Nerve impulses

Question 1

Describe how the movement of ions establishes the resting potential in an axon

Answer 1

sodium potassium pump moves 3 Na+ out of cell for every 2K+ into cell
process uses ATP
K+ diffuses back out of cell through channel proteins, down concentration gradient, while gated channel proteins for Na+ remain closed at rest so Na+ cannot move back into cell
therefore membrane more permeable to K+
creates resting potential of -70mV inside cell

Question 2

Describe how the movement of ions generates an action potential

Answer 2

Na+ channels open due to stimulus - Na+ diffuses in
If threshold potential reached, voltage gated Na+ channels open - faster Na+ diffusion
at +40mV Na+ channels close + voltage gated K+ channels open - K+ diffuses out
voltage gated K+ channels close when resting potential reached / membrane repolarises

Question 3

What is the refractory period?

Answer 3

time when you cannot generate another action potential
when voltage gated Na+ channels close, they become inactive
over 5mS change shape from inactive to closed

Question 4

What initiates an action potential?

Answer 4

must reach -55mV threshold potential for voltage gated Na+ channels to open

Question 5

What is a Pacinian corpuscle?

Answer 5

deep skin pressure receptor

Question 6

Describe how stimulation of a Pacinian corpuscle produces a generator potential

Answer 6

connective tissue presses against neurone membrane
pressure deforms membrane
Na+ channels open causing Na+ diffuse in
depolarisation leads to generator potential
a greater pressure causes more channels to open

Question 7

How is the intensity of a stimulus communicated?

Answer 7

frequency of action potentials

Question 8

Explain how resting potential is maintained?

Answer 8

membrane more permeable to potassium ions and less to sodium ions
sodium ions actively transported out and potassium ions in

Question 9

Why doesn’t every stimulus result in an action potential?

Answer 9

stimulus may not cause depolarisation to threshold potential - may not reach -55mV

Question 10

What is the importance of the refractory period?

Answer 10

prevents action potential spreading backwards along axon

Question 11

why is the transmission of action potentials unidirectional?

Answer 11

due to the refractory period where Na+ channels immediately behind region of membrane repolarising are inactive and cannot immediately reopen, preventing action potential from spreading backwards

Question 12

Explain how an action potential passes along an unmyelinated neurone

Answer 12

stimulus gated Na+ channels open when threshold potential
diffusion of Na+ inside axon
Depolarises neighbouring section of membrane to -55mV
voltage gated Na+ open and diffuses into second section
K+ diffuses into first section, repolarising
refractory period prevent action potential from travelling backwards as Na+ channels inactive

Question 13

What is the structure of a myelinated neurone?

Answer 13

myelin sheath = insulated part of membrane - no ion channels, no Na+/K+ movement across membrane
all ion channels at nodes of Ranvier

Question 14

How does the structure of the myelinated neurone affect transmission of the action potential?

Answer 14

Voltage gated Na+ channels open
Na+ moves in
Na+ diffuses inside axon to next node of Ranvier
membrane depolarises to -55mV
voltage gated Na+ channels open
Na+ moves in and diffuses

Question 15

Describe the effect that myelinated sheaths has on the way an action potential is conducted along an axon

Answer 15

action potential appears to jump between nodes of Ranvier
Saltatory Conduction

Question 16

Explain why the speed of transmission of impulses is faster along a
myelinated axon than along a non-myelinated axon

Answer 16

myelination provides insulation
depolarisation only at nodes of Ranvier so saltatory conduction
in non-myelinated depolarisation occurs along whole length of axon

Question 17

What factors affect the speed of conduction in neurones?

Answer 17

increase axon diameter, increase rate of conduction
increase temperature affects rate of diffusion