RECEPTORS IN ANIMALS

RICHARD LLOPIS GARCIA

A2 BIOLOGY

RECEPTORS IN ANIMALS

Are energy transducers (convert some kind of energy into a generator potential which may then initiate an action potential in nerve cell)

An action potential passing along a nerve cell is a nerve impulse.

There are many different receptors e.g.

PACINIAN CORPUSCLE and Pressure Detection

Pacinian corpuscles are pressure sensors found mainly deep in the dermis of the skin (also in some joints and tendons)

Question of the exam

Explain how pressure on the Pacinian corpuscle produces the changes in membrane potential recorded by a voltmeter

How the Pacinian corpuscle work...

If the pressure on the skin is great enough it will deform the corpuscle sufficiently to excite pressure-sensitive ions channels in the membrane of the nerve ending.

These open and positively charged Na ionsmove inward and alter the balance of charge across the membrane (membrane potential)

How the Pacinian corpuscle work...

This change in the membrane potential is called the generator potential.A greater pressure deform the Pacinian corpuscle more and opens more ions channels.This produces a larger generator potentialOn the other hand, only when the generator potential is large enough, a nerve inpulse will be initiated.

The eye and light detection

You must be able to identify key structures in the eye and know their funtions.

Please copy or label the diagram of the eye.

Does this table gives you some clues..

Part of the eye Description Function in focusing and detection of light

Conjunctiva Very thin, transparent membrane covering the cornea ans lining the eyelids

Transmission of light due to transparency

Cornea Transparent front part of the wall of the eye

Transmission of light due to transparency;

Refraction of light due to curvature

Aqueous and vitreous humours

Fluid in the eye; aqueous humour is less viscous than vitreous humour

Transmission of light due to transparency

iris Coloured disc in front of the lens

Control the amount of light entering the eye

lens Transparent crystalline structure held in place by suspensory ligaments

Transmission of light due to transparency;

Refraction of light due to curvature

Ciliary muscles Ring of muscle outside

Controls convexity of lens and therefore its ability to refract light; muscles contract to produce a more convex powerfull lens; relaxation produces a lens less convex

retina Inner layer of wall of eye containing rods and cones

Rods and cones transduce light energy into generator potentials; FOVEA has the greatest concentration of cells; BLIND SPOT has no sense cells

Choroid Black middle layer of the wall of eye

Dark colour prevent internal reflection of light rays

FOCUSING

The eye is able to focus all rays of light from one object to a single point on the retina.

The ability of the eye to adjust focusing from near to distant objects is called ACCOMODATION

(please copy the following diagrams...)

FOCUSING

The amount of refraction taking place at the cornea is more or less constant if the curvature remains the same.

The curvature of the lens is altered by the action of the ciliary muscles and so the amount of refraction changes.

Cones and rods

Cones and rods in the retina are linked to nerve cells by BIPOLAR CELLS.

Cones and rods differ in visual acuity (degree of detail, “pixels”), and in sensitivity (the intensity of light required to produce a sufficient large generator potential to iniciate an action potential)

All this is because of the way in which rods and cones are conected to bipolar cells.

Table of differences in cones and rods

PROPERTY CONES RODSsensitivy Low: light energy

transduced by a single cone must produce a generator potential large enough to exceed the threshold needed for and action potential (unlikely in low light intensities)

High: in low light intensity, generator potential from several rods can combine and so the threshold is more likely to be exceeded and action potential initiated (this is called SUMMATION). It is possible because several rods are liked to a single nerve cell (via bipolar cells). This is called retinal convergence.

Table of differences in cones and rods

PROPERTY CONES RODSAcuity High: each cone

is conected to a single bipolar cell, so in high light intensities each cone stimulated represent a separate part of the image which can be seen in detail

Low: several rods are conected to the same bipolar cell, so the individual parts of the image represented by each rod are merged into one (low detail distintion)

Distribution of rods and cones

Rods and cones are distributed unevenly across the retina (please see graph and diagram)

What conclusions can you deduce from the diagrams about the distribution of rods and cones in the eye?

Distribution of rods and cones

The greatest concentration of cones is found at the fovea in the centre of the retina.

Looking straight at an object focuses light intensity from it into the fovea, enabling to be seen in great detail if the light intensity is high

Distribution of rods and cones

The greatest concentration of rods is about 20 dregrees away from the fovea.

In low lifgt intensities, looking slightly to the side of an object causes the light rays to fall on this area of the retina. Summation by the rods allows better perception than if the light fell on the fovea.

Different types of cones

There are 3 different types of cone, sensitive to different wavelenght of light which are broadly equivalent to the 3 primary colours (red, blue and green.

TRICHROMATIC THEORY

We see everything by mixing only 3 colours in different proportions.Each cone is sensitive to one of these wavelengths. Any particular colour is experienced because the wavelength stimulates one, two, or all three types TO A DIFFERENT DEGREE.

Now look again to the graph and try to answer

when do we see the yellow colour?

Different types of cones

Answer: a wavelenght of 550 nm stimulates both red and green cones and it is interpreted by the brain as the yellow colour

How rods and cones are stimulated (at the molecular level)

When sense cells in the eye are stimulated by light, a change occurs in a photosensitive pigment.

This alters the membrane potential of the cell, creating a generator potential.

The pigment in RODS is RHODOPSIN and the changes that occur on stimulation are shown in the following diagram.

How Rhodopsin send impulses to the brain

Light energy is absorbed by a part of rhodopsin called RETINAL(can change shape from one form to another) and detaches from the other part OPSIN (bleaching)

Bleaching causes excess Na channels to close(reating potential is more –ve) 120mV

Less inhibitory neurotransmitter is released = less inhibition of bipolar neurone. Depolarisation of the membrane.

An action potential is formed in the bipolar neurone membrane, and transmitted trough the optic nerve.

How things go back to normal?

Retinal and opsin then join back together in an enzyme catalysed reaction that regenerates the original pigment ready to be used again.

The same happens with IODOPSIN in CONES but breaks less easily and joins back together more slowly (cones better for High Light Intensities)

So RODS are more sensitive

20 times more rods than conesFound outside the fovea (periphery of the retina)More sensitive because they converge onto the SAME BIPOLAR NEURONE.i.e. even the small responses from rods will be detected by the brain.They are not good at providing clarity or detail. (try to see an object from the corner of your eye)

But Cones let you see in more detail.

Mostly packed together in the FOVEAThey give good VISUAL ACUITY (clarity)And more accurately and in more detailBecause each cone synapses with its own individual bipolar synapse.Remember that also let you see in colour.(draw diagram of connections)

Click on the hyperlink to see an eye dissection

Eye Dissection Complete.wmv