Nervous System Overview

Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating body activities and transmitting signals between different parts of the body. It is divided into central and peripheral components, each with specialized functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord; processes information and coordinates activity.

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

Subdivisions of the Nervous System

• Visceral Sensory Division: Transmits sensory information from internal organs.

• Somatic Sensory Division: Transmits sensory information from skin, muscles, and joints.

• Motor Divisions: Includes somatic (voluntary) and autonomic (involuntary) systems.

Cellular Components of the Nervous System

Neurons

Neurons are excitable cells responsible for information transfer throughout the nervous system.

• Dendrites: Receive incoming signals.

• Soma (cell body): Contains the nucleus and organelles.

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

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

Glial Cells (Neuroglia)

Glial cells are support cells that provide structural and metabolic support to neurons. They are specific to either the CNS or PNS.

• Astrocytes (CNS): Maintain the blood-brain barrier, provide nutrients, and regulate the extracellular environment.

• Oligodendrocytes (CNS): Form myelin sheaths around axons in the CNS.

• Microglial Cells (CNS): Act as immune cells within the CNS.

• Ependymal Cells (CNS): Line ventricles and central canal; involved in cerebrospinal fluid production.

• Schwann Cells (PNS): Form myelin sheaths around axons in the PNS.

• Satellite Cells (PNS): Surround neuron cell bodies in ganglia, providing support and nutrients.

White matter consists mainly of myelinated axons, while gray matter contains neuron cell bodies and unmyelinated fibers.

Structural and Functional Categories of Neurons

Neuron Classification

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

• Bipolar Neurons: One dendrite, one axon; found in sensory organs.

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

Neuronal Organization

Neurons are organized into circuits to process and transmit information efficiently. These circuits can be simple (reflex arcs) or complex (integrative networks).

• Neural Circuit: A group of interconnected neurons working together for a specific function.

• Synapse: The junction where information is transmitted from one neuron to another, either electrically or chemically.

Action Potentials and Synaptic Transmission

Action Potentials

An action potential is a rapid change in membrane potential that travels along the axon, allowing neurons to transmit signals.

• Resting Membrane Potential (RMP): Typically around ; the inside of the neuron is more negative than the outside.

• Threshold: The critical level to which the membrane potential must be depolarized to initiate an action potential.

• Depolarization: Na+ channels open, Na+ enters the cell, making the inside more positive.

• Repolarization: K+ channels open, K+ leaves the cell, restoring the negative membrane potential.

• All-or-None Principle: Once threshold is reached, the action potential occurs fully; if not, it does not occur at all.

Saltatory conduction occurs in myelinated axons, where action potentials jump between nodes of Ranvier, increasing conduction speed.

Synaptic Transmission

• Chemical Synapse: Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic cell.

• Electrical Synapse: Direct passage of ions through gap junctions; less common in the nervous system.

• Example: The neuromuscular junction uses acetylcholine as its neurotransmitter.

Embryology: Early Development of the Nervous System

Neurulation and Neural Tube Formation

The nervous system develops early in embryogenesis through a process called neurulation. This involves the formation of the neural tube, which gives rise to the CNS.

• Neural Plate: Thickened region of ectoderm along the dorsal midline of the embryo.

• Neural Groove and Folds: The neural plate folds inward to form a groove flanked by neural folds.

• Neural Tube: The neural folds fuse to form the neural tube, which becomes the brain and spinal cord.

• Neural Crest Cells: Migrate away from the neural tube to form peripheral nerves, ganglia, and other structures.

Layers of the Developing Neural Tube

• Ependymal (Germinal) Layer: Lines the neural cavity; source of neurons and glia.

• Mantle Layer: Forms gray matter; contains developing neurons.

• Marginal Layer: Contains growing axons and some glial cells; forms white matter.

Development of the Spinal Cord

Formation and Organization

The spinal cord develops from the caudal portion of the neural tube. As it matures, it differentiates into distinct regions and structures.

• Gray Matter: Formed from the mantle layer; contains neuron cell bodies and dendrites.

• White Matter: Formed from the marginal layer; contains myelinated axons organized into tracts.

• Dorsal Root Ganglia: Derived from neural crest cells; contain sensory neuron cell bodies.

• Ventral Roots: Contain motor neuron axons exiting the spinal cord.

• Spinal Nerves: Formed by the union of dorsal (sensory) and ventral (motor) roots; branch into dorsal and ventral rami.

Spinal Cord Structure

• Meninges: Protective membranes (dura mater, arachnoid mater, pia mater) surrounding the spinal cord.

• Enlargements: Cervical and lumbar regions are enlarged to accommodate limb innervation.

• Columns (Funiculi): White matter is organized into dorsal, lateral, and ventral columns, each containing specific tracts.

• Horns: Gray matter forms dorsal, ventral, and (in some regions) lateral horns.

Peripheral Nerves and Plexuses

Spinal Nerves and Branches

• Spinal Nerves: 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal); each is a mixed nerve carrying sensory and motor fibers.

• Dorsal Ramus: Innervates muscles and skin of the back.

• Ventral Ramus: Innervates limbs and anterior/lateral trunk.

Plexuses

Plexuses are networks of intersecting nerves that supply specific body regions.

• Cervical Plexus: Supplies neck, shoulder, and diaphragm.

• Brachial Plexus: Supplies upper limb.

• Lumbar Plexus: Supplies anterior and medial thigh.

• Sacral Plexus: Supplies posterior thigh, most of the lower leg, and foot.

Reflexes and Neural Pathways

Reflex Arcs

A reflex is an involuntary, rapid response to a stimulus. Reflex arcs are the basic functional units of the nervous system.

• Components: Receptor, sensory neuron, integration center (CNS), motor neuron, effector.

• Example: The patellar (knee-jerk) reflex.

Tracts and Columns

• Ascending Tracts: Carry sensory information to the brain.

• Descending Tracts: Carry motor commands from the brain to the body.

Additional info: The notes reference the importance of myelination for rapid signal conduction, the role of neural crest cells in forming diverse structures, and the organization of spinal nerves and plexuses for efficient innervation of the body.