3.2 Receptors

RECEPTORS

Features of Receptors

• They are specific to a specific type of stimulus.

• They produce a generator potential by acting as a

transducer.

• The stimulus comes into the organism by one form of energy e.g. heat, sound or pressure. Receptors need to convert this energy into a form of energy the body can

understand – a nerve impulse. Since they convert one form of energy into another, they are known as transducers.

Pacinian Corpuscles

• Are sensitive to changes in mechanical pressure

• It contains layers of connective tissue around the unmyelinated end of a myelinated neurone.

• The pacinian corpuscle has stretch mediated Na+

Pacinian Corpuscles

• 1. In its resting phase the Pacinian corpuscle is round. The neurone has a resting potential

• 2. When the layers of connective tissue surrounding the corpuscle are compressed it becomes flattened and the surface membrane of the neurone is

stretched. This allow Na+ ions to flow into the neurone causing a generator potential to be produced.

THE EYE

• The human eye is a sense organ which contains

photoreceptors which act as transducers as they are able to convert light into a nervous impulse.

• The eye is adapted for controlling the amount of light entering the eye and for focusing the incoming light rays onto the rods and cones in the retina

• The eye is made up of two types of photoreceptors –

Eye Structure

Choroid: vascular, pigmented layer which absorbs light and

prevents internal reflection

Retina: photoreceptive layer containing rods and

cones

Fovea centralis: region of retina containing

only cones

Optic nerve: transmits impulses to brain

Blind spot: region of retina lacking both rods

and cones

Vitreous humour: jelly-like fluid filling the posterior chamber of the

eye that maintains the shape of the eyeball

Ciliary body: contains circular muscles that alter the shape of the

lens during focusing

Iris: controls the size of

the pupil

Pupil: circular opening for directing light

to the lens

Cornea: transparent portion of sclera across which the greatest refraction

of light occurs

Aqueous humour: watery fluid that

fills the anterior chamber of the eye

Conjunctiva: membrane that lines the eyelids and protects the

cornea

Suspensory ligaments: attach

Rod cell;

Contains rhodopsin (sensitive to low light intensities and not colour).

Cone cell

Bipolar neuron

Rod Cells – Basic Facts

• Do not respond to different wavelengths of light – can only produce images in black and white.

• Rod cells are more numerous – about 120 million per eye

• They have a low visual acuity – low ability to

distinguish between two points, but do respond to low intensity light

• They are rod-shaped and contain the pigment

Rod Cells – How they work

• Rod cells are able to respond to low light intensity.

• A certain threshold value has to be exceeded before a

generator potential is created in the bipolar that they are attached to.

• Since several rod cells are attached to a single bipolar cell (known as retinal convergence), there is a greater chance that the threshold value will be exceeded than if only one rod cells was attached to a bipolar cell.

Rod Cells – How they work

Rod Cells – Low Visual Acuity

• A consequence of several rod cells being attached to one bipolar cell is that light received from several rod cell will only generate a single impulse.

LIGHT Sclera Pigmented choroid epithelium Neurons of optic nerve

Cone cell; Contains iodopsin(sensitive to high light intensities and colour)

Rod cell;

Contains rhodopsin (sensitive to low light intensities and not colour).

Rods and Cones

Bipolar Neurons

Ganglion Cells

Cone Cells – Basic Facts

• Respond to different wavelengths of light

• Fewer cone cells – about 6 million per eye

• They have a high visual acuity – high ability to

distinguish between two points, but do

respond to low intensity light

• They are cone-shaped.

Cone cells – high Visual Acuity and

operate under high light intensity

• Each cone cell is connected to its own bipolar cell.

Therefore, if two adjacent cone cells are stimulated the brain receives two separate impulses and can therefore distinguish between two separate sources of light that are close together.

• Iodopsin, the pigment in cone cells requires higher light intensity that rhodopsin for it to break down. As a result cone cells only respond to higher light intensity. This

Cones

• _{Discrimination of colour}

• _{Good visual acuity}

• _{Poor sensitivity to light}

intensity

Rods

• _{Monochromatic vision}

• _{Poor visual acuity}

• _{Good sensitivity to light}

intensity

Visual Acuity and Sensitivity

Distribution of Cone and Rod cells

• Light is focused on the part of the eye

opposite the pupil known as the fovea. The

fovea therefore receives the highest light

intensity – only cone cells are found here.

• The number of cone cells reduces as you get

further from the fovea

Density of Rods and Cones in the

Retina

Cones are concentrated at the fovea centralis (yellow spot) of the retina

• By having different types of light receptor,

mammals can benefit from good all-round

vision both day and night.

• Cats possess a layer of the white compound, guanine, in the retina of the eye; this provides a mirror-like surface called the

tapetum lucidum, which reflects light outwards through the retina again.

Cones and Colour Vision

There are about six million cones in the human retina and these are the

photoreceptors responsible for our perception of colour

There are three types of cone (red, green and blue) each possessing a different form

of the pigment iodopsin

The existence of three forms of iodopsin, each sensitive to a different but

overlapping range of wavelengths, led to the proposal of the Trichromatic Theory of

Colour Vision

The cones are responsible for all high resolution vision, and the eyes move

continually to ensure that light from objects being viewed falls on the fovea

Light of wavelength

550 nm

Equal stimulation of red and green

sensitive cones; no stimulation of

blue cones

A yellow colour is perceived

Light of wavelength

560 nm

Unequal

stimulation of red and green

sensitive cones; optimal

absorption by red cones and 50%

absorption by green cones; no stimulation of

blue cones

An orange colour is perceived

Light of wavelength

485 nm

Equal stimulation of green and blue

sensitive cones; no stimulation of

red cones

A cyan colour is perceived

Light of an average

wavelength that

stimulates all cones

White is perceived

The ability to perceive colour is not limited

to humans

Although research has shown that colour vision in many primate mammals is fairly limited, the ability to perceive colour is known to be of particular importance in pollinating insects

The range of wavelengths perceived by bees extends into the

Fatigue of Receptor Cells

• _{A common property displayed by receptor cells is that of}

adaptation to a constantly applied stimulus

• _{Sensory adaptation describes the process whereby}

certain receptor cells fail to continue responding when subjected to a constant or rapidly repeating stimulus

• _{The failure of the receptor cell to continue responding is}

termed fatigue

• _{Sensory adaptation is a useful function in that it prevents}

the nervous system from being constantly activated by stimuli of an insignificant nature,

Fatigue of Receptor Cells

A yellow colour is perceived when there is equal

stimulation of the red and green cones

Staring at the red screen provides a constant

stimulus to the red cones which

subsequently adapt or fatigue

As the red cones cease to function, then a

yellow colour is perceived as green as only

Colour

Blindness

Colour blindness is a defect of colour vision that varies

enormously among individuals who are sufferers

Colour blindness is a reduced ability to distinguish between certain colours and may be due to a lack of specific cones

within the retina or an alteration in the properties of the colour-sensitive pigment, iodopsin

The most common type of colour blindness is red-green colour vision deficiency that affects around one in twenty

males; the condition is sex-linked and is due to an abnormal gene on the X-chromosome

Depending upon the exact cause of red-green colour blindness, sufferers may perceive different shades of

Both colour blind and ‘normal’ individuals see the number 12

Normal individuals see the number 8;

Normal individuals do not see any numbers;

red-green colour blind individuals see

the number 5

Photographed from Ishihara colour-blindness plates;