Neurophysiology: Axons, Potentials, and Synapses

Vocabulary flashcards covering axonal propagation, graded potentials, action potentials, synaptic transmission, and neuromuscular junctions.

Nodes of Ranvier

Openings of approximately $1\,mm$ between myelin wrapping cells on an axon where voltage-gated sodium channels are highly concentrated and action potentials regenerate.

Myelin

Electrical insulation formed by glial cells wrapping around an axon up to $50$ times to insulate electrical tissue and increase conduction velocity.

Saltatory Conduction

The mechanism of action potential propagation where electrical signals leap from one node of Ranvier to another, accelerating propagation speed tremendously.

Saltare

The Latin root word meaning 'to leap,' from which the term saltatory conduction is derived.

Voltage-Gated Sodium Channel Density at Nodes of Ranvier

The concentration of voltage-gated sodium channels at the nodes of Ranvier, which is $200$ to $2,000$ times higher than in other regions of the axon membrane.

Axon Diameter and Resistance Rule

Physical rule stating that axons with larger diameters offer lesser electrical resistance, allowing them to conduct action potentials faster than smaller diameter axons.

Unmyelinated Small-Diameter Axon Conduction Speed

The propagation velocity of action potentials in small-diameter axons without myelin, which is approximately $0.5\,m/s$.

Myelinated Large-Diameter Axon Conduction Speed

The propagation velocity of action potentials in large-diameter myelinated axons, reaching approximately $100\,m/s$.

Usain Bolt Speed Comparison

A benchmark mentioned to illustrate that myelinated large-diameter axon conduction speed ($100\,m/s$) is about $10$ times faster than Usain Bolt's $100\,m$ sprint record (around $9.50\,s$).

Multiple Sclerosis

An axonal disorder caused by demyelination that leads to current leakage, propagation delay, or complete block of action potentials.

Demyelination Delay Mechanisms

The two main reasons for action potential delay during myelin loss: current leak through exposed membrane and the absence of underlying voltage-gated channels beneath lost myelin.

Christina Applegate

A famous actress cited in the lecture who suffers from multiple sclerosis and walks with huge difficulty.

Graded Potentials

Local electrical currents generated in a small area of the membrane that decay with distance, do not propagate long distances, and serve to bring the membrane potential toward threshold.

Ligand-Gated Ion Channel Receptors

Membrane receptors that are insensitive to electrical stimulation but open upon binding a chemical ligand, allowing sodium entry to cause depolarization.

Synaptic Potentials

A specific class of graded potentials occurring at postsynaptic membranes caused by neurotransmitters, whose function is to bring the membrane potential toward threshold.

Neuromuscular Junction Synaptic Potential

A depolarizing graded potential generated at the junction between a motor neuron and a skeletal muscle cell.

Pacemaker Potentials

Local currents that spontaneously hyperpolarize and depolarize to bring the membrane potential to threshold, driving rhythmic firing of action potentials.

Medulla Respiratory Pacemaker Neurons

Specialized neurons in the medulla that generate action potentials once every several seconds throughout life to maintain continuous breathing.

Hyperpolarization-Activated Sodium Channels

Unique ion channels activated by hyperpolarization rather than depolarization that allow sodium influx to produce pacemaker potentials.

Slow Waves

Pacemaker potentials in the gastrointestinal tract that bring smooth muscle membrane potential to threshold, triggering muscle contractions to propel food.

Receptor Potentials

Local currents generated by the activation of sensory receptors in specialized sensory cells or nerve endings when exposed to external stimuli.

Somatosensory System Receptors

Sensory receptors activated by physical or chemical stimuli such as touch, pressure, strain, temperature, injury, or chemicals.

Chemical Sensory Systems

Specialized sensory systems that process chemical stimuli to provide the senses of smell and taste.

Vestibular Sensory System Stimulus

Gravity, which acts as the primary physical stimulus activating specialized vestibular receptors.

Somatosensory Pain Receptors

Receptors located in sensory nerve endings in the skin that are activated by heat or extreme cold to produce excitatory receptor potentials.

Initial Segment of Axon

The region of a sensory neuron where local receptor potentials passively spread to trigger the generation of the first action potential.

Specialized Non-Neuronal Sensory Cells

Epithelial cells in auditory, vestibular, and visual systems that generate receptor potentials and release neurotransmitters but cannot generate action potentials themselves.

Rods and Cones Visual Exception

Photoreceptors in the retina that generate hyperpolarizing receptor potentials when hit by light, unlike all other sensory cells which generate depolarizing potentials.

Graded Potential Amplitude

The magnitude of a graded potential, which varies directly depending on the strength or intensity of the stimulus applied.

Decremental Propagation

The characteristic of graded potentials to decay and decrease in amplitude as distance increases from the site of stimulation.

Action Potential Amplitude Constancy

The feature of action potentials maintaining a constant amplitude of approximately $100\,mV$ to $110\,mV$ along the entire length of the axon without decaying.

Inactivation Gate Role

Mechanism in voltage-gated sodium channels that closes to prevent action potentials from exceeding an amplitude of $100\,mV$ to $110\,mV$.

Temporal Summation

The additive effect of multiple EPSPs occurring in rapid succession from a single synapse before previous EPSPs have decayed.

Spatial Summation

The additive effect produced when multiple separate synapses are activated simultaneously at different locations on the postsynaptic cell membrane.

Action Potential Refractory Period

The period caused by sodium channel inactivation during which a second action potential cannot be generated on top of an existing one, preventing summation.

Graded Potential Threshold Absence

The property of graded potentials having no threshold or an extremely low threshold, allowing even minimal stimuli (like one photon of light) to trigger a response.

Presynaptic Cell

The neuron that transmits a signal by releasing neurotransmitters stored in synaptic vesicles into the synaptic cleft.

Postsynaptic Cell

The target cell expressing specific membrane receptors that bind released neurotransmitters to produce a synaptic potential.

Synaptic Cleft

The microscopic fluid-filled space separating the presynaptic membrane from the postsynaptic membrane across which neurotransmitters diffuse.

Active Zones

Specific regions in the presynaptic terminal characterized by the highest density of synaptic vesicles ready for release.

N-Type Voltage-Gated Calcium Channels

Voltage-gated calcium channels located at presynaptic axon terminals that open upon depolarization to allow calcium influx for neurotransmitter release.

Calcium Requirement for Exocytosis

The universal requirement of calcium influx for the release of all neurotransmitters across all synapses without exception.

SNARE Proteins

A family of approximately $60$ proteins in univalent cells responsible for docking, priming, and fusing synaptic vesicles with the presynaptic membrane.

Synaptotagmin

A specialized SNARE protein that functions as a calcium sensor to trigger vesicle fusion and neurotransmitter release upon sensing increased calcium concentration.

Active Transport of Exocytosis

The energy-dependent transport process of vesicle fusion and neurotransmitter release that requires ATP energy expenditure.

Neurotransmitter Elimination Purpose

The mandatory removal or breakdown of neurotransmitters from the synaptic cleft to prevent overstimulation of postsynaptic receptors.

Metabolism Mechanism of Elimination

The breakdown of neurotransmitter molecules into inactive metabolites by metabolic enzymes present in the synaptic cleft or synapse.

Reuptake Mechanism of Elimination

The recycling process where sodium-dependent transporters transport intact neurotransmitter molecules back into the presynaptic terminal to prevent overstimulation.

Proton Pump in Synaptic Vesicles

A transporter that uses proton gradients to repackage reuptaken neurotransmitters into synaptic vesicles inside the presynaptic terminal.

Skeletal Muscle Innervation Principle

The rule that each individual skeletal muscle fiber receives only one synapse from a single motor neuron.

Motor Unit

A single alpha motor neuron and all the individual skeletal muscle cells or fibers that it innervates.

Alpha Motor Neuron

A motor neuron with its cell body located in the spinal cord or brainstem whose axon extends to innervate skeletal muscle fibers.

End Plate

The specialized, flat area of the postsynaptic muscle membrane where the neuromuscular junction synapse is located.

Junctional Folds Function

Structural folds in the end plate membrane that function specifically to increase surface area to accommodate a high density of nicotinic receptors.

Nicotinic Receptor Density Comparison

The concentration of nicotinic receptors, which is approximately $10,000$ times higher within the end plate region than outside the end plate membrane.

Choline Acetyltransferase (ChAT)

An enzyme expressed in the presynaptic terminal that synthesizes acetylcholine from choline and acetyl-CoA.

Acetylcholinesterase

An enzyme located in the synaptic cleft of the neuromuscular junction that rapidly hydrolyzes acetylcholine into inactive choline and acetate.

Acetylcholine Vesicle Molecule Count

The quantitative amount of neurotransmitter contained in a single synaptic vesicle at the neuromuscular junction, equal to approximately $10,000$ molecules of acetylcholine.

Nicotinic Acetylcholine Receptor

A postsynaptic ionotropic receptor channel at the neuromuscular junction permeable to both sodium and potassium ions upon acetylcholine binding.

Neuromuscular Transmission Duration

The brief timeframe of synaptic transmission at the neuromuscular junction, lasting approximately $1$ to $2\,ms$.

End Plate Potential (EPP)

A massive depolarizing graded potential of approximately $70\,mV$ generated at the muscle end plate membrane.

Choline Transport

The reuptake process by which inactive choline produced from acetylcholine breakdown is transported back into the presynaptic terminal for acetylcholine resynthesis.

Acetate Fate in Synaptic Cleft

The breakdown product of acetylcholine hydrolysis that diffuses away from the synaptic cleft rather than being reuptaken.

Vesicle Release Count per Motor Action Potential

The simultaneous exocytosis of over $200$ synaptic vesicles induced by a single action potential arriving at the motor neuron terminal.

CNS EPSP vs. Neuromuscular EPP Amplitude

The comparative potential size where typical CNS excitatory postsynaptic potentials are $1\,mV$ to $10\,mV$, while neuromuscular end plate potentials are huge ($70\,mV$).

End Plate Electrical Excitability

The non-excitable nature of the end plate membrane itself, due to its total lack of voltage-gated sodium and potassium channels.

Muscle Cell Resting Membrane Potential

The resting potential of skeletal muscle fibers, which is approximately $-90\,mV$.

Off-End-Plate Action Potential Generation

The generation of muscle action potentials occurring strictly outside the end plate membrane, where high concentrations of voltage-gated sodium channels exist.

One-to-One Correspondence in Neuromuscular Junction

The reliable relationship where every single action potential in an alpha motor neuron produces one EPP that invariably triggers one muscle fiber action potential.

Bidirectional Propagation in Muscle Fibers

The pattern of muscle action potential propagation, spreading outward in both directions along the muscle fiber from the centrally located end plate.

Axodendritic Synapse

A common type of central nervous system synapse formed between an axon terminal of a presynaptic neuron and a dendrite of a postsynaptic neuron.

Axosomatic Synapse

A synapse formed between an axon terminal of a presynaptic neuron and the cell body (soma) of a postsynaptic neuron.

Axoaxonic Synapse

A modulating synapse formed between the axon terminal of one neuron and the axon terminal of another neuron to regulate neurotransmitter release.

Autoreceptors

Receptors located on the presynaptic terminal that bind the cell's own released neurotransmitter to provide feedback modulation of transmitter release.

Peptide Neurotransmitters

Neuropeptide signaling molecules synthesized from protein precursors that exhibit a long duration of action (seconds to hours) and long latency.

Peptide Neurotransmitter Elimination

The removal mechanism for neuropeptides, which occurs exclusively through enzymatic degradation in the extracellular space without reuptake.

Botulinum Toxins

A group of $7$ toxin serotypes that selectively degrade SNARE proteins and inhibit excitatory neuromuscular synaptic transmission.

Electromyogram (EMG)

A diagnostic recording technique used to measure electrical activity and response amplitude in muscle tissue.

One Photon of Light

The minimal quantum of light energy sufficient to activate a single rod photoreceptor cell in the retina.

Equilibrium Potential for Potassium

The membrane potential level where there is no net movement of potassium ions across the membrane.