Comprehensive Study Notes: Muscular and Nervous Systems, Homeostasis, and Cellular Foundations

Muscle Tissue and Contraction

Muscle Structure and Organization

Muscle tissue is organized in a hierarchical structure that allows for efficient contraction and force generation.

• Epimysium: The outermost connective tissue layer surrounding the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers called fascicles.

• Endomysium: Thin connective tissue surrounding each individual muscle fiber.

• Bundling Anatomy: Myofibrils (contractile elements) are inside muscle fibers (cells), which are grouped into fascicles, and fascicles are bundled to form the whole muscle.

Sarcomere Structure

The sarcomere is the functional unit of muscle contraction, composed of repeating patterns of thick and thin filaments.

• Actin: Thin filament protein.

• Myosin: Thick filament protein.

• H zone: Central region of the A band where only myosin is present; shortens during contraction.

• I band: Region containing only actin; shortens during contraction.

• A band: Length of the myosin filaments; remains the same during contraction.

Which bands change length during contraction? The I band and H zone decrease in length, while the A band remains constant. This is due to the sliding filament mechanism.

Cross Bridge Cycle

The cross bridge cycle describes the molecular events that lead to muscle contraction.

1. Cross Bridge Formation: Myosin head attaches to actin (requires calcium to expose binding sites).

2. Power Stroke: Myosin head pivots, pulling actin filament toward the center of the sarcomere; ADP and Pi are released.

3. Cross Bridge Detachment: ATP binds to myosin, causing it to detach from actin.

4. Cocking of Myosin Head: ATP is hydrolyzed to ADP and Pi, re-energizing the myosin head.

Role of ATP: ATP is required for detachment of myosin from actin and for re-cocking the myosin head.

What prevents the cycle in relaxed muscle? In the absence of calcium, tropomyosin blocks the binding sites on actin, preventing cross bridge formation.

Role of Action Potential in Muscle Contraction

An action potential in a motor neuron triggers the release of acetylcholine at the neuromuscular junction, leading to depolarization of the muscle fiber membrane and initiation of contraction.

Tension, Load, and Types of Contraction

• Tension: The force generated by a muscle.

• Load: The force exerted by the object to be moved.

• Isometric Contraction: Muscle generates tension without changing length (e.g., holding a weight steady).

• Isotonic Contraction: Muscle changes length while moving a load (e.g., lifting a weight).

Summation and Tetanus

• Summation: Increased force of contraction due to multiple stimuli in rapid succession.

• Unfused (Incomplete) Tetanus: Muscle fibers partially relax between stimuli.

• Fused (Complete) Tetanus: No relaxation between stimuli; sustained maximal contraction.

Factors Affecting Muscle Force

• Number of muscle fibers recruited

• Frequency of stimulation

• Degree of muscle stretch

• Size of muscle fibers

Skeletal Muscle Fiber Types

Muscle fibers differ in contraction speed, metabolic pathway, and appearance.

Type	Metabolism	Color	Example
Slow Oxidative	Aerobic	Red (high myoglobin)	Postural muscles
Fast Glycolytic	Anaerobic	White (low myoglobin)	Jumping, sprinting
Fast Oxidative	Aerobic	Pink	Walking

Color differences are due to myoglobin content and capillary density.

Nervous System Fundamentals

Glial Cells

Glial cells support and protect neurons in the central and peripheral nervous systems.

• Astrocytes: Maintain blood-brain barrier, regulate nutrients.

• Oligodendrocytes: Form myelin in CNS.

• Schwann Cells: Form myelin in PNS.

• Microglia: Immune defense in CNS.

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

• Satellite Cells: Support neurons in PNS ganglia.

Membrane Potential and Resting State

Neurons maintain a voltage difference across their membrane, called the resting membrane potential (typically about -70 mV).

• Sodium (Na+): Higher concentration outside the cell.

• Potassium (K+): Higher concentration inside the cell.

• Channels: K+ leak channels are mostly open; Na+ channels are mostly closed at rest.

• Sodium-Potassium Pump: Maintains gradients by pumping 3 Na+ out and 2 K+ in, using ATP.

$V_{rest} \approx -70\ \mathrm{mV}$

Graded Potentials vs Action Potentials

• Graded Potentials: Local changes in membrane potential; vary in size; decay with distance.

• Action Potentials: All-or-none electrical impulses; propagate along axons without decrement.

Phases of the Action Potential

• Threshold: Membrane potential at which AP is triggered.

• Depolarization: Rapid Na+ influx.

• Repolarization: K+ efflux restores negative potential.

• Hyperpolarization: Membrane potential becomes more negative than resting.

$\text{AP: Threshold} \rightarrow \text{Depolarization} \rightarrow \text{Repolarization} \rightarrow \text{Hyperpolarization}$

Stimulus Intensity and Refractory Period

• Intensity: Encoded by frequency of action potentials, not amplitude.

• Refractory Period: Time during which a neuron cannot fire another AP; ensures one-way propagation and limits firing rate.

Synapse and Synaptic Transmission

• Synapse: Junction between two neurons or a neuron and effector cell.

• Events: AP triggers Ca2+ influx, neurotransmitter release, postsynaptic response.

• EPSP (Excitatory Postsynaptic Potential): Depolarization due to Na+ influx.

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarization due to K+ efflux or Cl- influx.

Somatic vs Autonomic Nervous System

• Somatic: Voluntary control of skeletal muscles.

• Autonomic: Involuntary control of smooth muscle, cardiac muscle, glands.

Sympathetic ("fight or flight"): Uses norepinephrine; increases heart rate, dilates pupils. Parasympathetic ("rest and digest"): Uses acetylcholine; slows heart rate, stimulates digestion.

Dual innervation: Most organs receive input from both divisions, allowing fine control.

Sensory Receptors and Adaptation

• Modality: Type of stimulus detected (e.g., light, pressure).

• Adequate Stimulus: Specific type of stimulus a receptor is most sensitive to.

• Sensory Adaptation: Decreased response to a constant stimulus over time.

Grey Matter vs White Matter

• Grey Matter: Neuron cell bodies, dendrites, unmyelinated axons.

• White Matter: Myelinated axons; organized into ascending (sensory), descending (motor), and commissural (crossing) tracts.

Major Brain Regions and Functions

• Cerebrum: Higher cognitive functions, voluntary movement.

• Ventricles: Fluid-filled spaces; produce and circulate cerebrospinal fluid.

• Cerebellum: Coordination and balance.

• Thalamus: Sensory relay station.

• Brainstem: Includes midbrain, pons, medulla oblongata; controls vital functions.

• Pons: Relays signals, regulates breathing.

• Cortex: Outer layer of cerebrum; involved in perception, thought, and voluntary movement.

• Hypothalamus: Regulates homeostasis, endocrine system.

• Diencephalon: Contains thalamus and hypothalamus.

• Pituitary Gland: Master endocrine gland.

• Medulla Oblongata: Controls heart rate, breathing, blood pressure.

• Blood Brain Barrier: Protects brain from harmful substances.

Spinal Cord Roots

• Ventral Root: Motor (efferent) fibers.

• Dorsal Root: Sensory (afferent) fibers.

Homeostasis and Cellular Foundations

Homeostasis and Feedback Mechanisms

• Homeostasis: Maintenance of a stable internal environment.

• Negative Feedback: Response reduces or shuts off original stimulus (e.g., body temperature regulation).

• Positive Feedback: Response enhances original stimulus (e.g., blood clotting).

Atoms, Isotopes, and Ions

• Subatomic Particles: Protons (positive, in nucleus), neutrons (neutral, in nucleus), electrons (negative, orbit nucleus).

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Ions: Atoms that have gained or lost electrons; cations are positive, anions are negative.

• Major Elements in Human Body: Oxygen, carbon, hydrogen, nitrogen.

Cell Structure and Function

• Major Parts: Plasma membrane, cytoplasm, nucleus, organelles.

• Plasma Membrane: Selectively permeable barrier; composed of phospholipid bilayer with embedded proteins.

Proteins and ATP

• Proteins: Polymers of amino acids; perform structural, enzymatic, and regulatory functions.

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Metabolic Reactions

• Catabolic: Breakdown of molecules; releases energy.

• Anabolic: Synthesis of molecules; requires energy.

Membrane Transport Mechanisms

• Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Passive transport via membrane proteins.

• Active Transport: Movement against concentration gradient; requires ATP.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Cell Division: Mitosis vs Meiosis

• Mitosis: Division of somatic cells; produces two identical daughter cells.

• Meiosis: Division of germ cells; produces four genetically unique gametes with half the chromosome number.

Key difference: Mitosis maintains chromosome number; meiosis reduces it by half.