Nervous System (2): Neurotransmitters, Neuroglia, and Neuronal Circuits

Comprehensive practice flashcards defining key vocabulary terms on neurotransmitters, neuromodulators, neuroglia, and neuronal circuits based on lecture notes.

Neurotransmitter

A chemical messenger synthesized in neurons, stored in synaptic vesicles, released from the presynaptic neuron upon stimulation, and bound to specific postsynaptic receptors to produce rapid electrical responses such as EPSPs or IPSPs.

Neuromodulator

A messenger often co-released with classical neurotransmitters that acts primarily through metabotropic (GPCR) receptors to amplify or dampen synaptic activity, alter presynaptic release or re-uptake, or regulate gene transcription and enzyme activity over seconds to days.

Large Dense Core Vesicle

A type of vesicle formed in the soma and transported down the axon via axonal transport that stores and releases neuropeptides following cleavage and post-translational processing.

Glutamate

The primary excitatory neurotransmitter in the central nervous system, released by approximately $80-90\%$ of excitatory neurons and synthesized from glutamine by the enzyme glutaminase.

NMDA Receptor

An ionotropic glutamate receptor and ligand-gated ion channel permeable to $Na^+$, $K^+$, and $Ca^{2+}$; its activation is essential for synaptic plasticity and long-term potentiation, but excessive stimulation triggers excitotoxicity.

Synaptic Plasticity

The ability of synapses to strengthen or weaken over time in response to activity, fundamentally driven by $Ca^{2+}$ influx triggering intracellular cascades such as Long-Term Potentiation (LTP).

Excitotoxicity

Neuronal injury and death triggered by the excessive accumulation of glutamate, which hyperactivates glutamate receptors (particularly NMDA receptors) and floods neurons with toxic levels of intracellular $Ca^{2+}$, generating reactive oxygen species (ROS), inflammation, apoptosis, and necrosis.

Glutamate-Glutamine Cycle

The recycling process where extracellular glutamate is cleared from the synaptic cleft by astrocytic transporters (EAATs / GLT-1), converted into glutamine by glutamine synthetase, and returned to presynaptic neurons to be converted back into glutamate by glutaminase.

GABA ((\gamma)-aminobutyric acid)

The most widespread inhibitory neurotransmitter in the brain, synthesized from glutamate via glutamic acid decarboxylase (GAD) and removed from the synaptic cleft by GABA transporters (GAT).

GABA(_A) Receptor

An ionotropic receptor complex functioning as a ligand-gated $Cl^-$ channel that causes fast membrane hyperpolarization and neuronal inhibition; it contains allosteric binding sites for benzodiazepines, barbiturates, propofol, and ethanol.

GABA(_B) Receptor

A metabotropic G-protein-coupled receptor that produces slower inhibitory effects by opening $K^+$ channels and reducing $Ca^{2+}$ influx, targeted clinically by agonists such as baclofen to treat muscle spasticity.

Glycine

The primary inhibitory neurotransmitter in the spinal cord and brainstem that activates ionotropic receptors coupled to $Cl^-$ channels to regulate motor reflexes and pain pathways.

Strychnine

A toxic alkaloid derived from the seeds of Strychnos nux-vomica that acts as a potent, selective antagonist of glycine receptors, blocking inhibitory control and resulting in severe muscle spasms and hyperexcitability.

Acetylcholine (ACh)

A major neurotransmitter in both the CNS and PNS, synthesized from choline and acetyl-CoA by choline acetyltransferase, that acts on nicotinic (ionotropic) and muscarinic (metabotropic) receptors to mediate motor contraction, attention, arousal, and memory.

Acetylcholinesterase (AChE)

An extracellular enzyme localized in the synaptic cleft that rapidly terminates acetylcholine signaling by hydrolyzing it into acetate and choline, the latter of which is recycled into the presynaptic terminal.

Alzheimer's Disease

The most common neurodegenerative cause of dementia, pathologically marked by widespread degeneration of cortical and basal forebrain cholinergic neurons, notably those originating in the nucleus basalis.

Myasthenia Gravis

An autoimmune disorder characterized by muscle weakness due to autoantibodies directed against nicotinic acetylcholine receptors at the neuromuscular junction, blocking and destroying receptor sites.

Botulinum Toxin

A bacterial neurotoxin produced by Clostridium botulinum that prevents the exocytosis of acetylcholine-containing synaptic vesicles at the neuromuscular junction, blocking muscle contraction and resulting in flaccid paralysis.

Monoamines

A class of neurotransmitters derived from aromatic amino acids containing an $-NH_2$ group (including catecholamines, serotonin, and histamine) that function predominantly as neuromodulators via reuptake transporters and enzymatic degradation by MAO and COMT.

Dopaminergic System Pathways

Four major neural tracts originating from midbrain dopaminergic nuclei (substantia nigra and VTA): the nigrostriatal pathway (motor regulation), mesolimbic pathway (emotion and reward), mesocortical pathway (learning and memory), and tuberoinfundibular pathway (prolactin regulation).

Mesolimbic Pathway

The dopaminergic reward pathway projecting from the ventral tegmental area (VTA) to the nucleus accumbens that mediates motivation, incentive drive, and the perception of pleasure.

Locus Coeruleus

A noradrenergic nucleus situated in the pons that provides widespread ascending projections throughout the brain to mediate arousal, vigilance, focused attention, and fight-or-flight signaling.

Serotonin (5-HT)

A monoamine synthesized from tryptophan by tryptophan hydroxylase and aromatic L-amino acid decarboxylase, primarily concentrated in the brainstem raphe nuclei, that regulates mood stabilization, sleep, pain inhibition, and appetite.

Histamine

A biogenic amine synthesized from histidine, highly concentrated centrally within the tuberomammillary nucleus of the hypothalamus, that promotes wakefulness and cortical arousal.

Neuropeptides

A diverse class of over $80$ slow-acting peptide messengers cleaved enzymatically from larger precursors, packed in dense-core vesicles, and acting via GPCRs to fine-tune neural circuits over minutes to hours.

Endogenous Opioids

A family of peptide neuromodulators including endorphins, enkephalins, and dynorphins that bind to $\mu$, $\delta$, and $\kappa$ opioid receptors to produce analgesia, stress reduction, and reward.

Glucagon-like Peptide-1 (GLP-1)

A peptide metabolic messenger secreted by intestinal L-cells and brainstem nucleus of the solitary tract (NTS) neurons that triggers glucose-dependent insulin release, delays gastric emptying, promotes satiety, and diminishes the rewarding value of food.

Astrocytes

The most abundant glial cells in the CNS ($20-50\%$), featuring star-shaped processes that maintain the blood-brain barrier, supply energy to neurons via lactate shuttle, buffer extracellular $K^+$, and clear synaptic glutamate.

Microglia

The resident immune defense cells of the CNS ($10-20\%$), derived from modified macrophages, that transform from a ramified resting state into an amoeboid phagocytic form to clear pathogens, cellular debris, and prune synapses.

Oligodendrocytes

Glial cells of the central nervous system that wrap myelin sheaths around multiple axon segments simultaneously, facilitating rapid saltatory action potential conduction.

Ependymal Cells

Ciliated epithelial glial cells lining the cerebral ventricles and the central canal of the spinal cord that produce, monitor, and circulate cerebrospinal fluid (CSF).

Schwann Cells

The myelinating glial cells of the peripheral nervous system, each enclosing a single axonal segment, which play an indispensable role in promoting axonal regeneration following peripheral nerve injury.

Satellite Cells

Peripheral glial cells that closely envelop neuronal cell bodies located within sensory and autonomic ganglia, regulating the surrounding microenvironment, ion levels, and nutritional exchange.

Glial Scar

A dense structural barrier formed predominantly by reactive astrocytes and infiltrating macrophages after CNS injury that impedes successful axonal regeneration in the brain and spinal cord.

Neuronal Circuit

A functional collection of interconnected neurons organized to receive, process, and propagate specific modalities of information within the nervous system.

Divergent Circuit

A neuronal circuit configuration where axonal collaterals of a single presynaptic neuron synapse onto multiple postsynaptic neurons, distributing and amplifying a signal to several targets simultaneously.

Convergent Circuit

A neuronal circuit organization where synaptic inputs originating from multiple presynaptic neurons converge onto a single postsynaptic neuron, integrating diverse inputs into one common output.

Reverberating Circuit

An oscillating feedback circuit where neurons in a pathway project axon collaterals back to re-excite preceding neurons in the chain, generating sustained rhythmic activity (e.g., breathing, locomotion, sleep-wake cycles) until terminated by fatigue or inhibition.

Parallel After-Discharge Circuit

A circuit in which an incoming signal diverges into multiple parallel chains containing varying numbers of synapses that reconverge on a common output neuron, producing a prolonged discharge after the stimulus stops due to differing synaptic delays.

Feed-Forward Inhibition

An inhibitory circuit motif where an excitatory afferent neuron branches to activate an inhibitory interneuron that suppresses downstream target neurons, precisely regulating the timing and duration of excitation.

Feedback (Recurrent) Inhibition

A negative feedback mechanism in which an active excitatory neuron sends a recurrent axon collateral to activate an inhibitory interneuron, which loops back to inhibit the originating neuron and prevent excessive excitation.

Renshaw Cell

A specialized inhibitory interneuron within the spinal cord that receives cholinergic collateral inputs from an $\alpha$-motor neuron and releases glycine back onto the same motor neuron to mediate recurrent negative feedback inhibition.

Reciprocal Inhibition

A spinal cord reflex mechanism whereby sensory afferents from a contracting agonist muscle activate inhibitory interneurons to relax the antagonistic muscle across the same joint, preventing co-contraction.

Lateral Inhibition

An inhibitory circuit design where strongly activated sensory neurons recruit interneurons to inhibit adjacent, less active neighboring pathways, sharpening borders and enhancing sensory contrast.