Objective 8: Describe the following types of muscle responses to stimulation: twitch, graded muscle contraction (wave summation, incomplete tetanus, complete tetanus, multiple motor unit summation), treppe, isotonic contraction and isometric contraction

wave (temporal) summation

a second stimulus is applied to muscle before it finishes relaxing. 2nd contraction is greater because muscle has already been stimulated

size principle of recruitment

motor units fire in order from small to large. largest motor units are controlled by largest and least excitable neurons only when powerful contraction is necessary - allows gradual increase in strength through small steps. ex: hand can slap lightly or hit a volleyball hard

isometric contraction

muscle attached to weight that exceeds muscle's peak tension-developing capabilities. when stimulated, increases to peak but does not shorten - muscle does not shorten or lengthen because load is too heavy, but its internal tension increases - L > T - ex: trying to lift a piano

eccentric contraction

muscle lengthens, generates force; isotonic - ex: lowering a weight during bicep curl

concentric contractions

muscle shortens; isotonic - ex: bicep curl pick up, kicking a ball

latent (lag) period

short period of time after stimulation when excitation/coupling is happening. cross bridges begin to cycle but tension is not measurable and myogram shows no response - Time between stimulus application and measurable tension

unfused (incomplete) tetanus

some relaxation between contractions, stimuli delivered more frequently. sustained but quivering

increased contractile force due to recruitment results from

stimulation of a muscle by increasing numbers of activated somatic motor neurons

muscle tone

the state of balanced muscle tension, low levels of contractility, that makes normal posture, coordination, and movement possible

what happens physiologically during latent phase

- AP causes Ach release, binds to receptors, initiates AP in sarcolemma. AP propagated by T tubules, causes release of Ca2+ from SR. Ca2+ binds to troponin, troponin/tropomyosin blockade is removed, cross bridge binding sites on G actin exposed

changes in rate of muscle stimulation

1. wave summation 2. unfused (incomplete) tetanus 3. fused (complete) tetanus

what occurs during relaxation phase of muscle twitch to Ca2+?

Ca2+ is actively transported back into SR

what happens physiologically during relaxation phase

Ca2+ transported into SR and cross bridge attachment is prevented. muscle fibers lengthen

graded muscle contractions

Contractions that vary in strength. Variations in degree of muscle contraction by changing the frequency or strength of the stimulus

myogram

a graph of a muscles mechanical contractile activity (tension or shortening versus time) - latent period, contraction, relaxation

period of contraction

active cross bridges, myogram peaks. cross bridge activity leads to tension; amount of tension depends on number of motor units activated. myogram raises to peak

load (resistance)

any force that opposes the tension (effort) generated by muscle contraction - force exerted on muscle by weight of object

relaxation period

cross bridge activity stops as Calcium is pumped back into SR; muscle tension falls

what happens physiologically during contraction phase

cross bridges between actin and myosin cross bridge heads form, move, release, reform causing sarcomeres to shorten

isotonic contraction

on stimulation, muscle develops enough tension (force) to lift the load (weight). when resistance is overcome, muscle shortens and tension remains constant through contraction - muscle tension stays constant at a given load, muscle shortens. tension remains constant through contraction - tension > load - concentric and eccentric contractions - ex: pick up 5 lbs

fused (complete) tetanus

prolonged, smooth contraction from rapid stimulation. no relaxation, strongest contraction. smooth and sustained - rare IRL

to increase strength (tension) of contraction, _ rate of stimulation, and strength

rate, strength - rate of stimulation: go from twitch contraction to complete tetanus - strength of stimulus: more and larger motor units

muscle twitch contraction

response of muscle to a single, brief, threshold stimulus - 3 phases: latent, contraction, relaxation periods

multiple motor unit summation

strength of contraction depends upon recruitment of motor units - increasing voltage/strength of stimulation to motor units increases recruitment, stronger muscle contractions - subthreshold stimuli: no contractions. first contraction is threshold. then maximal. in laboratory, done by increasing voltage therefore calling more motor units into play

Treppe (staircase effect)

strength of muscle contraction increases with repeated stimulation. first few contractions cause muscle to warm up, temperature rises, blood flow increases, etc.

rapid, strong stimuli lead to _ contractions

strong

faster rate of stimulation, _ the contraction

stronger (single fiber)

stronger stimulus, _ contraction

stronger; more motor units recruited

increase in frequency of stimulation causes; increase in strength causes

temporal/wave summation (higher frequency, greater strength); recruitment (stronger stimulation, more motor units)

tension (effort)

the force exerted by a contracting muscle on a load (object)

Contraction

the process of generating force in a muscle as a result of cross-bridge activity. May or may not lead to shortening

low frequency, weak stimuli lead to _ contractions

weak