3 - Nerve Physio

Question 1

What is a nerve?

Answer 1

Cell whose main function is to transmit signals along the nervous system

Question 2

Illustrate organization of the nervous system

Answer 2

Two divisions
I. Central: Brain and Spinal Cord
II. Peripheral:
A. Afferent neurons
B. Efferent neurons
1. Somatic
2. Autonomic neuron
a. Sympathetic
b. Parasympathetic

Question 3

Segments of the neuron? Other terms? Arrange according to flow of signal transmission.

Answer 3

1. Dendrites/ Receptive segment - receives signal
2. Cell body/ Integrative segment - integrates received signal
3. Initial segment - first segment that will initiate signal traveling along the long process
4. Axon/ Conductive segment - long process; allows electrical signals to flow through it
5. Terminal buttons Transmissive segment - transmits signals from conductive segment to end of cell (usually connected to another cell)
   * Axon hillock

Question 4

Types of neuron based on number of poles; based on myelination

Answer 4

A. Unipolar - cell body attached to a single process (either dendrite or axon)
B. Bipolar cell - two poles, one as dendrite, the other one as axon
C. Pseudounipolar - has a cell body, single process which divides into two (single bifurcated process)
D. Multipolar - each pole would have lots of branches

A. Myelinated - there’s node of Ranvier (part of axon not surrounded by myelin sheath)
B. Unmyelinated

Question 5

Function and types of neuroglia/glial cells

Answer 5

Function: Nourishment, structural support, and protection of neurons

Types:
CNS contains:
A. Astrocytes
B. Ependymal cells
C. Oligodendrocytes
D. Microglia

PNS contains:
A. Satellite
B. Schwann cells

*Take note:
Oligodendrocytes and Schwann cells - responsible for producing myelin sheath

Question 6

Part of axon not surrounded by myelin sheath

Answer 6

Node of Ranvier

Question 7

Distribution of ions in a cell

Answer 7

Na+ (& Cl-) - predominantly outside the cell
K+ - predominantly inside the cell
*PISO

Question 8

Ion channels involved in maintaining the resting state of the cell

Answer 8

1. K+ leak channels

2. Na+-K+ pump/ATPase

Question 9

What will happen if

1. K+ leak channels are open?
2. Na+/K+ pump is activated?
3. Voltage-gated Na+ and K+ channels

Answer 9

1. K+ will go out along the conc. gradient (passive transport)
2. Transport ions against the concentration gradient (active transport)
3. Passive transport so along conc. gradient; channel is opened by changes in membrane potential/voltage

Question 10

How is the resting membrane potential generated and maintained?

Answer 10

Cell has K+ through K+ leak channels
*diffusion potential – tendency to move/diffuse i.g. tendency of K+ to move
*when this happens, K+ movement makes the extracellular area more positive and intracellular area becomes more negative
*Potential difference
As long as diffusion potential is greater in magnitude, K+ will still move out until such time intracellular site becomes more -; K+ will now have no net movement if super laki nung negative inside, it can counteract the outward movement/direction of diffusion because attracting charge (known as the equilibrium potential – pot diff across the membrane that in terms of magnitude counteracts diffusion potential; = -diffusion potential); if charge equalizes equilib potential (= no net movement)

Question 11

What dictates diffusion of ions?

Answer 11

Electrochemical gradient

Question 12

Equilibrium potential
A. Define
B. Factors affecting this using Nernst Equation? Problem with this equation?
D. Way to get exact membrane potential

Answer 12

A. Aka Nernst Potential - membrane potential wherein there is no net movement of ions; potential that opposes direction of diffusion; every ion has a specific equilibrium potential; inside the cell!!

B. EMF (Vm) = +- 61/z x log (conc. inside/conc. outside)
> Affected by ratio of concentration of ion inside and outside the cell (conc gradient)

Problem: You have many ions

C.

1. Goldman-Hodgkin-Katz equation (GHK equation)
   - makes use of P (permeability of ion)

2 Chord Conductance equation
-uses conductivity/conductance of the ions as the contributor on the membrane potential

Question 13

True or False. Different cells have different resting membrane potentials and close to Nernst potential of K+.

Answer 13

True!

Question 14

What is the resting membrane potential?

Answer 14

Membrane potential when the cell is at rest.

Question 15

Compare local and action potential.

Answer 15

Local potential
-changes in membrane potential that are non-propagating (does not move/travel long distances), graded (increasing stimulus changes the response; e.g. stronger stimulus = stronger response), decremental (e.g. when amplitude travels very short distances = amplitude decreases)

Action potential
-changes in membrane potential that are propagating , exhibits “all-or-nothing” response, decrementless (as it propagates, amplitude does not diminish)

Question 16

Events in an action potential

Answer 16

1. Resting state
2. Depolarization
3. Overshoot*
4. Repolarization
5. Hyperpolarization*
   * Threshold of excitation - usually +15-30 of resting membrane potential
6. Plateau phase* - voltage stays the same (usually seen in cardiac muscles)

Question 17

IV. Determine physiological properties exhibited by the nerve cells

Answer 17

A. Excitability

*Giant Squid Axon Experiment (Hodgkin-Huxley)

Question 18

Ionic basis of the phases of the action potential?

Answer 18

*Voltage-gated

Resting states: Na+ activation gate closed, inactivated open and K+ channel closed; K+ leaks occur; Na+-K+ pump works (to maintain low Na+ intracellularly)

Depolarization: Open up Na+ channel (along conc. Gradient = mem pot become less negative until you reach threshold), if enough stimulus, open more Na+ channels = less negative

Overshoot: Mem pot becomes positive: Na+ channel becomes inactivated (open activation but inactivation gate is closed = Na+ cannot flow); K+ starts to open (when mem pot becomes positive; slow opening) = K+ will move from inside to outside = mem pot becomes more negative = Repolarization (opening of more K+ channel)

Hyperpolarization (*in some cells): delayed closure of K+ channels

Question 19

Absolute refractory period vs relative refractory period

Answer 19

Absolute refractory period - point/period no amount of stimulation can elicit an action potential regardless of strength of stimulus (because Na+ channel is inactivated)

Relative refractory period - if you apply stimulus stronger than initial stimulus applied, you can elicit an action potential

Question 20

Why is the propagation of an action potential in neurons unidirectional?

Answer 20

Because of absolute refractory period.

Question 21

True or False. Threshold stimulus = threshold potential.

Answer 21

False.

Question 22

Compare stimulating a nerve on threshold stimulus for 30 seconds vs. sustained supra-threshold stimulus. Is this possible in neurons? Why?

Answer 22

Threshold stimulus = few peaks but sustained supra-threshold constant peaks because of relative refractory period. This can’t happen in neurons because of the absolute refractory period.

Question 23

What do local anesthetics do?

Answer 23

Inactivates Na+ channels so if inactivated, depolarization phase won’t occur.

Question 24

True or False. Both graded and action potential have a threshold and a refractory period.

Answer 24

False. Only the action potential has the two.

Question 25

Define:

1. Rheobase
2. Chonaxie
3. Utilization time

Relationship of the three.

Answer 25

1. Rheobase - minimum stimulus strength needed to elicit a response given at an infinite duration (inf duration usually: 300 ms) (*like a threshold stimulus)
2. Utilization time - time it takes to have a response when you apply a stimulus of rheobase strength
3. Chronaxie - duration needed in order to excite a nerve cell when you give a stimulus strength of twice the rheobase strength

Neurons with similar rheobases: the chronaxie gives a measure of the relative excitability of the neurons - shorter chronaxies = more excitable neurons

Question 26

Compute for excitability of nerves

Answer 26

Product of stimulus strength and the duration by which the stimulus is given (they are inversely; at high stimulus strength, need shorter duration applied and vice versa)

Question 27

What is conductivity? It is affected by?

Answer 27

Conductivity - property of a neuron to transmit an electrical signal (action potential)

Factors affecting conductivity:
1. Presence of the myelin sheath
Myelin sheath - non-polar, made up of lipids, acts as an INSULATOR (—> Saltatory conduction) = myelinated nerve fibers would conduct fibers

2. Diameter
   Larger diameter of nerve = faster conduction velocity

Question 28

Correlate duration of latent period and the rate of conduction of impulses.

Answer 28

Conduction velocity = distance/time

Question 29

What is the latency period?

Answer 29

Time you applied the stimulus up to the time just before you have the response

Question 30

Factors which may impair conduction of impulses?

Answer 30

A. Sedatives, anesthetics - slower if you have a blocker

B. Interruption of the blood flow to neuronal processes (neurons rely on energy/ATP to power Na+/K+ pump, if supply of nutrition is reduced = ATP is lower = can’t activate the pump)

C. Conditions that affect myelin sheath formation (e.g. Mutiple Sclerosis)