Study Notes: The Muscular System (Anatomy & Physiology)

The Muscular System

Introduction

The muscular system is essential for all types of body movement and is composed of three basic muscle types: skeletal muscle, cardiac muscle, and smooth muscle. Each type has distinct structural and functional characteristics that contribute to the body's ability to move, maintain posture, and regulate internal processes.

Muscle Types

General Features

• Muscle cells are elongated and are also called muscle fibers.

• Contraction and shortening of muscles are due to the movement of microfilaments.

• Common terminology includes prefixes myo- and mys- (meaning "muscle"), and sarco- (meaning "flesh").

Comparison of Skeletal, Cardiac, and Smooth Muscles

Characteristic	Skeletal	Cardiac	Smooth
Body Location	Attached to bones or skin	Walls of the heart	Walls of hollow organs (other than the heart)
Cell Shape & Appearance	Long, cylindrical, multinucleate, striated	Branching chains, uninucleate, striated, intercalated discs	Spindle-shaped, uninucleate, no striations
Connective Tissue Components	Epimysium, perimysium, endomysium	Endomysium attached to fibrous skeleton of heart	Endomysium
Regulation of Contraction	Voluntary, via nervous system	Involuntary, internal pacemaker, nervous system, hormones	Involuntary, nervous system, hormones, chemicals, stretch
Speed of Contraction	Slow to fast	Slow	Very slow
Rhythmic Contraction	No	Yes	Yes, in some

Skeletal Muscle

• Most skeletal muscle fibers are attached by tendons to bones.

• Cells are large, cigar-shaped, and multinucleate.

• Also known as striated muscle due to visible stripes.

• Also known as voluntary muscle because it is subject to conscious control.

Connective Tissue Organization

• Endomysium: Encloses a single muscle fiber.

• Perimysium: Wraps around a fascicle (bundle) of muscle fibers.

• Epimysium: Covers the entire skeletal muscle.

• Fascia: Located on the outside of the epimysium.

Connective Tissue Attachments

• Tendons: Cordlike structures, mostly collagen fibers, often cross joints due to toughness and small size.

• Aponeuroses: Sheetlike structures, attach muscles indirectly to bones, cartilages, or connective tissue coverings.

Smooth Muscle

• No striations.

• Involuntary—not under conscious control.

• Located mainly in the walls of hollow visceral organs (e.g., stomach, urinary bladder, respiratory passages).

• Spindle-shaped fibers, uninucleate.

• Contractions are slow and sustained.

Cardiac Muscle

• Striated appearance.

• Involuntary control.

• Found only in the walls of the heart.

• Uninucleate cells.

• Branching cells joined by gap junctions called intercalated discs.

• Contracts at a steady rate set by a pacemaker.

Muscle Functions – Skeletal Muscle

Roles Beyond Movement

• Maintain posture and body position

• Stabilize joints

• Generate heat

Microscopic Anatomy of Skeletal Muscle

Cellular Structure

• Sarcolemma: Specialized plasma membrane of muscle fiber.

• Myofibrils: Long organelles inside muscle cell; responsible for striated appearance due to alternating light (I) bands and dark (A) bands.

Banding Pattern of Myofibrils

• I band: Light band, contains only thin filaments, interrupted by Z disc (midline interruption).

• A band: Dark band, contains entire length of thick filaments, includes H zone (lighter central area) and M line (center of H zone).

Sarcomere Structure

• Sarcomere: Contractile unit of a muscle fiber; structural and functional unit of skeletal muscle.

• Organization:

  • Thick filaments = myosin filaments

  • Thin filaments = actin filaments

Thick Filaments (Myosin)

• Composed mostly of the protein myosin.

• Contain ATPase enzymes to split ATP and release energy for contraction.

• Possess projections called myosin heads that form cross bridges during contraction.

Thin Filaments (Actin)

• Composed mostly of the protein actin.

• Actin is anchored to the Z disc.

• At rest, the H zone within the A band lacks actin filaments; during contraction, H zones disappear as actin and myosin overlap.

Sarcoplasmic Reticulum (SR)

• Specialized smooth endoplasmic reticulum.

• Surrounds the myofibril.

• Stores and releases calcium, which provides the final "go" for contraction.

Stimulation and Contraction of Single Skeletal Muscle Cells

Functional Properties

• Irritability (Responsiveness): Ability to receive and respond to a stimulus.

• Contractility: Ability to forcibly shorten when an adequate stimulus is received.

• Extensibility: Ability of muscle cells to be stretched.

• Elasticity: Ability to recoil and resume resting length after stretching.

The Nerve Stimulus and Action Potential

Motor Neuron Stimulation

• Skeletal muscles must be stimulated by a motor neuron to contract.

• Motor unit: One motor neuron and all the skeletal muscle cells it stimulates.

Neuromuscular Junction

• Association site of axon terminal of the motor neuron and sarcolemma of a muscle.

• Neurotransmitter: Chemical released by nerve upon arrival of nerve impulse in the axon terminal.

• Acetylcholine (ACh) is the neurotransmitter that stimulates skeletal muscle.

Synaptic Cleft

• Gap between nerve and muscle filled with interstitial fluid.

• Nerve and muscle do not make direct contact.

Events at the Neuromuscular Junction

1. Nerve impulse reaches the axon terminal of the motor neuron.

2. Calcium channels open, and calcium ions enter the axon terminal.

3. Calcium ion entry causes synaptic vesicles to release acetylcholine (ACh).

4. ACh diffuses across the synaptic cleft and attaches to receptors on the sarcolemma of the muscle cell.

5. If enough ACh is released, the sarcolemma becomes temporarily more permeable to sodium ions (Na+).

6. More sodium ions enter than potassium ions leave, producing depolarization and opening more Na+ channels.

7. An action potential is created and conducted along the muscle cell; acetylcholinesterase (AChE) breaks down ACh, ending muscle contraction.

Restoration of Resting State

• Potassium ions (K+) diffuse out of the cell.

• Sodium-potassium pump moves sodium and potassium ions back to their original positions.

Mechanism of Muscle Contraction: The Sliding Filament Theory

Overview

• Calcium ions (Ca2+) bind regulatory proteins on thin filaments, exposing myosin-binding sites.

• Myosin heads attach to actin, forming cross bridges.

• Each cross bridge pivots, causing thin filaments to slide toward the center of the sarcomere.

• Contraction occurs, and the cell shortens.

• ATP provides the energy for the sliding process, which continues as long as calcium ions are present.

Schematic Representation

• Regulatory proteins block myosin binding sites on actin in a relaxed muscle.

• Action potential triggers Ca2+ release, which binds to regulatory proteins, exposing binding sites.

• Myosin heads bind to actin, initiating contraction.

Key Equation

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

$\text{Ca}^{2+} + \text{Troponin} \rightarrow \text{Conformational change exposing myosin-binding sites}$

Example

During muscle contraction, calcium released from the sarcoplasmic reticulum binds to troponin, causing tropomyosin to move and expose binding sites on actin. Myosin heads then attach and pull actin filaments inward, shortening the sarcomere and producing movement.

Additional info: These notes are based on textbook-style lecture slides and are suitable for exam preparation in Anatomy & Physiology, specifically Chapter 6: The Muscular System.