Muscle Tissue: Structure, Function, and Physiology

The Muscular System

Introduction

The muscular system is responsible for movement, posture, and heat production in the human body. Skeletal muscles, which are under voluntary control, are organized in various patterns that influence their function and efficiency. Understanding the classification and organization of muscles is essential for comprehending how movement is produced and controlled.

Classification of Muscles by Fascicle Organization

Fascicles and Connective Tissue Organization

• Fascicles are bundles of muscle fibers (cells) grouped together within a muscle.

• Each fascicle is surrounded by a connective tissue layer called the perimysium.

• The perimysium contains blood vessels and nerves that supply the muscle fibers within the fascicle.

• Muscle fibers within a fascicle are typically parallel to each other.

• Tendons connect muscles to bones, transmitting the force generated by muscle contraction.

Example: The biceps brachii muscle contains fascicles surrounded by perimysium, with tendons attaching the muscle to bones of the forearm and shoulder.

Types of Skeletal Muscle Fascicle Arrangements

Parallel Muscles

In parallel muscles, the fascicles are aligned parallel to the long axis of the muscle. This arrangement is common in many skeletal muscles and allows for efficient contraction and movement.

• When a parallel muscle contracts, it shortens and increases in diameter.

• Example: Biceps brachii (arm muscle), rectus abdominis (with tendinous bands).

Key Features:

• Produce greater range of motion but less force compared to other arrangements.

• Muscle body (belly) thickens as it contracts.

Convergent Muscles

Convergent muscles have a broad origin and fascicles that converge toward a single tendon or insertion point. This arrangement allows the muscle to pull in different directions, depending on which fascicles are activated.

• Fascicles spread out like a fan or broad triangle, with the tendon at the apex.

• Activation of different portions of the muscle can change the direction of the pull, making the muscle versatile.

• Example: Pectoralis major (chest muscle).

Pennate Muscles

Pennate muscles have fascicles that form a common angle with the tendon, resembling a feather. This arrangement allows for more muscle fibers in a given area, increasing force production.

• Fascicles pull at an angle relative to the tendon.

• Types of pennate muscles include:

  • Unipennate: Fascicles on one side of the tendon (e.g., extensor digitorum).

  • Bipennate: Fascicles on both sides of the tendon (e.g., rectus femoris).

  • Multipennate: Fascicles arranged around multiple tendons (e.g., deltoid).

Key Features:

• Produce more force than parallel muscles but have a smaller range of motion.

Circular Muscles (Sphincters)

Circular muscles, also known as sphincters, have fascicles arranged concentrically around an opening. When these muscles contract, the diameter of the opening decreases.

• Control the opening and closing of body passages.

• Examples: Orbicularis oris (mouth), external anal sphincter.

Muscle Attachments: Origins and Insertions

Definitions

• Origin: The fixed, less movable attachment point of a muscle, usually proximal or closer to the trunk.

• Insertion: The movable attachment point, usually distal or farther from the trunk. During contraction, the insertion is pulled toward the origin.

• Some muscles have more than one origin (e.g., biceps brachii has two heads; triceps brachii has three heads).

Muscle Actions and Interactions

Functional Roles of Muscles

• Muscles typically function in groups to produce coordinated movements.

• Prime Mover (Agonist): The muscle primarily responsible for a specific movement.

• Antagonist: A muscle whose action opposes that of the agonist, allowing for controlled movement and return to the original position.

• Synergist: A muscle that assists the agonist, making its action more efficient.

Example: During elbow flexion, the biceps brachii is the prime mover, the triceps brachii is the antagonist, and the brachialis acts as a synergist.

Naming of Muscles

Descriptive Criteria

• Location: e.g., temporalis (temporal bone), brachialis (arm).

• Relative Position: e.g., external oblique, internal intercostals.

• Fascicle Organization/Direction: e.g., external oblique, transversus abdominis.

• Origin and Insertion: e.g., sternocleidomastoid (originates at sternum and clavicle, inserts at mastoid process).

• Number of Origins: e.g., biceps brachii (two heads), triceps brachii (three heads).

• Shape: e.g., deltoid (triangular), orbicularis (circular), trapezius (trapezoid).

• Size: e.g., adductor longus (long), fibularis brevis (short), gluteus maximus (largest), gluteus minimus (smallest).

• Action: e.g., extensor digitorum (extends fingers), adductor longus (adducts thigh).

Summary Table: Types of Muscle Fascicle Arrangements

Type	Fascicle Arrangement	Example	Functional Characteristic
Parallel	Fascicles parallel to long axis	Biceps brachii, rectus abdominis	Greatest range of motion, less force
Convergent	Fascicles converge to a single tendon	Pectoralis major	Versatile direction of pull
Pennate	Fascicles at an angle to tendon	Deltoid, rectus femoris, extensor digitorum	More force, less range of motion
Circular	Fascicles arranged in concentric rings	Orbicularis oris, external anal sphincter	Control openings, constrict passages

Additional info: The connective tissue layers merge at the ends of muscles to form tendons, which attach muscles to bones and transmit the force of contraction.