Receptors

Question 1

What are receptors?

Answer 1

specialised cells that detect changes in the environment

Question 2

What is a transducer?

Answer 2

converts the energy of a stimulus into the start of a nerve impulse - generator potential

Question 3

Name a type of receptor that is sensitive to changes in pressure

Answer 3

pancinian corpuscle

Question 4

When does a generator potential become an action potential in a pancinian corpuscle?

Answer 4

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

Question 5

What is an adaption of the pancinian corpuscle?

Answer 5

-with continuous pressure, the frequency of action potentials decreases and soon stops
-prevents the nervous system from being bombarded with insignificant information

Question 6

Describe what happens in a pancinian corpuscle?

Answer 6

-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

Question 7

What is a Schwann cell?

Answer 7

cell wrapped around the axon, forming myelin sheath

Question 8

What is the node of Ranvier?

Answer 8

a gap in the myelin sheath where the axon is exposed

Question 9

What is the normal resting state of the axon called?

Answer 9

the resting potential

Question 10

What is the normal potential difference across an axon?

Answer 10

-70mV

Question 11

What does the -70mV change to in an action potential?

Answer 11

+40mV

Question 12

How is resting potential maintained?

Answer 12

-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

Question 13

What else helps to maintain resting potential?

Answer 13

negatively charged ions on the inside like proteins which can’t move across the membrane

Question 14

What are the three stages of an action potential?

Answer 14

-depolarisation
-repolarisation
-hyperpolarisation

Question 15

Explain depolarisation

Answer 15

-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

Question 16

Explain repolarisation

Answer 16

-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

Question 17

Explain hyperpolarisation

Answer 17

-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

Question 18

What is the role of the nervous system?

Answer 18

it detects, relays, and coordinates information about an organisms environment

Question 19

What is the all or nothing principle?

Answer 19

-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

Question 20

How do we perceive the strength of a stimulus?

Answer 20

increased frequency of action potentials

Question 21

What is the refractory period?

Answer 21

-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

Question 22

Why is the refractory period important?

Answer 22

-ensures action potential moves only in one direction
-ensures no overlap between impulses as it limits the number of action potentials

Question 23

Describe the propagation of action potential

Answer 23

-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

Question 24

What are the three factors that effect the speed of transmission?

Answer 24

-myelination
-axon diameter
-temperature

Question 25

How does myelination impact action potentials?

Answer 25

action potentials move faster along myelinated neurones than unmyelinated ones

Question 26

What is the importance of the nodes of Ranvier?

Answer 26

-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

Question 27

What is saltatory conduction?

Answer 27

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

Question 28

How does axon diameter impact the speed of a neurone impulse?

Answer 28

the larger the diameter of the axon, the faster the impulse travels

Question 29

Why do axons with a smaller diameter generate a slower impulse speed?

Answer 29

-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

Question 30

How does temperature impact the speed of the impulse?

Answer 30

-temperature affects the rate of diffusion
-rate of diffusion of ions across the axon is affected