Intrinsic primary afferent neuronsof the intestine
Section snippets
The enteric nervous system and intrinsic intestinal reflexes
This review describes the properties of recently identified primary afferent neurons that have cell bodies within the wall of the gastrointestinal tract. These neurons are referred to as intrinsic primary afferent neurons, to differentiate them from the spinal and vagal primary afferent neurons that innervate the gastrointestinal tract (Fig. 1). They have a morphology that is referred to as Dogiel type II and belong to an electrophysiological class known as AH neurons.
The enteric nervous
Identification of Dogiel type II neurons as intrinsic primary afferent neurons
Considerations of their morphology led Dogiel (1899)to suggest that the type II neurons might be intrinsic primary afferent neurons. Dogiel traced processes (which he called dendrites) from these nerve cells to the mucosa and processes he called axons to other ganglia; he deduced that the projections to the mucosa were sensory and the projections in the ganglia provided outputs to other neurons. In a slightly later morphological study, Kuntz (1913)came to the same conclusion. He identified
Electrophysiological properties of AH/Dogiel type II neurons
The electrophysiological properties of the Dogiel type II neurons are influenced by the recording conditions, which can determine whether the intrinsic primary afferent neurons are active and whether there is ongoing synaptic transmission that affects their properties. Under conditions in which spontaneous activity of neurons and, therefore, background synaptic transmission are suppressed, they exhibit properties that identify them as AH neurons in the terminology of Hirst et al. (1974). In
Relation of recent findings to earlier observations
The first experiments in which recordings were made directly from intrinsic primary afferent neurons appear to be those of Wood (1970)and Wood (1973). In these experiments, records were taken from myenteric ganglia with extracellular electrodes pressed against the ganglion surface. Action potentials were recorded from a population of neurons referred to as phasic mechanoreceptors in the initial work, but referred to later as slowly adapting mechanoreceptors (Wood, 1975, Wood, 1994). The slowly
Intrinsic primary afferent neurons have characteristics that differentiate them from other mammalian primary afferent neurons
Both the morphology and the physiological characteristics of the intrinsic primary afferent neurons have some unusual features, when compared with the historically more thoroughly studied primary afferent neurons with cell bodies in cranial or spinal (dorsal root) ganglia (Fig. 1; Fig. 9). An obvious difference is that the majority of the intrinsic primary afferent neurons are multipolar, whereas neurons of the cranial and spinal ganglia are pseudounipolar. Action potentials that arise in one
Acknowledgements
The authors' studies reported in this review were supported by the National Health and Medical Research Council of Australia (grant 963213). Dr Paul Bertrand held a National Institutes of Health (NIDDK) Research Training Fellowship, # DK 09162 (USA). Dr Nadine Clerc was a Visiting Research Fellow of the University of Melbourne, on leave from the Centre National de la Recherche Scientifique, France. We thank Siobhan Lavin and Heather Woodman for excellent assistance with the illustrations and
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