Page 4, 5- Action Potential (Big) including transmission of nervous impulses

Question 1

Transmission across a Synapse

Answer 1

1. A wave of action potential travels along the presynaptic neuron, until it reaches the synaptic end bulb.
2. Depolarisation of the membrane at the synaptic end bulb causes the Ca2+ channels to open.
3. Ca2+ ions flow into the presynaptic membrane.
4. Increase in Ca2+ inside the cell activates vesicles containing the neurotransmitter.
5. Vesicles fuse with membrane, releasing neurotransmitter (via exocytosis) into synaptic cleft
6. Neurotransmitter diffuses across the synaptic cleft and binds to a neurotransmitter receptor molecule (specific to neurotransmitter) on the postsynaptic neuron.
7. Binding of neurotransmitter causes the receptor molecule to be activated. This causes an action potential in the postsynaptic neuron or stimulus for an effector (muscle or gland).
8. Neurotransmitter molecules eventually break loose from receptor molecule and drift away (through synaptic cleft towards the presynaptic neuron).
9. The neurotransmitter is deactivated by an enzyme in the synaptic cleft and is reabsorbed (endocytosis) by the presynaptic neuron to be reused.

Question 2

Depolarisation

Answer 2

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Question 3

Repolarisation

Answer 3

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Question 4

Refractory Period

Answer 4

-The period of time during action potential and very briefly afterwards, in which the part of a nerve affected cannot be stimulated to respond again.
When the membrane reaches the threshold (-55mV) and until it returns to RMP (-70mV).

Question 5

What does Refractory period mean for nerve signals?

Answer 5

-Resting potential must be restored before the next impulse can be carried out.
-This causes the impulse to only travel in one direction.
-Therefore the refractory period effectively sets the limit of frequency of impulses along the neuron

Question 6

Transmission of Nervous Impulses

Answer 6

Unmyelinated: Depolarisation of one area of the membrane causes the area adjacent to also depolarise, creating a “domino effect” that propagates the action potential along the fibre

Myelinated: The myelin sheath insulates most of the membrane, separating it from the extracellular fluid.

Thus, depolarisation can only occur at the Nodes of Ranvier (the gaps in the sheath) as they are the only areas that allow for ion exchange between the intracellular and extracellular fluid.

As a result action potential jumps from one node to the next (saltatory conduction), making conduction faster than unmyelinated fibres.

Question 7

Nervous Synapse Diagram

Answer 7

check diagram