Topic 6—A: Stimuli and responses- 4. Receptors Flashcards
How are receptors specific?
- They only detect one particular stimulus
Example- light
What do receptors in the nervous system convert the energy of the stimulus into?
- They convert the energy of the stimulus into the electrical energy used by neurones
The resting potential
- When a nervous system receptor is in its resting state there’s a difference in charge between the inside and the outside of the cell
- The inside is negatively charged relative to the outside
- This means there’s a voltage across the membrane
- Voltage is also known as potential difference
- The potential difference when a cell is at rest is called its resting potential
- The resting potential is generated by ion pumps and ion channels
Generator potential
- 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 potential difference due to a stimulus is called the generator potential
- a bigger stimulus excites the membrane more causing a bigger movement of ions and a bigger change in potential difference- so a bigger generator potential is produced
Action potential
- If the generator potential is big enough, it will trigger an action potential- an electrical impulse along a neurone
- An action potential is only triggered if the generator potential reaches a certain level called the thresh hold level
- action potentials are all one size so the strength of the stimulus is measured by the frequency of action potentials
- If the stimulus is too weak, the generator potential won’t reach the threshold so there’s no action potential
Pacinian corpuscles
- They are mechano receptors
- They detect mechanical stimuli e.g. pressure and vibrations
- found in your skin
- Pacinian corpuscles contain the end of a sensory neurone called a sensory nerve ending
- The sensory nerve ending is wrapped in loads of layers of connective tissue called Lamellae
What happens when a pacinian corpuscle is stimulated?
- pressure distorts the pacinian corpmuscles
- deforming stretch mediated na+ channels
- channels open and sodium ions diffuse into the cell creating a generator potential
- If the generator potential reaches the threshold, it triggers an action potential
Photoreceptors
- Receptors in your eye that detects light
- Light enters the eye through the pupil and the amount of light that enters is controlled by the muscles of the iris
- Light rays are focused by the lens onto the retina which lines the inside of the eye
- The retina contains the photoreceptor cells
- Fovea is an area of the retina where there are lots of photoreceptors
- Nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve which is a bundle of neurones
- Where the optic nerve leaves the eye it is called the blindspot- there aren’t any photoreceptor cells so it’s not sensitive to light
How photoreceptors work?
- Light enters the eye, 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
In the human eye, what are the two types of photo receptor?
- Rods
- Cones
Where are rods mainly found?
- In the peripheral parts of the retina
Where are cones mainly found?
- Packed together in the fovea
Why are rod and cones sensitive to different wavelengths of light?
- They contain different optical pigments
What do rods only give information in?
- Black and white (monochromatic vision)
What do cones give information in?
- In colour (trichromatic vision)
What are the three types of cones containing a different optical pigment?
- Red sensitive.
- Green sensitive.
- Blue sensitive.
- When they’re stimulated in different proportions, you see different colours
Sensitivity (rods)
- Rods cells are very sensitive to light (they work well in dim light)
- This is because many rods join one bipolar neurone so many week generator potentials combine to reach the threshold and trigger and action potential
Sensitivity (cones)
- Cones are less sensitive than rods (they work best in bright lights)
- This is because one cone joins one bipolar neurone so it takes more light to reach the threshold and trigger an action potential
Definition of visual acuity
- It is the ability to tell apart points that are close together
Visual acuity (rods)
- Rods give low visual acuity because many rods join the same bipolar neurone which means light from two points close together can’t be told apart
Visual acuity (cones)
- cones give high visual acuity because cones are close together and one cone joins one bipolar neurone
- When light from two points hits two cones, two action potentials one from each cone goes to the brain- so you can distinguish two points that are close together as two separate points
Rods
- Mainly located in the peripheral parts of the retina
- Give information in black and white
- Many rods join one bipolar neurone
- High sensitivity to light
- Give low visual acuity
- contains pigment Rhodopsin
Cones
- Mainly located in the fovea
- Give information in colour
- One cone joins one bipolar neurone
- Low sensitivity to light
- give high visual acuity
- Contains pigment Iodopsin
How do rods allow us to see in lowlight intensities?
- Lower threshold potential (less Rhodopsin) needs to be broken down to initiate an action potential
- several rods share a sensory neurone
What is always open during resting potential?
- k+ channels are open so potassium can leave
- Pump is going the whole time
- Na+ channels are closed
What is open when there is a stimulus?
- Na+ channels
What is open when there is an action potential?
- More na+ channels
Resting potential (channels)
- Outside the neurone= positive due to presence of Na+ and k+
- Inside of neurone= negative due to presence of anions
- Gated na+ channels are closed so little diffusion of na+ into neurone
- k+ channels are open, so k+ can diffuse out of neurone
- Na+ and k+ pump, pumping 3 na+ out of neurone and 2k+ into neurone
Generator potential (channels)
- Outside of neurone- gets more negative due to loss of na+ and k+ ions
- Inside the neurone is more positive due to the presence of na+ and K+ ions
- Stimulus causes gated na+ channels to open, increasing the permeability of the membrane to na+
- 2k+ na+ diffuses down its concentration gradient into the neurone making potential difference more positive if there is enough na+ diffusing across membrane then threshold potential is reached and membrane is depolarised