Nervous System Basics

Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is essential for sensation, movement, and cognition.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and processing of information.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; connects the CNS to limbs and organs.

Divisions of the Nervous System

CNS vs. PNS

• CNS: Integrates sensory input and coordinates motor output.

• PNS: Transmits sensory and motor signals between the CNS and the rest of the body.

Example: Touching a hot object activates sensory neurons in the PNS, which send signals to the CNS for processing, resulting in a motor response to withdraw the hand.

Functional Organization

• Sensory Input: Detection of stimuli (e.g., touch, temperature).

• Integration: Processing and interpretation of sensory input in the CNS.

• Motor Output: Activation of effector organs (muscles, glands) to produce a response.

Cells of the Nervous System

Neurons and Supporting Cells

• Neurons: Specialized cells that transmit electrical impulses; high metabolic rate and require abundant oxygen and glucose.

• Supporting Cells (Neuroglia): Non-neuronal cells that support, protect, and insulate neurons (e.g., Schwann cells, oligodendrocytes).

Structure of a Typical Neuron

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Conducts electrical impulses away from the cell body.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

Myelin Sheaths

Function and Structure

• Function: Myelin sheaths insulate axons, allowing action potentials to travel more quickly.

• PNS Myelination: Schwann cells wrap around axons to form myelin sheaths.

• CNS Myelination: Oligodendrocytes extend processes to myelinate multiple axons.

Example: Demyelinating diseases such as multiple sclerosis disrupt myelin, impairing nerve signal transmission.

Myelinated vs. Unmyelinated Axons

Neurons vs. Nerves

Definitions and Differences

• Neuron: A single nerve cell that transmits electrical impulses.

• Nerve: A bundle of axons (nerve fibers) in the PNS, often containing both sensory and motor fibers.

• Tract: A bundle of axons in the CNS.

Example: The sciatic nerve is a large nerve in the PNS composed of many axons from different neurons.

Classification of Neurons

Structural Classification

• Multipolar Neurons: Many dendrites, one axon; most common type in CNS.

• Bipolar Neurons: One dendrite, one axon; found in special sensory organs (e.g., retina).

• Unipolar (Pseudounipolar) Neurons: Single process that splits into two branches; primarily sensory neurons in the PNS.

Functional Classification

• Afferent (Sensory) Neurons: Carry impulses toward the CNS.

• Efferent (Motor) Neurons: Carry impulses away from the CNS to effectors (muscles/glands).

• Interneurons: Connect sensory and motor neurons within the CNS.

Peripheral Nervous System: Sensory and Motor Divisions

Somatic vs. Visceral Divisions

• Somatic Division: Controls voluntary movements (skeletal muscles).

• Visceral Division (Autonomic): Regulates involuntary functions (smooth muscle, cardiac muscle, glands).

Example: The autonomic nervous system controls heart rate and digestion, while the somatic nervous system controls limb movement.

Types of Sensory and Motor Information

Structure of Nerves

Organization of a Nerve

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds bundles (fascicles) of axons.

• Epineurium: Surrounds the entire nerve.

Example: The connective tissue layers provide protection and support for nerve fibers.

Key Terms and Definitions

• Neuron: A nerve cell specialized for the transmission of electrical signals.

• Nerve: A bundle of axons in the PNS.

• Tract: A bundle of axons in the CNS.

• Myelin Sheath: Insulating layer around axons that increases the speed of nerve impulse conduction.

• Schwann Cell: Glial cell in the PNS that forms myelin.

• Oligodendrocyte: Glial cell in the CNS that forms myelin.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nucleus (in CNS): Cluster of neuron cell bodies in the CNS.

Summary Table: Neuron vs. Nerve

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard Anatomy & Physiology content.