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Journal of Neuroscience, Vol 11, 505-518, Copyright © 1991 by Society for Neuroscience
Synaptic responses evoked by mechanical stimulation of the mucosa in morphologically characterized myenteric neurons of the guinea-pig ileum
JC Bornstein, JB Furness, TK Smith and DC Trussell
Centre for Neuroscience, Flinders Medical Centre, Bedford Park, South Australia.
Recordings were made from myenteric neurons of the guinea-pig ileum during
reflexes evoked by mechanical stimulation of the mucosa. Impaled neurons
were injected with dye (Lucifer yellow or biocytin), and their shapes were
determined. All neurons were 5-12 mm from the stimulus, a brush stroke that
deformed the mucosal villi. Neurons were classified as S-neurons or
AH-neurons (Hirst et al., 1974). About 40% of S-neurons oral to a stimulus
responded with bursts of fast EPSPs (average frequency, 15-40 Hz); these
neurons were in ascending reflex pathways. About 60% of S-neurons anal to a
stimulus responded with similar bursts of fast EPSPs or slow
depolarizations; these neurons were in descending pathways. Only 2 of 48
AH-neurons responded, both in descending pathways. Most S-neurons in either
ascending or descending pathways received inputs from at least 2 or 3 other
neurons. Action potentials evoked during a response averaged 3-10 Hz in
frequency, with occasional bursts at up to 100 Hz. The speed of conduction
along the reflex pathways was about 0.5 m/sec. All S-neurons were
uniaxonal, but they differed in size, dendritic morphology, and
projections. The axons of S- neurons injected with biocytin were followed
up to 7 mm within the myenteric plexus. Three S-neurons projected to the
tertiary plexus and were probably longitudinal muscle motor neurons; 2 of
these were in descending pathways. Five S-neurons projected along the
intestine and had varicose collaterals in some ganglia. These neurons were
probably interneurons; 3 were descending and 2 ascending, and all responded
in the appropriate reflex pathway. Many S-neurons had short axons that
entered the circular muscle and were probably circular muscle motor
neurons. Others projected several millimeters along the intestine before
entering the circular muscle or fading beyond detection. From this study,
we have been able to deduce the circuits mediating ascending and descending
mucosa-to-muscle reflexes. It is concluded that AH-neurons are primary
sensory neurons and S-neurons are interneurons and muscle motor neurons in
the circuits.
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