Paper 2 - Nervous Coordination

Question 1

Explain how the resting membrane is maintained by the sodium potassium pump

Answer 1

NOKIA

The Na+/K+ pump actively transports 3 Na+ out of the axon and 2 K+ in using ATP.

Voltage gated Na+ and K+ channels are closed

K+ channels leak = membrane slightly permeable to K+ and not permeable to Na+

Question 2

Describe and explain the process of an action potential

Answer 2

1. Stimulus causes membrane to become more permeable to Na+ as some voltage gated Na+ channels open
2. Threshold is reached - 55mV = more Na+ gates to open and more Na+ to rapidly diffuse into the axon. Membrane potential becomes positive - depolarisation.
3. Colgate gated Na+ channels close. K+ channels open and K+ diffuse out of the neurone.
4. K+ channels close = slight overshoot as K+ channels are slow to close. Membrane potential is slighter lower = hyperpolarisation.
5. The Na+/K+ pump restores resting potential by moving 3 Na+ out and 2 K+ in by AT.

Lasts milliseconds

Question 3

Explain the importance of the refractory period

Answer 3

Acts as a time delay, making sure action potentials cannot overlap and can pass along as separate impulses.

Action potentials are also unidirectional.

Question 4

Explain how depolarisation travels as a wave along a neurone

Answer 4

1. When action potential is fired, some Na+ that entered diffuses sideways .
2. Voltage gated Na+ channels open in adjoining part of axon = more sodium diffuse depolarising the next section
3. Wave of depolarisation along length of the neurone

Question 5

Describe the all or nothing principle

Answer 5

If the stimulus isn’t large enough, less sodium channels will open and the axon membrane will not become full depolarised = no action potential generated.

If the stimulus is large enough then the impulse will be generated at a constant size and speed.

A stronger stimulus will increase the frequency of action potentials

Question 6

How does myelination impact the speed of conduction

Answer 6

Voltage gated sodium channels are found at the nodes of Ranvier.

Cytoplasm conducts electrical charge to allow depolarisation of the next node.

Action potential jumps from node to node = saltatory conduction and increases speed of nerve impulse.

Question 7

How does temperature impact the speed of conduction?

Answer 7

Higher temperature = faster conduction of action potentials along the axon.

Increase in rate of diffusion of ions and the rate of enzyme controlled reactions.

Above 40 degrees proteins denature and speed decreases.

Question 8

How does axon diameter impact the speed of conduction?

Answer 8

Greater diameter = faster conduction of action potentials.

Less resistance to flow of ions than in cytoplasm of smaller axon

Depolarisation reaches other parts of axon membrane quicker

Question 9

Describe spatial summation

Answer 9

When 2 or more presynaptic neurones release their neurotransmitter at the same time onto one postsynaptic neurone.

Question 10

Describe the sequence of events involved in transmission across a cholinergic synapse

Answer 10

1. Action potential arrives at presynaptic membrane = depolarisation
2. Calcium channels open and calcium diffuses into neurone.
3. This causes synaptic vesicles to fuse with the presynaptic membrane and release ACh
4. ACh diffuses across the synaptic cleft and binds to receptors on postsynaptic membrane.
5. This causes the sodium channels to open so sodium diffuses in = depolarisation
6. If threshold is reached, new action potentials will be fired in the postsynaptic neurone

Question 11

Describe temporal summation

Answer 11

When two or more impulses arrive in quick succession from the same presynaptic neurone