Nervous System Overview

Central and Peripheral Nervous System

The nervous system is divided into two main components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each plays a distinct role in processing and transmitting information throughout the body.

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the main control center for processing information and coordinating activity.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. It 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 Division: Controls voluntary and involuntary muscle movements.

Cells of the Nervous System

Neurons

Neurons are excitable cells responsible for information transfer in the nervous system. They have specialized structures for receiving and transmitting signals.

• Dendrites: Receive incoming signals.

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

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

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

Glial Cells

Glial cells (neuroglia) are support cells that provide structural and metabolic support to neurons. They are essential for maintaining homeostasis and protecting neural tissue.

• Astrocytes: Maintain the blood-brain barrier (BBB), provide nutrients, and regulate ion balance.

• Oligodendrocytes: Form myelin sheaths in the CNS, creating white matter.

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

• Ependymal Cells: 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, provide nutrients.

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

Structural & Functional Categories of Neurons

Classification

Neurons are classified based on their structure and function:

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

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

• Unipolar Neurons: Single process; common in sensory neurons of PNS.

Functionally, neurons can be:

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

• Motor (Efferent) Neurons: Transmit impulses away from the CNS to effectors.

• Interneurons: Connect neurons within the CNS.

Neuronal Organization

Neural Circuits

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

• 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 and Signal Transmission

Types of Synapses

A synapse is the junction between two neurons where information is transmitted. There are two main types:

• Electrical Synapse: Direct flow of ions through gap junctions; rapid transmission.

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

Directionality is important: the presynaptic neuron sends the signal, and the postsynaptic neuron receives it.

Action Potentials

Generation and Propagation

Action potentials (APs) are rapid changes in membrane potential that allow neurons to transmit signals over long distances.

• Resting Membrane Potential (RMP): Typically -70 mV; inside of neuron is negative relative to outside.

• Threshold: If membrane potential reaches a critical value, an AP is triggered.

• Depolarization: Na+ channels open, Na+ enters cell, membrane becomes more positive.

• Repolarization: K+ channels open, K+ exits cell, membrane returns to negative.

• All-or-None Principle: APs either occur fully or not at all.

Myelination increases the speed of AP propagation via saltatory conduction, where APs jump between nodes of Ranvier.

Equation:

Additional info: The action potential is essential for neural communication and underlies all nervous system functions.

Embryology: Early Development of the Nervous System

Neural Plate and Neural Tube Formation

The nervous system develops from the neural plate in the embryo, which folds to form the neural tube. This process is called neurulation.

• Notochord: Induces formation of the neural plate.

• Neural Groove: Forms along the midline of the neural plate.

• Neural Folds: Elevate and fuse to form the neural tube, which becomes the CNS.

Neural Crest Cells

Neural crest cells migrate away from the neural tube and give rise to diverse cell types, including sensory neurons, Schwann cells, and components of the peripheral nervous system.

• Form dorsal root ganglia, autonomic ganglia, and other structures.

• Contribute to cranial nerves and peripheral organs.

Development of the Spinal Cord

Spinal Cord Layers and Differentiation

The spinal cord develops from the neural tube and differentiates into distinct layers:

• Ependymal Layer: Lines the neural cavity.

• Mantle Layer: Forms gray matter; contains neuron cell bodies.

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

Axons from the mantle layer extend into the marginal layer, forming tracts that become columns of white matter in the spinal cord.

Spinal Cord Structure

The mature spinal cord consists of:

• Gray Matter: Central region shaped like a butterfly; contains neuron cell bodies and dendrites.

• White Matter: Surrounds gray matter; contains myelinated axons organized into columns (dorsal, lateral, ventral).

• Meninges: Protective membranes (dura mater, arachnoid mater, pia mater) that provide stability and shock absorption.

Spinal Nerves and Plexuses

Spinal Nerve Organization

Spinal nerves emerge from the spinal cord and branch into dorsal and ventral rami, providing sensory and motor innervation to the body.

• Dorsal Root: Contains sensory (afferent) fibers.

• Ventral Root: Contains motor (efferent) fibers.

• Mixed Nerves: Spinal nerves contain both sensory and motor fibers.

Plexuses

Nerve plexuses are networks of intersecting nerves that supply specific regions of the body.

• Cervical Plexus: Innervates neck and diaphragm.

• Brachial Plexus: Supplies the upper limb.

• Lumbosacral Plexus: Supplies the lower limb and pelvic region.

Additional info: Plexuses ensure redundancy and flexibility in nerve supply to limbs and body regions.

Reflexes and Neural Pathways

Reflex Arcs

Reflexes are involuntary motor responses to specific stimuli, mediated by neural circuits called reflex arcs.

• Receptor: Detects stimulus.

• Sensory Neuron: Transmits signal to CNS.

• Integration Center: Processes information in CNS.

• Motor Neuron: Sends signal to effector.

• Effector: Muscle or gland that responds.

Reflexes can be simple (monosynaptic) or complex (polysynaptic), depending on the number of neurons involved.

Summary Table: Nervous System Components