Nervous System

Question 1

What happens when a patch of excitable membrane generates an AP?

Answer 1

Na+ influx reverses potential difference (inside becomes +). This depolarizes adjacent membrane → opens Na+ channels → AP propagates.

Question 2

How does an AP spread to adjacent membrane?

Answer 2

Passive depolarizing current spreads via electromagnetism → opens Na+ channels in next patch.

Question 3

What happens if depolarizing current is too weak?

Answer 3

The AP dies out before reaching the axon.

Question 4

What makes a cell excitable?

Answer 4

Presence of voltage-gated Na+ channels.

Question 5

Which cells propagate APs?

Answer 5

Neurons with axons and muscle cells.

Question 6

Where is the AP usually initiated?

Answer 6

Trigger zone (axon hillock)

Question 7

What carries APs away from the cell body?

Answer 7

The axon

Question 8

What’s the functional region where neurons communicate?

Answer 8

Synapse (presynaptic terminal, synaptic cleft, postsynaptic neuron).

Question 9

What is λ (length constant)?

Answer 9

measures how quickly a potential difference disappears (decays to zero) as a function of distance

In other words it measures how far we can carry the potential distance

Distance where voltage drops to ~37% of its original value

Question 10

How can λ be increased?

Answer 10

Increase axon diameter (↓ internal resistance) or increase membrane resistance (↓ current leakage).

Question 11

What increases membrane resistance most efficiently?

Answer 11

Myelination by glial cells (Schwann cells in PNS, oligodendrocytes in CNS).

Question 12

What are Nodes of Ranvier?

Answer 12

Gaps between myelin where voltage-gated channels cluster; APs regenerate here.

Question 13

What happens in multiple sclerosis (MS)?

Answer 13

Loss of myelin → slowed or blocked AP conduction

Question 14

How does saltatory conduction work?

Answer 14

APs “jump” node to node, with passive spread between nodes.

Question 15

Safety factor of saltatory conduction?

Answer 15

Can skip damaged nodes and still propagate APs.

Question 16

What about unmyelinated axons?

Answer 16

No myelin → more leakage, slower velocity. But Schwann cells still partially insulate axons in bundles (Remak bundle).

Question 17

Why can’t APs go backwards?

Answer 17

Refractory period (Na+ channels inactivated).

Question 18

What happens at the end of the axon?

Answer 18

AP dies out; cannot regenerate beyond terminal.

Question 19

What are the two types of synapses?

Answer 19

Electrical (gap junctions) & chemical (neurotransmitter release).

Question 20

Example of electrical synapse?

Answer 20

Cardiac muscle cells (synchronous contraction).

Question 21

What defines a chemical synapse?

Answer 21

Presynaptic bouton (vesicles), synaptic cleft, postsynaptic receptors.

Question 22

What does Ri represent in the length constant equation?

Answer 22

Internal resistance of the axon.

Question 23

What does Ro represent in the length constant equation?

Answer 23

Extracellular fluid resistance (low, doesn’t vary much, usually ignored).

Question 24

What does Rm represent in the length constant equation?

Answer 24

Membrane resistance.

Question 25

How can λ (length constant) be increased?

Answer 25

By decreasing Ri (increase axon diameter) or increasing Rm (increase membrane resistance).

Question 26

What triggers neurotransmitter vesicle release?

Answer 26

Ca++ influx via voltage-gated Ca++ channels at bouton.

Question 27

How does exocytosis occur?

Answer 27

Ca++ entry → vesicles dock & fuse → NT released into cleft.

Question 28

Is vesicle release guaranteed?

Answer 28

No, it’s probabilistic (1 AP has 10–90% chance of releasing 1 vesicle).