Muscle and Nervous System Anatomy

MUSCLES

TYPES OF MUSCLE

  • Skeletal Muscle

    • Attaches to the bones

    • Voluntary: under conscious control

    • Microscopically exhibits a striated appearance

  • Smooth Muscle

    • Involuntary

    • Not striated

  • Cardiac Muscle

    • Striated

    • Involuntary

    • A specialized type of muscle

PROPERTIES OF MUSCLE

  • Contractility

    • The ability of muscle to shorten forcefully

    • Causes movement of attached structures or increases pressure inside hollow organs or vessels

  • Excitability

    • The capacity of muscle to respond to a stimulus

    • Skeletal muscle contracts via stimulation by nerves

    • Smooth and cardiac muscles can contract without outside stimuli but also respond to stimulation by nerves and hormones

  • Extensibility

    • Muscle can be stretched beyond its normal resting length and still contract

  • Elasticity

    • The ability of muscle to recoil to its original resting length after being stretched

FUNCTIONS OF SKELETAL MUSCLE

  1. Produce Skeletal Movement

    • Contractions pull on tendons to move bones

  2. Maintain Posture and Body Position

    • Tension maintains posture (e.g., holding head still while reading)

  3. Support Soft Tissues

    • Layers of skeletal muscle form the abdominal wall and floor of the pelvic cavity

  4. Guard Entrances and Exits

    • Skeletal muscles encircle openings of digestive and urinary tracts

  5. Maintain Body Temperature

  6. Store Nutrient Reserves

    • Inadequate diet leads to breakdown of contractile proteins in skeletal muscle, releasing amino acids into circulation, which can be used by the liver for glucose synthesis or energy production

    • Kwashiorkor: severe malnutrition, common in developing countries, presenting as swelling under the skin (oedema) due to fluid retention

STRUCTURE OF SKELETAL MUSCLE

  • Each muscle composed of various fibers covered by connective tissue (fascia), blood vessels, and nerves

  • Each skeletal muscle fiber is a long, cylindrical cell with multiple nuclei found near the plasma membrane

  • Fibers exhibit striated appearance due to alternating light and dark bands

  • Terminology

    • Sarco: refers to muscle

    • Mysium: refers to muscle

  1. Epimysium

    • Dense layer of collagen fibers

    • Covers entire muscle surface, connects to deep fascia

  2. Perimysium

    • Penetrates the muscle to form bundles (fasciculi)

  3. Endomysium

    • Thin fascia covering each muscle fiber

    • Muscles anchored to bones via tendons formed from extensions of endomysium, perimysium, and epimysium

MICROSCOPIC ANATOMY OF SKELETAL MUSCLE

  • Myofibrils run parallel to the muscle fiber length

    • Sarcomere: functional unit of muscle

    • A Band: dark band with thick and thin filaments

    • I Band: light band

    • H Zone: region containing only thick filaments

  • Thick filaments mainly consist of myosin

  • Thin filaments mainly consist of actin, troponin, and tropomyosin

SARCOPLASMIC RETICULUM AND T TUBULES

  • Skeletal muscle fibers contain two intracellular tubule sets:

    1. Sarcoplasmic Reticulum (SR): system of membranous tubules surrounding myofibrils; releases and sequesters calcium ions

    2. T Tubules: invaginations of the sarcolemma that run between terminal cisternae of the SR; deliver electrical stimulus deep into the cell

    • Sarcoplasm: cytoplasm of muscle fibers

MUSCLE CONTRACTION

  1. Isotonic Contraction: muscles shorten; contraction force is constant

  2. Isometric Contraction: contraction occurs without sarcomere shortening; thin myofilaments slide over thick ones

  3. Cross-Bridge Cycle:

    • Actin myofilaments contain tropomyosin and troponin

    • Tropomyosin blocks cross-bridge attachment in relaxed muscle

    • Calcium ions (Ca++) in sarcoplasm cause tropomyosin to move and expose binding sites for myosin to form cross-bridges

    • Muscle relaxation is produced by active transport of Ca++ out of the sarcoplasm back into SR

ATP AND ENERGY SUPPLY IN MUSCLE

  • ATP: main energy source; high-energy molecule

    • Hydrolysis of ATP yields ADP (Adenosine Diphosphate) and inorganic phosphate

    • ATP can be recycled by adding phosphate to ADP

    • Equation:
      ATP<br>ightleftharpoonsADP+P+Energy+H2OATP <br>ightleftharpoons ADP + P + Energy + H_2O

CATEGORIES OF MUSCLE

  • Cardiac Muscle:

    • All myocardial cells electrically joined to form functional units

  • Smooth Muscle:

    • Contains actin and some myosin (16:1 smooth vs. 2:1 striated)

    • Contraction is initiated by increases in Ca++

    • Calmodulin replaces troponin in smooth muscle for contraction regulation

MUSCLE METABOLISM

  • ATP sources include:

    • Direct Phosphorylation of ADP by creatine phosphate (makes 1 ATP at a time)

    • Aerobic Respiration: occurs in mitochondria, breaks down glucose, yields 34 ATP for prolonged energy, requires oxygen

    • Anaerobic Glycolysis: breaks glucose to lactic acid yielding 2 ATP; lactic acid accumulation leads to fatigue; provides energy for 30 seconds of activity

NERVOUS SYSTEM DIVISIONS

  1. Central Nervous System (CNS): brain and spinal cord

  2. Peripheral Nervous System (PNS): includes sensory receptors, nerves, ganglia, plexuses

CELLS OF THE NERVOUS SYSTEM

  • Composed of neurons and non-neuronal cells (neuroglia)

  • Neurons: receive stimuli and conduct action potentials

  • Neuroglia: support cells more numerous than neurons

    • Types of Neurons:

    • Multipolar: many dendrites, one axon

    • Bipolar: one dendrite, one axon

    • Unipolar: one process extending from the cell body

NEUROGLIA OF THE CNS

  1. Astrocytes: star-shaped supporting cells, form blood-brain barrier

  2. Ependymal Cells: line ventricles of the brain; produce cerebrospinal fluid

  3. Microglia: mobile phagocytic cells in response to inflammation

  4. Oligodendrocytes: surround axons, form myelin sheaths

NEUROGLIA OF THE PNS

  1. Schwann Cells: wrap around axons in PNS

  2. Satellite Cells: surround neuron cell bodies in ganglia; provide support and nutrients

MYELINATED AND UNMYELINATED AXONS

  • Myelinated axons have phospholipid-rich membranes; interruptions create nodes of Ranvier for faster conduction (Saltatory conduction)

GRAY AND WHITE MATTER

  • White Matter: aggregates of myelinated neuron processes

  • Gray Matter: consists of neuron cell bodies, dendrites, unmyelinated axons, and neuroglia; appears gray due to lack of myelin

ACTION POTENTIAL

  • Describes rapid changes in membrane potential:

    • Depolarization: rapid Na+ influx changes membrane potential from -55 mV to +30 mV

    • Repolarization: K+ channels open, recovering resting membrane potential

    • Refractory Periods:

    • Absolute: no second action potential can occur

    • Relative: a second action potential can occur with stronger stimulus

SYNAPSES

  • Electrical Synapses: ionic current flows directly through gap junctions; faster communication

  • Chemical Synapses: neurotransmitters released from presynaptic neurons into synaptic cleft to bind to receptors on postsynaptic neurons, generating graded potentials

BRAIN AND SPINAL CORD

STRUCTURE OF BRAIN

  • Parts: cerebrum, diencephalons, brain stem, cerebellum

  • Meninges: three connective tissue layers protecting the brain and spinal cord

    • Dura mater (outer), Arachnoid mater (middle), Pia mater (inner)

    • Subarachnoid space contains cerebrospinal fluid

CEREBROSPINAL FLUID (CSF)

  • Clear liquid that protects CNS against chemical/physical injury

  • Carries nutrients to neurons and glial cells

FUNCTIONS OF THE BRAIN

  • Cerebrum: interprets sensory impulses, controls movement, and functions in emotional/intellectual processes

  • Diencephalon: regulates autonomic nervous system and pituitary gland activity

  • Cerebellum: smooths and coordinates skilled movements

  • Brain Stem (Medulla Oblongata): relays impulses between brain parts and regulates heartbeat

SPINAL CORD FUNCTIONS

  • Conducts sensory nerve impulses to the brain and motor impulses from the brain

  • Integrates spinal reflexes

REFLEXES AND REFLEX ARCS

  • The reflex arc is the simplest neural pathway;

    • Components: sensory receptor, sensory neuron, interneuron, motor neuron, effector organ

    • Somatic reflexes involve skeletal muscle contraction; autonomic reflexes regulate smooth/cardiac muscle and glands

AUTONOMIC NERVOUS SYSTEM (ANS)

  • Operates via reflex arcs

  • Involves sympathetic (fight or flight) and parasympathetic (rest and digest) divisions