9.1: Overview of Skeletal Muscles

I. Structure of a Skeletal Muscle

A. Contains blood vessels that supply muscle cells with oxygen and glucose and remove wastes, as well as nerves that coordinate muscle contraction.

B. Also contains connective tissue.

  1. Endomysium - a thin layer of extracellular matrix that surrounds each individual muscle cell (fiber).

  2. Perimysium - bundles together several muscle cells into a fascicle.

  3. Epimysium - encloses all fascicles that make up a muscle.

  4. Fascia: is continuous with the epimysium, which separates different muscles

C. Interconnected connective tissues taper down and connect to tendons or other connective tissues, attaching muscle to bone or other structures to be moved.

II. Fascicles and Muscle Shapes

A. Fascicles – bundles of 10 to 100 muscle cells whose specific arrangement affects both the appearance and function of the whole skeletal muscle.

B. Shapes of a Skeletal Muscle:

  1. Parallel – strap-like muscle with evenly spaced fascicles; muscle and tendon are the same width.

    • Examples: sartorius, rectus abdominis, zygomaticus major.

  2. Convergent – broad triangular-shaped muscle that tapers down into a single tendon.

    • Examples: pectoralis major, temporalis.

  3. Pennate – fascicles attach to tendon at an angle, giving it a feather-like appearance.

    • Unipennate – fascicles that are only attached to one side of the associated tendon.

      • Example: flexor pollicis longus.

    • Bipennate – fascicles that are attached to both sides of the associated tendon.

      • Example: rectus femoris.

    • Multipennate – several regions of fascicles joined by connective tissue, where each section contributes to form a single tendon.

      • Example: deltoid.

  4. circular or sphincters– circular fascicle arrangements that surround body openings; provide voluntary control over defecation and urination.

    • Examples: orbicularis oculi, external anal sphincter.

  5. Spiral – found in muscles that wrap around another structure such as a bone.

    • Example: supinator.

  6. Fusiform – muscle midsection or belly is thicker than each tapered end.

    • Examples: biceps brachii, gastrocnemius.

III. Naming Skeletal Muscles

A. Muscles can be named based on:

  1. Location (anterior/posterior).

  2. Body region or anatomical structures where they are attached (abdominis, oris).

  3. Actions performed when contracted (flexors, extensors, adductors, abductors, and levators).

  4. Fiber orientation (rectus, oblique).

  5. Shape (deltoid, rhomboid).

  6. Size (longus, brevis).

  7. Heads of origin (triceps, biceps).

IV. Functions of Skeletal Muscles

A. Movement of bones at a joint.
B. Contractions generate heat as a by-product, which can be used as a homeostatic mechanism for maintaining body temperature.
C. Contraction of diaphragm muscle is a vital function associated with the respiratory system.
D. Skeletal muscles attached to facial skin allow for facial expressions and nonverbal communication.
E. Muscles in the throat assist with swallowing.
F. Stapedius modifies the level of sound we perceive.
G. Sphincters allow conscious control over the opening and closing of body openings.

V. Functional Groups

A. Agonists (prime movers)

  1. Provide most force for a given muscle action.

B. Antagonists

  1. Usually on the opposite side of bones and joints where they meet.

  2. Have the opposite action of the agonist.

  3. Allows for modulation and control of agonist movement.

C. Synergists

  1. Aid agonists by supplying supplemental force, minimizing unwanted movement, and helping to stabilize joints.

D. Fixators

  1. Provide a stabilizing force that anchors a bone.

  2. Provides movement efficiency and protection from injury due to unnecessary movements.

VI. Origin and Insertion

A. Origin

  1. Anchoring point on a bone.

  2. Typically not involved directly with the movement of the joint.

B. Insertion

  1. Moving end of the muscle, whose tendon attaches to a bone or other structures.

  2. Usually located on the far side of the joint.

I. Overview of Levers in the Human Body

Levers are simple machines that amplify an applied force to move a load. In the human body, levers involve bones serving as levers, joints functioning as fulcrums, and muscles providing the force.

II. First Class Levers

A. Features:

  1. The fulcrum is located between the effort (force) and the load (resistance).

  2. They can change the direction of the applied force.

B. Examples in the Human Body:

  1. Neck Extension:

    • When the head is tilted back, the atlas (first cervical vertebra) acts as a fulcrum, the neck muscles provide the force, and the weight of the head is the load.

  2. Seesaw Motion:

    • This lever system is exemplified when lifting the body during a push-up with the hands resting on the ground and feet elevated.

III. Second Class Levers

A. Features:

  1. The load is situated between the effort and the fulcrum.

  2. They provide a mechanical advantage, enabling a small effort to move a large load.

B. Examples in the Human Body:

  1. Calf Raises:

    • The ball of the foot acts as the fulcrum, the weight of the body is the load, and the calf muscles generate the effort to lift the body.

  2. Gastrocnemius Muscle:

    • Acts as a second-class lever when performing movements like standing on tiptoes.

IV. Third Class Levers

A. Features:

  1. The effort is applied between the fulcrum and the load.

  2. They increase the range of motion and speed, but require more effort to move a given load.

B. Examples in the Human Body:

  1. Bicep Curl:

    • The elbow joint serves as the fulcrum, the biceps contract to apply effort, and the weight (load) is held in the hand.

  2. Knee Extension:

    • The quadriceps muscles apply force to extend the knee, with the knee joint acting as the fulcrum and the load being the leg.