3.6.1.2 Receptors

A receptor responds to a specific type of stimulus. The central nervous system receives sensory information through receptors.

Sensory reception = the function of these receptors
Sensory perception = the understanding/making sense of the information from receptors.


The Pacinian corpuscle
Okay so we need to know quite a it about the Pacinian corpuscle - this is just one type of receptor that responds to changes in mechanical pressure. they are most abundant on toes/fingers//the hand/soles of the feet/genitalia but also occur in joints/tendons/ligaments to enable the organism to know which joints are changing direction. It is specific (ie it will not respond to other stimuli, such as a change in light/heat/sound). It also produces a generator potential by acting as a transducer. In fact, all receptors do both of these. All stimuli involve a change of energy. The receptor (acting as a transducer) converts the energy into a nerve impulse known as a generator potential.

The structure of the Pacinian corpuscle:
It contains a single sensory neurone at the centre of many layers of tissue. Each layer is separated by a little bit of gel. the sensory neurone here contains stretch-mediated sodium channels (whose permeability to sodium ions changes when stretched/deformed - makes sense tbh).

Function of the Pacinian corpuscle:

1. In it's resting/usual state the stretch-mediated sodium channels are closed (well, they're not strictly closed but they're too narrow to allow sodium ions to pass along them. The Pacinian corpuscle has a resting potential (this refers to its potential difference/voltage).
2. Pressure is applied
3. The Pacinian corpuscle is deformed and the membrane around the neurone is stretched
4. The stretch-mediated sodium channels open
5. Sodium ions diffuse into the neurone via facilitated diffusion
6. The potential of the membrane changes due to the influx of sodium ions
7. This produces a generator potential
8. The generator potential creates an action potential (if it is above threshold value)

The human retina

The retina holds the light receptor cells of our eyes (rods and cones are the two main types, you might have come across these at GCSE). As all receptors do, they convert the energy from the stimulus into the electrical energy of a nerve impulse (in this case the stimulus is light).

Rod cells:

• Rod shaped
• Absent at the fovea but plentiful at the periphery of the retina
• Cannot distinguish different wavelengths of light therefore only see black and white.
• More numerous than cone cells (~120 million per eye)
• Used to detect low level/dim light
• As per usual, a certain threshold value must be exceeded before a generator potential is created in the bipolar cells to which they are created. Many rod cells are connected to a single sensory neurone in the optic nerve. This means that there is a greater chance that the threshold value is exceeded as a number of rod cells are connected to each bipolar cell = summation (this is known as retinal convergence).
• Rhodopsin must be broken down to create a generator potential (this is the pigment in the cells) - there is enough energy in low light to break down this pigment hence rods let us see in low light.
• There is a downside, since many rods are connected to one bipolar cell (which is good as it means summation/retinal convergence) it means that light received by many rods that are all connected to one bipolar cell will only produce one nerve impulse so the brain cannot distinguish between separate sources of light e.g there is low resolution (this means poor visual acuity and is why things might look a little sort of blurry in the night).

Cone cells:

• Cone shaped
• Fewer at the retinal periphery but plentiful at the fovea
• Three different types which all respond to different wavelengths of light = images can be perceived in full colour
• Less numerous than rod cells (~6 million per eye)
• Detect brighter light
• Often connected to just one bipolar cell meaning that stimulation is not combined to exceed a threshold value - so only light of a high intensity produces a generator potential.
• Iodopsin must be broken down - each of the three types of cone cells contains a specific type of iodopsin. Only light of high intensity will break down iodopsin and create a generator potential.
• Since each cone cell is connected to its own bipolar cell, if two adjacent cones are stimulated two nerve impulses will be received by the brain (this means high resolution so good visual acuity)

The fovea

Okey so a little bit about the fovea. So basically the lens of the eye focuses light on the part of the retina that is opposite the pupil - this is the fovea. This means that the fovea gets the highest intensity of light hence lots of cone cells and not many rod cells are situated there. Where there is lower light intensity 9the retinal periphery) lots of rod cells are situated.