1. Sensory Receptors and the Eye

2. 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:

3. Exteroceptors: Interoceptors Proprioceptors Chemoreceptors
respond to stimuli from outside of the body
Interoceptors
respond to stimuli inside the body
Proprioceptors
Sensitive to relative positions of skeleton and muscle contractions
Chemoreceptors
Sensitive to chemical stimuli eg smell, taste and blood pH

4. Mechanoreceptors Photoreceptors Thermoreceptors
Sensitive to mechanical stimuli like pressure, tension, movement and gravity
Photoreceptors
Sensitive to electromagnetic stimuli – that’s visible light to us!
Thermoreceptors
Respond to heat stimuli, particularly effective at detecting change

5. 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.

6. Action
potential
stimulus
Local change in
permeability
Generator
current
Generator
potential
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.

7. 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?

8. The Human eye

9. Anatomy of the eye… go!

11. 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

12. 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

14. 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

15. 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

16. 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

17. 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

18. 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)

20. 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

21. 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:

23. 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:

25. 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

26. The nerve impulses synapse with the parasympathetic cranial nerve (occulomotor nerve)
This transmits impulses to the iris
These stimulate the effectors
The circular muscles in the iris contract, radial muscles relax
So the pupil constricts

27. 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 ;)

28. Comparing Mechanisms of Coordination
Chemical control vs Nervous Control
Comparing Mechanisms of Coordination

29. 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

30. 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