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The Muscular System: Microscopic Anatomy and Physiology of Skeletal Muscle

The Muscular System

Microscopic Anatomy and Physiology of Skeletal Muscle

  • Skeletal Muscle
    • Characteristics:
    • Voluntary
    • Striated appearance
    • Components:
    • Skeletal muscle fibers: Muscle cells that contain myofibrils, the contractile elements of muscle.
    • Striations: Alternating light and dark bands that are visible under a microscope, indicating layers of thin (actin) and thick (myosin) filaments.
    • Nuclei: Multiple, located peripherally within the fiber.

Muscle Structure

  • Definitive Muscle: Composed of many muscle bundles.
    • Example: Triceps muscle.
  • Muscle Bundles: Groups of muscle fibers (muscle cells).
    • Each muscle bundle is made up of multiple muscle fibers.
  • Muscle Fiber:
    • Definition: A single muscle cell.
    • Contains numerous myofibrils (contractile units).
  • Myofibril:
    • Basic unit of muscle structure.
    • Characteristic: Striations present, allowing for contraction.
    • Composed of myofilaments.

Myofilament Types

  • Myofilaments:
    • Two Types:
    • Thin Myofilaments: Comprised mainly of actin.
    • Thick Myofilaments: Comprised mainly of myosin.
  • Striated Appearance:
    • Result of the overlapping arrangement of thin and thick myofilaments.

Muscle Contraction Mechanism

  • Initiation:
    • Triggered by an electrical impulse.
  • Process:
    • Thin actin myofilaments slide in between thick myosin filaments during contraction.
    • Energy Requirement: Energy is sourced from ATP (Adenosine Triphosphate).

Neuromuscular Junction

  • Function: Allows communication between the nervous system and muscle for contraction.
  • Process:
    • A nerve impulse travels from the central nervous system (CNS) down the axon of a motor neuron.
    • The end of the axon contacts the muscle cell at the neuromuscular junction.

Motor Components

  • Motor: Refers to nerves or muscles involved in producing movement.
  • Motor Neuron: A type of neuron involved in stimulating muscle contraction.
  • Motor Unit:
    • Definition: A motor neuron and all muscle cells it innervates.
  • Terminal Fibers:
    • A branch of a single motor neuron axon that innervates individual muscle cells.
  • Neuromuscular Junction Structure:
    • Consists of:
    • End of motor neuron terminal fiber.
    • Synaptic Cleft: Gap between the neuron and muscle cell, they do not touch directly.
    • Motor End Plate: Specialized area on the sarcolemma with receptors for neurotransmitters.

Neurotransmitter Dynamics

  • Neurotransmitter:
    • Definition: A chemical substance synthesized and released by nerve cells to transmit or inhibit nerve impulses.
    • Example: Acetylcholine (Ach).
  • Acetylcholine (Ach):
    • A specific neurotransmitter at the neuromuscular junction critical for muscle contraction.
    • Also associated with the parasympathetic nervous system.
  • Acetylcholinesterase (Ach-ase):
    • Enzyme that breaks down acetylcholine in the synaptic cleft to prevent continuous stimulation of the muscle.

Synapse Overview

  • Synapse:
    • Definition: The junction between
    • Two neurons,
    • A neuron and a muscle cell, or
    • A neuron and a gland cell.
    • Function: Site of action for neurotransmitters, facilitating signal transmission between cells.