Fundamentals of the Nervous System: Neurons, Electrical Activity, and Synaptic Transmission

Fundamentals of the Nervous System and Nervous Tissue

Overview

This section introduces the essential concepts of the nervous system, focusing on the structure and function of neurons, the basis of electrical activity in nervous tissue, and the mechanisms of synaptic transmission. Understanding these topics is foundational for studying human anatomy and physiology, especially in relation to the cardiovascular, lymphatic, and respiratory systems.

Neurons: Structure and Function

Regions of the Neuron

Neurons are the basic functional units of the nervous system, specialized for the reception, propagation, and transmission of nerve impulses.

• Dendrites: Receive incoming signals from other neurons or sensory receptors. They are the primary receptive region of the neuron.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals and supports metabolic activities.

• Axon Hillock: The trigger zone where action potentials are initiated.

• Axon: Conducts action potentials away from the cell body toward axon terminals; the propagative region.

• Axon Terminals: Release neurotransmitters to communicate with other neurons, muscles, or glands; the transmissive region.

Example: Sensory neurons receive stimuli via dendrites, integrate the signal in the soma, and transmit the response through the axon to the central nervous system.

Electrical Activity of Neurons

Ion Diffusion and Ion Channels

Neuronal electrical activity is governed by the movement of ions across the plasma membrane through specialized channels.

• Diffusion of Ions: Ions move down their concentration gradients through channels, generating electrical signals.

• Types of Ion Channels:

  • Leak Channels: Always open, contribute to resting membrane potential.

  • Ligand-Gated Channels: Open in response to chemical signals (e.g., neurotransmitters).

  • Voltage-Gated Channels: Open in response to changes in membrane potential, crucial for action potentials.

  • Mechanically-Gated Channels: Open in response to physical deformation (e.g., touch receptors).

Key Terms: Resting membrane potential (RMP), graded potential, action potential.

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the neuron's plasma membrane when the cell is not actively transmitting a signal.

• Typical Value: Approximately -70 mV in neurons.

• Established by:

  • Na+/K+ ATPase pump (maintains high Na+ outside, high K+ inside).

  • Selective permeability of the membrane to K+ and Na+.

Equation:

$E_{ion} = \frac{61}{z} \log \left(\frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}}\right)$

Where $E_{ion}$ is the equilibrium potential for a given ion, and $z$ is the ion's valence.

Graded Potentials

Graded potentials are localized changes in membrane potential that vary in magnitude depending on the strength of the stimulus.

• Occur in dendrites and cell bodies.

• Can be depolarizing (making the membrane less negative) or hyperpolarizing (making it more negative).

• Decay with distance from the site of origin.

Example: A neurotransmitter binding to a dendritic receptor may cause a small depolarization, leading to a graded potential.

Action Potentials

Action potentials are rapid, long-distance electrical signals generated by neurons and muscle cells.

• Initiated at the axon hillock when membrane potential reaches threshold (~-55 mV).

• All-or-none events: once threshold is reached, the action potential is always the same size.

• Do not decay with distance; propagate along the axon.

• Involve sequential opening and closing of voltage-gated Na+ and K+ channels.

Example: Sensory neurons transmit pain signals from the skin to the spinal cord via action potentials.

Propagation of Action Potentials

Factors Influencing Propagation

The speed and efficiency of action potential propagation depend on several factors:

• Axon Diameter: Larger diameter axons conduct impulses faster due to lower resistance.

• Myelination: Myelinated axons conduct impulses more rapidly via saltatory conduction, where the action potential jumps between nodes of Ranvier.

Example: The squid giant axon is large and unmyelinated, allowing rapid conduction; human motor neurons are myelinated for efficient signal transmission.

Synaptic Transmission

Mechanisms of Synaptic Transmission

Neurons communicate with each other and with effector cells at synapses, where electrical signals are converted to chemical signals.

• Presynaptic Neuron: Releases neurotransmitters into the synaptic cleft.

• Postsynaptic Neuron: Receives neurotransmitters, leading to postsynaptic potentials.

• Synaptic Integration: The summation of multiple postsynaptic potentials determines whether the postsynaptic neuron will fire an action potential.

Example: Excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) integrate at the axon hillock to determine neuronal output.

Summary Table: Key Concepts in Neuronal Function

Concept	Description	Example/Application
Resting Membrane Potential	Stable charge difference across the membrane	-70 mV in neurons
Graded Potential	Localized, variable change in membrane potential	EPSP in dendrites
Action Potential	Rapid, all-or-none electrical signal	Nerve impulse along axon
Synaptic Transmission	Chemical communication between neurons	Neurotransmitter release at synapse
Myelination	Insulation of axons for faster conduction	Saltatory conduction in motor neurons

Additional info:

• Neurons are highly specialized, non-mitotic cells with high metabolic rates, requiring continuous oxygen and glucose supply.

• Glial cells support, protect, and nourish neurons, and are essential for nervous system function.