Neurophysiology

Question 1

Neurons convey messages using

____________.

Answer 1

electrical signals

Question 2

Ohm’s Law

Answer 2

I = V/R

Question 3

How is the membrane lipid-bilayer also a diffusion bilayer?

Answer 3

• Impermeable to ions
• Functions as a capacitor
• Membrane potential:
separation of the +ve and –ve
charges
• Ions flow through channels

Question 4

Ion flow is dependent on _______________.

Answer 4

size & charge

Question 5

Common characteristics of ion

channels

Answer 5

• Multiple states:
– Open
– Closed

• Selectivity:
– Minimal selectivity: nature of amino acid residues lining its pore
(anions vs cations)
– Highly selective: size of the pore
(e.g. Na+ >>>K+)

• Gating

Question 6

Gating of ion channels:

Answer 6

• Channels flip between open and closed states
• Factors can change this probability:
– Voltage gated
– Ligand gated
– Thermally gated
– Mechanically gated

Question 7

Membrane potential

Answer 7

• Impermeable PM will not allow the ions to flow down their concentration gradient
• Negative and positive charges would be identical in each side

Question 8

Extracellular concentrations:

Answer 8

Na: 140 mM
K: 4 mM
Cl: 120 mM
Ca: 2.5 mM

Question 9

Intracellular concentrations:

Answer 9

Na: 15 mM
K: 130 mM
Cl: 5 mM
Ca: 0.0001 mM

Question 10

Equilibrium potential:

Answer 10

the potential at which an ion is in electrochemical equilibrium

Question 11

Resting membrane potential is heavily influenced, but not determined, by ____________.

Answer 11

K gradient

Question 12

Factors that influence the net passive diffusion:

Answer 12

1. Electrochemical gradient
   – Chemical gradient
   – Difference in electrical potential, (positively charged ions will tend to move toward regions of more negative potential)
2. Conductance of the channel
   – At resting membrane potential, Na conductance is small

Question 13

Electrical properties of neurons

Answer 13

• Passive (cable properties)

* Active

Question 14

Cable properties of neurons

Answer 14

• Amplitude of the potential change
decays exponentially as it moves away from
its source
• Change in Vm passively spreads in
both directions along the axon/dendrite

Question 15

Length constant:

Answer 15

• distance over which the potential falls by
  1-(1/e) or 63% from its original value.
• depends on the Rm (resistance of the membrane) and the Rl (longitudinal resistance).

Question 16

Action potential (AP)

Answer 16

Actively propogates over long distances

Question 17

Properties of an Action Potential:

Answer 17

• Threshold
• All-or-none response
• Latency
• Refractory Period
• Propagation

Question 18

Refractory Periods:

Answer 18

• Absolute: no way to generate an AP

* Relative: requires higher stimulus and produces lower AP

Question 19

Tetrodotoxin

Answer 19

• Blocking Na channels of the peripheral nerves causes weakness and numbness
• Larger doses can result in respiratory paralysis and death
• Example: puffer fish poison

Question 20

How does the longitudinal resistance affect the length constant?

Answer 20

Increasing the diameter will decrease the longitudinal resistance, thus increasing the length constant

Question 21

How does the membrane resistance affect the length constant?

Answer 21

Increasing the membrane resistance increases the length constant

Question 22

What determines the speed of the AP?

Answer 22

Length constant

Question 23

How can the length constant be increased?

Answer 23

1. Increasing the diameter of the neuron
2. Increasing the resistance of the axonal membrane
   - insulating the axon - myelination

Question 24

Properties of the length constant:

Answer 24

• Cable property:
Processes with larger diameter
(i.e. higher length constant)

• Active property (AP):
Increasing the length constant will increase the speed of AP propagation (faster).

Question 25

Myelination saves _________, which allows for quicker propagation.

Answer 25

space

Question 26

Demyelinating diseases

Answer 26

• Guillain-Barre syndrome

* Multiple sclerosis

Question 27

Channelopathy

Answer 27

Defective ion channels
• Hyperkalemic periodic paralysis:
– Episodes of weakness and decreased muscle tone
(may follow exercise or K-rich food)
– During the attacks, the involved muscle fibers are
unable to fire APs (cells are depolarized by 30-40 mV)
– Mutation of the muscle voltage-gated Na
channels that prevents some % of them from being
completely inactive after depolarization
– Small but constant inward current…
inactivating normal Na channel…render muscle
inexcitable

Question 28

Myotonia (muscle tone):

Answer 28

Having difficulty getting muscle to relax once they have contracted

Question 29

What are the forms of myotonia?

Answer 29

• Two similar forms: Thomsen’s disease and
Becker’s disease
– The muscles of affected individuals relax unusually slowly following a sudden contraction

Question 30

Myotonic goats

Answer 30

• “fainting” goats
• Cl channels abnormality:
– Normal skeletal muscles: relatively high Cl
permeability; resting membrane potential is influenced by the Cl concentration gradients.
– Mutation in the Cl channel results in decreased conductance….. reduces the amount of depolarizing current required to reach threshold