Spinal Cord Segments and Tracts

The spinal cord is the elongated portion of the central nervous system housed within the vertebral canal. It is organized into 31 segments, each giving rise to a pair of spinal nerves, with a central butterfly of gray matter surrounded by white matter carrying ascending sensory and descending motor tracts.

Definition

The spinal cord is the part of the central nervous system extending from the medulla through the vertebral canal, divided into cervical, thoracic, lumbar, sacral, and coccygeal segments and organized into central gray matter and surrounding white-matter tracts.

Scope

This topic covers spinal cord segmentation, the internal arrangement of gray and white matter, and the principal long tracts — dorsal columns, spinothalamic, and corticospinal pathways. It is reference anatomy, not clinical management guidance.

Key concepts

• 31 spinal segments and spinal nerves
• Dorsal and ventral horns
• Dorsal column-medial lemniscus pathway
• Spinothalamic tract
• Lateral corticospinal tract
• Dermatomes and myotomes

Mechanisms

Each spinal segment receives sensory input through a dorsal root and sends motor output through a ventral root. Sensory information is routed in distinct ascending systems: fine touch and proprioception travel in the dorsal columns, while pain and temperature cross near their entry level and ascend in the spinothalamic tract. Descending motor commands travel principally in the lateral corticospinal tract, which Lemon's review describes as central to skilled voluntary movement, especially of the hand. The crossing patterns of these tracts explain why cord lesions produce dissociated sensory and motor deficits.

Clinical relevance

Knowledge of segmental levels and tract locations underlies the mapping of sensory levels and weakness to specific cord regions. This entry presents anatomy for educational reference and is not intended to guide individual diagnosis or treatment.

History

The segmental plan of the cord and the dermatomal map were established through nineteenth- and early twentieth-century anatomical and clinical studies. Twentieth-century tract-tracing and physiological work clarified the course and decussation of the major ascending and descending pathways.

Key figures

• Roger Lemon

Seminal works

• lemon-2008
• kandel-2021

Frequently asked questions

How many segments does the spinal cord have?

The spinal cord is divided into 31 segments — 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal — each associated with a pair of spinal nerves.

Which tract carries pain and temperature sensation?

Pain and temperature are conveyed by the spinothalamic tract, whose fibers cross to the opposite side near their level of entry before ascending to the thalamus.