To investigate extrinsic origins of calcitonin gene-related peptide immunoreactive (CGRP-IR) nerve fibres in the sheep ileum, the retrograde fluorescent tracer Fast Blue (FB) was injected into the ileum wall. Sections of thoraco-lumbar dorsal root ganglia (DRG) and distal (nodose) vagal ganglia showing FB-labelled neurons were processed for CGRP immunohistochemistry. The distribution of CGRP-IR in fibres and nerve cell bodies in the ileum was also studied. CGRP-IR enteric neurons were morphometrically analysed in myenteric (MP) and submucosal plexuses (SMP) of lambs (2-4 months). Sensory neurons retrogradely labelled with FB were scattered in T5-L4 DRG but most were located at the upper lumbar levels (L1-L3); only a minor component of the extrinsic afferent innervation of the ileum was derived from nodose ganglia. In the DRG, 57% of retrogradely labelled neurons were also CGRP-IR. In cryostat sections, a dense network of CGRP-IR fibres was observed in the lamina propria beneath the epithelium, around the lacteals and lymphatic follicles (Peyer's platches), and along and around enteric blood vessels. Rare CGRP-IR fibres were also present in both muscle layers. Dense pericellular baskets of CGRP-IR fibres were observed around CGRP-negative somata. The only CGRP-IR nerve cells were well-defined Dogiel type II neurons localised in the MP and in the external and internal components of the SMP. CGRP-IR neurons in the myenteric ganglia were significantly larger than those in the submucosal ganglia (mean profile areas: about 1,400 mum(2) for myenteric neurons, 750 mum(2) for submucosal neurons). About 6% of myenteric neurons and 25% of submucosal neurons were CGRP-IR Dogiel type II neurons. The percentages of CGRP-IR neurons that were also tachykinin-IR were about 9% (MP) and 42% (SMP), whereas no CGRP-IR neurons exhibited immunoreactivity for vasoactive intestinal peptide, nitric oxide synthase or tyrosine hydroxylase in either plexus. Thus, CGRP immunoreactivity occurs in the enteric nervous system of the sheep ileum (as in human small intestine and MP of pig ileum) in only one morphologically defined type of neuron, Dogiel type II cells. These are probably intrinsic primary afferent neurons.

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Intact aneurysms of the carotid siphon at the point of take-off of the posterior communicating artery may exhibit orbital pain, whether associated with oculomotor palsy or not as a warning symptom prior to rupture. In order to explain this symptom the hypothesis of a sensory pathway within the third cranial nerve, which is liable to compression by the enlarging aneurysm sac, has been investigated. Data from human autopsy material show evidence of sensory ganglion cells within the rootlets of the oculomotor nerve; furthermore, studies in animals prove that the third nerve contains sensory fibers which run proximally along the nerve bundles, enter the brainstem and reach the spinal trigeminal nucleus. These fibers come from the ophthalmic division of the fifth nerve and join the third nerve at the level of the lateral wall of the cavernous sinus. Although a number of questions remain to be solved, the presence of a sensory pattern within the third nerve could account for fronto-orbital pain from enlarging aneurysms impinging on the third nerve itself.

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In the present investigation, the fiber content and the diameter spectra of the intracranial portion of the three oculomotor nerves (oculomotor, trochlear, and abducens nerves) were analysed in sheep by light and electron microscopy. It was determined that up to 14.98% of fibers in the oculomotor nerve, 17.01% in the trochlear nerve, and 11.87% in the abducens nerve were unmyelinated. The myelinated fibers showed a bimodal distribution in their size spectrum in all three nerves, with a majority of large myelinated axons, but a considerable proportion of small myelinated fibers, as well. The sensory function of the unmyelinated fibers present in the three oculomotor nerves is discussed also on the basis of our previous morphofunctional investigations. Anat Rec 260:294–298, 2000. © 2000 Wiley‐Liss, Inc.

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