The distribution of nerves immunoreactive to protein gene product
9.5
(PGP 9.5), tyrosine hydroxylase
(TH), neuropeptide Y (NPY), substance P (SP) and calcitonin gene related
peptide (CGRP) antisera was
investigated in the atrioventricular valves of the Sprague–Dawley
rat and the Dunkin–Hartley guinea pig
using confocal and epifluorescence microscopy. No major differences were
noted between the innervation of
the mitral and tricuspid valves in either species. For all antisera the
staining
was more extensive in the
guinea pig valves. Two distinct nerve plexuses separated by a ‘nearly
nerve free’ zone were identified in both
species with each antiserum tested. This was most apparent on the anterior
cusp of the mitral valve. The
major nerve plexus extends from the atrioventricular ring through the
basal, intermediate and distal zones of
the valves towards the free edge of the valve cusp. These nerve bundles,
arranged as primary, secondary and
tertiary components, ramify to the free edge of the valve and extend to
the
attachment of the chordae. They
do not contribute to the innervation of the chordae tendineae. The second,
minor chordal plexus, runs from
the papillary muscles through the chordae tendineae and passes parallel
to the
free edge of the cusp. The
nerves of this minor plexus are interchordal, branching to terminate mainly
in the distal zone, free edge of
the valve cusp and adjacent chordae tendineae. Some interchordal nerve
fibres
loop from a papillary muscle
up through a chorda, along the free edge and pass down an adjacent chorda
into another papillary muscle.
The nerve fibres of the major and minor plexuses intermingle although no
evidence was found for
interconnectivity between them. In the distal zone between the major plexus
which
extends from the base of
the valve and the minor chordal plexus there is a zone completely free
of
nerves staining with antisera to
TH and NPY. Occasional nerves which stained positive for PGP 9.5, SP and
CGRP immunoreactivities
crossed this ‘nearly nerve free zone’ passing either from
the chordal/free edge nerves to the intermediate
and basal zones or vice versa. An additional small nerve plexus which displayed
immunoreactivity to CGRP
antiserum extended from the atrioventricular ring into the basal zone of
the valve cusp. Not all chordae
tendineae displayed immunoreactive nerve fibres. It is concluded that
the innervation patterns of the sensory
and sympathetic neurotransmitters and neuropeptides examined in the
atrioventricular valves of the rat and
guinea pig are ubiquitous in nature. The complexity of the terminal innervation
network of the mammalian
atrioventricular valves and chordae tendineae may contribute to the
complex functioning of these valves in the cardiac cycle.