Sensory Receptors and the Eye

How sensory receptors work

• Sensory receptors transfer the energy of a stimulus into the action potential in a sensory nerve fibre

• There are various types of receptor:

• Exteroceptors: respond to stimuli from outside of the body

• Interoceptorsrespond to stimuli inside the body

• ProprioceptorsSensitive to relative positions of skeleton and muscle

contractions• Chemoreceptors

Sensitive to chemical stimuli eg smell, taste and blood pH

• MechanoreceptorsSensitive to mechanical stimuli like pressure,

tension, movement and gravity• Photoreceptors

Sensitive to electromagnetic stimuli – that’s visible light to us!

• ThermoreceptorsRespond to heat stimuli, particularly effective at

detecting change

How do receptors work?

• Similar to nerves• They start with a resting potential

(interior –ve to exterior maintained by Na-pumps)• On stimulation, Na ions move across the membrane

(along the electrochemical gradient) • This generator current sets up a generator potential • If the potential is large enough to reach the threshold

of the sensory neurone, an action potential is generated in the neurone.

• In sensory organs (eg eye) several receptors may join with one sensory neurone

• so it may need more than 1 receptor to be stimulated to send an impulse

• It may need several potentials to add together (summate) to trigger an AP

• This is convergence – useful for changing the sensitivity of a sensory system.

stimulus Local change in permeability Generator

current Generator potential

Action potential

Adaptation of the Receptors• Most important info carried by sensory system is

about the environment• If a stimulus does not change for a while, it will

continue to stimulate the system, but this info is of no use to the animal

• It only need to know when it changes• When a steady stimulus is presented, most

receptors begin to decline in generator potentials• This is adaptation• Can you think of examples?

THE HUMAN EYE

Anatomy of the eye… go!

The Role of the Retina• Light entering the eye must first be focused• But then the retina must perceive the light

and provide information to the brain to interpret.

• 100million light sensitive cells (photoreceptors) are in the retina

• All of those synapse with neurones• 2 main types of photoreceptor:– Rods– Cones

Rods

• 120million in each eye• Spread evenly in retina except in the Fovea

(6million cones there, tightly packed)• Black and white only• Used mainly for low light vision• Rods contain a visual pigment called

rhodopsin

• Rods are not tightly packed• Several synapse with each neurone• They do not give a very clear picture• But because of summation, they are very

sensitive in low light

• Note back to front arrangement

• Neurones are on the outer side of the eye

• Means you get a blind spot where optic nerve has to pass out of the eye

How do the Rods Work?

• Rhodopsin is made of 2 components:– Opsin (a lipoprotein)– Retinal (derived from Vit. A)

• Retinal exists in 2 forms, cis- and trans-• In the dark it is cis-retinal, a photon of light

converts it to trans-retinal• This changes its shape and therefore it breaks

off from opsin• This breaking up is called bleaching

• Rod cell membranes are usually permeable to Na ions

• They move into the rod cells all the time and are removed by sodium pumps.

• When trans-retinal is made by bleaching, it sets of a cascade which closes the Na channels

• The sodium pumps keeping working• Thus, the interior becomes more –ve than

usual• This hyperpolarisation is the generator

potential

• Once the visual pigment has been bleached, the rod cannot be stimulated again until it is resynthesised

• It takes ATP to do this (from the many mitochondria in the inner segment)

• In normal daylight, nearly all rods are fully bleached (the eye is light-adapted)

• 30minutes of dark and they have regenerated, the eyes are sensitive to low light (the eye is dark-adapted)

The Cones and Colour Vision• Cones work similarly to rods• Their pigment is iodopsin• There are three types of iodopsin, each

sensitive to one of the primary colours of light• It takes a lot of light energy to break down

iodopsin, so they are not sensitive in low light• But they do provide colour vision• This compliments the low light and movement

sensitivity of the rods

Coordination at Work

• Reflexes are the basis of most nervous systems

• These are unconditioned, fast fixed responses to stimuli

• They are controlled by the simplest type of nerve pathway in the body:

The Iris as an Effector

• The iris is a muscular diaphragm with a hole in the middle

• Pigments in it absorb light• This means light can only enter the eye

through the pupil• The size of this is controlled by muscles:

• Light enters the eye• Sensory receptors of the retina fire• Impulses travel down the optic nerve to the

brain• The brighter the light, the bigger the

frequency of the impulses• Detected by the control centre in the midbrain• Impulses sent to 2 further control centres

• The circular muscles in the iris contract, radial muscles relax

• So the pupil constricts

• The nerve impulses synapse with the parasympathetic cranial nerve (occulomotor nerve)

• This transmits impulses to the iris

• These stimulate the effectors

• When light levels drop, frequency of APs fall• Impulses then travel from the control centres

via sympathetic nerves to the iris• The circular muscles relax• Radial muscles contract• Thereby widening the pupil

• The reflex of either eye controls both• The pupils respond to emotional cues as well ;)

COMPARING MECHANISMS OF COORDINATION

Chemical control vs Nervous Control

Chemical control

• Chemical control is slow but long lasting• Hormones are carried in animals’ blood

plasma• They move to target cells by diffusion and are

picked up by receptors on cell membranes• Plant hormones move in phloem• Chemical control is often linked to growth but

can be used in day-to-day control as well

Nervous control

• Usually very rapid• Makes it ideal for organisms that move their

whole bodies about• If you need to respond to environmental cues,

it gives you the speed you need