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

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Nervous System – Part 2

Comparison of Structural Classes of Neurons

Neurons are classified based on the number and arrangement of their processes, as well as their functional roles in the nervous system.

Structural Classes of Neurons

Multipolar Neurons: Many processes extend from the cell body; all are dendrites except for a single axon.
Bipolar Neurons: Two processes extend from the cell body; one is a fused dendrite, the other is an axon.
Unipolar (Pseudounipolar) Neurons: One process extends from the cell body and forms central and peripheral processes, which together comprise an axon.

Functional Regions of Neurons

Receptive region: Receives stimulus (dendrites and cell body).
Conducting region: Generates and transmits action potentials (axon).
Secretory region: Axon terminals release neurotransmitters.

Note: Many bipolar neurons do not generate action potentials; in those that do, the location of the trigger zone is not universal.

Relative Abundance and Location in Human Body

Multipolar: Most abundant; major neuron type in the CNS.
Bipolar: Rare; found in special sensory organs (olfactory mucosa, eye, ear).
Unipolar: Mainly in the PNS; common in dorsal root ganglia of spinal cord and sensory ganglia of cranial nerves.

Structural Variations

Multipolar: Purkinje cell (cerebellum), pyramidal cell.
Bipolar: Olfactory cell, retinal cell.
Unipolar: Dorsal root ganglion cell.

Functional Classes: Direction of Impulse Conduction

Multipolar Neurons: Mostly interneurons (integrate sensory input or motor output within CNS); some are motor neurons (conduct impulses from CNS to effectors).
Bipolar Neurons: Essentially sensory neurons located in special sense organs (e.g., retina for vision).
Unipolar Neurons: Mostly sensory neurons conducting impulses along afferent pathways to CNS for interpretation.

Basic Principles of Electricity in Neurons

Definitions

Voltage: Measure of potential energy generated by separated charge; measured in volts (V) or millivolts (mV). Also called potential difference or potential.
Current: Flow of electrical charge (ions) between two points; can be used to do work; flow depends on voltage and resistance.
Resistance: Hindrance to charge flow. Insulator: high resistance; Conductor: low resistance.

Ohm's Law

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

Current is directly proportional to voltage: greater voltage means greater current.
No net current flows between points with the same potential.
Current is inversely proportional to resistance: greater resistance means smaller current.

Formula:

Membrane Ion Channels

Large proteins serve as selective membrane ion channels, allowing specific ions to pass through the plasma membrane.

Leakage (nongated) channels: Always open.
Gated channels: Open and close in response to specific stimuli.

Types of gated channels:

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 quickly when channels are open, along chemical concentration gradients (from high to low concentration) and electrical gradients (toward opposite charge).
Electrochemical gradient: Combination of electrical and chemical gradients.
Ion flow creates electrical current and voltage changes across the membrane.
Expressed by rearranged Ohm's law equation.

Resting Membrane Potential

Generation of Resting Membrane Potential

Measured using a voltmeter; typical value for a resting neuron is approximately -70 mV (cytoplasmic side is negative relative to outside).
Actual voltage difference varies from -40 mV to -90 mV.
The membrane is said to be polarized.
Generated by differences in ionic composition of intracellular fluid (ICF) and extracellular fluid (ECF), and differences in plasma membrane permeability.

Establishing Resting Membrane Potential

Depends on differences in potassium (K+) and sodium (Na+) concentrations inside and outside the cell, and differences in membrane permeability to these ions.
More potassium diffuses out than sodium diffuses in, making the inside of the cell more negative.
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.

Changes in membrane potential are used as signals to receive, integrate, and send information.

Membrane Potential Changes Relative to Resting Potential

Depolarization: Decrease in membrane potential (moves toward zero and above); inside becomes less negative; increases probability of impulse.
Hyperpolarization: Increase in membrane potential (away from zero); inside becomes more negative; decreases probability of impulse.

Graded Potentials

Short-lived, localized changes in membrane potential.
Stronger stimulus produces greater voltage changes and farther current flows.
Triggered by stimulus that opens gated ion channels; can result in depolarization or hyperpolarization.
Receptor potential (generator potential): Graded potentials in sensory receptors.
Postsynaptic potential: Graded potential in a neuron after synaptic input.

Action Potentials (APs)

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.

Main Steps of Action Potential

Resting state: All gated Na+ and K+ channels are closed; only leakage channels are open.
Depolarization: Na+ channels open; Na+ rushes in, causing further depolarization.
Repolarization: Na+ channels inactivate; K+ channels open, K+ exits cell, restoring negative membrane potential.
Hyperpolarization: Some K+ channels remain open, causing excessive K+ efflux; membrane becomes more negative than resting state.

Threshold and All-or-None Phenomenon

Depolarization must reach threshold (~15–20 mV above resting) to trigger AP.
AP either happens completely or not at all.

Propagation of Action Potential

AP is transmitted from origin down entire axon length toward terminals.
Local currents cause opening of Na+ voltage gates in adjacent areas, leading to depolarization.
AP is self-propagating and occurs only in a forward direction.

Coding for Stimulus Intensity

All APs are alike; intensity is coded by frequency of impulses (number of APs per time).
Higher frequency means 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; only strong stimulus can trigger AP.

Conduction Velocity

Factors Affecting Conduction Velocity

Axon diameter: Larger diameter fibers conduct impulses faster due to less resistance.
Degree of myelination: Myelinated axons conduct impulses faster.

Types of Conduction

Continuous conduction: Slow; occurs in nonmyelinated axons.
Saltatory conduction: Fast; occurs in myelinated axons; APs jump from gap to gap.

Classification of Nerve Fibers

Group	Diameter	Myelination	Speed	Function/Location
Group A	Largest	Myelinated	150 m/s (~300 mph)	Somatic sensory and motor fibers (skin, skeletal muscles, joints)
Group B	Intermediate	Lightly myelinated	15 m/s (~30 mph)	Autonomic fibers
Group C	Smallest	Unmyelinated	1 m/s (~2 mph)	Pain and temperature fibers

Organization of Neurons: Neuronal Pools and Circuits

Neuronal Pools

Functional groups of neurons that integrate incoming information and forward processed information to other destinations.
Discharge zone: Neurons closer to incoming fiber are more likely to generate impulse.
Facilitated zone: Neurons farther from incoming fiber; 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; different parts of circuitry deal simultaneously with information; important for higher-level mental functioning.

Types of Circuits

Circuit Type	Description	Example
Diverging	One input, many outputs; amplifying circuit	Single neuron activating multiple motor units
Converging	Many inputs, one output; concentrating circuit	Different sensory stimuli eliciting same memory
Reverberating	Signal travels through chain of neurons, each feeding back to previous neurons; oscillating circuit	Breathing, sleep-wake cycle, repetitive motor activities
Parallel after-discharge	Signal stimulates neurons arranged in parallel arrays that converge on a single output cell; impulses reach output cell at different times	Mathematical calculations, exacting mental processes

Developmental Aspects of Neurons

Origin and Differentiation

Nervous system originates from neural tube and neural crest (ectoderm).
Neuroepithelial cells proliferate to form necessary number of cells.
Neuroblasts become amitotic and migrate; sprout axons to connect with targets and become neurons.

Axon Guidance and Synapse Formation

Growth cone at axon tip interacts with environment via cell surface adhesion molecules (e.g., N-CAMs).
Neurotropins attract or repel growth cone; nerve growth factor (NGF) supports axon growth.
Astrocytes provide physical support and cholesterol for synapse construction.

Neuronal Death and Pruning

Two-thirds of neurons die before birth if axons do not form synapses with targets (apoptosis).
During childhood and adolescence, learning reinforces certain synapses and prunes others.
Genes promoting excessive synaptic pruning may predispose to schizophrenia.

Neurogenesis After Birth

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

Additional info: Some content was inferred and expanded for clarity and completeness, especially regarding the functional significance of neuron types, circuit examples, and developmental processes.