Enteric motor neurons form synaptic-like junctions with interstitial cells of Cajal in the canine gastric antrum

Morphological studies have shown synaptic-like structures between enteric nerve terminals and interstitial cells of Cajal (ICC) in mouse and guinea pig gastrointestinal tracts. Functional studies of mice lacking certain classes of ICC have also suggested that ICC mediate enteric motor neurotransmission. We have performed morphological experiments to determine the relationship between enteric nerves and ICC in the canine gastric antrum with the hypothesis that conservation of morphological features may indicate similar functional roles for ICC in mice and thicker-walled gastrointestinal organs of larger mammals. Four classes of ICC were identified based on anatomical location within the tunica muscularis. ICC in the myenteric plexus region (IC-MY) formed a network of cells that were interconnected to each other and to smooth muscle cells by gap junctions. Intramuscular interstitial cells (IC-IM) were found in muscle bundles of the circular and longitudinal layers. ICC were located along septa (IC-SEP) that separated the circular muscle into bundles and were also located along the submucosal surface of the circular muscle layer (IC-SM). Immunohistochemistry revealed close physical associations between excitatory and inhibitory nerve fibers and ICC. These contacts were synaptic-like with pre- and postjunctional electron-dense regions. Synaptic-like contacts between enteric neurons and smooth muscle cells were never observed. Innervated ICC formed gap junctions with neighboring smooth muscle cells. These data show that ICC in the canine stomach are innervated by enteric neurons and express similar structural features to innervated ICC in the murine GI tract. This morphology implies similar functional roles for ICC in this species.     

3-D illustration of network orientations of interstitial cells of Cajal subgroups in human colon as revealed by deep-tissue imaging with optical clearing

Morphological changes of interstitial cells of Cajal (ICC) have been proposed to characterize motility disorders. However, a global view of the network orientations of ICC subgroups has not been established to illustrate their three-dimensional (3-D) architectures in the human colon. In this research, we integrate c-kit immunostaining, 3-D microscopy with optical clearing, and image rendering to present the location-dependent network orientations with high definition. Full-depth colonic tissues were obtained from colectomies performed for nonobstructing carcinoma. Specimens of colon wall were prepared away from the tumor site. C-kit and nuclear fluorescent staining were used to identify the ICC processes and cell body. Optical clearing was used to generate transparent colon specimens, which led to panoramic visualization of the fluorescence-labeled ICC networks at the myenteric plexus (ICC-MY), longitudinal (ICC-LM) and circular (ICC-CM) muscles, and submucosal boundary (ICC-SM) up to 300 μm in depth via confocal microscopy with subcellular level resolution. We observed four distinct network patterns: 1) periganglionic ICC-MY that connect with ICC-LM and ICC-CM, 2) plexuses of ICC-LM within the longitudinal muscle and extending toward the serosa, 3) repetitive and organized ICC-CM layers running parallel to the circular muscle axis and extending toward the submucosa, and 4) a condensed ICC-SM layer lining the submucosal border. Among the four patterns, the orderly aligned ICC-CM layers provide an appropriate target for quantitation. Our results demonstrate the location-dependent network orientations of ICC subgroups and suggest a practical approach for in-depth imaging and quantitative analysis of ICC in the human colon specimen.     

Distribution and morphological characteristics of the interstitial cells of Cajal in the ileocaecal junction of the guinea-pig

The guinea-pig ileocaecal junction including the valve was studied by immunohistochemistry to clarify the organization of the muscle bundles, the enteric nerves and the interstitial cells of Cajal (ICC). This region clearly exhibited characteristic features in the distribution patterns of ICC in a proximal to distal direction: (1) the thickened portion of the terminal ileum immediately adjacent to the ileocecal junction contained many ICC throughout the circular (ICC-CM) and longitudinal (ICC-LM) muscle layers, but ICC were few or absent in the rest of the ileum; (2) the ileal side of the valve contained ICC associated with the deep muscular plexus (ICC-DMP) as in the small intestine, whereas ICC-DMP were absent in the caecal side as in the caecum; (3) the valve contained many ICC-CM and ICC-LM in both the ileal and caecal sides; (4) many ICC associated with the myenteric plexus were observed in both the ileal and caecal sides of the valve, whereas they were only sparsely found in the caecum; (5) ICC were also observed around the submucosal plexus in a confined area of the terminal ileum and the ileocaecal valve. These observations provide morphological evidence that the terminal ileum and ileocaecal valve are specially equipped for their active involvement in the movement of the junctional area.     

Relationship between interstitial cells of Cajal and enteric motor neurons in the murine proximal colon

Interstitial cells of Cajal (ICC) are interposed between enteric neurons and smooth muscle cells in gastrointestinal (GI) muscles. The specific relationships between these cells in the murine proximal colon were studied with conventional and immunoelectron microscopy and immunohistochemistry. Intramuscular interstitial cells (IC-IM) formed discrete networks within the circular muscle layer of the murine proximal colon. Nerve trunks ran in close association with IC-IM and individual nerve trunks came into close contact with multiple IC-IM. Conventional electron microscopy revealed very close (< or = 20 nm) associations between nerve fibers and IC-IM. Processes of IC-IM also formed close contacts with neighboring smooth muscle cells. At the points of close association between neurons and IC-IM, areas of membrane densification in both pre- and postjunctional cells were present, suggesting specialized contacts or synaptic-like structures. Similar points of contact between neurons and smooth muscle cells were extremely rare. Immunoelectron microscopy demonstrated that IC-IM formed close associations with neurons containing nitric oxide synthase-like immunoreactivity (NOS-LI) or vesicular acetylcholine transporter-like immunoreactivity (vAChT-LI), suggesting innervation by both inhibitory and excitatory motor neurons. IC-IM were also labeled with anti-NOS antibodies. These observations suggest that IC-IM are an integral part of the neuromuscular junction in the colon. These cells may be the primary site of innervation, and neural regulation of the musculature may occur via IC-IM.     

Fine structural study of interstitial cells associated with the deep muscular plexus of the rat small intestine,with special reference to the intestinal pacemaker cells

Two types of interstitial cells have been demonstrated in close association in the deep muscular plexus of rat small intestine, by electron microscopy. Cells of the first type are characterized by a fibroblastic ultrastructure, i.e. a well-developed granular endoplasmic reticulum, Golgi apparatus and absence of the basal lamina. They form a few small gap junctions with the circular muscle cells and show close contact with axon terminals containing many synaptic vesicles. They may play a role in conducting electrical signals in the muscle tissue. Cells of the second type are characterized by many large gap junctions that interconnect with each other and with the circular muscle cells. Their cytoplasm is rich in cell organells, including mitochondria, granular endoplasmic reticulum and Golgi apparatus. They show some resemblance to the smooth muscle cells and have an incomplete basal lamina, caveolae and subsurface cisterns. However, they do not contain an organized contractile apparatus, although many intermediate filaments are present in their processes. They also show close contacts with axon terminals containing synaptic vesicles. These gap-junction-rich cells may be regular components of the intestinal tract and may be involved in the pacemaking activity of intestinal movement.     

Development of the myoneural junction in the rat

Summary The structure of the myoneural junction in the striated muscle of rat embryos and postnatal rats was studied by electron microscopy in order to assess at ultrastructural level the roles of neuronal and muscular elements and the sequence of events resulting in the formation of a functionally mature synaptic organization.From the observations it is concluded that the axon terminals enveloped by Schwann cells contain vesicles prior to apposition of the prospective synaptic membranes. Subsequently, subsarcolemmal thickening of the postsynaptic membrane takes place after the synaptic gap has been formed by disappearance of the teloglial cell from between the synaptic membranes but before the primary synaptic cleft in the strict sense is formed. Secondary synaptic clefts are formed later, when the primary synaptic cleft is regular in width, by local finger-like invaginations of the postsynaptic membrane, which thereafter expand basally, in a plane transverse to the axis of the axon terminal, to resemble flattened flasks. The junction is formed between multinucleated muscle cells and multiple axons, which at first lie side by side and later, when formation of adult-type secondary synaptic clefts is in progress, become separated by folds of the sarcoplasm and the teloglia. In extraocular muscles of adult rats the sarcoplasmic reticulum is closely associated with the postjunctional sarcoplasm.In the light of earlier observations on the development of contractibility after nerve stimulation, cholinesterase histochemistry and muscle fibre physiology, these observations are interpreted to indicate that functional differentiation of the myoneural synapse results from induction by the motor axon and that the association of the sarcoplasmic reticulum with the postjunctional sarcoplasm in adult extraocular muscles is related to modified fibre physiology.The author wishes to thank Prof. Antti Telkkä, M.D., Head of the Electron Microscope Laboratory, University of Helsinki, for placing the electron microscopic facilities at his disposal.     

Three-dimensional observation of the fibroblast-like cells associated with the rat myenteric plexus,with special reference to the interstitial cells of Cajal

Summary An extensive cellular network becomes visible over the myenteric plexus of the rat after removal of the overlying tissues under the scanning electron microscope. The cells are mainly stellate and have many slender processes via which they interconnect. They form a three-dimensional network and are closely associated with the ganglia and nerve bundles, and also extend over the smooth muscle cells. They are considered to correspond to the interstitial cells of Cajal because of their peculiar arrangement and their topography. Transmission electron-microscopic evidence demonstrates that the majority of those cells have features of fibroblasts. Gap junctions and intermediate junctions are observed between these fibroblast-like cells, and also between them and smooth muscle cells. Examination of serial thin sections reveals that single fibroblast-like interstitial cells connect to both circular and longitudinal muscle cells via gap junctions. It is suggested that the network of interstitial cells conducts electrical signals.     

Smooth muscle cells, interstitial cells of Cajal and myenteric plexus interrelationships in the human colon

The plane between longitudinal and circular muscle of human colon, as revealed on examination with light and electron microscopes, has no clear-cut border. Some groups of smooth muscle cells, obliquely oriented and with features similar to both circular and longitudinal ones--the connecting muscle bundles--run from one muscle layer to another. Other groups of smooth muscle cells, possessing their own specific ultrastructural features--the myenteric muscle sheaths--, make up envelopes of variable thickness around some myenteric ganglia and nerve strands, partially or completely embedding them in one or other muscle layer. Non-neuronal, non-muscular cells (interstitial cells of Cajal, covering cells, fibroblast-like and macrophage-like cells) complicate the texture of the myenteric muscle sheaths, creating an intricate, interconnected cellular network inside them, widespread among nerve bundles and smooth muscle cells; however, only interstitial cells have cell-to-cell junctions also with the smooth muscle cells and nerve endings. These data document the existence in this colonic area of two different types of muscle cell arrangements, one of which, the myenteric muscle sheath, only contains putative pacemaker cells.     

Electron microscopic observations on the innervation of the sphincter of oddi in the dog

The ultrastructure and acetylcholinesterase activity of the intrinsic innervation of the sphincter of Oddi of eight adult dogs was studied by electron microscopy. A rich distribution of unmyelinated axons embedded individually or as groups within Schwann cell cytoplasm ("innervation fasciculee"), is to be observed. A few myelinated fibres were also observed. Many of the axons are acetylcholinesterase-positive. Three main types of nerve terminals are distinguished according to their vesicle populations. Individual nerve cells or small groups of nerve cells were scattered between the smooth muscle bundles and in the lamina glandularis mucosae. The cytoplasm of some neurons contains many electron dense spherical bodies resembling "myeloid bodies", and many lysosomes. Nerve terminals synapse onto both neuronal perikarya and their dendrites. Within the nerve fascicles, close appositions between the terminals occur frequently probably representing the most peripheral inter-neuronal integrative link in the neural regulation of the function of the sphincter of Oddi. -- The gap between nerve terminals and smooth muscle cells usually measures several thousands of A. Closer appositions are seldom seen, and no synaptic complexes can be observed.     

The fine structure of smooth muscle cells and their relationship to connective tissue in the rabbit portal vein

Summary The smooth muscle of rabbit portal vein was studied by electron microscopy with particular emphasis on the mechanical linkage between the muscle cells and on the distribution of connective tissue.The media of this vein is composed of inner circular and outer longitudinal muscle layers which are orientated almost perpendicularly to each other. The muscle of the inner circular layer shows very irregular contours with much branching and anastomosing of the cytoplasmic processes, which often make membrane contacts with neighbouring cells to form an extensive network of cytoplasmic processes. The muscle cells of the outer longitudinal layer are arranged in densely packed bundles and are spindle-shaped, with no branching processes. Opposing dense areas from neighbouring cells, with variable gap distances (30–100 nm) and close membrane contacts (intermediate junctions) with a gap of 11 nm were observed in both circular and longitudinal muscle layers.In the terminal regions of muscle cells in both circular and longitudinal layers a specialized anchoring structure was present which was closely related to extracellular elastic tissue. Muscle cells in the longitudinal layer showed the most elaborate structure, the tapering end of the muscle cell showing a honeycomb-like structure penetrated by columns of connective tissue compounds. The functional implications of these structures are discussed.     

Structural characterization of interstitial cells of Cajal in myenteric plexus and muscle layers of canine colon

We have carried out a detailed ultrastructural study of the interstitial cells near the myenteric plexus of the canine colon and defined the structural characteristics which distinguish them from other resident non-neural cells. We have also examined the interconnections of these interstitial cells with nerves, the longitudinal muscle, and the circular muscle. In addition, we sought connections between interstitial cells of the myenteric plexus and those described earlier at the inner border of the circular muscle in proximal and distal colon. The interstitial cells of the myenteric plexus were structurally distinctive, and made gap junctions with one another and occasionally with smooth muscle. There seemed to be two subsets of these interstitial cells, one associated with the longitudinal muscle and the other with the circular muscle. Cells of both subsets were often close (less than or equal to 20 nm) to nerve profiles. The interstitial cells near the longitudinal muscle layer penetrated slightly into the muscle layer, but those near the circular muscle did not and neither set contacted the other. Moreover, interstitial cells of Cajal located near the myenteric plexus were never observed to contact those at the inner border of circular muscle. The interstitial cells of Cajal at the canine colon myenteric plexus are structurally organized to provide independent pacemaking activities for the longitudinal and adjacent circular muscle. Their dense innervation suggests that they mediate neural modulation of intestinal pacemaker activities. Moreover, they lack direct contacts with the interstitial cell network at the inner border of circular muscle, which is essential for the primary pacemaking activity of circular muscle. The structural organization of interstitial cells in canine colon is consistent with their proposed role in pacemaking activity of the two muscle layers.     

Interstitial cells of Cajal: mediators of communication between circular and longitudinal muscle layers of canine colon

The network of interstitial cells of Cajal associated with Auerbach’s (myenteric) plexus in the canine colon was investigated to determine its role in facilitating communication between circular and longitudinal muscle layers. Electrical coupling between the muscle layers was demonstrated by propagating extracellularly evoked electrotonic pulses from circular muscle cells to nearby longitudinal muscle cells. The likelihood of cytoplasmic continuity across Auerbach’s plexus was further demonstrated by the ability of neurobiotin to spread between the interstitial cells and the circular and longitudinal muscle cells. Importantly, direct neurobiotin spread between circular and longitudinal muscle cells was not observed even when they were in close proximity as determined by confocal microscopy. When neurobiotin did spread across the two muscle layers, the intervening interstitial cells were always neurobiotin-positive. In regions where circular and longitudinal muscle cells approach each other closely, electron microscopy revealed the presence of close appositions between interstitial cells and smooth muscle cells. Gap junctions between interstitial cells and smooth muscle cells of both layers, as judged by electron microscopy, were extremely rare. Neither gap junctions nor close appositions were observed between longitudinal and circular muscle cells. The special arrangement for electrotonic coupling across Auerbach’s plexus through interstitial cells of Cajal suggests controlled coupling between the two muscle layers, explaining the preservation of their distinct electrical activities. Received: 21 July 1995 / Accepted: 22 April 1998     

Neuronal distribution and subcellular localization of HCNP-like immunoreactivity in rat small intestine

A novel peptide, hippocampal cholinergic neurostimulating peptide (HCNP), originally purified from young rat hippocampus, affects the development of specific cholinergic neurons of the central nervous system in vitro. In this study, HCNP-like-immunoreactive nerve processes and nerve cell bodies were identified by electron microscopic immunocytochemistry in the rat small intestine. Labeled nerve processes were numerous in the circular muscle layer and around the submucosal blood vessels. In the submucosal and myenteric plexuses, some HCNP-like-immunopositive nerve cell bodies and nerve fibers were present. The reaction product was deposited on the membranes of various subcellular organelles, including the rough endoplasmic reticulum, Golgi saccules, ovoid electron-lucent synaptic vesicles in axon terminals associated with submucosal and myenteric plexuses, and the outer membranes of a few mitochondria. The synaptic vesicles of HCNP-like-positive terminals were 60–85 nm in diameter. The present data provide direct immunocytochemical evidence that HCNP-like-positive nerve cell bodies and nerve fibers are present in the submucosal and myenteric plexuses of the rat small intestine. An immunohistochemical light microscopic study using mirror-image sections revealed that in both the submucosal and myenteric ganglia, almost all choline acetyltransferase (ChAT)-immunoreactive neurons were also immunoreactive for HCNP. These observations suggest (i) that HCNP proper and/or HCNP precursor protein is a membrane-associated protein with a widespread subcellular distribution, (ii) that HCNP precursor protein may be biosynthesized within neurons localized in the rat enteric nervous system, and (iii) that HCNP proper and/or HCNP precursor protein are probably stored in axon terminals.     

Ultrastructural morphometric study of efferent nerve terminals on murine bone marrow stromal cells, and the recognition of a novel anatomical unit: the "neuro-reticular complex"

In order to extend our understanding of the role of nerve fibers in the structure and function of bone marrow stroma, we have examined nerve terminals, arterioles, and capillaries in femoral bone marrow tissues of 50 C57BL strain mice, using electron microscopy and morphometric methods. Within the adventitia of arterioles, a particular type of cell, termed periarterial adventitial (PAA) cell, is characterized by a thin veil-like cytoplasm which concentrically surrounds both nerves and arterioles. Nerve fibers containing both unmyelinated and myelinated axons are distributed mainly between the layers of PAA cells, but are found rarely on the sinus walls or within the hematopoietic parenchyma. Quantitatively, the efferent nerve terminals with many synaptic vesicles are distributed mainly beside arterial smooth muscle cells (Type I: 58.8%) or between the layers of PAA cells (Type III: 33.2%), and rarely in hematopoietic parenchyma (Type II: 5.3%) or on sinus walls (Type IV: 2.7%). In the case of Type II-IV nerve terminals, efferent (autonomic) nerves and bone marrow stromal cells which are connected by gap junctions (sinus adventitial reticular cells, intersinusoidal reticular cells, and PAA cells) appear to constitute a potential functional unit for signal conduction. We would like to propose a new term for this anatomical unit in marrow, the "neuro-reticular complex."     

Innervation of the ureterovesical junction musculature of man

The ultrastructure of the innervation of the human ureterovesical junction was studied. Three different nerve terminals were distinguished among the smooth muscle cells. 1. Nerve processes containing predominantly small granular vesicles (40--60 nm in diameter). 2. Other nerve fibres contained predominantly small round agranular vesicles (30--50 nm in diameter). 3. Processes with large granulated vesicles (80--120 nm in diameter). The first type may be adrenergic, the second cholinergic and the third may originate from the local nerve cells. The gap between the nerve fibres and muscle cells was 300 to 500 nm wide and no synaptic thickenings were observed. This suggests that the transmitter may influence several muscle cells, and the different nerve fibres may directly innervate the smooth muscle cells.     

Light and electron microscopic localization of GABA-like immunoreactivity in myenteric plexus of chicken

Whole-mounts of 1-day-old chicken midgut were incubated with an antiserum against GABA-glutaraldehyde-BSA conjugate. The immunoreaction was visualized by using the peroxidase-antiperoxidase method, and processed for consecutive light and electronmicroscopic observation. GABA was selectively localized in some of the varicose and nonvaricose nerve fibres of the myenteric plexus. The varicose fibres formed dense networks within the myenteric ganglia, some of which--mainly in duodenum--also contained immunopositive nerve cell bodies. Some of the varicose fibres projected out from the myenteric plexus into the circular muscle layer. At the electronmicroscopic level, labelled axon terminals formed synaptic contact with unlabelled perikarya and vica versa. At the same time, no labelled terminals were found on immunostained cells. In a few cases, axon terminals with GABA positivity were situated close to the smooth muscle cells in the circular muscle layer, suggesting a prejunctional GABA effect on the neighbouring nerve terminals on the release of their transmitters.     

Electronmicroscopical localization of nitric oxide synthase-containing nerve elements in cat pylorus.

Intrinsic nitric oxide synthase (NOS)-containing nerve cells and fibers were studied in the wall of the pylorus of cat at the ultrastructural level using ABC immunocytochemistry. Large numbers of NOS immunoreactive (IR) nerve cell bodies were observed in the myenteric and in the submucous plexuses, and few in the tunica propria mucosa. The NOS IR nerve fibers were most abundant in the inner circular muscle layer and in the tunica mucosa. They were found in very close vicinity to the smooth muscle cells of the inner circular muscular layer as well as to the blood vessels and the epithelial lining. The gap between the NOS IR nerve fibers and the membrane of the target cells was 20 to 250nm. Apparent synaptic contacts were observed between the IR nerve fibers and unlabelled nerve processes and other non IR nerve cell body. It is confirmed that NO might influence smooth muscle cell activity, regulate blood flow and modulate the function of the epithelial cells. Our ultrastructural study suggested that some of the NOS containing neurons belong to the intrinsic interneurons and have a regulatory effect on other intrinsic nerve elements involved in local neuronal reflexes.     

Light and electron microscopic localization of GABA-like immunoreactivity in myenteric plexus of chicken

Summary Whole-mounts of 1-day-old chicken midgut were incubated with an antiserum against GABA-glutaraldehyde-BSA conjugate. The immunoreaction was visualized by using the peroxidase-antiperoxidase method, and processed for consecutive light and electronmicroscopic observation. GABA was selectively localized in some of the varicose and nonvaricose nerve fibres of the myenteric plexus. The varicose fibres formed dense networks within the myenteric ganglia, some of which — mainly in duodenum — also contained immunopositive nerve cell bodies. Some of the varicose fibres projected out from the myenteric plexus into the circular muscle layer. At the electronmicroscopic level, labelled axon terminals formed synaptic contact with unlabelled perikarya and vica versa. At the same time, no labelled terminals were found on immunostained cells. In a few cases, axon terminals with GABA positivity were situated close to the smooth muscle cells in the circular muscle layer, suggesting a prejunctional GABA effect on the neighbouring nerve terminals on the release of their transmitters.     

Do gap junctions play a role in nerve transmissions as well as pacing in mouse intestine?

Varicosities of nitrergic and other nerves end on deep muscular plexus interstitial cells of Cajal or on CD34-positive, c-kit-negative fibroblast-like cells. Both cell types connect to outer circular muscle by gap junctions, which may transmit nerve messages to muscle. We tested the hypotheses that gap junctions transmit pacing messages from interstitial cells of Cajal of the myenteric plexus. Effects of inhibitors of gap junction conductance were studied on paced contractions and nerve transmissions in small segments of circular muscle of mouse intestine. Using electrical field stimulation parameters (50 V/cm, 5 pps, and 0.5 ms) which evoke near maximal responses to nitrergic, cholinergic, and apamin-sensitive nerve stimulation, we isolated inhibitory responses to nitrergic nerves, inhibitory responses to apamin-sensitive nerves and excitatory responses to cholinergic nerves. 18beta-Glycyrrhetinic acid (10, 30, and 100 microM), octanol (0.1, 0.3, and 1 mM) and gap peptides (300 microM of (40)Gap27, (43)Gap26, (37,43)Gap27) all failed to abolish neurotransmission. 18beta-Glycyrrhetinic acid inhibited frequencies of paced contractions, likely owing to inhibition of l-type Ca(2+) channels in smooth muscle, but octanol or gap peptides did not. 18beta-Glycyrrhetinic acid and octanol, but not gap peptides, reduced the amplitudes of spontaneous and nerve-induced contractions. These reductions paralleled reductions in contractions to exogenous carbachol. Additional experiments with gap peptides in both longitudinal and circular muscle segments after N(G)-nitro-l-arginine and TTX revealed no effects on pacing frequencies. We conclude that gap junction coupling may not be necessary for pacing or nerve transmission to the circular muscle of the mouse intestine.     

Ultrastructure of sea urchin tube feet

Summary An analysis of the ultrastructure of the tube feet of three species of sea urchins (Strongylocentrotus franciscanus, Arbacia lixula and Echinus esculentus) revealed that the smooth muscle, although known to be cholinoceptive, receives no motor innervation.The muscle fibers are attached to a double layer of circular and longitudinal connective tissue which surrounds the muscle layer and contains numerous bundles of collagen fibers. On its outside, the connective tissue cylinder is invested by a basal lamina of the outer epithelium to which numerous nerve terminals are attached. These are part of a nerve plexus which surrounds the connective tissue cylinder. The plexus itself is an extension of a longitudinal nerve that extends the whole length of the tube foot. It is composed of axons, but nerve cell bodies and synapses are conspicuously lacking, suggesting that the axons and terminals derive from cells of the radial nerve. Processes of the epithelial cells penetrate the nerve plexus and attach to the basal lamina. There is no evidence that the epithelial cells function as sensory cells.On the basis of supporting evidence it is suggested that the transmitter released by the nerve terminals diffuses to the muscle cells over a distance of several microns and in doing so affects the mechanical properties of the connective tissue.Supported by the Sonderforschungsbereich 138 of the Deutsche Forschungsgemeinschaft