Nervous System – Part 2: Neuron Structure, Electrical Principles, and Signal Transmission

Neuron Structure and Classification

Structural Classes of Neurons

Neurons are specialized cells of the nervous system responsible for transmitting electrical and chemical signals. They are classified based on the number and arrangement of their processes (dendrites and axons).

• Multipolar Neurons:

  • Many processes extend from the cell body; all are dendrites except for a single axon.

  • Most abundant in the body; major neuron type in the CNS.

  • Examples: Purkinje cell of cerebellum, Pyramidal cell.

• Bipolar Neurons:

  • Two processes extend from the cell body: one dendrite and one axon.

  • Rare; found in special sensory organs (e.g., olfactory mucosa, eye, ear).

  • Examples: Olfactory cell, Retinal cell.

• Unipolar (Pseudounipolar) Neurons:

  • One process extends from the cell body and forms central and peripheral processes, which together comprise an axon.

  • Mainly found in the PNS; common in dorsal root ganglia of spinal cord and sensory ganglia of cranial nerves.

  • Example: Dorsal root ganglion cell.

Functional Regions of Neurons

Neurons have distinct functional regions:

• Receptive region: Receives stimuli (dendrites and cell body).

• Conducting region: Generates and transmits action potentials (axon).

• Secretory region: Axon terminals release neurotransmitters.

Functional Classes: Direction of Impulse Conduction

• Multipolar Neurons:

  • Most are interneurons (association neurons) that conduct impulses within the CNS, integrating sensory input or motor output.

  • Some are motor neurons that conduct impulses along efferent pathways from the CNS to effectors (muscles/glands).

• Bipolar Neurons:

  • Essentially all are sensory neurons located in special sense organs.

  • Example: Bipolar neuron of retina relays visual input from the eye to the brain.

• Unipolar Neurons:

  • Most are sensory neurons that conduct impulses along afferent pathways to the CNS for interpretation.

Table: Comparison of Structural Classes of Neurons

Neuron Type	Processes	Location	Example
Multipolar	Many dendrites, one axon	CNS	Pyramidal cell, Purkinje cell
Bipolar	One dendrite, one axon	Special sensory organs	Retinal cell, Olfactory cell
Unipolar	One process (axon with peripheral and central branches)	PNS (sensory ganglia)	Dorsal root ganglion cell

Basic Principles of Electricity in Neurons

Definitions

• Voltage: A measure of potential energy generated by separated charge.

  • Measured between two points in volts (V) or millivolts (mV).

  • Also called potential difference or potential.

  • Greater charge difference between points = higher voltage.

• Current: Flow of electrical charge (ions) between two points.

  • Can be used to do work.

  • Flow is dependent on voltage and resistance.

• Resistance: Hindrance to charge flow.

  • Insulator: Substance with high electrical resistance.

  • Conductor: Substance with low electrical resistance.

Ohm's Law

Ohm's law describes the relationship between voltage, current, and resistance:

• Current is directly proportional to voltage.

• Current is inversely proportional to resistance.

Formula:

$I = \frac{V}{R}$

• Where $I$ is current, $V$ is voltage, and $R$ is resistance.

• No net current flows between points with the same potential.

Ion Channels and Membrane Potentials

Types of Ion Channels

• Leakage (nongated) channels: Always open; allow ions to diffuse across the membrane.

• Gated channels: Open and close in response to specific stimuli. Types include:

  • Chemically gated (ligand-gated): Open with binding of a specific chemical (e.g., neurotransmitter).

  • Voltage-gated: Open and close in response to changes in membrane potential.

  • Mechanically gated: Open and close in response to physical deformation of receptors (e.g., sensory receptors).

Electrochemical Gradients

• Ions diffuse along chemical concentration gradients (from high to low concentration).

• Ions also move along electrical gradients (toward opposite electrical charge).

• The combination of these gradients is called the electrochemical gradient.

• Ion flow creates electrical current and voltage changes across the membrane.

Resting Membrane Potential

Establishing the Resting Membrane Potential

• Measured as the potential (charge) difference across the membrane of a resting neuron.

• Typical resting membrane potential: approximately -70 mV (cytoplasmic side is negative relative to outside).

• The membrane is said to be polarized.

• Generated by:

  • Differences in ionic composition of intracellular fluid (ICF) and extracellular fluid (ECF).

  • Differences in plasma membrane permeability.

Role of Ion Concentrations and Permeability

• More potassium (K+) diffuses out than sodium (Na+) diffuses in, making the inside of the cell more negative.

• The sodium-potassium pump ($Na^+/K^+$ ATPase) maintains concentration gradients by pumping three Na+ out and two K+ in.

Changes in Membrane Potential

Types of Signals

• Graded potentials: Incoming signals operating over short distances.

• Action potentials: Long-distance signals of axons.

Membrane Potential Changes

• Depolarization: Decrease in membrane potential (moves toward zero and above); inside becomes less negative. Increases probability of producing an impulse.

• Hyperpolarization: Increase in membrane potential (away from zero); inside becomes more negative. Decreases probability of producing an impulse.

Graded Potentials

Characteristics

• Short-lived, localized changes in membrane potential.

• Stronger stimulus produces greater voltage changes and farther current flow.

• Triggered by stimulus that opens gated ion channels.

• Can result in depolarization or hyperpolarization.

• Types:

  • Receptor (generator) potential: Graded potentials in sensory receptors.

  • Postsynaptic potential: Graded potential in postsynaptic neuron.

Action Potentials (APs)

Characteristics

• Principal way neurons send signals; means of long-distance neural communication.

• Only occur in muscle cells and axons of neurons.

• Involve reversal of membrane potential (~100 mV change).

• Do not decay over distance.

• Require opening of specific voltage-gated channels.

Phases of Action Potential

1. Resting State: All gated Na+ and K+ channels are closed; only leakage channels are open. Maintains resting membrane potential.

2. Depolarization: Voltage-gated Na+ channels open; Na+ rushes into cell, causing further depolarization. At threshold (~ -55 to -50 mV), all Na+ channels open, resulting in a spike.

3. Repolarization: Na+ channels inactivate; K+ channels open, K+ exits cell, returning membrane to resting potential.

4. Hyperpolarization: Some K+ channels remain open, causing excessive K+ efflux; membrane becomes more negative than resting state.

Threshold and All-or-None Principle

• APs are generated only if depolarization reaches threshold (~15-20 mV above resting potential).

• All-or-none: An AP either happens completely or not at all.

Propagation of Action Potential

• AP is transmitted from origin down entire axon length toward terminals.

• In nonmyelinated axons, each segment depolarizes and repolarizes sequentially.

• In myelinated axons, APs occur only at gaps (nodes of Ranvier) via saltatory conduction (faster).

Stimulus Intensity and Refractory Periods

Stimulus Intensity

• APs are independent of stimulus strength; intensity is coded by frequency of impulses.

• Higher frequency = stronger stimulus.

Refractory Periods

• Absolute refractory period: Neuron cannot trigger another AP; ensures one-way transmission.

• Relative refractory period: Follows absolute period; threshold for AP generation is elevated, requiring a stronger stimulus.

Conduction Velocity

Factors Affecting Velocity

• Axon diameter: Larger diameter = faster conduction.

• Degree of myelination: Myelinated axons conduct faster (saltatory conduction) than nonmyelinated (continuous conduction).

Types of Fibers

Group	Diameter	Myelination	Speed	Function
Group A	Largest	Myelinated	150 m/s	Somatic sensory & motor fibers
Group B	Intermediate	Lightly myelinated	15 m/s	Autonomic fibers
Group C	Smallest	Unmyelinated	1 m/s	Autonomic fibers

Neuronal Pools and Neural Processing

Neuronal Pools

• Functional groups of neurons that integrate incoming information and forward processed information to other destinations.

• Discharge zone: Neurons closer to incoming fiber; more likely to generate impulse.

• Facilitated zone: Neurons farther away; usually not excited to threshold unless stimulated by another source.

Patterns of Neural Processing

• Serial processing: Input travels along one pathway to a specific destination; all-or-none response. Example: spinal reflex.

• Parallel processing: Input travels along several pathways; important for higher-level mental functioning. Example: smell triggering memories.

Types of Circuits

Circuit Type	Description	Example
Diverging	One input, many outputs	Single neuron activating multiple muscle fibers
Converging	Many inputs, one output	Sensory stimuli eliciting a memory
Reverberating	Chain of neurons feeding back to previous neurons	Breathing, sleep-wake cycle
Parallel after-discharge	Signal stimulates neurons in parallel arrays converging on one output	Mathematical calculations

Developmental Aspects of Neurons

Origin and Differentiation

• Nervous system originates from neural tube and neural crest (ectoderm).

• Neuroepithelial cells proliferate and migrate; neuroblasts sprout axons and become neurons.

• Growth cones at axon tips interact with environment via cell adhesion molecules (CAMs) and neurotropins (e.g., nerve growth factor).

Synapse Formation and Cell Death

• Axons must find correct targets to form synapses; astrocytes provide support and cholesterol for synapse construction.

• Many neurons die before birth if they fail to form synapses (apoptosis).

• During childhood/adolescence, learning reinforces certain synapses and prunes others.

• Genes promoting excessive synaptic pruning may predispose to schizophrenia.

• Most neurons are amitotic after birth, but some populations (olfactory neurons, hippocampus) continue to divide.

Example: Olfactory neurons regenerate throughout life, supporting the sense of smell.

Additional info: The notes above expand on the original content with academic context, definitions, and examples for clarity and completeness.