Muscle Anatomy Notes

The Functions of Muscles

  • Muscle Functions:
    • Movement:
    • Muscles help in moving from place to place, body parts, and contents such as air during breathing, blood during circulation, and food during digestion.
    • Also plays a significant role in communication through speech, writing, and facial expressions.
    • Stability:
    • Maintains posture and prevents unwanted movements, particularly through antigravity muscles which prevent falls.
    • Stabilizes joints by maintaining tension in muscles around them.
    • Control of Openings and Passageways:
    • Sphincters: Internal muscular rings that control the movement of substances (e.g., food, blood) within the body.
    • Heat Production:
    • Skeletal muscles generate as much as 85% of body heat through their activity.
    • Glycemic Control:
    • Muscles absorb and store glucose, regulating blood sugar levels within a normal range.

Structural and Functional Organization of Muscles

  • Muscle Count:

    • There are approximately 600 human skeletal muscles, constituting about 50% of body weight.

  • Types of Muscle Tissue:

    • Skeletal Muscles: Mostly voluntary, responsible for movement.
    • Cardiac Muscles: Involuntary muscles found in the heart.
    • Smooth Muscles: Involuntary muscles found in walls of hollow organs (e.g., intestines, blood vessels).

  • Myology: The study of the muscular system.

Muscle Connective Tissues, Fascicles, and Compartments

  • Epimysium:

    • A fibrous sheath surrounding the entire muscle; outer surface transitions to fascia, while inner projections form the perimysium.

  • Fascia:

    • A sheet of connective tissue that separates muscles or muscle groups.

  • Endomysium:

    • A thin sleeve of loose connective tissue surrounding each muscle fiber, allowing space for capillaries and nerve fibers, and providing a chemical environment for the muscle fibers.

  • Perimysium:

    • A thicker connective tissue layer that wraps bundles of muscle fibers known as fascicles.

Classification of Muscles According to Fascicle Orientation

  • Fascicle Orientation Influences Strength:

    • Fusiform Muscles: Thick in the middle and tapered at each end.
    • Parallel Muscles: Uniform width with parallel fascicles.
    • Triangular (Convergent) Muscles: Broad at one end, narrow at the other.

  • Pennate Muscles:

    • Unipennate: Fascicles approach the tendon from one side.
    • Bipennate: Fascicles approach the tendon from both sides.
    • Multipennate: Bundles of feathers converge to a single point.

  • Circular Muscles (Sphincters): Form rings around body openings.

Muscle Compartmentalization

  • Muscle Compartment:

    • A group of muscles that are functionally related, enclosed by fascia along with nerves and blood vessels that supply them.

  • Intermuscular Septa: Thick fascia that separate muscle compartments from one another.

Muscle Attachments

  • Indirect Attachment:

    • Tendons connect muscles to bones; collagen fibers of muscle connective tissue transform into tendons and attach to bone periosteum.
    • Aponeurosis: A broad, flat tendon (e.g., palmar aponeurosis).
    • Retinaculum: A connective tissue band that tendons of separate muscles pass under.

  • Direct Attachment:

    • Minimal separation between muscle and bone, appearing as if the muscle emerges directly from the bone.

  • Attaching to Other Structures:

    • Some muscles can attach to fascia or the tendons of other muscles (e.g., facial muscles may attach to the skin).

Muscle Types Based on Function

  • Intrinsic Muscles: Entirely contained within a specific region (e.g., muscles of the hands).
  • Extrinsic Muscles: Act on a designated region but have attachments elsewhere (e.g., forearm muscles control finger movements).

Functional Groups of Muscles

  • Categories of Muscle Action:
    • Prime Mover (Agonist): Muscle that produces the most force during a specific action.
    • Synergist: Assists the prime mover and helps stabilize nearby joints.
    • Antagonist: Opposes the prime mover, helps to prevent excessive movement and sometimes provides control over motion.
    • Fixator: Prevents movement of bones, stabilizing references during muscle actions.

Muscle Innervation

  • Innervation: Refers to the identity of the nerve that stimulates a muscle, crucial for diagnosing injuries through muscle tests.

  • Spinal Nerves:

    • Emerge from the spinal cord, split into posterior and anterior rami, innervating muscles below the neck.
    • Plexus: A network of spinal nerves adjacent to the vertebral column.

  • Cranial Nerves:

    • Emerge from the brain, innervating head and neck muscles, numbered CN I to CN XII.

Blood Supply to Muscles

  • The muscular system receives 1.24 L of blood per minute at rest, accounting for about one-quarter of the heart's output; during intense exercise, it can increase to over 11.6 L/min.
  • Capillaries extensively branch through the endomysium to supply individual muscle fibers.

Muscle Naming Conventions

  • Muscles are typically named using Latin terms that describe:
    • Distinctive structural features, location, or function (e.g., tibialis anterior, sternocleidomastoid).