the motor unit and graded muscle contractions

week 4 unit 2

what is the motor unit

• the motor neuron and all the muscle fibers it innervates form the motor unit

action potential in neuron

in an ap is generated in one of the (purple) neurons, then an ap will be generated in all the purple skeletal muscle cells it innervates. same rule for the rest of the motor units

one ap in the motor neuron results in 

one ap in all the muscle fibers it innervates

muscle twitch

• a contraction in response to one ap on the motor unit 

• muscle twitch can be as short as 10ms or as long as 100ms

what are the periods/phases of a single muscle twitch

latent period, contraction period, and relaxation period

latent period ( muscle twitch period)

• a short 1-2 msec delay from when the ap was generated in muscle cell

• to when tension can be measured in the muscle

• delay is due to the time it takes for calcium to be released from sr into cytoplasm

• to reach and bind to troponin c, causing tropomyosin to shift and expose the myosin binding site on actin and formation of cross bridges

motor unit ordering 

how our muscle move so smoothly even tho twitching is a period of contraction. the interpersed arrangement skeletal muscle cells ensures that the muscle contracts smoothly even though not all motor units contract at the same time

motor unit firing

• if all motor units would contract at the same time, we would produce a jerky movement

• however, the motor units fire asychronously (not firing all at the same time)

• the relaxation period of one may overlap with the contraction period of another motor unit, therfore ensuring smooth movement

why is the relaxation period of the muscle twitch longer than the contraction period

• relaxation period is dependant on calcium being pumped back into sr by calcium atpase at the end of contraction

• rather than it does to be released from the sr, binding to troponin c, expose myosin binding site

• during contraction period, calcium goes down its concentration gradient as its released from the sr.

• during relaxation period, calcium is going against its concentration gradient

graded muscle contraction

• the increase in muscle contraction force thru motor unit recruitment  and/or summation of twitches

motor unit recruitment

• as more motor units are recruited due to increase load or demand on the muscle to generate force,

• more muscle fibers contract, leading to a higher contractile force

summation

• as ap freq is increased, each muscle twitch cannot reach full relaxation before the next twitch occurs

• as consequence, twitched start to stack up on one another, generating more force

• force generated by the twitches add up on top of previous generated force, producing muscle summation

treppe

• increase in ap freq increases the force of contraction in a stair like fashion referred to as treppe

unfused tetanus

when the freq of the ap still allows for partial relaxation between twitches, the tension in the muscle plateaus 

complete tetanus

when ap freq is so high that there is no relaxation between muscle twitches, the muscle is in complete tetanus. all the twitches summate to produce a smooth sustained contraction

muscle twitches

each twitch occurs following an ap (black arrow). all twitches come to relaxation before next twitch starts. tension of power generated by each twitch is same

summed twitches

• first twitch completely relaxes before the next twitch, following an ap.

• second twitch completes contraction, but does not reach full relaxation before the third twitch begins

• amplitude of the third twitch is higher than the previous ones as tension is increased due to summation

• increase in ap freq= summation