Muscle Physiology

Neuromuscular Junction

outer layer - sacrolema

inside - actin, myosin

The Twitch

response within the muscle in one AP

Period of The Twitch

Latent period

Contraction phase

Relaxation phase

Latent Period

delay between the recipe of a stimulus by a sensory nerve and the response to it

Contraction Phase

cross-bridges form between actin and myosin, causing sarcomere shortening

as long as calcium is in the muscle fibres the contraction will continue

Relaxation Phase

occurring after peak tension, where muscle fiber returns to its resting length

Motor Unit

motor neurons and all the muscle fibres that the motor neuron innervates

What is the relationship between the Stimulus Strength and the Twitch Peak Force

Relationship is direct, but not perfectly linear

• As stimulus strength increases, twitch peak force increase up to max point

• at very low stimulus strengths there may be little or no force

• as the stimulus becomes stronger the force rises quickly then eventually levels off when max force is reached

What is the physiological basis of this relationship in the twitch response and recruitment exercise?

the response changes with stimulus strength because the nervous system is activating more or fewer muscle fibers

• At low stimulus strengths → only a few motor neurons reach threshold and fire so only a small number of muscle fibers contract → low force 

• Stimulus strength increases → more motor neurons reach threshold and more motor units are recruited → more fibers contract simultaneously → greater force 

• At high stimulus strengths, all motor units are already recruited, so forces reach a max plateau and cannot increase further

During data collection, at the lower stimulus strengths, did you have any occurrences where you applied a stimulus, but no response was observed? What is the physiological reason for this?

Threshold stimulus 

• At very low stimulus strengths, the electrical stimulus is below threshold 

• So action potential is generated, no Ca+ is released inside the muscle fibers, not contraction occurs → called a subthreshold stimulus

During data collection, at the higher stimulus strengths, did you have any occurrences where multiple stimuli of varying strength elicited roughly the same response? What is the physiological reason for this?

Maximal recruitment 

• At high stimulus, all motor units are already activated 

• Once every motor unit is firing the muscle is producing maximal force 

• Increasing the stimulus further cannot recruit additional fibers because there are none left to recruit → producing a plateau

According to the Summation Exercise, why does the peak force of the 2nd force response increase as the time between stimulation pulses decreases? Specifically, what happens within the muscle fiber to make the second and third responses bigger than the first response?

• This occurs due to temporal summation 

• The muscle doesn’t get time to fully relax so the contractions stack on top of each other, making the second and third responses larger than the first

• More calcium → more cross-bridges between actin and myosin → stronger contraction