Neuroscience Terms Starting With I
Neuroscience Glossary: I
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Inhibitory Synapse
/ in-HIB-ih-tor-ee SIN-aps / · Latin inhibere (to restrain) + Greek synapsis
Inhibitory Synapse is a synaptic connection at which neurotransmitter release decreases the probability that the postsynaptic neuron will fire an action potential by hyperpolarizing or stabilizing its membrane potential.
GABA is the principal inhibitory neurotransmitter in the brain, acting on GABA-A receptors to open chloride channels that hyperpolarize postsynaptic cells. Glycine serves the equivalent role in the spinal cord. Inhibitory interneurons shape neural circuit dynamics by controlling the timing, selectivity, and synchrony of neural activity, and imbalance between excitation and inhibition is a central feature of epilepsy, anxiety disorders, and schizophrenia.
Benzodiazepines, among the most prescribed drugs in the world, enhance GABA-A receptor activity at inhibitory synapses, increasing the frequency of chloride channel opening and reducing anxiety, preventing seizures, and producing sedation.
Fun Facts About the Nervous System →Inhibitory synapses are not merely brakes on neural activity. They actively sculpt the temporal patterns of neural firing, enable contrast enhancement in sensory systems, and generate the synchronized oscillations that characterize different brain states.
Chandelier cells, highly specialized inhibitory interneurons in the cortex, contact the axon initial segment of pyramidal neurons, placing them in the most powerful position to prevent action potential generation. A single chandelier cell can form inhibitory synapses onto the axon initial segments of roughly 300 to 500 pyramidal neurons, giving it outsized influence over cortical output.
Interneuron
/ IN-ter-NYOOR-on / · Latin inter (between) + Greek neuron
Interneuron is a neuron that connects other neurons within the same region of the nervous system, integrating and modulating signals between sensory, motor, and other interneurons without directly contacting sensory receptors or muscle fibers.
Interneurons constitute about 20 to 25 percent of cortical neurons and are predominantly inhibitory, using GABA or glycine as their neurotransmitter. They are the most diverse neuronal cell type, with dozens of distinct subtypes marked by different morphologies, firing patterns, electrophysiological properties, and synaptic targets. Parvalbumin-positive fast-spiking interneurons are critical for generating gamma oscillations associated with attention and working memory, and their dysfunction is implicated in schizophrenia.
The spinal cord interneurons mediating the knee-jerk reflex include Ia inhibitory interneurons that simultaneously inhibit the antagonist muscle when the agonist contracts, enabling reciprocal inhibition that prevents the knee from buckling.
Interneurons are not a single uniform cell type but an enormously diverse collection of cell subtypes. The mouse cortex alone contains over 20 molecularly distinct interneuron subtypes, each with characteristic axonal targeting, firing properties, and roles in circuit computation.
Hippocampal interneurons coordinate the firing of thousands of pyramidal cells into theta oscillations at 4 to 8 Hz and gamma oscillations at 30 to 80 Hz during spatial navigation and memory encoding. Parvalbumin-positive basket cells and somatostatin-positive dendrite-targeting cells control pyramidal neuron activity at different phases of each oscillation cycle, often within windows shorter than 10 milliseconds.
Ion Pump
/ EYE-on pump / · Greek ion (going) + Old English pumpe
Ion Pump is a membrane protein that uses metabolic energy from ATP hydrolysis to transport ions against their concentration gradients across the neuronal membrane, maintaining the ionic conditions necessary for neural signaling.
The primary neuronal ion pump is the sodium-potassium ATPase, which expels three sodium ions from the cell interior for every two potassium ions it imports, maintaining the resting membrane potential and restoring ion gradients after repeated action potentials. This pump consumes a significant fraction of the brain’s total ATP budget, explaining the brain’s extraordinary energy demand relative to its mass. Without continuous pump activity, neurons would gradually depolarize as ions leak across the membrane, eventually becoming unable to generate action potentials.
The sodium-potassium ATPase belongs to a family of P-type ATPases found across all domains of life, and the version in animal cells is closely related to the calcium pump that fills the sarcoplasmic reticulum of muscle fibers. This evolutionary kinship is why cardiac drugs that target one pump, such as the digitalis glycosides used to treat heart failure, often produce neural side effects through cross-reaction with the neuronal sodium-potassium ATPase.
Ion pumps are not the same as ion channels. Channels are passive pores through which ions flow down their electrochemical gradients, while pumps are active transporters that use energy to move ions against their gradients.
Facilitated Diffusion →Cardiac glycosides including digitalis inhibit the cardiac sodium-potassium ATPase, increasing intracellular sodium, which secondarily raises intracellular calcium and strengthens cardiac contraction. At therapeutic doses the effect is beneficial in heart failure, but the margin is narrow: plasma concentrations above roughly 2 nanograms per milliliter produce toxic arrhythmias because the same pump maintains electrical stability in both cardiac and neural tissue.