Site author Richard Steane
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In the body there are several sorts of receptors, each responding to specific stimuli.

A stimulus is a change that can be detected in the external environment (surroundings) or in the internal environment (within the organs of the body).

Receptors are actually parts of sensory neurones, and they function by initiating a nerve impulse (or a chain of impulses) which can be passed on to other neurones. As such, they are the first link in a chain of nerve cells.

In the peripheral nervous system, sensory neurones typically have a cell body displaced to one side of the nerve fibre, often at some distance from the 'nerve ending', and these are collected together with other cell bodies from sensory neurones to form a structure known as a ganglion (plural ganglia).

This topic concentrates on the structure and function of a skin pressure receptor, the Pacinian corpuscle, and the light-sensitive cells in the retina, together with the associated neural connections.

Subsequent topics (links in dropdown menu above and at end) cover conduction of nerve impulses, and synaptic transmission.

With sensory neurones, an ending is just the beginning

Sensory receptors respond to one of four primary stimuli:
Baroreceptors in the aorta and carotid arteries detect blood pressure which causes stretching of the walls of these blood vessels.
Chemoreceptors in the same areas detect increases in dissolved carbon dioxide levels as a result of decreased pH.
Both of these types of receptors contribute to the control of heart rate.

The structure of a Pacinian corpuscle

Pacinian corpuscles, also known as lamellar corpuscles, are rounded structures composed of many layers of connective tissue, containing collagen and gelatinous material, which resemble an onion when sectioned. From the centre of this emerges a tubular process which becomes the dendron of a sensory neurone.
PacinianCorpuscle (58K)
A Pacinian corpuscle and section of sensory neurone

Pacinian corpuscles are found on the skin (not near the surface), and also in various parts of the body. They are known as mechanoreceptors, and detect pressure or vibration.

The outer layers of the capsule become squashed when compressed by (sudden) pressure. To some extent they shield the inner nerve fibre, and spring back afterwards.

It is assumed that the central nerve fibre membrane is polarised by the pumping of sodium ions out of the cytoplasm, resulting in a membrane potential (near to -70mV), like the resting potential in other nerve cells.

The membrane here contains embedded sodium (ion) channels which are said to be stretch-mediated. In other words, they only allow sodium ions to pass through when subjected to pressure which deforms their shape. The entry of sodium ions into the nerve fibre causes a certain amount of depolarisation - raising the membrane potential (perhaps above -50mV) - and this will be proportional to the intensity of the stimulus. This electrical activity shows as a fairly broad pulse, generator potential or receptor potential, lasting longer than a typical nervous impulse, and it remains localised within the end section of the nerve fibre.

If there is sufficient pressure, the generator potential within the 'nerve ending' exceeds a threshold value, and a local electrical circuit is formed with the section of dendron leading out of the corpuscle. This produces a nervous impulse which passes along the whole dendron towards the central vervous system.

If the stimulus is stronger, more stretch-mediated sodium channels open, so more sodium ions enter the 'nerve ending', resulting in further depolarisation (perhaps -40mV). As a result, other impulses quickly follow and move along the dendron, with increased frequency.

Finger pinching, with wires attached

pacinian_electrodes (12K)

A Pacinian corpuscle and section of sensory neurone
Five Schwann cells are shown but in 2 the nuclei are out of the plane of section.

These graphs show the electrical activity simultaneously recorded from the Pacinian corpuscle and its sensory neurone when increasing pressure was applied to a fingertip.
Microelectrode A effectively records the generator potential within the sensitive section ('nerve ending') of the Pacinian corpuscle, and microelectrode B shows the membrane resting potential and action potential in the dendron of the sensory neurone.
Note that these graphs have a different scale on the Y-axis.

paclightpressure (11K)
A generator potential (peaking at -52mV) has been produced as a result of pressure causing deformation of the pressure-mediated sodium (ion) channels so that they open, and allow sodium ions to enter by diffusion. This causes depolarisation. However, the generator potential is not sufficient to cause an impulse to pass along the sensory neurone.
pacmediumpressure (12K)
Extra pressure means more sodium ion channels open , allowing more sodium ions to enter, causing more depolarisation (-46 mV). This is enough to cause an action potential in the sensory neurone - not once but twice. Note that the impulses are the same (+37 mV?)
pacheavypressure (11K)
Even more pressure results in more depolarisation (-41 mV) and this generator potential results in three impulses, but these are more closely spaced together. Each impulse has the same action potential. This is the all-or-nothing principle. Greater stimulation of nervous sensors causes increased frequency of impulses along the dendron, not an increase in amplitude.

The human retina

The retina (at the back of the eye) is composed of several types of cells, as well as blood vessels. Surprisingly, light has to pass through at least 2 layers of neurones before reaching the photoreceptor cells: 'rods' and 'cones'. These are so called because of the shape of the outer segment. Here is a stack of membranes, containing a pigment which makes them light-sensitive.

Rod cells are responsible for monochrome ("black and white") vision, whilst cones give colour vision. They also vary in sensitivity, as explained below.
Impulses from the photoreceptors are passed via synapses to intermediate bipolar neurones - so called because each has two processes on either side of the cell body - and these pass impulses on to ganglion cells which feed into the optic nerve. The left and right optic nerves pass into the optic chiasma, effectively crossing to the other side of the brain and into the thalamus, which passes impulses on to the visual cortex, at the back of the cerebral hemispheres of the brain, where the nervous information is interpreted as vision.

Rods and cones are typically connected differently to their bipolar cells. One bipolar cell normally receives impulses from several rod cells, and sends impulses on to more than one ganglion cell. On the other hand, each cone cell normally sends its impulses on to a single bipolar cell and then on to a single ganglion cell. This means that rod cells are particularly useful in low light levels, but they do not give such clear and high-definition vision as cones, which require higher levels of illumination.
The distribution of cones and the pattern of nerves in the fovea are responsible for vision with the highest definition - visual acuity - as well as discriminating colours of light reflected from objects under view.

It also explains why it is necessary to (turn the head and) line up the visual axis of the eye with the object being studied, in class or reading a book, for example. However it is known that in the dark it is easier to see stars more clearly by looking slightly to one side.


The pigment in rods is called rhodopsin, and its molecule consists of a protein part - opsin - attached to retinal - a derivative of vitamin A. When this absorbs light energy, its molecule changes shape (from 11-cis to all-trans) and this alters membrane permeability, producing a generator poential.
The cone cells themselves are not differently coloured - they just respond to different colours of light

Similarly, cone cells have retinal attached to different opsin proteins. There are actually three types of cone cell. Each one has a slightly different opsin, so that they respond to different wavelengths of light, corresponding to different colours of light: blue, green and red. This is called the trichromatic theory of colour vision.

cones_sensitivity (33K)
This graph shows the percentage of light of different wavelengths absorbed by the three types of cone cell found in the retina of the human eye. Light of different colours is detected by stimulation of different combinations of these cells. This has a parallel to the system by which computer users can specify colours of pixels and text on screen by reference to numbers for rgb (red, green and blue components of light produced).

Layers of cells in the retina

retina_cells (52K) Section through the retina, showing types of neurones
Photoreceptors labelled 1-6 are rods, 7 & 8 are cones

Rod cells 1 and 2 share the same ganglion cell and neurone, so light falling on both of these only registers as a single spot. Rod cell 3 has a separate neurone so when light falls on 2 and 3, two separate spots may be seen.

When a small amount of light falls on a cell, e.g. 3 , it may not be seen because the receptor potential is sub-threshold.

However, if a similar amount of light falls on several, e.g. rod cells 3, 4, and 5, this may be seen, because collectively they are above threshold, i.e. they cause sufficient depolarisation to stimulate an action potential in the next cell. This is an example of spatial summation, with 3 receptors releasing enough neurotransmitter to generate an impulse.

Don't use the term "messages" or "signals" when referring to nerve impulses
This phenomenon is called retinal convergence, as light falling on several nearby rod cells results in each of them producing impulses which are brought together to a single bipolar cell and outgoing neurone, resulting in a single action potential leaving the eye in one 'nerve fibre' of the optic nerve, which is interpreted in the brain as a single point of light.

Large numbers of cone cells similar to 7 and 8 are found in the fovea. These give higher visual acuity than rod cells 1-6, because each photoreceptor connects to a single bipolar cell and ganglion, and pass action potentials out along separate nerve fibres, so they are each are registered in the brain as a distinct point of light. They also give colour vision, because there are 3 different types of cones, each responding to different colours/wavelengths of light

All in the back of the eye

The retina consists of about 120 million rods, widely spread out, and about 5 million cones. About 2000 cones (and practically no rods) are concentrated in an area in the centre of the optical axis at the back of the eye. This region of maximal sensitivity is called the fovea centralis , and it is served by about half the nerve fibres that make up the optic nerve (consequently feeding on to half the visual cortex). There is less obstruction by blood vessels and overlying connective neurones in the central region. There are several zones, varying in the packing density of cones and bipolar cells. The term macula may be used interchangeably with fovea, or it may represent a slightly larger area. Macular degeneration is an (age related) condition in which cellular changes in this area lead to reduced clarity of (central) vision.

There are no photoreceptors where the optic nerve leaves the eye, seen below as the optic disc. This is the 'blind spot'.

Can you see through this?

retina_unlabeled (36K)

A photograph of the back of the eye ('fundus') showing the surface of the retina, and the dark orange colour is mainly due to visual pigments.
The fovea is the small blob in the indistinct dark area on the left.
The optic disc (composed of axons from ganglia) shows as a light circular zone on the right, with blood vessels emerging.

rods_and_cones_distribution (38K)
The distribution of rod cells and cone cells across the retina of a human eye
N.b. Even though there are high numbers of rods in some regions of the retina, they do not give such good resolution as cones do, because of the way their associated neurones are 'connected'.

Colour blindness

There are several forms of colour blindness, differing in the range of colour loss and severity.

Red-green colour blindness is a fairly common sex-linked condition due to a recessive allele which causes problems in distinguishing between hues in the red-green area.
The most common form deuteranomaly results from a change in the green-sensitive pigment and is only a minor inconvenience to some. The frequency in males is about 6-8%.
A more severe form of red-green colour blindness deuteranopia results from the complete absence of one set of cones - the green-sensitive ones.
This is also known as Daltonism - after John Dalton - who introduced the atomic theory in Chemistry. The frequency in males is about 1%.

When analysing allele and phenotype frequencies by the Hardy-Weinberg methodology, it must be modified to reflect the limitation caused by the X chromosome:
in males allele frequencies for affected and unaffected are identical to phenotype frequencies, typically p and q, not squared,
whereas in females phenotype frequencies are the 'normal' 3 categories: p2, 2pq and q2, as there are 2 copies of the X chromosome.

Total colour blindness achromatopsia is caused by the absence of all cone cells, resulting in monochromatic vision, in which everything is seen in shades of grey - like on a black and white television. This is inherited as an autosomal recessive condition, and is very uncommon - in the USA the frequency of individuals affected is about 0.003%.

However on the small tropical Micronesian island of Pingelap, the frequency is said to be about 10%, a consequence of a founder effect/genetic bottleneck resulting from the repopulation of the island following a tornado in 1775 which greatly reduced the population. All affected individuals today can trace their ancestry back to one individual who survived.
It is said that the eyes of some affected individuals are sensitive to daylight sun levels, but they are at an advantage when night fishing.

There are some inconsistencies in online accounts, such as the suggestion that 30% of the island's population (of 250) are carriers.
Hardy-Weinberg analysis suggests q2 = 0.1 so q = 0.32, and p = 0.68. The 2pq value for heterozygote frequency should therefore be (2 x 0.32 x 0.68) = 0.4352 - in other words 43.5%.
But perhaps there is another explanation, such as immigrants upsetting the equilibrium.

What do you see?

Plate17 (28K)

The answer to the Ultimate Question of Life, the Universe, and Everything?

Colour blindness is a sex-linked condition, so it is more common in males.
If you can see two numbers, you have normal colour vision.
If you see only one number clearly, you probably have 'green colour blindness' or 'red colour blindness'.
If you see one number clearly and another faintly, you probably have a mild version 'mild green colour blindness' or ' mild red colour blindness'.

For more tests see web references below.

Other related topics on this site

(also accessible from the drop-down menu above)

Nerve cells, Nerve impulses - similar level - more detail about impulses, Schwann cells etc
Synaptic transmission - similar level - more detail about types of synapse, neurotransmitter substances, temporal and spatial summation, effects of specific drugs
Hardy Weinberg Equation - explaining phenotype and allele frequencies

The eye and vision - simpler, more general but scroll down to the section on the blind spot
Senses, sensitivity, stimuli and skin - simpler, more general

Interactive 3-D molecular graphic models on this site

(also accessible from the drop-down menu above)

Web references

Sensory neuron From Wikipedia, the free encyclopedia
Lamellar corpuscle From Wikipedia, the free encyclopedia
Ishihara Color Test
General vision tests
Graded Potentials versus Action Potentials

Color blindness From Wikipedia, the free encyclopedia

The island of colour blindness

Pingelap From Wikipedia, the free encyclopedia

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