Rj. Phillips et Tl. Powley, Tension and stretch receptors in gastrointestinal smooth muscle: re-evaluating vagal mechanoreceptor electrophysiology, BRAIN RES R, 34(1-2), 2000, pp. 1-26
Electrophysiological and morphological analyses of vagal mechanoreceptors i
n the gut wall suggest conflicting conclusions. Electrophysiology has disti
nguished a single general class of ending in smooth muscle, one characteriz
ed as an 'in series' tension receptor. Morphology, in contrast, has charact
erized two distinct specializations of vagal afferent endings in the muscle
wall of the gastrointestinal (GI) tract. These two structures differ in te
rms of their target tissues, terminal architectures and regional distributi
ons; they also develop on different ontogenetic timetables and depend on di
fferent trophic support in the muscle wall. On the basis of these features,
we have proposed that one of the putative mechanoreceptors, the intragangl
ionic laminar ending (IGLE), has characteristics of a tension receptor and
the other, the intramuscular array (IMA), has features of a stretch or leng
th receptor. In a functional analogy with striated muscle proprioceptors, I
GLEs should have similarities to Golgi tendon organs, whereas IMAs should h
ave equivalencies with muscle spindle afferents. The present survey re-exam
ines the recording analyses in light of the structural observations. This r
eview indicates that previous electrophysiological studies are too inconclu
sive to refute the inference that the vagus supplies two distinct types of
mechanoreceptors to the muscle wall of the GI tract. Multiple methodologica
l constraints and sources of variance have limited the resolution of electr
ophysiological experiments. Specifically, these experiments have convention
ally used distension stimuli that confound tension and stretch. In addition
, sampling strategies have biased recording experiments towards a focus on
one type of ending, the IGLE. Furthermore, putative functional properties (
e.g., broad tuning) of vagal mechanoreceptors suggest that distinguishing t
wo recording patterns will require exacting protocols. Combining a recognit
ion of the methodological difficulties Chat have limited electrophysiologic
al analyses with an understanding of the structural features of the endings
, however, suggests several critical electrophysiological experiments with
the resolution to distinguish two classes of response profiles. Until such
experiments can be conducted, sensory physiology's axiom that 'function Var
ies with form', taken together with a re-assessment of the existing data, s
uggests that the vagus nerve supplies stretch receptors as well as tension
receptors to the wall of the GI tract. (C) 2000 Elsevier Science B.V. All r
ights reserved.