1.1 Cellular Neuroscience: Neurons, Action Potentials, and Synaptic Transmission

Cellular Neuroscience: Neurons, Action Potentials, and Synaptic Transmission

At the foundational level of biological psychology lies the neuron doctrine, formulated by Santiago Ramón y Cajal using Camillo Golgi's silver nitrate staining technique. Cajal established that the nervous system is not a continuous, syncytial reticulum (as Golgi argued), but rather a network of discrete, individual cells that communicate across minute specialized junctions termed synapses by Charles Sherrington.

1. Neuronal Morphology and Structural Classification

Neurons are specialized for the reception, integration, conduction, and transmission of electrochemical signals. The prototypical multipolar neuron consists of four primary structural compartments:

1. Soma (Cell Body / Perikaryon): Contains the cell nucleus, mitochondria, Golgi apparatus, and extensive rough endoplasmic reticulum known as Nissl bodies, reflecting high rates of protein and neurotransmitter peptide synthesis. The soma maintains metabolic viability and integrates input signals.
2. Dendrites and Dendritic Spines: Highly branched arborizations extending from the soma that serve as the primary receptive field. Many excitatory synapses terminate on dendritic spines—small, bulbous actin-rich protrusions whose morphological plasticity (dynamic enlargement, retraction, and density changes) serves as the primary cellular substrate for learning and synaptic plasticity.
3. Axon and Axon Hillock: A single slender cylindrical process extending from the soma. The conical region connecting the soma to the initial segment of the axon is the axon hillock. Because the initial segment contains the highest biological density of voltage-gated sodium channels, it exhibits the lowest threshold for membrane excitation and serves as the spike trigger zone where graded electrical inputs are converted into an all-or-none action potential.
4. Axon Terminals (Terminal Boutons / Synaptic Knobs): Specialized distal swellings containing synaptic vesicles packed with neurotransmitters, mitochondria to support high metabolic demand, and active zones where vesicular exocytosis occurs into the 20–40 nm synaptic cleft.

                    [ Dendrites ]
                          │
                          ▼
      [ Soma / Perikaryon (Nissl Bodies) ]
                          │
                          ▼
      [ Axon Hillock / Trigger Zone ] ──> Threshold Check (~-55 mV)
                          │
                          ▼
             [ Myelinated Axon ]
           (Nodes of Ranvier Gaps)
                          │
                          ▼
              [ Axon Terminals ] ──> Ca2+ Influx & Exocytosis

Structural and Functional Typologies

• Multipolar Neurons: Possess one axon and multiple dendritic trees. Multipolar morphology characterizes the vast majority of central nervous system (CNS) neurons, including cortical pyramidal cells and spinal motor neurons.
• Bipolar Neurons: Possess exactly two distinct processes extending from opposite poles of the cell body—one dendritic branch and one axon. These occur in specialized sensory systems, notably retinal bipolar cells and olfactory sensory neurons.
• Unipolar / Pseudounipolar Neurons: Possess a single neurite that bifurcates into a peripheral sensory branch and a central axonal branch projecting into the spinal cord. Somatosensory neurons located in the dorsal root ganglia represent classic pseudounipolar cells.
• Interneurons: Local circuit neurons with short axons that do not leave their regional brain nucleus or spinal segment, serving critical modulatory, inhibitory, and feedback functions.

2. Neuroglial Cells: Functions and Pathology

Glial cells outnumber or roughly equal neurons in the human brain, providing metabolic, structural, and immunologic support:

3. The Biophysical Basis of the Resting Membrane Potential

In an unstimulated neuron, the electrical potential difference across the plasma membrane is termed the resting membrane potential ($V_m$), typically measured at approximately -70 mV (the intracellular face is electrically negative relative to the extracellular fluid).

Ionic Distribution and Concentration Gradients

The resting potential is an electrochemical equilibrium governed by four major ions distributed unequally across the phospholipid bilayer:

• Potassium ($K^+$): Highly concentrated inside the cell (~140 mM intracellular vs. ~4–5 mM extracellular).
• Sodium ($Na^+$): Highly concentrated outside the cell (~145 mM extracellular vs. ~10–15 mM intracellular).
• Chloride ($Cl^-$): Highly concentrated outside the cell (~110 mM extracellular vs. ~10 mM intracellular).
• Organic Anions ($A^-$): Negatively charged proteins, sulfates, and phosphate molecules confined exclusively to the intracellular cytoplasm (~100 mM).

Selective Permeability and the Nernst / Goldman Equations

At rest, the neuronal membrane is not equally permeable to all ions. The membrane contains constitutively open potassium leak channels (two-pore domain $K^+$ channels, $K_{2P}$), rendering the resting membrane roughly 20 to 30 times more permeable to $K^+$ than to $Na^+$:

$P_K \gg P_{Cl} > P_{Na}$

Driven by its steep chemical concentration gradient, $K^+$ diffuses out of the cell via leak channels. However, the large impermeable organic anions ($A^-$) cannot follow, leaving a net excess of negative charge along the inner leaflet of the membrane. This accumulating intracellular negativity exerts an inward electrostatic force that opposes further $K^+$ efflux.

The membrane potential at which the inward electrical gradient exactly balances the outward chemical gradient for a single permeable ion is that ion's equilibrium potential ($E_{ion}$), calculated using the Nernst Equation:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right) \approx \frac{61.5}{z} \log_{10} \left( \frac{[ion]_{out}}{[ion]_{in}} \right) \text{ at } 37^\circ\text{C}$

• $E_K \approx -90\text{ mV}$
• $E_{Na} \approx +60\text{ mV}$
• $E_{Cl} \approx -70\text{ mV}$

Because multiple ions are permeable simultaneously, the true resting membrane potential is calculated using the Goldman-Hodgkin-Katz (GHK) Voltage Equation, which weights each ion's equilibrium potential by its relative membrane permeability. Because resting permeability to $K^+$ predominates, $V_m$ (-70 mV) settles close to, but slightly less negative than, the equilibrium potential of potassium ($E_K = -90\text{ mV}$) due to a small inward baseline leak of $Na^+$.

The Sodium-Potassium ($Na^+/K^+$) ATPase Pump

Without an active counter-mechanism, the continuous inward leak of $Na^+$ down its electrochemical gradient and the outward leak of $K^+$ would eventually dissipate these vital concentration gradients, causing $V_m$ to collapse to 0 mV. The $Na^+/K^+$ ATPase pump prevents this dissipation:

• Hydrolyzes one molecule of intracellular ATP to pump 3 $Na^+$ ions outward against their electrochemical gradient in exchange for 2 $K^+$ ions inward.
• Because it moves a net positive charge outward (3 positive charges out for every 2 positive charges in), it is an electrogenic pump, contributing roughly -3 to -5 mV directly to the resting potential while maintaining the ionic batteries required for neural signaling.

4. Action Potential Dynamics and Biophysics

An action potential is a rapid, regenerative, all-or-none reversal of the membrane potential from -70 mV to approximately +35 or +40 mV, followed by rapid restoration of the resting negative state.

 Membrane Potential (mV)
   +40 ───┐             Peak / Na+ Inactivation (+35 mV)
          │  /\ 
          │ /  \  Repolarization (K+ Efflux)
     0 ───┼/────\──────────────────────────
          /      \
   -55 ──/────────\──────── Threshold of Excitation
        /          \   /
   -70 ─┘           \_/     Resting Potential (-70 mV)
                     ▲
                     └─ Undershoot / Hyperpolarization (-85 to -90 mV)

Sequential Phases of the Action Potential

1. Threshold Depolarization: Synaptic inputs produce graded, local depolarizations that spread electrotonically to the axon hillock. If the membrane potential depolarizes to the threshold of excitation (~-55 mV), an action potential is triggered.
2. Depolarization Phase (Upstroke): Depolarization to -55 mV causes the activation gates ($m$-gates) of voltage-gated $Na^+$ channels ($Na_V$) to swing open rapidly. Driven by both a concentration gradient and an electrical gradient, $Na^+$ rushes inward into the cytoplasm. This influx further depolarizes the membrane, opening additional $Na_V$ channels in an explosive positive-feedback cascade known as the Hodgkin cycle.
3. Overshoot and Peak: The membrane potential sweeps past 0 mV toward the sodium equilibrium potential, peaking at approximately +35 to +40 mV. It does not reach $E_{Na}$ (+60 mV) because two events occur simultaneously: (a) the inactivation gates ($h$-gates) of $Na_V$ channels snap shut via a cytosolic ball-and-chain mechanism, terminating $Na^+$ conductance, and (b) delayed rectifier voltage-gated $K^+$ channels ($K_V$) slowly open in response to sustained depolarization.
4. Repolarization Phase (Downstroke): With $Na^+$ channels inactivated and $K_V$ channels fully open, $K^+$ rapidly exits the cell down its steep outward electrochemical gradient (driven outward by positive intracellular charge and chemical concentration). This massive efflux of positive charge repolarizes the membrane back toward negative values.
5. After-Hyperpolarization (Undershoot): Because voltage-gated $K^+$ channels close slowly with a noticeable temporal delay, potassium permeability remains elevated even after reaching -70 mV. The membrane potential transiently dips toward the potassium equilibrium potential, reaching -85 to -90 mV.
6. Restoration of Resting State: As delayed rectifier $K^+$ channels finish closing, background $K^+$ leak channels and the ongoing $Na^+/K^+$ pump re-establish the resting potential at -70 mV.

5. Refractory Periods and Axonal Conduction

During and immediately following an action potential, the axonal membrane exhibits temporary states of diminished excitability:

• Absolute Refractory Period:
  • Mechanism: Occurs from the initial threshold depolarization throughout the entire upstroke and early repolarization. During this window, voltage-gated $Na^+$ channels are either already open or locked in the inactivated state ($h$-gates closed). The inactivation gate cannot reopen until the membrane has repolarized to near-resting levels.
  • Significance: No stimulus, regardless of intensity, can elicit a second action potential. This enforces two critical physiological rules: it sets a strict upper ceiling on the maximum firing frequency of a neuron (~1,000 Hz / impulses per second), and it mandates unidirectional propagation of the action potential along the axon away from the soma, preventing backward back-propagation.
• Relative Refractory Period:
  • Mechanism: Coincides with the hyperpolarizing undershoot. By this point, $Na_V$ channels have transitioned from inactivated to their closed-resting conformation and can be reopened. However, delayed rectifier $K^+$ channels remain open, producing elevated membrane conductance to $K^+$ and holding the potential further away from threshold (e.g., at -85 mV).
  • Significance: A second action potential can be triggered, but only by a suprathreshold depolarizing stimulus stronger than what is ordinarily required from rest.

Saltatory Conduction vs. Continuous Conduction

In unmyelinated axons, action potentials propagate via continuous conduction, in which inward current from each segment depolarizes the immediately adjacent patch of membrane to threshold. This continuous opening and closing of channels along the entire axonal length is metabolically expensive and slow (conduction velocities ranging from 0.5 to 2.0 m/s).

In myelinated axons, lipid-rich myelin sheaths act as an electrical insulator:

• Biophysical Effect: Myelin dramatically increases membrane resistance ($R_m$) (preventing charge leakage across the lipid bilayer) and decreases membrane capacitance ($C_m$) (reducing the amount of charge stored along the membrane).
• Mechanism: Intracellular electrotonic current spreads passively and rapidly down the internode with minimal decay. Regenerative action potentials occur only at the unmyelinated gaps known as the Nodes of Ranvier, which are densely packed with voltage-gated $Na^+$ channels and $Na^+/K^+$ pumps.
• Conduction Velocity: This process of the electrical impulse "jumping" from node to node is termed saltatory conduction (from Latin saltare, to leap), achieving conduction speeds of up to 100 to 120 m/s while conserving massive amounts of ATP.

6. Synaptic Transmission and Postsynaptic Integration

When an action potential arrives at the presynaptic terminal, electrical energy is converted into a chemical message across the chemical synapse:

[ Action Potential Arrives ] ──> Depolarizes Presynaptic Terminal
                                           │
                                           ▼
                             [ Voltage-Gated Ca2+ Influx ]
                                           │
                                           ▼
                             [ SNARE-Mediated Exocytosis ]
                               (Synaptotagmin / Vesicle Fusion)
                                           │
                                           ▼
                             [ Neurotransmitter in Cleft ]
                                           │
             ┌─────────────────────────────┴─────────────────────────────┐
             ▼                                                           ▼
[ Ionotropic Receptors ]                                    [ Metabotropic Receptors ]
(Ligand-Gated Ion Channel)                                   (GPCR / Second Messengers)
Fast EPSP (Na+ influx) / IPSP (Cl- influx)                  Slow, modulatory, prolonged cascades

The Presynaptic Cascade

1. Depolarization of Terminal: The arriving action potential depolarizes the presynaptic bouton membrane.
2. $Ca^{2+}$ Influx: Depolarization opens voltage-gated calcium channels ($Ca_V$). Because extracellular calcium is roughly 10,000 times higher than free intracellular calcium (~2 mM extracellular vs. 100 nM intracellular), $Ca^{2+}$ enters the terminal.
3. Vesicular Exocytosis: Intracellular $Ca^{2+}$ binds to the sensor protein synaptotagmin, triggering a conformational change in the SNARE core complex (comprising vesicle-associated synaptobrevin/VAMP and plasma membrane-associated syntaxin and SNAP-25). The synaptic vesicle membrane fuses with the presynaptic active zone, dumping neurotransmitter molecules into the synaptic cleft.

Postsynaptic Receptors: Ionotropic vs. Metabotropic

Once in the cleft, neurotransmitter molecules diffuse across the 20–40 nm space and bind to specific postsynaptic receptors:

• Ionotropic Receptors (Ligand-Gated Ion Channels): The receptor protein contains an intrinsic ion pore. Neurotransmitter binding directly opens the channel pore, producing rapid, brief postsynaptic potentials (onset within milliseconds; duration tens of milliseconds). Classic examples include the nicotinic acetylcholine receptor (nAChR) and the AMPA glutamate receptor.
• Metabotropic Receptors (G-Protein Coupled Receptors, GPCRs): The receptor is coupled to a heterotrimeric guanine nucleotide-binding protein ($G_\alpha, G_\beta, G_\gamma$). Binding induces GDP-GTP exchange on the $G_\alpha$ subunit, which dissociates to modulate ion channels directly or stimulate second-messenger enzymatic cascades (e.g., adenylyl cyclase $\rightarrow$ cyclic AMP $\rightarrow$ protein kinase A; or phospholipase C $\rightarrow$ $IP_3$ / DAG). Metabotropic signaling has a slower onset (hundreds of milliseconds to seconds) but produces long-lasting, amplified, and widespread structural effects, including transcriptional alterations.

Postsynaptic Potentials: EPSPs and IPSPs

• Excitatory Postsynaptic Potentials (EPSPs): Neurotransmitter binding causes inward flow of cations (predominantly $Na^+$ or $Ca^{2+}$), producing a local, graded depolarization that moves the membrane potential closer to threshold.
• Inhibitory Postsynaptic Potentials (IPSPs): Neurotransmitter binding opens channels permeable to $Cl^-$ (causing an inward flux of negative ions) or $K^+$ (causing an outward flux of positive ions), producing a graded hyperpolarization of $V_m$. When the chloride equilibrium potential sits close to the resting potential, opening these channels barely changes the voltage but clamps the membrane near rest and "short-circuits" nearby EPSPs; this voltage-neutral form is termed shunting inhibition.

Neural Integration: Spatial and Temporal Summation

Individual EPSPs are small (~0.5 to 2 mV) and insufficient on their own to reach threshold. The postsynaptic neuron performs neural integration at the axon hillock:

• Spatial Summation: Graded potentials originating nearly simultaneously from multiple distinct synaptic inputs across different dendritic locations propagate electrotonically and add together at the axon hillock.
• Temporal Summation: Successive graded potentials originating in rapid succession from a single presynaptic synapse arrive before previous potentials fully decay, compounding additively over time.

Spatial Summation:   Synapse A (EPSP) + Synapse B (EPSP) arriving together ──> Reaches Threshold
Temporal Summation:  Synapse A (EPSP) + Synapse A (EPSP) in rapid sequence ──> Reaches Threshold

Termination of Synaptic Signaling

To prevent continuous receptor desensitization and allow high-fidelity transmission of subsequent impulses, neurotransmitter signaling is rapidly terminated via three primary mechanisms:

1. Enzymatic Degradation: Specific enzymes in the cleft cleave the neurotransmitter into inactive metabolites (e.g., acetylcholinesterase [AChE] cleaving acetylcholine into acetate and choline).
2. Presynaptic Reuptake: High-affinity plasma membrane transporter proteins harness the electrochemical gradient of $Na^+$ to pump intact neurotransmitter back into the presynaptic terminal (e.g., the serotonin transporter SERT, dopamine transporter DAT, and norepinephrine transporter NET).
3. Glial Uptake: Surrounding astrocytes express excitatory amino acid transporters (EAATs) to clear neurotransmitters (especially glutamate) from the extrasynaptic space.