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; serves as the main control center for processing information.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; connects the CNS to limbs and organs.

Subdivisions of the Nervous System

The nervous system is organized into functional and structural subdivisions to efficiently process sensory input and motor output.

• Central Nervous System (CNS):

  • Brain: Integrates and processes information; controls voluntary and autonomic activities.

  • Spinal Cord: Relays information to and from the brain; coordinates reflexes.

• Peripheral Nervous System (PNS):

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

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

  • Motor Division: Carries signals from the CNS to effectors (muscles and glands).

Neural Cells and Their Functions

Neurons

Neurons are highly specialized cells responsible for the transmission of electrical and chemical signals throughout the nervous system.

• Dendrites: Receive incoming signals from other neurons.

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

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

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

Action Potential: If the summed signals at the axon hillock reach a threshold, an electrical impulse (action potential) is generated and propagated along the axon.

• Resting Membrane Potential (RMP): The typical neuron has a RMP of about , with the inside of the cell more negative than the outside.

• Threshold: If the membrane potential reaches a certain value, the neuron fires an action potential.

• All-or-None Principle: Once initiated, the action potential proceeds to completion and cannot be stopped.

Glial Cells

Glial cells are non-neuronal cells that provide support, protection, and nutrition to neurons. They play crucial roles in maintaining homeostasis and forming myelin.

• Central Nervous System (CNS) Glial Cells:

  • Astrocytes: Maintain the blood-brain barrier (BBB), provide structural support.

  • Oligodendrocytes: Form myelin sheaths in the CNS; myelinated regions are called white matter, unmyelinated regions are gray matter.

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

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

• Peripheral Nervous System (PNS) Glial Cells:

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

  • Satellite Cells: Surround neuron cell bodies in ganglia; provide nutrients and structural support.

Structural & Functional Categories of Neurons

Neuron Classification

Neurons are classified based on their structure and function.

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

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

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

Functionally, neurons are categorized as:

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

• Efferent (Motor) Neurons: Carry information away from the CNS to effectors.

• Interneurons: Connect neurons within the CNS; involved in processing and integration.

Neuronal Organization and Synapses

Neural Circuits

Neurons are organized into circuits to process information efficiently. These circuits can be simple (reflex arcs) or complex (networks for higher processing).

• Converging Circuits: Multiple inputs converge on a single neuron.

• Diverging Circuits: One neuron sends output to multiple targets.

• Serial Processing: Information passes through a sequence of neurons.

• Parallel Processing: Information is processed simultaneously by multiple pathways.

Synapses

A synapse is the junction between two neurons or between a neuron and an effector cell. Synapses can be electrical or chemical.

• Electrical Synapse: Direct flow of ions between cells via gap junctions; allows rapid transmission.

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

Example: The neuromuscular junction (NMJ) is a chemical synapse where acetylcholine is the neurotransmitter released to stimulate muscle contraction.

Embryology: Early Development of the Nervous System

Neural Plate and Neural Tube Formation

The nervous system develops from the ectodermal layer of the embryo. The process begins with the formation of the neural plate, which folds to create the neural tube.

• Neural Plate: Specialized region of ectoderm that gives rise to the nervous system.

• Neural Groove and Folds: The neural plate forms a groove flanked by neural folds.

• Neural Tube: The neural folds fuse to form the neural tube, which becomes the CNS.

Neurulation: The process of neural tube formation along the embryo's dorsal midline.

Neural Crest Cells

Cells at the edge of the neural folds become neural crest cells, which migrate to form various structures in the PNS and other tissues.

• Neural Crest Derivatives: Sensory neurons, autonomic ganglia, Schwann cells, melanocytes, craniofacial cartilage and bone, adrenal medulla.

Spinal Cord Development

As the neural tube closes, it differentiates into distinct layers that give rise to the spinal cord's structure.

• Ependymal Layer: Lines the neural cavity; forms ependymal cells.

• Mantle Layer: Contains neuroblasts; forms gray matter.

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

Motor neurons grow out from the ventral roots, while sensory neurons connect via the dorsal roots. The dorsal and ventral roots combine to form spinal nerves.

Spinal Cord Structure and Organization

Gray Matter and White Matter

The spinal cord consists of central gray matter surrounded by white matter.

• Gray Matter: Contains neuron cell bodies and dendrites; forms the butterfly-shaped central region with dorsal, ventral, and lateral horns.

• White Matter: Composed of myelinated axons organized into columns (funiculi) for ascending and descending tracts.

Enlargements: Cervical and lumbar enlargements accommodate increased neural input/output for limbs.

Spinal Nerves and Plexuses

Spinal nerves emerge from the spinal cord and branch to innervate various body regions.

• Spinal Nerves: 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal); each nerve is formed by the union of dorsal (sensory) and ventral (motor) roots.

• Plexuses: Networks of nerves formed by the ventral rami of spinal nerves; major plexuses include cervical, brachial, lumbar, and sacral.

Meninges and Protection

The spinal cord is protected by three connective tissue membranes called meninges.

• Dura Mater: Outermost, tough layer.

• Arachnoid Mater: Middle, web-like layer; contains cerebrospinal fluid in the subarachnoid space.

• Pia Mater: Innermost, delicate layer adhering to the spinal cord.

Reflexes and Neural Pathways

Reflex Arcs

A reflex arc is a simple neural circuit that mediates an automatic response to a specific stimulus.

• Receptor: Detects stimulus.

• Sensory Neuron: Transmits signal to CNS.

• Integration Center: Processes information in CNS.

• Motor Neuron: Carries response to effector.

• Effector: Muscle or gland that executes the response.

Example: The knee-jerk (patellar) reflex is a classic example of a monosynaptic reflex arc.

Ascending and Descending Tracts

Information travels in the spinal cord via specific tracts:

• Ascending Tracts: Carry sensory information to the brain.

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

Additional info: Ascending tracts include the dorsal columns (touch, proprioception), while descending tracts include the corticospinal tract (voluntary motor control).