Muscle Tissue and Muscular System: Study Guide

Muscle Tissue

Gross Anatomy of Skeletal Muscle

Skeletal muscles are complex organs composed of muscle fibers, connective tissue, blood vessels, and nerves. The connective tissue layers organize and protect the muscle.

• Epimysium: Dense connective tissue that surrounds the entire muscle, providing structural integrity and separating it from surrounding tissues.

• Perimysium: Connective tissue that groups muscle fibers into bundles called fascicles.

• Endomysium: Thin connective tissue that surrounds each individual muscle fiber, providing support and insulation.

• Tendons: Dense regular connective tissue that connects muscle to bone, transmitting the force generated by muscle contraction to produce movement.

Anatomy of the Neuromuscular Junction

The neuromuscular junction (NMJ) is the synapse between a motor neuron and a skeletal muscle fiber, enabling neural control of muscle contraction.

• Motor End Plate: Specialized region of the muscle fiber's sarcolemma with receptors for the neurotransmitter acetylcholine (ACh).

• Synaptic Cleft: Small gap between the neuron and muscle fiber where neurotransmitters are released.

• Synaptic Vesicles: Contain ACh, released from the neuron upon stimulation.

Ultrastructure of Skeletal Muscle Fibers

Skeletal muscle fibers are long, multinucleated cells containing myofibrils, which are composed of repeating units called sarcomeres.

• Myofibrils: Cylindrical structures within muscle fibers, made up of actin (thin) and myosin (thick) filaments.

• Sarcomeres: The basic contractile units of muscle, defined by Z-discs. Sarcomeres contain overlapping thick and thin filaments.

Structure and Function of a Motor Unit

A motor unit consists of a single motor neuron and all the muscle fibers it innervates. Motor units allow graded control of muscle force.

• Small Motor Units: Control fine movements (e.g., eye muscles).

• Large Motor Units: Control gross movements (e.g., thigh muscles).

Sliding-Filament Theory of Muscle Contraction

This theory explains how muscles contract by the sliding of actin and myosin filaments past each other, shortening the sarcomere.

• Myosin heads bind to actin, forming cross-bridges.

• ATP hydrolysis provides energy for the myosin head to pivot, pulling actin filaments toward the center of the sarcomere.

• Repeated cycles result in muscle contraction.

Ultrastructure of Cardiac Muscle Tissue

Cardiac muscle cells are striated, branched, and interconnected by intercalated discs, which facilitate synchronized contraction.

• Intercalated Discs: Specialized junctions containing desmosomes and gap junctions, allowing electrical and mechanical coupling between cells.

Ultrastructure of Smooth Muscle

• Multiunit Smooth Muscle: Muscle fibers operate independently, found in the iris of the eye and large arteries.

• Single-Unit Smooth Muscle: Muscle fibers contract as a unit due to gap junctions, found in the walls of hollow organs (e.g., intestines).

Functional Differences: Multiunit vs. Single-Unit Smooth Muscle

• Multiunit: Precise, graded contractions; less common.

• Single-Unit: Synchronized, wave-like contractions (peristalsis); more common.

Control of Muscle Contraction

• Skeletal Muscle: Voluntary, controlled by somatic nervous system.

• Cardiac Muscle: Involuntary, controlled by autonomic nervous system and intrinsic pacemaker cells.

• Smooth Muscle: Involuntary, controlled by autonomic nervous system, hormones, and local factors.

General Functions of Muscle Tissue

• Skeletal Muscle: Movement, posture, heat production.

• Cardiac Muscle: Pumping blood.

• Smooth Muscle: Movement of substances through hollow organs.

Muscular System

Origin, Insertion, and Action of Major Skeletal Muscles

Each skeletal muscle has a specific origin (fixed attachment), insertion (movable attachment), and action (movement produced).

• Origin: The stationary attachment point of the muscle.

• Insertion: The movable attachment point, where the muscle exerts force.

• Action: The specific movement produced by muscle contraction.

Examples of major muscles and their actions:

• Sternocleidomastoid: Flexes and rotates the head.

• Pectoralis Major: Adducts and flexes the arm.

• Rectus Abdominis: Flexes the vertebral column.

• Trapezius: Elevates, retracts, and rotates the scapula.

• Deltoid: Abducts the arm.

• Biceps Brachii: Flexes the elbow and supinates the forearm.

• Triceps Brachii: Extends the elbow.

• Gluteus Maximus: Extends and laterally rotates the hip.

• Quadriceps Group (e.g., Vastus Lateralis): Extends the knee.

• Hamstring Group (e.g., Biceps Femoris): Flexes the knee.

• Gastrocnemius: Plantar flexes the foot.

Muscles Responsible for Specific Movements

Movement	Primary Muscles
Abduction of the arm	Deltoid, Supraspinatus
Adduction of the arm	Pectoralis Major, Latissimus Dorsi
Flexion of the elbow	Biceps Brachii, Brachialis
Extension of the elbow	Triceps Brachii
Flexion of the wrist	Flexor Carpi Radialis, Flexor Carpi Ulnaris
Extension of the wrist	Extensor Carpi Radialis, Extensor Carpi Ulnaris
Extension of the digits	Extensor Digitorum
Flexion of the hip	Iliopsoas, Rectus Femoris, Sartorius
Extension of the hip	Gluteus Maximus, Hamstrings
Abduction of the thigh	Gluteus Medius, Tensor Fasciae Latae
Adduction of the thigh	Adductor Longus, Adductor Magnus, Gracilis
Extension of the knee	Quadriceps Group
Flexion of the knee	Hamstring Group
Dorsiflexion of the foot	Tibialis Anterior
Plantar flexion of the foot	Gastrocnemius, Soleus

Principles of Levers and Leverage

Muscles and bones act as levers to produce movement. The three classes of levers differ in the arrangement of the fulcrum, load, and effort.

• First-Class Lever: Fulcrum between load and effort (e.g., neck extension).

• Second-Class Lever: Load between fulcrum and effort (e.g., standing on tiptoe).

• Third-Class Lever: Effort between fulcrum and load (e.g., biceps curl).

Lever Equation:

$\text{Effort} \times \text{Effort Arm Length} = \text{Load} \times \text{Load Arm Length}$

Mechanical advantage is achieved when a small effort moves a large load.

Muscle Roles: Antagonist, Synergist, Fixator, Stabilizer

• Antagonist: Muscle that opposes the action of the agonist (prime mover).

• Synergist: Muscle that assists the agonist in performing its action.

• Fixator: Muscle that stabilizes the origin of the agonist.

• Stabilizer: General term for muscles that maintain posture or joint position.

Arrangement of Muscle Fibers

• Strap: Long, parallel fibers (e.g., sartorius).

• Fusiform: Spindle-shaped, with a thick belly (e.g., biceps brachii).

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

• Bipennate: Fibers insert into both sides of a tendon (e.g., rectus femoris).

• Multipennate: Multiple tendons with fibers arranged obliquely (e.g., deltoid).

• Convergent: Broad origin, fibers converge to a single tendon (e.g., pectoralis major).

Fiber Orientation and Contractile Force

The arrangement of muscle fibers affects the strength and range of motion. Pennate muscles generate more force, while parallel muscles allow greater range of motion.

Criteria for Naming Muscles

• Number of Origins: e.g., biceps (two origins), triceps (three origins).

• Relative Size: e.g., maximus (largest), minimus (smallest).

• Relative Length: e.g., longus (long), brevis (short).

• Shape: e.g., deltoid (triangular), trapezius (trapezoid-shaped).

• Origin and Insertion: e.g., sternocleidomastoid (originates on sternum and clavicle, inserts on mastoid process).

• Action: e.g., flexor, extensor, adductor.

Origin and Insertion of a Muscle

• Origin: The fixed, less movable attachment point of a muscle.

• Insertion: The movable attachment point, where the muscle exerts its action.

Additional info: The above content expands on the provided objectives with academic context, definitions, and examples to ensure a comprehensive study guide for exam preparation.