Neurophysiology: Axons, Potentials, and Synapses

Axon Anatomy, Myelination, and Saltatory Conduction

• Axonal Structure and Myelin Sheath:

  • Axons function analogously to electrical cables, requiring insulation to prevent signal dissipation.
  • Glial cells provide electrical insulation by wrapping around the axon up to $50$ times.
  • Nodes of Ranvier: Spaces or uninsulated gaps located between adjacent myelin wrap segments. The distance between two consecutive nodes is approximately $1\,\text{mm}$.

• Saltatory Conduction:

  • Derived from the Latin verb saltare, meaning "to leap".
  • Instead of regenerating continuously along every adjacent membrane segment, action potentials leap from one Node of Ranvier to the next.
  • Saltatory conduction tremendously accelerates the propagation velocity of action potentials.

• Ion Channel Distribution at Nodes of Ranvier:

  • The density of voltage-gated sodium ($\text{Na}^+$) channels is $200$ to $2,000$ times higher at the Nodes of Ranvier than in unmyelinated membrane segments.
  • Underneath the myelin sheath, the axonal membrane lacks voltage-gated ion channels completely.
  • The extremely high channel density at the nodes renders the nodal membrane highly excitable electrically. This ensures that even weak passive currents traveling beneath the myelin sheath reach the next node with sufficient strength to trigger an action potential.

Axonal Conduction Velocity and Demyelinating Disorders

• Determinants of Conduction Velocity:

  • Axon Diameter: Larger axon diameters exhibit lower internal electrical resistance, which increases conduction velocity. While a theoretical axon diameter of $3\,\text{feet}$ would yield maximum speed, physical body size imposes strict anatomical limits.
  • Myelination: Myelin coverage enables saltatory conduction, increasing propagation speed far beyond what unmyelinated fibers can achieve.

• Velocity Comparisons:

  • Small-diameter, unmyelinated axons propagate action potentials at approximately $0.5\,\text{m/s}$ (equivalent to a slow walking pace).
  • Large-diameter, heavily myelinated axons conduct action potentials at speeds up to $100\,\text{m/s}$ ($100\,\text{meters}$ in $1\,\text{second}$).
  • A velocity of $100\,\text{m/s}$ is approximately $10$ times faster than Usain Bolt's world record sprint speed ($100\,\text{meters}$ in $9.58\,\text{seconds}$, approximated around $9.50\,\text{seconds}$).

• Multiple Sclerosis (MS) and Demyelination:

  • Multiple Sclerosis is a neurodegenerative disorder characterized by progressive autoimmune demyelination of axons.
  • In normal myelinated axons, signal propagation to a given point (e.g., Node 3) takes a short time interval $t_1$.
  • With partial myelin loss, current leakage occurs through the newly exposed membrane. Because the membrane unmasked by myelin breakdown lacks voltage-gated ion channels, action potentials cannot regenerate in those segments, causing propagation delay ($t_2 > t_1$).
  • With complete loss of myelin, current leak is severe enough that action potentials fail to reach threshold at subsequent nodes, causing total conduction block. Clinical complications range from impaired motor control to severe respiratory failure, depending on which axonal tracts are demyelinated (e.g., patients like Christina experience extreme difficulty walking).

Physiological Mechanisms of Depolarization and Graded Potentials

• Initiation of Depolarization in Neurons:

  • The human nervous system contains approximately $100,000,000,000$ ($10^{11}$) neurons.
  • Without artificial electrical stimulation, initial depolarization is driven by local currents generated by ligand-gated ion channels or sensory receptors.

• Mechanism of Ligand-Gated Ion Channels:

  • Ligand-gated receptor channels are insensitive to transmembrane voltage changes but respond to chemical ligands.
  • Binding of a specific ligand opens the ion channel pore, allowing rapid influx of sodium ($\text{Na}^+$) into the intracellular compartment.
  • $\text{Na}^+$ influx drives the local membrane potential toward the sodium equilibrium potential, generating a localized depolarization known as a graded potential.

• Definition and Core Characteristics of Graded Potentials:

  • Graded potentials are local, non-regenerative electrical currents generated in a small localized region of the plasma membrane.
  • They spread passively to adjacent membrane segments and decay exponentially over distance (decremental conduction).
  • Their physiological role is to bring the axon hillock/initial segment membrane potential to threshold for initiating an all-or-none action potential.

Functional Classification of Graded Potentials

• 1. Synaptic Potentials:

  • Generated on postsynaptic membranes following neurotransmitter release (e.g., glutamate, acetylcholine).
  • Function to alter postsynaptic membrane potential toward or away from threshold.
  • Example: Excitatory postsynaptic potentials (EPSPs) at the neuromuscular junction produce local depolarizing currents.

• 2. Pacemaker Potentials:

  • Rhythmic local currents that automatically bring membrane potential to threshold without external neural input.
  • Medullary Respiratory Neurons: Neurons in the medulla oblongata generate spontaneous pacemaker potentials every few seconds throughout life, maintaining continuous involuntary respiration.
  • Channel Dynamics: Hyperpolarization activates specialized hyperpolarization-activated cation channels permeable to $\text{Na}^+$. Intracellular $\text{Na}^+$ entry slowly depolarizes the cell to threshold, firing an action potential.
  • Gastrointestinal (GI) Slow Waves: Pacemaker potentials in the GI tract produce continuous rhythmic depolarizations ("slow waves") that reach threshold to trigger smooth muscle contraction, propelling food along the digestive tract.

• 3. Receptor Potentials:

  • Generated by sensory receptors in response to environmental stimuli (touch, pressure, strain, temperature, tissue injury, chemicals, smell, taste, gravity, sound).
  • Sensory Nerve Terminals (e.g., Nociceptors): Pain receptors in the skin respond to extreme heat or cold. The sensory stimulus generates a depolarizing local receptor potential at the nerve terminal, which spreads passively to the initial segment to trigger an action potential that propagates along the axon to the Central Nervous System (CNS).
  • Specialized Non-Neuronal Epithelial Receptor Cells: Found in the auditory (cochlear), vestibular, and gustatory systems. These epithelial cells do not fire action potentials. Stimulus-induced receptor potentials modulate neurotransmitter release onto secondary sensory neurons, generating postsynaptic potentials.
  • Exception in Visual Systems: Virtually all sensory receptor cells produce excitatory/depolarizing receptor potentials upon stimulation, except photoreceptors (rods and cones) in the retina, which hyperpolarize when exposed to light photons.

Comparative Analysis: Graded Potentials vs. Action Potentials

• Amplitude Dependency:

  • Graded Potentials: Amplitude varies directly with stimulus intensity. Light touch produces a low-amplitude depolarization; firm pressure recruits more receptors and induces a larger amplitude depolarization.
  • Action Potentials: Amplitude is constant ($100\,\text{mV}$ to $110\,\text{mV}$) and independent of stimulus strength due to uniform voltage-gated channel dynamics and inactivation gate behavior.

• Spatial Propagation and Decay:

  • Graded Potentials: Decremental propagation. Maximum amplitude occurs at the site of stimulation ($0\,\text{mm}$). Amplitude drops significantly at $1\,\text{mm}$ distance and dissipates completely by $2\,\text{mm}$.
  • Action Potentials: Non-decremental propagation. Active continuous regeneration maintains a constant amplitude ($100\,\text{mV}$) from the axon initial segment to the terminal.

• Summation Capabilities:

  • Graded Potentials: Capable of spatial and temporal summation.
    • Temporal Summation: Rapid successive stimulation across a single synapse occurs before previous EPSPs decay, stacking amplitudes sequentially.
    • Spatial Summation: Simultaneous activation of multiple independent synapses across a cell body or dendritic tree sums their individual local currents.
  • Action Potentials: Absolute summation is impossible due to voltage-gated channel refractory states.

• Refractory Periods and Thresholds:

  • Graded Potentials: No refractory period. No strict minimum threshold (e.g., a single photon of light can excite a single retinal rod cell).
  • Action Potentials: Possess strict refractory periods (absolute and relative) and a defined activation threshold determined by voltage-gated sodium channel kinetics.

• Summary Comparison Matrix:

  • Amplitude Varies: Action Potentials = No; Graded Potentials = Yes.
  • Summation Occurs: Action Potentials = No; Graded Potentials = Yes.
  • Refractory Period: Action Potentials = Yes; Graded Potentials = No.
  • Voltage Threshold Required: Action Potentials = Yes; Graded Potentials = No (or extremely low).

Synaptic Transmission and Vesicular Release Dynamics

• Synaptic Ultrastructure:

  • Presynaptic Terminal: Contains high densities of mitochondria and membrane-bounded synaptic vesicles.
  • Active Zones: Regions of the presynaptic membrane displaying the absolute highest density of synaptic vesicles docked and ready for exocytosis.
  • Synaptic Cleft: Extracellular space separating presynaptic and postsynaptic membranes, housing degradation enzymes.

• Excitation-Secretion Coupling:

  1. An action potential arrives at the presynaptic terminal, causing localized depolarization.
  2. Depolarization activates presynaptic N-type voltage-gated calcium channels (distinct from L-type channels found in muscle fibers).
  3. Extracellular calcium ($\text{Ca}^{2+}$) rushes down its concentration gradient into the presynaptic terminal.
  4. Intracellular $\text{Ca}^{2+}$ binding triggers exocytotic fusion of neurotransmitter vesicles with the presynaptic membrane.
  5. Vesicular release of all neurotransmitters (including gaseous messengers like nitric oxide) strictly requires $\text{Ca}^{2+}$ influx.

• SNARE Protein Complex and Synaptotagmin:

  • Approximately $60$ distinct proteins participate in vesicle docking, priming, and fusion within eukaryotic cells.
  • Synaptotagmin: A specialized presynaptic SNARE protein that acts as the primary calcium sensor, triggering physical vesicle membrane fusion upon binding $\text{Ca}^{2+}$.
  • Vesicle exocytosis is an active transport process dependent on ATP hydrolysis energy.

Neurotransmitter Elimination Mechanisms

• Physiological Necessity:

  • Rapid removal of neurotransmitters from the synaptic cleft is vital to prevent postsynaptic receptor desensitization and toxic cell overstimulation.

• Primary Clearance Pathways:

  • Enzymatic Metabolism: Free neurotransmitters dissociate from postsynaptic receptors and are cleaved into inactive metabolites by specific enzymes in the cleft (e.g., acetylcholinesterase). Metabolites diffuse away or undergo cellular reuptake.
  • Transporter-Mediated Reuptake: Active $\text{Na}^+$-dependent plasma membrane transporters pump intact neurotransmitters back into the presynaptic terminal or surrounding glial cells. Inside the terminal, vesicular proton ($\text{H}^+$) pumps use ATP to reload neurotransmitters back into storage vesicles for recycling.

Structure and Physiology of the Neuromuscular Junction (NMJ)

• Motor Unit Architecture:

  • An $\alpha$-motor neuron originates in the spinal cord anterior horn or brainstem motor nuclei.
  • A motor unit consists of a single $\alpha$-motor neuron and every individual skeletal muscle cell (fiber) it innervates.
  • Rule of Skeletal Muscle Innervation: Every individual skeletal muscle fiber receives exactly one neuromuscular synapse.

• Postsynaptic End Plate Morphology:

  • Unlike typical CNS synapses, the postsynaptic membrane of a skeletal muscle fiber forms a wide, specialized structure called the end plate.
  • The end plate contains deep junctional folds that dramatically increase total membrane surface area.
  • Nicotinic acetylcholine receptors (nAChRs) are localized at ultra-high density within these junctional folds ($10,000$ times higher density inside the end plate than outside it).

Biochemical Synthesis, Release, and Degradation of Acetylcholine

• Biosynthesis:

  • Synthesized inside the presynaptic nerve terminal by the cytosolic enzyme Choline Acetyltransferase (ChAT).
  • Reaction: $\text{Choline} + \text{Acetyl-CoA} \xrightarrow{\text{ChAT}} \text{Acetylcholine (ACh)} + \text{CoA}$.

• Vesicular Storage and Release:

  • Concentrated into synaptic vesicles, with each single vesicle containing approximately $10,000$ molecules of ACh.
  • Arrival of a single motor neuron action potential triggers simultaneous exocytosis of over $200$ synaptic vesicles into the neuromuscular cleft.

• Enzymatic Degradation:

  • Acetylcholinesterase (AChE) is localized directly within the synaptic cleft extracellular matrix.
  • AChE rapidly hydrolyzes released ACh into inactive metabolites: $\text{ACh} \xrightarrow{\text{AChE}} \text{Choline} + \text{Acetate}$.
  • Choline is actively transported back into the presynaptic terminal via $\text{Na}^+$-coupled transporters to resynthesize ACh. Acetate diffuses away into surrounding interstitial fluid.

End Plate Potential (EPP) and Muscle Action Potential Generation

• Nicotinic Acetylcholine Receptor Kinetics:

  • Ligand-gated ion channels requiring the binding of two ACh molecules to open.
  • Upon opening, the channel is non-selectively permeable to both $\text{Na}^+$ and $\text{K}^+$.
  • At a resting potential of $-70\,\text{mV}$ to $-90\,\text{mV}$, the electrochemical driving force for $\text{Na}^+$ entry far exceeds the outward force for $\text{K}^+$, causing massive net $\text{Na}^+$ influx and rapid localized depolarization.
  • Synaptic transmission duration is brief, lasting only $1\text{ to }2\,\text{ms}$.

• Characteristics of the End Plate Potential (EPP):

  • Simultaneous release of $>200$ vesicles produces a massive depolarizing graded potential called the End Plate Potential (EPP), with an amplitude of approximately $70\,\text{mV}$.
  • By contrast, typical neuronal EPSPs in the central nervous system range from only $1\,\text{mV}$ to $10\,\text{mV}$.

• Conversion to Muscle Action Potential:

  • The specialized end plate membrane itself is electrically inexcitable—it contains zero voltage-gated $\text{Na}^+$ or $\text{K}^+$ channels.
  • The large $70\,\text{mV}$ EPP spreads passively from the end plate to surrounding perijunctional sarcolemma regions containing high densities of voltage-gated $\text{Na}^+$ channels.
  • Because the EPP amplitude is so large, passive spread invariably exceeds threshold (100% security factor).
  • This triggers a self-propagating action potential that spreads bidirectionally from the central end plate toward both ends of the muscle fiber, inducing muscle contraction.

• Unique NMJ Properties vs. Standard Graded Potentials:

  • No Spatial Summation: Only a single synapse exists per muscle cell.
  • No Temporal Summation Needed: A single EPP is naturally suprathreshold.
  • 1-to-1 Correspondence: Exactly 1 action potential in a motor neuron yields 1 EPP, which triggers 1 action potential in the target muscle fiber.

Central Nervous System Synaptic Architecture and Presynaptic Modulation

• Structural Synapse Types in the CNS:

  • Axosomatic: Axon terminal innervates a cell body (soma).
  • Axodendritic: Axon terminal innervates a dendritic spine or shaft.
  • Axoaxonic: Axon terminal innervates another presynaptic axon terminal.

• Presynaptic Modulation:

  • In an axoaxonic arrangement, Neuron B forms a synapse directly onto the presynaptic terminal of Neuron A, modulating Neuron A's neurotransmitter release onto Postsynaptic Cell C.
  • Autoreceptors on presynaptic terminals provide local feedback control over transmitter release.

• Neurotoxin Action (Botulinum Toxins):

  • Produced across $7$ to $8$ gene variants/serotypes ($A$ through $H$).
  • Botulinum toxins selectively cleave and degrade specific presynaptic SNARE proteins, blocking vesicle fusion and ACh release at neuromuscular junctions, causing flaccid paralysis.

Student Discussions and Class Dialogue

• Student Exchange on Fatigue, Class Attendance, and Personal Anecdotes:

  • Student 1: Expressed extreme fatigue from staying up until 3:00 AM, mentioning cravings for barbecue brisket.
  • Student 1: Shared a story about threatening to skip class so her dad would make brisket, noting her dad would react strongly if she skipped.
  • Student 1: Recounted previously attending Seminole State College where her father is the IT Director. After a relationship breakup, she skipped class to get tattoos with her brother. Her dad tracked her down via text regarding her absence right before finals week.
  • Student 1: Defended skipping by stating she had straight A's and found the professor unhelpful.

• Exam Logistics and Course Schedule Clarification:

  • Course Schedule: The class discussed an upcoming review session tomorrow, followed by the final portion of the lecture covering hemoglobin and quaternary protein structure.
  • Exam Dates: Clarified that the biotechnology exam is scheduled for the current week, while the physiology exam covering membrane potentials occurs on the 14th.

• Discussion on Action Potential Summation Diagrams:

  • Clarification: Students debated a lecture slide diagram showing apparent action potential summation.
  • Conclusion: Confirmed that action potentials cannot summate due to inactivation gates. The diagram illustrated multiple rapid incoming action potentials generating overlapping postsynaptic graded potentials at the same membrane site, resulting in temporal summation of the graded potentials, not the action potentials themselves.