Muscle Physiology Quiz

Muscle Physiology

• Muscle cells, like neurons, respond to chemical, electrical, or mechanical stimuli to generate an action potential that propagates along the cell membrane.
• Unlike neurons, muscle cells possess a contractile mechanism triggered by action potentials (excitation-contraction coupling).

Types of Muscles

• Skeletal Muscle:

  • Exhibits well-defined cross-striations.
  • Requires nervous stimulation for contraction.
  • Lacks direct connections between muscle fibers.
  • Controlled voluntarily.

• Cardiac Muscle:

  • Also displays cross-striations but contracts rhythmically without external stimulation due to pacemaker cells.

• Smooth Muscle:

  • Lacks cross-striations.
  • Actin and myosin are present but not arranged in regular arrays.
  • Contains dense bodies analogous to Z-lines in skeletal and cardiac muscle.
  • Troponin is absent.
  • Sarcoplasmic reticulum is poorly developed.

Skeletal Muscle Structure

• Skeletal muscles are composed of muscle fibers arranged in parallel and joined to tendons.
• Each fiber is a long, cylindrical, multinucleated cell surrounded by the sarcolemma.
• Muscle fibers are made up of myofibrils further divided into filaments (actin and myosin).
• Contractile proteins in skeletal muscle include myosin II, actin, tropomyosin, and troponin.

Skeletal Muscle Morphology

• Striations: Created by the arrangement of thick (myosin) and thin (actin, tropomyosin, troponin) filaments.
• Myosin: Possesses globular heads, each with an actin-binding site and ATPase activity.
• Thin filaments: Consist of two actin chains forming a double helix. Troponin and tropomyosin regulate muscle contraction by interacting with myosin.

Troponin

• Troponin molecules are globular proteins located along tropomyosin.
• Troponin T connects troponin to tropomyosin.
• Troponin I inhibits myosin-actin interaction.
• Troponin C has calcium-binding sites, which initiate muscle contractions.

Arrangement of Contractile Proteins

• Diagram illustrating the arrangement of actin, myosin, and associated proteins within a sarcomere structure.

Sarcotubular System

• Made up of T-tubules and sarcoplasmic reticulum (SR).

• T-tubules: Transverse tubules that are continuous with the sarcolemma; crucial for rapid transmission of action potentials throughout the muscle fiber.

• SR: Stores calcium ions (Ca²⁺), crucial for muscle contraction; has terminal cisternae and T-tubules

• Action potential travels down T tubules, triggering release of Ca++ from SR.

Neuro-Muscular Junction

• Specialized synapse between a motor neuron and a skeletal muscle fiber.

Skeletal Muscle Action Potential (AP)

• Resting membrane potential (RMP) of skeletal muscle is −80 to −90 mV.
• Action potentials must penetrate myofibrils along T-tubules triggering the release of Ca++.
• Excitation-contraction coupling is the process connecting the nerve signal to muscle contraction.

General Mechanisms of Skeletal Muscle Contraction

• Action potentials travel along a motor neuron.
• Acetylcholine (ACh) is released, activating membrane channels.
• Sodium (Na⁺) ions influx initiating action potential in muscle fiber.
• Action potentials propagate along the sarcolemma and T-tubules.
• This triggers calcium (Ca²⁺) release from the sarcoplasmic reticulum (SR).
• Sliding filament theory: Myosin heads bind to actin filaments causing the sliding of actin filaments toward the center of the sarcomere, which shortens the muscle fiber and leads to contraction.
• Ca²⁺ is returned to SR, muscle relaxes.

The Muscle Twitch

• A single action potential causes a brief contraction (twitch) followed by relaxation.
• Twitch duration varies based on muscle type (fast or slow).

Muscle Fiber Types

• Fast Fibers: Fine, rapid movements; short twitch duration (approximately 7.5 ms).
• Slow Fibers: Long, sustained contractions used in posture maintenance (e.g., back muscles); longer twitch duration.

Types of Contraction

• Isometric: Muscle contracts without changing length (e.g., holding a weight).

Electromyography (EMG)

• Recording and analyzing the electrical activity of muscles.

Drugs Affecting Neuromuscular Junction

• Stimulatory Drugs: Mimic or enhance acetylcholine (ACh) action (e.g., methacholine, carbachol, nicotine)
• Inhibitory Drugs (Anticholinesterase Inhibitors): Inhibit the breakdown of ACh, increasing its concentration at the synapse(e.g., neostigmine, physostigmine)
• Curariform Drugs: Block action of ACh on receptors preventing nerve impulse transmission to muscle .

Myasthenia Gravis

• Autoimmune condition causing muscle weakness due to impaired neuromuscular transmission.
• Treated with anticholinesterase drugs.

Cardiac Muscle Morphology

• Striated muscle with branching fibers connected via intercalated discs.
• Myofibrils are composed of actin, myosin, tropomyosin, and troponin.

Cardiac Muscle Electrical Properties

• Resting membrane potential is approximately −90 mV.
• Action potential phase 0 is rapid depolarization due to opening voltage-gated Na + channels.
• Phase 1: Initial rapid repolarization.
• Phase 2: Prolonged plateau caused by slow opening voltage-gated Ca 2+ channels.
• Phase 3 & 4: Final repolarization to resting membrane potential due to closing of Ca++ channels and K+ channels efflux.

Refractory Period of Cardiac Muscle

• Cardiac muscle, like other excitable tissues exhibits refractory periods preventing repetitive contractions during AP.
• Absolute refractory period is the time muscle cannot respond to stimulation.
• Relative refractory period is the time when muscle is more difficult to stimulate than normally but possible with a stronger stimulus

Smooth Muscle Structure

• Smooth muscle lacks striations, consisting of actin and myosin filaments arranged in irregular arrays.
• Contraction mechanism differs from skeletal and cardiac muscle.
• Dense bodies are anchored to the cell membrane and are analogous to Z-lines.

Types of Smooth Muscles

• Visceral (Unitary): Contract as a single unit interconnected via gap junctions (e.g., gut, blood vessels).
• Multi-unit: Individual muscle cells contracting independently under nervous stimulation (e.g., eye muscles).

Electrical and Mechanical Activities of Visceral Smooth Muscle

• resting membrane potential is approximately -50 to -60mV.
• Slow-wave potentials are fluctuations in membrane potential.

Control of Smooth Muscle Contraction

• Smooth muscle contraction is regulated via nervous and hormonal signals.

Test your knowledge on muscle fibers, excitation-contraction coupling, and neuromuscular junction actions. This quiz covers various aspects of muscle physiology, including differences between muscle fiber types and cardiac muscle action potentials. Challenge yourself with a series of questions that gauge your understanding of these critical topics.