Unit 10: Key Terms A and P
Types of muscle tissue
skeletal (voluntary), cardiac (involuntary), smooth (involuntary)
Structure of the skeletal muscle fiber
skeletal muscle tissue consists of many fibers and surrounding endomysium (Extracellular matrix)
Parts of muscle cell
muscle cell (myocyte), sarcolemma, sarcoplasm, myofibril, sarcoplasmic reticulum, mitochondrion, nucleus
Myofibrils
unique structures found in all muscle cells, made up of bundles of specialized proteins: allow for contraction
Cylindrical organelles
100s to 100s in each myocyte, 50-80% of cell volume
Sarcoplasmic reticulum (SR)
surrounds myofibrils: stores and releases calcium ions
Myofilaments
myofibril is made up of these, consist of one or more types of proteins
3 types of myofilaments
thick filaments, thin filaments, elastic filaments
Thick filaments
bundles of contractile protein myosin
Thin filaments
proteins actin, tropomyosin and troponin
Elastic filaments
single massive, spring-like structural protein (titin), stabilizes myofibril structure, resists excessive stretching
Thick filaments cont.
globular heads at each end linked by intertwining tails heads are connected to tails by hinge-like neck, each head has active site that bins with actin
Thin filaments cont.
actin, tropomyosin, and troponin, multiple actin subunits string together, form two intertwining strands in functional thin filament, each head-shaped actin has active site, binds with myosin heads
The sarcomere
microscopically, striations are alternating light and dark bands with specific regions
The sarcomere: I band
"i" in light mnemonic, only thin filaments
The sarcomere: Z disc
in the middle of I band, composed of structural proteins that anchor thin filaments in place to one another, serves as attachment points for elastic filaments
The sarcomere: A band
"a" in dark mnemonic, contains zone of overlap, both thick and thin filaments, generate tension during contraction
The sarcomere: H zone
HA mnemonic, H is in the A band, middle of A band where only thick filaments exist
The sarcomere: M line
M is in middle mnemonic, dark line in middle of A band, structural proteins hold thick filaments in place, serve as anchoring point for elastic filaments
The sliding-filament mechanism of contraction
sarcomere extends from on z-disc to the next, functional unit of contraction, explains tension generation during muscle contraction, both I band and H zone narrow, A band unchanged, Myosin heads attach to actin pulling thin filaments toward M line, bring Z-discs closer together, shorten whole muscle fiber
Innervated
includes all skeletal muscles, connected to a neuron
A single motor neuron
communicates with many muscle fibers
Synapse
each connection is called this
Neuromuscular Junction (NMJ)
synapse where a single motor neuron communicates with many muscle fibers, transmits signal (nerve impulse/action potential) from neuron to sarcolemma of muscle fiber
NMJ: Axon Terminal
contains synaptic vesicles filled with the neurotransmitter acetylcholine (ACh), neurotransmitters are chemicals that trigger changes in a target tissue, allow for cell to cell communication
Synaptic cleft
space b/w axon terminal and muscle fiber, filled with collagen fibers and gel that anchors neuron in place
Skeletal Muscle Contraction 1
In preparation for muscle contraction, 1. calcium ions released from terminal cisternae bind to troponin 2. tropomyosin moves and active sites of actin are exposed
Skeletal muscle contraction
a) at rest, tropomyosin blocks actin's active sites b)after stimulation calcium release cause the active sites of actin to be exposed
Contraction Phase
begins when actin's active site is exposed to initiate cross-bridge cycle
The first Step in the bridge cycle
Myosin head cocked once ATP is bound and energy is gathered by hydrolysis, ATP -> ADP+P
The second step in cross-bridge cycle
In high energy position (with ADP+P still attached), head is able to bind to active site of actin, cross-bridge is at 90 degree angle relative to thick filament
The third step in the cross-bridge cycle
Power stroke-released from head, myosin pulls actin toward M line as it pivots to relaxed (low energy) position, cross-bridge is at 45 degree angle relative to thick filament
The fourth step in the cross-bridge cycle
Myosin can bind to another ATP, breaks link with actin active site, detachment does not allow thin filaments to slide backward (some myosin heads still attached to action)
Contraction Cycle repetition
may repeat as long as stimulus to contract continues and ATP is available, myosin head recocked, binds to first actin molecule, and power stroke repeats, myosin binds to second action, and so on, over and over, for leverage contraction, process will repeat about 20-40 times for each myosin head in each sarcomere
Muscle relaxation
2 components: motor neuron action potentials stop signaling for release of acetylcholine from axon terminals, calcium ions are actively pumped back into SR terminal cisternae, in absence of calcium, tropin and tropomyosin black sites of actin, therefore muscle relaxes, myofilaments slide back into original positions
Rigor Mortis
progressive stiffening (contraction) of skeletal muscles, begins 3-4 hours after death, pumps that drive calcium back into SR no longer have ATP to fuel their activity, muscle fibers are unable to relax without ATP, myosin heads cannot detach from actin, muscles remain contracted until myofilaments proteins being to degenerate, about 48-72 hours after death
Muscle twitch
smallest muscle contraction, occurs in laboratory, not in whole muscle of body
3 phases of twitch on myogram
latent period, contraction period, relaxation period
Latent period
time for action potential to propagate across sarcolemma
Contraction period
repeated cross-bridge cycles generate tension
Relaxation period
calcium ion levels reduced in cytosol by SR pumps, tension diminishes
Wave summation
increase in tension caused by repeated stimulation of muscle fiber by motor neuron
Greater tension production
repeated stimulation results in progressively greater tension production pumps in SR membranes have inadequate time to pump all released calcium ions back into SR before the fiber is restimulated, concentration of calcium ions cytosol increases with each stimulation
Tension Production
depends on frequency of motor neuron stimulation, results in 2 possible myogram patterns: unfused tetanus, fused (complete) tetanus
Unfused tetanus
fibers are stimulated about 50 times per second, fiber partially relaxes between stimuli, tension pulsates (individual twitches remain visible)
Fused (complete) tetanus
fiber is stimulated at a rate of 80-100 stimuli per second, does not relax between stimuli, the tension stays constant at nearly 100% of maximum, increased availability of calcium allows more cross-bridges to form, contributing to increase in tension