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

Anatomy of the Muscular System

Overview

The muscular system consists of skeletal muscles that enable movement, maintain posture, and support bodily functions. Muscles are classified by their structure, function, and location, and are named according to specific conventions.

Skeletal Muscles: Functional Groups

Main Roles in Movement

• Prime Movers (Agonists): Provide the major force for producing a specific movement. Example: Biceps brachii during elbow flexion.

• Antagonists: Oppose or reverse a particular movement. Example: Triceps brachii during elbow flexion.

• Synergists: Add force to a movement and reduce undesirable or unnecessary movement. Example: Brachialis assists biceps brachii.

• Fixators: Synergists that immobilize a bone or muscle’s origin, stabilizing the joint. Example: Rotator cuff muscles stabilizing the shoulder.

Naming Skeletal Muscles

Criteria for Muscle Names

• Location: Indicates the bone or body region associated with the muscle (e.g., temporalis over the temporal bone).

• Shape: Describes the muscle’s form (e.g., deltoid = triangle).

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

• Direction of Fibers or Fascicles: Rectus (straight), transversus (transverse), oblique (angled).

• Number of Origins: Biceps (2 origins), triceps (3 origins).

• Location of Attachments: Named for origin or insertion points.

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

Muscle Mechanics: Arrangement of Fascicles

Types of Fascicle Arrangements

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

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

• Parallel: Fascicles parallel to the long axis of a straplike muscle (e.g., sartorius).

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

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

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

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

  • Multipennate: Multiple fascicle arrangements (e.g., deltoid).

Major Skeletal Muscles of the Body

Information for Each Muscle

• Name and Description: Often indicates location, shape, or function.

• Origin and Insertion: The origin is the fixed attachment, while the insertion moves during contraction. Usually, a joint is located between these points.

• Action: The movement produced when the muscle contracts (e.g., flexion, extension).

• Innervation: The major nerve supplying the muscle.

Muscles of the Head

Groups

• Muscles of Facial Expression: Insert into the skin, important for nonverbal communication. All are innervated by cranial nerve VII (facial nerve).

• Muscles of Mastication and Tongue Movement: Responsible for chewing and moving the tongue.

Key Muscles of Facial Expression

• Epicranius (frontal and occipital bellies): Moves scalp and eyebrows.

• Orbicularis oculi: Closes eyelids.

• Zygomaticus major and minor: Elevate corners of mouth (smiling).

• Buccinator: Compresses cheek.

• Platysma: Tenses skin of neck.

Muscles of Mastication

• Temporalis: O: Temporal fossa I: Coronoid process of mandible A: Closes jaw

• Masseter: O: Zygomatic arch I: Angle and ramus of mandible A: Prime mover of jaw closure

• Buccinator: O: Maxilla and mandible I: Orbicularis oris A: Compresses the cheek

• Medial Pterygoid: O: Pterygoid plate of sphenoid I: Mandible medial surface A: Protraction and grinding movements

• Lateral Pterygoid: O: Greater wing of sphenoid I: Condylar process of mandible A: Grinding movements

Tongue Musculature

Innervation and Function

• Innervated by cranial nerves V (Trigeminal), IX (Glossopharyngeal), and XII (Hypoglossal).

• Genioglossus: O: Inner mandibular symphysis I: Body of hyoid bone A: Protracts the tongue

• Styloglossus: O: Styloid process I: Inferior aspect of the tongue A: Retracts and elevates the tongue

• Hyoglossus: O: Hyoid bone I: Inferior aspect of the tongue A: Depresses the tongue

• Stylohyoid: O: Styloid process I: Hyoid bone A: Elevates hyoid during swallowing

• Geniohyoid: O: Mandibular symphysis I: Hyoid bone A: Elevates and protracts hyoid during swallowing

Muscles of the Neck

Key Muscles

• Sternocleidomastoid (SCM): O: Manubrium I: Mastoid process A: Flexes and laterally rotates the head

• Scalenes (anterior, middle, posterior): O: Transverse processes of cervical spine I: Ribs 1 and 2 A: Elevate ribs 1 and 2 (assist in respiration)

• Splenius capitis and cervicis: O: Nuchal ligament I: Occiput and mastoid process, spinous processes C2-C4 A: Extension and hyperextension of the head

Muscles of Respiration

Primary Muscles

• Inspiration: Scalenes, Pectoralis Minor, External Intercostals, Diaphragm

• Expiration: Abdominals, Internal Intercostals, Diaphragm

Key Points

• External Intercostals: Elevate the rib cage during inspiration.

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

• Diaphragm: Prime mover of inspiration; flattens on contraction to increase thoracic volume. Innervation: Phrenic nerve

Table: Major Skeletal Muscles and Their Functions

Muscle	Origin	Insertion	Action	Innervation
Masseter	Zygomatic arch	Angle and ramus of mandible	Jaw closure	Trigeminal nerve (V)
Temporalis	Temporal fossa	Coronoid process of mandible	Closes jaw	Trigeminal nerve (V)
Buccinator	Maxilla and mandible	Orbicularis oris	Compresses cheek	Facial nerve (VII)
Sternocleidomastoid	Manubrium	Mastoid process	Flexes and rotates head	Accessory nerve (XI)
Diaphragm	Inferior rib cage, sternum	Central tendon	Inspiration	Phrenic nerve

Summary

The muscular system is organized into functional groups that work together to produce movement, maintain posture, and support vital functions such as respiration and facial expression. Understanding muscle naming conventions, fascicle arrangements, and the major muscles of the body is essential for mastering human anatomy and physiology.