Study Guide: The Muscular System (Anatomy & Physiology) Chapter 10A

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The Muscular System

Muscle Actions and Interactions

The muscular system is composed of contractile tissues, including skeletal, cardiac, and smooth muscle. This section focuses on skeletal muscle, examining how muscles interact to produce movement, the criteria for naming muscles, and principles of leverage.

Muscles can only pull; they never push. Movement is achieved by the coordinated action of muscle groups.
Three main functional groups:
- Prime mover (agonist): Major responsibility for producing a specific movement.
- Antagonist: Opposes or reverses a particular movement. Located on the opposite side of the joint from the prime mover.
- 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.
The action of a muscle can be inferred by its position relative to the joint it crosses.

Examples of Muscle Actions Relative to Joint Position

Anterior side: Muscles crossing on the anterior side of a joint produce flexion.
Posterior side: Muscles crossing on the posterior side produce extension.
Lateral side: Muscles crossing on the lateral side produce abduction.

Muscle crossing anterior side produces flexion Muscle crossing posterior side produces extension Muscle crossing lateral side produces abduction

Naming Skeletal Muscles

Skeletal muscles are named according to several criteria, which often provide clues to their location, shape, size, and function.

Location: Named for the bone or region they are associated with (e.g., temporalis over the temporal bone).
Shape: 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 (origin first).
Action: Named for the action they produce (e.g., flexor, extensor).
Multiple criteria can be combined (e.g., extensor carpi radialis longus).

Fascicle Arrangements in Skeletal Muscles

All skeletal muscles consist of fascicles (bundles of muscle fibers). The arrangement of fascicles determines the muscle's shape and functional capabilities.

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 of the muscle (e.g., sartorius).
Fusiform: Spindle-shaped with parallel fibers (e.g., biceps brachii).
Pennate: Short fascicles attach obliquely to a central tendon.
- Unipennate: Fascicles attach to one side (e.g., extensor digitorum longus).
- Bipennate: Fascicles attach to both sides (e.g., rectus femoris).
- Multipennate: Multiple fascicles attach to one tendon (e.g., deltoid).

Patterns of fascicle arrangement in muscles Circular fascicle arrangement Convergent fascicle arrangement Fusiform fascicle arrangement Parallel fascicle arrangement Multipennate fascicle arrangement Bipennate fascicle arrangement Unipennate fascicle arrangement

Major Skeletal Muscles of the Body

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

Description: Location relative to other muscles.
Origin and insertion: Usually a joint between these points.
Actions: Movement caused by contraction.
Innervation: Major nerve supplying the muscle.

Superficial muscles of the body: Anterior view

Muscles of the Head: Facial Expression

Muscles of facial expression are unique because they insert into the skin, not bone, and are important for nonverbal communication. All are innervated by cranial nerve VII (facial nerve).

Two groups:
- Muscles of the scalp
- Muscles of the face

Muscle	Description	Origin & Insertion	Action	Nerve Supply
Corrugator supercilii	Small muscle; activity causes eyebrows to move medially and downward	Arch of frontal bone above nasal bone; skin of eyebrow	Pulls eyebrows medially and downward	Facial nerve
Orbicularis oculi	Thin, flat sphincter muscle of eyelid	Frontal and maxillary bones; skin around eyelids	Closes eyes	Facial nerve
Zygomaticus (major and minor)	Muscle pair extending diagonally from cheekbone to corner of mouth	Zygomatic bone; skin and muscle at corner of mouth	Raises lateral corners of mouth (smiling)	Facial nerve
Risorius	Slender muscle; runs inferior and lateral to zygomaticus	Fascia of masseter muscle; skin at angle of mouth	Pulls corner of lip laterally	Facial nerve

Table of facial expression muscles Lateral view of muscles of the scalp, face, and neck Muscles used in facial expressions

Muscles of Mastication and Tongue Movement

Four pairs of muscles are involved in mastication (chewing), all innervated by cranial nerve V. The tongue is anchored and moved by three extrinsic muscles.

Prime movers of jaw closure: Temporalis and masseter
Grinding movements: Pterygoids
Chewing role: Buccinator
Tongue movement: Genioglossus, hyoglossus, styloglossus

Muscle	Description	Origin & Insertion	Action	Nerve Supply
Temporalis	Fan-shaped muscle covering parts of temporal, frontal, and parietal bones	Temporal fossa; coronoid process of mandible	Closes jaw	Trigeminal nerve
Masseter	Powerful muscle covering lateral aspect of mandibular ramus	Zygomatic arch; angle and ramus of mandible	Closes jaw	Trigeminal nerve
Medial pterygoid	Deep two-headed muscle; runs along internal surface of mandible	Medial surface of lateral pterygoid plate; medial surface of mandible	Grinding movements	Trigeminal nerve
Lateral pterygoid	Deep two-headed muscle; runs along internal surface of mandible	Lateral surface of lateral pterygoid plate; condylar process of mandible	Grinding movements	Trigeminal nerve
Buccinator	Horizontal muscle; forms cheek	Mandible and maxilla; orbicularis oris	Compresses cheek	Facial nerve

Table of mastication and tongue movement muscles Muscles of mastication and tongue movement

Muscles of the Anterior Neck and Throat: Swallowing

The sternocleidomastoid divides the neck into anterior and posterior triangles. Muscles are grouped as suprahyoid and infrahyoid based on their relation to the hyoid bone. These muscles facilitate swallowing by moving the hyoid bone and larynx.

Suprahyoid muscles: Four deep muscles that elevate the hyoid bone and move the larynx during swallowing.
Infrahyoid muscles: Four straplike muscles that depress the hyoid bone and larynx during swallowing and speaking.

Muscle	Description	Origin & Insertion	Action	Nerve Supply
Mylohyoid	Flat, triangular muscle forms floor of oral cavity	Medial surface of mandible; hyoid bone	Elevates hyoid bone and floor of mouth	Mandibular branch of trigeminal nerve
Digastric	Two bellies united by intermediate tendon	Lower margin of mandible; hyoid bone	Opens mouth and depresses mandible	Mandibular branch of trigeminal nerve; facial nerve
Sternohyoid	Most medial muscle of neck	Manubrium and medial end of clavicle; hyoid bone	Depresses hyoid bone	Ansa cervicalis (C1-C3)

Table of anterior neck and throat muscles Muscles of the anterior neck and throat used in swallowing

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

Muscles in this region are divided into anterolateral neck muscles (move the head) and intrinsic muscles of the back (extend the trunk and maintain posture).

Anterolateral neck muscles: Move the head.
Intrinsic back muscles: Extend the trunk and maintain posture.

Muscle	Description	Origin & Insertion	Action	Nerve Supply
Sternocleidomastoid	Two-headed muscle; key landmark in neck	Manubrium of sternum and medial portion of clavicle; mastoid process of temporal bone	Flexes and laterally rotates the head	Accessory nerve (cranial nerve XI)
Scalenes	Three muscles (anterior, middle, posterior) located laterally on neck	Transverse processes of cervical vertebrae; first two ribs	Elevate first two ribs; flex and rotate neck	Cervical spinal nerves
Splenius	Broad, two-part muscle; extends or hyperextends head	Ligamentum nuchae and spinous processes of vertebrae; mastoid process and occipital bone	Extend or hyperextend head	Cervical spinal nerves

Table of neck and vertebral column muscles Muscles of the neck and vertebral column that move the head and trunk

Summary Table: Functional Groups of Skeletal Muscles

Group	Function	Example
Prime mover (agonist)	Produces specific movement	Biceps brachii (elbow flexion)
Antagonist	Opposes or reverses movement	Triceps brachii (elbow extension)
Synergist	Assists prime mover	Brachialis (assists elbow flexion)
Fixator	Stabilizes origin	Muscles stabilizing scapula

Additional info: Academic context was added to expand brief points and tables, ensuring completeness and clarity for exam preparation.