Nervous System I: Structure, Function, and Development (CNS & PNS, Embryology, Spinal Cord)

Nervous System Overview

Introduction

The nervous system is a complex network responsible for coordinating and regulating bodily functions, including sensation, movement, and cognition. It is divided into the central nervous system (CNS) and peripheral nervous system (PNS).

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

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

Subdivisions of the Nervous System

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

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

• Somatic Motor Division: Controls voluntary movements via skeletal muscles.

• Visceral Motor Division (Autonomic Nervous System): Regulates involuntary functions (e.g., heart rate, digestion) and is further divided into sympathetic and parasympathetic divisions.

Cell Types in the Nervous System

Neurons

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

• Structure: Consist of dendrites, cell body (soma), axon, and axon terminals.

• Function: Generate and conduct electrical impulses.

Neuroglia (Glial Cells)

Glial cells provide support, protection, and nutrition to neurons. Their types and functions vary between CNS and PNS.

Central Nervous System (CNS) Glial Cells

• Astrocytes: Form the blood-brain barrier (BBB), regulate ion balance, and support neurons.

• Oligodendrocytes: Produce myelin sheaths in the CNS; myelinated axons form white matter, unmyelinated axons form gray matter.

• Microglia: Act as phagocytes, removing debris and pathogens.

• Ependymal Cells: Line ventricles and central canal, involved in cerebrospinal fluid production.

Peripheral Nervous System (PNS) Glial Cells

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

• Schwann Cells: Form myelin sheaths around axons in the PNS, aiding in rapid signal transmission.

Structural & Functional Categories of Neurons

Neuron Classification

Type	Structure	Function
Anaxonic	Multiple processes, indistinguishable axon	Integrative functions, found in CNS
Bipolar	One dendrite, one axon	Sensory organs (e.g., retina)
Pseudounipolar	Single process splits into two branches	Sensory neurons in PNS
Multipolar	Multiple dendrites, one axon	Most common, motor neurons

Neuronal Organization

Neural Circuits

• Divergence: One neuron spreads information to multiple neurons.

• Convergence: Multiple neurons send information to a single neuron.

• Serial Processing: Neurons arranged in a sequence.

• Parallel Processing: Information processed simultaneously by several pathways.

• Reverberation: Positive feedback loop within a circuit.

Synapse

Types and Function

• Chemical Synapse: Neurotransmitter release allows signal transfer across synaptic cleft.

• Electrical Synapse: Direct ion flow via gap junctions; less common.

Synapses are classified by their position: presynaptic neuron (sending signal) and postsynaptic neuron (receiving signal).

Action Potentials

Mechanism

Action potentials are rapid changes in membrane potential that propagate along neurons.

• Resting Membrane Potential (RMP): Typically about -70 mV; maintained by ion gradients.

• Threshold: If membrane depolarization reaches threshold, an action potential is triggered.

• All-or-None Principle: Action potential either occurs fully or not at all.

• Irreversible: Once started, cannot be stopped.

Key Equations

• Nernst Equation: $E_{ion} = \frac{RT}{zF} \ln \frac{[ion]_{out}}{[ion]_{in}}$

Ion Channels in Action Potentials

• Na+ Channels: Open during depolarization, allowing Na+ influx.

• K+ Channels: Open during repolarization, allowing K+ efflux.

Myelination

Role in Signal Transmission

• Myelin Sheath: Insulates axons, increasing speed of action potential propagation.

• Nodes of Ranvier: Gaps in myelin where ion channels are concentrated; enable saltatory conduction.

• Unmyelinated Neurons: Action potentials propagate continuously and more slowly.

Chemical Synapse

Mechanism

• Arrival of action potential at axon terminal triggers neurotransmitter release into synaptic cleft.

• Neurotransmitters bind to receptors on postsynaptic cell, initiating a response.

• Example: Acetylcholine is released at the neuromuscular junction.

Embryology: Early Development of the Nervous System

Neurulation

• Neural Plate: Forms from ectoderm along dorsal midline of embryo (day 20).

• Neural Groove: Crease develops along neural plate (day 21).

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

Neural Crest Cells

• Cells dorsal to neural tube migrate to form PNS and other structures.

Layers of Neural Tube

Layer	Location	Function
Ependymal	Inner	Lines neural cavity
Mantle	Middle	Forms gray matter
Marginal	Outer	Forms white matter

Development of the Spinal Cord

Part I: Formation of Gray and White Matter

• Neural tube closes; mantle layer forms gray matter.

• Axons from mantle layer extend to targets, forming white matter tracts in marginal layer.

Part II: Differentiation and Organization

• Neural crest cells migrate to form autonomic ganglia and other structures.

• Motor nerve fibers emerge and form spinal nerves, which divide into dorsal and ventral rami.

• Spinal nerves innervate developing muscles and organs.

Spinal Cord Structure and Function

Gray Matter and White Matter

• Gray Matter: Contains neuron cell bodies, dendrites, glial cells; forms 'horns' in spinal cord.

• White Matter: Composed of myelinated axon tracts; surrounds gray matter.

• Central canal runs length of spinal cord, contains cerebrospinal fluid.

Organization

• Dorsal horns: Receive sensory information.

• Ventral horns: Contain motor neuron cell bodies.

• Dorsal columns: Ascending sensory tracts.

• Ventral/lateral columns: Motor and sensory tracts.

Meninges of the Spinal Cord

Protective Membranes

• Dura Mater: Outermost tough layer.

• Arachnoid Mater: Middle layer; subarachnoid space contains cerebrospinal fluid.

• Pia Mater: Innermost layer; adheres to spinal cord.

Spinal Nerves

Structure and Function

• 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

• Ventral root (motor) joins dorsal root (sensory) to form mixed spinal nerve.

• Spinal nerves branch into dorsal and ventral rami.

Ramus Communicans

Definition and Function

• Communicating branch connecting nerves.

• White ramus: Path for preganglionic sympathetic fibers.

• Gray ramus: Contains postganglionic sympathetic fibers.

Nerve Plexuses

Cervical Plexus

• Located under sternocleidomastoid muscle; innervates neck, diaphragm, and parts of head.

Brachial Plexus

• Supplies upper limb; divided into roots, trunks, divisions, cords, and branches.

• Nerves from plexus innervate arm, forearm, and hand.

Lumbar Plexus

• Formed by lower thoracic and lumbar ventral roots; innervates lower limb and pelvic girdle.

Sacral Plexus

• Located on posterolateral pelvic wall; nerves pass through sciatic foramen to lower limb.

Reflexes

Classification and Mechanism

• Involuntary motor responses to stimuli.

• Information processed in CNS; response via effector (muscle or gland).

Classification	Types
Development	Innate, acquired
Response	Somatic, visceral
Complexity	Monosynaptic, polysynaptic
Processing Site	Spinal, cranial

Recall: Tracts, Columns, Ascending, Descending

Definitions

• Tracts: Bundles of axons in CNS with common origin and destination.

• Columns: Groups of tracts in spinal cord.

• Ascending Tracts: Carry sensory information to brain.

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

Additional info: These notes cover foundational concepts in nervous system anatomy and physiology, including embryological development, cellular structure, and organization of the spinal cord and peripheral nerves, suitable for exam preparation in a college-level Anatomy & Physiology course.