Skeletal Muscle Tissue and Contraction: Structure and Function (Chapter 9, Human Anatomy & Physiology)

Study Guide - Smart Notes

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Muscle: The entire organ, composed of bundles called fascicles.
Fascicles: Bundles of muscle fibers (cells) within the muscle.
Muscle fibers: Individual muscle cells, long and multinucleated.
Myofibrils: Rod-like structures within muscle fibers, composed of repeating units called sarcomeres.
Sarcomeres: The functional contractile units of myofibrils, defined by Z discs.
Myofilaments: Protein filaments within sarcomeres, including thin filaments (actin) and thick filaments (myosin).

Sheaths: Connective tissue layers surround muscle structures:

Thin filaments: Composed primarily of actin.
Thick filaments: Composed primarily of myosin.
When muscle is relaxed, thin and thick filaments overlap only slightly at the ends of the A band.

During contraction, thin filaments slide past thick filaments, increasing their overlap without changing filament length.
Myosin heads bind to actin, forming cross bridges and initiating sliding.
Each myosin head binds and detaches several times, acting like a ratchet to pull actin filaments toward the center of the sarcomere.

Z discs are pulled toward the M line and get closer together.
I bands (regions with only actin) shorten.
H zones (regions with only myosin) disappear.
A bands (length of myosin) move closer together but do not change in length.

The brain or spinal cord sends a signal (nerve impulse or action potential) down the spinal cord to motor neurons that activate muscle fibers.
Both neurons and muscle fibers are excitable cells capable of generating and propagating action potentials (APs).
APs cannot spread directly from neuron to muscle cell; instead, the signal crosses the neuromuscular junction via the neurotransmitter acetylcholine (ACh).

Resting membrane potential is typically -70 mV in excitable cells, maintained by ion pumps (e.g., Na+/K+ ATPase).
Action potential (AP): A rapid, transient electrical voltage change that travels along the cell membrane.
APs are generated by the opening of ion channels (Na+ or Ca2+), which allow ions to flow across the membrane.
The AP consists of two main phases: depolarization (membrane potential becomes less negative) and repolarization (returns to resting state).

Equation:

Skeletal muscles are stimulated by motor neurons.
The axon of a motor neuron travels from the central nervous system to the muscle, branching as it enters the muscle.
Each axon branch ends on a muscle fiber, forming a neuromuscular junction (NMJ) or motor end plate.
Each muscle fiber has only one NMJ.

Synaptic cleft: Gel-filled space between the axon terminal and muscle fiber.
Synaptic vesicles in the axon terminal contain acetylcholine (ACh).
Infoldings of the sarcolemma, called junctional folds, contain millions of ACh receptors.
NMJ consists of the axon terminal, synaptic cleft, and junctional folds.

AP arrives at the neuron's axon terminal.
Voltage-gated Ca2+ channels open, Ca2+ enters the axon terminal.
Ca2+ causes release of ACh into the synaptic cleft.
ACh diffuses across the cleft and binds to ACh receptors on the sarcolemma.
Binding opens Na+ channels, allowing Na+ to enter, resulting in end plate potential (depolarization).
Acetylcholinesterase degrades ACh, terminating the signal.

Excitation: Generation of an AP that travels down the sarcolemma along the length of the fiber.

Three Steps:

Generation of end plate potential: Local depolarization as ACh opens Na+ channels.
Depolarization: If threshold is reached, voltage-gated Na+ channels open, AP is generated and propagates.
Repolarization: Restoration of resting ionic and voltage conditions (K+ channels open, Na+ channels close).

Refractory period: Muscle fiber cannot be stimulated again until repolarization is complete.

E-C coupling: The AP along the sarcolemma (excitation) is coupled to the sliding of myofilaments (contraction).
AP moves along the sarcolemma and down into T tubules, where voltage-sensitive proteins stimulate Ca2+ release from the sarcoplasmic reticulum (SR).

Structure	Function
Muscle fiber	Contractile cell of skeletal muscle
Myofibril	Contains sarcomeres; responsible for striations
Sarcomere	Functional contractile unit; contains actin and myosin
Myofilament	Actin (thin) and myosin (thick) filaments; slide during contraction
Motor neuron	Stimulates muscle fiber contraction via APs
Neuromuscular junction	Site of signal transmission from neuron to muscle fiber
Acetylcholine (ACh)	Neurotransmitter that initiates muscle fiber depolarization
Sarcoplasmic reticulum (SR)	Stores and releases Ca2+ for contraction
T tubule	Conducts APs into muscle fiber interior

Depolarization: Decrease in membrane potential (inside becomes less negative).
Repolarization: Return to resting membrane potential after depolarization.
End plate potential: Local depolarization at the NMJ due to Na+ influx.
Excitation-contraction coupling: Sequence linking AP to muscle contraction.
Cross bridge: Connection formed between myosin head and actin filament during contraction.

Motor neuron releases ACh at NMJ.
ACh binds to receptors, opening Na+ channels and generating end plate potential.
AP propagates along sarcolemma and down T tubules.
Ca2+ is released from SR, initiating cross bridge cycling and contraction.

Additional info: For more details on action potentials and their propagation, see Chapter 11 (Nervous System).