Features of the sensory recetion as illustrated by
A Pacinian corpuscle:
- -Is specific to a single type of stimulus. In this case it only responds to mechanical pressure.
- -Produces a generator potential by acting as a transducer. All stimuli are forms of information but not forms the body can understand. Transducer converts the forms of information (nerve impulses). Receptors transduce one form of energy into another; a nervous impulse known as generator potential.
- Pacinian corpuscle tranduces the mechanical energy into a generator potential.
Structure of Pacinian corpuscle
Pacinian corpuscles occur deep in the skin and are most abundant on the fingers, soles of the feet and external genitalia. They also occur in joints, Ligaments and tendons, enable the organism to know which joints are changing direction.
The single sensory neurone is at the centre of layers of tissue, each seperated by a gel. This has a sepcial type of sodium channel in its plasma membrane. stretch-mediated sodium channel. This is becausetheir permeability to sodium changes when they change shape by stretching.
Function of Pacinian corpuscle
- -In its resting state, stretch mediated sodium channels around the neurone are too narrow to allow sodium ions to pass along them. In this state the neurone of the pacinian corpuscle has a resting potential.
- -When pressure is applied, it changes shape and the membrane around its neurones become stretched.
- -This widens the sodium channels in the membrane and sodium ions diffuse into the neurone.
- -The influx of sodium ions changes the potential of the membrane (ie, it becomes depolarised), thereby producing a generator potential.
- - generator potential creates an action potential that passes along the neurone and then, via other neurones, to the central nervous system.
Receptors working together in the eye
- A receptor also only responds to a certain intensity of stimulus. This means the body has a range of receptors, each responding to a different range of stimulus.
- The lights receptor cells of the mammalian eye are found on its inner most layer: the retina. There are two main types: rod cells and cone cells.
- Both act as transducers by converting light energy into electrical energy of a nerve impulse.
- Rod cells cannot distinguish different wavelengths of light and therefore produce images only in black and white. More rod cells than cone cone cells; there around 120 million in each eye.
- Rod cells share a single neurone this means they can respond to light of very low intensity. A certain threshold value has to be exceeded before a generator potential is created in the bipolar cells to which they are attachted. As a number of rod cells are attacged to each bipolar cell (= retinal convergence), there is a much greater chance thath the threshold value will be exceeded than if only attachted to a single cell.
- As a result rod cells allow us to see in low light intensity (ie, night) but only in black and white.
- In order to create a generator potential, the pigment in rod cells (rhodopsin) must be broken down. Low-intensity light causes this.
- Rod cells generate a single impulse regardless of how many neurones are stimulated.
- they cannot distinguish between the seperate sorces of light that stimulated them.
- two dots close together will appear as a single blob. this is visual acuity
- There are 3 different types- each responding to a different wavelength of light.
- because of this we can perceive images in colour.
- there are about 6 million cone cells in each eye- with there own bipolar cell connected to a sensory neurone.
- The stimulation of a number of cone cells cannot be combined to help exceed the threshold value and so create a generator potential.
- they cant respond to low light intesity (that is why we see in black and white in low intensity light)
- cone cells contain a different pigment (iodopsin) this requires higher light intensity for its breakdown..
- Each cone cell has its own connection to a single bipolar cell, if two adjacent cone cells are stimulated, the brain receives 2 seperate impulses.
- the brain can distinguish between the two seperate sources of light
- two dots close together will appear as two dots.
- good visual acuity
Difference between rod cells and cone cells
- Rod shaped
- Greater numbers than cone cells
- Distribution- more at the periphery of the retina, absent at the fovea
- Poor visual acuity
- Sensitive to low intensity light
- Fewer numbers than rod cells
- fewer at the periphery of the retina, concentrated at the fovea
- good visual acuity
- not sensitive to low light-intensity