Intramural neural elements in components of the carotid bifurcation

A histological basis for differential function

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✓ The histology of the specialized region of the carotid bifurcation in man was studied with Orcein stain for elastic tissue, Masson's trichrome for muscle and connective tissue, and Bodian's silver method for neural elements. Four distinct regions exist: the common carotid, which appears to be solely a conduit; the carotid sinus, which is thin and very elastic with its baroreceptors in the medial wall; the external carotid, which is highly muscular and presumably active; and the internal carotid, also highly muscular and presumably active. The transition between zones is abrupt. Prominent clusters of up to 30 or 40 multipolar nerve cells, 15 to 25 µ in diameter, were found in the subintimal region of the internal carotid artery, some in the common, and a few in the external carotid. Occasional rounder cells of similar size with only one process were seen, possibly of sensory type. No cell bodies were seen in the adventitia of any vessel. No relation to the perivascular plexus was established. It is speculated that the neurons may be related to the reported local differential response to environment by the internal and external carotid systems.

Article Information

Present address for Dr. Hall: Department of Anatomy, College of Medicine, University of Cincinnati, Cincinnati, Ohio.

Address reprint requests to: William E. Hunt, M.D., Division of Neurological Surgery, Ohio State University Hospitals, 410 W. 10th Avenue, Columbus, Ohio 43210

© AANS, except where prohibited by US copyright law.

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Figures

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    Carotid bifurcation showing common carotid (CC); external carotid (EC); internal carotid (IC); medial (m), and lateral (l) walls; angle of bifurcation (A); carotid body (CB); start of muscular internal carotid artery (M); and segment of wall containing elastic sense organ or carotid sinus (-CS-). Masson, × 5.

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    Comparison of walls of common carotid (CC); external carotid (EC); internal carotid (IC); and carotid sinus (CS); showing intima (I), media (M), and adventitia (A). The media of the common carotid is composed predominantly of elastic membranes and collagenous tissue with little smooth muscle. In the internal and external carotid the media is composed mostly of compact smooth muscle. The carotid sinus has the thinnest media, composed of circular elastic fibers. Masson, × 120.

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    Media of common carotid showing junction of the intima and media, with layering of elastic membranes (E) and interspersed collagenous tissue (C); scattered muscle fibers (M); and compact outer longitudinal muscle fibers (LM). Masson, × 219.

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    Left: Portion of media near intima showing muscle fibers (M) oriented in different directions, and elastic membranes (E). Masson, × 600. Right: Portion of media of common carotid showing muscle fibers running between (S) and inserting into the transition zone (T) where fenestrated elastic membrane (E) is seen. Masson, × 1350.

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    Medial wall of internal carotid showing angle of bifurcation (A); the media of carotid sinus (CS) composed of circular elastic fibers; transition zone (T) where muscle and longitudinal elastic reappear and large pool of autonomic neurons are located; well-developed muscular internal carotid artery (M) about 8.0 mm from angle of bifurcation; and carotid body (CB). Masson, × 5.

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    Wall of carotid sinus and medial wall of internal carotid showing intima (I); media composed of circular elastic fibers (M); and adventitia (A). Masson, × 400.

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    Media of internal carotid showing compact smooth muscle (M) layer between internal (IEL) and external elastic layers (EEL); and intima (I). Masson, × 400.

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    Neuron pool in the subintimal portion of the media of the common carotid. These presumably autonomic neurons (N) are angular, multipolar cells measuring 15 to 25 µ with several processes (p), eccentric nucleus (n), and granular cytoplasm. Intima (I); junction of intima and media (IM). Bodian, × 1450.

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    Example of small cluster of autonomic neurons (N) found in the wall of the internal carotid rostral to large pool; process (p). Bodian, × 2800.

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    Round or unipolar neuron (UN) which might be receptor neuron in reflex arc. This cell is in the common carotid not far from the neuron collection in Fig. 8. Process (p), nucleus (n). Bodian, × 2800.

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    Left: Portion of neuron pool in the wall of the internal carotid at the junction of the carotid sinus and muscular artery showing intima (I); internal elastic layer (IEL); external elastic layer (EEL); junction of intima and media (IM); and neurons (N). Bodian, × 1450. Right: higher power view of same neuron pool. Bodian, × 2800.

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    Photomicrograph of small cluster of autonomic neurons (N) found in wall of internal carotid rostral to large pool; process (p). Bodian, × 2800.

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    Photomicrograph of small cluster of autonomic neurons (N) found in wall of internal carotid rostral to large pool; process (p); nucleus (n). Bodian, × 2800.

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    Left: Photomicrograph of oval unipolar neuron near the intima of the internal carotid artery. Bodian, × 2800. Right: Oval unipolar neuron in the wall of the internal carotid artery. Bodian, × 2800.

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