The Muscular System: Structure, Function, and Major Muscles

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The Muscular System

Introduction

The muscular system is essential for movement, posture, and various bodily functions. This chapter focuses on skeletal muscles, their organization, naming, and the principles underlying their actions and interactions.

Muscle Actions and Interactions

Functional Groups of Muscles

Prime Mover (Agonist): The main muscle responsible for a specific movement.
Antagonist: Muscle that opposes or reverses a movement; located on the opposite side of the joint from the agonist.
Synergist: Assists the prime mover by adding force or reducing unwanted movement.
Fixator: A type of synergist that stabilizes the origin of the prime mover.

Muscles can only pull, not push. The action of a muscle can be inferred by its position relative to the joint it crosses.

Inferring Muscle Action by Position

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.

Anterior side muscle action: flexion Posterior side muscle action: extension Lateral side muscle action: abduction Medial side muscle action: adduction

Origin and Insertion

Origin: The attachment site on the bone that moves the least (usually proximal or medial).
Insertion: The attachment site on the bone that moves the most (usually distal or lateral).

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), triceps (three).
Location of attachments: Named for origin and insertion (e.g., sternocleidomastoid).
Action: Named for the movement produced (e.g., flexor, extensor).
Names may combine several criteria (e.g., extensor carpi radialis longus).

Fascicle Arrangements

Types of Fascicle Arrangements

Fascicles are bundles of muscle fibers. Their arrangement affects muscle shape and function.

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 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: unipennate, bipennate, multipennate.

Patterns of fascicle arrangement in muscles Convergent and circular fascicle arrangements Fusiform and parallel fascicle arrangements Pennate fascicle arrangements

Functional Implications: Long, parallel fibers allow greater range of motion but less power. Pennate muscles, with more fibers, are more powerful but have less range of motion.

Lever Systems in the Muscular System

Basic Components

Lever: Rigid bar (bone) that moves on a fixed point (fulcrum, usually a joint).
Effort: Force applied by muscle contraction.
Load: Resistance moved by the effort (bone, tissue, or external weight).

Power vs. Speed Levers

Mechanical Advantage (Power Lever): Load is close to fulcrum, effort is far. Small effort moves large load.
Mechanical Disadvantage (Speed Lever): Load is far from fulcrum, effort is close. Load moves rapidly over a large distance.

Power lever: mechanical advantage Speed lever: mechanical disadvantage

Classes of Levers

First-Class Lever: Fulcrum between load and effort (e.g., seesaw, scissors, raising head off chest).

First-class lever diagram First-class lever in the body

Second-Class Lever: Load between fulcrum and effort (e.g., wheelbarrow, standing on tiptoe).

Second-class lever diagram Second-class lever in the body

Third-Class Lever: Effort between fulcrum and load (e.g., tweezers, most skeletal muscles, flexing forearm).

Third-class lever diagram Third-class lever in the body

Summary: Power levers are slower but stronger; speed levers are faster but less powerful.

Major Skeletal Muscles of the Body

Overview

There are over 600 skeletal muscles, grouped by function and location. Muscle tables typically include description, origin and insertion, actions, and innervation.

Superficial muscles can be identified from anterior and posterior views of the body.

Superficial muscles of the body: anterior view

Muscles of the Head and Neck

Facial Expression

Muscles insert into skin, not bone, allowing for facial expressions.
All are innervated by the facial nerve (cranial nerve VII).
Divided into muscles of the scalp and face.

Lateral view of muscles of the scalp, face, and neck (cadaver) Muscles used in facial expressions

Mastication and Tongue Movement

Prime movers of jaw closure: temporalis and masseter.
Grinding movements: pterygoids.
Chewing role: buccinator.
Extrinsic tongue muscles: genioglossus, hyoglossus, styloglossus.

Superficial muscles of mastication Deep muscles of mastication Extrinsic muscles of the tongue and associated suprahyoid muscles

Muscles of the Anterior Neck and Throat: Swallowing

Suprahyoid muscles: Elevate hyoid bone and larynx during swallowing.
Infrahyoid muscles: Depress hyoid bone and larynx during swallowing and speaking.
Pharyngeal constrictor muscles: Propel food into the esophagus.

Muscles of the anterior neck and throat used in swallowing

Muscles of the Neck and Vertebral Column: Head Movements and Trunk Extension

Sternocleidomastoid: Main flexor of the head.
Splenius muscles: Main extensors of the head.
Erector spinae group: Main extensors of the trunk and maintain posture.

Muscles of the neck and vertebral column that move the head and trunk *Additional info: For a complete understanding, students should also review the innervation and specific actions of each muscle group, as well as their clinical significance in injury and disease.*