Neurophysiology: Action Potential Propagation, Synaptic Transmission, and Spinal Pathways

Vocabulary practice flashcards covering action potential propagation, electrotonic currents, electrical vs. chemical synapses, vesicular release mechanisms, second-messenger GPCR pathways, and spinal cord motor/sensory tracts.

Electrotonic Current

The passive spread of electrical charge through the intracellular fluid of an axon that depolarizes adjacent regions of the membrane toward threshold without immediate regeneration by voltage-gated channels.

Saltatory Conduction

The rapid mode of action potential propagation in myelinated axons, where depolarization jumps from one Node of Ranvier to the next as electrotonic current flows beneath the insulating myelin sheath.

Demyelination

The pathological destruction or loss of the insulating myelin sheath, which causes electrotonic current to leak through the bare axonal membrane, drastically slowing or blocking action potential propagation.

Refractory Period

The period following an action potential during which voltage-gated $Na^+$ channels are inactivated, preventing re-excitation of recently depolarized membrane patches and enforcing unidirectional propagation.

Electrical Synapse

A direct specialized connection between neurons formed by gap junctions that allows fast, bidirectional ionic current flow, but lacks modifiability and plasticity.

Chemical Synapse

A neuronal junction where an electrical signal triggers the exocytosis of chemical neurotransmitters across a synaptic cleft to bind postsynaptic receptors; it is slower than an electrical synapse but highly modifiable.

Quantal Release

The exocytosis of neurotransmitters in discrete, multimolecular packets termed quanta, reflecting the uniform neurotransmitter content of individual synaptic vesicles and generating postsynaptic potentials in integer multiples of a unit size.

Miniature End Plate Potential (mEPP)

A small, spontaneous postsynaptic depolarization recorded in muscle fibers that corresponds to the release of a single quantum (one synaptic vesicle) of acetylcholine in the absence of presynaptic stimulation.

Readily Releasable Pool

The subpopulation of synaptic vesicles docked at the active zone of the presynaptic terminal that undergo rapid exocytosis immediately upon $Ca^{2+}$ influx through voltage-gated channels.

Reserve Pool

The depot of synaptic vesicles tethered to the presynaptic cytoskeleton that must be mobilized by $Ca^{2+}$-dependent signaling before transitioning to the active zone for release.

$Ca^{2+}$ ATPase

An active transport pump that uses ATP hydrolysis to extrude two $Ca^{2+}$ ions out of the cytoplasm into the extracellular fluid or into internal stores such as the sarcoplasmic reticulum, restoring low basal cytosolic $Ca^{2+}$ levels.

End Plate Potential (EPP)

The graded, non-propagated postsynaptic depolarization produced at the neuromuscular junction when acetylcholine binds to nicotinic acetylcholine receptors on the motor end plate.

Acetylcholinesterase (ACh-ase)

An enzyme located in the synaptic cleft of cholinergic synapses that rapidly terminates neurotransmission by hydrolyzing acetylcholine into acetate and choline.

Dopamine Transporter (DAT)

A plasma membrane transport protein on presynaptic terminals that terminates dopamine signaling by clearing dopamine from the synaptic cleft via active reuptake.

Monoamine Oxidase Inhibitors (MAOIs)

A class of pharmaceutical agents that inhibit the enzymatic degradation of monoamine neurotransmitters (dopamine, epinephrine, serotonin), prolonging their synaptic lifetime to treat depression and anxiety.

Selective Serotonin Re-uptake Inhibitors (SSRIs)

Therapeutic compounds that selectively block the serotonin reuptake transporter on the presynaptic membrane, elevating serotonin availability in the synaptic cleft.

Ionotropic Receptor

A ligand-gated ion channel receptor that directly opens a transmembrane pore upon neurotransmitter binding, allowing rapid ion flux and immediate postsynaptic electrical responses.

Metabotropic Receptor

A G-protein-coupled receptor (GPCR) that regulates cellular functions indirectly through an intracellular second-messenger cascade, producing slower and longer-lasting postsynaptic modifications.

$G_s$ Protein

A stimulatory G protein alpha subunit that activates adenylyl cyclase, promoting the conversion of ATP into cyclic AMP (cAMP) and activating Protein Kinase A (PKA).

$G_i$ Protein

An inhibitory G protein alpha subunit that inhibits adenylyl cyclase activity, thereby lowering intracellular cAMP production and dampening downstream PKA activation.

$G_q$ Protein

A heterotrimeric G protein alpha subunit that activates phospholipase C (PLC), stimulating the hydrolysis of membrane phospholipids into diacylglycerol (DAG) and inositol trisphosphate ($IP_3$).

Inositol Trisphosphate ($IP_3$)

A water-soluble second messenger generated by PLC activity that diffuses into the cytosol and binds ligand-gated channels on the endoplasmic reticulum, releasing stored $Ca^{2+}$ into the cytoplasm.

Diacylglycerol (DAG)

A lipid second messenger produced by PLC-mediated cleavage of membrane phospholipids that remains embedded in the plasma membrane and activates Protein Kinase C (PKC).

$\beta_2$-Adrenergic Receptor

A metabotropic adrenergic receptor that couples to $G_s$ to activate the cAMP-PKA pathway, resulting in vascular smooth muscle relaxation and vasodilation.

$\alpha_1$-Adrenergic Receptor

A metabotropic adrenergic receptor that couples to $G_q$ to stimulate the PLC-$IP_3$-$Ca^{2+}$ pathway, causing vascular smooth muscle contraction and vasoconstriction.

Excitatory Postsynaptic Potential (EPSP)

A graded depolarization of the postsynaptic membrane caused by ion fluxes with an equilibrium potential ($E_{\text{ion}}$) more positive than the action potential threshold, bringing the neuron closer to firing.

Inhibitory Postsynaptic Potential (IPSP)

A graded change in postsynaptic membrane potential that stabilizes or hyperpolarizes the membrane toward an equilibrium potential (such as $E_K$) below threshold, decreasing the probability of an action potential.

Dorsal Root Ganglion

A peripheral cluster of cell bodies belonging to pseudounipolar primary afferent (sensory) neurons situated adjacent to the posterior entry of the spinal cord.

Dorsal Horn

The posterior gray matter column of the spinal cord containing somatic and visceral sensory relay nuclei where central axons of primary afferent fibers synapse.

Ventral Horn

The anterior gray matter region of the spinal cord containing the cell bodies of somatic lower motor neurons whose axons exit via ventral roots to innervate skeletal muscle fibers.

Lateral Horn

The intermediate gray matter projection found primarily in thoracic and lumbar spinal levels that contains preganglionic cell bodies of autonomic efferent neurons.

Lemniscal Pathway

An ascending sensory pathway (dorsal columns) that carries ipsilateral discriminative touch, vibration, and proprioception up the spinal cord before decussating in the medulla oblongata and projecting via the medial lemniscus to the thalamus.

Spinothalamic Pathway

An ascending sensory pathway (anterolateral tract) carrying pain, temperature, and crude touch that decussates across the midline within the spinal cord near the level of entry and ascends contralaterally to the thalamus.

Decussation

The anatomical crossing of nerve tracts across the midline of the central nervous system, occurring in the medulla for the lemniscal and lateral corticospinal pathways and in the spinal cord for the spinothalamic pathway.

Lateral Corticospinal Tract

The descending motor tract whose fibers cross at the pyramidal decussation in the medulla and descend in the lateral spinal cord white matter to control voluntary movements of ipsilateral limb muscles.

Anterior Corticospinal Tract

A descending motor pathway in the anterior funiculus of the spinal cord that remains uncrossed in the brainstem and mediates bilateral control of axial and postural muscles.

Dermatome

A distinct segment of the skin surface whose sensory receptor fields are mapped to and innervated by the sensory afferents of a single spinal nerve root level.