Neurobiology: Key Terms and Glossary

For about a millisecond after a spike begins, sodium-channel inactivation is widespread enough that nothing can start another spike. That silence also stops the impulse from turning round on an ordinary axon.

The enzyme anchored in the basal lamina of the neuromuscular junction that hydrolyses acetylcholine during the brief end-plate current, so the terminal recovers choline rather than acetylcholine.

The presynaptic specialisation at which vesicles are held ready and calcium entry is coupled to their fusion.

The undershoot of membrane voltage towards the potassium equilibrium while delayed-rectifier channels opened by the spike are still closing.

The signal released by a motor axon that, together with LRP4, activates muscle MuSK and clusters nicotinic receptors under the developing terminal.

The behaviour of a spike that, once threshold is passed, reaches the full amplitude set by that axon, so strength is carried by firing rate and by how many axons are recruited.

The ionotropic glutamate receptor that carries the fast excitatory current, passing sodium and potassium and usually excluding calcium when an edited GluA2 subunit is present.

Delivery of further AMPA receptors into the postsynaptic membrane, which is how the CA1 synapses stressed here express long-term potentiation and how a previously silent synapse comes to conduct.

Plaque peptide of Alzheimer disease, freed from its precursor when β-secretase and γ-secretase cut it out, whereas an α cut inside the same stretch stops the peptide forming. Rare dominant mutations that raise the aggregation-prone form put the peptide upstream in those families, and the causal order in common sporadic disease is still disputed here.

Loss of the ability to form new declarative memories after medial-temporal removal, as in H.M., with remote memory, language, and undistracted immediate memory largely intact.

Potentiation of a weak synaptic input when another input, or current passed into the cell, provides the depolarisation that the weak input cannot supply alone.

A central glial cell that takes up potassium and glutamate, supports neurons metabolically, and induces the endothelial blood–brain barrier through its endfeet without itself being the seal.

The membrane just beyond the axon hillock where voltage-gated sodium channels are packed on a scaffold that includes ankyrin-G, and where the decision to fire is usually taken.

The forebrain nuclei to which these notes assign the selection of actions and habits, rather than a single exclusive job.

In a mammal, the endothelial lining of cerebral capillaries, closed by continuous tight junctions and little of the transcytosis seen in leakier vessels, and induced by astrocyte endfeet and pericytes. Those endfeet are not the seal, and a few circumventricular organs are left open on purpose.

A spinal circuit that can alternate flexor and extensor activity without a cortical command on each cycle, and that can keep a rhythm in reduced preparations even without sensory drive.

The antagonistic receptive-field arrangement in which light in a small retinal centre and light in the surrounding ring drive a ganglion cell in opposite directions, so even illumination of both does little.

The nucleotide that keeps cation channels open in a photoreceptor in darkness; light lowers it, the channels close, and the cell hyperpolarises.

The ventricular epithelium that secretes cerebrospinal fluid, sitting on fenestrated capillaries and therefore not the same interface as the blood–brain barrier.

The inferior-olive axon that wraps one Purkinje cell, drives its complex spike, and, on the Marr–Albus–Ito account used here, reports an error in the movement.

A requirement, fulfilled by the NMDA receptor, that the event of interest occurs only when transmitter is bound and the postsynaptic membrane is depolarised together.

Failure of a spike to reappear at the next node because current leaks and capacitance is high along a bare stretch of axon, so threshold is never reached.

The path from cerebral cortex, including large layer-V axons, most of whose lateral fibres cross in the medullary pyramid and that these notes associate with fractionated voluntary movement.

A developmental window during which correlated activity can still capture whole territories of synaptic input, after which that large-scale reassignment ends even though synapses are not frozen.

A voltage-gated potassium conductance that opens more slowly than the sodium conductance and supplies the outward current of the falling phase.

The neuronal process specialised to receive synapses, often studded with spines on cortical, hippocampal, and cerebellar cells.

A following impulse moves less transmitter, or meets less sensitive receptors, because the ready pool of vesicles has been used up before it can refill. That paired-pulse fall is what a high initial chance of release tends to produce.

The striatal projection that inhibits the internal pallidum, briefly lifting that nucleus’s inhibition of motor thalamus and so facilitating the act that was chosen.

The root by which sensory axons enter the spinal cord, with their cell bodies in the dorsal root ganglion; cutting it removes afferent drive and does not, by itself, paralyse the muscles.

Cajal’s reading of the neuron as carrying signals from dendrites towards the axon terminal.

The motor that, with dynactin, carries organelles and signalling endosomes from the axon terminal back to the soma.

The positive potential of potassium-rich endolymph relative to perilymph, maintained by the stria vascularis, which drives potassium into the hair cell across the apical membrane even though potassium is abundant on both sides.

A ligand of Eph receptors, commonly laid out as a repulsive gradient that assigns axons their places in a topographic map.

An excitatory postsynaptic potential, a depolarisation towards spike threshold, such as the inward current at a glutamate synapse whose reversal lies near 0 mV.

Memory for episodes or for facts, of a kind that can be declared.

A short-term increase in release when a second spike arrives before calcium from the first has cleared, largest at terminals whose initial probability of release was low.

The ionotropic GABA receptor, a pentameric chloride channel whose usual adult action is inhibition, either as a hyperpolarisation or as a shunt.

The metabotropic GABA receptor, coupled through Gi/o, which opens potassium channels on the postsynaptic side and reduces transmitter release on the presynaptic side.

The motor neuron that tightens the poles of an intrafusal fibre so the spindle’s sensory ending does not fall slack when the muscle shortens.

An electrical synapse, a pair of docked connexons, that passes ionic current with almost no synaptic delay and usually in both directions.

The olfactory target on which all receptor neurons that express one odorant-receptor gene converge.

Clearance of glutamate into astrocytes, its conversion to glutamine, and the return of glutamine for neurons to rebuild glutamate or GABA, which keeps transmitter from spilling.

The resting voltage expected when more than one ion leaks, each Nernst potential weighted by permeability, so the membrane can sit near the potassium potential without equalling it.

A force sensor lying in the tendon, in series with the muscle, which reports load during both passive stretch and active pull, and which inhibits that muscle through Ib interneurons.

Central tissue composed of cell bodies, dendrites, and synapses.

The crawling tip of an axon, with actin filopodia and invading microtubules, whose receptors decide whether a given cue attracts or repels.

A decline in response to a repeated mild stimulus which, in the gill-withdrawal reflex of *Aplysia*, is homosynaptic depression of transmitter release.

A synapse is strengthened when presynaptic activity helps make the postsynaptic cell fire. At many excitatory synapses the presynaptic spike has to lead by a few tens of milliseconds, so transmitter is bound as the returning spike clears magnesium from NMDA receptors.

A change in what a subject can do, including skills, priming, conditioning, and habituation, that does not require the subject to recall the training.

Block of an open voltage-gated sodium channel by an internal particle while depolarisation continues, which shuts off the sodium current before the membrane has repolarised.

The striatal route that inhibits the external pallidum, disinhibits the subthalamic nucleus, and thereby drives the internal pallidum to suppress competing actions.

The order of cortical settlement in which deep-layer neurons stop first and later-born neurons migrate past them towards the pia.

A receptor that is itself a ligand-gated ion channel and that acts within about a millisecond of transmitter binding.

A synaptic effect counted as inhibitory because it lowers the probability of a spike, whether or not the voltage trace moves away from threshold.

The motor that walks towards the plus ends of axonal microtubules, carrying vesicles, mitochondria, and cytoskeletal cargo out to the terminal.

The doctrine that stimulus quality is fixed by which receptor feeds an axon and which central neurons receive it, not by a special symbol inside the spike.

Antibodies aimed at P/Q-type calcium channels on the motor terminal cut the number of vesicles released by each spike, the presynaptic opposite of the receptor loss in typical myasthenia gravis.

An inclusion rich in α-synuclein, often found in remaining dopamine neurons of the substantia nigra in Parkinson disease. Why those neurons die is not settled in these notes.

A lasting fall in synaptic strength, produced at CA1 by a modest calcium rise that leads phosphatases to remove AMPA receptors, and at cerebellar parallel-fibre synapses by a different route to the same kind of removal.

A lasting rise in synaptic strength, induced at the CA1 connection emphasised here only when glutamate and depolarisation together let NMDA receptors admit calcium. Mossy-fibre potentiation onto CA3 is a different, presynaptic mechanism and does not use those receptors.

The last neuron on the path to a muscle fibre. If it is cut, both willed movement and reflexes of that fibre stop, tone goes slack, and the fibre is denervated.

A seven-transmembrane receptor that acts through a G protein, starting over tens of milliseconds and able to alter channels, the probability of release, or transcription.

Yolk-sac-derived myeloid cells of the central nervous system that survey intact tissue, remove weak synapses during development, and phagocytose debris after injury.

A small spontaneous depolarisation at the neuromuscular junction, the postsynaptic sign of one quantum released without a nerve impulse.

Closure of one eye while ocular territories can still be captured, which shifts cortical cells towards the open eye and shrinks the closed eye’s columns although the retina continues to work.

One motor neuron plus the fibres it innervates. A fibre has only a single end-plate, so the muscle grades force by calling up more of these sets and by their firing rate, not by a graded end-plate potential.

A metabotropic acetylcholine receptor, with M1, M3, and M5 coupled through Gq and M2 and M4 through Gi/o, including the receptors at parasympathetic targets and the cardiac M2 receptor that slows the pacemaker.

A stretch receptor set in parallel with the working fibres. Its Ia axons both report lengthening and excite, across a single synapse, the motor neurons of that same muscle.

The muscle kinase through which agrin clusters nicotinic receptors; antibodies to MuSK are a less common myasthenic mechanism in which receptors can be present yet fail to gather under the nerve.

An end-plate disease in which, in most people who have it, antibodies remove, damage, or block muscle nicotinic receptors, so each quantum opens fewer channels and the end-plate potential misses threshold as use continues.

The equilibrium voltage of a single ion, the point at which its concentration gradient and the electrical force on it cancel.

A guidance cue that draws commissural axons towards the floor plate when they express DCC, and that repels axons expressing Unc5.

Cells that migrate from the crest of the closing neural tube and give rise to dorsal-root and autonomic neurons, enteric neurons, Schwann cells, melanocytes, and the adrenal medulla.

The tube formed when the neural plate rolls up. Its cavity will be the ventricles, and its wall will become brain, spinal cord, and retina.

An intracellular mass of paired helical filaments built from hyperphosphorylated tau that has left the microtubules, one of the classical lesions of Alzheimer disease.

The claim that the nervous system is made of separate cells, not a fused reticulum, with signals passing at synapses between them; electron microscopy settled the question by showing two membranes and a cleft.

A transmitter synthesised in the soma as a prepropeptide, carried out in dense-core vesicles, released when firing is intense, and destroyed by peptidases and dilution rather than by a reuptake pump.

The ionotropic class of acetylcholine receptor, covering the muscle end-plate channel and the related neuronal channels that pass the signal at autonomic ganglia.

The ligand-gated cation channel of the skeletal neuromuscular junction, opened when two acetylcholine molecules bind its α subunits, with a reversal potential near 0 mV.

A gaseous signal made from arginine by neuronal nitric oxide synthase when calcium–calmodulin is active, never stored in vesicles, and commonly detected by a rise in cGMP.

An ionotropic glutamate receptor that passes calcium and is blocked by magnesium while the membrane is negative, so the calcium signal requires bound glutamate and depolarisation at once, plus a co-agonist. That conjunction is what makes the receptor a coincidence detector.

The short gap between myelin sheaths where sodium channels are densely packed and where the regenerative spike of a myelinated axon is rebuilt.

Preference for one eye, in a cell or in a patch of visual cortex, produced because inputs that fire together are retained and the two eyes rarely do so.

The central glial cell that myelinates several internodes, commonly on more than one axon.

A bipolar cell that is held off by glutamate in darkness, through mGluR6, and that depolarises when light hyperpolarises the photoreceptor and glutamate release falls.

The crossing at which nasal retinal axons change sides and temporal axons do not, so that one visual field is carried to the opposite hemisphere.

The utricular or saccular macula, weighted by otoconia, which reports linear acceleration, including the continuous acceleration of gravity during a tilt.

The craniosacral autonomic division, whose long preganglionic axon releases acetylcholine onto a nicotinic ganglion near the organ and whose short postganglionic axon releases acetylcholine onto muscarinic receptors.

The long depolarisation, with a burst of spikes on top, that cortical neurons show in a focal seizure, fed by recurrent excitation and normally cut short by potassium currents and nearby inhibition.

The inhibitory neuron that is the sole output of cerebellar cortex and that uses GABA to suppress the deep nuclei.

The packet of transmitter released from one vesicle, of which the evoked end-plate potential, in low calcium, is an integer multiple.

Early progenitors that both generate neurons beside the ventricle and extend a process to the pia along which cortical neurons migrate.

The patch of skin, or of the sensory world, inside which a stimulus changes the firing of one neuron.

The graded voltage produced by a sensory transducer, which triggers spikes in some afferents and, in photoreceptors, hair cells, and taste cells, changes glutamate release without a spike in the receptor cell itself.

The path from a sense organ through an afferent axon, at least one central synapse, and an efferent axon to an effector.

The interval after the absolute refractory period, during the afterhyperpolarisation, when a spike can be forced only by a stronger than usual stimulus.

The steady inside-negative voltage of a neuron that is not spiking, near −65 to −70 mV in the cells these notes take as typical, and held by high potassium permeability on gradients the pump maintains. It is not the equilibrium potential of any one ion, because sodium and other ions also leak and pull the voltage off the potassium Nernst potential.

Transmitter is carried back into a cell by a membrane pump, which clears glutamate, GABA, glycine, and the monoamines, whereas acetylcholine is instead destroyed in the cleft.

The photopigment of rods, an opsin holding 11-cis-retinal; light isomerises the retinal, the resulting cascade destroys cGMP, cation channels close, and the photoreceptor hyperpolarises.

The surplus by which current reaching the next node still exceeds what is required for threshold, a surplus that falls if myelin is lost and that warming can shrink further by shortening channel currents.

Conduction in a myelinated axon in which axial current flows along the internode but the spike itself is regenerated only at successive nodes of Ranvier.

The glial cell of peripheral nerve that either wraps one myelin internode around one axon or holds several unmyelinated axons in a Remak bundle.

A guidance molecule, sensed by plexin and neuropilin, whose typical action is repulsion that excludes axons from the wrong region.

One of three ducts on each side that report angular acceleration: at the start of a rotation the endolymph lags and bends the cupula, and at constant speed the signal fades.

An increase in the *Aplysia* gill-withdrawal reflex after a shock, produced in the short term by serotonin acting through cAMP to broaden the sensory spike, with new protein required only if the change is to last.

Inhibition by a conductance whose reversal potential sits at rest, so excitatory current is drained locally even though the recording shows no hyperpolarisation.

A repulsive cue read by Robo that, once a commissural axon has crossed the midline, stops it from crossing back.

The proteins that zipper a vesicle to the plasma membrane for fusion; synaptobrevin on the vesicle pairs with syntaxin and SNAP-25 on the terminal membrane.

The brisk, velocity-dependent rise in stretch-reflex tone that follows loss of descending restraint, as distinct from flaccid denervation and from the rigidity of dopamine loss.

The adding together, at the site that decides on a spike, of synaptic currents that are active at the same moment.

A small dendritic protrusion whose narrow neck confines a local rise in calcium.

The ascending path for pain, temperature, and crude touch, which crosses in the cord within about a segment or two of the dorsal horn.

The midbrain nucleus whose pars compacta supplies dopamine to the dorsal striatum; loss of that supply is the motor lesion of Parkinson disease in these notes, leaving the internal pallidum over-inhibiting the thalamus.

The thoracolumbar autonomic division, with short cholinergic preganglionic axons ending on nicotinic receptors and, in the usual case, long postganglionic axons that release noradrenaline, sweat glands and the adrenal medulla excepted.

The fast calcium sensor on the synaptic vesicle that triggers synchronous fusion when the spike opens calcium channels at the terminal.

The proposed passage of a declarative episode from hippocampal binding to a neocortical form that can be recalled alone. Other accounts keep rich episodic detail dependent on the hippocampus, and both accept that new declarative memories can fail while some skills remain.

The adding of successive postsynaptic potentials that arrive before the previous one has decayed, on a timescale set largely by the membrane time constant.

Smell is the only major sense that reaches cortex without passing through this structure. Axons returning from cortex outnumber those that arrive, higher-order nuclei tie cortical areas together, and the inhibitory reticular nucleus can shut through-traffic, which is why calling the whole arrangement a relay is too small.

The condition in which inward sodium current outruns outward potassium and leak, so a small depolarisation becomes a regenerative spike instead of dying as a local response.

The extracellular filament, of cadherin-23 and protocadherin-15, that opens the hair-cell transduction channel when a bend towards the tallest stereocilium tensions it.

The place code of frequency along the cochlea, high tones peaking on the narrow stiff base of the basilar membrane and low tones on the wider apex, and kept in order through to auditory cortex.

A central representation in which neighbouring points on a sensory surface stay neighbours, expanded for regions with dense innervation and small receptive fields rather than for regions that are physically large.

A cation channel on many nociceptors that opens above about 43 °C and also to capsaicin and to protons, so that chilli and damaging heat share an axon already committed to pain.

The rate-limiting enzyme of catecholamine synthesis, which hydroxylates tyrosine to L-DOPA.

Clinical name for a descending cell that gives orders to the motor neuron, especially a corticospinal cell. After spinal shock, reflexes are brisk and tone is spastic, and the muscle does not show acute denervation atrophy.

The root by which motor axons leave the spinal cord.

A technique that holds membrane potential constant so the current the experimenter injects equals the ionic current through open channels, used on the squid axon to separate the sodium and potassium components of the spike.

Central tissue composed of myelinated axons that connect the synaptic neuropil of grey matter.