Muscle and Muscle Tissue Flashcards
Muscle Tissue
- nearly half of body’s mass
- capable of transforming chemical energy (ATP) into directed mechanical energy
- mechanical energy is capable of exerting force
3 types of muscle tissue
- skeletal
- smooth
- cardiac
*both skeletal and smooth muscle cells are elongated and referred to as “muscle fibers”
skeletal muscle tissue - voluntary muscle
- organs attached to bones and skin
- longest and have striations
- contract rapidly and powerfully but tire easily
- remarkably adaptable
- subject to conscious control and voluntary movement
cardiac muscle tissue
- only in heart; makes up bulk of heart walls
- striated
- involuntarily controlled; contracts @ steady rate due to heart’s own pacemaker
- nervous system can alter heart rate, but it is not consciously controlled
Four characteristics of all muscle tissue
- excitability (responsiveness)
- contractibility
- extensibility
- elasticity
excitability (responsiveness)
ability to receive/respond to stimuli by changing its membrane potential
contractibility
ability to forcibly shorten when stimulated
extensibility
ability to stretch or extend - even beyond resting length
elasticity
ability to recoil to resting length
four functions of muscle tissue
- produce movement
- locomotion & manipulation
- contraction of the heart
- blood vessel dilation/constriction
- movement of all fluids/substances
through tracts
- maintain posture and body position
- stabilize joints
- generate heat
anatomy and features of a skeletal muscle
each skeletal muscle is an organ made of different tissues
features
- nerve and blood supply
- connective tissue sheaths
- attachments
nerve and blood supply
- each muscle receives a nerve, arteries, and veins
- consciously controlled skeletal muscle have nerves supplying every muscle fiber
- contracting muscle requires huge amounts of oxygen and nutrients + quick removal of waste
- cellular respiration: C6H12O6 + 6O2 –> 6CO2 + 6H2O + ATP
connective tissue sheaths
every skeletal muscle and muscle fiber is covered in connective tissue
- support muscle cells and reinforce whole muscle
- become the tendons that join muscles to bones
3 parts:
- epimysium
- perimysium
- endomysium
epimysium (connective tissue sheath)
most external; dense irregular connective tissue surrounding the entire muscle; may blend with fascia
perimysium (connective tissue sheath)
fibrous connective tissue surrounding fascicles (groups of muscle fibers)
endomysium
most internal; fine areolar connective tissue surrounding each muscle fiber
attachments
muscles span joints and attach to bones
- muscles attach in at least to places
- origin: immoveable/less moveable bone
- insertion: moveable bone
- attachments can be direct or indirect
- direct (fleshy): epimysium fused to periosteum or perichondrium
- indirect: connective tissue wrappings extend beyond a muscle as ropelike tendon or sheetlike aponeurosis (more common)
muscle fiber microanatomy
- long cylindrical cells w multiple nuclei
- sarcolemma & sarcoplasm
- specialized organelles:
- myofibrils
- sarcoplasmic reticulum
- t-tubules
sarcolemma
plasma membrane of a muscle fiber
sarcoplasm
cytoplasm of a muscle fiber
- contains many glycosomes - granules of stored glycogen - and myoglobin - a red pigment that stores oxygen
myofibrils
densely packed, rodlike elements
- single muscle fiber can contain 1000s
- accounts for 80% of muscle cell volume
- made of chains of sarcomeres
parts of myofibrils
- striations: stripes formed from repeating series of dark and light bans along the length of each myofibril
- a bands: dark regions
- h zone: lighter region in the middle of the dark a band
- m line: dark line of protein (myomesin) that bisects the H zone vertically
- i bands: lighter regions
- z disc/line: sheet of proteins on midline of light i band
sarcomere
muscle segment
- smallest contractile unit of a muscle fiber
- contains a band with half of an i band at each end - the area between z discs
- individual sarcomeres align end-end along myofibril
myofilaments
- contain actin and myosin
- arranged in an orderly pattern within a sarcomere
- actin myofilaments = thin filaments
- myosin myofilamets = thick filaments
actin myofilaments
- thin filaments
- lateral, extend across i band - partway into a band
- anchored by z discs
myosin myofilaments
- thick filaments
- central, extend length of a band
- connected at m line
thick filaments
composed of the protein myosin, each myosin molecule contains 2 heavy and 4 light polypeptide chains
- heavy chains intertwine to form myosin heads
- light chains form globular myosin heads
during contraction, myosin heads link thick+thin filaments to form cross bridges
- myosins are often offset from each other - staggered array of heads along thick filament
regulatory proteins
tropomyosin and troponin
- bound to actin that control muscle contraction
tropomyosin
rod-shaped protein, spiral about the actin core and block myosin-binding sites
troponin
gloular protein, able to bind to 1-actin, 2-tropomyosin, 3-calcium
other proteins in myofibril
- elastic filament: composed of protein titin; holds thick filaments in place; helps to resist excessive stretch and assists with recoil
- dystrophin: structural protein that links the thin filaments to the integral proteins of the sarcolemma
- nebulin, myomesin, c proteins: bind filaments or sarcomeres together; maintain alignment of the sarcomere
duchenne muscular dystrophy
caused by defective gene for dystrophin - a protein that links thin filaments to extracellular matrix and stabilizes sarcolemma
- loss of muscle mass over time from apoptosis of muscle cells (falling and weakness)
sarcoplasmic reticulum
network of smooth endoplasmic reticulum tubules surrounding each myofibril
- stores and releases calcium on demand
- functions in regulation of intracellular calcium levels
- most tubules run longitudinally
- terminal cisterns: SR tubules that form perpendicular cross channels at the a-i band junction; always occur in pairs
t (transverse) tubules
- tube formed by protrusion of the sarcolemma deep into the cell’s interior - pass from 1 myofibril to another
- occur at a-i band junction - between terminal cisterns
- t tubule lumen is continuous with extracellular space, greatly increases the muscle fiber’s surface area
- allow electrical nerve transmissions to reach deep into the interior of each muscle fiber and trigger the release of calcium
- triad: area formed from the terminal cistern of one sarcomere, a t tubule, and the terminal cistern of the neighboring sarcomere
triad relationships
- t tubules are a rapid messaging system - work to ensure every myofibril in the muscle fiber contracts simultaneously
- t tubules and SR are linked together via membrane spanning proteins
- protruding proteins from the t tubules act as voltage sensors - change shape in response to an electrical current
- protruding proteins from the SR form gated channels through which calcium can be released
- when an electrical impulse passes by, t tubule proteins change shape, and calcium is released into the cytoplasm
contraction
the activation of myosin’s cross bridges to generate force
sliding filament model of muscle contraction part 1
muscle fiber shortening only occurs if the cross bridges generate enough tension on the thin filaments to exceed the forces that oppose shortening
- ex. lifting a bowling ball
- contraction ends when cross bridges become inactive, tension decline, and the muscle fiber relaxes
sliding filament model of muscle contraction part 2
- in relaxed state, thin + thick filaments overlap only slightly at the ends of the a band
- during contraction, thin filaments slide past thick filaments - actin and myosin overlap more
- actin + myosin don’t change their length
- when nervous system stimulates a muscle fiber, myosin heads are allowed to bind to actin, cross bridges are formed, and the sliding process begins
sliding filament model of muscle contraction part 3
cross bridge attachments form and break several times, each time pulling the thin filaments a little closer towards the center of the sarcomere
- ratcheting action shortens the muscle fiber
- z discs are pulled toward the m line
- i bands shorten
- h zones disappear
- a bands move closer together
