3.6.1.2 Receptors(A-level only) Flashcards
Examples of sensory receptors and their specific stimuli
Mechanoreceptor: pressure and movement └ e.g Pacinian corpuscle, skin Chemoreceptor: chemicals └ e.g. olfactory receptor, nose Thermoreceptor: heat └ e.g. end-bulbs of Krause, tongue Photoreceptors: light └ e.g. cone cell, eye
Function of sensory receptors
└act as transducers converting the stimulus into electrical energy and producing a generator potential. When the generator potential gets big enough, and the threshold is reached it is converts into an action potential.
Transducer role definition
└detects stimulus and converts into a nerve impulse called a generator potential
Generator potential definition
└change in potential difference
Action potential definition
└a change in the membrane potential that produces a nerve impulse
Threshold definition
└ the minimum amount of stimulus a neuron needs to respond
Location of Pacinian corpuscle
Skin
└mainly fingers, genitals, hands, feet
└also ligaments, tendons, joints
Structure of pacinian corpuscle
└an end of a sensory neurone/sensory nerve ending
└wrapped in/surrounded by connective tissue (lamellae) which is separated by a gel
How a generator potential is created by a pacinian corpuscle
No pressure/resting state
└stretch mediated sodium channels are closed/ too narrow for sodium ions (Na+) to pass through
└found in membrane of sensory nerve endings in the pacinian corpuscle
└maintains resting potential difference in Na+ across membrane
└more Na+ outside than inside
Pressure
└deforms/ changes pacinian corpuscles shape
└stretches the surrounding membrane
└widens stretch mediated Na+ channels
└causes Na+ influx as they diffuse into neurone/cell down the concentration gradient
└the positive charge on the Na+ changes the membrane potential
└causes depolarisation to occur
└more Na+ inside than outside
└depolarisation creates a generator potential
└if the generator potential gets large enough, the threshold is reached and an action potential occurs (wave of depolarisation) along the sensory neuron
└aka electrical impulse
└transmitted along neurone to CNS
Stretch mediated sodium channels
…
Resting potential definition
└potential difference between the inside of the non-stimulated neuron and the surrounding interstitial fluid (humans: -70 mV)
Depolarisation definition
└sodium ions rushing into the cell down their concentration gradient, changing the electric charge of the interior
Generator potential definition
└change in potential difference
Threshold definition
└ the minimum amount of stimulus a neuron needs to respond
Action potential definition
└a change in the membrane potential that produces a nerve impulse
Lots of pressure
Pacinian corpuscle
└increase pressure
└more sodium ion channels open
└more Na+ diffuse into cell/neurone
└bigger generator potential
└threshold is more likely to be reached so action potential is met more often
└increases the frequency of impulses sent along sensory neurone
Slight pressure
Pacinian corpuscle
└slight pressure- may go unnoticed
└fewer sodium ion channels open
└less Na+ diffuse into cell/neurone
└lower generator potential
└threshold is less likely to be reached so no action potentials
└decreases the frequency of impulses sent along sensory neurone/ no impulses
Adaptation
Pacinian corpuscle
└continuous stimulation of a sensory neurone
└ decreased frequency of impulses/ stop them-
└useful as it prevents overloading the nervous system with insignificant information
└e.g. the pressure from clothing
Light receptor cell/ photoreceptor definition
A specialized structure or cell that is sensitive to light
e.g. rod and cone cells in the eyes retina
Light receptor cell location
The retina
Light receptor cell/ photoreceptor types
rod cells
cone cells
Light receptor cell- transducers
They convert light energy into an electrical impulse.
Rod cell location
On the retina.
Rod cells most abundant at the peripheries of the retina
Rod cell distribution
Periphery of retina = mostly rod cells
Rod cell light intensity
Can see in the dark (low light intensity)
└ Responds to low intensity light because the threshold value can be exceeded as a number of rod cells are attached to a single bipolar cell (retinal convergence)
Rod cell creating a generator potential
Rhodopsin must be broken down into retinal and opsin. Low light intensity is sufficient
Rod cell sensitivity to colour
black and white
└as they cannot distinguish between different wavelengths of light
Rod cell exhibit summation
Number of them connected to single bipolar cell (retinal convergence) - neurotransmitters released by each combine to exceed threshold and trigger an action potential which is called spatial summation
Rod cell visual acuity
Low
└ Multiple rod cells share the same bipolar neurone so light received by many rod cells sharing the same neurone only generates a single impulse
└ many rods join the same neurone which means light from 2 objects close together cannot be told apart
Rod cell sensitivity
High sensitivity
└ many rods join one neurone so many week generator potentials combine to reach the threshold
Rod cell optical pigments
Black and white
Different types of cone cell
3 └ respond to different wavelengths of light
└ red, green, and blue
Cone cell location
On the retina.
Cone cells most abundant on the fovea.
└because Fovea receives the highest intensity of light and cone cells can only respond to high intensity light
Cone cell distribution
High concentration of cone cells around fovea as high light intensity.
Cone cell light intensity
High (not low)
└Each connected to different bipolar neuron so stimulation of a number of cone cells cannot be combined to exceed threshold
└Also contains pigment (iodopsin) which can only be broken down by high light intensities
Cone cell sensitivity to colour
Can see in colour
└have three different cone cells (red, green and blue) that can respond to different wavelengths of light
Cone cell visual acuity
High
└ Each cone cell has its own single bipolar neurone so the brain receives separate impulses allowing it to distinguish between the separate sources of light.
└ cones are close together and 1 cone joins 1 neurone
Cone cell sensitivity
Low sensitivity
└1 cone joins 1 neurone so it takes more light to reach the threshold and trigger an action potential
Cone cell optical pigments
red, blue, green
Sensitivity to light definition
..
Sensitivity to colour definition
Color vision is the ability of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they reflect, emit, or transmit.
Visual acuity definition
The ability to distinguish between two separate points
Summation definition
Retinal summation describes the relationship between different types of cells in the retina
High retinal summation is an adaptation to low light levels, and low retinal summation to high light levels (thus sharpening the images).
Optic nerve definition
a bundle of neurones
carries nerve impulses from the retina to the brain
Photoreceptor definition
Receptors that detect light
Generator potential definition
generates action potentials in a sensory neuron
How rods and cones (light receptor cells) react to light as a stimulus
1) Light is absorbed by light-sensitive optical pigments
2) Pigments are bleached causing a chemical change
3) Permeability of membrane to Na⁺ is increased
4) Generator potential is created
5) If threshold is reached, action potential is sent along a bipolar neurone to the optic nerve and then the brain
Fovea definition
an area of the retina where there are lots of photoreceptor
How light enters the eye
What happens when light enters the eye
through the pupil
it hits the photoreceptors and is absorbed by the light sensitive pigments
Role of iris muscles
control the amount of light that enters the eye
What light rays are focused by
the lens onto the retina which lines the inside of the eye
What is the blind spot?
Why isn’t it sensitive to light?
where the optic nerve leaves the eye
there aren’t any photoreceptor cells
What does light bleach? What does this cause?
the pigments
a chemical change and alters the membrane’s permeability to sodium
What happens when a generator potential is created and it reaches the threshold?
a nerve impulse is sent along a bipolar neurone
What happens when light from 2 points hits 2 cones?
2 action potentials go to the brain so you can distinguish 2 points that are close together as separate points
