Skeletal Muscle Practical

Question 1

When you deliver a single supra-threshold stimulus to the nerve innervating a skeletal muscle, the muscle undergoes a _______contraction - a ______. Some important points can be learnt by considering the timing of this event.

Answer 1

single brief; twitch

Question 2

If you were recording electrical activity from the nerve you would find that stimulation of the nerve with increasingly large pulses produces _____ (larger/smaller) and _____ (larger/smaller) compound action potentials until an upper limit is reached.

Answer 2

larger; larger

Question 3

What is an upper limit?

Answer 3

This is the point at which the stimulation threshold for all available axons has been exceeded and thus all axons have been recruited.

Question 4

Each axon arising from a motoneuron makes synaptic contact with a number of muscle fibres. The actual number of muscle fibres innervated by a motor neuron varies with the _____ of the muscle

Answer 4

size

Question 5

What is a motor unit?

Answer 5

A single motor neuron and the muscle fibres it innervates

Question 6

For example: When a stimulus is applied to the sciatic nerve, the size of the contraction produced in the gastrocnemius muscle will depend upon how many motor neuron axons, and thus how many ______, have been recruited.

Answer 6

motor units

Question 7

Why does the force of contraction increase as stimulus strength increased?

Answer 7

Increased stimulus –> increase recruitment of motor units (increased tension)

Increase stimulus –> amount of Ca2+ (triggers link) released from sacroplasmic reticulum –> binding of actin and mysoin –> more sacromere overlap –> greater muscle response

Drop- too much overlap (actin and myosin)

Question 8

What does testing the recruitment of a muscle mean?

Answer 8

Test how intensity of stimulus affects muscle

Question 9

What does testing the tetanus of a muscle mean?

Answer 9

Test how frequency of stimulus affects muscle

Question 10

What is a mechanical summation?

Answer 10

If a second stimulus is delivered before the muscle has time to completely relax the second response may add to the first.

When repetitive contraction, tension does not return to base line, rather increased on previous tension

Question 11

What is tetanus and what causes it?

Answer 11

A maintained contraction

Repetitive stimulation at a frequency which prevents any relaxation between stimuli

Question 12

At lower stimulation frequencies the tension is ______ (constant/fluctuates). Above a certain stimulation frequency – ___________ – a smooth tetanic contraction is produced.

Answer 12

fluctuates; the fusion frequency

Question 13

The tension produced during tetanus may be several times as great as that produced in a _____.

Answer 13

twitch

Question 14

What is a fusion frequency?

Answer 14

where can relaxation can’t occur and maintain contraction

Flats line

Question 15

Why is the force of contraction greater in tetanus than in a single twitch?

Answer 15

Builds on previous amount of tension

Motor units have no chance to relax before next stimulus needs to recruited them (no chance to return to base line) 40-50Hz

Question 16

By what two mechanisms can the body increase the force of contraction of a skeletal muscle?

Answer 16

1. Increase frequency

2. Intensity of stimulus

Question 17

The tension generated by skeletal muscle during an isometric contraction depends on the ________ of the muscle

Answer 17

initial length

Question 18

Two forces can be measured: the ___ force, and the _____ force (when the muscle contracts).

Answer 18

passive; active

Question 19

The passive force, measured when the muscle is not contracting, increases _______ as the muscle is stretched - this is due to the _______ nature of the muscle.

Answer 19

exponentially; elastic

Question 20

The active force, however, varies with muscle ______ in a manner which results from the functional organization of skeletal muscle.

Answer 20

length

Question 21

In skeletal muscle, what is the basic contractile unit?

Answer 21

Sarcomere

Question 22

Sarcomeres are composed of overlapping filaments of ____ and _____ and the degree of overlap is a primary determinant of the amount of ______ generated by the muscle.

Answer 22

actin; myosin; active tension

Question 23

How does stretching the muscle, to increase the passive tension, affect the sarcomere length and the overlap between actin and myosin filaments?

Answer 23

Sacromeres are being pulled apart

Question 24

Explain the relationship between passive tension and active tension

Answer 24

As length increase –> passive increase

As length increase –> active increase to optimal and then decrease

Question 25

What is the relationship between actin and myosin and how does that have an effect on muscle contraction :
Optimal zone
Too close
Too far

Answer 25

Optimal zone- actin and myosin are perfect (max. binding) –> able to contract

Too close –> can’t bind to as many myosin –> less pull (no room to oull)

Too far –> not enough binding sites –> less pull