Physio: Overview of the Nervous System

Nervous System Overview

Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating body activities by transmitting signals throughout the body. It is essential for sensation, movement, cognition, and homeostasis.

• Neurons use action potentials to transmit electrical signals.

• The nervous system is divided into two main parts:

  • Central Nervous System (CNS): Composed of the brain and spinal cord.

  • Peripheral Nervous System (PNS): Consists of nerves outside the CNS.

Structure of Neurons

General Features of Neurons

Neurons are the principal cells of the nervous system, specialized for communication. They vary greatly in shape and size but share common structural features.

• Cell Body (Soma): Contains the nucleus and most organelles. It is the metabolic center of the neuron.

• Cluster of Cell Bodies: In the CNS, these clusters are called nuclei; in the PNS, they are called ganglia.

Dendrites

Dendrites are branched cytoplasmic extensions from the cell body. They serve as the main receptive (input) regions of the neuron, receiving electrochemical signals from other neurons.

• Increase the surface area for synaptic input.

• Transmit signals toward the cell body.

Axon

The axon is a long, singular cytoplasmic projection that conducts action potentials away from the cell body toward other neurons or effector cells.

• Originates from an expanded region of the cell body called the axon hillock.

• Can be very long, extending up to a meter in some cases (e.g., sciatic nerve).

Axon Collaterals and Synaptic Connections

Axons can branch into collateral axons, allowing a single neuron to communicate with many other neurons.

• A single neuron can form synapses with 30,000 to 60,000 other neurons.

• Impulses typically travel from the dendrites, through the cell body, and down the axon to the axon terminals.

Classification of Neurons

Functional Classification

Neurons can be classified based on their function within the nervous system:

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors toward the CNS.

• Motor (Efferent) Neurons: Carry impulses away from the CNS to effector organs (muscles and glands).

• Interneurons: Relay signals between sensory and motor neurons; located entirely within the CNS.

Structural Classification

Neurons are also classified by the number of processes extending from the cell body:

• Multipolar Neurons: Most abundant type; possess several dendrites and one axon.

• Bipolar Neurons: Have one dendrite and one axon; found in sensory organs such as the retina.

• Pseudounipolar Neurons: Have a single process that splits into two branches; primarily sensory neurons in the PNS.

Summary Table: CNS vs. PNS

Feature	Central Nervous System (CNS)	Peripheral Nervous System (PNS)
Main Components	Brain, Spinal Cord	Nerves, Ganglia
Cluster of Cell Bodies	Nuclei	Ganglia
Primary Function	Integration, Processing	Transmission of signals to/from CNS

Key Terms

• Action Potential: A rapid, transient electrical signal propagated along the axon of a neuron.

• Axon Hillock: The region where the axon originates from the cell body; site of action potential initiation.

• Collateral Axon: A branch of an axon that allows communication with multiple targets.

• Synapse: The junction between two neurons or between a neuron and an effector cell.

Example: Signal Transmission in a Neuron

• A sensory neuron receives a stimulus at its dendrites.

• The signal is integrated at the cell body and, if threshold is reached, an action potential is generated at the axon hillock.

• The action potential travels down the axon, possibly branching via collateral axons, to communicate with other neurons or effector cells.

Additional info: The provided images reinforce the structural features of neurons, including the cell body, dendrites, axon, axon hillock, and collateral axons. The distinction between CNS and PNS is fundamental for understanding nervous system organization.