Skeletal Muscle Structure and Function: Anatomy & Physiology Study Notes

Structure of Skeletal Muscle

Fascicles and Muscle Shapes

Skeletal muscles are composed of bundles of muscle fibers called fascicles. The arrangement of these fascicles determines the muscle's shape and function. Different arrangements allow for various types of movement and force generation.

• Parallel arrangement: Fascicles run parallel to the long axis of the muscle, resulting in muscles that contract over a great distance but are not very strong. Example: Sartorius.

• Convergent arrangement: Fascicles converge from a broad origin to a single tendon. This allows for versatile movement. Example: Pectoralis major.

• Pennate arrangement: Fascicles are short and attach obliquely to a central tendon. Types include:

  • Unipennate: Fascicles insert into one side of the tendon. Example: Extensor digitorum longus.

  • Bipennate: Fascicles insert into both sides of the tendon. Example: Rectus femoris.

  • Multipennate: Several tendons with fascicles arranged around them. Example: Deltoid.

• Sphincters: Fascicles are arranged in concentric rings, forming a circular muscle. Example: Orbicularis oris.

• Spiral arrangement: Fascicles twist between their points of attachment. Example: Supinator.

• Fusiform arrangement: Muscle is spindle-shaped with a thick belly and tapered ends. Example: Biceps brachii.

Naming Muscles

Muscles are named based on their appearance, size, location, function, or the structures they are associated with. Names may use Greek or Latin roots.

• Some names describe shape (e.g., deltoid = triangle).

• Others describe location (e.g., pectoralis = chest).

• Some are named for their action (e.g., flexor, extensor).

Functions of Skeletal Muscles

Movement and Other Functions

Skeletal muscles do more than move bones; they also play roles in posture, heat production, and protection.

• Movement: Muscles contract to move bones at joints.

• Heat production: Muscle contractions generate heat, helping maintain body temperature.

• Respiration: Muscles like the diaphragm are essential for breathing.

• Facial expression: Muscles attached to the skin allow for facial movements.

• Swallowing and sphincter control: Muscles in the throat and sphincters control swallowing and the opening/closing of body orifices.

Functional Groups of Muscles

Muscles often work in groups to perform movements. The main roles are:

• Agonist (Prime mover): The main muscle responsible for a movement.

• Antagonist: Opposes the action of the agonist, allowing for modulation and control.

• Synergist: Assists the agonist by adding force or reducing unnecessary movement.

• Fixator: Stabilizes the origin of the agonist muscle.

Muscle Attachments

Muscles attach to bones at two main points:

• Origin: The fixed attachment, usually proximal.

• Insertion: The movable attachment, usually distal.

Levers in the Musculoskeletal System

Muscles and bones act together as levers to produce movement. A lever system includes:

• Load (Resistance): The weight to be moved.

• Effort (Applied force): The force applied by the muscle.

• Fulcrum (Pivot point): The joint around which movement occurs.

There are three classes of levers:

• First-class lever: Fulcrum is between the load and the effort. Example: Neck extension. $\text{Effort} - \text{Fulcrum} - \text{Load}$

• Second-class lever: Load is between the fulcrum and the effort. Example: Standing on tiptoe. $\text{Fulcrum} - \text{Load} - \text{Effort}$

• Third-class lever: Effort is between the fulcrum and the load. Example: Biceps curl. $\text{Fulcrum} - \text{Effort} - \text{Load}$

Muscles of the Face, Head, and Neck

Muscles of Facial Expression

These muscles insert into the skin and allow for facial movements.

• Epicranius (epicranial aponeurosis): Raises eyebrows and wrinkles forehead.

• Orbicularis oculi: Closes the eyelids.

• Orbicularis oris: Closes and protrudes the lips.

• Zygomaticus major/minor: Raises the corners of the mouth (smiling).

• Buccinator: Compresses the cheek (as in blowing or sucking).

• Platysma: Tenses skin of the neck and depresses the mandible.

Muscles of Mastication (Chewing)

These muscles move the jaw for chewing and biting.

• Masseter and temporalis: Elevate the mandible.

• Medial pterygoid: Elevates and protrudes the mandible.

• Lateral pterygoid: Protracts and moves the mandible side-to-side.

Muscles of Swallowing

These muscles move food from the mouth to the pharynx and esophagus.

• Suprahyoid muscles: Elevate the hyoid bone and larynx during swallowing.

• Infrahyoid muscles: Depress the hyoid bone and larynx after swallowing.

• Pharyngeal constrictors: Constrict the pharynx to move food into the esophagus.

Muscles That Move the Head and Neck

• Sternocleidomastoid: Flexes and rotates the head.

• Scalenes: Assist in neck flexion and lateral movement.

• Splenius capitis and cervicis: Extend and rotate the head.

Muscles of the Trunk and Pelvic Floor

Muscles of Respiration

• Diaphragm: Main muscle of inspiration.

• External intercostals: Elevate ribs during inspiration.

• Internal intercostals: Depress ribs during forced expiration.

Abdominal Muscles

• Rectus abdominis: Flexes the vertebral column.

• External oblique: Rotates and laterally flexes the trunk.

• Internal oblique: Same as external oblique, but deeper.

• Transversus abdominis: Compresses abdominal contents.

• Linea alba: Midline connective tissue separating left and right rectus abdominis.

Muscles of the Pelvic Floor and Perineum

• Levator ani: Supports pelvic organs.

• Coccygeus: Supports pelvic floor.

• External anal sphincter: Voluntary control of defecation.

• External urethral sphincter: Voluntary control of urination.

Muscles of the Pectoral Girdle and Upper Limb

Muscles That Move the Scapula

• Serratus anterior: Protracts and stabilizes the scapula.

• Pectoralis minor: Depresses and protracts the scapula.

• Trapezius: Elevates, retracts, and rotates the scapula.

• Rhomboids (major and minor): Retract the scapula.

Muscles That Move the Arm at the Shoulder Joint

• Pectoralis major: Flexes, adducts, and medially rotates the arm.

• Latissimus dorsi: Extends, adducts, and medially rotates the arm.

• Deltoid: Abducts the arm.

• Rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis): Stabilize and move the shoulder joint.

Muscles That Move the Forearm

• Biceps brachii: Flexes the elbow and supinates the forearm.

• Brachialis: Flexes the elbow.

• Triceps brachii: Extends the elbow.

Muscles That Move the Wrist, Hand, and Fingers

• Flexor group: Flexes the wrist and fingers.

• Extensor group: Extends the wrist and fingers.

Summary Table: Muscle Arrangements and Examples

Arrangement	Description	Example
Parallel	Fascicles run parallel to muscle's long axis	Sartorius
Convergent	Fascicles converge to a single tendon	Pectoralis major
Unipennate	Fascicles insert into one side of tendon	Extensor digitorum longus
Bipennate	Fascicles insert into both sides of tendon	Rectus femoris
Multipennate	Fascicles insert into multiple tendons	Deltoid
Sphincter	Fascicles arranged in concentric rings	Orbicularis oris
Fusiform	Spindle-shaped muscle with thick belly	Biceps brachii

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard Anatomy & Physiology content.