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90 terms

Muscle Contraction

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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
acetylcholine
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
ions
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
ATP
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
repeats
as long as ATP and calcium are present to shorten the sarcomere
cessation
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
acetylcholinesterase
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
dissociation
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
death
calcium ions leak into the sarcoplasm and bind to toponin molecules, which initiates cross-bridge formatino and triggors rigor mortis within three-four hours
death
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
force
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
over-stretched
there is little overlap between the thick filaments and the thin filaments and few cross bridges can form, so muscle tension falls to zero
shortened
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
contraction
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
twitch
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
strength
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
recruitment
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)
treppe
calcium ions should be more available because sarcoplasmic reticulum doesn't have time to completely reabsorb them
treppe
subsequent twitches become stronger
treppe
heat released by each twitch causes msucle enxymes to work more efficiently
treppe
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
oxygen
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)
intermediate
type IIA fibers
intermediate
produce a fast twich and are relatively resistant to fatigue
intermediate
contain many mitochondria, large amounts of myoglobin, and many blood capillaries
intermediate
generate ATP by glycolysis and aerobic respiration
intermediate
utilize ATP at a fast rate
intermediate
rather uncommon, except in highly trained athletes