Muscle Physiology

Question 1

What is the name of the motor unit used in this experiment?

Answer 1

The Gastrocnemius

Question 2

What is the largest nerve in the body?

Answer 2

The sciatic nerve

Question 3

What is the sciatic nerve a major nerve of in humans?

Answer 3

The sacral plexus

Question 4

What is the sciatic nerve comprised of?

Answer 4

2 nerves: the common peroneal and the tibial nerves

Question 5

Where do the nerve fibres arise from in the body?

Answer 5

From spinal roots L4-S2

Question 6

How do you destroy the brain and spinal cord of the frog?

Answer 6

The frog is stunned and pithed

Question 7

What are the final steps to removing the sciatic nerve and preparing the gastrocnemius muscle?

Answer 7

The in-situ length of the gastrocnemius muscle is measured and a ligature tied around the achilles tendon. This should be left long to facilitate attachment to the strain gauge.
The tendon is cut and the gastrocnemius muscle carefully dissected free up to the knee joint. The limb bones either side of the knee-joint are cut with coarse scissors allowing the muscle, nerve and knee-joint to be lifted free of the animal.

Question 8

What does the trigger lead do?

Answer 8

The trigger lead co-ordinates delivery of the stimulus to the muscle with the start of the trace (beam) moving across the oscilloscope screen. Without this lead you would not be able to see the muscle contraction or the compound nerve action potential (CNAP).

Question 9

What does the isolated stimulator do?

Answer 9

The stimulator can deliver electrical pulses via stimulating electrodes to either the sciatic nerve or the gastrocnemius muscle. Controls allow the stimulus parameters to be varied; voltage (V), duration (ms), frequency (Hz) and number (single or paired stimuli with a variable interval between them).

Question 10

What do the stimulating electrodes 1 do?

Answer 10

These are used to electrically activate nerve fibres in the sciatic nerve to produce the CNAP.

Question 11

What is the function of stimulating electrodes 2?

Answer 11

These are used to electrically activate the gastrocnemius muscle directly

Question 12

What is the function of the recording electrodes?

Answer 12

These are used to record the CNAP of the sciatic nerve evoked by electrical stimulation.

Question 13

Are the characteristics of the muscle twitch similar in all muscle?

Answer 13

No, the duration of the twitch varies according to the type of muscle in “slow fibres”, the twitch may last over 150ms, whereas in “fast fibres” it may be less than 10ms. Temperature also affects the duration of the twitch contraction.

Question 14

What is the smallest functional unit of contraction in muscle?

Answer 14

A motor unit. A motor unit comprises a motor neurone and the muscle fibres it innervates. Each motor neurones branches and supplies several muscle fibres: thus activation of one motor neurone will activate several muscle fibres.

Question 15

Are all motor units comprised of a motor neurone whose branches innervates 5-10 muscle fibres?

Answer 15

No, this may be typical of motor units of muscles concerned with fine graded movements but in large muscles of the leg and back, each motor neurone may innervate 120-170 muscle fibres

Question 16

What effect does Curare have on the amplitude of the CNAP?

Answer 16

Curare has no effect on the amplitude of the CNAP.

Question 17

TRUE/FALSE - Curare has no effect on amplitude of muscle contraction evoked by stimulation of the muscle:

Answer 17

True - Curare has no effect if the muscle is stimulated directly

Question 18

Is curare is a neuromuscular blocking agent?

Answer 18

Yes - Curare is a neuromuscular blocking agent which works by competing with acetylcholine; which is the neurotransmitter released at motor nerve endings, for receptor sites at the end plate membrane of skeletal muscle fibres. It thus reduces the activation of muscle fibres and thus the amplitude of contraction.

Question 19

Is Curare a muscle relaxant and is it used clincially?

Answer 19

Yes, it reduces the activation of muscle fibres and thus the amplitude of contraction. Derivatives of curare such as d-tubocurarine are clinically used as muscle relaxants given with general anaesthetics. However the dose is critical.

Question 20

Is Curare life threatening to humans?

Answer 20

Yes, it is an antagonist of acetylcholine receptors at the skeletal neuromuscular junction. It was discovered being used by South American Indians as poison in blow darts killing prey by stopping respiration. That is why the dosage of curare clinically is critical as it depresses respiration by blocking neuromuscular junctions of respiratory muscles.

Question 21

Are tetanic contractions only important when we preform sustained muscle activity?

Answer 21

No, the activation of muscles in the body is achieved via motor neurons which generate action potentials at frequencies much greater than 1Hz. Thus, however brief the contraction of a muscle in the body it will be a tetanic contraction.