Muscular System Histology and Physiology

Types of Muscle Tissue
- Skeletal Muscle:
- Responsible for locomotion, facial expressions, posture, respiratory movements, and other body movements.
- Voluntary control, regulated by the nervous system.
- Smooth Muscle:
- Found in walls of hollow organs, blood vessels, eyes, glands, and skin.
- Functions include propelling urine, mixing food in the digestive tract, and regulating blood flow; autorhythmic in some locations.
- Involuntary control by the endocrine and autonomic nervous systems.
- Cardiac Muscle:
- Located in the heart, major source of blood movement.
- Autorhythmic and involuntary control.

Skeletal Muscle
- Location: Attached to bones.
- Appearance: Striated, multiple nuclei, peripherally located.
- Control: Voluntary and involuntary (reflexes).
Smooth Muscle
- Location: Walls of hollow organs, blood vessels, eyes, glands, and skin.
- Appearance: Spindle-shaped, single centrally located nucleus.
- Control: Involuntary, capable of spontaneous contraction.
Cardiac Muscle
- Location: Heart.
- Appearance: Cylindrical and branched, single centrally located nucleus, striations present.
- Control: Involuntary, autorhythmic.

Contractility: Ability of muscle tissue to shorten with force.
Excitability: Capacity to respond to stimuli (typically from nerves).
Extensibility: Ability to stretch beyond normal resting length and still contract.
Elasticity: Ability to recoil to original resting length post-stretch.

Connective Tissue Coverings:
- Epimysium: Connective tissue surrounding an entire muscle, merges with muscular fascia.
- Perimysium: Loose connective tissue around fascicles, allows for blood vessels and nerves passage.
- Endomysium: Loose connective tissue separating individual muscle fibers in a fascicle.
- Collagen from these connective tissues forms tendons or aponeuroses attaching muscle to bone.

Motor neurons stimulate skeletal muscle contraction; each controls several muscle fibers.
Each muscle fiber connects with branches of the motor neuron.
Blood supply includes one artery and 1-2 veins alongside nerves through connective tissue layers, with extensive capillary beds surrounding muscle fibers.

Develop from the fusion of myoblasts into large, multinucleated muscle cells, averaging 1 to 4 mm in length and 10 to 100 microns in diameter, exhibiting striated appearance.
Number of fibers remains constant post-birth; muscle growth is due to fiber hypertrophy

Electrical Component Structures:
- Sarcolemma: Plasma membrane surrounds muscle cell content.
- Transverse Tubules (T tubules): Inward folds of sarcolemma projecting into the cell interior.
- Sarcoplasmic Reticulum (SR): Specialized smooth endoplasmic reticulum storing calcium; terminal cisternae lie adjacent to T tubules, with two terminal cisternae forming a triad.

Myofibrils: Bundles of protein filaments causing contraction; made from myofilaments.
Myofilaments:
- Thin (Actin) Filaments: Consisting of G actin monomers forming F actin double helix, with tropomyosin and troponin complexes regulating interaction with myosin.
- Thick (Myosin) Filaments: Structures resembling golf clubs with myosin heads capable of forming cross-bridges and ATPase activity for energy during contraction.
Sarcomeres: Basic functional units of muscle fibers; striated appearance due to aligned A and I bands. In each sarcomere:
- Z Disk: Attachment point for actin.
- I Bands: Light regions containing Z disk; only actin.
- A Bands: Dark central area with overlapping actin and myosin.
- H Zone: Area in A band without overlap.
- M Line: Center of H zone stabilizing myosin alignment.

Site where motor neuron and muscle fiber connect.
Components:
- Presynaptic Terminal: Axon terminal with synaptic vesicles containing the neurotransmitter acetylcholine (ACh).
- Synaptic Cleft: Space between NMJ components.
- Postsynaptic Membrane: Motor end-plate receiving ACh, leading to muscle fiber activation.

Mechanism where actin myofilaments slide over myosin to shorten sarcomeres; during contraction, lengths of actin and myosin do not change, but overlap increases.

Action Potentials generated by the nervous system control contractions through voltage changes. Key phases include:
- Resting Membrane Potential: Inside of cell remains more negative than the outside due to ion distribution, primarily maintained by sodium-potassium pumps.
- Action Potential Phases:
- Depolarization: Voltage-gated sodium channels open, reducing negative charge inside the cell.
- Repolarization: Return to resting potential as potassium channels open, allowing K+ to exit.
- Hyperpolarization: Potential briefly becomes more negative than the resting potential before stabilizing.

ATP Production:
- Limited storage (~5-6 seconds of contraction); need for replenishment through:
- Adenylate Kinase: Converts 2 ADP to ATP.
- Creatine Kinase: Transfers phosphate from creatine to ADP.
- Anaerobic Respiration: Glucose breakdown yielding lactic acid and ATP without oxygen.
- Aerobic Respiration: Requires oxygen, yielding ATP, CO2, and H2O with greater efficiency.

Fatigue: Decreased performance capacity resulting from acidosis, oxidative stress, and potential loss of ATP.
Rigor Mortis: Post-mortem muscle stiffness due to calcium influx and cross-bridge formation without subsequent detachment.

Distinct features including:
- Not striated; smaller fibers; spindle-shaped cells with a single nucleus.
- Cross-bridges formed via an alternative mechanism through calmodulin.
Contraction regulation via G protein mechanisms, responding to neural and hormonal stimulation.

Unique characteristics:
- Striated and branched fiber structure; intercalated disks for cell communication.
- Autorhythmic properties with longer action potential durations compared to skeletal muscle.

Aging effects include decreased muscle mass, slower contraction times, increased recovery periods, and loss of fast-twitch fibers.
Duchenne Muscular Dystrophy (DMD) symptoms extend beyond muscle weakness to skeletal and respiratory complications, significantly impacting quality of life.