Membrane Physiology, Neuronal Function, and Nervous System Organization

Membrane Structure and Physiology

• Lipid Bilayer Composition:

  • Formed by amphipathic phospholipid molecules consisting of polar (hydrophilic) heads and non-polar (hydrophobic) lipid tails.

  • In aqueous environments, lipid molecules spontaneously organize into bilayers or spherical structures such as liposomes, placing polar heads in contact with extracellular and intracellular water ($H_2O$) while insulating hydrophobic tails inside.

• Functional Role of Cell Membrane:

  • Represents the primary barrier and first layer of physiological machinery for cellular regulation.

  • Demonstrates selective permeability: large, highly polar, or charged molecules cannot freely diffuse through the lipid core.

  • Embedded transport proteins provide specialized pathways across the membrane.

Mechanisms of Membrane Transport

• Passive Transport and Simple Diffusion:

  • Unassisted movement of substances directly across the lipid membrane or through channel proteins down their concentration gradient (from higher concentration to lower concentration).

  • Does not require expenditure of metabolic energy (ATP).

• Transporter Proteins:

  • Highly organized integral membrane proteins facilitating the movement of specific solutes across the membrane.

• Active Transport:

  • Transport of ions or molecules AGAINST their chemical or electrochemical concentration gradient (from lower concentration to higher concentration).

  • Requires metabolic fuel, typically through the hydrolysis of adenosine triphosphate (ATP).

• Cotransport Mechanisms:

  • Simultaneous transport of multiple solute species across the membrane, driven either directly or indirectly by ion gradients.

• Ion Channels:

  • Transmembrane proteins providing selective pore pathways for ions such as Sodium ($Na^+$), Potassium ($K^+$), and Chloride ($Cl^-$).

  • Leak Channels: Continuously open channels that permit constant passive diffusion of ions down electrochemical gradients.

  • Gated Channels: Opened or closed in response to specific environmental stimuli:

  • Chemically-Gated (Ligand-Gated) Channels: Open upon binding of specific chemical messengers or neurotransmitters.

  • Mechanically-Gated Channels: Open in response to physical deformation, stretch, or mechanical pressure on the cell membrane.

  • Voltage-Gated Channels: Open or close in response to changes in membrane electrical potential.

Electrochemical Gradients and Membrane Potential

• Electrochemical Equilibrium:

  • The state at which the driving force of an ion's chemical concentration gradient is exactly equal and opposite to the driving force of its electrical charge gradient across the membrane.

  • Calculated mathematically as the equilibrium potential ($E_{\text{ion}}$) for a specific ion.

• Sodium Equilibrium Potential ($E_{Na^+}$):

  • Sodium ($Na^+$) exists in higher concentration outside the cell than inside.

  • Inflow of $Na^+$ down its concentration gradient brings positive charges into the cell, creating an increasingly positive internal charge that electrically repels further $Na^+$ entry.

  • Electrochemical equilibrium for Sodium is reached at $E_{Na^+} = +55\,\text{mV}$.

• Potassium Equilibrium Potential ($E_{K^+}$):

  • Potassium ($K^+$) exists in higher concentration inside the cell than outside.

  • Outflow of $K^+$ down its concentration gradient carries positive charges out of the cell, leaving behind an accumulation of negative charge inside.

  • Electrochemical equilibrium for Potassium is reached at $E_{K^+} = -90\,\text{mV}$.

• Resting Membrane Potential ($V_m$):

  • The baseline electrical potential difference across the plasma membrane of a resting cell, typically around $-70\,\text{mV}$.

  • Differential Permeability: The resting cell membrane is significantly "leakier" to $K^+$ than to $Na^+$ because $K^+$ leak channels far outnumber $Na^+$ leak channels. Consequently, the resting membrane potential sits much closer to $E_{K^+}$ ($-90\,\text{mV}$) than to $E_{Na^+}$ ($+55\,\text{mV}$).

  • Primary Active Maintenance ($Na^+/K^+$ ATPase Pump):

  • Operates continuously to maintain fundamental intracellular and extracellular concentration gradients.

  • Actively pumps $3\,Na^+$ ions out of the cell and $2\,K^+$ ions into the cell per ATP molecule consumed.

  • Contributes directly to electrogenicity by net movement of positive charge out of the cell.

• Charge Distribution Across Membrane:

  • In a resting state, the interior of the cell is slightly negative relative to the exterior.

  • Unbalanced positive charges align along the outer leaflet of the lipid bilayer while negative charges align along the inner leaflet.

Excitable Cells and Electrical Potentials

• Definition of Excitable Cells:

  • Specialized cells (such as neurons and muscle cells) capable of using membrane potentials and gated ion channels to produce rapid, controlled changes in cell membrane charge.

• Graded Potentials:

  • Small, localized fluctuations in membrane potential caused by localized ion flux across gated channels.

  • Amplitude varies directly with stimulus intensity; signals decay over space and time.

  • Do not independently propagate long distances and do not reach action potential threshold on their own.

• Action Potentials:

  • Rapid, large, stereotyped electrical impulses ("all-or-nothing" phenomenon) characterized by complete membrane depolarization, repolarization, and hyperpolarization.

  • Triggered once membrane depolarization reaches a specific threshold value ($-55\,\text{mV}$).

Voltage-Gated Channels and Action Potential Kinetics

• Voltage-Gated Channel Characteristics:

  • Voltage-Gated Sodium ($Na^+$) Channels:

  • Possess two functional gates: a fast activation gate and a slow inactivation gate.

  • At Resting State ($-70\,\text{mV}$): Fast gate is closed, slow gate is open.

  • At Threshold ($-55\,\text{mV}$): Fast activation gate opens rapidly, causing explosive $Na^+$ influx and rapid depolarization toward $+55\,\text{mV}$.

  • At Peak Potential ($+55\,\text{mV}$): Slow inactivation gate closes completely, halting further $Na^+$ influx.

  • Voltage-Gated Potassium ($K^+$) Channels:

  • Possess a single gate that is triggered to open at $-55\,\text{mV}$, but opens slowly.

  • Full channel opening occurs at peak depolarization ($+55\,\text{mV}$), allowing rapid $K^+$ efflux out of the cell.

• Phases of the Action Potential:

  1. Resting State: Membrane potential maintained at $-70\,\text{mV}$ by $Na^+/K^+$ ATPase and leak channels.

  2. Threshold: Local depolarization reaches $-55\,\text{mV}$, triggering voltage-gated channel dynamics.

  3. Rapid Depolarization: Fast opening of $Na^+$ channels causes massive $Na^+$ influx, driving membrane voltage from $-55\,\text{mV}$ to peak potential at $+55\,\text{mV}$.

  4. Rapid Repolarization: $Na^+$ channel slow gates close (inactivation) while $K^+$ channels fully open, driving massive $K^+$ efflux and dropping potential back toward resting negative values.

  5. Hyperpolarization: Slow closure of $K^+$ channels allows membrane potential to temporarily drop below $-70\,\text{mV}$ toward $E_{K^+}$ ($-90\,\text{mV}$).

  6. Return to Resting Potential: $Na^+/K^+$ ATPase and baseline leak channels restore resting ion distributions and potential at $-70\,\text{mV}$.

• Refractory Period:

  • Period during and immediately following an action potential during which the cell membrane cannot fire another action potential (Absolute Refractory Period due to closed $Na^+$ inactivation gates) or requires a substantially stronger depolarizing stimulus to reach threshold (Relative Refractory Period due to ongoing $K^+$ efflux and hyperpolarization).

Neuronal Physiology and Signal Propagation

• Structural Components of a Neuron:

  • Dendrites: Branching input structures that receive incoming chemical or electrical signals from other neurons.

  • Cell Body (Soma): Metabolic center containing the cell nucleus; integrates incoming electrical inputs.

  • Axon Hillock: Specialized region where the soma connects to the axon. Acts as the neuronal "calculator" or decision center by integrating all incoming graded potentials; fires an action potential if local voltage reaches $-55\,\text{mV}$.

  • Axon: Long cylindrical process that conducts action potentials away from the soma (anterograde conduction). Retrograde propagation is non-physiological under normal conditions.

  • Nerve Terminal (Axon Terminal): Distal terminus that converts electrical action potentials into chemical neurotransmitter release for outgoing intercellular communication.

• Myelination and Saltatory Conduction:

  • Schwann Cells: Glial cells in the Peripheral Nervous System that wrap around axon segments to form insulating myelin sheaths.

  • Nodes of Ranvier: Unmyelinated gaps along the axon between adjacent Schwann cells.

  • Mechanism: Myelin prevents charge leakage across the axonal membrane, forcing action potentials to skip from one Node of Ranvier to the next, dramatically increasing propagation velocity.

Synaptic Transmission and Postsynaptic Potentials

• Postsynaptic Signal Integration:

  • Excitatory Postsynaptic Potential (EPSP): Localized depolarization at the post-synaptic membrane that makes the membrane potential more positive, bringing it closer to the $-55\,\text{mV}$ threshold.

  • Inhibitory Postsynaptic Potential (IPSP): Localized hyperpolarization at the post-synaptic membrane that makes the membrane potential more negative, moving it further away from threshold.

  • Axon Hillock Summation: Summation of all incoming EPSPs and IPSPs determines whether total membrane voltage reaches $-55\,\text{mV}$ to initiate propagation down the axon.

• Electrical Synapses:

  • Cells joined directly via gap junctions, enabling direct physical flow of ionic current between cytoplasm.

  • Allows extremely rapid, bidirectional communication driven directly by local depolarization.

• Chemical Synapses:

  • Presynaptic and postsynaptic cells separated by a fluid-filled gap called the synaptic cleft.

  • Sequence of Events:

  1. Action potential propagates down axon to presynaptic nerve terminal.

  2. Depolarization opens Voltage-Gated Calcium ($Ca^{2+}$) Channels at the terminal.

  3. Influx of extracellular $Ca^{2+}$ into the terminal triggers exocytosis of neurotransmitter-filled synaptic vesicles.

  4. Neurotransmitters diffuse across the synaptic cleft and bind specific receptors on the postsynaptic cell membrane.

Organization of the Nervous System

• Primary Structural Divisions:

  • Central Nervous System (CNS): Composed of the Brain and Spinal Cord.

  • Peripheral Nervous System (PNS): Composed of all neural tissue outside the CNS (cranial and spinal nerves).

• Directional Signal Flow:

  • Afferent Nerves (Sensory Pathways): Carry sensory inputs from peripheral sensory receptors to the CNS ("arrives at central hub").

  • Efferent Nerves (Motor/Autonomic Pathways): Carry motor commands from the CNS to peripheral target tissues and effector organs ("exits central hub").

  • Interneurons: Neurons residing entirely within the CNS that integrate incoming afferent information and coordinate efferent outputs.

• Efferent PNS Subdivisions:

  • Somatic Nervous System:

  • Innervates skeletal (striated) muscle under voluntary control.

  • Uses a single motor neuron chain extending from CNS directly to muscle effector.

  • Always excitatory (+) in effect.

  • Autonomic Nervous System (ANS):

  • Innervates visceral effectors (cardiac muscle, smooth muscle, glands) under involuntary control.

  • Uses a two-neuron chain sequence (preganglionic neuron and postganglionic neuron).

  • Can produce either excitatory (+) or inhibitory (-) effects depending on target receptors.

Autonomic Nervous System Architecture and Functional Divisions

• Autonomic Tone:

  • Continuous baseline balance between sympathetic and parasympathetic input to visceral organs, establishing steady-state physiological function.

• Sympathetic Division ("Fight or Flight"):

  • Anatomical Origin: Thoracic and Lumbar regions of the spinal cord (Thoracolumbar division).

  • Neuron Chain Architecture:

  • First neuron (preganglionic) emerges from spinal cord and terminates in the sympathetic trunk/ganglion outside the spinal cord, releasing Acetylcholine ($ACh$).

  • Second neuron (postganglionic) travels to target effector organs and releases Norepinephrine ($NE$) (also known as noradrenaline).

  • Specialized Sympatho-Adrenal Division:

  • Direct sympathetic preganglionic innervation of the Adrenal Medulla (inner core of adrenal gland).

  • Triggers mass secretion of a mixture of epinephrine (adrenaline) and norepinephrine directly into the bloodstream during extreme stress or physical exertion.

• Parasympathetic Division ("Rest and Digest"):

  • Anatomical Origin: Cervical (brainstem) and Sacral regions of the spinal cord (Craniosacral division).

  • Neuron Chain Architecture:

  • First neuron (preganglionic) has a long axon extending close to or inside the target organ wall before terminating. Releases Acetylcholine ($ACh$).

  • Second neuron (postganglionic) is short and releases Acetylcholine ($ACh$) directly onto effector tissue.

• Comparative Physiological Organ Responses:

  • Heart: Sympathetic stimulation increases rate/contractility (+); Parasympathetic stimulation decreases rate (-).

  • Stomach & Intestines: Parasympathetic stimulation enhances digestion (+); Sympathetic stimulation inhibits activity (-).

  • Exercise State: High Sympathetic activity, Low Parasympathetic activity.

  • Homeostatic / Rest State: High Parasympathetic activity, Low Sympathetic activity.

Central Nervous System Organization and Glial Support

• Cellular Distribution in CNS:

  • Neurons: Account for approximately $10\%$ of total CNS cells; perform specialized electrical signaling.

  • Glial Cells (Neuroglia): Account for approximately $90\%$ of total CNS cells; perform structural, metabolic, and protective support functions.

• Neuroglia Types and Functions:

  • Oligodendrocytes: Form myelin sheaths around axons within the CNS. Unlike Schwann cells in the PNS, a single oligodendrocyte extends processes to wrap around multiple adjacent CNS axons simultaneously.

  • Ependymal Cells: Epithelial-like cells lining brain ventricles and the central canal of the spinal cord. Synthesize and secrete Cerebrospinal Fluid (CSF), a clear fluid that surrounds and cushions the brain and spinal cord, providing hydraulic protection and enabling neural tissue to "float".

  • Microglia Cells: Functions as the primary immune defense system of the CNS, clearing cellular debris and fighting infections within brain tissue.

  • Astrocytes: Star-shaped glial cells that envelop brain capillaries with foot processes, forming tight junctions with capillary endothelial cells to establish the Blood-Brain Barrier (BBB). The BBB strictly restricts fluid, ion, and solute movement into brain tissue, protecting neurons while assisting in metabolic support and structural layout.

CNS Protective Structures, Ventricular System, and Functional Cortical Mapping

• Protective Meningeal Layers:

  • Dura Mater: Outermost, thick, tough fibrous connective tissue layer; contains dural venous sinuses for fluid drainage.

  • Arachnoid Mater: Middle, web-like avascular layer; subarachnoid space contains circulating cerebrospinal fluid (CSF).

  • Pia Mater: Innermost, delicate, highly vascular layer adhering directly to the surface contours of the brain and spinal cord.

• Ventricular System and CSF Flow Pathway:

  • CSF synthesized by ependymal cells flows from the Lateral Ventricles $\rightarrow$ Third Ventricle $\rightarrow$ Fourth Ventricle $\rightarrow$ Central Canal of spinal cord $\rightarrow$ Subarachnoid Space $\rightarrow$ reabsorbed into dural venous sinuses.

• Functional Brain Regions:

  • Motor Cortex: Located anteriorly; controls voluntary skeletal muscle movement.

  • Sensory Cortex (Somatosensory): Processes touch, tactile, and somatic sensory inputs.

  • Auditory Cortex: Converts sound wave energy into neural action potentials to process hearing.

  • Visual Cortex: Processes visual perception.

  • Top / Front Cortical Regions: Executive functions, abstract reasoning, and higher cognitive brain activity.

  • Brainstem & Cerebellum: Regulates vital involuntary functions including respiration, vision adjustments, and motor balance.

• Spinal Cord Architecture and Reflex Arcs:

  • Protected surrounding by vertebrae and meninges.

  • Gray Matter: Central butterfly-shaped core containing neuron cell bodies and unmyelinated interneurons.

  • Dorsal Horns: Entry region for sensory inputs via afferent sensory nerves (dorsal fin orientation).

  • Ventral Horns: Exit region for motor commands via efferent motor and autonomic nerves.

  • White Matter: Outer regions containing bundles of myelinated axons ascending to or descending from the brain.

  • Reflex Arc: Anatomical relationship between dorsal afferent sensory input and ventral efferent motor output within gray matter, enabling immediate reflex responses without requiring cortical processing.

Neurotransmitter Systems and Receptor Subtypes

• Acetylcholine ($ACh$) Signaling:

  • Binds to Cholinergic Receptors:

  • Nicotinic Cholinergic Receptors: Ligand-gated ion channels that directly open upon $ACh$ binding, producing rapid excitatory action potentials in postganglionic autonomic neurons and skeletal muscle motor endplates.

  • Muscarinic Cholinergic Receptors: G-protein coupled receptors found on autonomic parasympathetic effector tissues; can mediate excitatory or inhibitory responses depending on downstream effector pathways.

• Norepinephrine ($NE$) Signaling:

  • Binds to Adrenergic Receptors:

  • Alpha ($\alpha$) Adrenergic Receptors: Subtypes (\alpha_1) and (\alpha_2).

  • Beta ($\beta$) Adrenergic Receptors: Subtypes (\beta_1) and (\beta_2).

  • Mediate sympathetic target responses, exerting excitatory or inhibitory effects across cardiovascular, respiratory, and visceral organ systems.

Membrane Structure and Physiology

• Lipid Bilayer: Made of phospholipid molecules with polar, water-loving (hydrophilic) heads on the outside and non-polar, water-fearing (hydrophobic) tails on the inside.

• Cell Membrane Role: Functions as the primary barrier for cellular regulation.

  • Selective Permeability: Large, highly polar, or charged molecules cannot freely pass through.

  • Transport Proteins: Embedded proteins create pathways for specific molecules to cross.

Mechanisms of Membrane Transport

• Passive Transport (Simple Diffusion): Unassisted movement of substances down their concentration gradient (from high to low concentration) without spending ATP energy.

• Transporter Proteins: Integral proteins that help specific solutes move across the membrane.

• Active Transport: Movement of ions or molecules AGAINST their concentration gradient (from low to high concentration). Requires metabolic energy ($ATP$).

• Cotransport: Moving two or more solute species at the same time, driven by ion gradients.

• Ion Channels: Pore pathways for specific ions like Sodium ($Na^+$), Potassium ($K^+$), and Chloride ($Cl^-$).

  • Leak Channels: Permanently open channels that allow constant passive movement of ions.

  • Gated Channels: Open or close in response to specific triggers:

  • Chemically-Gated (Ligand-Gated): Opened by binding neurotransmitters or chemical signals.

  • Mechanically-Gated: Opened by physical pressure or stretching.

  • Voltage-Gated: Opened by changes in electrical voltage across the membrane.

Electrochemical Gradients and Membrane Potential

• Electrochemical Equilibrium: The state where the chemical push (concentration gradient) equals the electrical pull (charge gradient) for an ion ($E_{\text{ion}}$).

• Sodium Equilibrium Potential ($E_{Na^+}$): High $Na^+$ outside. $Na^+$ flows in down its concentration gradient until the internal positive charge repels further entry. Equilibrium is reached at $+55\,\text{mV}$.

• Potassium Equilibrium Potential ($E_{K^+}$): High $K^+$ inside. $K^+$ flows out down its concentration gradient until the internal negative charge prevents further exit. Equilibrium is reached at $-90\,\text{mV}$.

• Resting Membrane Potential ($V_m$): The baseline voltage across a resting cell membrane, typically $-70\,\text{mV}$.

  • Differential Permeability: Resting membranes have many more $K^+$ leak channels than $Na^+$ leak channels, making resting voltage sit close to $E_{K^+}$ ($-90\,\text{mV}$).

  • $Na^+/K^+$ ATPase Pump: Consumes $ATP$ to actively pump $3\,Na^+$ out and $2\,K^+$ into the cell, maintaining essential ion gradients.

Excitable Cells and Electrical Potentials

• Excitable Cells: Neurons and muscle cells that use membrane voltage changes to send quick electrical signals.

• Graded Potentials: Small, local voltage changes that weaken over distance and time; do not trigger an action potential on their own.

• Action Potentials: Fast, all-or-nothing electrical signals triggered when membrane voltage reaches the threshold of $-55\,\text{mV}$.

Voltage-Gated Channels and Action Potential Kinetics

• Voltage-Gated Sodium ($Na^+$) Channels:

  • Possess a fast activation gate and a slow inactivation gate.

  • At Rest ($-70\,\text{mV}$): Fast gate is closed, slow gate is open.

  • At Threshold ($-55\,\text{mV}$): Fast gate opens quickly, causing a rapid $Na^+$ influx toward $+55\,\text{mV}$.

  • At Peak ($+55\,\text{mV}$): Slow inactivation gate closes, stopping $Na^+$ entry.

• Voltage-Gated Potassium ($K^+$) Channels:

  • Triggered at $-55\,\text{mV}$, but open slowly.

  • Fully open at peak depolarization ($+55\,\text{mV}$), allowing rapid $K^+$ efflux out of the cell.

• Phases of an Action Potential:

  1. Resting State: Maintained at $-70\,\text{mV}$ by leak channels and the $Na^+/K^+$ pump.

  2. Threshold: Voltage reaches $-55\,\text{mV}$.

  3. Depolarization: Fast opening of $Na^+$ channels causes massive $Na^+$ influx up to $+55\,\text{mV}$.

  4. Repolarization: $Na^+$ channels close while $K^+$ channels open, letting $K^+$ rush out to lower the voltage.

  5. Hyperpolarization: Slow closing of $K^+$ channels temporarily drops voltage below $-70\,\text{mV}$ toward $-90\,\text{mV}$.

  6. Return to Rest: $Na^+/K^+$ pump and leak channels restore resting voltage to $-70\,\text{mV}$.

• Refractory Period:

  • Absolute: Impossible to fire another action potential because $Na^+$ inactivation gates are closed.

  • Relative: Requires a stronger stimulus to fire because the cell is hyperpolarized.

Neuronal Physiology and Signal Propagation

• Neuron Structure:

  • Dendrites: Receive incoming chemical or electrical signals.

  • Soma (Cell Body): Integrates incoming electrical signals.

  • Axon Hillock: Connects soma to axon; acts as the decision center that fires an action potential if voltage reaches $-55\,\text{mV}$.

  • Axon: Long fiber that carries action potentials away from the soma.

  • Nerve Terminal: End of the axon that releases neurotransmitters.

• Myelination and Saltatory Conduction:

  • Schwann Cells: Glial cells in the PNS that wrap around axons to form insulating myelin sheaths.

  • Nodes of Ranvier: Unmyelinated gaps between adjacent Schwann cells.

  • Mechanism: Myelin prevents current leakage, forcing the action potential to jump from node to node, speeding up signal propagation.

Synaptic Transmission and Postsynaptic Potentials

• Postsynaptic Signal Integration:

  • EPSP (Excitatory): Local depolarization that shifts voltage closer to threshold ($-55\,\text{mV}$).

  • IPSP (Inhibitory): Local hyperpolarization that shifts voltage further from threshold.

  • Summation: The Axon Hillock totals all EPSPs and IPSPs to determine if threshold is met.

• Synapses:

  • Electrical Synapses: Direct connection via gap junctions for immediate, bidirectional current flow.

  • Chemical Synapses: Cells separated by a fluid-filled synaptic cleft.

  1. Action potential arrives at the nerve terminal.

  2. Depolarization opens Voltage-Gated Calcium ($Ca^{2+}$) channels.

  3. $Ca^{2+}$ influx triggers exocytosis of neurotransmitters into the cleft to bind postsynaptic receptors.

Organization of the Nervous System

• Main Structural Divisions:

  • Central Nervous System (CNS): Brain and Spinal Cord.

  • Peripheral Nervous System (PNS): All neural tissue outside the CNS.

• Signal Directions:

  • Afferent (Sensory): Carries sensory input from body to CNS ("arrives").

  • Efferent (Motor): Carries commands from CNS to body effectors ("exits").

  • Interneurons: Reside entirely inside CNS to process and connect sensory and motor pathways.

• Efferent PNS Subdivisions:

  • Somatic Nervous System: Controls voluntary skeletal muscle; uses a single motor neuron; always excitatory ($+$).

  • Autonomic Nervous System (ANS): Controls involuntary visceral organs (cardiac muscle, smooth muscle, glands); uses a two-neuron chain; can be excitatory ($+$) or inhibitory ($-$).

Autonomic Nervous System (ANS) Divisions

• Autonomic Tone: Continuous balance between sympathetic and parasympathetic baseline inputs.

• Sympathetic Division ("Fight or Flight"):

  • Originates in Thoracic and Lumbar regions of the spinal cord.

  • Preganglionic neuron releases Acetylcholine ($ACh$); Postganglionic neuron releases Norepinephrine ($NE$).

  • Adrenal Medulla: Direct sympathetic activation releases epinephrine (adrenaline) and norepinephrine into the bloodstream during emergencies.

• Parasympathetic Division ("Rest and Digest"):

  • Originates in Brainstem and Sacral regions of the spinal cord.

  • Both Preganglionic and Postganglionic neurons release Acetylcholine ($ACh$).

• Organ Responses:

  • Heart: Sympathetic increases rate ($+$); Parasympathetic decreases rate ($-$).

  • Digestion: Parasympathetic increases digestive activity ($+$); Sympathetic decreases it ($-$).

CNS Glial Support Cells

• Cell Composition: $10\%$ Neurons (electrical signaling), $90\%$ Glial cells (structural/metabolic support).

• Types of Glia:

  • Oligodendrocytes: Form myelin sheaths around multiple axons inside the CNS.

  • Ependymal Cells: Line ventricles and produce Cerebrospinal Fluid (CSF) to cushion the brain and spinal cord.

  • Microglia: Act as the primary immune defense in the CNS to clear debris and fight infection.

  • Astrocytes: Form tight junctions around brain capillaries to create the Blood-Brain Barrier (BBB), protecting brain tissue from blood-borne substances.

CNS Structures and Functional Mapping

• Protective Meninges:

  • Dura Mater: Tough, fibrous outer layer.

  • Arachnoid Mater: Web-like middle layer; subarachnoid space holds circulating CSF.

  • Pia Mater: Delicate inner layer adhering directly to brain and spinal cord surface.

• CSF Flow Pathway: Lateral Ventricles $\rightarrow$ Third Ventricle $\rightarrow$ Fourth Ventricle $\rightarrow$ Central Canal / Subarachnoid Space $\rightarrow$ Reabsorbed into dural venous sinuses.

• Brain Regions:

  • Motor Cortex: Controls voluntary movement.

  • Sensory Cortex: Processes touch and physical sensations.

  • Auditory Cortex: Processes hearing.

  • Visual Cortex: Processes vision.

  • Frontal/Top Cortex: Higher cognitive function and decision making.

  • Brainstem & Cerebellum: Vital involuntary functions (breathing) and motor balance.

• Spinal Cord & Reflex Arcs:

  • Gray Matter: Core region of cell bodies; Dorsal horn receives sensory input, Ventral horn sends motor output.

  • White Matter: Outer regions of myelinated axon tracts ascending to or descending from the brain.

  • Reflex Arc: Direct link between dorsal sensory input and ventral motor output in gray matter, enabling instant responses without waiting for cortical processing.

Neurotransmitter Systems and Receptors

• Acetylcholine ($ACh$) Receptors:

  • Nicotinic Receptors: Ligand-gated ion channels causing fast excitatory responses on skeletal muscles and autonomic neurons.

  • Muscarinic Receptors: G-protein coupled receptors on parasympathetic target organs (excitatory or inhibitory).

• Norepinephrine ($NE$) Receptors:

  • Adrenergic Receptors (Alpha $\alpha<em>1, \alpha</em>2$ and Beta $\beta<em>1, \beta</em>2$): Mediate sympathetic fight-or-flight responses across body systems.