Muscle Physiology Flashcards
What is the only source of energy for contractile activities?
ATP
characteristics of muscles tissue
excitable: can be stimulated; able to receive stimulus and respond
contractible: able to contract and become shorter
extensible: able to lengthen
elastic: able to go back to og shape/length after being deformed
muscle functions
produce movement
maintain posture/position
stabilize joints: more active than tendons and ligaments
generate heat: when muscles contract it creates heat (glycolysis)
skeletal muscle anatomy
connective tissue sheath: epimysium, endonysium
attachment
motor fibers innervate muscle fibers
blood supplies O2 and nutrients and removes waste products
sarcolemma
around the whole fiber
muscle fiber plasma membrane
sarcoplasm
muscle fiber cytoplasm
structure for metabolism
contains:
- glycosome: storage for glycogen (energy source for muscles contraction)
- myoglobin: carry O2 in muscle
- ribosome: synthesizes protein (RNA, DNA)
- mitochondria: creates ATP which produces energy
myofibril
densely packed, rod-like elements
a single muscle fiber can contain 1000s
has myofilaments, striations, and sarcomeres
myosin
thick filaments (16 nm diameter)
motor protein in all types of muscle
globular head and myosin tail
converts chem energy (ATP) to mechanical energy (contraction)
has actin binding sites
actin
thin filament (8 nm diameter)
double helix formation
has indentation for myosin head
tropomyosin: string
troponin: at regular intervals binds to tropomyosin and actin
myofibril striations
repeating series of dark and light bands along length of myofibril
A band, H zone, M line, I band, and Z disc
myosin and actin in hexagonal arrangement with myosin filaments surrounded by 6 actin filaments
A band
dark region-dark bc of the density of myosin and actin filaments
length of thick filament (myosin)
myosin and actin
has H zone and M line
H zone
lighter area in A band w/only myosin and no thin filaments (actin)
contains M line
M line
connects myosin vertically within the H zone in the A band
I band
light region
thin filaments (actin) only
actin extends across the I band and partway into the A band
has the Z disc
Z disc
anchors actin together within the I band
sarcomere
smallest contractile unit
area b/w Z discs
align along myofibril
A band and 1/2 I band on each side
shortened muscle=less sarcomeres in series (less=can’t lengthen)
2 sarcomeres connect at the Z disc
titan filament
sarcomeres and muscles length
less sarcomeres=shorter
add sarcomeres=lengthen muscle
titan filament
acts as spring tethering myosin to Z disc
contributes to stability of myosin
doesn’t contribute to contraction of muscles but is important to MUSCLE TONE
contractile proteins
myosin and actin
regulatory proteins
tropomyosin and troponin
tropomyosin
covers binding sites, blocks the cross bridge
troponin
holds tropomyosin in place
high affinity for calcium
sarcoplasmic reticulum (SR)
web like structure
fluid filled intracellular organelle
spans the sarcomere and wraps up the contractile myofilaments
stores and releases Ca2+ which binds to troponin, moving tropomyosin off myosin binding sites so actin is ready to bind to it
has terminal cistern (cisterna)-enlarged space where the sarcomere ends.
T tubules
vertical; overlies Z disc
continuation of sarcolemma
allows AP to reach deep into each muscle fiber
b/w terminal cistern
triad
2 terminal cistern and 1 T-tubule
transmission of AP along T-tubules cause Ca2+ release from the SR (terminal cistern)
Why is Ca2+ important?
troponin has calcium binding sites and high affinity for calcium
when Ca2+ binds to troponin, it moves the tropomyosin and uncovers myosin binding sites on actin so it’s ready to bind
sliding filament theory
relaxation and contraction is caused by actin and myosin sliding past each other
relaxation: filaments only overlap slightly
- H zone wide in middle of A band
- lighter band is wider
contraction: thin filaments slide past thick filaments
- myosin fits into spaces made by actin
- in fully contracted muscle, everything comes together and fully overlap
- no/very little I band bc actin and myosin overlap more
- light region gets much shorter, dark region remains pretty unchanged
- sarcomere shortens
- progressive pulling of actin towards myosin
what triggers muscle contractions?
AP in somatic motor neurons
AP arrives at axon terminal
voltage-gated calcium channels open
release ACh into synaptic cleft
- acetylcholinesterase degrades ACh later
ACH binds to receptors on sarcolemma that surrounds the fiber
opens voltage-gated ion channels
end plate potential (EPP) (local depolarization/excitation) triggers an AP
AP propogates along sarcolemma and T-tubules
voltage sensitive proteins in tubules stimulate Ca2+ release from SR
Ca2+ binds to troponin, moves tropomyosin off binding sites so myosin can bind to actin
activation of cross-bridges
cross bridge formation: high energy myosin head with ADP and P attaches to actin
power (working) stroke: myosin head pulls actin towards in at the M line
- ADP and P detach from myosin
cross bridge detachment: ATP attaches to myosin head, causing bridge to detach
cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head into high energy state
- hydrolysis of ATP-ADP and P that’s ready to attach to myosin head and allow it to attach to actin and start cycle again
role of ATP in muscles
both contraction and relaxation
what does no ATP do to muscles
rigid muscles (rigor mortis)
relaxation phase
APs of motor neurons stop
Ca2+ pumped back into SR
- requires ATP (active transport)
- prevents myosin cross bridge formation
muscle innervation
each muscle innervated by one motor nerve (bundle of motor neurons axons)
motor unit
one motor neuron and all muscle fibers it supplies
fibers innervated are dispersed throughout muscle, so an AP would only contract those fibers (very weak contraction)
motor unit innervation ratio
muscle fibers per motor neuron
low motor unit innervation ratio
20:1 for example
fine motor control
finger muscles, extraocular muscles, face muscles, larynx
need great amount of precision, control, and stabilization
high motor unit innervation ratio
2000: 1 for example
gross motor controls
quads, hamstrings, thigh muscles, gastrocs, etc.
muscle twitch
brief activation of a single motor unit w/a single action potential
just 1 AP lasts only a couple hundred milliseconds
VERY small force generated
summation of twitches makes things functional
latency period: (2 ms)
contraction period (10-100 ms) -max tension
relaxation period: (10-100 ms)
contracts faster than relaxes
frequency (temporal) summation
muscle response to increases in stimulus frequency
increased frequency (decreased interval b/w twitches)=twitches summate
b4 motor unit completely relaxes, give another stimulus
high frequency=high force
unfused tetanus
high tension w/ wave-like pattern (quiver) on top
20-40 Hz
fused tetanus
over 20-40 Hz
max tension that can be generated from that motor unit
no quiver, one smooth contractions
multiple motor unit summation
muscle response to increased stimulus strength
increased strength=increased motor units recruited
subthreshold stimulus: below 5 volts; not going to recruit
threshold stimulus: 5 volts; start to recruit
max stimulus: no more motor units to recruit
high stimulus=high recruitment= high force
what determines which motor units are recruited 1st vs later?
size principle
size principle
smaller motor units recruited 1st, larger motor units recruited later w/stronger stimulus
smallest, highly excitable (lower threshold) motor neurons innervate the smaller muscle fibers with small amount of force generated
larger, less excitable (higher threshold) motor neurons innervate larger muscle fibers with larger amount of force generated
benefit of recruiting motor units asynchronously
helps reduce muscle fatigue
isometric muscle contraction
tension at cross bridge is equal to resistance
constant muscle length
exercise when holding a a position (holding a dumbbell for a few minutes)
isotonic muscle contraction
muscle tension remains constant
concentric and eccentric contractions
concentric muscle contraction
tension at cross bridges is enough to overcome resistance
muscle shortens: biceps shorten with bicep curls
eccentric muscle contraction
tension at cross bridges is less than resistance
muscle lengthens: slowly lower down the dumbbell, the biceps contracts eccentrically
