AC 2.3 Explain Mechanisms Of Impulse Propagation

Question 1

What is a neurone?

Answer 1

Specialised cells that generate and conduct electrical impulses

Question 2

What is resting potential / state? (-70mV)

Answer 2

1. Neuron is not conducting impulse (POLARISED)
2. Inside neurone more -ve charged than outside
3. Neuron becomes polarised due to movement of Na+ and K+ ions across cell membrane

Question 3

What is a threshold potential?

Answer 3

1. To generate an AP, an axon requires a stimulus of a certain minimum strength (-55mV)

Question 4

What is an action potential / depolarisation?

Answer 4

Rapid reversal of membrane potential

Question 5

What is the all or nothing principle?

Answer 5

Each AP has same amplitude regardless of the strength of the stimulus

Question 6

What is the refractory period?

Answer 6

A second action potential cannot occur during this period as the channels are still open

Question 7

What is hyperpolarisation?

Answer 7

Membrane potential increases, becoming more negative than resting potential (-70mV)

Question 8

What is absolute refractory period?

Answer 8

Second stimulus (no matter how strong) will not excite the neurone

Question 9

What is relative refractory period?

Answer 9

A stronger than normal stimulus needed to elicit neuronal excitation

Question 10

Explain each component of an AP graph

Answer 10

1. Up: Na+ channels open, Na+ enters cell
2. Up: K+ channels open
3. Peak: Na+ channels close, No more Na+ enters cell
4. Down: K+ leaves cell, causes membrane potential to return to resting potential
5. Down: K+ channels close
6. Overshoot: Extra K+ outside diffuses away

Question 11

What is a graded potential?

Answer 11

1. Changes in membrane potential that vary in size, as opposed to being all-or-none.
2. Arise from summation of individual actions of ligand-gated ion channel proteins, decreasing over time and space.
3. Magnitude of a graded potential is determined by the strength of the stimulus.

Question 12

What are the effects of myelination?

Answer 12

1. Myelination: process coating axon of each neuron with fatty coating called myelin
2. Protects neurone, helps it conduct signals more efficiently via Saltatory conduction.

Question 13

How does an impulse pass down the neurone?

Answer 13

1. Only cells with excitable membranes can generate APs (neuron, muscle)
2. Depolarisation > repolarisation > hyperpolarisation
3. At trigger zone, Na+ channels open, membrane becomes more permeable to Na+ ions and AP occurs

Question 14

Explain the Na+/K+ pump

3Na+ OUT

2K+ IN

Answer 14

• Na+ ions naturally leak into neurons through ion channels
• K+ ions naturally leak out
• membrane more permeable to K+ than Na+
• activity of the pump moves the ions against their conc. gradient, maintaining a membrane potential of -70mV

Question 15

Explain how an action potential happens

Answer 15

1. Stimulus causes Na+ channels to open (ligand/mechanically gated) in dendrites of sensory neurones.
2. Stimulus produces graded potential
   (can vary in size)
3. If stimulus large enough, cause depolarisation of membrane to reach threshold potential (-55mV), AP triggered.
4. Voltage-gated Na+ channels open in trigger zone of axon
5. Na+ rushes in, reversing membrane potential (depolarisation/AP)
6. K+ ions flow out cell, but K+ channels open more slowly than Na+ channels
7. Outflow of K+ repolarises membrane together with Na+/K+ pump