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Muscles

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All cells contain specific proteins for movement
microtubules - flagella, cytoskeleton, spindle fibers
microfilaments - sytoskeleton, microvilli, muscle contraction
types of microfilaments
1. actin- thin
2. myosin- thick
3. tyoponin
4. tropomyosin
Types of muscle
1. skeletal muscle
2. cardiac muscle
3. smooth muscle
skeletal muscle
striated (repeating longitudinal arrangement of muscle fibers, creates a striped or banded appearance)
multinucleated (one cell merges into the next)
voluntary - movement usually under conscious control
moves skeleton
cardiac muscle
striated
involuntary
only in heart
has many gap junctions that allow ions to pass from one cell to another - muscle cells are electrically coupled- one cell strongly stimulated will cause all muscle cells to contract
some specialized cardiac muscle can contract without nerve stimulation = pacemaker
smooth muscle
nonstriated (not arranged in an orderly, longitudinal fashion)
long, spindle-shaped cells that form sheets
single nucleus
involuntary
present in internal organs - digestive tract, blood vessels, urinary reproductive tract
skeletal muscle cell structure
muscle fiber = muscle cell
1. sarcolemma
2. sarcoplasma
3. SR / Sarcoplasmic Reticulum
4. transverse tubules
5. sarcomere
sarcolemma
cell membrane
Sarcoplasma
cytoplasm of muscle cell
sarcoplasmic reticulum (SR)
similar to SER
membranous network that wraps around myofibrils
stores Ca++
transverse tubules
extension of sarcolemma that penetrates deep into the muscle
sarcomere
functional or contractile unit of the muscle cell
a repeating arrangement of actin and myosin
arranged along the length of each myofibril
sliding filament theory of muscle contraction
actin and myosin slide past each other producing a shortening of the sarcomere = contraction
because all sarcomeres contract as a unit, this causes shortening of the whole muscle fiber. When enough muscle fibers contract, this enables the entire muscle to contract with great force
the heads o the myosin filament form cross bridges with actin, and then pull the actin molecule, let go, grap another, and repeat
muscle movement involves:
1. excitation (nerve stimulation)
2. contraction
3. relaxation
before contraction
myosin bidnign site on actin is blocked by presence of tropomyosin- so actin and myosin can't interact
Ca++ is stores in SR
ATP is tores in head of myosin
contraction
1. action potential in motor neuron activates muscle cell= axon knobs release NT acetylcholine into neuromuscular junction (synapse between neuron and muscle cell)
2. acetylcholine binds receptors on sarcolemma- causes depolarization, and (if stimulus is strong enough) an action potential that spreads across muscle cell membrane
3. depolarization of membrane- spreads from sarcolemma to transverse tubules, and then to SR
4. Membrane permeability of SR is altered and CA++ is released (lot of Ca++ is stored in SR) and binds to troponin and alters its shape
5. Change in troponin alters configuration of tropomyosin. Movement of tropomyosin from actin exposes region on actin that can bind to myosin head.
6. Myosin head binds ATP and hydrolyzes it. the ADP and P remain attached. This alters the shape of the myosin molecule, allowing it to make a cross-bridge with actin. This stimulates the release of ADP + P. When those are released from myosin molecule, the myosin head swivels, and pulls the actin filament toward the center of the sarcomere.
7. myosin head binds another ATP. This is necessary in order for the myosin/actin cross bridge to break, returning tone myosin head to its original position (like the na/K pump, myosin is also an ATP are enzyme that can hydrolyze ATP)
8. Myosin head can attach to new actin monomer and repeat the process as long as CA++ and ATP are available
9. Each of the many cross-bridges will break and reform many times per second, producing a smooth sliding movement
10. When stimulus ceases, CA++ returns to SR by active transport (requires ATP)
Uses of ATP
1. rovides energy for contraction cycle - moves myosin head
2. used to release myosin head forom actin so cycle can continue
3. needed to return Ca++ to the SR - active transport - this occurs when muscle is no longer receiving AP's / stimulation
rigor mortis
rigidity of muscles after death
muscles remain contracted because no ATP is produced
1. actin/myosin complex can't dissociate
2. Ca++ can't be returned to SR by active transport
Msucle energy
1. stored glucose (glycogen)
2. O2 (only in muscle cells- some O2 is bound to myoglobin)
3. Creatine phosphate - a compound formed and stored in a resting muscle cell. When needed, it is broken down to generate ATP
Creatine-P + ADP
stored energy
creatine + ATP
useable energy