Neuroscience Terms Starting With N

Nerve is a bundle of axons enclosed in connective tissue sheaths that transmits electrical signals between the central nervous system and peripheral tissues including muscles, skin, and organs.

Nerves contain sensory axons carrying signals toward the CNS, motor axons carrying signals away from it, or both in mixed nerves. Each axon is surrounded by the endoneurium, groups of axons are bundled into fascicles surrounded by the perineurium, and the entire nerve is enclosed in the epineurium. Peripheral nerves can regenerate after injury because Schwann cells guide regenerating axon tips back toward their targets, though recovery proceeds at roughly one millimeter per day.

Nerve Impulse is the self-propagating wave of electrochemical change traveling along an axon membrane, equivalent to an action potential, that carries information from one part of the nervous system to another.

The nerve impulse travels by sequential depolarization and repolarization of adjacent membrane segments, with each segment triggering the next through local current spread. In myelinated axons, saltatory conduction between nodes of Ranvier dramatically accelerates this propagation. Each impulse is identical in amplitude regardless of stimulus strength; information is encoded in the frequency and pattern of impulses rather than their size, a principle called the all-or-none law.

Neural Circuit is an interconnected group of neurons that processes specific information or generates specific behaviors through the pattern of their synaptic connections and activity.

Neural circuits range from the simplest two-neuron reflex arc to large-scale brain networks involving millions of neurons distributed across multiple regions. The function of a circuit emerges from its connectivity, the properties of its neurons, and the neurotransmitters at its synapses. Modern optogenetics allows researchers to activate or silence genetically defined circuit elements with light, enabling precise dissection of circuit function in behaving animals and revealing causal links between specific neurons and specific behaviors.

Neural Plasticity is the capacity of the nervous system to change its structure, function, and connectivity in response to experience, learning, injury, or development, allowing the brain to adapt throughout the lifespan.

Plasticity operates at multiple levels, from molecular changes in synaptic strength through structural changes in axon and dendrite morphology to large-scale reorganization of cortical maps. Hebbian plasticity at individual synapses, embodied in long-term potentiation and long-term depression, provides the cellular mechanism for learning-induced changes. Critical periods in development represent windows of heightened plasticity during which experience has disproportionate effects on neural circuit formation, as demonstrated by the permanent loss of binocular vision in cats deprived of patterned light to one eye during the first weeks of life.

Neuroglia are all non-neuronal cells of the nervous system that support, protect, and modulate the function of neurons, including astrocytes, oligodendrocytes, microglia, ependymal cells, Schwann cells, and satellite cells.

Astrocytes regulate extracellular ion concentrations, recycle neurotransmitters at synapses, and help maintain the blood-brain barrier through their endfeet. Oligodendrocytes produce the myelin sheaths that insulate central nervous system axons, with each cell myelinating up to 50 axon segments simultaneously. Microglia are the resident immune cells of the brain, surveying the tissue continuously and phagocytosing debris, pathogens, and excess synapses during development.

Ependymal cells line the ventricles and central canal of the spinal cord and contribute to cerebrospinal fluid circulation, while satellite cells surround neuronal cell bodies in peripheral ganglia.

Neurology is the branch of medicine and science concerned with the structure, function, and diseases of the nervous system, encompassing diagnosis and treatment of disorders affecting the brain, spinal cord, and peripheral nerves.

Neurologists evaluate and treat conditions including stroke, epilepsy, multiple sclerosis, Parkinson’s disease, Alzheimer’s disease, migraine, neuropathy, and neuromuscular disorders. The field bridges basic neuroscience research and clinical medicine, with advances in neuroimaging, genetics, and electrophysiology continuously transforming diagnostic and therapeutic capabilities. Neurosurgery, psychiatry, and neuroradiology are closely related specialties that together address the full spectrum of nervous system disorders.

Board-certified neurologists complete four years of medical school followed by at least three years of residency training, with many pursuing additional fellowship subspecialization in fields such as stroke, epilepsy, or movement disorders.

Neuromodulator is a chemical messenger released by neurons that alters the excitability or responsiveness of large populations of target neurons rather than directly causing rapid excitation or inhibition at a single synapse.

Neuromodulators including dopamine, serotonin, norepinephrine, acetylcholine, and histamine are released diffusely from axonal varicosities and act on G-protein-coupled receptors that modulate ion channels and intracellular signaling cascades over seconds to minutes. They effectively reconfigure neural circuit dynamics by changing the gain, timing, and connectivity of circuit elements, so the same circuit can produce different outputs depending on the behavioral state. Disruption of neuromodulatory systems underlies major psychiatric conditions including depression, schizophrenia, ADHD, and Parkinson’s disease.

Neuromuscular Junction is the specialized synapse between a motor neuron axon terminal and a skeletal muscle fiber where acetylcholine release triggers muscle contraction.

When a motor neuron fires, an action potential invades the axon terminal and opens voltage-gated calcium channels, triggering the fusion of approximately 200 synaptic vesicles with the presynaptic membrane and releasing acetylcholine into the synaptic cleft. Acetylcholine diffuses across the roughly 50-nanometer cleft and binds to nicotinic receptors on the muscle fiber’s motor end plate, depolarizing the membrane and initiating a muscle action potential. Acetylcholinesterase in the cleft then rapidly hydrolyzes acetylcholine within about one millisecond, terminating the signal and preventing sustained muscle activation.

The NMJ is the target of clinically important toxins and drugs: botulinum toxin cleaves SNARE proteins to block vesicle fusion, curare competes with acetylcholine at nicotinic receptors, and neostigmine inhibits acetylcholinesterase to prolong receptor activation.

Neuron is a specialized nerve cell that receives, integrates, and transmits information through electrical changes in its membrane and chemical signaling at synapses.

Most neurons contain a cell body with the nucleus, branching dendrites that receive synaptic input, and a single axon that carries output to other neurons, muscles, or glands. Electrical signals arise from ion channels that change the membrane potential, while chemical communication occurs when neurotransmitters are released from axon terminals and bind receptors on target cells. Neurons vary enormously in form, from tiny interneurons with local axons to human motor neurons whose axons extend more than one meter from spinal cord to muscle.

Because most mature mammalian neurons do not divide, nervous system repair often depends on axon regeneration, synaptic reorganization, or compensation by surviving circuits rather than replacement of lost cells.

Neurotransmitter is a chemical messenger released by a neuron or related excitable cell that binds receptors on a target cell to transmit, inhibit, or modulate a signal across a synapse.

Neurotransmitters are stored in synaptic vesicles or synthesized near release sites and are released when calcium enters the presynaptic terminal after an action potential. Once in the synaptic cleft, they bind to ionotropic receptors that open channels within milliseconds or metabotropic receptors that trigger slower intracellular signaling cascades. Signal termination occurs through reuptake transporters, enzymatic degradation, or diffusion away from the cleft, preventing continuous receptor activation.

Major neurotransmitters include glutamate, GABA, acetylcholine, dopamine, serotonin, norepinephrine, glycine, and neuropeptides, each acting through receptor subtypes that can produce different effects depending on the target cell.

Nodes of Ranvier are small, regularly spaced gaps in the myelin sheath along myelinated axons where the axon membrane is exposed and voltage-gated sodium channels are densely concentrated, enabling saltatory conduction.

Each node is about one micrometer in length and is flanked by paranodal regions where the myelin loops are anchored to the axon membrane. The high density of sodium channels at nodes compensates for the current lost through the myelin-covered internodes, regenerating the action potential at each node as it jumps down the axon. Spacing between nodes is about 100 times the axon diameter, a ratio optimized for maximum conduction velocity with minimum metabolic cost.

Norepinephrine is a catecholamine neurotransmitter and hormone that increases arousal, attention, and cardiovascular output by binding to adrenergic receptors in the brain and peripheral tissues.

In the brain, norepinephrine is synthesized and released primarily by neurons in the locus coeruleus, a compact brainstem nucleus containing roughly 15,000 neurons in humans that projects axons to the cerebral cortex, hippocampus, cerebellum, and spinal cord. At peripheral sympathetic nerve terminals, norepinephrine binds to alpha and beta adrenergic receptors on target organs, raising heart rate, constricting blood vessels, and dilating airways during the fight-or-flight response. Drugs that block norepinephrine reuptake, such as atomoxetine, treat attention-deficit/hyperactivity disorder by increasing norepinephrine availability in prefrontal circuits that regulate sustained attention.