Neurotransmitters & Neuroreceptors

Vocabulary flashcards covering neural transmission, chemical and electrical synapses, synaptic vesicle exocytosis, neurotransmitter termination mechanisms, major neurotransmitter systems, associated clinical disorders, and pharmacological agonists and antagonists.

Neurotransmitter

A chemical substance synthesized and released by neurons into the synaptic cleft to stimulate adjacent neurons, muscle cells, or gland cells, allowing electrical impulses to pass through the nervous system.

Neurotransmitter Receptor (Neuroreceptor)

A specific membrane protein on a target cell that binds an endogenous chemical messenger or drug to initiate a cellular response, operating via high specificity comparable to a lock and key.

Dendrite

A branching projection extending from the neuronal cell body (soma) specialized to receive incoming electrical or chemical signals from other neurons.

Axon Hillock

The specialized region connecting the soma to the axon where incoming graded potentials are integrated to initiate an action potential if threshold is reached.

Myelin Sheath

An insulating lipid-rich layer wrapped around some neuronal axons that increases the velocity of action potential propagation.

Presynaptic Terminal (Axon Terminal)

The specialized distal ending of an axon (also called a synaptic bouton or end bulb) where neurotransmitter molecules are stored in vesicles and released upon terminal depolarization.

Synaptic Cleft (Synaptic Gap)

The narrow extracellular fluid-filled space that physically separates the presynaptic membrane from the postsynaptic receptive membrane.

Synapse

The functional junction consisting of the presynaptic axon terminal, the synaptic cleft, and the receptive postsynaptic membrane (such as a dendrite) of the target cell.

Chemical Synapse

A junction between electrically active cells where incoming depolarization triggers the exocytosis of neurotransmitters across a synaptic cleft to activate receptors on the adjacent cell.

Electrical Synapse

A direct physical connection between electrically active cells where membranes are touching, allowing ions to pass directly between cells so that depolarization in one instantly depolarizes the other.

Voltage-Gated $Ca^{2+}$ Channels

Ion channels located in the presynaptic terminal membrane that open in response to action potential depolarization, allowing $Ca^{2+}$ ions to rush into the terminal and stimulate vesicle fusion.

Stages of Synaptic Transmission

The sequence depicted where vesicles dock in the resting state, presynaptic depolarization opens $Ca^{2+}$ channels, increased $[Ca^{2+}]$ causes vesicle exocytosis, neurotransmitters diffuse to bind postsynaptic receptors, and signal is terminated by diffusion, degradation, or active reuptake.

Exocytosis

The process whereby neurotransmitter-containing membranous vesicles fuse with the presynaptic plasma membrane to release chemical messengers into the synaptic cleft.

Ligand

A specific substance or chemical molecule that binds to a complementary receptor protein under high specificity (one ligand, one receptor).

Transporter Protein (Reuptake Molecule)

A membrane protein on the presynaptic terminal that reabsorbs intact neurotransmitter molecules out of the synaptic cleft and pumps them back into the cell for reuse.

Enzymatic Degradation

A termination pathway in the synaptic cleft where specific catabolic enzymes cleave and digest neurotransmitter molecules to prevent continuous postsynaptic stimulation.

Acetylcholine (ACh)

The primary neurotransmitter of the PNS (also found in the CNS and NMJ) responsible for muscle control, memory formation, and sensory response.

Myasthenia Gravis

An autoimmune disorder in which a patient produces antibodies against acetylcholinesterase (AChE), hindering acetylcholine breakdown and resulting in persistent muscle stimulation and contraction issues.

Acetylcholinesterase (AChE)

The degradation enzyme that catabolizes and inactivates acetylcholine within the synaptic cleft.

Dopamine

A neurotransmitter found in the hypothalamus and other brain regions involved in reward pathways, cognition, and voluntary motion.

Parkinson's Disease

A neurological condition characterized by the degeneration of dopamine-releasing neurons, often treated with the dopamine precursor L-dopa.

L-dopa

A chemical precursor to dopamine that crosses into the brain and converts into dopamine to treat Parkinson's disease.

Schizophrenia

A mental disorder linked to excessive activity of dopamine-releasing neurons in certain areas of the brain, managed with drugs that block dopamine receptors.

GABA (Gamma-Aminobutyric Acid)

The primary inhibitory neurotransmitter of the brain that dampens central nervous system activity; low levels are associated with anxiety, epilepsy, and chronic pain.

Norepinephrine

An autonomic neurotransmitter produced in the adrenal medulla that drives the fight or flight response, increasing heart rate, blood glucose, and $O_2$ delivery to the brain and muscles.

Autonomic Failure

A disorder resulting from the loss of norepinephrine-secreting sympathetic neurons, seen in diabetes, Parkinson's disease, or pure autonomic failure.

Serotonin

A neurotransmitter located in the gut and CNS that regulates mood, appetite, sleep, intestinal movement, and muscle control.

Clinical Depression

A psychiatric condition linked to insufficient release of serotonin in the synaptic cleft, treated with medications that prevent serotonin clearance.

Fluoxetine (Prozac)

An antidepressant medication and related compound that treats clinical depression by specifically blocking the elimination of serotonin from the synaptic cleft.

Agonist

A drug that binds to and activates a neurotransmitter receptor, mimicking the endogenous chemical's action to turn on the target cell (e.g., opiate painkillers).

Antagonist

A drug that binds to a neurotransmitter receptor and blocks chemical signaling, turning off or slowing down the target cell's response.

Clozapine (Clozaril)

An antagonist medication used to treat schizophrenia by binding to and blocking dopamine receptors in the brain.

Haloperidol (Haldol)

An antagonist drug prescribed to manage schizophrenia and related disorders by blocking dopamine receptors and mitigating excessive dopamine activity.