factors that affect the speed of a nervous impulse

Question 1

define refractory period

Answer 1

the period after an action potential where all sodium voltage gated channels are closed and no further action potentials can occur

Question 2

what is the all or nothing principle

Answer 2

if the threshold value (-55mV) is reached an action potential is always generated

below threshold value no action potential is generated regardless of the strength of a stimulus

action potentials are always the same size (regardless of stimulus)

Question 3

explain how the brain determines the strength of a stimulus

Answer 3

• number of impulses in a given time
• specific neurons have different threshold values - the brain then interprets the type and number of neurones action potentials pass along

Question 4

suggest the importance of the all or nothing principle

Answer 4

prevents central nervous system from becoming “overstimulated” by very small/less important stimuli

Question 5

what 3 factors affect the speed of a nervous impulse

Answer 5

myelin sheath

diameter of the axon

temperature

Question 6

how does myelin affect the speed of the nervous impulse

Answer 6

myelin sheath acts as an electrical insulator

Na+ ion channels are concentrated in nodes of Ranvier so depolarisation 1 action potential only occurs here

Impulse jumps from node to node = “saltatory conduction”

Question 7

explain how the axon diameter affects the speed of the nervous impulse

Answer 7

The wider diameter, the greater the speed of conductance

This is because there is less resistance to the flow of ions in the cytoplasm (compared to a narrower axon) therefore, depolarisation reaches other parts of cell membrane quicker

Less leakage of ions from a large axon - so easier to maintain a potential difference

Question 8

how does temperature affect the speed of the nervous impulse

Answer 8

The higher the temperature, the faster the nerve impulse (up to a certain point before denaturation of membrane protein occurs)

this is because…

• higher temperature increases the rate of ion diffusion
• increased respiration rate (which releases energy for the protein pumps/active transport) is controlled by enzymes which function more quickly at higher temperatures