contraction and excitation of skeletal m. Flashcards
what are the 2 types of filament involved in the contraction of skeletal m.
actin and myosin
what do skeletal muscles surround and whats are they composed of
muscles that surround the skeleton (bones)
composed of numerous m. fibers(or m. cells)
what are m. fibers, what are they innervated by and what do they do
excitable cells that generate and propagate action potentials
are innervated by nerve fibers (motor neurons) to cause m. contraction
one n. fiber usually innervates ____ = ____
multiple m. fibers
=motor unit
which disease is characterized by the degeneration of alpha motorneurons in the spinal cord and brain stem
amyotrophic lateral sclerosis
each ____ contains several hundred to several thousand ____
m. fiber (cell), myofibrils
what is the mem of the muscle fiber that surrounds myofibrils
sarcolemma
each myofibril is composed of __ ___ elements called ____ (proteins)
2 contractile, myofilaments
what are the two myofilaments
thick filament: myosin
thin filament: actin
what is the biggest to the smallest unit in a m.
m > m. fiber > myofibril > actin/myosin
where does the contraction take place
in the sarcomeres
what are the smallest contractile unit of m. fibers (2 micrometers during contraction)
sarcomeres
what is aligned end to end in myofibrils and are separated by Z discs
sarcomeres
what causes the actin filaments to slide inwards among the myosin filaments
forces generated by the interaction btw the 2 filaments= power stroke
what happens to the actin filaments during m. contraction
the actin filaments are pulled by the myosin filaments inward toward the center of the sarcomere
is there any E required for the sliding filament mechanism
ATP which is converted into ADP+ Pi
are the actin filaments overlapping in the contracted or relaxed state
contracted, they don’t touch in the relaxed state
what are myosin composed of
body and cross-bridges(head and arm)
what are the actin filaments composed of
actin, tropomyosin and troponin
what is tropomyosin
a mol that hides the active sites (during relaxation)
what is troponin
a mol that moves tropomyosin from the active sites
what is the interaction btw actin and myosin filaments in a relaxed state
active sites on the actin filaments are covered by tropomyosin
what is the interaction btw actin and myosin filaments during m. contraction
-Ca2+ is released and Ca2+ ions bind on troponin
-troponin undergoes a conformational change, moving away the tropomyosin and uncovering the active sites
-myosin cross-bridges (heads) are attracted to the active sites of the actin
describe the first step of cross-bridge cycling
describe the second step of cross-bridge cycling
describe the third step of cross-bridge cycling
describe the fourth step of cross-bridge cycling
describe cross bridge cycling
1-ATP binds to myosin making it detach from actin
2-ATP turns into ADP, releasing E to convert myosin heads into cocked state
3-Calcium binds to troponin, which exposes sites on actin that myosin can grab
4-myosin then completes a power stroke, pushing on the actin
what is a muscle twitch (contraction)
is the tension developed in response to one n. stimulation
what is summation
means the adding together of individual m. twitches to increase the intensity of overall m. contraction
what are the 2 ways to reach summation (can also be less)
multiple fiber summation: increase the number of motor units contracting at the same time (recruitement)
frequency summation: increasing the frequency of stimulation of 1 motor unit
what is the size principle
depending on the intensity of the stimulation, motor units are recruited in an orderly fashion according to their size
which motor units are recruited first and for what kind of stimulation
smallest motor units are recruited first for weak stimulation
largest motor units are recruited last for strong stimulation
what allows the size principle for m. force
it allows the gradation of m. force from small steps (weak contraction) to great steps (Strong contraction)
what happens to the individual twitches when the frequency of signals increase
they summate bc of no recovery from previous contractions
what happens to the individual twitches when the frequency reaches a critical level
the summated twitches fuse together to form one continuous contraction (tetanization)
what happens during tetanization (to contraction and to Ca2+ ions)
contraction reaches a max level, no response to further stimulations
Ca2+ ions are maintained in the sarcoplasm, the contractile state of the muscle is sustained (myosin and actin stay attached)
what is an example of a sustained contractile state of the m.
crouching or holding up a heavy box (can lead to cramps, spasms)
m. fibers are innervated by ____ to cause ___
n. fibers (motor neurons)
m. contraction
one ___ ___ innervates multiple ___ = ____
n. fiber innervates multiple m. fibers=motor unit
describe small m. (what do they require, e.g, innervation ratio, n. innervates ___ m. fibers)
require precision
hand and eye
low innervation ratio
1n. innervates 2-3 m. fibers
describe large m. (what do they require, e.g, innervation ratio, n. innervates ___ m. fibers)
doesn’t require precision
gastrocnemius
high innervation ratio
1n. innervates 1000-2000 m. fibers
describe slow fibers
-type 1, red m.
-smaller than fast fibers
-innervated by smaller n. fibers
-have a more extensive blood vessel system and more capillaries to supply extra amounts of O2
-have more mitochondria to support high levels of oxidative metabolism
describe fast fibers
-type 2, white m.
-larger for greater strength of contraction
-an extensive sarcoplasmic reticulum is present for rapid release of Ca2+ to initiate contraction
-large amounts of glycolytic enzymes are present for rapid release of energy
-less extensive blood supply and fewer mitochondria vs slow fibers
what is the contraction speed for type 1 (slow oxidative), type 2A (fast oxidative glycolytic) and type 2B (fast oxidative glycolytic)
type1: slow (100 ms)
type 2A: fast (50ms)
type 2B: fast (25ms)
what is the fatigue rate for type 1 (slow oxidative), type 2A (fast oxidative glycolytic) and type 2B (fast oxidative glycolytic)
type1: slow
type 2A: intermediate
type 2B: fast
what is the diameter for type 1 (slow oxidative), type 2A (fast oxidative glycolytic) and type 2B (fast oxidative glycolytic)
type1: small
type 2A: intermediate
type 2B: large
what is the aerobic capacity for type 1 (slow oxidative), type 2A (fast oxidative glycolytic) and type 2B (fast oxidative glycolytic)
type1: high
type 2A: intermediate
type 2B: low
what is the anaerobic capacity for type 1 (slow oxidative), type 2A (fast oxidative glycolytic) and type 2B (fast oxidative glycolytic)
type1: low
type 2A: intermediate
type 2B: high
what types of fibers are human muscles made of
genetically determined mixture of both slow and fast type fibers
in most muscles used for mvt on average: __ % slow fibers and __%fast fibers
50/50
how would you describe speed and strength of contraction of muscle fibers
which of the two has more slow twitch fibers than fast twitch fibers: sprinters or marathon runners
marathon runners
can exercise training change muscle fiber types
yes but very limited changes (± 10%)
t/f fibers cannot change from type 2A to type 2B
false, they can change within their own types only
what is unknown about fiber types
IT IS UNKNOWN WHETHER:
-muscle types are more malleable early in life
-certain m., like the biceps, are more adept at changing fiber types than others
-muscle fibers can truly change btw type 1 and type 2 and how long it takes to do so
most studies thus far have examined the effects of exercise on muscle fiber types over just ___ to __ ___. In addition, we still don’t know the exact mechanism behing __ ___ _____, though some researchers believe it has to do with the ___ that ___ the m.
5 to 6 months
fiber type conversion
nerve, activates
why do you think soleus and gastrocnemius muscles have a greater % of slow twitch vs fast twitch fibers
to sustain prolonged periods of standing and walking
specify which fibers are used in each:
walking
lifting a sofa
lifting a sofa whilst going up stairs
type 1
type 2A
type 2B
what is another name for neuromuscular junction
motor end plate
what is a neuromuscular junction (contact btw what and what, equivalent to what, where do the n. terminals go, what is the released neurotransmitter)
-contact btw n. terminals (branching from motor neuron axon) and m. fiber plasma mem (sarcolemma)
-equivalent of chemical synapse in the CNS
-n. terminals invaginates into the surface of the m. fiber
-acetylcholine
what are the steps at the neuromuscular junction
1-AP arrives at presynaptic terminal
2-Local depolarization opens voltage gated Ca2+ channels and Ca2+ ions flow into terminal
3-Ca2+ ions trigger the fusion of synaptic vesicles with the presynaptic mem
3-release of the NT into synaptic cleft (exocytosis)
at the nmj; m. fiber mem has ___ receptors that contain ___ channels (___, ___)
acetylcholine
cation
(Na+, K+)
ACh mol attach to what in the m. fiber mem
binding sites
what happens when the cation channels open
-positive ions (Na+, K+, Ca2+) flow thru channels
-but far more Na+ ions flow inside
how many Na+ ions in 1 ms can 1 Na+ channel transmit
15 000 to 30 000 Na+ ions
the entry of the Na+ ions creates what
an end plate potential (equivalent to EPSP)
what does the endplate pot initiate
AP
where are we here
n. terminals and myofibrils
what is sarcoplasmic reticulum
-surrounds myofibrils as a net
-contains Ca2+ (necessary for m. contraction process)
-contains voltage-sensitive Ca2+ receptors (diffusion of Ca2+ into the sarcoplasm)
-contains a Ca2+ pump to remove the Ca2+ from the sarcoplasm
what are T-tubules (transverse tubules)
-begin at cell mem (sarcolemma) and penetrate all the way from one side of the mem to the opposite side
-run transverse to myofibrils
-are in contact with the sarcoplasmic reticulum
-for easy propagation of APs inside the m. fiber (rich w Na+/K+ channels)
what is the first step of the neuromuscular transmission and excitation-contraction coupling
AP arrives at the axon terminal and causes the release of ACh on the synaptic cleft. ACh binds to receptors on the sarcolemma and activates cation (Na+) channels
what is the second step of the neuromuscular transmission and excitation-contraction coupling
Net entry of Na+ initiates an AP which is propagated along the sarcolemma and down the T tubules
what is the third step of the neuromuscular transmission and excitation-contraction coupling
AP in T tubule activates voltage-sensitive receptors, which in turn trigger Ca2+ release from sarcoplasmic reticulum into sarcoplasm
what is the fourth step of the neuromuscular transmission and excitation-contraction coupling
Ca2+ ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed
what is the fifth step of the neuromuscular transmission and excitation-contraction coupling
cross-bridge cycling: myosin heads alternately attach to actin and detach pulling the actin filaments towards the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process
what is the sixth step of the neuromuscular transmission and excitation-contraction coupling
removal of Ca2+ by a Ca2+ pump (active transport, ATP) from sarcoplasmic into the SR after the AP ends
what is the seventh step of the neuromuscular transmission and excitation-contraction coupling
without Ca2+, tropomyosin blockage restored (blocking myosin heads to attach to the active sites of actin); contraction ends and m. fiber relaxes
what is myasthenia gravis
neuromuscular disease caused by an auto-immune response: antibodies block or destroy ACh receptors
what are the symptoms of MG
m. weakness, that develops progressively across the day (worse towards the end of the day)
affects facial m. (eyelid drooping), limb m., respiratory m. ,etc.
what is the tx for MG
immunosuppressant tx
symptomatic tx: acetylcholinesterase (enzyme that catalyzes the breakdown of ACh) inhibitors
what is the cause for rigor mortis and when does it occur
body rigidity due to substained m. contraction after death
occurs within the first day (3-12 hours), and decreases gradually (depending on the ambient temperature; cold slows
the process down)
what mechanisms cause rigor mortis
Lack of ATP,
Ca2+ pump cannot transport Ca2+ back into the sarcoplasmic reticulum, thus Ca2+ keeps the actin active sites continuously exposed
myosin head cannot detach from actin (needs ATP)
1- myofibrils
2- Z disk
3- sarcolemma
4- terminal cisternae
5- sarcoplasmic reticulum
6- transverse tubules
describe each letter
point D: actin filament has pulled all the way out to the end of myosin filaments, no tension
point C: sarcomere shortens and actin filament begins to overlap myosin, Tension increases until sarcomere reaches 2.2 micrometers
point B: sarcomere maintains full tension
point A: the end of actin filaments overlap each other. As the sarcomere length decreases from 2.0 micrometers down to 1.65 micrometers, the strength of contraction decreases rapidly
what is active tension
interaction btw myosin heads and actin filaments determines the tension; when m. is stimulated to contract
due to overlapping of actin and myosin heads
involves E (ATP)
what is passive tension
when m. fibers are pulled apart (no contraction)
stretching
tension/force generated in absence of E (no ATP) when m. fibers are pulled apart (e.g stretching fingers after stroke, MS, arthritis)
what is total tension
summation of passive and active tension
what are each of the lines
red is active tension
blue is passive tension
purple is total tension
what comprises the passive structures that elongate elastically
titin (inside m. cell)
collagen (surrounding m. cell)
what is titin
protein believed to be the greatest contributor to passive force throughout normal ranges of motion
connects the Z disk to the M line in the sarcomere
limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle
examples of increase in active and passive stiffness of hand m.
hand contracture after immobilization due to forearm fracture
hand contracture due to spasticity (neurological condition)
what is the resting length
the length at rest from which a muscle develops MAXIMUM isometric tension
m. tension is dependent on what
muscle length; the more flexible a person, the longer a m. can be stretched
when a m. becomes more flexible (as a result of stretching), what happens to the passive tension curve
it moves to the right of the graph
maximum strength of contraction of a muscle operating at a normal length averages ___
3.5 kg/cm2 (sq. cm) of m.
since a quad m. can have up to 103 cm2 of m. belly, as much as ___ can be applied to the patellar tendon
360.5 kg
describe ocular movement
ocular mvt must be extremely rapid to maintain fixation of the eyes on specific objects to provide accuracy of vision
describer gastrocnemius movement
gastroc must contract moderately rapidly to provide enough velocity of limb mvt for running and jumping
describe soleus mvt
soleus is concerned maily w slow contraction for continual, long term support of the body against gravity
what is m. hypertrophy
increase in total m. mass
what causes m. hypertrophy
-results from an increase in number of actin and myosin filaments in each m. fiber, causing enlargement of the individual m. fibers
-caused when the m. is loaded during the contractile process
what is required to cause significant hypertrophy
only a few strong contractions each day for 6 to 10 weeks
what are the training effect of m. hypertrophy
-nb of actin and myosin filaments in myofibrils can increase by as much as 50%
-myofibrils can split to form new myofibrils and increase fiber and muscle size
