Nervous System V

Question 1

What types of ion channels does conduction of AP require?

Answer 1

Voltage gated Na+ and K+ channels as well as leak channels

Question 2

List the steps of the conduction of an AP:

Answer 2

1. Resting membrane potential
2. Depolarization stimulus
3. Membrane depolarizes to threshold. Voltage gated Na+ and K+ channels begin to open
4. Rapid Na+ entry depolarizes cell
5. Na+ channels close and slower K+channels open
6. K+ moves from cell to ECF
7. K+ channels remain open and additional K+ leaves cell, hyperpolarizing it
8. Voltage gated K+ channels close slowly, less K+ leaks out of cell
9. Cell return to resting ion permeability and membrane potential

Question 3

What channels are activated by depolarization?

Answer 3

Na+ and K+ channels, K+ open more slowly

Question 4

What is the rising phase of an AP?

Answer 4

• depolarization
• depolarizing stimuli open voltage gated Na+ channels (-55 mV), allow Na+ travel down electrochemical gradient
  -at +30 mV Na+ channels inactivate

Question 5

What is falling phase of an AP?

Answer 5

• repolarization
• voltage gated K+ channels also open in response to depolarization, but slowly causing delayed efflux

Question 6

What is the after-hyperpolarization phase of an AP?

Answer 6

• undershoot
• voltage gated K+ do not immediately close when reaching -70 mV causing membrane potential to dip below resting membrane potential
• leak channels bring back to -70 mV

Question 7

How do voltage gated Na+ channels suddenly close at peak of AP?

Answer 7

They contain two gates: activation gate (quick) and inactivation gate (slower, on cytoplasmic side)

Question 8

What does the double gating of Na+ create?

Answer 8

A refractory period

Question 9

What is an absolute refractory period?

Answer 9

A second AP cannot be initiated 1-2 msec (inactivation gates locked)

Question 10

What is a relative refractory period?

Answer 10

A second AP can be initiated but requires a larger than normal depolarizing stimulus (graded potential)
- 2-5 msec
- need to overcome more negative membrane potential (-80 to +30)
- need to overcome after depolarization phase

Question 11

What is the purpose of a refractory period?

Answer 11

• ensures an AP travels in one direction
• limits the rate at which signals can be transmitted down a neuron
• creates space so brain can interpret stimulus

Question 12

How is information encoded from AP’s?

Answer 12

The frequency of the APs

Question 13

What is AP potential conduction (propagation)?

Answer 13

• travel over long distances without losing energy, size and amplitude is identical at trigger zone and axon terminal

Question 14

Steps of AP conduction?

Answer 14

1. Graded potentials enters trigger zone
2. Voltage gated Na+ channels open, and Na+ enters axon
3. Positive charges spread along adjacent sections of axon by local current flow
4. Local current flow causes a new section of the membrane to depolarize
5. Loss of K+ repolarizes membrane
6. Refractory period prevents backward conduction

Question 15

What is the first parameter determining velocity of APs?

Answer 15

The diameter of the axon: larger diameter axon will offer less internal resistance to current flow
- bringing adjacent regions of membrane to threshold faster

Question 16

What is the second parameter determining velocity of APs?

Answer 16

The resistance of the axon membrane to ion leakage: current will spread to adjacent sections more rapidly if it is not lost via leak channels

Question 17

Why is conduction velocity more rapid in myelinated axon?

Answer 17

No leakage in myelinated parts
- nodes of ranvier contain abundance of Na+ channels
- larger diameter axons (~120 m/sec)
- unmyelinated are smaller (~2 m/sec)
10um myelinated has same velocity of 500um unmyelinated

Question 18

What is saltatory conduction?

Answer 18

Conduction from node to node

Question 19

Demyelination?

Answer 19

• only nodes contain Na+ channels, AP cannot be maintained in unmyelinated region due to lack of Na+ channels
• current leaks out of unmyelinated region

Question 20

Where do neurons communicate?

Answer 20

At synapses: presynaptic cell (neuron) to postsynaptic cell (neuron, muscle, target cell)

Question 21

How many synapses can a postsynaptic neuron contain?

Answer 21

Up to 150,000

Question 22

What are electrical synapses?

Answer 22

Some CNS neurons, cardiac muscle, smooth muscle

Question 23

What are chemical synapses?

Answer 23

Majority of neurons in the nervous system use chemical signals to communicate

Question 24

What is a neurocrine?

Answer 24

A chemical substance released from neurons used for cell-cell communication: neurotransmitters, neuromodulators, and neurohormones

Question 25

What are neurotransmitters?

Answer 25

Chemical that is released, acts on a postsynaptic cell in close vicinity and causes rapid response in postsynaptic cell
- paracrine or autocrine

Question 26

Paracrine vs autocrine?

Answer 26

Acts in local vicinity vs acts on themselves

Question 27

What are neuromodulators?

Answer 27

Chemical that is released, acts on a postsynaptic cell in close vicinity and causes a slow response in postsynaptic cell
- paracrine or autocrine

Question 28

What does different receptions at synapses cause?

Answer 28

Same neurocrine can act as a neurotransmitter at one and neuromodulator at another

Question 29

What do neurohormones do?

Answer 29

Secreted into blood stream and act on targets throughout body