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what are the 3 types of muscle tissue?
Skeletal
Cardiac
Smooth
what is skeletal muscle?
longest fibers, striated, voluntary
what is cardiac muscle?
only found in heart, striated, involuntary
what is smooth muscle?
found in walls of hollow organs (visceral), nonstriated, involuntary
what are the 4 main characteristics shared by all muscles?
Excitability, Contractility, Extensibility, Elasticity
what is excitability?
responsiveness
ability to receive and respond to stimuli
what is contractility?
ability to shorten forcibly when stimulated
what is extensibility?
ability to be stretched
what is elasticity?
ability to recoil to resting length
what are the 4 important functions of muscles?
Produce movement: responsible for all locomotion and manipulation
Maintain posture and body position
Stabilize joints
Generate heat as they contract
what is the order of muscle from largest to smallest?
fascicle
muscle fiber
myofibril
myofilaments
what is the epimysium?
connective tissue surrounding entire muscle
what is the perimysium?
connective tissue surrounding fascicles (groups of muscle fibers)
what is the endomysium?
connective tissue surrounds each muscle fiber
what is the attachment to moveable bone called?
insertion
what is the attachment to immoveable/less moveable bone called?
origin
what is direct attachment?
epimysium fused to the bone
what is indirect attachment?
muscle attached by tendon or sheetlike aponeurosis
Indirect is the most common type of attachment
what is the sarcolemma?
muscle fiber plasma membrane
what is the sarcoplasm?
cytoplasm, contains a lot of glycosomes that store glycogen which provide glucose during muscle cell activity for ATP, contains myoglobin for oxygen storage
what are myofibrils?
80% of cell volume, hold actin and myosin, they dont ever shorten just slide
what is a sarcomere?
The functional unit of skeletal muscle, classified from z-disc to z-disc, this is the actual part of the muscle that actually shortens
Contains A band with half of an I band at each end
what is the purpose of t tubules?
increase muscle fiber surface area, allos changes in membrane potential to rapidly penetrate the fibers
what are the 2 types of myofilaments?
myosin and actin
what is the central thick myofilament?
myosin → “dark” A-bands
attached by the H zone, connected at the M line
what are the thin myofilaments?
actin → “light” I-bands
attached by z discs
what does each myosin filament contain?
head and tail (myosin head that links myosin and actin together)
what are the regulatory proteins bound to actin?
Tropomyosin and troponin
what blocks myosin from binding to actin?
Tropomyosin
what facilities binding along with the help of calcium ions?
troponin
what us the sarcoplasmic reticulum?
network of smooth endoplasmic reticulum tubules surrounding each myofibril
Main function is storage and regulation of calcium ions
what is acetylcholine (ACh)?
the neurotransmitter that allows an action potential to cross from neuron to muscle cell
what is not visible when muscles contracts?
the M-line
what is the sliding filament model of contraction?
states that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more
Thick (myosin) and thin (actin) filaments NEVER change length, just overlap more
how are thin filaments pulled closer towards the center of a sarcomere?
Cross bridge attachments form and break several times
Z discs are pulled toward M line
I bands shorten
Z discs become closer
H zones disappear
A bands move closer to each other
***actin and myosin never shorten the only slide past one another***
what happens when there is a low concentration of calcium (Ca2+)
Tropomyosin blocks active sites on actin
Myosin heads cannot attach to actin
Muscle fiber remains relaxed
what happens that causes the sarcoplasmic reticulum (SR) to release Ca2+ to cytosol?
Voltage-sensitive proteins in T tubules change shape
what happens so that tropomyosin moves away from myosin-binding sites
Ca2+ binds to troponin and changes shape
Step 1 of Cross Bridge Cycling
Cross bridge formation: high-energy myosin head attaches to actin thin filament
active site
Step 2 of Cross Bridge Cycling
Working (power) stroke: myosin head pivots and pulls thin filament toward M line
Step 3 of Cross Bridge Cycling
Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to
detach
Step 4 of Cross Bridge Cycling
Cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head
into high-energy state
This energy will be used for power stroke in next cross bridge cycle
what does a skeletal muscle fiber NEED to contract?
Calcium
ATP
Stimulation from a motor neuron
what is a chemically gated ion channel?
opened by chemical messengers such as neurotransmitters
Example: ACh receptors on muscle cells
what is a voltage-gated ion channel?
open or close in response to voltage changes in membrane potential
how are skeletal muscles stimulated?
somatic motor neurons
what is a neuromuscular junction?
an axon splitting a bunch (getting smaller and smaller) and then coming into “contact” with a muscle fiber.
what is the synaptic cleft?
gel-filled space between axon terminal and muscle fiber
what are the purpose of junctional folds in the sarcolemma?
increase the surface area to contain millions of ACh receptors
what are the events at the neuromuscular junction?
AP arrives at axon terminal
Voltage-gated calcium channels open, calcium enters motor neuron
Calcium entry causes release of ACh neurotransmitter into synpatic cleft
ACh diffuses across to ACh receptors (Na+ chemical gates) on sarcolemma
ACh binding to receptors, opens gates, allowing Na+ to enter resulting in end plate
potential
Acetylcholinesterase degrades ACh
what is end plate potential
ACh released from motor neuron binds to ACh receptors on sarcolemma
Causes chemically gated ion channels on sarcolemma to open
what is depolarization?
generation and propagation of an action potential (AP)
If end plate potential causes enough change in membrane voltage to reach critical
level called threshold, voltage-gated Na+ channels in membrane will open
what is repolarization?
Restoration of resting conditions
Na+ voltage-gated channels close, and voltage-gated K+ channels open
K+ efflux out of cell rapidly brings cell back to initial resting membrane voltage
what is the refractory period?
muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete
Ionic conditions of resting state are restored by Na+-K+ pump
Na+ that came into cell is pumped back out, and K+ that flowed outside is pumped back into cell
what is excitation-contraction (E-C) coupling?
events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction)
AP is propagated along sarcolemma and down into T tubules, where voltage-sensitive proteins in tubules stimulate Ca2+ release from SR
Ca2+ release leads to contraction
what is rigor mortis?
3–4 hours after death, muscles begin to stiffen
Peak rigidity occurs about 12 hours postmortem
Intracellular calcium levels increase because ATP is no longer being synthesized, so calcium cannot be pumped back into SR
Results in cross bridge formation
