Lecture 6A - Neuromuscular Stimulation - Physiology Review

Question 1

What is resting membrane potential

Answer 1

-60 to -90mV membrane resting potential

Question 2

Describe the active transport mechanism

Answer 2

Expels three (3) Na+ ions and takes in two (2) K+ ions

Higher Na+ outside the cell and higher K+ inside the cell

Question 3

What channels exist in excitable cells?

Answer 3

Voltage-gated sodium and voltage-gated potassium channels exist in excitable cells

Question 4

Where in the neurone are excitable cells found?

Answer 4

In the neuron, these cells are found on the axon and are essential for the generation of an action potential

Question 5

What changes are voltage gated channels sensitive to and when do they open?

Answer 5

These channels are sensitive to changes in the membrane potential and open when the inside of the cell becomes more positive

Question 6

Describe the 4 steps of depolarization of nerve fibre

Answer 6

1.Adequate stimulus
•thermal, chemical, mechanical or electrical
2.Explosive Na+ influx
•open Na+ channels
•Creates depolarization
3.Impulse travels along fibre
4.Out flux of K+
•re-polarization

Question 7

Describe depolarization

Answer 7

• When the channels open NA+ rushes into the neuron and the membrane depolarizes rapidly (+35 mV)
• NA+ channels then become inactivated while the K+ channels open and K+ rushes out of the cell

Question 8

Describe action potential

Answer 8

•The action potential is the rapid reversal of the resting membrane
•The membrane changes from resting
(-70mV) to roughly +35mV
–This is depolarization
•After this phase, the membrane rapidly returns to –70mV
•This is re-polarization

Question 9

Describe threshold

Answer 9

to generate an action potential an axon requires a stimulus of a certain minimum strength

Question 10

describe all or none principle

Answer 10

each action potential has the same amplitude independently from the strength of the stimulus

Question 11

Describe the refractory period

Answer 11

a second action potential cannot occur during this period

Question 12

Describe the different myelinated and unmyelinated nerve fibres

Answer 12

•Myelinated nerve fibres
–A alpha (proprioception)
–A beta (touch)
–A gamma (pain and temperature)
•Unmyelinated nerve fibres
–C fibres (pain / temperature / itch)

Question 13

describe the neuromuscular junction

Answer 13

• Where the motor neuron and the muscle fibre meet
• Myelinated axon divides into terminal branches (no longer myelinated) then forms a terminal button
• Terminal button contains neurotransmitters that project close to the motor end plate
• Neurotransmitters change electrical impulses into chemical stimuli
• Impulse travels to the terminal button causing an opening of positive calcium channels which causes the release of acetylcholine from its vesicles
• In the muscle, the muscle fibre membrane at this junction is specialized –forms motor end plate –membrane is folded with multiple mitochondria

Question 14

Describe a muscle contraction with regard to fibre recruitment

Answer 14

During normal voluntary contraction the brain estimates the number of fibres it needs for a task, then recruits in an all-or-none fashion with the smallest fibres called on first and the largest fibres recruited last.

Question 15

Describe acetylcholine’s role in muscle contraction

Answer 15

Acetylcholine released into the gap, combines with the transmitter-receptor complex, causing the post-synaptic membrane to become more permeable to sodium ions inward and potassium ions outward and will lead to depolarization.

Acetylcholine also enters transverse tubercles of the muscle fibre –causes release of Calcium (Ca)

• Ca stimulates actin and myosin filaments and causes the contraction
• Current flow in the muscle fibre causes a contraction
• Within 5 milliseconds after acetylcholine is released it is then destroyed by an enzyme called acetylcholinesterase

Question 16

Describe motor unit recruitment - voluntary muscle contraction

Answer 16

During a voluntary muscle contraction,
–the body recruits motor units asynchronously
–according to its needs
–allows a smooth gradual contraction
–less fatigue , less energy

Question 17

describe motor unit recruitment - low frequency muscle stimulation

Answer 17

Low frequency Muscle
Stimulation causes:
•same nerve fibres to be stimulated repeatedly
•causing rapid fatigue and excessive metabolic energy
•fibres closest to pad contract first

Question 18

describe motor unit recruitment - medium frequency NMES

Answer 18

Medium frequency NMES units may allow an asynchronous depolarization of nerve fibres because the depolarization is a result of the short duration negative half cycles of sine wave