Muscles of the Head, Neck, and Trunk: Structure, Function, and Identification

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Muscles of the Head, Neck, and Trunk

Introduction

This study guide covers the anatomy, terminology, and functional organization of the major skeletal muscles of the head, neck, and trunk. It includes muscle naming conventions, mechanisms of muscle action, and the identification and function of key muscle groups. Understanding these muscles is essential for comprehending movement, support, and protection provided by the musculoskeletal system.

Naming Skeletal Muscles

Muscle Naming Conventions

Skeletal muscles are named based on several criteria, including location, position, structure, and function. Recognizing these conventions aids in identifying and understanding muscle roles.

Location: Indicates the region (e.g., abdominis for abdomen, femoris for femur).
Position/Direction: Describes orientation (e.g., anterior, lateralis, rectus for straight, transversus for transverse).
Structural Characteristics: Includes number of origins (e.g., biceps = two heads), shape (e.g., deltoid = triangle), and size (e.g., maximus = largest).
Origin and Insertion: Muscles are often named for their attachment points (e.g., sternocleidomastoid attaches to sternum, clavicle, and mastoid process).
Action: Describes the primary movement (e.g., flexor, extensor).

Example: The rectus abdominis is a straight muscle of the abdomen.

Labeled anterior view of major skeletal muscles Labeled posterior view of major skeletal muscles

How Skeletal Muscles Produce Movement

Muscle Action and Levers

Skeletal muscles produce movement by exerting force on tendons, which pull on bones or other structures. Most muscles cross at least one joint and are attached to the bones forming that joint. When a muscle contracts, it draws one articulating bone toward the other, creating movement at the joint.

Lever: A rigid object (bone) that moves on a fixed point (fulcrum) when force is applied.
Fulcrum: The fixed point (joint) on which the lever moves.
Tendon: Cordlike structure of dense regular connective tissue connecting muscle to bone.
Aponeurosis: Broad, flat connective tissue sheet connecting muscle to bone or other muscles.
Origin: The stationary attachment point of the muscle.
Insertion: The movable attachment point of the muscle.
Action: The specific movement produced by muscle contraction.

Example: The biceps brachii flexes the forearm at the elbow joint by pulling on the radius.

Diagram of muscle lever system and functional classification

Functional Classification of Muscles

Agonist (Prime Mover): The main muscle responsible for a specific movement.
Antagonist: Muscle that opposes the action of the agonist.
Synergist: Muscle that assists the agonist in performing its action.

Muscles of Facial Expression and Mastication

Muscles of Facial Expression

These muscles are responsible for movements of the skin and soft tissues of the face, allowing for expressions such as smiling, frowning, and blinking. They are primarily innervated by the facial nerve (cranial nerve VII).

Frontalis: Raises eyebrows and wrinkles forehead.
Orbicularis oculi: Closes eyelids.
Zygomaticus major/minor: Elevates corners of the mouth (smiling).
Orbicularis oris: Closes and protrudes lips (kissing).
Buccinator: Compresses cheek (whistling, blowing).
Platysma: Tenses skin of neck and depresses mandible.
Depressor anguli oris: Depresses corners of the mouth (frowning).
Mentalis: Protrudes lower lip (pouting).

Muscles of facial expression, anterior view

Muscles of Mastication

The muscles of mastication are responsible for movements of the jaw during chewing. They act on the temporomandibular joint (TMJ).

Temporalis: Elevates and retracts mandible.
Masseter: Elevates mandible (primary chewing muscle).
Medial pterygoid: Elevates mandible and moves it side to side.
Lateral pterygoid: Depresses and protracts mandible, moves it side to side.

Example: The masseter is the strongest muscle for jaw elevation during chewing.

Muscles of mastication: temporalis, masseter, pterygoids

Model Images of Facial Muscles

Model: frontalis, procerus, orbicularis oculi Model: zygomaticus minor, levator labii superioris Model: nasalis, levator labii superioris, zygomaticus minor, orbicularis oris, zygomaticus major, buccinator Model: buccinator, depressor anguli oris, depressor labii inferioris, mentalis Model: temporalis, masseter Model: temporalis, lateral pterygoid Model: lateral and medial pterygoid

Extrinsic Eye Muscles

Overview

Extrinsic eye muscles control the movement of the eyeball within the orbit. Each muscle has a specific origin, insertion, and action, allowing for precise eye movements.

Superior rectus: Elevates and turns eye medially.
Inferior rectus: Depresses and turns eye medially.
Lateral rectus: Moves eye laterally.
Medial rectus: Moves eye medially.
Superior oblique: Depresses and turns eye laterally.
Inferior oblique: Elevates and turns eye laterally.

Extrinsic eye muscles, lateral and medial views Extrinsic eye muscles, anatomical model

Muscles of the Neck

Major Muscles and Triangles of the Neck

The neck contains several important muscles that facilitate head movement, swallowing, and support of the hyoid bone. The neck is divided into anatomical triangles for clinical reference.

Sternocleidomastoid (SCM): Flexes and rotates the head.
Scalene muscles (anterior, middle, posterior): Elevate first two ribs, assist in neck flexion and rotation.
Suprahyoid muscles (geniohyoid, mylohyoid, stylohyoid, digastric): Elevate hyoid bone, assist in swallowing.
Infrahyoid muscles (sternohyoid, omohyoid, sternothyroid, thyrohyoid): Depress hyoid bone and larynx during swallowing and speech.

Muscles of the neck: lateral view Scalene muscles, sternocleidomastoid Suprahyoid muscles, anterior triangle Suprahyoid muscles, model Infrahyoid muscles, model Sternocleidomastoid and scalene muscles, posterior triangle Scalene group, model Muscles of the neck, anterior triangle

Muscles of the Thorax

Intercostal Muscles

The intercostal muscles are located between the ribs and are essential for respiration.

External intercostals: Elevate ribs during inspiration.
Internal intercostals: Depress ribs during forced expiration.

External and internal intercostal muscles, model Diagram of intercostal muscle fiber orientation

Diaphragm

The diaphragm is the primary muscle of respiration, separating the thoracic and abdominopelvic cavities.

Origin: Xiphoid process, costal cartilages, lumbar vertebrae.
Insertion: Central tendon of the diaphragm.
Action: Contracts to increase thoracic volume during inspiration.

Diaphragm, model view

Pectoral Muscles

Pectoralis major: Flexes, adducts, and medially rotates the arm.
Pectoralis minor: Depresses and protracts the scapula.

Pectoralis major and minor, model

Muscles of the Abdominal Wall

Major Muscles and Structures

The abdominal wall is composed of several layers of muscle that protect abdominal organs, assist in trunk movement, and maintain intra-abdominal pressure.

Rectus abdominis: Flexes vertebral column, compresses abdomen.
External oblique: Compresses abdomen, flexes and rotates vertebral column.
Internal oblique: Compresses abdomen, flexes and rotates vertebral column.
Transversus abdominis: Compresses abdomen.
Linea alba: Midline tendinous seam joining the abdominal muscles.

Muscles of the abdominal wall, model

Summary Table: Key Muscles of the Head, Neck, and Trunk

Muscle	Origin	Insertion	Action
Temporalis	Temporal bone	Coronoid process of mandible	Elevates and retracts mandible
Masseter	Zygomatic arch	Mandibular ramus	Elevates mandible
Sternocleidomastoid	Sternum and clavicle	Mastoid process of temporal bone	Flexes and rotates head
Rectus abdominis	Pubic crest and symphysis	Costal cartilages of ribs 5-7, xiphoid process	Flexes vertebral column, compresses abdomen
External intercostals	Inferior border of rib above	Superior border of rib below	Elevates ribs during inspiration
Diaphragm	Xiphoid process, lower ribs, lumbar vertebrae	Central tendon	Increases thoracic cavity volume

Key Terms and Concepts

Origin: Fixed attachment point of a muscle.
Insertion: Movable attachment point of a muscle.
Action: Movement produced by muscle contraction.
Agonist, Antagonist, Synergist: Functional roles in movement.
Tendon, Aponeurosis: Connective tissue structures for muscle attachment.

Additional info: For exam preparation, students should be able to identify the origin, insertion, and action of each muscle listed, as well as recognize these muscles on anatomical models and diagrams.