S+R Receptors Flashcards
What do you need to remember about receptors?
Receptors are specific - they only detect one particular stimulus, e.g. light, pressure or glucose concentration.
- Some receptors are cells, e.g. photoreceptors
- some receptors are proteins on cell surface membranes
Describe a receptor in its resting state.
In its resting state (not being stimulated), there’s a difference in charge between the inside and the outside of the cell -this is because there are more +ve ions outside than inside the cell.
The membrane is polarised -there’s a diff. in charge/p.d/voltage across it.
The potential difference when a cell is at rest is called its resting potential.
The resting potential is created and maintained by by Na+/k+ pumps and because the neurone cell membrane is more permeable to potassium (more k+ channels).
What is a generator potential?
The change in potential difference due to a stimulus.
How is a generator potential formed?
When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell-altering the potential difference.
- the change in p.d. due a stimulus is known as a generator potential.
How does the size of the stimulus affect the generator potential?
Bigger stimulus -excites membrane more - bigger movement of ions - bigger change in p.d - bigger GP
What is an action potential?
an electrical impulse along a neurone.
How is an action potential formed?
If the generator potential is big enough it’ll trigger an A.P - an electrical impulse along a neurone.
An A.P is only triggered if the generator potential reaches a certain level called the threshold level.
What do you need to remeber about A.Ps?
A.Ps are all one size, so the strength of the stimulus is measured by the frequency of A.Ps (the no. of A.Ps triggered during a certain time period).
What is a pacininian corpuscle?
a mechanoreceptor (they detect mechanical receptor) e.g. pressure and vibrations.
Where’s the pacinian corpsule found?
What does it contain?
Skin
contain the end of the sensory neurone ( the sensory nerve ending).
the sensory nerve ending is wrapped in loads of layers connective tissue called lamellae.
Describe what happens when a pacinian corpuscle is stimulated?
Lamellae deformed and press on the sensory nerve ending.
deforms the stretch mediated sodium ion channels.
Na+ ion channels open.
Na+ ions diffuse into the cell.
Describe light entering the eye.
light enters through pupil.
the amount of light that enters is controlled by the muscles of the iris.
light rays are focused by the lens on to the retina which lines the inside of the eye.
the retina contains photoreceptor cells that detect light.
How does information from photoreceptors travel to the brain?
nerve impulses from the photoreceptor cells are carried fom the retina to the brain ny the optic nerve (a bundle of neurones).
What do you need to remeber about the optic nerve?
where the optic nerve leaves the eye is called the blind spot - there’s no photoreceptor cells here, so it’s not sensitive to light.
How do photoreceptors convert light into an electrical impulse.
- light hits the photoreceptors and is absorbed by light sensitive optical pigments.
- light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium ions.
- a generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.
- bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.
Diagram of the eye.
Describe rods’ sensitivity.
Rods are very sensitive to light (they work well in dim light).
This is because many rods join to one neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential.
Describe cone sensitivity.
Cones are less sensitive than rods (they work best in bright light). This is because one cone joins to one neurone, so it takes more light to reach the threshold and trigger an A.P.
Describe visual acuity in rods.
Rods give low visual acuity because many rods join to the same neurone, which means light from two points close together can’t be told apart.
Describe visual acuity in cones.
Cones give high visual acuity because cones are close together and one cone joins one neurone.
When light from two points hits two cones, two action potentials (one from each cone) go to the brain
- so you can distinguish two points that are close together as two separate points.
