Muscle System Notes

The Muscle System Overview

  • Muscular Tissue Types:
    • Skeletal: Voluntary muscle with striations; attached to bones.
    • Cardiac: Involuntary muscle found in the heart; also striated.
    • Smooth: Involuntary muscle found in organs; lacks striations.

Functions of Muscles

  1. Movement:
    • Enables movement of body parts (e.g., limbs, trunk).
    • Facilitates processes like breathing and digestion.
    • Pumps blood through the circulatory system.
  2. Stability:
    • Maintains body posture and stability during movement.
  3. Control of Openings:
    • Regulates passage through various body openings (sphincters).
  4. Heat Generation:
    • Produces heat via metabolic processes, crucial for enzymatic functions.
  5. Glycemic Control:
    • Regulates blood glucose levels, supporting overall homeostasis.

Skeletal Muscle Fibers

  • Definition: Voluntary striated muscles under conscious control (muscle fibers).
  • Characteristics:
    • Long, slender cells with multiple nuclei located near the membrane.
    • Contain myofibrils filled with contractile proteins; packed with mitochondria.
  • Key Components:
    • Sarcoplasmic Reticulum (SR): Stores calcium; aids in muscle contraction.
    • Sarcolemma: Plasma membrane of muscle fiber with T tubules for electrical signals.

Myofilaments and Striations

  • Myofibrils: Arranged in bundles containing thick and thin filaments.
  • Filament Types:
    • Thick Filaments: Made of myosin; have a club-shaped head.
    • Thin Filaments: Mainly actin; include tropomyosin and troponin which regulate contraction.
  • Striated Appearance:
    • Alternating dark A bands (thick filaments) and light I bands (thin filaments).
    • Z discs serve as anchoring points for thin filaments.

Nerve-Muscle Relationship

  • Motor Neurons: Stimulate skeletal muscles; originate in the brain/spinal cord.
  • Motor Unit: Comprises a motor neuron and the muscle fibers it controls.
  • Neuromuscular Junction (NMJ): Connection point between a motor neuron and muscle fiber, involving:
    • Axon Terminal: Releases neurotransmitter acetylcholine (ACh).
    • Synaptic Clef: Small gap between neuron and muscle cell.
    • Acetylcholinesterase (AChE): Enzyme that breaks down ACh to halt muscle stimulation.

Muscle Contraction Process

  1. Excitation: Nerve impulse releases ACh, opening ion channels in sarcolemma, leading to action potentials.
  2. Calcium Release: Action potentials trigger calcium release from SR, allowing contraction.
  3. Contraction Mechanism:
    • Myosin heads use ATP to bind to actin, forming cross-bridges, leading to power strokes that pull filaments.
    • Cycle of binding, power stroke, and release continues with ATP consumption.
  4. Relaxation:
    • Cessation of nerve signals reduces ACh, triggers SR calcium reabsorption, and returns muscle to resting state.

Types of Muscle Contraction

  • Muscle Twitch: A single contraction-relaxation cycle.
  • Tetanus: Continuous muscle contraction with no relaxation.
  • Isometric Contraction: Muscle contraction without a change in length.
    • Concentric: Shortening while maintaining tension.
    • Eccentric: Lengthening while maintaining tension.

Energy for Muscle Contraction

  • All muscular activity relies on ATP production.
  • ATP Generation:
    • Anaerobic fermentation converts glucose to lactate.
    • Aerobic respiration uses pyruvate in mitochondria to generate energy.

Muscle Fatigue and Recovery

  • Muscle Fatigue: Weakness following prolonged use.
  • Endurance Factors:
    • Supply of myoglobin, glycogen, and number of mitochondria; oxygen uptake capacity.
  • Recovery: Increased oxygen consumption post-exercise for ATP and creatine phosphate replenishment.

Types of Muscle Fibers

  • Slow Twitch Fibers: Endurance-oriented, resist fatigue, suited for aerobic respiration.
  • Fast Twitch Fibers: Quick response, fatigue quickly.

Muscular Strength and Conditioning

  • Muscular Strength: Dependent on anatomical and physiological factors.
  • Types of Exercise:
    • Resistance Exercise: Builds muscle mass (e.g., weight lifting).
    • Endurance Exercise: Improves fatigue resistance (e.g., running).

Cardiac and Smooth Muscle

  • Cardiac Muscle:
    • Striated, involuntary, self-regulating (autorhythmic), must contract rhythmically and without conscious control.
    • Fibers are shorter with intercalated discs connecting cells.
  • Smooth Muscle:
    • Involuntary, non-striated, single nucleus; found in hollow organs.
    • Connects using dense plaques, allowing twisting contraction; relies mainly on extracellular calcium.