The Muscular System: Structure, Function, and Major Muscles

The Muscular System

Introduction

The muscular system is essential for movement, posture, and various bodily functions. This chapter focuses on skeletal muscles, their actions, naming conventions, fascicle arrangements, and major muscle groups. Understanding these concepts is fundamental for students of anatomy and physiology.

Muscle Actions and Interactions

Functional Groups of Muscles

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

• Antagonist: The muscle that opposes or reverses the action of the prime mover. Prime movers and antagonists are typically located on opposite sides of a joint.

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

• Fixator: A type of synergist that stabilizes the origin of the prime mover, providing a stable base for movement.

Muscles can only pull; they cannot push. The coordinated action of these groups allows for smooth and controlled movements.

Muscle Action Relative to Joints

The action of a muscle can often be inferred by its position relative to the joint it crosses. The following general rules apply (with exceptions for the knee and ankle):

• A muscle crossing the anterior side of a joint produces flexion.

• A muscle crossing the posterior side of a joint produces extension.

• A muscle crossing the lateral side of a joint produces abduction.

• A muscle crossing the medial side of a joint produces adduction.

Example: The pectoralis major (anterior) flexes the shoulder, while the latissimus dorsi (posterior) extends it.

Origin and Insertion

• Origin: The attachment site on the bone that moves the least during muscle contraction (usually proximal or medial).

• Insertion: The attachment site on the bone that moves the most (usually distal or lateral).

Movement occurs as the insertion is pulled toward the origin during contraction.

Naming Skeletal Muscles

Criteria for Naming Muscles

• Location: Named for the bone or region (e.g., temporalis over the temporal bone).

• Shape: Named for distinctive shapes (e.g., deltoid = triangle).

• Size: Terms like maximus (largest), minimus (smallest), longus (long).

• Direction of Fibers: Rectus (straight), transversus (right angles), oblique (angles).

• Number of Origins: Biceps (two origins), triceps (three origins).

• Location of Attachments: Named for origin and insertion (e.g., sternocleidomastoid attaches to sternum and clavicle, inserts on mastoid process).

• Action: Named for the movement produced (e.g., flexor, extensor).

• Combined Criteria: Some names combine several features (e.g., extensor carpi radialis longus).

Example: The biceps brachii is named for its two origins and location in the arm.

Fascicle Arrangements in Skeletal Muscles

Patterns of Fascicle Arrangement

Fascicles are bundles of muscle fibers. Their arrangement determines muscle shape and function. The main patterns are:

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

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

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

• Fusiform: Spindle-shaped with parallel fibers (e.g., biceps brachii).

• Pennate: Short fascicles attach obliquely to a central tendon. Types include:

  • Unipennate: Fascicles attach to one side (e.g., extensor digitorum longus).

  • Bipennate: Fascicles insert from both sides (e.g., rectus femoris).

  • Multipennate: Multiple feather-like fascicles (e.g., deltoid).

Functional Implications: Fascicle arrangement affects a muscle’s range of motion and power. Long, parallel fibers allow greater movement but less power, while pennate muscles are more powerful but have less range of motion.

Major Skeletal Muscles of the Body

Overview and Study Tips

• There are over 600 skeletal muscles, grouped by function and location.

• Muscle tables typically include: description, origin and insertion, actions, and innervation.

• Tips for learning muscles:

  • Use the muscle’s name to infer location and function.

  • Identify muscles on anatomical figures.

  • Relate muscle attachments to their actions.

  • Palpate muscles on your own body to feel contractions.

Muscles of Facial Expression

Muscles of facial expression are unique because they insert into the skin rather than bone, allowing for a wide range of nonverbal communication. All are innervated by the facial nerve (cranial nerve VII) and are divided into muscles of the scalp and face.

Example: The orbicularis oris encircles the mouth and allows for movements such as puckering the lips.

Muscles of the Abdominal Wall

The abdominal wall muscles support the trunk, allow movement, and help maintain posture. They also play a role in processes such as respiration, urination, and childbirth.

Example: The rectus abdominis is a prominent muscle of the anterior abdominal wall, responsible for flexing the vertebral column.

Summary Table: Fascicle Arrangements

Arrangement	Example Muscle	Functional Feature
Circular	Orbicularis oris	Closes body openings
Convergent	Pectoralis major	Broad origin, focused insertion
Parallel	Sartorius	Great range of motion
Fusiform	Biceps brachii	Spindle-shaped, moderate power
Pennate (uni-, bi-, multi-)	Deltoid, rectus femoris	High power, less range of motion