Muscle Structure and Attachments

SKELETAL MUSCLES

  • Definition: Skeletal muscles are complex organs primarily composed of contractile muscle tissue, but also contain several other essential components:

    • Connective tissue: Provides structural support, organizes muscle fibers, and transmits force.

    • Blood vessels: Supply nutrients (glucose, oxygen) and remove metabolic waste products (lactic acid, carbon dioxide).

    • Nerves: Innervate muscle fibers, controlling contraction and providing sensory feedback.

COMPONENTS OF SKELETAL MUSCLE

  • Epimysium:

    • A dense, irregular connective tissue layer that surrounds the entire gross muscle.

    • It helps to protect the muscle from friction against other muscles and bones.

    • Is continuous with the tendon at the ends of the muscle.

  • Perimysium:

    • A fibrous connective tissue sheath that divides the muscle into bundles of muscle fibers called fascicles.

    • Each fascicle is a functional unit containing 10-100 muscle fibers.

    • Contains blood vessels and nerves that supply the individual fascicles.

  • Endomysium:

    • A delicate, thin layer of areolar connective tissue that surrounds individual muscle cells (fibers).

    • Contains capillaries and nerve fibers to directly supply the muscle cells.

    • Provides an extracellular matrix for muscle fibers.

CONNECTIVE TISSUE LAYERS

  • The three connective tissue layers (Epimysium, Perimysium, Endomysium) are continuous with each other and merge at the ends of the muscle to form tendons or aponeuroses, effectively transferring the force of muscle contraction to bone.

MUSCLE SHAPE AND FASCICLE ARRANGEMENT

  • The fascicle arrangement by perimysia significantly influences:

    • The force generated by a muscle: Muscles with more fibers packed into a given volume (e.g., pennate) can generate greater force.

    • The range of motion of the muscle: Muscles with long, parallel fascicles allow for a greater range of shortening and thus a larger range of motion.

TYPES OF MUSCLE SHAPES

  • Parallel Muscle:

    • Most common type, characterized by fascicles arranged parallel to the long axis of the muscle.

    • This arrangement allows for a large range of motion but less power compared to pennate muscles for the same volume.

    • Characteristics:

      • Strap muscles: Flat sheets with broad attachments, like the sartorius.

      • Large central bellies: Tapering to origin (the less movable point of attachment) and insertion (the more movable point of attachment).

    • Subgroup of Parallel: Fusiform muscles have a spindle shape, wider in the middle (belly) and tapering at both ends (e.g., biceps brachii).

  • Circular Muscles (Sphincters):

    • Bundles of muscle fibers arranged concentrically around an opening.

    • Functionality:

      • Relaxation: Increases the size of the opening (e.g., when you open your mouth).

      • Contraction: Decreases the size of the opening to the point of closure (e.g., orbicularis oris around the mouth or orbicularis oculi around the eye).

  • Convergent Muscles:

    • Characterized by widespread expansion over a sizable area (fan-shaped muscles).

    • Fascicles converge to a single, common attachment point, which can be a tendon, a raphe (seam of fibrous tissue), or an aponeurosis.

    • This arrangement allows versatile pull in different directions, though not all fascicles can contract simultaneously to their full extent (e.g., pectoralis major).

  • Pennate Muscles:

    • Named from the Latin word “penna” meaning feathers, as their fascicles attach obliquely to a central tendon.

    • Tendon runs along the central length of the muscle like a feather’s quill.

    • Characteristics:

      • Can only pull at an angle relative to the tendon.

      • Do not move tendons significantly far due to shorter fiber length.

      • Contain more muscle fibers per unit area (higher physiological cross-sectional area), which allows them to produce more tension (force) for their size compared to parallel muscles.

      • Types:

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

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

        • Multipennate: Tendon branches within the muscle, and fascicles insert at multiple angles (e.g., deltoid).

CONNECTIVE TISSUE BUNDLES

  • The collagen fibers of the three connective tissue layers (epimysium, perimysium, endomysium) are interwoven and continuous.

  • At each end of a muscle, these collagen fibers converge to form:

    • A dense regular connective tissue bundle called a tendon.

    • A broad, flat sheet of dense regular connective tissue called an aponeurosis.

  • Functions:

    • These structures serve to securely attach skeletal muscles to bones, and sometimes to other muscles or skin.

    • The strong collagen fibers extend into the periosteum of the bone and then into the bone matrix itself, forming a strong, direct connection.

    • Crucially, they transmit the contractile force generated by muscle fibers to bones, thereby producing movement around joints.

TENDONS

  • Definition:

    • Connective tissue structures primarily composed of tightly packed collagen fibers (Type I collagen) that connect muscle to bone or other structures (e.g., eyeball, skin).

    • Serves to efficiently transfer the pulling force generated by muscle contraction to the skeletal system, resulting in movement.

    • Characteristics:

      • Resistant to tearing: Due to the high tensile strength of collagen, allowing them to withstand significant pulling forces.

      • Limited stretch: They are relatively inelastic, ensuring that muscular force is transmitted effectively with minimal energy loss.

      • Rope-like structure: Often appear as cord-like bundles.

      • Contain fibroblasts and a sparse blood supply, making healing a slower process.

APONEUROSIS

  • Definition:

    • A type of broad, flat sheet of dense regular connective tissue connecting muscle to muscle, muscle to bone, or muscle to other structures (e.g., fascia).

    • Provides strength and stability to the muscle attachments over a wider area.

    • Functions to absorb and transmit energy when muscles move, often serving as an origin or insertion point for multiple muscles.

    • Examples include the aponeurosis connecting the abdominal muscles or the epicranial aponeurosis on the scalp.

LIGAMENTS

  • Definition:

    • A type of dense regular connective tissue, similar to tendons but with a higher proportion of elastic fibers in some cases.

    • Primarily functions to bind bone to bone, forming part of a joint capsule or acting independently.

    • Serves several crucial functions:

      • Helps hold structures together, providing stability to joints (e.g., cruciate ligaments in the knee).

      • Keeps joints stable by restricting excessive or undesirable movements.

      • Guide joint movements within a physiological range.

TENDON SHEATHS

  • Definition:

    • Tendons are often protected by tendon sheaths, which are specialized synovial membranes that form a tunnel around tendons subjected to friction.

    • Made of a double-layered connective tissue membrane, with an inner visceral layer wrapped around the tendon and an outer parietal layer.

    • The space between these layers is filled with lubricating fluid (synovial fluid), which reduces friction during tendon movement.

    • Risks associated with tendon sheaths:

      • If overused or irritated, they can become inflamed and swollen, a condition known as tenosynovitis, causing pain and restricted movement.

    • Example: The carpal tunnel, which is formed by the carpal bones and the flexor retinaculum ligament, contains several tendons encased in synovial sheaths.

COMPARATIVE ANATOMY: HUMAN VS. CHICKEN

  • Musculoskeletal Differences:

    • Chicken femur: Typically smaller and more delicate compared to the human femur, reflecting different postural and locomotor adaptations.

    • Chicken tibia (tibiotarsus): Is significantly larger and forms the main leg bone, adapted for bipedal locomotion and powerful jumping/flight take-off.

    • Patella: Chicken has no true patella (kneecap), while humans do; the patella in humans acts as a fulcrum to increase the mechanical advantage of the quadriceps muscle.

    • Similarities: Despite differences, similar fundamental muscles and tendons are present in humans and chickens, performing analogous functions. Tendon sheaths are notably present at the chicken ankle joint, indicating shared biomechanical principles for smooth tendon gliding.

DISSECTION NOTES

  • Thigh Muscles:

    • During dissection, multiple muscles are observed, both deep and superficial, each with distinct origins, insertions, and fiber directions.

    • The Epimysium appears shiny and tough, surrounding individual muscles and keeping them functionally separate while allowing them to slide past each other.

    • The direction of fascicles is critical in understanding the potential action and mechanical advantage of the muscle (e.g., longitudinal fascicles for range of motion, pennate for force).

  • Dissection Procedure:

    • To understand muscle function, identify a deep muscle still attached to its tendon.

    • Pulling on the tendon should result in the observable movement of the lower leg or relevant limb segment, demonstrating the muscle's mechanical action.

JOINT OBSERVATIONS

  • Hip Joint:

    • During careful dissection, the head of the femur should be observed articulated within the acetabulum of the pelvis.

    • Note the remains of the ligamentum teres (ligament of the head of the femur) holding it into the acetabulum, providing both stability and housing small blood vessels.

  • Knee Joint:

    • Upon opening the joint, observe:

      • Various ligaments (e.g., collateral and cruciate ligaments) that reinforce the joint capsule and prevent excessive movement.

      • The meniscus (or menisci), which are crescent-shaped fibrocartilaginous pads that improve congruency between the femoral and tibial condyles, distribute load, and absorb shock.

      • The extremely smooth articular cartilage at the ends of the bones (femur and tibia), which minimizes friction and allows for fluid movement within the joint.

BONE MARROW

  • Observation: Inside certain long bones (e.g., femur or tibia), within the medullary cavity, bone marrow can be seen. Red marrow is responsible for hematopoiesis (blood cell formation), while yellow marrow is primarily adipose tissue.