7.Excitability and excitation: general characteristics. Propagation of action potentials along the nerve fibers. Nerve fiber types.

Question 1

What are the main biological properties of neurons

Answer 1

Excitability and conductivity

Question 2

What is the significance of excitability and conductivity

Answer 2

transmission of information
afferent fibers transmit from peripheral organs to CNS
Efferent fibers transmit from CNS to periheral organs

Question 3

How does propagation of action potentials occur

Answer 3

spread of local currents from active regions to inactive regions

ELECTROTONIC CONDUCTION

Question 4

What is conduction velocity

Answer 4

The speed at which action potentials travel along nerve fiber

Question 5

What are the cable properties that are major concepts in conduction velocity

Answer 5

Time constant and length constant

the higher the permeability, the lower the resistance, and vice versa.

Longitudinal resistance (Rl ): is the resistance that counteracts electric
current in the cytosol and ECF.

Question 6

What are 2 factors that affect time constant

Answer 6

membrane resistance + membrane capacitance

Time constant is highest when both of these are high

Question 7

How can you increase the conduction velocity of a nerve

Answer 7

Increasing nerve diamter- the larger the fiber, the lower the internal resistance

Myelination- myelin increases membrane resistance but decreases membrane capacitance. High resistance means it forces the action potential to take path of least resistance
At nodes of Ranvier, the resistance is low-saltatory conduction

Question 8

What is rheobase

Answer 8

The lowest intensity of stimulus that can cause A.P after prolonged stimulation

Question 9

What is chronaxie

Answer 9

Time requires for a stimulus that is 2x rheobase to induce a A.P

Question 10

What is the Erlanger and Gasser classification of nerve fibres

Answer 10

A- alpha, beta, delta, gamma
B
C

Question 11

Which is the fastest nerve fiber

Answer 11

A
largest diameter

skeletal muscle, receptors of muscle spindle etc

Question 12

If the action potential is stimulated in the middle of the nerve fiber, how will the impulse travel

Answer 12

Action potential travels in both directions, centrally and peripherally

Question 13

What is the threshold potential

Answer 13

-55mv

Question 14

What determines the excitability of given excitable tissue

Answer 14

Difference between resting membrane potential (-70mV) and threshold potential (-55mV)

Question 15

What is the law for all or nothing stating

Answer 15

Upon threshold or above threshold all APs have the same amplitude

Question 16

What is the biological significance of refractory periods

Answer 16

1. Limit the max frequency of effective stimuli
2. determine direction of nerve propogation

Question 17

Can an AP generated in 1 nerve fibre be transmitted to neighbouring nerve fibers within a single nerve

Answer 17

NO IT CANT- law for isolated conduction

the resistance in the extracellular space is much lower than the transmembrane resistance of adjacent unexcited nerve fibers

Question 18

Differences between myelinated and unmyelinated AP conduction

Answer 18

unmyelinated - Na+ channels are evenly distributed along axonal membrane- slow conduction

Myelinated- myelin sheath increases transmembrane resistance. The nodes have the highest conc of Na+ channels with lowest resistance

Question 19

What is time constant

Answer 19

The time constant (τ) gives us an idea about ​​the time it takes for initially
applied voltage to drop to 37% of its original value.

It is usualy 2-15 ms

Question 20

How can physiological continuity of nerve be interrupted

Answer 20

Blocking voltage gated Na+ channels eg lidocaine

Cooling

Demyelination leading to decrease of length constant

Compression of nerve leading to resistance for local currents

Question 21

Mechanism of neuromuscular transmission

Answer 21

• Depolarization of presynaptic terminal by AP
• Voltage gated Ca2+ channels open and Ca2+ flows into presynaptic terminal
• Ach extruded by exocytosis into synaptic cleft
• Ach binds to receptors on postsynaptic membrane (motor end plate)
• Na+/K+ channels open leading to depolarization and excitory postsynaptic potential
• This EPSP causes generation of AP
  in the adjacent muscle tissue, were
  voltage-gated Na+ channels are
  again available.
• Ach is then degraded to choline
  and acetate by acetylcholinesterase
  (AchE).