an electrical change across its own membrane. The change is called a generator potential, and it is the single most useful idea in this lesson.

The Pacinian corpuscle is the example boards keep returning to, partly because its mechanism is unusually easy to see. It sits deep in the skin, in joint capsules, in ligaments and tendons, and it responds to pressure — specifically to changes in pressure. Under the microscope it looks like an onion about a millimetre long: rings of connective tissue called lamellae, separated by a gel, wrapped round the ending of a single sensory neurone.

The graph on the right is the part worth studying. Press harder and the generator potential is larger; that is what graded means, and it is precisely what an action potential does not do.

Press on the corpuscle and the lamellae deform, which stretches the membrane of the nerve ending inside them. That membrane contains stretch-mediated sodium channels: channels whose shape, and therefore whether they are open, depends on being physically pulled. Stretched open, they let sodium ions diffuse in down their electrochemical gradient, and the inside of the ending becomes less negative. That depolarisation is the generator potential.

Now the important bit. A bigger pressure deforms more lamellae, stretches more channels, admits more sodium and gives a bigger generator potential. If the generator potential is large enough to bring the membrane to about −55 mV — the threshold — an action potential is triggered in the sensory neurone. If not, the depolarisation simply fades away and nothing is sent. So the receptor's response is graded and the neurone's is all-or-nothing, and the threshold is the step between them.

Transducer
Something that converts energy from one form into another; a receptor converts the energy of a stimulus into a change in membrane potential.
Generator potential
The graded depolarisation produced in a receptor by a stimulus, whose size depends on the strength of that stimulus.
Threshold
The membrane potential, around −55 mV, that must be reached before an action potential is triggered.

Working out what will and will not fire

In one Pacinian corpuscle a pressure of one arbitrary unit produces a generator potential of 6 mV of depolarisation, and the relationship is proportional. The resting potential is −70 mV and the threshold is −55 mV. Will pressures of 2 units and of 3 units each trigger an impulse?

First find what is needed. Going from −70 mV to −55 mV is a depolarisation of 15 mV, so that is the size of generator potential required.

Two units gives 2 × 6 = 12 mV, which reaches −58 mV. That is short of threshold, so no action potential is triggered and the depolarisation dies away.

Three units gives 3 × 6 = 18 mV, which takes the membrane past −55 mV. An action potential is triggered — and it is worth adding that it will be exactly the same size as one triggered by ten units, because the generator potential is graded but the impulse is not.

Two kinds of photoreceptor

The retina carries two receptor types, and every comparison question about them can be answered from four differences: the pigment, the light needed, the detail resolved and where in the retina they sit.

Rods contain the pigment rhodopsin, which is broken down — bleached — by relatively little light. There are roughly 120 million of them in a human retina, they respond to all wavelengths together and so give a monochrome image, and they are spread across the peripheral retina with none at all in the fovea.

Cones contain iodopsin, which needs a good deal more light to break down. There are about 6 million, in three types whose pigments absorb best in the red, green and blue parts of the spectrum, and colour vision comes from comparing how strongly the three are stimulated. They are concentrated at the fovea, directly behind the centre of the lens, and thin out towards the edges.

Because rhodopsin is broken down in dim light and iodopsin is not, only the rods report anything at night — which is why moonlit surroundings look grey rather than colourless-by-choice. Rhodopsin then has to be resynthesised, and that takes time: the twenty minutes or so your eyes need to adjust when you walk into a dark room is the chemistry of that resynthesis, not a mental adjustment.

RodsCones
PigmentRhodopsinIodopsin, in three types
Light neededLow: bleached by dim lightHigh: needs bright light
NumberAbout 120 millionAbout 6 million
DistributionPeripheral retina, none in the foveaConcentrated in the fovea
ImageMonochromeColour, from three pigments
ConnectionSeveral rods share one bipolar neuroneOne cone to one bipolar neurone
SensitivityHighLow
Visual acuityLowHigh

Convergence, and the trade it forces

The last two rows of that table are not independent facts to be memorised. They both follow from the wiring, and a question asking you to explain rod sensitivity or cone acuity is asking about convergence.

Six rods feeding one fibre on the left, one cone to one fibre on the right. Everything in the last two rows of the table above follows from the difference between those two diagrams.

Take sensitivity first. In dim light a single rod produces only a small generator potential, nowhere near enough on its own. But several rods — in the peripheral retina, sometimes a hundred of them — connect to the same bipolar neurone, so their contributions are added together by spatial summation. Enough small potentials arriving at once will reach threshold, and one impulse is sent. A cone has a bipolar neurone to itself, so nothing is added to its contribution; it needs enough light to reach threshold single-handed.

Now acuity, which is the same wiring read the other way. If two points of light fall on two different rods that share a neurone, the brain receives one impulse from one fibre and cannot possibly tell whether one point or two caused it: the two points are seen as one. Two points falling on two cones send impulses along two separate fibres, and are resolved as two. High sensitivity and high acuity pull in opposite directions, and the retina solves the problem by having both kinds of receptor in different places.

TRY IT — Explaining an observation with the wiring

Astronomers use a technique called averted vision: to see a very faint star, they look slightly to one side of it rather than straight at it. Explain why this works.

Check your answer

Looking straight at something focuses its image on the fovea, and the fovea contains cones and almost no rods. Cones need bright light to break down iodopsin, and a faint star does not provide it, so the cones do not reach threshold and nothing is sent.

Looking slightly to one side moves the image onto the peripheral retina, which is rod territory. Rhodopsin is broken down by much less light, and several rods converge on one bipolar neurone, so their generator potentials sum. Threshold is reached, an action potential is generated, and the star is seen.

There is a price, and it is worth stating because it shows you understand the trade rather than half of it. The image formed by the rods has low acuity, so the star is visible but blurred and its position is imprecise — you cannot make out anything fine about it. Sensitivity has been bought with detail.

In the exam

Check yourself

A student presses gently on the back of their hand and feels nothing; pressing harder in the same place, they feel it clearly. Using what a Pacinian corpuscle does, explain both observations, and explain why the harder press does not produce larger impulses in the sensory neurone.

Answer

Gentle pressure deforms the lamellae only slightly, so few stretch-mediated sodium channels are opened, few sodium ions enter and the generator potential is small. It does not reach the threshold of about −55 mV, no action potential is triggered, and nothing is sent to the central nervous system.

The harder press deforms the lamellae much more, so more channels open, more sodium ions diffuse in and the generator potential is larger. This time the membrane reaches threshold and action potentials are triggered in the sensory neurone.

Their size, though, is fixed. An action potential is all-or-nothing: once threshold is passed, the voltage-gated channels open fully and the same peak is reached every time, whatever the stimulus. What a stronger stimulus changes is the frequency of impulses, and how many neurones are firing at once.

The graded step and the all-or-nothing step are different stages of the same journey — receptor first, neurone second — and questions on this topic are usually testing whether you have kept them apart.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

Question 15 marks

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

Mark scheme
  1. B1 the pressure deforms the lamellae of connective tissue, which stretches the membrane of the nerve ending inside them
  2. B1 stretch-mediated sodium channels in that membrane are pulled open
  3. B1 sodium ions diffuse into the ending down their electrochemical gradient, so the inside becomes less negative
  4. B1 this depolarisation is the generator potential, and it is graded, so a firmer pressure opens more channels and gives a larger one
  5. 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 24 marks

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.

Mark scheme
  1. B1 the pathway is short: the impulses enter the spinal cord and leave it again without travelling up to the brain first
  2. B1 there are only two synapses to cross, and each synapse adds a delay of about half a millisecond
  3. B1 the pathway is fixed, so no conscious decision has to be made before the effector is stimulated
  4. 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 34 marks

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.

Mark scheme
  1. B1 rods contain rhodopsin, which is broken down by dim light, whereas cones contain iodopsin, which needs much brighter light to break down
  2. 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
  3. 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
  4. 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 44 marks

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

Mark scheme
  1. B1 several rods converge on a single bipolar neurone, whereas each cone has a bipolar neurone to itself
  2. B1 in dim light one rod produces only a small generator potential, but the contributions of several rods are added together by spatial summation
  3. 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
  4. 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 53 marks

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.

Mark scheme
  1. B1 sodium ions would diffuse into the nerve ending continuously even with no pressure applied, so the membrane would be permanently depolarised
  2. 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
  3. 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 62 marks

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.

Mark scheme
  1. B1 the sensory neurone carries impulses from the receptor into the spinal cord
  2. B1 the motor neurone carries impulses from the spinal cord to the effector

Worth remembering

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