Skeletal Muscle Structure, Function, and Major Muscle Groups

Structure of Skeletal Muscle

Fascicles and Muscle Shapes

Skeletal muscles are composed of bundles of muscle cells called fascicles. The arrangement of fascicles determines the muscle's shape and function. Several fascicle arrangements are found in the human body:

• Parallel arrangement: Fascicles run parallel to the long axis of the muscle, producing a strap-like shape. Example: sartorius.

• Convergent arrangement: Fascicles converge from a broad origin to a single tendon. Example: pectoralis major.

• Pennate arrangement: Fascicles attach obliquely to a central tendon, resembling a feather. Types include:

  • Unipennate: Fascicles attach to one side of the tendon.

  • Bipennate: Fascicles attach to both sides of the tendon.

  • Multipennate: Several fascicle regions join to form a single tendon.

• Sphincter (circular) arrangement: Fascicles are arranged in concentric rings, forming muscles that close openings. Example: orbicularis oris.

• Spiral arrangement: Fascicles wrap around another structure. 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, or function. Naming conventions include:

• Location (e.g., temporalis near the temporal bone)

• Shape (e.g., deltoid is triangular)

• Size (e.g., maximus for largest, minimus for smallest)

• Direction of fibers (e.g., rectus for straight)

• Number of origins (e.g., biceps has two origins)

• Attachment points (e.g., sternocleidomastoid attaches to sternum, clavicle, and mastoid)

• Action (e.g., flexor, extensor, adductor, levator)

Functions of Skeletal Muscles

Movement and Other Functions

Skeletal muscle contractions are responsible for movement of bones and joints, but also serve other functions:

• Generate heat as a by-product, helping maintain body temperature.

• Contraction of the diaphragm is essential for respiration.

• Facial muscles allow for facial expression.

• Sphincter muscles control openings and closings (e.g., swallowing).

Functional Groups of Muscles

Muscles often work in groups to produce movement. Key terms include:

• Agonist (prime mover): Main muscle responsible for a movement.

• Antagonist: Muscle that opposes the action of the agonist.

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

• Fixator: Stabilizes the origin of the agonist for efficient movement.

Muscle Attachments and Lever Systems

Muscles attach to bones at two main points:

• Origin: The fixed attachment point, usually proximal.

• Insertion: The movable attachment point, usually distal.

Muscle action often involves lever systems, which consist of:

• Load (resistance): The weight or force to be moved.

• Applied force (effort): The force exerted by the muscle.

• Fulcrum: The pivot point (joint).

Lever classes:

• First-class lever: Fulcrum between load and applied force. Example: neck muscles raising the head. $\text{Effort} - \text{Fulcrum} - \text{Load}$

• Second-class lever: Load between fulcrum and applied force. Example: standing on tiptoe. $\text{Fulcrum} - \text{Load} - \text{Effort}$

• Third-class lever: Applied force between fulcrum and load. Example: biceps flexing the forearm. $\text{Fulcrum} - \text{Effort} - \text{Load}$

Major Muscle Groups and Their Functions

Muscles of Facial Expression

These muscles insert into the skin and connective tissue of the face, allowing for facial expressions.

• Epicranius aponeurosis: Connects frontal and occipital muscles.

• Orbicularis oris: Closes lips.

• Levator palpebrae superioris: Raises upper eyelid.

• Corrugator supercilii: Draws eyebrows together.

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

• Buccinator: Compresses cheek (chewing).

• Platysma: Tenses skin of neck.

Extrinsic Eye Muscles

Six muscles control eye movement:

• Superior, inferior, medial, and lateral rectus: Move the eye up, down, medially, and laterally.

• Superior oblique: Rotates eye downward and laterally.

• Inferior oblique: Rotates eye upward and laterally.

Muscles of Mastication and Swallowing

• Masseter and temporalis: Elevate the mandible for chewing.

• Medial and lateral pterygoid: Move jaw side-to-side and forward.

• Digastric, stylohyoid, mylohyoid, geniohyoid: Push food toward the pharynx.

• Genioglossus, hyoglossus, styloglossus: Manipulate and push food with the tongue.

Muscles of the Vertebral Column

• Erector spinae: Extends and laterally bends the spine.

• Spinalis, longissimus, iliocostalis: Subgroups of erector spinae, each with specific actions.

• Transversospinalis group: Includes semispinalis, multifidus, rotatores; stabilize and move vertebral column.

• Quadratus lumborum: Laterally flexes the spine.

Muscles of the Trunk and Pelvic Floor

Muscles of Respiration

• Diaphragm: Main muscle of inspiration.

• Intercostal muscles: External and internal intercostals assist with breathing.

Abdominal Muscles

• Rectus abdominis: Flexes vertebral column.

• External and internal obliques: Rotate and laterally flex trunk.

• Transversus abdominis: Compresses abdominal contents.

• Linea alba: Connective tissue line separating left and right rectus abdominis.

Muscles of the Pelvic Floor, Urogenital Diaphragm, and Perineum

• Levator ani and coccygeus: Support pelvic organs.

• External urethral sphincter: Controls urination.

• Deep transverse perineal muscle: Supports pelvic floor.

• External anal sphincter: Controls defecation.

• Bulbospongiosus and ischiocavernosus: Found in the perineum, involved in sexual function.

Muscles of the Pectoral Girdle and Upper Limb

• Serratus anterior, pectoralis minor, trapezius, levator scapulae, rhomboid major/minor: Move the scapula.

• Pectoralis major, latissimus dorsi, deltoid, teres major/minor, supraspinatus, infraspinatus, subscapularis: Move the arm at the shoulder joint.

Table: Muscle Arrangements and Examples

Arrangement	Description	Example
Parallel	Fascicles run parallel to long axis	Sartorius
Convergent	Fascicles converge to a single tendon	Pectoralis major
Unipennate	Fascicles attach to one side of tendon	Extensor digitorum longus
Bipennate	Fascicles attach to both sides of tendon	Rectus femoris
Multipennate	Multiple fascicle regions join to form tendon	Deltoid
Circular (Sphincter)	Fascicles arranged in rings	Orbicularis oris
Fusiform	Spindle-shaped muscle	Biceps brachii

Additional info: Lever systems in the body are important for understanding how muscles produce movement efficiently. The arrangement of muscle attachments and the type of lever affect the force and speed of movement. Muscles of the trunk and pelvic floor are essential for core stability, posture, and functions such as breathing, urination, and defecation.