Overview of Skeletal Muscle Attachments and Structure

Muscle Attachments

Skeletal muscles typically span joints and are attached to bones at two or more points. These attachments are essential for movement and stability.

• Origin: The immovable (or less movable) bone to which the muscle is attached.

• Insertion: The movable bone where the muscle exerts its action.

• Most muscle attachments are indirect, involving either tendons or an aponeurosis (a flat, broad tendon-like sheet).

Arrangement of Fascicles

All skeletal muscles are composed of bundles of muscle fibers called fascicles. The arrangement of these fascicles determines the muscle's shape and function. There are four basic patterns:

• Parallel: Fascicles run parallel to the long axis of the muscle. Can be straplike (e.g., sartorius) or fusiform (e.g., biceps brachii).

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

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

  • Bipennate: Fascicles insert into the tendon from both sides (e.g., rectus femoris).

  • Multipennate: Fascicles attach from many directions to several tendons (e.g., deltoid).

• Convergent: Muscle is triangular or fan-shaped, with fascicles converging toward a single tendon (e.g., pectoralis major).

• Circular: Fascicles are arranged in concentric rings (e.g., orbicularis oris around the mouth).

Functional Groups of Muscles

Prime Movers, Antagonists, and Synergists

Muscles are grouped by their functional roles during movement. A single muscle may serve different roles depending on the movement.

1. Prime Mover (Agonist): The muscle that provides the major force for producing a specific movement. Example: Biceps brachii in elbow flexion.

2. Antagonist: The muscle that opposes or reverses the movement of the prime mover. It can provide resistance to prevent overshoot or control the rate of movement. Example: Triceps brachii is the antagonist to biceps brachii during elbow flexion.

3. Synergist: Muscles that help the prime mover by:

   • Promoting the same movement.

   • Reducing undesirable or unnecessary movements (e.g., finger flexor muscles stabilize the wrist when making a fist).

Criteria for Naming Skeletal Muscles

Muscle names often describe key features, including shape, size, location, and function.

• Location: Named for the bone or region with which they are associated (e.g., temporalis muscle over the temporal bone).

• Shape: Describes the muscle's shape (e.g., deltoid is triangular; right and left trapezius muscles form a trapezoid).

• Relative Size: Terms like maximus (largest), minimus (smallest), longus (long), brevis (short) (e.g., gluteus maximus vs. gluteus minimus).

• Direction of Muscle Fibers: Named for the orientation of fibers relative to the body's midline (e.g., rectus = straight, parallel to midline; transversus abdominis, oblique).

• Number of Origins: Prefixes indicate the number of origins (e.g., biceps = two, triceps = three, quadriceps = four).

• Location of Attachments: Named for the origin and insertion (e.g., sternocleidomastoid: origin on sternum and clavicle, insertion on mastoid process of temporal bone).

• Action: Named for the movement produced (e.g., flexor, extensor, adductor).

Muscles of the Head: Facial Expression

General Features

• Insert into skin or other muscles, not bones.

• Innervated by the facial nerve.

Major Muscles

• Epicranius: Main muscle of the scalp, consisting of:

  • Frontal belly: Raises eyebrows, wrinkles forehead.

  • Occipital belly: Pulls scalp posteriorly.

• Orbicularis oculi: Surrounds rim of orbit; protects eyes from light and injury by closing eyelids.

• Mentalis: Protrudes lower lip (pout), wrinkles chin.

• Buccinator: Deep to masseter; whistling, sucking, holds food in place during chewing, especially in nursing infants.

• Platysma: Helps depress mandible; tenses skin of neck.

Muscles That Move the Tongue

• Genioglossus: (mandible to inferior tongue & hyoid bone) – Prime mover of tongue protrusion; anchors tongue to prevent airway obstruction.

• Hyoglossus: (hyoid bone to inferolateral tongue) – Depresses tongue, especially lateral margins.

• Styloglossus: (styloid process of temporal bone to inferolateral tongue) – Retracts and elevates tongue.

Summary Table: Muscle Naming Criteria

Key Equations and Concepts

• Muscle Action: Movement occurs when a muscle contracts and its insertion moves toward its origin.

• Lever Systems: Muscles act on bones as levers to produce movement. The arrangement of the fulcrum, effort, and load determines the type of lever and mechanical advantage.

Example Equation (Lever Law):

Additional info: The notes also reference the importance of muscle fiber arrangement in determining the range of motion and power of a muscle. Parallel muscles have greater range of motion, while pennate muscles generate more power due to a higher number of fibers packed into a given length.