Overview and Functions of Skeletal Muscles: Anatomy & Physiology Study Notes

Module 9.1 Overview of Skeletal Muscles

Structure of Skeletal Muscle

The shape, size, placement, and arrangement of fibers in a skeletal muscle contribute to the function of that muscle. Muscle structure is closely related to its function.

• Fascicles and Muscle Shapes: Fascicles are bundles of muscle cells whose specific arrangement affects both the appearance and function of the whole skeletal muscle. The following are the different arrangements in which fascicles are found in the human body:

  • Parallel arrangement: Fascicles form a strap-like muscle with evenly spaced fascicles where muscle and tendon are the same width. Example: Sartorius muscle

  • Convergent arrangement: Fascicles form a broad triangular-shaped muscle that tapers down into a single tendon. Example: Pectoralis major

  • Pennate arrangement: Fascicles attach to the tendon at an angle, giving a feather-like appearance. Types include:

    • Unipennate: Fascicles are only attached to one side of the associated tendon.

    • Bipennate: Fascicles are attached to both sides of the associated tendon.

    • Multipennate: Several regions of fascicles joined by connective tissue, each section contributes to forming a single tendon.

  • Sphincters: Circular fascicle arrangements that surround body openings, allowing for closing and opening actions. Example: Orbicularis oris

  • Spiral arrangement: Fascicles wrap around another structure, such as a bone.

How Muscles Are Named

Muscles are named based on appearance, size, position, or function. Some muscles have retained historical names. Naming conventions include:

• Location using directional terms (e.g., anterior, posterior).

• Anatomical structures where they are attached.

• Actions performed when contracted (e.g., flexors, extensors, abductors, adductors, levators).

Functions of Skeletal Muscles

Muscle contractions are involved in more than just the movement of bones at joints. Other functions include:

• Movement and Other Functions:

  • Muscle contractions generate heat as a by-product, helping maintain body temperature.

  • Diaphragm contraction is vital for respiration.

  • Facial muscles assist with swallowing and expression.

  • Sphincters control the opening and closing of body openings.

• Functional Groups of Muscles: Muscles often work in groups to perform movements.

  • Agonists (prime movers): Provide most of the force for a given muscle action.

  • Antagonists: Oppose the action of the agonist, allowing for modulation and control.

  • Synergists: Assist the agonist by supplying supplemental force and stabilizing joints.

  • Fixators: Stabilize the joint, protecting it from injury due to unnecessary movements.

• Muscle Origin and Insertion:

  • Origin: The anchoring point on a bone, typically not involved directly with joint movement.

  • Insertion: The moving end of the muscle, where the tendon attaches to a bone or other structure.

• Levers in Muscle Action: Muscles act on bones as levers. The three components of a lever system are:

  1. Load or resistance

  2. Applied force (effort)

  3. Pivot point (fulcrum)

  Types of levers:

  • First-class lever: The Fulcrum is between the load and the applied force. The load moves in the opposite direction from the applied force. $\text{Fulcrum} \rightarrow \text{Load} \rightarrow \text{Applied Force}$

  • Second-class lever: The Fulcrum is at one end, the load is between the fulcrum and the applied force. The load moves in the same direction as the applied force.

  • Third-class lever: The Fulcrum and applied force are close together, load is at the other end. The load moves in the same direction as the applied force.

Module 9.2 Muscles of the Head, Neck, and Vertebral Column

Muscles of Facial Expression

These muscles insert into the skin and connective tissue of the face to produce subtle changes in facial structure.

• Epicranius aponeurosis: Connective tissue linking the frontal and occipitalis muscles, raising the eyebrows and skin of the forehead.

• Muscles around the eyes: Orbicularis oculi, levator palpebrae superioris, corrugator supercilii

• Muscles around the mouth: Zygomaticus major, zygomaticus minor, levator labii superioris, risorius

• Depressor muscles: Depressor anguli oris, depressor labii inferioris, mentalis

• Buccinator: Assists in manipulating food while chewing.

• Platysma: Pulls the chin down, helps open the mouth.

Extrinsic Eye and Orbit Muscles

These muscles attach to the outer surface of the eye and control its movement.

• Rectus muscles: Superior, inferior, medial, lateral

  • Superior rectus: Moves the eye upward.

  • Inferior rectus: Moves the eye downward.

  • Lateral rectus: Moves the eye laterally.

  • Medial rectus: Moves the eye medially.

• Oblique muscles: Superior and inferior oblique

  • Superior oblique: Rotates the eye downward and laterally.

  • Inferior oblique: Rotates the eye upward and laterally.

Muscles of the Head and Neck

This group of muscles moves the head and neck and assists with chewing and swallowing.

• Muscles of chewing (mastication):

  • Masseter and temporalis: Elevate the mandible for biting and chewing.

  • Medial and lateral pterygoid: Assist with elevation, protraction, and side-to-side movement of the mandible.

• Muscles of swallowing:

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

  • Tongue muscles: Genioglossus, hyoglossus, styloglossus

  • Pharyngeal constrictors: Sternohyoid, sternothyroid, omohyoid, thyrohyoid, pharyngeal constrictor muscles

• Muscles that move the head and neck: Sternocleidomastoid, scalenes, splenius capitis

Module 9.3 Muscles of the Trunk and Pelvic Floor

Muscles of Ventilation

These muscles are involved in breathing and are grouped by anatomical proximity and function.

• Muscles of respiration: Diaphragm, internal intercostals, external intercostals

Abdominal Muscles

These muscles support the trunk and assist in movements such as flexion, rotation, and compression of the abdomen.

• Rectus abdominis, external obliques, internal obliques, transversus abdominis

• Linea alba: Connective tissue at the midline separating left and right abdominal muscles.

Muscles of the Pelvic Floor, Urogenital Diaphragm, and Perineum

These muscles support pelvic organs and control urination and defecation.

• Pelvic floor muscles: Pubococcygeus, iliococcygeus, coccygeus

• Urogenital diaphragm: External urethral sphincter, deep transverse perineal, superficial transverse perineal

• External anal sphincter: Controls voluntary defecation.

• Bulbospongiosus, ischiocavernosus: Found in the perineum, associated with external genitalia.

Module 9.4 Muscles of the Pectoral Girdle and Upper Limb

Muscles That Move the Scapula at the Pectoral Girdle

These muscles attach to the scapula and clavicle, moving the shoulder and arm.

• Serratus anterior, pectoralis minor, trapezius, levator scapulae, rhomboid major, rhomboid minor

Muscles That Move the Arm at the Shoulder Joint

These muscles attach to the humerus and move the arm at the shoulder.

• Deltoid, pectoralis major, latissimus dorsi, teres major, coracobrachialis

Table: Muscle Arrangements and Functions

Arrangement	Example Muscle	Function
Parallel	Sartorius	Uniform contraction, flexibility
Convergent	Pectoralis major	Versatile movement, strength
Pennate (Unipennate, Bipennate, Multipennate)	Rectus femoris, deltoid	Increased force production
Sphincter	Orbicularis oris	Control of openings
Spiral	Supinator	Rotation

Key Equations

• Lever Law: $\text{Force} \times \text{Distance from Fulcrum} = \text{Load} \times \text{Distance from Fulcrum}$

Additional info: Academic context and examples have been expanded for clarity and completeness. Table entries and some muscle functions have been inferred and grouped for study purposes.