Study Notes on Muscular Tissue and System

Overview of Muscular Tissue and the Muscular System

  • Muscle Tissue Types

    • There are three types of muscle tissue found in the human body:

    • Skeletal Muscle

    • Cardiac Muscle

    • Smooth Muscle

    • Only skeletal and smooth muscle cells are elongated and referred to as muscle fibers.

  • Goals of the Laboratory Study

    • Distinguish between the three types of muscle tissues.

    • Describe the structure of muscle cells (fibers) and muscle tissue.

    • Identify the connective tissue structures associated with skeletal muscles.

    • Describe the anatomy of the sarcomere and the neuromuscular junction (NMJ).

    • Locate the major structures of skeletal muscle cells on figures and laboratory models.

    • Identify the major muscles of the human muscular system.

    • Identify the origin and insertion of selected muscles.

    • Describe the major actions of selected skeletal muscles.

Functions of Muscle Tissue

  • Mass and Energy Transformation

    • Muscle tissue constitutes almost half of the human body’s mass.

    • It can transform chemical energy (ATP) into directed mechanical energy capable of exerting force.

  • Key Functions of Muscle Tissue

    • Producing body movements.

    • Stabilizing body positions.

    • Storing and mobilizing substances within the body.

    • Generating heat.

  • Characteristics of Muscle Tissue

    • Excitability/Responsiveness: Ability to receive and respond to stimuli.

    • Contractility: Ability to shorten forcibly when stimulated.

    • Extensibility: Ability to be stretched.

    • Elasticity: Ability to recoil to resting length.

Level of Organization in Skeletal Muscle

  • Components of skeletal muscle hierarchy:

    • Skeletal Muscle (largest)

    • Muscle Fascicle

    • Muscle Fiber

    • Myofibril

    • Filaments/Myofilaments (smallest)

Connective Tissue of Skeletal Muscle

  • Each skeletal muscle and muscle fiber is enveloped by connective tissue.

  • Purpose of Connective Tissue: Supports cells and reinforces the whole muscle.

  • Connective Tissue Sheaths (from external to internal):

    • Epimysium: Dense irregular connective tissue that surrounds the entire muscle; may blend with fascia.

    • Perimysium: Fibrous connective tissue surrounding fascicles, which are groups of muscle fibers.

    • Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

Anatomy of Skeletal Muscle Fiber

  • Microscopic Features

    • Contains multiple nuclei and striations.

    • Components include:

    • Sarcolemma: The muscle fiber plasma membrane.

    • Sarcoplasm: The cytoplasm of the muscle fiber.

    • Glycosomes: Store glycogen.

    • Myoglobin: Stores oxygen.

    • Myofibrils: Contractile organelles.

    • Sarcoplasmic Reticulum (SR): Stores Ca²⁺ and wraps around the myofibrils.

    • Transverse (T) Tubules: Conducts muscle action potentials toward the center of muscle fibers.

Sarcomere: The Contractile Unit

  • Definition: The sarcomere is identified as the basic contractile unit of the skeletal muscle fiber.

  • Structure:

    • Contains an A band with half of an I band at each end.

    • Area between Z discs marks the boundaries of individual sarcomeres, aligning end to end along each myofibril similar to boxcars of a train.

  • Striations: Formed from a repeating series of dark and light bands along each myofibril.

    • A bands: Dark regions.

    • H band: Lighter region in the middle of dark A band.

    • M Line: Protein (myomesin) bisecting the H zone vertically.

    • I bands: Lighter regions.

    • Z disc/Line: Coin-shaped sheet of proteins at the midline of the light I band.

Myofilaments in the Sarcomere

  • Types:

    • Actin Myofilaments: Thin filaments that extend across the I band and partway into the A band; anchored to Z discs.

    • Myosin Myofilaments: Thick filaments that extend the length of the A band; connected at the M line.

  • Arrangement: Sarcomere cross-section shows a hexagonal arrangement with one thick filament surrounded by six thin filaments.

Muscle Contraction Cycle

  • Four Main Steps of the Contraction Cycle:

    • Step 1: ATP Hydrolysis

    • Myosin hydrolyzes ATP to produce ADP and inorganic phosphate (Pi).

    • This reaction's energy rotates the myosin head to a cocked position.

    • Myosin head binds weakly to actin.

    • Step 2: Cross-Bridges Form

    • Calcium is released from the Sarcoplasmic Reticulum (SR).

    • Calcium binds to troponin, inducing a conformational change that pulls tropomyosin away from myosin-binding sites on actin.

    • Myosin binds tightly to actin, with one head per binding site.

    • Step 3: Power Stroke Occurs

    • After the cross-bridge forms, the myosin head pivots, pulling the thin filament past the thick filament, generating tension/force termed the power stroke.

    • Myosin remains tightly bound to ADP during this step.

    • Step 4: Detachment of Myosin from Actin

    • ADP is released from the myosin head, while another ATP molecule binds to the myosin head.

    • Upon ATP binding, myosin head detaches from actin.

Summation of Contractions

  • Types of Muscle Twitches:

    • Single Twitches: Muscle relaxes completely between stimuli.

    • Summation: Occurs when stimuli are delivered closer together, preventing complete relaxation.

    • Unfused Tetanus: Stimuli allow muscles to relax slightly between contractions.

    • Complete Tetanus: Steady tension is achieved without relaxation.

    • Muscle Fatigue: In cases of fatigue, tension decreases rapidly.

Functional Groups of Skeletal Muscles

  • Types of Functional Groups:

    1. Prime Mover (Agonist): Major responsibility for producing a specific movement.

    2. Antagonist: Opposes or reverses a particular movement; found on opposite sides of the joint relative to the prime mover.

    3. Synergist: Aids prime movers by adding extra force to the same movement or reducing undesirable movements.

    • Fixators: A subtype of synergists that immobilize the bone or muscle’s origin to provide a stable base for prime movers.

  • Example of Functional Groups:

    • Prime Mover: Biceps brachii.

    • Antagonist: Triceps brachii.

    • Synergists: Brachioradialis and brachialis.

    • Fixators: Subscapularis.

Contraction Variability Within Functional Roles

  • Skeletal muscles can serve multiple functional roles in different movements, acting as a prime mover in one scenario while being an antagonist in another.

Fascicular Arrangement of Muscles

  • Fascicular Arrangement: Refers to how the muscle fibers are arranged in relation to the tendon, contributing to muscle shape and functional characteristics.

  • Common Patterns of Fascicular Arrangement:

    • Circular: Fascicles arranged in concentric circles.

    • Convergent: Fascicles converge towards a single insertion point.

    • Parallel: Fascicles run parallel to the muscle's long axis.

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

  • Correlations: Fasciculus arrangement correlates with

    • The amount of power a muscle can produce.

    • The range of motion a muscle can produce.

Naming of Muscles

  • Criteria for Muscle Naming:

    1. Muscle Location: Named for the bone or body region it is associated with (e.g., temporalis).

    2. Muscle Shape: Named according to distinctive shapes (e.g., deltoid muscle).

    3. Muscle Size: Descriptive terms include maximus (largest), minimus (smallest), longus (long).

    4. Fiber Direction: Names indicate the direction of muscle fibers (e.g., rectus, transversus).

    5. Number of Origins: Named after the number of origins (e.g., biceps, triceps).

    6. Attachment Locations: Named according to the origin and insertion points (e.g., sternocleidomastoid).

    7. Function: Named for the action they perform (e.g., flexor, extensor).

Detailed Muscular Anatomy

  • Facial Muscles:

    • Occipitofrontalis, Nasalis, Orbicularis oris, Depressor anguli oris.

    • Primary muscles for facial expressions and movements of the mouth and eyebrows.

  • Muscles of the Neck and Back: Involved in stabilizing and moving the vertebral column.

  • Muscles of the Thorax:

    • Function: Alter size of thoracic cavity for breathing.

  • Muscles of the Arm: Move the radius and ulna, as well as the humerus.

    • Involve notable muscles such as Biceps brachii, Triceps brachii, Brachialis.

  • Muscles of the Leg:

    • Important for foot and toe movement; inclusive of Gastrocnemius, Soleus, Tibialis anterior.

Note: Each muscle group serves specific functions, contributing to overall movement and stability within the human muscular system.

  • Abdominal Muscles: Protect the viscera and assist in movements involving the vertebral column.

Conclusion

  • Understanding the structure and functionality of these muscles is crucial for various applications, including exercise science, rehabilitation, and anatomy education.