Muscle Structure and Function Notes

Muscle Structure and Function

Muscle Types

• Multinucleate Fibers: Striated appearance due to the arrangement of sarcomeres.

  • Voluntary vs. Involuntary: Striated (skeletal) is voluntary; non-striated (smooth) is involuntary.

  • Intercalated Discs: Unique to cardiac muscle, allowing synchronized contractions.

Muscle Organization

• Epimysium: Surrounds the whole muscle.

• Perimysium: Surrounds bundles of muscle fibers (fascicles).

• Endomysium: Surrounds individual muscle fibers.

  • Myofilaments: Actin (thin) and Myosin (thick) are the key proteins for contractions.

• Sarcomere: The smallest functional unit of a muscle, consisting of overlapping actin and myosin filaments.

Nervous System Role in Muscle Contraction

• Innervation: Nerves control muscle contraction, with two main types:

  • Sensory Neurons: Bring information to the CNS.

  • Motor Neurons: Relay commands from the CNS to effectors (muscles).

Muscle Cell Structure

• Sarcoplasm: Fluid inside muscle cells, rich in potassium and other ions.

• Sarcolemma: The muscle cell membrane; transmits action potentials.

• Transverse Tubules (T-tubules): Invaginations of the sarcolemma that carry action potentials deep into the muscle fiber.

• Sarcoplasmic Reticulum (SR): Stores calcium ions critical for muscle contraction.

  • Terminal Cisternae: Enlarged ends of the SR, associated with T-tubules to form a triad.

Excitation-Contraction Coupling

• Actin and Myosin Interaction:

  • G-actin: Globular actin units that bind with myosin heads.

  • Tropomyosin: Covers binding sites on actin when muscle is relaxed.

  • Troponin: Regulates the location of tropomyosin, moving it in response to calcium ions.

  • Cross-Bridge Formation: Myosin heads attach to active sites on actin, initiating contraction through power strokes.

Synaptic Transmission

• Synapse: The site of communication between a motor neuron and a muscle fiber.

  • Neurotransmitter: Acetylcholine (ACh) is released from the axon terminal, binding to receptors on the sarcolemma, enabling contraction.

  • Acetylcholinesterase: Enzyme that breaks down ACh after contraction to stop muscle action.

Muscle Contraction Mechanics

Phases of Muscle Twitch

1. Latent Period: Time before contraction begins, action potential spreads through the sarcolemma, leading to Ca2+ release.

2. Contraction Phase: Ca2+ binds to troponin, exposing active sites on actin; cross-bridges form, tension builds.

3. Relaxation Phase: Ca2+ levels drop, tropomyosin recovers the active sites, muscle tension decreases.

Muscle Tension Production

Muscle Relaxation and Rigor Mortis

• Relaxation: Triggered when Ca2+ concentration falls, leading to the restoration of muscle length.

• Rigor Mortis: Stiffness post-mortem from calcium buildup and lack of ATP, locking cross-bridges in place.

• All-or-None Principle: Individual muscle fibers either fully contract or do not contract at all.

• Recruitment: More muscle fibers are recruited for a stronger contraction, depending on the frequency of stimuli.

  • A single neural stimulation results in a twitch; multiple stimuli lead to sustained contractions through repeated and rapid firing of motor neurons.