Figure 1: The anatomy of the spinal cord
The spinal cord consists of grey matter and white matter. The white matter contains ascending and descending fibres, the grey matter contains cells and central terminals of primary afferents from the periphery.
The dorsal horn is divided into 6 layers (laminae) and processes sensory information.
Lamina I is the most dorsal and is a thin layer of large cells, together with small inhibitory interneurons. The axons from the large cells form part of the spinothalamic tract.
The second layer is lamina II or the “substantia gelatinosa”. Many of the cells are inhibitory but excitatory cells exist as well. This region is believed to control the “connectivity” of the other laminae in the dorsal horn. Together, laminae I-II are known as the superficial dorsal horn and receive input from C and Ad fibres. Functionally, they receive input from the nociceptors (high threshold C and Ad fibres) and contain cells that are nociceptive specific, NS (respond only to noxious stimuli) or wide dynamic range, WDR (respond to both innocuous and noxious stimuli).
Laminae III-VI receive input from the cutaneous Ab non-nociceptive afferents and contain cells with low-threshold (LT) receptive fields that respond to innocuous sensations. Some lamina V cells are WDRs that receive input from both low-threshold (Ab) sensory fibres and high-threshold (C, Ad) fibres as their dendrites project dorsally into laminae I-II.
The dorsal horn is not just a relay station for the transmission of innocuous and noxious messages. It has an important role in modulating pain transmission through spinal and supraspinal mechanisms. These regulatory circuits involve primary afferents, spinal interneurons and descending fibres.