Neurons and Electrical Signaling in the Nervous System

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Neurons: The Structural Units of the Nervous System

Neuron Structure and Function

Neurons are the fundamental units of the nervous system, specialized for the transmission of electrical and chemical signals. Each neuron consists of a cell body and cytoplasmic processes called axons and dendrites.

Cell Body (Soma): Contains the nucleus and most organelles; the biosynthetic and receptive center of the neuron. In the CNS, clusters of cell bodies are called nuclei; in the PNS, they are called ganglia.
Processes: Dendrites receive incoming signals; axons transmit signals away from the cell body. Bundles of axons in the CNS are called tracts, and in the PNS, nerves.
Myelin Sheath: In the CNS, formed by oligodendrocytes wrapping their plasma membranes around axons. Myelin insulates axons and increases the speed of impulse transmission.

Key Point: Neurons are classified by the number of processes (multipolar, bipolar, unipolar) and by function (sensory, motor, interneurons).

Definitions and Comparisons

Nucleus (in brain): Cluster of neuronal cell bodies in the CNS.
Nucleus (in neuron): Organelle containing genetic material.
Nerve Fiber: Refers to a long axon. In contrast, fibers in connective tissue are extracellular proteins, and muscle fibers are muscle cells.

Neuron Types and Classifications

Structural: Multipolar (many processes), bipolar (two processes), unipolar (one process).
Functional: Sensory (afferent), motor (efferent), interneurons (association neurons).
Example: Bipolar sensory neurons are involved in olfaction (smell); multipolar neurons transfer impulses to the brain for integration.

Electrical Properties of Neurons

Basic Principles of Electricity

Neurons use electrical signals to communicate. Understanding the relationship between current, voltage, and resistance is essential.

Voltage (V): Potential energy generated by separated electrical charges; measured in volts (V) or millivolts (mV).
Current (I): Flow of electrical charge; can do work.
Resistance (R): Hindrance to charge flow; high resistance = insulator, low resistance = conductor.

Ohm's Law:

Current is directly proportional to voltage and inversely proportional to resistance.

Membrane Potentials

Although the body is electrically neutral overall, local differences in ion concentration across the plasma membrane create a membrane potential.

Resting Membrane Potential: In neurons, typically around -70 mV (inside negative relative to outside).
Generated by differences in ionic composition (mainly Na+ and K+) and membrane permeability.
Maintained by the sodium-potassium pump and selective ion channels.

Membrane Ion Channels

Leak Channels: Always open; allow ions to move down their gradients.
Gated Channels: Open or close in response to stimuli (chemical, voltage, or mechanical).

Electrochemical Gradient: The combined effect of concentration and electrical gradients determines ion movement.

Graded Potentials

Definition and Properties

Graded potentials are short-lived, localized changes in membrane potential, usually occurring in dendrites or the cell body.

Can be depolarizations (inside becomes less negative) or hyperpolarizations (inside becomes more negative).
Magnitude varies with stimulus strength; decays with distance.
Types include receptor (generator) potentials (sensory receptors) and postsynaptic potentials (synapses).
Essential for initiating action potentials.

Example: The end plate potential (EPP) at the neuromuscular junction is a graded potential that triggers an action potential in muscle fibers.

Action Potentials

Definition and Phases

Action potentials (APs) are brief, long-distance electrical signals generated by neurons and muscle cells.

Consist of depolarization (Na+ influx), repolarization (K+ efflux), and brief hyperpolarization.
All-or-none phenomenon: APs occur fully if threshold is reached; otherwise, not at all.
Do not decay with distance; regenerated at each patch of membrane.

Phases of an Action Potential:

Depolarization: Increased Na+ permeability; membrane potential becomes less negative.
Repolarization: Decreased Na+ permeability, increased K+ permeability; membrane returns to resting potential.
Hyperpolarization: K+ channels remain open longer, causing membrane potential to become more negative than resting.

Refractory Periods

Absolute Refractory Period: No new AP can be generated, regardless of stimulus strength (Na+ channels inactivated).
Relative Refractory Period: AP possible only with a very strong stimulus (some Na+ channels reset, K+ channels still open).

Propagation and Coding of Stimulus Intensity

APs propagate by local currents depolarizing adjacent membrane regions.
Stimulus intensity is coded by AP frequency (not amplitude).

Conduction Velocity and Myelination

Axon Diameter: Larger diameter = faster conduction.
Myelination: Myelinated axons conduct APs faster via saltatory conduction (APs jump between nodes of Ranvier) compared to continuous conduction in unmyelinated axons.

Fiber Type	Diameter	Myelination	Conduction Velocity	Function
Group A	Largest	Thick myelin	Up to 150 m/s	Somatic sensory/motor (skin, skeletal muscle)
Group B	Intermediate	Light myelin	~15 m/s	Autonomic fibers
Group C	Smallest	Unmyelinated	1 m/s or less	Pain, touch, visceral sensory

Summary Table: Graded vs. Action Potentials

Feature	Graded Potential	Action Potential
Location	Dendrites, cell body	Axon
Distance	Short	Long
Amplitude	Variable	All-or-none (~100 mV)
Decay with distance?	Yes	No
Initiation	Stimulus or neurotransmitter	Threshold depolarization

Check Your Understanding: Key Questions

Which is bigger, a graded potential or an action potential? Which travels farther? Which initiates the other? Action potentials are larger and travel farther; graded potentials generally initiate action potentials.
Why does a myelinated axon conduct action potentials faster than a nonmyelinated axon? Myelin insulates the axon, allowing rapid voltage changes only at nodes of Ranvier, enabling saltatory conduction.
If an axon receives two stimuli close together in time, why does only one AP occur? During the absolute refractory period, Na+ channels are inactivated, preventing a second AP.
What is measured on the x and y axes of an action potential graph? X-axis: Time (ms); Y-axis: Membrane potential (mV).
Name the phases of an action potential and the permeability changes responsible: (a) Depolarization: Increased Na+ permeability; (b) Repolarization: Decreased Na+, increased K+ permeability; (c) Hyperpolarization: Delayed closure of K+ channels.

Additional info:

Some context and definitions were expanded for clarity and completeness.
Tables were inferred and constructed based on standard textbook content for this topic.