Unit 4 - Muscle Physiology Flashcards
Three types of muscles
skeletal
cardiac
smooth
skeletal
voluntary/striated
cardiac
heart/involuntary/striated
smooth
involuntary/unstriated
internal organs
controlled muscle contraction allows
purposeful movement
manipulation of external objects
movement of contents through hollow internal organs
removal of wastes
Skeletal muscle
muscle fibres lying parallel to one another
single skeletal muscle cells
held together by dense, connective tissue
multinucleated
large, elongated, and cylindrically shaped
fibres usually extend entire length of muscle
Structure of skeletal muscle
each muscle covered by dense connective tissue 3 anatomic connective tissues 1.epimysium 2.perimysium 3.endomysium
Epimysium
covers whole muscle
Perimysium
divides muscle fibers into bundles(fascicles)
Endomysium
innermost
covers each muscle fiber and cell
Muscle cell terminologies
Sarcoplasmic reticulum: endoplasmic reticulum
sarcolemma: plasma membrane of a muscle cell
sarcoplasm: cytoplasm
Structure of Muscle fibres
myofibrils are contractile elements of a muscle fibre
protein fibers run parallel to cell’s long axis
Myosin
thick filaments
Actin
thin filaments
Dark band
A band - thick
Light band
I band - thin
Sarcomere
functional unit for muscle contraction
along myofibril
between two Z lines (connects thin filaments of 2 adjoining sarcomeres)
A band
thick filaments and portions of thin filaments that overlap on both ends of thick filaments
H zone
lighter area within middle of A band where thin filaments don’t reach
I band
remaining portion of thin filaments that don’t project into A band
M line
extends vertically down middle of A band within center of H zone to stabilize thick filament
Thick filaments
composed of protein myosin Golf club lie parallel tail ends point toward centre globular heads project out at one end form cross bridges with 2 binding sites ATPase and Actin
Thin filaments
composed of protein actin (two strands twisted together)
two other proteins, tropomyosin and troponin
Tropomyosin
lies along groove of actin spiral
cover binding sites -> actin can’t bind to myosin -> no contraction
Troponin
3 polypeptides keep tropomyosin in blocking position
1 binds to tropomyosin
1 binds to actin
1 binds with Ca++
Muscle proteins
myosin and actin
contractile proteins but don’t actually contract
not unique to skeletal muscle
each myosin molecule surrounded by 6 actin molecules
Titin
giant, highly elastic protein
largest protein in body
extends in both directions from M line along length of thick filament to Z lines at opposite ends of sarcomere
Two important roles of Titin
with M line proteins helps stabilize position of thick filaments in relation to thin filaments
augments muscle’s elasticity by acting like a spring
Sarcoplasmic Reticulum
modified ER in muscle cell
interconnected compartments that surround myofibril
wrapped around each A and I band
Tranverse T tubules
extension of sarcolemma to spread action potential and release Ca2+
run perpendiculary from sarcolemma into central portions of muscle fibre
Sepment ends form sacs - lateral sacs store calcium
Motor Unit
one motor neuron and the muscle fibres it innervates
1 motor neuron innervates a varied number of muscle fibres, but each muscle fibre supplied by only 1 motor neuron
muscles that produce precise, delicate movements contain fewer fibres per motor unit
muscles performing powerful, coarsely controlled movement have larger number of fibres per motor unit
Neuromuscular junction
space between motor unit axon terminal and muscle cell
Ach diffused across synaptic cleft
ACh binds with specific receptors sites on sarcolemma -> action potential
AP speads down muscle fibre into T-tubule
Ca+ released from lateral sacs to sarcoplasm
Sliding filament mechanism
acetylcholine released at neuromuscular junction
sarcoplasmic reticulum releases Ca2+ into sarcoplasm
Ca2+ -> binds to troponin on actin filaments -> tropomyosin and troponin move aside to uncover binding site on actin -> myosin heads bind to actin
Myosin and actin cross bridge binding site moves inward -> actin ‘rowed’ inward -> muscle shortens (contracts)
Actin molecules in thin myofilament
BINDING
POWER STROKE
DETACHMENT ATP
BINDING
Binding 1
Myosin cross bridge binds to actin molecule
Power stroke
cross bridge bends, pulling thin myofilament inward
Detachment ATP
attaches to ATPase sire; cross bridge detaches at end of poer stroke and returns to original conformation
Binding 2
cross bridge binds to more distal actin molecule; cycle repeated
Cross Bridge Cycle
myosin heads swivel toward centre of sarcomere (power stroke) pulling actin inward
splitting of ATP gives energy for power stroke of cross bridge
binding of new ATP to myosin site lets bridge detach from actin filament at end of power stroke so cycle can be repeated
Power stroke
cross bridges don’t power stroke in unison (asynchronous cycling -> prevent thin filaments from slipping back)
with cross bridge, thin filaments slide inward over stationary thick filaments toward centre of sarcomere
power stroke pulls thin filament inward
ATP during contractions
ATP and Ca2+ (both available -> sliding mechanism and contraction, one or neither available -> sliding mechanism and contraction steps)
increase in Ca2+ starts filament sliding
decrease in CA2+ turns off sliding process
Relaxation
Acetylcholinesterase breaks down ACh at neuromuscular junction
Muscle fibre action potential stops (when local action potential is no longer present Ca2+ moves back into sarcoplasmic reticulum
Troponin and tropomyosin cover actin binding sites
myosin can’t bind -> relaxation
Rigor and Muscle Tone
between power stroke and detachment, myosin and actin tightly bound together
If no ATP produced, cross bridge cycle gets stuck at this step -> “rigor mortis”
Muscle tone
some of the motor units contract continuously, producing a resting tension in a skeletal muscle
Tension
produced internally within sarcomeres during contraction
tension must be transmitted to bone via connective tissue and tendons during contraction before bone can move against load
Two types of contraction
isotonic
isometric
Isotonic
muscle tension remains constant as muscle changes length
isometric
muscle is prevented from shortening
tension develops at constant muscle length
cardiac muscle
heart interconnected by gap junctions ANS CN X Hormones Local factors more sarcoplasm and mitochondria larger T-tubules but less developed SR
Cardiac muscle
initiates own AP potential without neuron stimulation ( AP travels throughout the entire cell network)
Limited intracellular Ca++ to sarcoplasm produces a contraction that last 10-15 times longer than in skeletal muscle
Smooth Muscle
hollow organs and tubes no striations (thick and thin filaments don't form myofibrils, lack sarcomeres) muscle fiber contracts and twists into a helix as it shortens (relaxes by untwisting)
Smooth muscle
contraction/relaxation controlled by
ANS
CN X
Hormones
Local factors
longer to initiate and terminate contraction
Ca2+ dissociation required enzyme phosphatase
Muilti-Unit smooth muscle
many separate muscle units function independently and are separately stimulated to contract
Neurogenic
contraction only in response to stimulation of ANS nerves supplying the muscle
Multi-Unit smooth muscle found in
in walls of large blood vessels in large airways in muscle of eye that adjusts lens for near or far vision in iris of eye at base of hair follicles
