Lecture 5: Mechanics

motor unit

a single motor neuron + the cells it controls

as myosin pulls on actin during the power stroke, it produces

tension

the muscle twitch

a response to a single electrical stimulation

phases of muscle twitch

latent period, contraction period, relaxation period

latent period

• first few milliseconds

• cross bridges formed but no measurable shortening

contraction period

• reach peak of muscle tension

• muscle may shorten

relaxation period

• initiated by Ca2+ reentry into the SR

• muscle tension decreases to zero

• muscle returns initial length

muscles responses are _______, allowing for fluid and appropriate movement

graded

mechanisms for grading muscle contractions

• changing the frequency of stimulation (temporal summation)

• changing the strength of stimulation (motor unit recruitment)

single muscle twitch

a single stimulus is delivered and the muscle contracts and relaxes

low stimulation frequency

another stimulus is applied before the muscle relaxes completely, resulting in increased tension via summation

temporal summation works because there is

insufficient time to remove all Ca2+

high stimulation frequency

no muscle relaxation between stimuli resulting in complete tetanus

in high stimulation frequency, muscle reaches

max tension

motor unit recruitment

more motor unit activation increases the number of muscle fibers activated, increasing the force of contraction up to the maximal contraction.

muscle tone

relaxed muscles are always slightly contracted

muscle tone generated by involuntary neural stimulation of skeletal muscle driven by

spinal reflexes

isotonic contractions

muscle generates force and changes length

isometric contractions

muscle stays the same length as it generates force

eccentric isotonic contraction

the muscle lengthens as it generates force

concentric isotonic contraction

the muscle shortens as it generates force