BISC 101 Lecture Notes - Lecture 20: Neuromuscular Junction, Skeletal Muscle, Motor Neuron

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BISC 101 – Lecture 20 – Muscular System
Vertebrate Skeletal Muscle
Vertebrate Skeletal Muscle: Moves bones and body and is characterized by a
hierarchy of smaller and smaller units
A skeletal muscles consists of a bundle of long fibers
oLong fibers are a single cell that run parallel to the length of the muscle
Each muscle fiber is a bundle of smaller myofibrils arranged longitudinally
Myofibrils are composed of two kinds of myofilaments
oThin Filaments: Consists of two strands of actin and one strand of
regulatory protein
oThick Filaments: Staggered arrays of myosin molecules
Skeletal muscle is also called striated muscle
oArrangement of myofilaments create pattern of light and dark bands
Sarcomere: Functional unit of muscle bordered by Z lines
oZ Line: Boundary
oM Line: Midline of Sarcomere
oH Zone: Myosin only
oI Band: Actin Only
oA Band: Contains both actin and myosin
When a sarcomere is fully contracted, the following happens:
oThe H Zone and I band disappears
oThe Z line moves closer together
oA band does not change length
oM line is unaffected
Muscle Contraction
Sliding – Filament Model: Filaments side past each other longitudinally,
producing more overlap between thin and thick filaments
Sliding of filaments is based on interaction between actin of the thin filaments
and myosin of the thick filaments
The head of a myosin molecule binds to an actin filament, forming a bridge
oThe thin filament will be pulled toward the center of the sarcomere
Muscle contraction requires repeated cycles of binding and release
Glycolysis and aerobic respiration generate ATP needed to sustain contraction
Calcium and Regulatory Proteins
A skeletal muscle fiber contracts only when stimulated by a motor neuron
When a muscle is at rest, myosin binding sites on thin filaments are blocked by
regulatory protein
Myosin – binding sites are exposed when Ca2+ is released
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For a muscle fiber to contract, myosin binding sites must be uncovered
oThis occurs when Ca2+ ions bind to the troponin complex and expose the
myosin head binding sites
Muscle fiber contraction occurs when the concentration of Ca2+ ions is high
Contractions stop when concentration of Ca2+ is lo
Neuromuscular Junction
Neuromuscular Junction: Specialized chemical synapse that serves to transmit
electrical impulses from the motor neuron nerve terminal to the skeletal muscle
oEx: Action potential from pre to post synaptic membranes
Acetylcholine or ACh: The primary neurotransmitter in the neuromuscular junction
The following steps of what goes on at the neuromuscular junction:
1. The action potential reaches the nerve terminal in the presynaptic region
2. Calcium channels open to facilitate the influx of Ca2+ into the presynaptic
nerve terminal
3. The increase in intracellular Ca2+ ion concentration triggers fusion of
synaptic vesicles (containing ACh) with the nerve terminal membrane
4. Once fused with this membrane, vesicles release ACh into the synaptic
5. ACh binds to its receptors on the postsynaptic membrane (i.e., motor end
plate) and opens ligand-gated Na+ /K + channels
6. The opening of these ligand-gated channels will ultimately lead to the
opening of voltage-gated Na+ channels to initiate the action potential in
muscle fiber (postsynaptic membrane)
7. Acetylcholinesterase degrades ACh into acetic acid and choline
oCholine is reabsorbed back into the presynaptic nerve terminal so
that more acetylcholine can be synthesized
Skeletal Muscle Contraction
1. At the neuromuscular junction, acetylcholine (ACh) is released at synaptic
oACh diffuses across synaptic cleft and binds to receptor proteins on
muscle fiber’s plasma membrane
oThis triggers an action potential in muscle fiber
2. The AP is propagated along the plasma membrane and down the transverse
3. The AP triggers Ca2+ ions to be released from sarcoplasmic reticulum
4. Ca2+ ions bind to troponin associated with the thin filament (actin)
oThe tropomyosin moves, which exposes the myosin head binding sites
5. The sliding filament cycling begins with myosin cross-bridge formation and
breakdown (coupled with ATP hydrolysis)
oAs a result, the actin slides toward the center of the sarcomere
6. Cytosolic Ca2+ ions are removed by the calcium pump (via active transport) into
the SR after the action potential ends
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