Nervous System: Synapses, Graded Potentials, and Neurotransmitter Signaling

Vocabulary practice flashcards covering membrane potential phases, neuron anatomy and functional segments, electrical and chemical synapses, graded potentials (EPSP/IPSP), summation types, and neurotransmitter receptor pathways.

Depolarization

A phase in which the membrane potential becomes more positive, changing from $-70\,mV$ to $+30\,mV$.

Repolarization

A phase in which the membrane potential returns toward the resting membrane potential (RMP), moving from $+30\,mV$ back down to $-70\,mV$.

Hyperpolarization

A phase in which the membrane potential drops below the resting membrane potential (becoming more negative), shifting from $-70\,mV$ down to $-80\,mV$ or $-90\,mV$.

Receptive Segment

The neuronal region consisting of the dendrites and soma, responsible for binding neurotransmitters released from presynaptic neurons and producing graded potentials.

Initial Segment

The region at the axon hillock responsible for the summation of graded potentials and the initiation of an action potential.

Conductive Segment

The axon of a neuron, dedicated to the propagation of all-or-none action potentials.

Transmissive Segment

The axon terminals and synapses, where an action potential triggers the release of neurotransmitters.

Dendrites

Neuronal projections that receive action potentials from other neurons or react to specialized stimuli.

Soma

The cell body of a neuron that generates graded potentials to promote or inhibit depolarization.

Axon Hillock

The specialized region of a neuron that serves as the origin of all-or-none action potentials.

Axon

The long nerve fiber responsible for the propagation of all-or-none action potentials.

Axon Terminals & Synapses

Structures that relay action potentials to another neuron or an effector cell via neurotransmitter release.

Chemical Synapses

Physical and functional junctions between neurons or effectors where arriving action potentials trigger neurotransmitter release, initiating graded depolarizations at dendrites and soma.

Electrical Synapses

Synapses that do not use neurotransmitters, directly propagating membrane depolarization from one cell to another through physical junctions.

Gap Junctions

Physical channel connections formed by connexin proteins located in areas of low electrical resistance between neighboring cells, found in cardiac and smooth muscle.

Intercalated Discs

Specialized structures in the heart containing gap junctions that permit action potentials to spread rapidly across the myocardium.

Neurotransmitter Exocytosis

The presynaptic release sequence where action potentials open voltage-gated $\text{Ca}^{2+}$ channels, allowing $\text{Ca}^{2+}$ to bind a cytoplasmic sensor protein (synaptotagmin) and activate the SNARE complex for vesicle docking and fusion.

Graded Potentials

Local changes in membrane potential that possess no threshold potential, have no refractory period, and have an overall magnitude directly dependent on the number of signals received.

Spatial Summation

The additive combination of presynaptic potentials originating from different neurons to increase the magnitude of a graded potential.

Temporal Summation

The increase in the magnitude of a graded potential produced by a high frequency of successive stimuli delivered by a single presynaptic neuron.

Inhibitory Postsynaptic Potential (IPSP)

A graded potential that hyperpolarizes the postsynaptic membrane (e.g., moving resting membrane potential from $-70\,mV$ to $-90\,mV$), moving the potential further away from threshold.

Excitatory Postsynaptic Potential (EPSP)

A graded potential that depolarizes the postsynaptic membrane (e.g., moving resting membrane potential from $-70\,mV$ to $-40\,mV$), moving the potential closer to threshold.

Ligand Receptors

Receptor channels where neurotransmitters (such as serotonin, GABA, or glycine) bind directly to the ion channel to mediate opening.

G-Protein Coupled Receptors (GPCRs)

Receptors where a neurotransmitter binds to a protein complex, prompting the dissociation of G-protein subunits ($\alpha$, $\beta$, $\gamma$) which migrate to activate target ion channels or enzymes.

Muscarinic ACh Receptors

G-protein coupled acetylcholine receptors prevalent in the parasympathetic nervous system that stimulate smooth muscle contraction (e.g., stomach) or regulate pacemaker activity via $\text{K}^+$ channel opening in the SA node.

Nicotinic ACh Receptors

Ligand-gated channel receptors active in the sympathetic nervous system that stimulate skeletal muscle contractions upon binding acetylcholine.

Acetylcholinesterase

An enzyme in the synaptic cleft that hydrolyzes acetylcholine into acetate and choline, promoting presynaptic reuptake and regulating receptor function.

Catecholamines

A group of monoamine neurotransmitters synthesized from the amino acid tyrosine, comprising dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline).

Serotonin

A monoamine neurotransmitter derived from the amino acid tryptophan.

Histamine

A monoamine neurotransmitter derived from the amino acid histidine.

Monoamine Oxidase (MAO)

An enzyme that breaks down monoamines, facilitating presynaptic reuptake and regulating receptor function.

Adenylate Cyclase Pathway

A GPCR signaling pathway activated by norepinephrine binding, causing G-protein dissociation to activate adenylate cyclase, convert $\text{ATP}$ to cyclic $\text{AMP}$ ($\text{cAMP}$), and activate protein kinase to phosphorylate proteins and open ion channels.