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nerve signal

arrives at a synaptic knob and causes calcium channels to open, which allows the calcium ions to enter the synaptic knob

calcium ions

entry into the synaptic knob triggers exocytosis of synaptic vesicles, which release acetylcholine into the synaptic cleft


diffuses across the synaptic cleft and binds to receptors on the motor end plate, which causes them to change shape

ion channels

open and sodium ions enter the muscle fiber while potassium ions leave the muscle fiber

muscle action potential

spreads through the T-tubules into the sarcoplasm


movement of these across sarcolemma creates muscle action potential to excite muscle fiber

calcium-release channels

in the sarcoplasmic reticulum open and calcium ions diffuse into the sarcoplasm within one millisecond

calcium ions

bind to toponin molecules on thin filaments, causing them to change shape

troponin-topomyosin complex

shifts to a new position, which exposes the myosin-binding sites on the actin filament so cross-bridges can form

during contraction

the thin filaments slide inward past the thick myosin filaments and cause a sarcomere to shorten; neither thin nor thick filaments change their length


attaches to an ATP-binding site on a myosin head and gets split by myosin ATPase, which energizes the myosin head

cross bridge

the result of an energized myosin head attaching to an actin binding site

power stroke

the myosin head swivels toward the center of the sarcomere, which draws the thin filament past the thick filaments and generates this

second ATP molecule

attaches to the ATP-binding site on the myosin head, which allows it to detach from the actin filament

new cross bridge

forms along thin filaments and generates another power stroke


as long as ATP and calcium are present to shorten the sarcomere


when nerve signals no longer arrive at the neuromuscular junction, teh synaptic knob stops releasing acetylcholine

nerve signal

no longer transferred to the motor end plate; calcium channels close


brekas acetylcholine molecules into fragments, which get absorbed into synaptic knob

calcium ions

are actively transported out of sarcoplasm and back into the sarcoplasmic reticulum using ATP energy, where they bind to the protein calsequestrin


of calcium ions from troponin molecules and the troponin-tropomyosin complex moves back into its original position, blocking the myosin-binding sites on the actin strand


calcium ions leak into the sarcoplasm and bind to toponin molecules, which initiates cross-bridge formatino and triggors rigor mortis within three-four hours


since ATP can't be synthesized by dead people, myosin cross bridges can't detach from actin molecules until myofilaments break down

rigor mortis

lasts up to 60 hours following death because muscles cannot relax


of a muscle contraction depends on the length of the sarcomeres within a muscle fiber before the muscle contraction begins

maxium tension

develops when the resting length of the sarcomere is optimal due to degree of overlap between thick and thin filaments


there is little overlap between the thick filaments and the thin filaments and few cross bridges can form, so muscle tension falls to zero


there is too much overlap between teh thick filaments and the thin filaments; the thck filaments crumpe as they get cmopressed against the z disc, so muscle tension falls to zero


the principles that govern the contraction of a single muscle fiber also govern the contraction of a whole muscle

nerve signal

will stimulate an individual muscle fiber to contract with less than maximum force, so the total tension produces depends on the frequency of stimulation of the fiber

nerve signal

will stimulate all muscle fibers in a motor unit to contract in unison

muscle fibers

in motor unit are dispersed and not clustered together, muscle contraction will be weak over a wide are rather than localized

small motor units

may have only 3-6 muscle fibers per motor neurons (fingers; eyes)

small motor units

easy to stimualte so they can control precise muscle movements

large motor units

may contain up to 1,000 muscle fibers per motor neuron (calf)

large motor units

are harder to stimulate so they control muscle movements that provide strength


a biref contraction of all the muscle fibers in a motor unit in response to a single stimulus form a motor neuron

muscle fiber

obeys an "all-or-nothing" law when electrically excited; either contracts to its maximum extend or doesn't contract at all

latent period

is a time interval between the application of a threshold stimulus and the beginnign of the twitch contraction

latent period

calcium ions are released from teh sarcoplasmic reticulum into the sarcoplasm which allows cross bridges to form

latent period

the force generated creates internal tension without shortening muscle

contraction period

is a time interval when peak muscle tension gets produced (can last up to 100 milliseconds)

contraction period

during this time the sliding filament mechanism is occuring

contraction period

the force generated creates external tension used to move a load

relaxation period

is a time interval when muscle tension decreases (can last up to 100 milliseconds)

relaxation period

calcium ions are actively transported out of the sarcoplasm back into the sarcoplasmic reticulum


varies for constant stimulus voltage (electrical excitation)

varies strength of twitch contraction

frequency of stimulation

varies strength of twitch contraction

concentration of calcium ions

varies strength of twitch contraction

length-tension relationship

varies strength of twitch contraction

temperature of the muscle

varies strength of twitch contraction

pH of the sarcoplasm

varies strength of the twitch contraction

state of hydration of the muscle that affects cross bridge formation

individual twitches

are usually insufficient to do any useful work, but stronger twitches can be produced by increasing the voltage of the stimulus


higher voltages excite more nerve fibers in motor nerve causing more motor units to contract (multiple motor unit summation)

muscle twitches

single, jerky contractrions

healthy muscle contraction

relatively smooth and varies in strength according to the demands placed upon it

below 10 stimuli per second

identical twitches will be produced for each stimulus and the msucle will fully recover between twitches

between 10 and 20 stimuli per second

the muscle fully recovers between twitches, each twitch can develop more tension than the preceding twitch because of treppe (staircase)


calcium ions should be more available because sarcoplasmic reticulum doesn't have time to completely reabsorb them


subsequent twitches become stronger


heat released by each twitch causes msucle enxymes to work more efficiently


stronger twitches are produced as muscle "warms up"

oxygen debt

the difference between teh resting rate of oxygen consumption adn the elevated rate of oxygen consumption following exercise; "it must be repaid"

lactic acid

can be reconverted to pyruvic acid or converted back into glucose, which gets stored as glycogen in the liver

phosphagen system

CP and ATP are restored to replenish this


gets returned to the myoglobin

skeletal muscle fibers

vary according to the number of mitochondria they contain

skeletal muscle fibers

vary according to the amount of myoglobin they contain

skeletal muscle fibers

vary according to the blood supply

skeletal muscle fibers

vary according to the rate at which they use ATP

red fibers

have many mitochondria, a high myoglobin content, an extensive capillary network, and are adapted for aerobic respiration

white fibers

have fewer mitochondria, a lower myoglobin content, and fewer capillaries than red fibers; adapted for anaerobic respiration

slow oxidative

type I fibers

slow oxidative

produce a slow twitch and do not fatigue very easily

slow oxidative

contain many mitochondria, large amount of myoglobin, and many blood capillaries (red fibers)

slow oxidative

generate ATP by aerobic respiration and utilize ATP at a relatively slow rate

slow oxidative

usually present in muscles responsible for posture or endurance activities

fast glycolytic

type IIB fibers

fast glycolytic

produce a fast twitch and tend to fatigue easily

fast glycolytic

contain fewer mitochondria, less myoglobin, and relatively fewer blood capillaries (white fibers)

fast glycolytic

contain large amounts of glycogen and produce ATP by anaerobic respiration

fast glycolytic

split ATP at a faster rate to produce contractions that are strong and rapid

fast glycolytic

found in arm muscles used for short duration to produce more force (weight lifting)


type IIA fibers


produce a fast twich and are relatively resistant to fatigue


contain many mitochondria, large amounts of myoglobin, and many blood capillaries


generate ATP by glycolysis and aerobic respiration


utilize ATP at a fast rate


rather uncommon, except in highly trained athletes

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