Muscular System


1. Overview & Functions of the Muscular System


Main Functions:

  • Movement: Produces body movements and facial expressions.

  • Stability: Maintains posture and joint stability.

  • Control of Body Openings and Passages: Manages internal movement via sphincters and tubular structures.

  • Heat Production: Generates body heat through contractions.

2. Types of Muscle Tissue

  • Skeletal Muscle:

  • Location: Attached to bones and skin of the face.

  • Function: Produces body movements and facial expressions.

  • Control: Voluntary.

  • Contraction Rate: Fast to contract and relax.

  • Features: Striated; no intercalated discs; multinucleated.

  • Smooth Muscle:

  • Location: Walls of hollow organs, blood vessels, and iris.

  • Function: Moves contents through organs; vasoconstriction.

  • Control: Involuntary.

  • Contraction Rate: Slow to contract and relax.

  • Features: Non-striated; no intercalated discs.

  • Cardiac Muscle:

  • Location: Wall of the heart.

  • Function: Pumps blood through the heart.

  • Control: Involuntary.

  • Contraction Rate: Groups of muscle fibers contract as a unit (self-exciting).

  • Features: Striated; connected by intercalated discs.

3. Structure & Mechanisms of Skeletal Muscle

  • Connective Tissue Coverings:

  • Epimysium: Surrounds the entire muscle.

  • Perimysium: Surrounds muscle fascicles (bundles).

  • Endomysium: Surrounds individual muscle fibers.

  • Muscular Fascia: Separates and compartmentalizes individual or groups of muscles.

  • Fiber Microstructure:

  • A muscle fiber is a single cell consisting of a plasma membrane (sarcolemma), cytoplasm (sarcoplasm), several nuclei, and myofibrils.

  • Myofibrils are made up of two major protein filaments:

  • Actin (thin) myofilaments: Consist of two strands of F actin (composed of G actin), tropomyosin, and troponin.

  • Myosin (thick) myofilaments: Consist of myosin molecules with a head (containing ATPase), a hinge region, and a rod.

  • Sarcomere: The functional unit bounded by Z disks. Myofibrils appear striated due to alternating A bands and I bands.

  • Cross-Bridge Formation: Occurs when a myosin head binds to the active site on G actin in the presence of Calcium ions (Ca2+) and ATP.

  • Neuromuscular Junction:

  • Motor neurons release Acetylcholine (ACh) to trigger muscle contraction.

  • The enzyme Acetylcholinesterase breaks down ACh to allow muscle relaxation.

4. Types and Mechanisms of Smooth Muscle

  • Types of Smooth Muscle:

  • Multi-unit Smooth Muscle: Contracts in response to neurotransmitters and hormones. Located in the iris of the eye and blood vessel walls.

  • Visceral (Single-unit) Smooth Muscle: Cells closely contact each other in sheets within hollow organs (stomach, intestines, bladder, uterus). Fibers stimulate each other to produce peristalsis (rhythmic wave-like contractions).

  • Neurotransmitters in Smooth Muscle:

  • Uses both Acetylcholine (ACh) and Norepinephrine (NE), which can cause or inhibit contraction depending on the receptor type.

5. Cardiac Muscle Specifics

  • Intercalated Discs: Contain gap junctions and desmosomes, allowing impulse transmission so groups of fibers contract together as a pump.

  • Self-Exciting: Does not require nerve stimulation to initiate contraction. Nerves only modulate rate:

  • Acetylcholine: Slows heart rate.

  • Norepinephrine: Increases heart rate.

6. Energy Production for Muscle Contraction

Muscles generate ATP (adenosine triphosphate) through three primary pathways:

Creatine Phosphate Pathway:
- Creatine phosphate + ADP --(Creatine kinase)--> Creatine + ATP

  • Provides rapid ATP restoration during immediate activity.

Aerobic Respiration:
- Glucose + O2 --> CO2 + H2O + ATP

  • Glucose breaks down into pyruvic acid, which converts to acetyl coenzyme A to enter the Krebs cycle (citric acid cycle) in mitochondria.

  • Requires large amounts of oxygen; muscle cells store extra oxygen using the pigment myoglobin.

Anaerobic Respiration:

  • Glucose --> Lactic acid + ATP

  • Occurs when oxygen is low; produces small amounts of ATP. Lactic acid buildup contributes to muscle fatigue and cramps.

7. Pathophysiology & Muscle Disorders

  • Botulism:

  • Cause: Toxin from Clostridium botulinum (found in improperly canned foods, honey, or open wounds).

  • Mechanism: Blocks Acetylcholine (ACh) release.

  • Symptoms: Muscle weakness, paralysis, double vision, dry mouth, difficulty swallowing/breathing, nausea, vomiting, constipation.

  • Fibromyalgia:

  • Cause: Poorly understood; linked to sleep disturbances, emotional distress, decreased muscle blood flow, or viral infection. Most common in women aged 20–50.

  • Symptoms: Chronic pain in joints, muscles, and tendons lasting at least 3 months, fatigue, tenderness, facial pain.

  • Muscular Dystrophy:

  • Cause: Hereditary / genetic defect (e.g., X-linked recessive genetic defect).

  • Symptoms: Progressive muscle weakness, loss of muscle mass, difficulty walking, frequent falls, delayed motor skills, curved spine, claw hand/clubfoot.

  • Myasthenia Gravis:

  • Cause: Autoimmune disorder; antibodies block acetylcholine receptors at the neuromuscular junction.

  • Symptoms: Muscle weakness, ptosis (drooping eyelids), double vision, slurred speech, difficulty chewing/swallowing/breathing, fatigue.

  • Rhabdomyolysis:

  • Cause: Severe muscle damage (from trauma, extreme exercise, drug overdose, or alcoholism) releasing excess myoglobin into the bloodstream.

  • Symptoms: Very dark / tea-colored urine, muscle tenderness, weakness, stiffness, joint pain, kidney damage.

  • Tetanus (Lockjaw):

  • Cause: Toxin from Clostridium tetani entering via open/deep puncture wounds.

  • Mechanism: Attacks inhibitory (Renshaw) cells in the central nervous system.

  • Symptoms: Lockjaw (trismus), facial/neck muscle spasms, risus sardonicus, severe bodily spasms (opisthotonos), fever, respiratory arrest. Preventable by vaccine.

8. Muscle Dynamics & Aging

  • Hypertrophy: Increase in overall muscle size.

  • Atrophy: Decrease in muscle size and loss of myofibrils due to disuse or disease.

  • Rigor Mortis: Postmortem muscle stiffening caused by ATP depletion, decreased intracellular pH, and calcium leaking into sarcomeres to lock actin and myosin cross-bridges.

  • Effects of Aging:

  • Muscle fiber loss begins around age 25.

  • By age 80, up to 50% of muscle mass can be lost.

  • Reduced capillary density leads to longer recovery times after exercise.

9. Muscle Attachments & Body Movements

  • Attachments:

  • Origin: Attachment site that does not move during contraction.

  • Insertion: Attachment site that moves during contraction.

  • Body Movement Definitions:
    Abduction: Moving a body part away from the anatomical position.

  • Adduction: Moving a body part toward the anatomical position.

  • Flexion: Bending a body part.

  • Extension: Straightening a body part.

  • Hyperextension: Extending past the normal anatomical position.

  • Dorsiflexion: Pointing toes upward.

  • Plantar flexion: Pointing toes downward.

  • Rotation: Twisting a body part around its axis.

  • Circumduction: Moving a body part in a circular motion.

  • Pronation: Turning the palm downward.

  • Supination: Turning the palm upward.

  • Inversion: Turning the sole of the foot medially (inward).

  • Eversion: Turning the sole of the foot laterally (outward).

  • Retraction: Moving a body part backward (posteriorly).

  • Protraction: Moving a body part forward (anteriorly).

  • Elevation: Raising a body part (e.g., shrugging shoulders).

  • Depression: Lowering a body part.

10. Major Skeletal Muscles

  • Muscles of the Head & Neck:

  • Sternocleidomastoid: Pulls head to one side and down to the chest.

  • Splenius capitis: Rotates and bends head to the side.

  • Frontalis: Raises eyebrows.

  • Orbicularis oculi: Closes eyes.

  • Orbicularis oris: Puckers lips.

  • Zygomaticus: Pulls corners of mouth up (smiling).

  • Platysma: Pulls corners of mouth down.

  • Masseter & Temporalis: Close the jaw during mastication (chewing).

  • Muscles Moving the Arm:

  • Pectoralis major: Pulls arm across chest; adducts and rotates arm medially.

  • Latissimus dorsi: Extends, adducts, and rotates arm medially.

  • Deltoid: Abducts and extends arm at the shoulder.

  • Subscapularis: Rotates arm medially.

  • Infraspinatus: Rotates arm laterally.