Neuroscience Terms Starting With A
Neuroscience Glossary: A
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Action Potential
/ AK-shun poh-TEN-shul / · Latin actio + potentia (power)
Action Potential is a rapid, self-propagating electrical signal generated when a neuron's membrane potential depolarizes past threshold, traveling along the axon to trigger neurotransmitter release at the synapse.
Action potentials follow an all-or-nothing rule: once threshold is reached, a full-amplitude spike is generated regardless of stimulus strength. Voltage-gated sodium channels open first, driving rapid depolarization, then potassium channels open to restore and briefly overshoot the resting potential in a phase called hyperpolarization. Myelinated axons conduct these signals far faster than unmyelinated ones, with the impulse jumping between nodes of Ranvier in a process called saltatory conduction.
Human motor axons can conduct at roughly 70 meters per second, while unmyelinated pain fibers conduct at less than 2 meters per second.
The squid giant axon, up to one millimeter in diameter, was the key experimental model that allowed Hodgkin and Huxley to record ionic currents in the 1950s and work out the mechanism of the action potential, earning them the 1963 Nobel Prize.
The nervous system encodes stimulus intensity through larger or stronger individual action potentials. Stimulus intensity is encoded by firing frequency: stronger stimuli generate more spikes per second, not bigger individual spikes.
The nervous system encodes stimulus intensity not through larger action potentials but through a higher frequency of firing, with stronger stimuli generating more spikes per second rather than bigger individual spikes. In the auditory system, neurons responding to loud sounds can fire at rates exceeding 300 spikes per second, compared to just a few spikes per second for near-threshold sounds.
Afferent Neuron
/ AF-er-ent NYOOR-on / · Latin afferens (carrying toward) + Greek neuron
Afferent Neuron is a nerve cell that carries sensory information from peripheral receptors toward the central nervous system, transmitting signals from the body and environment to the brain and spinal cord.
Afferent neurons form the sensory arm of the nervous system, relaying information about touch, pain, temperature, proprioception, vision, hearing, taste, and smell. Their cell bodies typically reside in dorsal root ganglia just outside the spinal cord, with one axon branch extending to a peripheral receptor and the other projecting into the spinal cord. Conduction speed varies with axon diameter and myelination: large-diameter, heavily myelinated Ia fibers from muscle spindles conduct at up to 120 meters per second, while thin, unmyelinated C fibers carrying slow pain conduct at only 0.5 to 2 meters per second.
This range of conduction velocities means that a single injury can produce both an immediate sharp pain and a delayed dull ache, carried by different afferent fiber types.
Sharks (Carcharhinus spp.) possess electroreceptive afferent neurons connected to ampullae of Lorenzini that detect electric fields as weak as 5 nanovolts per centimeter, allowing them to locate prey buried under sand by sensing the bioelectric fields generated by muscle contractions.
Afferent neurons only carry information about sensations people consciously perceive. Visceral afferents continuously relay unconscious information about gut pressure, blood oxygen levels, and organ status to the brainstem, and most of this signaling never reaches conscious awareness.
The knee-jerk reflex involves a single synapse between an Ia afferent neuron from the patellar tendon muscle spindle and the motor neuron controlling the quadriceps muscle, making it one of the fastest reflex arcs in the human body. The entire loop from stretch to contraction completes in roughly 25 to 50 milliseconds.
Fun Facts About the Nervous System →Amygdala
/ ah-MIG-dah-lah / · Greek amygdale (almond)
Amygdala is an almond-shaped cluster of nuclei deep in the temporal lobe that processes emotional responses, including fear conditioning, threat detection, and the consolidation of emotionally significant memories.
The amygdala receives direct input from the thalamus and sensory cortices to rapidly detect threats and coordinate physiological responses through outputs to the hypothalamus, brainstem, and autonomic nervous system. Sensory information enters the lateral nucleus, is processed through the basal nucleus, and projects to the central nucleus, which triggers fear responses including freezing, heart rate increases, and stress hormone release. Bilateral amygdala lesions from rare genetic conditions eliminate fear responses even to objectively dangerous stimuli, and the structure also encodes reward value and emotional significance beyond fear, responding to positive stimuli and social information.
A fast subcortical pathway from the thalamus directly to the amygdala can initiate a defensive response before the visual cortex has finished processing the triggering image.
A woman known in the literature as patient S.M., who has bilateral amygdala calcification caused by Urbach-Wiethe disease, reports no fear even when handling live snakes or watching horror films, and she rates frightened faces as approachable, providing direct evidence of the amygdala's role in fear processing.
The amygdala exclusively processes fear and negative emotions. Research shows it responds to positive emotional stimuli and rewards as well, attaching emotional significance to any salient experience regardless of its valence.
The amygdala's rapid thalamo-amygdala pathway can trigger a startle response to a threatening stimulus before the visual cortex has fully processed the image. In conditioned fear studies with rats (Rattus norvegicus), lesioning this direct thalamic pathway reduces the speed of fear responses by tens of milliseconds, demonstrating that the shortcut pathway, not the cortical route, drives the fastest defensive reactions.
Astrocyte
/ AS-troh-syt / · Greek astron (star) + kytos (cell)
Astrocyte is a star-shaped glial cell in the central nervous system that provides structural support, regulates neurotransmitter concentrations, maintains the blood-brain barrier, and modulates synaptic transmission.
Astrocytes are the most abundant glial cell type in the human brain and perform a remarkable diversity of functions beyond simple support. They ensheath synapses to form the tripartite synapse, taking up and recycling excess glutamate and other neurotransmitters, and regulating extracellular potassium concentrations that would otherwise disrupt neuronal firing. Astrocytes also respond to brain injury by becoming reactive, proliferating and forming glial scars that limit the spread of damage but simultaneously impair axon regrowth through the injured region.
Each astrocyte in the cerebral cortex contacts an estimated 100,000 synapses, positioning these cells to influence large numbers of neural connections simultaneously.
Astrocytes can propagate calcium waves across large areas of brain tissue, a form of slow signaling distinct from neuronal action potentials that may coordinate activity across brain regions over seconds to minutes. These waves travel at roughly 15 to 25 micrometers per second, far slower than electrical signals but capable of influencing blood vessel diameter and local blood flow.
Fun Facts About the Nervous System →Astrocytes are passive support cells that simply hold neurons in place. Selectively silencing astrocyte calcium signaling in mice alters memory formation and responses to stress, demonstrating that these cells actively shape circuit function rather than merely providing structural scaffolding.
Reactive astrogliosis following traumatic brain injury creates a dense barrier of astrocyte processes around the injury site that limits secondary damage by containing inflammatory cells and toxic debris. Studies in rodent spinal cord injury models show that this glial scar also blocks regenerating axons, and experimental deletion of the scar-forming gene STAT3 in mice reduces scar density and modestly improves axon regrowth.
Autonomic Nervous System
/ aw-toh-NOM-ik NER-vus SIS-tem / · Greek autonomos (self-governing) + Latin nervosus + systema
Autonomic Nervous System is the division of the peripheral nervous system that regulates involuntary bodily functions including heart rate, digestion, respiration rate, and glandular secretion through its sympathetic and parasympathetic branches.
The sympathetic branch prepares the body for action through the fight-or-flight response, increasing heart rate, dilating pupils, and diverting blood flow to skeletal muscles. Its counterpart, the parasympathetic branch, promotes rest and digestion by slowing heart rate, increasing gut motility, and stimulating glandular secretion. A third division, the enteric nervous system, governs gut function through a network embedded in the walls of the gastrointestinal tract and is sometimes considered a semi-independent component.
Sympathetic preganglionic neurons originate in the thoracic and lumbar spinal cord, while parasympathetic preganglionic neurons arise from cranial nerve nuclei in the brainstem and from the sacral spinal cord.
The enteric nervous system in the gut contains approximately 500 million neurons, more than in the spinal cord, and can regulate digestion independently of the brain, earning it the informal name "the second brain."
Fun Facts About the Nervous System →The autonomic nervous system is entirely beyond conscious control. Biofeedback training, meditation, and slow breathing exercises can measurably modulate heart rate variability and other autonomic outputs, and some trained practitioners can alter these functions voluntarily.
The Valsalva maneuver, used by musicians, divers, and athletes, demonstrates voluntary modulation of autonomic output: forcibly exhaling against a closed airway raises intrathoracic pressure, stimulates baroreceptors, and triggers a parasympathetic slowing of heart rate. In healthy adults this maneuver can drop pulse by 10 to 20 beats per minute within seconds, an effect the autonomic nervous system produces without conscious control.