Chapter 5: How Do Neurons Communicate and Adapt?

Comprehensive vocabulary flashcards covering the chemical and electrical mechanisms of synaptic transmission, neurotransmitter synthesis and classifications, receptor mechanisms, activating systems, and hormonal stress regulation based on Chapter 5.

Acetylcholine (ACh)

The first neurotransmitter discovered in the peripheral and central nervous systems; it activates skeletal muscles in the somatic nervous system and may excite or inhibit internal organs in the autonomic nervous system.

Otto Loewi (1921)

The pharmacologist who demonstrated chemical neurotransmission using a frog heart experiment, showing that stimulating the vagus nerve releases a chemical message that slows heart rate.

Epinephrine (EP)

A chemical messenger (also known as adrenaline) that acts as a hormone to mobilize the body for fight or flight during stress, and acts as a neurotransmitter in the central nervous system.

Norepinephrine (NE)

A neurotransmitter (also known as noradrenaline) found in the brain and autonomic nervous system that accelerates heart rate in mammals.

Chemical synapse

A junction where messenger molecules (neurotransmitters) are released from one neuron to excite or inhibit the adjacent neuron; first visually confirmed in the 1950s using electron microscopy.

Synaptic vesicle

A small, round presynaptic granule that encapsulates and stores neurotransmitter molecules awaiting release into the synaptic cleft.

Storage granule

A large membranous compartment within the presynaptic axon terminal that holds multiple synaptic vesicles.

Synaptic cleft

The microscopic extracellular space separating the presynaptic terminal membrane from the postsynaptic dendritic spine membrane.

Anterograde synaptic transmission

The five-step forward process across a chemical synapse consisting of: neurotransmitter synthesis, packaging and storage, release by exocytosis, receptor binding, and inactivation.

Transporter

A specialized protein embedded in the cell membrane that pumps neurotransmitter precursor molecules or intact neurotransmitters across the membrane.

Exocytosis

The mechanism triggered by $Ca^{2+}$ influx and calmodulin complex formation in which synaptic vesicles dock and fuse with the presynaptic membrane to discharge their contents into the synaptic cleft.

Gap junction

An electrical synapse where connexin proteins on adjacent cell membranes connect to form hemichannels, permitting direct, bidirectional passage of ions with virtually no transmission delay.

Axosecretory synapse

A specialized axon terminal connection that terminates directly onto a tiny blood vessel or capillary to secrete neurotransmitters directly into the bloodstream.

Excitatory synapse

A synapse typically located on dendritic shafts or spines characterized by round synaptic vesicles, dense material on both membranes, a wide synaptic cleft, and a large active zone.

Inhibitory synapse

A synapse typically located on the cell body (soma) characterized by flattened synaptic vesicles, sparse membrane material, a narrow synaptic cleft, and small active zones.

Dendritic spines

Small neuronal protrusions that independently undergo structural modifications to mediate learning and memory, the impairment or loss of which is linked to intellectual disability and Alzheimer disease.

Renshaw loop

A circular inhibitory motor circuit in which an axon collateral from a motor neuron excites a Renshaw interneuron via acetylcholine, which in turn releases glycine onto the motor neuron to inhibit it; blocked by strychnine.

Small-molecule transmitters

A class of quick-acting neurotransmitters synthesized directly in the axon terminal from dietary nutrients and packaged locally for immediate release.

Acetylcholinesterase (AChE)

The enzyme located within the synaptic cleft that terminates acetylcholine action by cleaving it into its precursors, acetate and choline, for presynaptic reuptake.

Amine synthesis pathway

The sequential enzymatic pathway in which the precursor amino acid tyrosine is converted successively into L-dopa, dopamine, norepinephrine, and epinephrine.

Tyrosine hydroxylase

The rate-limiting enzyme that catalyzes the initial conversion of dietary tyrosine to L-dopa in the catecholamine synthesis pathway.

Serotonin (5-HT)

An amine neurotransmitter synthesized from the dietary amino acid L-tryptophan that regulates mood, aggression, appetite, arousal, respiration, and pain perception.

Glutamate (Glu)

The primary amino acid neurotransmitter in the brain that typically opens $Na^+$ and $Ca^{2+}$ ion channels to produce rapid neuronal excitation.

Gamma-aminobutyric acid (GABA)

The main amino acid neurotransmitter in the brain that typically opens $Cl^-$ ion channels to hyperpolarize and inhibit postsynaptic neurons.

Adenosine

A purine neurotransmitter remaining after the removal of three phosphate groups from ATP that promotes sleep, suppresses arousal, and mediates vasodilation.

Neuropeptides

A diverse class of slow-acting peptide transmitters synthesized from mRNA on ribosomes in the cell body, packaged by Golgi bodies, and delivered to terminals via microtubules.

Endocannabinoids

Lipid neurotransmitters synthesized on demand at the postsynaptic membrane (such as anandamide and 2-AG) that travel retrogradely to activate presynaptic $CB_1$ receptors.

Gaseous transmitters

Unconventional messenger molecules (including nitric oxide, carbon monoxide, and hydrogen sulfide) that are synthesized on demand, freely diffuse across membranes, and are not stored in vesicles.

Ion transmitters

Inorganic transmitters, specifically zinc ($Zn^{2+}$), that are actively transported and packaged into synaptic vesicles (often alongside glutamate) and released into the synaptic cleft.

Ionotropic receptor

A transmembrane protein with a neurotransmitter binding site and an intrinsic ion pore that rapidly and directly alters membrane voltage upon ligand binding.

Metabotropic receptor

A slow, poreless transmembrane receptor coupled to an intracellular G protein that indirectly modulates ion channels or triggers cellular metabolic pathways via second messengers.

Second messenger

An intracellular molecule activated by a G protein subunit that can open membrane channels, incorporate new protein channels into the membrane, or alter DNA transcription.

Nicotinic ACh receptor

An ionotropic receptor found on skeletal muscle fibers and neurons whose pore opens upon binding acetylcholine or nicotine, allowing the simultaneous influx of $Na^+$ and efflux of $K^+$.

Enteric Nervous System (ENS)

An autonomous neural network governing the digestive tract that uses over 30 neurotransmitters (predominantly serotonin and dopamine) and can operate independently of the CNS.

Cholinergic activating system

A CNS ascending projection system originating in brainstem and basal forebrain nuclei that maintains normal waking behavior, attention, and memory; damaged in Alzheimer disease.

Nigrostriatal dopaminergic pathway

A dopamine pathway originating in the substantia nigra and projecting to the basal ganglia; loss of over $90\%$ of its dopamine causes rigidity and motor symptoms in Parkinson disease.

Mesolimbic dopaminergic pathway

A dopamine projection pathway associated with reward, repetition of behaviors, and addiction; hyperactivity within this system is linked to schizophrenia.

Homeostatic hormone

A functional category of hormones (such as insulin) responsible for maintaining internal metabolic equilibrium and physiological constancy across the body.

Fast-acting stress response

The rapid autonomic pathway in which the hypothalamus signals through the spinal cord to the adrenal medulla, provoking immediate epinephrine release into circulation.

Slow-acting stress response

The endocrine cascade where hypothalamic CRH stimulates pituitary ACTH release, directing the adrenal cortex to secrete cortisol into the circulatory system.

Hippocampal stress cycle (vicious circle)

A deleterious positive-feedback loop where prolonged stress causes excess cortisol secretion, destroying cortisol-sensitive hippocampal neurons and preventing the shutdown of further cortisol release.