Somatotopic organization of cutaneous afferent terminals and dorsal horn neuronal receptive fields in the superficial and deep laminae of the rat lumbar spinal cord
Authors: Woolf Clifford J. , Fitzgerald Maria 
Published: January 2005

The somatotopic organization of A‐ and C‐afferent fibre terminals in the dorsal horn of the rat lumbar spinal cord was compared with the spatial location of second‐order dorsal horn neuronal mechanoreceptive fields. The central terminal fields of the sural, saphenous, and tibial nerve were mapped by labelling the nerves with horseradish peroxidase (HRP). A previous study used the transganglionic transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA‐HRP) to produce a somatotopic map of highthreshold C‐fibre terminal fields in lamina II (Swett and Woolf: J. Comp. Neurol. 231:66–77, '85). In the present study the terminal fields of lowthreshold A beta afferents that terminate in laminae III and IV were mapped by using unconjugated HRP at prolonged survival times (72 hours). Unfixed tissue was used to increase the sensitivity of the tetramethylbenzidine reaction, thus allowing these afferent terminals to be clearly seen. The general spatial arrangement of the terminal fields in laminae III/IV closely resembled that found in lamina II in the mediolateral and rostrocaudal planes but because of a dorsoventral obliquity of the afferent terminals, the superficial and deeper fields are not in strict vertical register. The input to laminae II‐IV of the dorsal horn may therefore be viewed as two horizontally arranged sheets of afferent terminals both accurately representing the skin surface, the more superficial sheet representing the high‐threshold C‐afferents and the deeper sheet, low‐threshold A‐beta afferents. The spatial organization of high‐threshold A‐delta afferents in laminae I and V appears to be quite different, with a transverse rather than a longitudinal orientation.

To study dorsal horn cell receptive field organization two single units with mechanoreceptive fields were recorded extracellulary in each of 87 vertical tracks in the lumbar spinal cord, one unit in the superficial dorsal horn and the second in the deep dorsal horn. In general the somatotopic organization of the receptive fields of both sets of units followed that of the afferent terminal fields but there were cells with receptive fields that were anomalous relative to the recording site. No evidence of any vertical relation or columnar arrangement in receptive field size, threshold, or location on the body surface was found when comparing the two units in a pair. Furthermore, no laminar functional specialization was found, the majority of neurones having both low‐and high‐threshold inputs. The high‐threshold receptive fields of convergent cells were larger than the low‐threshold fields in the superficial laminae whereas the reverse was true in the deeper laminae.

The absence of a fixed somatopic relationship between the cutaneous mechanoreceptive field properties of a neurone recorded in a certain site and the spatial arrangement of the primary afferent terminals in that site indicates that whereas the order in the afferent termination pattern provides a geometric framework for the generation of cutaneous receptive fields, other factors also have an important role in contributing to the response properties of dorsal horn neurones.