BIO 105: Nervous System Lecture
Structural and Functional Divisions of the Nervous System
Primary Role of the Nervous System:
The nervous system is organized to monitor, process, and respond continuously to dynamic changes occurring within both the internal and external environments.
Receptor proteins receive sensory information across multiple modalities (including shifts in temperature, light intensity, and mechanical pressure).
Sensory inputs are routed directly to the integration center within the central nervous system, where signals are evaluated and coordinated into an appropriate motor output.
Primary Structural Classifications:
Central Nervous System (CNS): Comprises exclusively the brain and the spinal cord. It serves as the primary integration and decision-making center for all physiological processes.
Peripheral Nervous System (PNS): Encompasses all neural structures located outside of the brain and spinal cord, including cranial nerves, spinal nerves, sensory receptors, and peripheral ganglia.
Functional Flow of Neural Signaling:
Sensory (Afferent) Division: Conducts nerve impulses inward from sensory receptors in peripheral tissues toward the central nervous system for integration.
Motor (Efferent) Division: Transmits instructions and regulatory impulses outward from the central nervous system to peripheral target tissues (effectors), such as muscles or secretory glands.
Subdivisions of the Afferent (Sensory) Pathway:
Somatic Sensory: Conducts sensory impulses detected from the skin, skeletal muscles, joints, and the organs of special senses (vision, hearing, equilibrium, taste, and smell).
Visceral Sensory: Conveys internal sensory data originating from visceral body organs, including the stomach, pancreas, and intestines.
Subdivisions of the Efferent (Motor) Pathway:
Somatic Motor Nervous System: Responsible for voluntary motor commands directed exclusively to skeletal muscles, facilitating conscious bodily movement.
Autonomic Nervous System (ANS): Operates involuntarily to regulate involuntary effectors, including cardiac muscle, smooth muscle, and exocrine and endocrine glands. The ANS governs automatic homeostatic functions such as baseline respiratory rate, digestive motility, glandular secretions, and systemic blood pressure.
Subdivisions of the Autonomic Nervous System:
Sympathetic Division (Fight-or-Flight): Mobilizes physiological systems during acute stress, threat, or emergency conditions. It heightens alertness and metabolic readiness while downregulating vegetative activities.
Parasympathetic Division (Rest-and-Digest): Directs vegetative, energy-conserving, and restorative activities during non-threatening intervals, promoting digestion, nutrient assimilation, and tissue maintenance.
Antagonistic Relationship: The sympathetic and parasympathetic divisions function in an opposing, seesaw-like antagonistic relationship; high physiological tone cannot occur simultaneously in both divisions. Activation of one branch corresponds to the reciprocal inhibition of the other.
Cells of the Nervous Tissue: Neurons and Neuroglia
Cellular Composition of Neural Tissue:
Nervous tissue contains two primary cell populations: conducting cells (neurons) and supportive, non-conducting cells (neuroglia or glial cells).
Neuroglial cells outnumber conducting neurons in the brain by a ratio of approximately .
Glial Cells of the Central Nervous System:
Microglia: Function as specialized resident phagocytes within the CNS. They undergo phagocytosis to clear cellular debris, dead neurons, apoptotic matter, and invasive microorganisms, operating as a clean-up and defense mechanism.
Astrocytes: Distinctive star-shaped glial cells that provide mechanical and structural architecture for neurons. They participate in the blood-brain barrier and facilitate metabolic trafficking by supplying vital nutrients, such as glucose, from the vasculature directly to active neurons.
Oligodendrocytes: Large cells possessing a central body with multiple extending cytoplasmic processes (resembling an octopus) that reach out to wrap around multiple adjacent axons within the CNS. These wrappings deposit an insulating, multi-layered lipid coating known as the myelin sheath.
Glial Cells of the Peripheral Nervous System:
Schwann Cells: Serve as the functional equivalent of oligodendrocytes within the PNS. Instead of extending processes to multiple axons, individual Schwann cells wrap themselves sequentially along the longitudinal segment of a single peripheral axon to assemble the peripheral myelin sheath.
The Myelin Sheath and Conduction Properties:
Physical Appearance: Consists of concentrated layers of lipid membranes that impart a glistening, whitish sheen to nerves and myelinated neural tracts (readily visible during anatomical dissections of mammalian specimens, such as the sheep brain or fetal pig nervous pathways).
Functional Role: Provides electrical insulation that prevents ionic dissipation across the axonal membrane and markedly accelerates the velocity of action potential conduction.
Conduction Velocity: Enables rapid electrical propagation along myelinated fibers at speeds reaching to .
Functional Classes of Neurons:
Sensory (Afferent) Neurons: Specialized to detect internal and external environmental stimuli and carry those signals directly into the spinal cord and brain.
Motor (Efferent) Neurons: Transmit computational output instructions from the CNS out to somatic and visceral effectors.
Interneurons: Confined entirely within the gray matter of the central nervous system (predominantly the spinal cord and cerebral centers). They serve as communicative bridges, receiving signals from afferent neurons, processing the input locally, and relaying commands onward to appropriate efferent motor neurons.
Neurophysiology: Membrane Potentials and Action Potential Propagation
Gross Anatomy of the Multipolar Neuron:
Dendrites: Highly branched receptive processes extending from the soma that receive incoming biochemical signals from sensory receptors or the axon terminals of presynaptic neurons.
Cell Body (Soma): The metabolic and biosynthetic center containing the nucleus and primary organelles.
Axon (Nerve Fiber): A solitary, elongated process originating from the soma that conducts action potentials away from the cell body toward target tissues. Bundles of individual axons wrapped in connective tissue sheaths are organized into structural fascicles and nerve trunks that travel through anatomical pathways alongside blood vessels.
Axon Terminals: Specialized distal arborizations at the terminal end of an axon that form synaptic junctions with post-synaptic neurons, muscular endplates, or glandular cells.
Nodes of Ranvier: Unmyelinated, regularly spaced gaps occurring between adjacent Schwann cells (or oligodendrocyte wraps) along the length of an axon where ion channels are heavily concentrated.
Histological Organization of Neural Matter:
White Matter: Composed predominantly of bundles of myelinated axons; the elevated lipid content of the myelin sheaths imparts the white coloration. In the brain, white matter is located internally; in the spinal cord, it occupies the outer perimeter.
Gray Matter: Composed of unmyelinated axons, dendrites, neuronal cell bodies, and glial cells. It forms the outer cerebral cortex of the brain and the central butterfly-shaped core of the spinal cord.
Electrochemical Ionic Gradients at Rest:
A living neuronal membrane maintains steep electrical and chemical gradients across its phospholipid bilayer.
Potassium Ions (): Maintained at high concentrations inside the intracellular fluid and low concentrations in the extracellular fluid.
Sodium Ions (): Maintained at high concentrations in the extracellular fluid and low concentrations in the intracellular fluid.
If a potassium channel opens, passively diffuses down its chemical gradient out of the cell, carrying positive charge away and rendering the internal membrane face more negative.
If a sodium channel opens, passively diffuses down its electrochemical gradient into the cell, introducing positive charge and rendering the internal membrane face more positive.
Resting Membrane Potential ():
When unexcited, the neuron maintains a resting membrane potential across its membrane measured at via a voltmeter.
The negative sign signifies that the intracellular face of the plasma membrane possesses a net negative charge relative to the extracellular fluid.
The Sodium-Potassium ATPase Pump ( Pump):
Because ions continually leak across the plasma membrane down their respective concentration gradients, an active primary transport pump is required to preserve baseline conditions.
Driven by adenosine triphosphate () hydrolysis, the pump transports three sodium ions () out of the intracellular space while simultaneously importing two potassium ions () into the cell.
This electrogenic pumping activity moves ions against their steep concentration gradients, stabilizing the resting potential continuously at .
The Action Potential Event and Threshold:
An action potential represents a transient, all-or-none reversal of the electrical membrane potential that propagates along the axonal membrane.
Graded incoming stimuli stimulate the opening of local sodium channels, permitting localized influx of that shifts the membrane potential toward positive values.
Threshold Potential: Fixed at . It represents the minimum requisite depolarizing voltage necessary to trigger an action potential.
If an incoming depolarization shifts the voltage from only to , threshold is not achieved, voltage-gated channels remain closed, and no impulse is generated.
All-or-None Principle: If the local membrane reaches or surpasses , an action potential is irrevocably triggered. Action potentials do not vary in magnitude, height, or intensity regardless of stimulus strength; every action potential peaks at the identical voltage.
Sequential Phases of the Action Potential:
Depolarization: Crossing threshold () triggers the rapid opening of activation gates on voltage-gated sodium channels. Positively charged rapidly floods the intracellular compartment down its electrochemical gradient, shifting the membrane potential from negative to positive until reaching an apex of .
Repolarization: At the peak voltage of , the voltage-gated sodium channels inactivate and close, preventing further inward movement of . Concurrently, voltage-gated potassium channels open, allowing positively charged to rush out of the cell down its concentration gradient. The evacuation of positive charges restores the negative intracellular environment, dropping the potential back toward .
Refractory Period and Unidirectional Propagation: Immediately following depolarization, the preceding axonal segment enters a refractory period during which its sodium channel gates cannot reopen. This temporary inactivation guarantees that local currents stimulate only the adjacent resting segment ahead, enforcing strictly unidirectional forward propagation along the axon. The entire depolarization-repolarization cycle within a membrane patch occurs in merely a few milliseconds.
Modes of Axonal Propagation:
Continuous Conduction: Occurs along unmyelinated axons. Voltage-gated channels along every adjacent micrometer of membrane must sequentially depolarize and repolarize, yielding comparatively slow conduction velocities.
Saltatory Conduction: Occurs along myelinated axons. The insulating myelin blocks ionic flow through the internodal axon membrane. The electrical impulse leaps rapidly from one node of Ranvier to the next, drastically reducing transit times and permitting signals to travel from the brain to distal extremities within milliseconds.
Synaptic Transmission and Neurochemistry
Mechanism of Chemical Synaptic Transmission:
When an electrical action potential travels down an axon and reaches the terminal bouton (presynaptic terminal), the depolarizing wave prompts voltage-gated calcium channels to open.
Calcium ions () enter the presynaptic terminal down their concentration gradient.
Intracellular influx mobilizes membrane-bound synaptic storage vesicles, driving them to dock with the presynaptic plasma membrane and release their biochemical cargo into the synaptic cleft via exocytosis.
The chemical messengers (neurotransmitters) diffuse across the narrow, fluid-filled synaptic cleft (the physical space separating presynaptic and postsynaptic cells).
Neurotransmitters bind stereospecifically to corresponding receptor proteins embedded in the postsynaptic plasma membrane in a lock-and-key configuration.
Postsynaptic Effects:
Excitatory Postsynaptic Response: Neurotransmitter binding causes the opening of chemically gated sodium channels, allowing influx that depolarizes the postsynaptic membrane toward threshold, increasing the probability of generating a new action potential.
Inhibitory Postsynaptic Response: Neurotransmitter binding promotes hyperpolarization of the postsynaptic membrane, stabilizing or dropping the voltage further below resting levels and rendering the cell refractory to firing.
Neural Summation: A single postsynaptic neuron typically receives simultaneous inputs from hundreds or thousands of presynaptic terminals. The postsynaptic cell integrates (sums) all incoming positive (excitatory) and negative (inhibitory) electrical charges. If the collective sum of these inputs reaches the threshold at the axon hillock, an action potential is fired; if inhibitory inputs dominate or the sum remains below threshold, the signal terminates.
Major Neurotransmitters and Clinical Pharmacology:
More than distinct neurotransmitters have been identified in the nervous system, with hundreds of receptor subtypes mediating distinct physiological actions.
Acetylcholine (ACh): Operates throughout the somatic nervous system and represents the predominant neurotransmitter of the parasympathetic autonomic division.
Norepinephrine: Serves as the primary neurotransmitter of the sympathetic division, mediating emergency fight-or-flight signaling.
Dopamine: Involved in motor control, motivation, and neural reward circuits.
Serotonin: Regulates mood balance, emotional states, and sleep-wake cycles.
Glutamate: Represents the major, widespread excitatory neurotransmitter of the central nervous system.
Gamma-Aminobutyric Acid (GABA): Functions as the primary inhibitory neurotransmitter throughout the brain, acting as a brake on neuronal excitability.
Benzodiazepine Pharmacology: Anxiolytic and sedative pharmaceuticals—such as diazepam (Valium) and alprazolam (Xanax)—belong to the benzodiazepine drug class. These compounds exert their clinical effects by enhancing natural GABAergic inhibitory neurotransmission, thereby slowing down central synaptic transmission and cerebral processing to relieve pathological anxiety and induce sedation or drowsiness.
Gross Anatomy and Organization of the Central Nervous System
Protective Structures of the CNS:
Meninges: The brain and spinal cord are enclosed and protected by a series of three specialized connective tissue membranes. An infectious inflammation of these protective coverings is known clinically as meningitis. Because the cranial cavity is composed of unyielding, rigid bone, inflammatory edema or space-occupying lesions within the meninges rapidly elevate intracranial pressure, compressing cerebral tissue and compromising neurological function.
Cerebrospinal Fluid (CSF) and Ventricles: The brain features a network of interconnected internal chambers termed ventricles lined with specialized cells that synthesize cerebrospinal fluid. CSF circulates through the ventricles, flows within the subarachnoid space of the meninges, and envelops both the brain and spinal cord, providing mechanical cushioning, buoyancy, and shock absorption.
The Spinal Cord:
Anatomical Location: Extends inferiorly from the medulla oblongata through the protective vertebral canal formed by the vertebral column. Adjacent bony vertebrae are separated and cushioned by fibrocartilaginous intervertebral discs. Mechanical disruption or herniation of these discs can result in leakage of internal gel-like fluid, which impinges on adjacent spinal nerves to produce severe pain, paresthesias, or localized motor loss.
Dorsal vs. Ventral Organization:
The dorsal aspect of the spinal cord is identified anatomically by the spinous process of the vertebra, whereas the rounded vertebral body indicates the ventral orientation.
Dorsal Root: Carries incoming sensory (afferent) nerve fibers into the dorsal horn of the spinal gray matter. An anatomical swelling located on this root—the Dorsal Root Ganglion (DRG)—houses the clustered cell bodies of primary sensory neurons.
Ventral Root: Carries efferent motor nerve fibers emerging from the ventral horn of the spinal cord out to effector targets.
Spinal Nerves: Just distal to the ganglion, the dorsal and ventral roots fuse to form a spinal nerve. Consequently, all spinal nerves are categorized functionally as mixed nerves because they transport both sensory and motor axons.
Ascending and Descending Tracts: Bundles of myelinated axons within the spinal cord white matter are organized into directional pathways. Ascending tracts convey sensory data upward to the brain; descending tracts carry motor output downward from the brain. Tracts undergo decussation (crossing over) within the central neural axis, meaning sensory and motor control for the left side of the body is integrated within the right cerebral hemisphere, and vice versa.
The Withdrawal Reflex Arc:
A reflex represents a rapid, involuntary, automatic motor response to an acute stimulus mediated via a foundational reflex arc pathway.
Sequential Pathway:
Stimulation of Sensory Receptor: A noxious peripheral stimulus occurs (such as piercing bare skin on a sharp piece of broken glass while walking on a beach).
Afferent Signal Transmission: The sensory receptor generates an action potential that travels along an afferent neuron through the dorsal root ganglion into the dorsal horn of the spinal cord.
Central Integration: Within the central gray matter, the afferent neuron synapses directly with an interneuron.
Efferent Signal Transmission: The interneuron relays the impulse immediately to a motor neuron, which discharges an action potential outward through the ventral root.
Effector Action: The motor axon innervates the target skeletal muscle, causing immediate contraction that lifts the foot away from the glass.
Concurrently, collateral axonal branches ascend the spinal cord to sensory centers in the brain. As a result, the protective motor withdrawal occurs prior to conscious cognitive awareness of the pain stimulus, which is realized milliseconds later.
Functional Neuroanatomy of the Cerebrum and Cerebral Cortex
Gross Structure of the Cerebrum:
The cerebrum is the most superior and evolutionarily advanced division of the brain, responsible for complex cognitive processes, sensory interpretation, voluntary motor commands, and language.
Longitudinal Fissure: A prominent, deep midline groove that divides the cerebrum into right and left cerebral hemispheres.
Corpus Callosum: A prominent, thick horizontal bridge composed of dense white matter tracts that lies deep within the longitudinal fissure. It constitutes the primary anatomical structure that enables direct interhemispheric communication, passing signals between the left and right halves of the brain.
Convolutions: The outer shell of gray matter—termed the cerebral cortex—is intricately folded to increase surface area within the cranial vault. The upward elevations (bumps) are designated as gyri, and the intervening shallow grooves are designated as sulci.
Lobes of the Cerebrum:
Frontal Lobe: Positioned anteriorly; responsible for motor control, expressive language, and higher-order cognitive processing.
Parietal Lobe: Positioned superiorly and laterally behind the frontal lobe; specializes in somatosensory integration.
Occipital Lobe: Situated at the posterior aspect of the brain; contains primary processing networks for vision.
Temporal Lobe: Positioned laterally adjacent to the ears; coordinates auditory processing, hearing memories, and sensory language comprehension.
Insula: A deep, fifth cerebral lobe located internal to the lateral sulcus beneath the surrounding cortical lobes.
Cortical Functional Regions and Somatotopy:
Primary Motor Cortex: Located in the frontal lobe immediately anterior to the central sulcus (the precentral gyrus). It generates voluntary efferent commands to skeletal muscles.
Primary Somatosensory Cortex: Located in the parietal lobe immediately posterior to the central sulcus (the postcentral gyrus). It receives afferent sensory data from the skin, joints, and skeletal muscles.
Cortical Homunculus (Cortical Mapping):
The surface area of the primary motor and somatosensory cortices is disproportionately distributed across different anatomical structures according to function rather than anatomical size.
Regions that perform high-dexterity motor movements or possess high tactile sensitivity (such as the hands, fingers, thumbs, lips, and facial musculature) occupy massive swaths of cortical territory.
Conversely, regions with coarse motor control or sparse sensory innervations (such as the trunk, back, pelvis, neck, and legs) receive comparatively narrow strips of cortical real estate.
Cortical Association and Language Centers:
Prefrontal Cortex: The anterior portion of the frontal lobe; orchestrates executive functions, including strategic planning, judgment, critical decision-making, and complex behavioral reasoning.
Visual Processing Regions: The primary visual cortex is located in the occipital lobe at the posterior skull, where it receives retinal inputs via photoreceptors. The visual association area matches novel visual sensory patterns with stored visual memories to achieve cognitive recognition.
Auditory Association Area: Located in the temporal lobe; interprets acoustic signals and links complex sound patterns to auditory memories (evidenced by the visceral nostalgia and associative recall triggered when hearing musical tracks from one's youth).
Wernicke's Area: Situated within the temporal/parietal region; responsible for the comprehension and interpretation of both spoken and written language.
Broca's Area: Located in the frontal lobe (typically within the left hemisphere); coordinates the motor programming required for speech production and writing, directing the primary motor cortex to control the vocal apparatus and manual musculature.
Clinical Correlate: Broca's Area Aphasia:
Clinical trauma involving Broca's area highlights functional localization. For instance, following a multi-vehicle traffic collision in Port Jefferson involving four automobiles in a turning lane, a patient receiving anticoagulant medication sustained a intracranial hemorrhage due to mechanical deceleration.
Subsequent surgical intervention resulted in localized focal irritation and an ischemic stroke confined to Broca's area.
Upon awakening, the patient displayed preserved cognitive comprehension and retained the ability to read written language on an assistive communication board (intact Wernicke's processing). However, verbal expressive output was impaired; the patient could articulate only the single word "two" (repeating "two, two, and two") while displaying emotional awareness of the disconnect, and was completely unable to coordinate the hand muscles to write down words. The motor language deficit spontaneously resolved after four days of neurological recovery.
Functional Neuroimaging ():
Functional Magnetic Resonance Imaging () evaluates localized changes in cerebral blood flow as a direct proxy for metabolic and electrical neural activity in real time, identifying regions engaged during distinct sensory, cognitive, or motor tasks.
Subcortical Structures, Brainstem, and the Limbic System
The Diencephalon:
Represents a phylogenetically older central core situated beneath the cerebrum, consisting of three primary structures:
Hypothalamus: The primary visceral control and homeostatic integration center. It regulates internal body temperature, cellular metabolic rate, calcium balance, systemic water volume, thirst, hunger, and satiety. It functions as a neuroendocrine bridge, regulating the adjacent pituitary gland (the master endocrine gland) to coordinate hormonal secretions with autonomic demands.
Thalamus: A large, bilateral egg-shaped mass that acts as the primary sensory relay station of the brain. It filters and relays all ascending sensory information destined for the cerebral cortex, with the sole exception of olfactory (smell) inputs.
Pineal Gland: An endocrine component of the diencephalon that secretes the hormone melatonin, which governs circadian rhythms and regulates the normal sleep-wake cycle.
The Cerebellum:
Located in the posterior cranial fossa inferior to the occipital lobe.
Characterized by a distinctive branching internal arrangement of white matter tracts nestled inside gray matter folds, a pattern termed the arbor vitae (tree of life).
Functionally coordinates skeletal muscle contractions, refines voluntary movements to ensure smoothness, and continuously adjusts balance, posture, and equilibrium.
The Brainstem:
Composed of three continuous anatomical segments bridging the diencephalon with the spinal cord:
Midbrain: The most superior brainstem segment; serves as a conduction pathway and relay station routing visual, auditory, and tactile sensory information between the cerebrum, cerebellum, and spinal cord.
Pons: A prominent rounded bridge positioned between the midbrain and the medulla; functions as a bidirectional communication hub connecting the cerebellum to the broader CNS and houses auxiliary nuclei that regulate rhythmic breathing.
Medulla Oblongata: The most inferior component of the brainstem, which transitions directly into the spinal cord at the base of the skull. It serves as an autonomic reflex control center, housing primary nuclei that manage vital processes:
Cardiovascular and vasomotor regulation (modulating heart rate, contractile force, and blood pressure via vasoconstriction).
Primary respiratory rhythmicity.
Gastrointestinal and digestive reflex integration.
Involuntary protective visceral reflexes, including coughing, sneezing, swallowing, vomiting, and hiccuping.
The Limbic System and Synaptic Plasticity:
The limbic system constitutes a functional ring of interconnected structures that serves as the emotional center of the brain.
Amygdala: Mediates emotional appraisal, threat detection, and the autonomic expression of fear.
Hippocampus: Primary structure dedicated to memory consolidation and the acquisition of new learning.
Mechanisms of Memory Encoding: The process of learning and memory formation requires repetitive activation of specific neural circuits. Repeated recall stimulates structural neuroplasticity, yielding the proliferation of new dendrites, additional axonal collaterals, and new synaptic junctions. Unused pathways undergo synaptic pruning, whereas routinely reinforced pathways demonstrate strengthened synaptic efficacy, facilitating rapid recall.
Peripheral Nervous System, Reflex Arcs, and Autonomic Divisions
Organization of Peripheral Nerves:
Cranial Nerves: Twelve () pairs of nerves that originate directly from the brain and brainstem, designated sequentially by Roman numerals ( through ).
Cranial Nerve I (, Olfactory Nerve): Mediates the sense of smell via the olfactory bulbs (anatomically reduced in humans relative to the enlarged olfactory structures seen in macrosmatic mammals, such as cats, dogs, and mice).
Cranial Nerve II (, Optic Nerve): Transmits visual sensory input from the photoreceptors of the retina.
Cranial Nerve III (, Oculomotor Nerve): Governs extrinsic eye musculature to control ocular movements and pupillary constriction.
Cranial Nerve X (, Vagus Nerve): A far-reaching mixed nerve that exits the cranium to provide extensive parasympathetic innervation to internal thoracic and abdominal visceral organs, including the larynx, pharynx, lungs, heart, stomach, and intestines.
Spinal Nerves: Thirty-one () pairs of mixed nerves that emerge bilaterally from the spinal cord through the intervertebral foramina, innervating the musculoskeletal framework, skin, and peripheral tissues.
Ganglia: Structural aggregations of neuronal cell bodies situated in the peripheral nervous system (such as the dorsal root ganglia).
Sympathetic Division Physiology:
Functional Role: Mediates the catabolic fight-or-flight stress response, preparing the organism to confront or flee acute environmental hazards (such as an animal attack or sudden roadway near-miss requiring immediate braking).
Primary Neurotransmitter: Releases norepinephrine (alongside systemic hormonal secretion of epinephrine/adrenaline from the adrenal medulla) onto target effectors.
Physiological Adjustments:
Pupillary dilation (mydriasis) to maximize peripheral visual input.
Bronchodilation (opening airways) to maximize oxygen uptake and accelerate mitochondrial generation for skeletal muscle exertion.
Tachycardia (elevated heart rate) and increased myocardial contractility to raise cardiac output.
Vasoconstriction of non-essential vascular beds, elevating systemic blood pressure and shunting oxygenated blood into working skeletal muscles.
Transient inhibition of non-essential vegetative processes, including peristalsis and digestive glandular secretion.
Parasympathetic Division Physiology:
Functional Role: Mediates the rest-and-digest response, restoring baseline parameters and promoting anabolic, maintenance, and energy-storage functions.
Primary Neurotransmitter: Releases acetylcholine (ACh) at neuroeffector junctions, which counteracts sympathetic actions.
Physiological Adjustments:
Bradycardia (reduced heart rate) and lowered systemic blood pressure.
Bronchoconstriction and normalization of resting respiratory rate.
Stimulation of gastrointestinal motility, enzyme secretion, and nutrient absorption.
Stimulation of normal visceral elimination reflexes, including salivation (drooling), defecation, and urination.
Promotion of reproductive autonomic reflexes, such as penile erection.
None of these restorative parasympathetic responses are compatible with acute emergency survival; they are activated during low-stress, recuperative physiological states.