Pain

Major ascending and descending tracts in the spinal cordJohn Craven

AbstractThis article provides a synopsis of the anatomy of the spinal cord with

the emphasis on the position and function of the major tracts forming the

posterior, lateral and anterior columns of the white matter.

Keywords ascending pathways; descending pathways; grey matter;

white matter

The adult spinal cord is about 45 cm long. It is cylindrical in shape and has cervical and lumbar enlargements where the nerves sup­plying the upper limb (C5–T1) and lower limb (L1–S3) originate. The cord is shorter than the vertebral canal and thus the nerve roots descend with increasing obliquity within the canal to reach the appropriate intervertebral foramen. The cord has an anterior median fissure and a posterior median sulcus; the rootlets of the spinal nerves emerge from its side at anterolateral and postero­lateral sulci. The cord contains grey matter, largely neuronal cell bodies, and white matter, which contains the long ascending and descending tracts.

Grey matter

The grey matter is arranged around the central canal and pro­jects towards the anterolateral and posterolateral sulci to form paired anterior and posterior horns (Figure 1). The anterior horn contains mainly motor cells giving rise to the fibres of the anterior roots and the posterior horn, mainly sensory cells sub­serving touch, pain and temperature, the fibres of which enter from the posterior roots. In the thoracic region and, to a lesser extent, in the sacral region, there are lateral horns, which give rise to preganglionic cells of the sympathetic nervous system. In the thoracic region, a group of large cells, the thoracic nucleus, lies at the base of the posterior horn. Grey matter contains large numbers of inter­neurons, linking cells within a segment or across adjacent segments, and these contribute to local reflex arcs.

White matter

White matter is divided, by the anterior and posterior horns and emerging spinal rootlets, into three columns on each side: an

John Craven, FRCS, was formerly Consultant Surgeon at York Hospital,

York, UKQ1. He is past chairman of the primary examiners of the Royal

College of Surgeons of England.

anaESTHESia anD inTEnSiVE CaRE MEDiCinE 9:1 1

anterior column between the anterior horn and median fissure, a lateral column between the anterior and posterior columns, and a posterior column between the posterior horn and posterior median sulcus. The posterior columns comprise a medial (fas­ciculus gracilis) and lateral (fasciculus cuneatus) tract and these convey sensory fibres subserving fine touch and proprioception (Figure 1). The anterior columns are incompletely separated from each other by the median fissure; the residual communication is called the anterior commissure.

The posterior column contains intersegmental fibres and ascending fibres associated with light touch, pressure, vibration and proprioception; their cell bodies lie in the dorsal root gan­glia and the fibres pass to the gracile and cuneate nuclei in the medulla.

The lateral columnPeripherally placed ascending fibres

• The anterior and posterior spinocerebellar tracts are associ­ated with unconscious proprioception. The cell bodies of the an­terior tract lie in the opposite posterior horn and its fibres pass to the cerebellum. The posterior tract has its cell bodies in the thoracic nucleus of the same side and its fibres pass uncrossed to the cerebellum.• The dorsolateral fasciculus is a mixed bundle of fibres arising in the dorsal root ganglia and the gelatinous substance before ending in the posterior horn.• The lateral spinothalamic tract subserves pain and temper­ature. Its cell bodies lie in the opposite posterior horn and its fibres pass in the anterior commissure before ascending to the thalamus.

Intermediately placed, mainly descending fibres• The fibres of the lateral corticospinal (crossed pyramidal) tract originate from cell bodies in the cerebral motor cortex. On their descent, the fibres cross in the medulla before ending by entering the anterior horn to connect to motor cells.

Cross-section of spinal cord showing ascendingpathways on right and descending pathways on left

Posterior median sulcus

Lateral corticospinal tract

Rubrospinal tract

Olivospinal tract

Vestibulospinal tract

Tectospinal tract

Anterior corticospinal tractAnterior median fissure

Anteriorspinothalamic tract

Fasciculusproprius

Lateralspinothalamic

tract

Spinocerebellar tracts

Posterior

Anterior

Fasciculus gracilis

Fasciculus cuneatus

Figure 1

© 2007 Elsevier Ltd. all rights reserved.

Pain

• The reticulospinal and rubrospinal tracts contain extrapyram­idal motor fibres, which originate in the midbrain.

The anterior column contains the following.• The anterior corticospinal (uncrossed pyramidal) tract is a small tract, the fibres of which originate in the cerebral motor cortex of the same side and descend without medullary decussation. At their destination the fibres cross horizontally, in the anterior commis­sure, to synapse with cell bodies in the opposite anterior horn.

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• Vestibulospinal, olivospinal, reticulospinal and tectospinal tracts are of extrapyramidal fibres, which pass to the anterior horn cells from brainstem nuclei.• The anterior spinothalamic tract carries fibres subserving touch. Its cells lie in the posterior horn of the opposite side and its fibres ascend to the thalamus.• In the fasciculus proprius, surrounding the grey matter, lie fibres forming inter­ and intra­segmental connections. These connections form the basis of intersegmental reflexes in the spinal cord. ◆

© 2007 Elsevier Ltd. all rights reserved.