Receptors Flashcards
What are receptors?
specialised cells that detect changes in the environment
What is a transducer?
converts the energy of a stimulus into the start of a nerve impulse - generator potential
Name a type of receptor that is sensitive to changes in pressure
pancinian corpuscle
When does a generator potential become an action potential in a pancinian corpuscle?
greater pressure - more deformation
-more ion channels open
-larger generator potential
-if generator potential is large enough/above threshold, an action potential is passed along the axon
What is an adaption of the pancinian corpuscle?
-with continuous pressure, the frequency of action potentials decreases and soon stops
-prevents the nervous system from being bombarded with insignificant information
Describe what happens in a pancinian corpuscle?
-pressure causes the membrane around the neurone to become deformed
-this deformation causes stretch-mediated sodium ion channels to open, allowing sodium ions to diffuse into the neurone, down the electrochemical gradient
-this causes the inside of the membrane to become more positive, creating a difference in charge between both sides of the membrane
-initiates a generator potential that could lead to an action potential if it is above the threshold
What is a Schwann cell?
cell wrapped around the axon, forming myelin sheath
What is the node of Ranvier?
a gap in the myelin sheath where the axon is exposed
What is the normal resting state of the axon called?
the resting potential
What is the normal potential difference across an axon?
-70mV
What does the -70mV change to in an action potential?
+40mV
How is resting potential maintained?
-the sodium-potassium pump actively moves 2 K+ ions out in for every 3 Na+ ions out
-at rest, sodium and potassium ion channels are closed but leakage occurs
-the membrane has more potassium ion channels than sodium ion channels -membrane is more permeable to the loss of K+ ions so more K+ ions leak out
-causes outside of membrane to be more positive than the inside
What else helps to maintain resting potential?
negatively charged ions on the inside like proteins which can’t move across the membrane
What are the three stages of an action potential?
-depolarisation
-repolarisation
-hyperpolarisation
Explain depolarisation
-neurone is excited past threshold
-sodium ion channels open
-sodium ions quickly diffuse into the axon
-inside of axon becomes temporarily positive and outside negative
-nearby sodium ion channels continue the depolarisation as they are voltage-gated
Explain repolarisation
-potassium ion channels open
-potassium ions quickly diffuse out which generates a positive charge on the outside of the membrane
-sodium ion channels close
-sodium-potassium pump rapidly moves sodium ions out of the cell which further creates a positive charge on the outside and a negative charge on the inside
Explain hyperpolarisation
-potassium ion channels are slow to close
-slight overshoot where too many potassium ions diffuse out
-potential difference becomes more negative than the resting potential
-until ion channels shut fully and the resting potential can be restored by the sodium-potassium pump
What is the role of the nervous system?
it detects, relays, and coordinates information about an organisms environment
What is the all or nothing principle?
-stimulus must be above the threshold value for an action potential to be generated
-a stimulus will be below threshold if insufficient numbers of sodium ion channels open, so the axon is not fully depolarised
-if the threshold is reached, the action potential generated is always the same size regardless of the strength of the stimulus
How do we perceive the strength of a stimulus?
increased frequency of action potentials
What is the refractory period?
-once an action potential has passed the axon becomes in-excitable for a short period of time
-ion channels are recovering and cannot be opened which acts as a time delay
Why is the refractory period important?
-ensures action potential moves only in one direction
-ensures no overlap between impulses as it limits the number of action potentials
Describe the propagation of action potential
-stimulation causes influx on sodium ions leading to depolarisation and an action potential
-localised electrical circuits cause voltage-gated sodium ion channels to open further along the membrane
-behind this the Na+ channels close, K+ channels open
-K+ ions move out until resting potential occurs
What are the three factors that effect the speed of transmission?
-myelination
-axon diameter
-temperature
How does myelination impact action potentials?
action potentials move faster along myelinated neurones than unmyelinated ones
What is the importance of the nodes of Ranvier?
-ion channels present at the nodes of Ranvier allow the movement of Na+ and K+ ions across the membrane so ONLY at these points can an action potential be generated
What is saltatory conduction?
when the action potential moves from node to node - this is much quicker than in a non-myelinated neurone as channels do not need to be opened along the full length of the axon
How does axon diameter impact the speed of a neurone impulse?
the larger the diameter of the axon, the faster the impulse travels
Why do axons with a smaller diameter generate a slower impulse speed?
-axons with a smaller diameter have a larger surface area to volume ratio
-this causes a larger number of ions to leak out of the axon and makes it harder for an action potential to propagate.
-large axons have less resistance to passive flow of ions inside the neurone
How does temperature impact the speed of the impulse?
-temperature affects the rate of diffusion
-rate of diffusion of ions across the axon is affected
