Innervation of the heart
The first development of the heart takes place independently of its innervation. Later, though, three differing sources for cardiac innervation can be found.
The parasympathetic innervation (cholinergic system) arises from cardiac components of the cranial neural crest cells.
The neurons of the cardiac ganglia, which represent parasympathetic neurons of the second order, migrate directly from the neural crest into the heart. Somewhat later, the axons of the first order nerves obtain access to the heart via the vagus nerve. The parasympathetic innervation slows the heartbeat.
The sympathetic nerve fibers (adrenergic system), which speed up the heartbeat as well as promote the positive inotropism of the cardiac musculature, arise from the thoracic sympathetic ganglia that in their turn come originally from the thoracic neural crest cells.
The third component of the innervation comes directly from the vagus nerve. These are sensory nerves that arise from the ectodermal placode of the nodose ganglion.
- Neural crest cells in the area of the rhombencephalon (hindbrain)
parasympathetic, efferent - Ectodermal placode of the nodose ganglion (parasympathetic, afferent)
- Neural crest cells of the trunk forming sympathetic ganglia (efferent)
- First order parasympathetic neurons from cranial neural crest cells (1)
contained in the vagus nerve (X) - Second order parasympathetic neurons from the cranial neural crest
cells (1) that immigrate directly into the heart - Sensory ganglia that originate from the ectodermal placode of
the nodose ganglion (2) - Sympathetic nerve fibers that originate from sympathetic ganglia
of the trunk
Note also: cardiac outflow tract with neural crest cell material (1)
migrated via the pharyngeal arches
In the left-hand diagram, the source of the cardiac components of the cranial neural crest for the parasympathetic (1 + 2) and sympathetic (3) innervation of the heart are to be seen.
On the right, the heart is shown isolated with the two efferent and the one afferent components involved in the cardiac innervation.
This drawing also reminds us that the conotruncus is made up of derivatives of the neural crest cells (rhombencephalon) migrating through the pharyngeal arches.
During the early development of the brain, sideways thickenings of the ectoderm appear, the ectodermal placodes. Such thickening of the ectoderm mainly results from induction processes between various types of tissue. In the rhombencephalon various ectodermal thickenings also form. Together with the local neural crest cells, they contribute to forming the sensory ganglia of the cranial nerves.