The present study labels the neuronal cell bodies that give rise to afferent fibers that innervate the bladder of cat and rat. The method used was the retrograde transport of horseradish peroxidase (HRP) from its injection site in the bladder to cells in various dorsal root ganglia. In the rat, the labelled cells are located in the L1-L2 and L6-S1 dorsal root ganglia. In the cat, the labelled cells are located in the L2-L5 and S1-S4 dorsal root ganglia. This confirms older clinical findings, and for the first time directly demonstrates the afferent cell bodies for the bladder. The bladder afferents are small ganglion cells in both rat and cat, and because there is a correlation between the size of axon and the cell body from which it originates, we conclude that the great majority of bladder afferents are small myelinated or unmyelinated axons. In addition, by restricting the HRP to one side of the bladder, we are able to show that some afferent cell bodies send their distal processes across the midline. These results will be useful in considerations of the neural control of bladder function.

SUMMARY1. The receptive fields were determined for 118 afferent fibres in the S2, S3 and Ca (caudal) 1 ventral roots of the cat. Of these fibres, ninety-three were unmyelinated, another eleven were probably unmyelinated, and fourteen were myelinated, according to estimates of their conduction velocities.2. Confirmation that the recordings were from ventral root filaments came from electron microscopic inspection of ten of the filaments from which recordings of the activity of unmyelinated afferents were made.3. Receptive fields were demonstrated for twelve unmyelinated afferent fibres in the distal stumps of the S2 and S3 ventral roots which had been sectioned 3 weeks previously, indicating that the cell bodies giving rise to these fibres were not in the spinal cord.4. The action potentials of some of the unmyelinated ventral root afferent fibres were complex, suggesting branching of the afferents within the ventral root.5. One third of the unmyelinated ventral root afferents had receptive fields in somatic structures: the skin and deep tissues.6. Two thirds of the unmyelinated ventral root afferents had receptive fields in the viscera of the pelvis: the bladder, urethra, vagina, and lower bowel.7. Many of the unmyelinated afferents in the ventral roots, especially those with cutaneous receptive fields, had high thresholds and may participate in nociception.8. It is concluded that the cat ventral root contains a major sensory component and that the Law of Bell and Magendie is not an accurate description of the organization of the ventral root in this animal. L. CLIFTON AND OTHERS

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SUMMARY1. 29% of the axons in the S3 and Ca 1 caudal ventral roots of the cat are unmyelinated.2. Approximately one half of these unmyelinated axons survive in the proximal stump of a sectioned ventral root and are regarded as unmyelinated preganglionic efferents.3. At least 27 % of the preganglionic efferents in segments S3 and Ca 1 are unmyelinated.4. The other half of the ventral root unmyelinated axons arise from dorsal root ganglion cells and survive in the distal stump of a sectioned ventral root.5. The data in 4, combined with the receptive field data in our companion paper, show that these unmyelinated fibres are sensory.6. Since 15 % of the ventral root axons are sensory, the Law of Bell and Magendie cannot be regarded as an accurate description of the organization of these ventral roots.