Anatomy and Physiology Muscular System

Functions of Muscular System

• Produce movement

  • locomotion of the whole body, facial expressions, circulation of blood, passage of food, etc.

• Maintains posture

  • working against gravity to keep us upright

• Stabilizes joints

  • reinforces the connection of bones

• Generates heat

  • cellular respiration causes heat energy, allowing constant body temperature

Characteristics of muscular tissue

• Excitability (the ability to respond to a stimulus from a motor neuron to a hormone)

• Contractibility (the ability to shorten when stimulated)

• Extensibility (the ability to lengthen or stretch, even past their original shape)

• Elasticity (the ability to recoil or bounce back to the original shape and length after being stretched)

Skeletal Muscle

• Cylindrical

• Striated (stripes)

• Multinucleated

• Voluntarily contracted

• Contracts slowly or very quickly

• Connected to bones

Cardiac Muscle

• Branched

• Striated

• Uninucleated

• Involuntarily contracted

• Mostly slow and steady contractions except during short periods of activity

• Found in heart muscle

Smooth Muscle

• Uniform layers

• Nonstriated

• Uninucleated

• Involuntary contractions

• Slow contractions, long periods of time (for digestion, etc)

• Found in walls of internal organs (stomach)

Skeletal Muscle Basics

• Attach to bones and skin of face

• Voluntary

• Organ of muscular system

Skeletal Muscle Composition

• Skeletal muscle tissue

• Nervous tissue

• Blood

• Connective tissue

Coverings on Skeletal Muscle

• Fascia (a type of dense connective tissue that may project beyond the ends of its muscle fibers, which form tendons)

• Tendons (connect muscle to bone)

• Aponeurosis (type of fascia)

Microscopic Structure of Muscle Order (least complex —> most complex)

Myofilament —> Myofibril —> Muscle Fiber —> Fascicle —> Muscle

Myofilaments

• 2 types

  • Actin (thin) and myosin (thick)

  • Make up sliding filament theory

• Responsible for contracting activity of muscle fiber

• Located intracellularly

Myofibril

• Thread-like organelles of the muscle fibers

• Structured in long, striated units called sarcomeres

• Located intracellularly

• Overlapping parallel groups of thick and thin filaments in a repeating pattern; the underlying basis for the striation pattern

Muscle Fiber

• Long, thin muscle cells

• Each cell is covered by endomysium

• Sarcoplasmic reticulum supplies Ca2+ ions to disable tropomyosin/troponin complex

• A single muscle cell, multinucleated and may be many centimeters long

• Within a muscle, surrounded by a layer of connective tissue called endomysium

Fascicle

• Bundles within muscles

• Within a muscle, surrounded by a layer of connective tissue called perimysium

Muscle

• Skeletal Muscle is attached to bone by tendons and is made of many bundles of muscle fibers

• Surrounded by a layer of connective tissue called epimysium and in some cases an additional layer called muscle fascia

Skeletal Muscle Fibers Characteristics

• Also called muscle cell

• Multinucleated

• Sarcolemma (the muscle cell membrane)

• Sarcoplasm (muscle cell cytoplasm)

• Has many myofibrils (contain thin actin and thick myosin)

• Has sarcomeres (the muscle unit of contraction and relaxation)

• Has the sarcoplasmic reticulum (storage of calcium ions and plays a role in relaxation and contraction)

• Has the Transverse (T) Tubule (extension of skeletal and cardiac cell membranes)

Striation Pattern

Developed by arrangements of myofilaments in myofibrils

I Band

• Light bands

• Contain Actin

• Held together by direct attachments to structures called Z lines (center of I bands)

A band

• Dark band

• Composed of thick myosin filaments overlapping thin actin filaments

• Contains an H zone consisting of only myosin filaments

• The thickened part is called the M line that consists of proteins that help hold myosin in place.

• Myosin filaments are also held in place by the Z lines and are attached to them by a large protein called titin (connectin).

Protection of Muscle Fibers

Epimysium (covers the whole muscle) —> Perimysium (covers a fascicle) —> Endomysium (covers an individual muscle fiber)

• Having these membranes allow them to slide without bursting during contractions.

Myasthenia Gravis (MG)

• Autoimmune disorder

• Antibodies attack acetylcholine receptors on skeletal muscle fibers (specifically motor end plates = a chemical synapse between the terminal part of the motor neuron and the target muscle) in neuromuscular junctions

• 1/3 ACh receptors

• Widespread muscle weakness and fatigue

• Use drugs that inhibit acetylcholinesterase

• Immunosuppressant Drugs

• Administer antibodies that inhibit harmful ones

• Plasma exchange