Neurons, Action Potential, Synaptic Transmission & Neurotransmitters

The nervous system is the biological substrate of all cognition, emotion, perception, and voluntary behavior. Quantitative analysis of the human brain reveals approximately 86 billion neurons interconnected across 100 trillion synaptic junctions, supported and nourished by an equal number of glial cells.

For CUET PG Psychology (HUQP20), Unit 4 questions in this domain focus heavily on:

Structural Anatomy of the Neuron & Glial Taxonomies (Astrocytes, Oligodendrocytes vs Schwann cells, Microglia).
Biophysics of Resting Membrane Potential ( $-70\text{ mV}$ , ion gradients, and the electrogenic $3\text{ Na}^+ \text{ out} / 2\text{ K}^+ \text{ in}$ ATPase pump).
Action Potential Electrophysiology ( $-55\text{ mV}$ threshold, $\text{Na}^+$ influx depolarization, $\text{K}^+$ efflux repolarization, refractory periods, and Saltatory Conduction).
Synaptic Transmission Mechanics ( $\text{Ca}^{2+}$ influx, vesicular exocytosis, EPSPs vs IPSPs, spatial vs temporal summation).
Major Neurotransmitter Systems (ACh, DA, 5-HT, NE, GABA, Glutamate, Endorphins) and their clinical psychopathologies.

1. Cellular Taxonomy: Neurons and Glial Cells

The nervous system consists of two primary cellular populations: Neurons (excitable, electrochemically communicating signaling units) and Glia (non-excitable supporting and regulatory cells).

Structure	Physiological Role & Exam High-Yield Point
Dendrites	Branching receiving antennae covered in spines; generate graded postsynaptic potentials (EPSPs/IPSPs).
Soma (Cell Body)	Metabolic center containing nucleus and Nissl bodies; integrates converging dendritic inputs.
Axon Hillock (Trigger Zone)	Junction between soma and axon; highest density of voltage-gated $\text{Na}^+$ channels; primary site where action potential is initiated.
Axon	Conducts all-or-none electrical impulse away from the soma.
Myelin Sheath	Lipid insulation; increases conduction speed up to 120 m/s; prevents ion leakage.
Nodes of Ranvier	Periodic gaps (1–2 μm) along myelinated axon packed with voltage-gated channels; site of saltatory conduction.
Terminal Buttons (Axon Terminals)	Swollen axonal endings containing synaptic vesicles; voltage-gated $\text{Ca}^{2+}$ triggers exocytosis of neurotransmitters.

Functional Classifications of Neurons

Sensory (Afferent) Neurons: Transmit sensory information from peripheral somatic and visceral receptors inward to the Central Nervous System (CNS). Morphologically unipolar or bipolar.
Motor (Efferent) Neurons: Transmit motor impulses outward from the CNS to effector muscles and glands. Morphologically multipolar.
Interneurons (Association Neurons): Confined entirely within the CNS (brain and spinal cord); integrate sensory input and motor output; constitute >99% of all neurons in the human body.

Glial Cells (Neuroglia): Central vs. Peripheral Classification

Glial Cell Type	Location	Primary Functions & Clinical Pathology
Astrocytes	CNS	Star-shaped; forms Blood-Brain Barrier (BBB); regulates extracellular $\text{K}^+$ and clears glutamate; forms glial scars after neural trauma (gliosis).
Oligodendrocytes	CNS	Myelinates multiple axons simultaneously (up to 50 axonal segments per cell). Primary target of autoimmune demyelination in Multiple Sclerosis (MS). Cannot guide axonal regeneration.
Schwann Cells	PNS	Myelinates only ONE axonal segment per cell. Guides axonal regeneration following peripheral nerve laceration via endoneurial neurolemma tube.
Microglia	CNS	Resident macrophage/phagocyte of the brain; clears cellular debris; mediates neuroimmune inflammation and synaptic pruning.

[!WARNING] NTA Exam Watch: A classic question tests myelin formation: Oligodendrocytes = CNS (myelinates multiple axons; cannot regenerate), whereas Schwann Cells = PNS (myelinates single segment; facilitates regeneration).

2. Resting Membrane Potential (RMP)

In the absence of stimulation, the neuronal membrane maintains a stable electrical polarization called the Resting Membrane Potential, quantitatively standard at $-70\text{ mV}$ (inside negative relative to the outside).

Rendering diagram...

Ion	Extracellular Fluid (ECF)	Intracellular Fluid (ICF)	Primary Equilibrium / Resting Role
$\text{Na}^+$ (Sodium)	High (~145 mM)	Low (~12 mM)	Kept out by $\text{Na}^+/\text{K}^+$ pump; rushes in during depolarizing spike.
$\text{K}^+$ (Potassium)	Low (~4 mM)	High (~140 mM)	High resting permeability via leak channels; primary determinant of $-70\text{ mV}$ RMP.
$\text{Cl}^-$ (Chloride)	High (~110 mM)	Low (~4–10 mM)	Enters cell upon $\text{GABA}_A$ receptor activation, generating hyperpolarizing IPSPs.
$\text{A}^-$ (Organic Anions)	Negligible	Very High (~140 mM)	Non-diffusible proteins trapped inside soma; establishes fixed intracellular negativity.

1. Cellular Taxonomy: Neurons and Glial Cells

Functional Classifications of Neurons

Glial Cells (Neuroglia): Central vs. Peripheral Classification

2. Resting Membrane Potential (RMP)

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