Muscular System

Muscular System Overview

  • The muscular system consists of three types of muscle tissues:

    • Skeletal Muscle:

      • Responsible for voluntary movements.

      • Features striations (striped appearance) due to dark (A bands) and light (I bands) bands.

      • Made up of myofibrils and myofilaments.

    • Cardiac Muscle:

      • Found in the heart; involuntary control.

    • Smooth Muscle:

      • Found in organs; involuntary control.

Structure of Skeletal Muscle

  • Myofibrils:

    • Composed of myofilaments, namely myosin and actin.

  • Myosin:

    • Makes up the dark A bands.

  • Actin:

    • Constitutes the light I bands.

Sarcomere

  • The basic contractile unit of a muscle cell.

  • Sarcomeres are defined by the region between two Z lines.

Sliding Filament Theory

  • Describes how contractions occur in muscle fibers:

    1. Myofibrils contract by the filaments (myosin and actin) sliding past each other, rather than shortening in length.

    2. Myosin heads attach to actin, pulling them inward to shorten the sarcomere.

    3. This process requires ATP.

Energy Sources for Muscle Contraction

  • Adenosine Triphosphate (ATP):

    • Serves as the primary energy source for muscle contraction.

    • Involves several steps:

      1. Hydrolysis of ATP into Adenosine Diphosphate (ADP) + phosphate, releasing energy.

      2. When the muscle is relaxed, the binding site on actin is blocked, preventing binding of myosin.

      3. ATP binding to myosin head releases actin (releasing force).

      4. Hydrolysis of ATP resets myosin head to its original position.

Neural Control of Muscle Contraction

  • Neuromuscular Junction:

    • Connection between motor neuron and muscle fiber.

    • Action potential travels along the axon to the motor end plate, causing the release of calcium (Ca2+) into the cytosol from the sarcoplasmic reticulum.

    • Calcium ions bind to troponin, moving tropomyosin and exposing binding sites on actin.

  • T-tubules:

    • Extensions of the muscle cell membrane that transport the action potential into the fiber, facilitating contraction.

Calcium in Muscle Contraction

  • Calcium ions (Ca2+) are continuously being regulated:

    • During relaxation, they are sequestered in the sarcoplasmic reticulum.

    • Entry of calcium ions into the cytosol leads to contraction of the muscle.

Types of Muscle Fibers

  • Fast-twitch Fibers:

    • Adapted for explosive, short-duration activities.

    • Contain more sarcoplasmic reticulum for quick energy release.

  • Slow-twitch Fibers:

    • Designed for endurance, allowing for sustained activity over long periods.

    • Richer in mitochondria for aerobic metabolism, providing prolonged energy.