Anatomy of the Muscular System: Structure, Function, and Major Muscles

Anatomy of the Muscular System

Overview of Skeletal Muscles

The human body contains over 600 skeletal muscles, which are responsible for voluntary movements, posture, and heat production. These muscles are organized into regions such as the head and neck, back and thorax, abdomen, glutes and pelvis, shoulder and upper limb, and hip and lower extremity.

Functions and Characteristics of Skeletal Muscle Tissue

• Movement: Skeletal muscles contract to produce movement of the skeleton.

• Posture: Continuous muscle contractions maintain posture and body position.

• Stabilization: Muscles stabilize joints and support soft tissues.

• Heat Production: Muscle activity generates heat, helping to maintain body temperature.

Skeletal muscle tissue is characterized by excitability, contractility, extensibility, and elasticity.

Gross Structure of Skeletal Muscle

Skeletal muscles are composed of bundles of muscle fibers (cells) surrounded by connective tissue sheaths:

• Endomysium: Surrounds each individual muscle fiber.

• Perimysium: Encloses bundles of fibers called fascicles.

• Epimysium: Encloses the entire muscle.

• Deep fascia: Groups muscles into functional compartments.

Muscle Attachments

Types of Attachments

• Direct Attachment: The epimysium of the muscle fuses directly to the periosteum of a bone or perichondrium of cartilage.

• Indirect Attachment: The muscle connects to bone via a tendon or aponeurosis, which is more common.

Muscles typically attach to bones at two points:

• Origin: The fixed or less movable attachment.

• Insertion: The movable attachment, usually pulled toward the origin during contraction.

Muscle Actions and Interactions

Functional Groups of Muscles

• Prime Mover (Agonist): Main muscle responsible for a specific movement (e.g., brachialis in elbow flexion).

• Antagonist: Opposes or reverses a movement (e.g., triceps brachii in elbow flexion).

• Synergist: Assists the prime mover by adding force or reducing unnecessary movement.

• Fixator: A type of synergist that stabilizes the origin of a prime mover.

Muscles can change roles depending on the movement performed.

Arrangement of Fascicles in Skeletal Muscles

Types of Fascicle Arrangements

• Circular: Fascicles arranged in concentric rings (e.g., orbicularis oculi).

• Parallel: Fascicles run parallel to the long axis (e.g., sartorius).

• Fusiform: Spindle-shaped with an expanded belly (e.g., biceps brachii).

• Pennate: Short fascicles attach obliquely to a central tendon (unipennate, bipennate, multipennate).

• Convergent: Fascicles converge toward a single tendon (e.g., pectoralis major).

Muscles of Respiration

Intercostal Muscles

• External Intercostals: Elevate the rib cage during inspiration.

• Internal Intercostals: Depress the rib cage during forced expiration.

Diaphragm

The diaphragm is a dome-shaped muscle forming the floor of the thoracic cavity. It is the prime mover of inspiration, flattening to increase thoracic volume during inhalation.

Muscles of the Neck and Vertebral Column

Major Muscles

• Sternocleidomastoid: Flexes and rotates the head.

• Scalenes: Elevate the first two ribs and assist in neck flexion.

• Splenius: Extends or rotates the head.

Muscles Crossing the Shoulder Joint

Major Muscles

• Biceps Brachii: Flexes the elbow and supinates the forearm.

• Triceps Brachii: Extends the elbow.

Muscles of Facial Expression

Epicranius (Occipitofrontalis)

• Frontal Belly: Raises eyebrows and wrinkles forehead.

• Occipital Belly: Pulls scalp posteriorly.

All muscles of facial expression are innervated by the facial nerve (CN VII).

Summary Table: Types of Muscle Attachments and Actions

Term	Definition	Example
Origin	Fixed attachment point	Scapula for biceps brachii
Insertion	Movable attachment point	Radius for biceps brachii
Agonist	Main muscle causing movement	Brachialis in elbow flexion
Antagonist	Opposes movement	Triceps brachii in elbow flexion
Synergist	Assists agonist	Brachioradialis in elbow flexion
Fixator	Stabilizes origin	Muscles stabilizing scapula