A: Central Nervous System: Part 2

Anatomy of the Central Nervous System: Part 2

Overview

This study guide covers the anatomy and physiology of the spinal cord, including its structure, function, associated nerves, protective mechanisms, and relevant clinical procedures and pathologies. The spinal cord is a critical component of the central nervous system (CNS), serving as a conduit for neural signals between the brain and body and as a major reflex center.

Components of the CNS

Cerebro-Spinal Axis

• Brain (Encephalon): The control center for processing and integrating information.

• Spinal Cord: The main pathway for transmitting neural signals between the brain and peripheral body.

Vertebral Canal Contents

• Spinal Cord: Two-way impulse conduction pathway and major reflex center; shorter than the vertebral column in adults.

• Spinal Meninges: Protective membranes surrounding the spinal cord.

• Spinal Nerve Roots: Connect the spinal cord to peripheral nerves.

• Neurovascular Structures: Blood vessels and related tissues supporting spinal cord function.

Spinal Cord Structure

Gross Anatomy

• Origin: Begins as a continuation of the medulla oblongata.

• Location: Occupies the superior two-thirds of the vertebral canal.

• Enlargements:

  • Cervical Enlargement: Brachial plexus of nerves; innervates upper limbs.

  • Lumbar Enlargement: Lumbar and sacral plexuses; innervates lower limbs.

• Conus Medullaris: Tapering end of the spinal cord, typically terminating between T12 and L3 vertebrae.

Cauda Equina

• Spinal cord does not reach the end of the vertebral column.

• Lumbar and sacral spinal nerve roots travel inferiorly through the canal before reaching their intervertebral foramina.

• Collection of nerve roots resembles a horse's tail (cauda equina).

Spinal Nerves

Classification and Structure

• 31 pairs of spinal nerves:

  • 8 cervical

  • 12 thoracic

  • 5 lumbar

  • 5 sacral

  • 1 coccygeal

• Nerves exit the vertebral column via intervertebral foramina.

• Roots of spinal nerves:

  • Dorsal (posterior) root: sensory input

  • Ventral (anterior) root: motor output

Dermatome Map

A dermatome is an area of skin supplied by sensory fibers from a single spinal nerve root. Dermatome maps are used clinically to assess nerve function and diagnose nerve injuries.

Spinal Cord Trauma

Effects of Injury

• Loss of motor function: Paralysis

• Loss of sensation: Paresthesia (tingling, numbness, burning)

• Severe damage to ventral root/horn results in flaccid paralysis of innervated skeletal muscles.

• Transection: At any level results in total motor and sensory loss inferior to the site of damage.

Types of Spinal Cord Injury

Level of Injury	Resulting Paralysis
C4 (high-cervical)	Quadriplegia (paralysis below the neck)
C6	Partial paralysis of hands, arms, and lower body
T6	Paraplegia (paralysis below the chest)
L1	Paraplegia (paralysis below the waist)

Cross-Sectional Anatomy of the Spinal Cord

Transverse Sections

• Ventral median fissure and dorsal median sulcus divide the cord.

• H-shaped gray matter embedded in white matter.

• Posterior (dorsal) and anterior (ventral) gray horns.

• Lateral horns present in thoracic and lumbar segments.

Spinal Gray Matter Zones

Zone	Function
Somatic sensory (SS)	Receives sensory input from skin, muscles, joints
Visceral sensory (VS)	Receives sensory input from internal organs
Visceral motor (VM)	Controls autonomic motor functions
Somatic motor (SM)	Controls voluntary muscle movement

Spinal Nerve Roots and Branches

• Dorsal roots: Contain afferent axons from skin, subcutaneous and deep tissues, and viscera; cell bodies located in spinal ganglia.

• Ventral roots: Contain efferent axons to skeletal muscle; axons of somatic and visceral motor neurons.

• Spinal nerves divide into dorsal and ventral rami (branches).

Autonomic Nervous System Divisions

Division	Neurotransmitter	Target
Parasympathetic (PNS)	ACh	Tissue via ganglia
Sympathetic (SNS)	ACh, NE	Tissue, adrenal gland
Somatic	ACh	Muscle

White Matter of the Spinal Cord

• Contains myelinated and unmyelinated nerve fibers running in three directions:

  • Ascending: Sensory inputs

  • Descending: Motor outputs

  • Transverse: Commissural fibers

• Comprised of multi-neuron pathways connecting spinal cord, PNS, and brain.

Ascending Pathways to the Brain

• First-order neurons: Cell bodies reside in ganglion; conduct impulses from periphery to spinal cord.

• Second-order neurons: Cell bodies reside in dorsal horn; transmit impulses to thalamus or cerebellum.

• Third-order neurons: Cell bodies reside in thalamus; relay impulses to somatosensory cortex of cerebrum.

Descending Pathways from the Brain

Direct Pathways (Pyramidal System)

• Originate with pyramidal cells in the gyri of the primary motor cortex.

• Long axons project to the spinal cord from pyramidal tracts (corticospinal tracts).

• Permit voluntary movement via control of skeletal muscles.

• Stimulation of ventral horn neurons activates skeletal muscles.

Indirect Pathways (Extrapyramidal System)

• All motor pathways except pyramidal pathways (e.g., subcortical motor nuclei).

• Regulate muscles involved in balance, posture, and head/neck/eye movements that follow objects in the visual field.

Protecting the CNS

• Vertebrae: Bony protection for the spinal cord.

• Spinal Meninges: Three protective membranes (dura mater, arachnoid mater, pia mater).

• Cerebrospinal Fluid (CSF): Cushions and nourishes the spinal cord.

Spinal Meninges: The Membranes of the Spinal Cord

Spinal Dura Mater

• Outermost and strongest layer; single-layered sheet of fibrous connective tissue.

• Separated from vertebrae by extradural (epidural) space, which is fat-filled and contains a network of veins.

• Forms dural root sheaths to protect nerve roots; extends to level of S2.

Spinal Arachnoid Mater

• CSF fills the subarachnoid space between arachnoid and pia mater.

• CSF pressure forces the arachnoid against the inner dural surface.

• Extends to level of S2.

Spinal Pia Mater

• Innermost membrane; follows all surface features of the spinal cord.

• Directly covers roots of spinal nerves.

• Filum terminale: Fibrous extension of conus medullaris, covered by pia mater, extends to coccyx.

• Denticulate ligaments: Saw-toothed shelves of pia mater that secure spinal cord to dura mater.

Procedures and Pathophysiology

Lumbar Spinal Puncture (Spinal Tap)

• Needle traverses dura and arachnoid mater simultaneously.

• Used to obtain CSF sample from subarachnoid space.

• Needle inserted between L3-4 or L4-5, inferior to end of spinal cord, minimizing risk of damage.

Epidural Anesthesia (Blocks)

• Anesthetic agent injected into lumbar extradural space.

• Direct effect on spinal nerve roots of cauda equina after they exit dural sac.

• Patient loses sensation inferior to the level of the block.

Poliomyelitis

• Virus causes destruction of somatic neurons in ventral horn.

• Symptoms: Early (fever, headache, muscle pain, weakness); Late (paralysis, muscle atrophy).

• Medulla involvement can result in respiratory and cardiovascular complications and death.

• Mainly affects children under 5 years; prevention is key (oral and inactivated polio vaccines).

References and Further Study

• Agur AMR, Dalley AF. Moore's Essential Clinical Anatomy, 7e. Lippincott Williams & Wilkins, 2024.

• Medical illustrations: Servier Medical Art (CC BY 4.0).

Additional info:

• For more detailed anatomical images and clinical correlations, refer to Figures 1.23, 1.27, 1.28, and 1.29 in Moore's Essential Clinical Anatomy.

• Recommended videos for visual learning: cross-sectional anatomy, spinal nerves, epidural procedures, and spinal cord injury (YouTube links provided in original notes).