The nervous system and neuron/neural firing

Central nervous system (CNS)

The division of the nervous system consisting of the brain and spinal cord, serving as the primary control and processing center.

Peripheral nervous system (PNS)

The network of nerves branching outward from the brain and spinal cord to link the rest of the body to the central processing centers.

Autonomic nervous system (ANS)

The PNS subdivision responsible for regulating involuntary, automatic bodily functions such as heart rate, digestion, and respiration.

Sympathetic nervous system

The autonomic branch that mobilizes fight-or-flight responses under stress, elevating heart rate, dilating airways, and releasing adrenaline.

Parasympathetic nervous system

The autonomic division that calms the body post-stress to promote rest, digestion, and energy conservation (homeostasis).

Somatic nervous system

The PNS division directing voluntary skeletal muscle movements and transmitting sensory information to the central nervous system.

Dendrites

Branch-like projections of a neuron specialized in receiving biochemical signals from adjacent cells.

Soma

The central cell body containing the nucleus that maintains life-support processes and integrates incoming neural signals.

Axon

The elongated fiber that propagates electrical impulses away from the cell body toward terminal branches.

Myelin sheath

A fatty insulating layer surrounding axons that increases the transmission speed of electrical impulses.

Nodes of Ranvier

Microscopic gaps along the insulated axon where action potentials regenerate, enabling rapid saltatory conduction.

Schwann cells

Glial cells in the peripheral nervous system that synthesize and maintain the insulating lipid layer on axons.

Terminal buttons

Knob-like structures at the distal end of an axon that store and release chemical messengers across the synapse.

Glial cells

Non-neuronal nervous system cells that provide structural framework, metabolic support, insulation, and waste removal for neurons.

Sensory neurons (Afferent)

Nerve cells that transmit incoming sensory inputs from peripheral receptors toward the spinal cord and brain.

Interneurons

Neurons located exclusively within the central nervous system that integrate and relay signals between sensory and motor pathways.

Motor neurons (Efferent)

Nerve cells conveying outgoing motor commands from the central nervous system toward target muscles and secretory glands.

Resting potential

The electrical polarization across an inactive neuronal membrane, maintained at approximately $-70\,\text{mV}$ with high internal $K^+$ and high external $Na^+$.

Threshold of excitation

The critical membrane potential (approximately $-55\,\text{mV}$) required to trigger an all-or-none electrical impulse.

Depolarization

The phase of neural firing where voltage-gated sodium channels open, allowing an influx of $Na^+$ to shift the internal charge positive.

Repolarization

The phase restoring negative resting electrical polarity as potassium gates open and allow $K^+$ ions to exit the cell.

Refractory period

A temporary interval following an impulse during which a neuron cannot generate another action potential.

All-or-none principle

The physiological rule stating that an axon fires completely at full strength once threshold is reached or does not fire at all.

Reuptake

The reabsorption of excess chemical messengers from the synaptic cleft back into the presynaptic axon terminal for reuse.

Synapse

The microscopic junction across which electrochemical communication passes between an axon terminal and a target cell.

Agonist

A psychoactive chemical or drug that binds to receptor sites and mimics or enhances the effect of a natural neurotransmitter.

Antagonist

A chemical or pharmaceutical agent that binds to and blocks receptor sites, inhibiting normal neurotransmitter activity.

Dopamine

Neurotransmitter influencing reward, voluntary movement, and arousal; overactivity correlates with schizophrenia, while deficiency causes Parkinson's disease.

Serotonin

Neurotransmitter regulating mood, appetite, and sleep cycles; deficits are strongly associated with clinical depression.

Norepinephrine

Neurotransmitter and hormone modulating arousal, vigilance, and stress responses; underactivity is linked to depressive symptoms.

Glutamate

The central nervous system's primary excitatory neurotransmitter, critical for synaptic plasticity and memory; excess leads to excitotoxic migraines and seizures.

GABA (gamma-aminobutyric acid)

The primary inhibitory neurotransmitter that dampens central nervous system activity; deficiencies relate to anxiety, insomnia, and tremors.

Endorphins

Endogenous opioid neuropeptides that act as natural analgesics to suppress pain and promote feelings of pleasure.

Acetylcholine (ACh)

Neurotransmitter enabling skeletal muscle contraction, attention, and memory formation; deterioration of producing neurons underlies Alzheimer's disease.

Substance P

A neuropeptide synthesized in the spinal cord and brain involved in processing and transmitting noxious pain signals.

Pituitary gland

The primary endocrine 'master gland' controlled by the hypothalamus that regulates growth and directs other endocrine organs.

Pineal gland

The endocrine structure that synthesizes melatonin to govern circadian rhythms and sleep-wake timing.

Adrenal glands

Endocrine structures situated above the kidneys that secrete adrenaline and cortisol to manage acute and sustained stress.

Leptin

A hormone produced by adipose (fat) tissue that signals the hypothalamus to reduce appetite and accelerate basal metabolism.

Ghrelin

An appetite-stimulating peptide hormone secreted by an empty stomach lining that signals hunger to the brain.

Drug tolerance

A neuroadaptive state wherein repeated exposure to a psychoactive substance reduces its physiological effect, necessitating escalating doses.

Drug withdrawal

The adverse physical and psychological symptoms that emerge upon the sudden discontinuation or reduction of an addictive substance.

Depressants

A category of psychoactive substances (e.g., alcohol, barbiturates, opiates) that reduce neural activity and decelerate physiological processes.

Stimulants

A class of psychoactive substances (e.g., caffeine, amphetamines, cocaine) that elevate central nervous system activity, alertness, and autonomic function.

Hallucinogens

Psychoactive compounds (e.g., LSD, MDMA, THC) that alter perception, elicit sensory distortions, and shift subjective consciousness.