The Muscular System

Introduction

The muscular system is a fundamental component of human anatomy and physiology, responsible for movement, posture, and various bodily functions. This chapter focuses on the structure, classification, and function of skeletal muscles, including their arrangement, mechanics, naming conventions, and major muscle groups.

Fascicle Arrangement in Skeletal Muscles

Overview of Fascicle Arrangements

Skeletal muscle fibers are organized into bundles called fascicles. The pattern of fascicle arrangement determines the muscle's shape and functional capabilities. Muscles are classified into four main types based on fascicle arrangement:

• Parallel muscles

• Convergent muscles

• Pennate muscles

• Circular muscles

Parallel Muscles

In parallel muscles, fascicles run parallel to the long axis of the muscle. These muscles can be flat or spindle-shaped, often with a central body called the muscle belly.

• Key Features: Entire muscle can shorten up to 30% during contraction.

• Examples: Rectus abdominis, biceps brachii

Example: The biceps brachii is a parallel muscle that flexes the forearm.

Convergent Muscles

Convergent muscles have fascicles that spread out over a broad area and converge on a single attachment site, such as a tendon, aponeurosis, or raphe (a slender band of collagen fibers).

• Key Features: Fibers can pull in different directions depending on which fibers are active.

• Example: Pectoralis major

Example: The pectoralis major can flex, adduct, and rotate the arm depending on which fascicles contract.

Pennate Muscles

Pennate muscles have fascicles that form a common angle with the tendon, resembling a feather. They contain more myofibrils than parallel muscles and can generate more tension but move their tendons less distance.

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

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

• Multipennate: Tendon branches within the muscle (e.g., deltoid).

Circular Muscles (Sphincters)

Circular muscles have fascicles concentrically arranged around an opening. They act as valves to open and close passages in the body.

• Key Features: Contraction decreases the diameter of the opening.

• Example: Orbicularis oris (mouth sphincter)

Muscle Mechanics: Levers

Levers in the Musculoskeletal System

Most skeletal muscles attach to bones and act as levers to produce movement. A lever is a rigid structure (bone) that moves on a fixed point called the fulcrum (joint). Muscles provide an applied force to overcome a load.

• Levers can change:

  • Direction of applied force

  • Distance and speed of movement

  • Effective strength of the force

• Three classes of levers:

  • First-class lever

  • Second-class lever

  • Third-class lever

First-Class Lever

The fulcrum is between the applied force and the load, similar to a seesaw or crowbar.

• Example: Extension of the neck and lifting the head

Second-Class Lever

The load is between the applied force and the fulcrum, similar to a wheelbarrow. A small force can move a large weight.

• Example: Plantar flexion at the ankle by calf muscles

Third-Class Lever

The applied force is between the load and the fulcrum, similar to a pair of tongs. This is the most common lever type in the body, maximizing speed and distance at the expense of force.

• Example: Elbow flexion by the biceps brachii

Origins, Insertions, and Actions of Muscles

Definitions

• Origin: The fixed (less movable) point of attachment of a muscle to bone, usually proximal.

• Insertion: The more movable point of attachment, usually distal.

• Action: The specific movement produced by muscle contraction (e.g., flexion, adduction).

Actions are described in terms of their effect on bones or joints (e.g., flexion of the forearm or flexion at the elbow).

Functional Types of Muscles

• Agonist (Prime Mover): Main muscle responsible for a particular movement.

• Antagonist: Opposes the movement of the agonist.

• Synergist: Assists the agonist.

• Fixator: A synergist that stabilizes the origin of the agonist.

Agonists and antagonists work in pairs, such as flexors-extensors and abductors-adductors.

Naming Skeletal Muscles

Descriptive Information in Muscle Names

Muscle names often provide clues about their location, appearance, or function. The following categories are commonly used:

• Region of the body: e.g., temporalis (temple), femoris (thigh)

• Position, direction, or fascicle arrangement: e.g., externus (superficial), rectus (straight)

• Structural characteristics: origin/insertion, number of tendons, shape, size

• Action: e.g., flexor, extensor, abductor

Examples of Regional Terms

• Abdominal (abdomen), Brachial (arm), Temporal (temple), Popliteal (back of knee)

• Additional info: Other terms include glossal (tongue), hallux (great toe), palpebra (eyelid)

Examples of Position/Direction Terms

• Externus (superficialis): Visible at body surface

• Internus (profundus): Deeper muscles

• Transversus: Runs across the longitudinal axis

• Oblique: Runs at a slant

• Rectus: Runs straight along the axis

Structural Characteristics

• Origin and insertion: e.g., genioglossus (origin: chin, insertion: tongue)

• Number of tendons: biceps (two heads), triceps (three heads), quadriceps (four heads)

• Shape: deltoid (triangle), rhomboid (parallelogram), orbicularis (circle)

• Size: maximus (largest), minimus (smallest), longus (long), brevis (short)

Action Terms

• Abductor: Moves limb away from midline

• Adductor: Moves limb toward midline

• Flexor: Bends a joint

• Extensor: Straightens a joint

• Pronator: Turns palm downward

• Supinator: Turns palm upward

Axial Muscles

Overview

Axial muscles are grouped based on location and function. They include muscles of the head and neck, vertebral column, trunk (oblique and rectus muscles), and pelvic floor.

Muscles of the Head and Neck

• Orbicularis oris: Purses lips, closes mouth

• Buccinator: Compresses cheeks, aids in chewing and nursing

• Temporoparietalis: Tenses scalp, moves ears

• Occipitofrontalis: Raises eyebrows, wrinkles forehead, retracts scalp

• Platysma: Tenses skin of neck, depresses mandible

• Zygomaticus major: Elevates corners of mouth (smiling)

• Orbicularis oculi: Closes eyes

• Levator palpebrae superioris: Elevates upper eyelids

Extrinsic Eye Muscles

• Inferior rectus: Moves eye downward

• Medial rectus: Moves eye medially

• Superior rectus: Moves eye upward

• Lateral rectus: Moves eye laterally

• Inferior oblique: Moves eye upward and laterally

• Superior oblique: Moves eye downward and laterally

Muscles of Mastication

• Masseter: Elevates mandible, closes jaws

• Temporalis: Elevates mandible

• Pterygoid muscles: Elevate, depress, protract, and move mandible side to side

Muscles of the Tongue

• Palatoglossus: Elevates tongue, depresses soft palate

• Styloglossus: Retracts tongue, elevates sides

• Genioglossus: Depresses and protracts tongue

• Hyoglossus: Depresses and retracts tongue

Muscles of the Pharynx

• Pharyngeal constrictors: Move food into esophagus

• Palatal muscles: Elevate soft palate, open auditory tube

• Laryngeal elevators: Raise the larynx

Muscles of the Neck

• Digastric: Controls position of larynx

• Mylohyoid: Elevates floor of mouth and hyoid, depresses mandible

• Geniohyoid: Depresses mandible or elevates larynx

• Sternocleidomastoid: Flexes neck, rotates head

• Omohyoid and sternohyoid: Depress hyoid and larynx

Muscles of the Trunk

• External and internal oblique: Compress abdomen, depress ribs, flex spine

• Transversus abdominis: Compresses abdomen

• Rectus abdominis: Depresses ribs, flexes vertebral column, compresses abdomen

• Diaphragm: Expands thoracic cavity, compresses abdomen; major muscle of breathing

Appendicular Muscles

Muscles of the Shoulders and Upper Limbs

• Trapezius: Elevates clavicle, moves scapula, extends neck

• Serratus anterior: Protracts shoulder

• Subclavius: Depresses and protracts shoulder

• Pectoralis minor: Depresses and protracts shoulder, elevates ribs

• Rhomboid major/minor: Adduct and rotate scapula downward

• Levator scapulae: Elevates scapula

Muscles that Move the Arm

• Pectoralis major: Flexion, adduction, medial rotation at shoulder

• Latissimus dorsi: Extension, adduction, medial rotation at shoulder

• Rotator cuff: Supraspinatus, infraspinatus, teres minor, subscapularis (stabilize shoulder)

Muscles that Move the Forearm and Hand

• Triceps brachii: Principal extensor at elbow

• Anconeus: Extensor at elbow

• Biceps brachii: Principal flexor at elbow, supinates forearm, stabilizes shoulder

• Brachialis and brachioradialis: Flexors at elbow

Muscles of the Pelvis and Lower Limbs

• Gluteus maximus: Extends and laterally rotates hip

• Gluteus medius/minimus: Abduct and medially rotate hip

• Tensor fasciae latae: Lateral rotation of leg, pulls on iliotibial tract

Muscles that Move the Leg

• Flexors of the knee (Hamstrings): Biceps femoris, semitendinosus, semimembranosus, sartorius, popliteus

• Extensors of the knee (Quadriceps femoris): Rectus femoris, vastus intermedius, vastus lateralis, vastus medialis

Muscles that Move the Foot

• Gastrocnemius: Knee flexion, foot inversion, plantar flexion

• Soleus: Plantar flexion

• Fibularis brevis/longus: Foot eversion, plantar flexion

• Tibialis posterior: Foot adduction, inversion, plantar flexion

• Calcaneal tendon (Achilles tendon): Shared by gastrocnemius and soleus

Summary Table: Fascicle Arrangements

Key Equations

• Lever Law:

Additional info: These notes provide a comprehensive overview of the muscular system, suitable for exam preparation in a college-level Anatomy & Physiology course.