Nervous System: Central, Peripheral, and Autonomic Divisions

Comprehensive vocabulary flashcards covering the Central Nervous System, Peripheral Nervous System, Autonomic Nervous System, reflex arcs, spinal plexuses, and related neurological clinical conditions.

Central Nervous System (CNS)

The command and control center of the nervous system, housing approximately $85\text{ billion}$ neurons in the brain and roughly $100\text{ million}$ neurons in the spinal cord, that integrates sensory input, forms thoughts and memories, makes decisions, and executes motor actions.

Astrocytes

Star-shaped neuroglia in the central nervous system that represent the largest and most numerous glial cells.

Oligodendrocytes

Central nervous system neuroglia responsible for forming and maintaining the myelin sheath around CNS axons.

Microglia

Small central nervous system neuroglia that function as phagocytes to remove cellular debris and pathogens.

Ependymal Cells

Epithelial-like CNS neuroglia that produce, monitor, and assist in circulating cerebrospinal fluid (CSF) and form the blood–CSF barrier.

Cerebrospinal Fluid (CSF)

A clear filtrate rich in glucose, oxygen, vitamins, and ions ($\text{Na}^+$, $\text{Cl}^-$, $\text{Mg}^{2+}$) that cushions the brain and spinal cord, provides buoyancy to prevent the brain from crushing under its own weight, and nourishes brain tissue.

Choroid Plexus

Specialized vascular capillary networks situated within the brain ventricles, enveloped by ependymal cells with tight junctions, where cerebrospinal fluid is produced.

Blood–CSF Barrier

A selective physiological barrier formed by a single layer of choroid plexus ependymal cells joined by tight junctions that filters fluid leaking from porous capillaries.

Subarachnoid Space

The fluid-filled space between the arachnoid and pia mater through which cerebrospinal fluid circulates around the brain and spinal cord.

Arachnoid Granulations

Fingerlike projections of the arachnoid mater through which cerebrospinal fluid is reabsorbed from the subarachnoid space into the dural venous sinuses.

Median and Lateral Apertures

Openings in the ventricular system through which cerebrospinal fluid exits the ventricles to enter the subarachnoid space.

Hydrocephalus

A clinical condition resulting from an abnormal accumulation of cerebrospinal fluid that enlarges the brain ventricles when normal flow or reabsorption is impaired.

Cerebrum

The largest brain region, termed the 'seat of intelligence,' responsible for perception, voluntary motor actions, reasoning, and memory.

Cerebral Cortex

The outer superficial region of the cerebrum that localizes sensory input, executes voluntary movements, and manages high-level integrative functions like memory and reasoning.

Basal Nuclei

Subcortical clusters of gray matter in the cerebrum that help initiate and terminate movements and suppress unwanted muscle contractions.

Limbic System

A functional brain network that promotes and coordinates emotional responses and emotional processing.

Diencephalon

The central core of the forebrain situated between the cerebrum and brain stem, composed of the thalamus, hypothalamus, and epithalamus.

Thalamus

A diencephalic structure making up $80\%$ of the diencephalon that acts as the principal relay station for sensory impulses traveling to the cerebral cortex.

Hypothalamus

The chief autonomic integration center divided into four regions (mamillary, tuberal/infundibulum, supraoptic, and preoptic) that regulates temperature, hunger, thirst, sleep-wake cycles, controls anterior pituitary secretion, and synthesizes ADH and oxytocin.

Epithalamus

The posterior portion of the diencephalon that contains the pineal gland, which secretes melatonin to regulate sleep-wake cycles.

Pineal Gland

An endocrine gland located in the epithalamus that secretes melatonin to govern circadian rhythms and sleep-wake cycles.

Cerebellum

The brain region that processes motor commands and equilibrium sensory input to coordinate smooth, voluntary skeletal muscle movements, posture, and balance.

Brain Stem

The stalk of the brain consisting of the midbrain, pons, and medulla oblongata, which relays signals and contains vital autonomic and cranial nerve nuclei.

Midbrain (Mesencephalon)

The superior segment of the brain stem containing visual reflex centers (superior colliculi), auditory reflex centers (inferior colliculi), motor centers (substantia nigra, red nuclei), and nuclei for CN III and CN IV.

Superior Colliculi

Paired visual reflex centers located on the tectum of the midbrain.

Inferior Colliculi

Paired auditory reflex centers located on the tectum of the midbrain.

Substantia Nigra

A midbrain motor nucleus involved in motor control.

Red Nuclei

Motor center nuclei located in the midbrain that assist in coordinating skeletal muscle movements.

Pons

The brain stem bridge that relays signals between the cerebrum and cerebellum, assists the medulla in controlling breathing rate and depth, and houses nuclei for CN V, VI, VII, and VIII.

Medulla Oblongata

The inferior segment of the brain stem containing vital cardiovascular and respiratory rhythmicity centers, reflex centers for swallowing, coughing, and vomiting, and nuclei for CN VIII–XII.

Cardiovascular Center

A vital regulatory region in the medulla oblongata consisting of cardiac and vasomotor centers that regulate heart rate and blood pressure.

Respiratory Rhythmicity Center

A medullary center that controls the basic rate and rhythm of breathing.

Peripheral Nervous System (PNS)

All neural structures outside the central nervous system, composed primarily of cranial nerves, spinal nerves, and sensory receptors.

Schwann Cells

PNS neuroglial cells that form and maintain the myelin sheath around peripheral axons.

Satellite Cells

PNS neuroglial cells that envelop and support neuron cell bodies within peripheral ganglia.

Sensory (Afferent) Division

The functional division of the PNS that conveys sensory impulses from peripheral receptors toward the CNS.

Somatic Senses

General sensory modalities comprising tactile, thermal, pain, and proprioceptive sensations.

Special Senses

Specialized sensory modalities including smell, taste, vision, hearing, and equilibrium.

Motor (Efferent) Division

The functional division of the PNS that conveys motor output signals from the CNS to effector organs.

Somatic Nervous System (SNS)

The voluntary motor pathway of the PNS conducting motor impulses from the CNS directly to skeletal muscles.

Autonomic Nervous System (ANS)

The involuntary motor pathway of the PNS that regulates smooth muscle, cardiac muscle, and glands.

Sympathetic Division

The thoracolumbar branch of the ANS that drives the 'fight or flight' response, featuring short preganglionic and long postganglionic fibers with ganglia close to the spinal cord.

Parasympathetic Division

The craniosacral branch of the ANS that mediates 'rest and digest' energy conservation, featuring long preganglionic and short postganglionic fibers with ganglia located in or near visceral effectors.

Enteric Nervous System (ENS)

Known as the 'brain of the gut,' this third autonomic division consists of roughly $100\text{ million}$ neurons embedded in the gastrointestinal tract controlling motility and secretions.

Sensory Function

The core nervous system function dedicated to detecting internal and external environmental stimuli.

Integrative Function

The core nervous system function that processes, analyzes, and stores sensory data to formulate decisions.

Motor Function

The core nervous system function that executes appropriate responsive actions by activating effectors (muscles and glands).

Electrical Excitability

The physiological capacity of neural tissue to respond to environmental stimuli by converting them into action potentials.

Action Potential

An electrical impulse that propagates along the cell membrane of an excitable cell.

Cell Body (Soma / Perikaryon)

The biosynthetic center of a neuron containing the nucleus and Nissl bodies.

Nissl Bodies

Prominent clusters of rough endoplasmic reticulum situated within neuronal cell bodies that carry out protein synthesis.

Dendrites

Branching neuronal processes that function as the primary receiving input zones of a neuron.

Axon

A long cylindrical neuronal projection that originates at the axon hillock and conducts nerve impulses away from the cell body.

Axon Hillock

The cone-shaped elevation of the neuron soma where the axon originates.

Initial Segment (Trigger Zone)

The proximal segment of the axon adjacent to the axon hillock that functions as the trigger zone for generating action potentials.

Multipolar Neurons

A structural classification of neurons characterized by having several dendrites and a single axon.

Bipolar Neurons

A structural classification of neurons characterized by having one main dendrite and one axon.

Unipolar Neurons

A structural classification of neurons possessing a single process extending from the cell body that bifurcates into two branches.

Purkinje Cells

Specialized, highly branched neurons located in the cerebellar cortex.

Pyramidal Cells

Specialized pyramid-shaped neurons situated within the cerebral cortex.

Interneurons (Association Neurons)

Neurons located mainly within the central nervous system that integrate incoming sensory information and transmit signals to motor pathways.

Antidiuretic Hormone (ADH)

A peptide hormone synthesized by the hypothalamus that promotes renal water reabsorption.

Oxytocin

A hormone synthesized by the hypothalamus that stimulates uterine contractions and milk ejection.

Preganglionic Neuron

The first neuron in the two-neuron autonomic pathway, having its cell body in the CNS and its axon extending to an autonomic ganglion.

Autonomic Ganglion

A peripheral cluster of nerve cell bodies where preganglionic autonomic fibers synapse with postganglionic neurons.

Postganglionic Neuron

The second neuron in the autonomic pathway, whose unmyelinated axon extends from an autonomic ganglion directly to visceral effectors.

Reflex Arc

The specific neural wiring pathway executing a reflex, comprising a sensory receptor, sensory neuron, integrating center, motor neuron, and effector.

Sensory Receptor

The initial component of a reflex arc that detects a specific stimulus and produces a generator or receptor potential.

Integrating Center

One or more central nervous system regions within a reflex arc that process sensory impulses and relay signals to motor neurons.

Effector

The responding muscle or gland in a reflex arc that carries out the physiological action dictated by motor nerve impulses.

Reflex Grade 0

A reflex grading score denoting an absent response, indicative of a Lower Motor Neuron (LMN) lesion.

Jendrassik Maneuver

A reinforcement technique utilized during clinical examination to enhance or elicit diminished deep tendon reflexes.

Reflex Grade 4+

A reflex grading score denoting a hyperactive response with clonus, indicative of an Upper Motor Neuron (UMN) lesion.

Deep Tendon (Myotatic) Reflexes

Involuntary muscle stretch reflexes elicited by tapping tendons, including the patellar (knee jerk), Achilles, biceps, and triceps reflexes.

Corneal (Blink) Reflex

A superficial reflex testing the posterior fossa, medulla, and cranial nerves (CN V sensory, CN VII motor), which can also be elicited by sounds exceeding $>40\text{–}60\text{ dB}$.

Pharyngeal (Gag) Reflex

A superficial protective reflex mediated by CN IX (sensory) and CN X (motor) that is impaired by botulinum toxin and hyperactive in tetanus.

Spatula Test

A clinical examination procedure that evaluates the gag reflex, in which a hyperactive response confirms tetanus infection.

Superficial Abdominal Reflex

A superficial clinical reflex that assesses abdominal muscle contraction following cutaneous stimulation, typically absent in Multiple Sclerosis and Brown-Séquard syndrome.

Plantar Reflex

A superficial clinical reflex in which the normal adult response to stroking the lateral plantar surface of the foot is hallux flexion (downward curling of the toes).

Babinski Sign

An abnormal adult plantar reflex response consisting of dorsiflexion (upward extension) of the hallux with fanning of the toes, signifying central nervous system or upper motor neuron disease.

Pupillary Light Reflex

An autonomic cranial reflex wherein afferent light signals travel via CN II and efferent motor signals travel via CN III to constrict both pupils.

Accommodation Reflex

A triad visual reflex adjusting the eye for near vision via increased lens convexity (ciliary muscle), pupillary constriction, and eye convergence (medial rectus).

Edinger-Westphal Nucleus

A midbrain parasympathetic preganglionic nucleus that innervates the ciliary muscle via CN III to increase lens convexity during near accommodation.

Ciliospinal Reflex

An autonomic reflex wherein a painful stimulus applied to the face or neck causes ipsilateral pupillary dilation of $1\text{–}2\text{ mm}$.

Phrenic Nerve

A vital spinal nerve originating from the cervical plexus ($\text{C1–C5}$) that innervates the diaphragm; damage causes respiratory paralysis.

Axillary Nerve

A major branch of the brachial plexus ($\text{C5–T1}$) that innervates the deltoid muscle.

Radial Nerve

A major branch of the brachial plexus innervating upper limb extensor muscles; nerve damage results in wristdrop.

Median Nerve

A branch of the brachial plexus innervating forearm flexors and thumb intrinsic muscles; nerve damage impairs fine pinch grip.

Ulnar Nerve

A branch of the brachial plexus supplying medial hand flexors; nerve damage results in a clawhand deformity.

Musculocutaneous Nerve

A branch of the brachial plexus that innervates arm flexor muscles.

Femoral Nerve

A lumbar plexus nerve ($\text{L1–L4}$) innervating hip flexors and knee extensors; injury impairs leg extension.

Obturator Nerve

A lumbar plexus nerve ($\text{L1–L4}$) innervating thigh adductor muscles; injury prevents thigh adduction.

Sciatic Nerve

The largest nerve of the sacral plexus ($\text{L4–S4}$) that bifurcates into the common fibular and tibial nerves.

Common Fibular Nerve

A terminal branch of the sciatic nerve whose damage leads to footdrop.

Tibial Nerve

A terminal branch of the sciatic nerve whose damage results in a shuffling gait.

Upper Motor Neuron (UMN) Lesion

Damage occurring within CNS motor pathways characterized clinically by spastic paralysis and hypertonia.

Lower Motor Neuron (LMN) Lesion

Damage to motor neurons originating in the spinal cord or cranial nuclei extending to effectors, characterized clinically by flaccid paralysis and loss of muscle tone.

Afferent Pupillary Defect

A clinical defect caused by a CN II lesion resulting in an unresponsive pupillary light reflex on both sides when light is shone into the affected eye.

Efferent Pupillary Defect

A clinical defect caused by a CN III lesion resulting in a 'fixed and dilated' pupil on the affected side.

Horner Syndrome

A neurological condition caused by interruption of sympathetic cervical innervation, characterized clinically by miosis, partial ptosis, and anhidrosis (loss of hemifacial sweating).