Central Nervous System Anatomy

Central Nervous System:
The Spinal Cord

General anatomy

• CNS tissue within vertebral column from foramen magnum to L₁

• provides two-way communication

• protected by bone, meninges, fat, and CSF

Definitions

• Epidural space — space between the vertebrae and the dural sheath (dura mater) filled with fat and a network of veins

• Conus medullaris — terminal portion of the spinal cord

• Filum terminale — fibrous extension of the pia mater; anchors the spinal cord to the coccyx

• Denticulate ligaments — delicate shelves of pia mater; attach the spinal cord to the vertebrae

• Spinal nerves — 31 pairs attach to the cord by paired roots

• Cervical and lumbar enlargements — sites where nerves serving the upper and lower limbs emerge

• Cauda equina — collection of nerve roots at the inferior end of the vertebral canal

Embryonic Development of the Spinal Cord

design of embryonic spinal cord

• develops from caudal portion of neural tube

• by week 6, there are two clusters of neuroblasts:

• alar plate — will become interneurons

• basal plate — will become motor neurons

• neural crest cells form the dorsal root ganglia

Cross-Sectional Anatomy of the Spinal Cord

• cross-section of spinal cord within vertebral canal

• Anterior median fissure — separates anterior funiculi

• Posterior median sulcus — divides posterior funiculi

cross-sectional view of spinal cord and meninges

Gray Matter and Spinal Roots

• gray matter consists of soma, unmyelinated processes, and neuroglia

• gray commissure — connects masses of gray matter; encloses central canal

• posterior (dorsal) horns — interneurons

• anterior (ventral) horns — interneurons and somatic motor neurons

• lateral horns — contain sympathetic nerve fibers

Organization of Gray Matter

• dorsal half — sensory roots and ganglia

• ventral half — motor roots

• dorsal and ventral roots fuse laterally to form spinal nerves

• four zones are evident within the gray matter —

somatic sensory (SS)

visceral sensory (VS)

visceral motor (VM)

somatic motor (SM)

organization of gray matter in spinal cord

White Matter in the Spinal Cord

• fibers run in three directions — ascending, descending, and transversely

• divided into three funiculi (columns) — posterior, lateral, and anterior

• each funiculus contains several fiber tracks

fiber tract names reveal their origin and destination

fiber tracts are composed of axons with similar functions

organization of spinal cord tracts

White Matter: Pathway Generalizations

• pathways decussate

• most consist of two or three neurons

• most exhibit somatotopy (precise spatial relationships)

• pathways are paired (one on each side of the spinal cord or brain)

Ascending pathways

anterolateral (nonspecific ascending) pathways

lateral and anterior spinothalamic tracts

medial lemniscal system (specific ascending pathways)

dorsal white column

fasciculus cuneatus

fasciculus gracilis

anterior and posterior spinocerebellar tracts

medial lemniscal system and spinocerebellar tracts

lateral spinothalamic tract

Descending pathways

pyramidal system

lateral and anterior corticospinal tracts

extrapyramidal system

tectospinal tracts

vestibulospinal tracts

rubrospinal tracts

anterior, medial, and lateral reticulospinal tracts

pyramidal system

rubrospinal tract

Spinal Cord Trauma

Paralysis

• Paralysis — loss of motor function

• Flaccid paralysis — severe damage to the ventral root or anterior horn cells

• lower motor neurons are damaged and impulses do not reach muscles

• no voluntary or involuntary control of muscles

• Spastic paralysis — only upper motor neurons of the primary motor cortex are damaged

• spinal neurons remain intact and muscles are stimulated irregularly

• no voluntary control of muscles

Transection

• Cross sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut

• Paraplegia — transection between T₁ and L₁

• Quadriplegia — transection in the cervical region

Poliomyelitis ¹

• destruction of the anterior horn motor neurons by the poliovirus

• early symptoms — fever, headache, muscle pain and weakness, and loss of somatic reflexes

• vaccines — Salk and Sabin polio vaccines are available and can prevent infection

Amyotrophic Lateral Sclerosis (ALS) ²

• Lou Gehrig’s disease — neuromuscular condition involving destruction of anterior horn motor neurons and fibers of the pyramidal tract

• symptoms — loss of the ability to speak, swallow, and breathe

• death occurs within five years

• linked to malfunctioning genes for glutamate transporter and/or superoxide dismutase

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Central Nervous System:The Spinal Cord