Puzzle Piece 14: Skeletal Muscles Introduction

• Over 650 skeletal muscles in the human body.

• Most skeletal muscles attach to bones; some attach to other muscles and skin.

Fundamental Concepts Covered Previously

• Each skeletal muscle comprises several skeletal muscle cells.

• Each skeletal muscle cell (fiber) contains:

  • Multiple nuclei.

  • Striations due to the organized arrangement of filaments.

  • Myofibrils, which are sequences of functional units called sarcomeres.

  • Sarcomeres consist of:

    • Thick filaments (myosin).

    • Thin filaments (actin).

• Importance of cell division:

  • Skeletal muscle cells do not undergo cell division after birth.

  • Muscular enlargement is due to the increase in the number of myofibrils, not the number of muscle cells.

• Skeletal muscle cells have abundant mitochondria for ATP production:

  • Myoglobin stores oxygen for mitochondrial ATP production.

  • High endurance muscles possess more myoglobin, making them appear darker.

Connective Tissue Layers

• Endomysium: Surrounds each individual skeletal muscle cell.

• Perimysium: Encloses groups of skeletal muscle cells into a fascicle.

• Epimysium: Envelops all fascicles, epitomizing the muscle.

• Connective tissue is highly vascularized (exception: cartilage connective tissue!).

  • Contains blood vessels for:

  • Oxygen and nutrient supply.

  • Waste removal from muscle metabolism.

• Nerve fibers within the connective tissue layers serve two functions:

  • Detecting muscle conditions.

  • Triggering muscle cell contraction.

• Tendons: Formed from the fusion of endomysium, perimysium, and epimysium.

  • Tendons fuse with the periosteum of bone.

  • The fusion empowers a strong attachment of muscle to bone, essential for movement.

Fascia versus Fascicle

• Fascia: A connective tissue sheath wrapping around groups of skeletal muscles.

  • Synonyms: fascia = deep fascia = fascia of muscles.

  • Provides support (stability) and protection for groups of muscles.

• Fascicle: A connective tissue sheath that surrounds groups of skeletal muscle cells (formed by perimysium).

  • Muscles can be classified based on the pattern of fascicle organization.

Muscle Compartments of the Thigh

Compartments

• Anterior Compartment:

  • Contains:

  • Rectus Femoris

  • Vastus Medialis

  • Vastus Lateralis

  • Vastus Intermedius

  • Sartorius

  • The aforementioned muscles collectively form the Quadriceps Femoris.

  • Mnemonic for Quadriceps:

    • "Square" in Spanish is "cuadrado," correlating with the number four; hence, "quadriceps" signifies a group of four muscles.

    • "Ceps" is derived from "cephalic," indicating that quadriceps signify four heads.

  • Thus, the anterior compartment is comprised of the quadriceps and sartorius.

• Posterior Compartment:

  • Contains:

  • Biceps Femoris

  • Semitendinosus

  • Semimembranosus

  • The group of mentioned muscles constitutes the Hamstrings.

• Medial Compartment:

  • Contains:

  • Gracilis

  • Adductor Longus

  • Adductor Magnus

  • This compartment is centrally located, serving to divide the thigh into anterior and posterior sections.

Fascicle Arrangement and Muscle Classification

Classification of Fascicles

• Parallel Muscles:

  • Fascicles run parallel to the muscle's long axis, generating uniform force along the length.

  • Commonly formed as:

  • Fusiform Parallel Muscles: Exhibit a spindle-shaped appearance (e.g., biceps brachii).

  • Non-Fusiform Parallel Muscles: Display a rectangular shape (e.g., sartorius).

• Pennate Muscles:

  • Fascicles attach obliquely to the central tendon, maximizing force within a compact structure.

  • Types include:

  • Bipennate Muscles: (e.g., rectus femoris) attach at an angle to both sides of the central tendon.

  • Multipennate Muscles: (e.g., deltoid) with a central tendon branching into multiple tendons, each with fascicles at an angle on both sides.

  • Unipennate Muscles: (e.g., flexor pollicis longus) attach to a single side of the central tendon at an angle.

  • Mnemonic for Pennate Arrangement:

  • The arrangement resembles feathers, as "pena" translates to "feather" in Latin.

• Convergent Muscles:

  • Fascicles converge at a single attachment point, offering versatility in movement and strength.

  • Example: Pectoralis Major Muscle.

• Circular Muscles:

  • Fascicles are arranged concentrically around an opening, regulating passages.

  • Examples include:

  • Orbicularis oris

  • Orbicularis oculi

  • Sphincters throughout the body.

Muscle Characteristics: Origin, Insertion, and Belly

Key Components

• Origin: The proximal or less movable attachment site of a muscle.

• Insertion: The distal or more movable attachment site of a muscle.

• Belly: The middle part of the muscle that shortens during contraction.

General Rule of Muscle Contraction

• During skeletal muscle contraction, the insertion pulls toward the origin.

  • Important Concept:

  • Since bones are not flexible enough to fold, if a muscle starts and ends on the same bone, nothing will move upon contraction.

  • Therefore, a muscle must cross at least one joint.

  • If a muscle crosses more than one joint, it can move multiple joints.

  • Example: Biceps Brachii:

    • Originates in the scapula, crosses the shoulder joint, elbow joint, and proximal radioulnar joint to attach at the radial tuberosity of the radius bone.

    • Capable of flexing the shoulder, flexing the elbow, and supinating the forearm.

Muscle Interactions: Agonists, Antagonists, Synergists, and Fixators

Muscle Roles

• Agonist Muscle:

  • The primary mover initiating and executing a specific action.

• Antagonist Muscle:

  • Opposes the agonist's action, controlling speed and movement precision.

• Synergist Muscles:

  • Assist the agonist in generating movement and stability, optimizing force production.

  • Mnemonic for Synergists: Act in sync towards a common goal (i.e. fulfilling the movement).

• Fixator Muscles:

  • Stabilize the position of the bone from which the agonist muscle derives, facilitating more efficient muscle action.

Example of Flexion and Extension at the Elbow

Flex at the Elbow (Forearm Pronated):

• Agonist: Brachialis muscle (causes flexion).

• Synergist: Brachioradialis muscle (assists flexion).

• Antagonist: Triceps brachii and anconeus muscles (oppose flexion).

• Fixators: Muscles of the shoulder and scapula stabilizing the humerus for efficient flexion.

Extend at the Elbow:

• Agonist: Triceps brachii muscle (causing extension).

• Synergist: Anconeus muscle (assists extension).

• Antagonist: Brachialis and brachioradialis muscles (oppose extension).

• Fixators: Muscles of the shoulder and scapula stabilizing the humerus for efficient movement.

Important Note:

• Notably, the biceps brachii is not described in relation to elbow flexion because it primarily functions as a major supinator in the upper arm.

• The biceps brachii can only contract effectively when the forearm is in a supinated position.

• In a pronated forearm, the biceps brachii is rendered inactive and does not assist in the flexion action.