Question

Describe the submucosal plexus below. When it becomes activated what is the major physiological response? When would this plexus be activated?

Answer 1

Submucous Plexus and the Control of Secretion:

Neurons in the submucous plexus of the intestine tend to be smaller than those in the myenteric plexus and contain fewer functional cell types. Nevertheless, the anatomical arrangement of the plexus is complicated with conflicting naming conventions. The ganglia include multiple populations of secretomotor neurons projecting to the gut mucosa, as well as vasodilator neurons innervating the local microcirculation. Submucous ganglia also contain IPANs “intrinsic primary afferent neurons” with receptive endings projecting to the mucosa, and with outputs to other submucous neurons or to the myenteric plexus. About two-thirds of submucosal neurons are likely to be cholinergic, whilst both secretomotor and vasodilator neurons contain some combination of VIP and NOS. Other submucous neurons contain 5-HT, substance P, or somatostatin. In the colon, some neurons in the submucous plexus project to the deeper layers of the circular muscle. These neurons are located closer to the circular muscle layer; some contain NOS and VIP and are probably inhibitory. Functions of other submucosal neurons projecting to the muscle are unknown.

Activity of submucosal secretomotor neurons is tonically inhibited by sympathetic neurons projecting from prevertebral ganglia that are separate from those responsible for the tonic inhibition of myenteric motor neurons.

The cell bodies of enteric neurons are localized in two extensive networks: the myenteric plexus (sandwiched between the circular and longitudinal muscle layers of the muscularis externa) and the submucous plexus  (or plexuses), which lies within the submucosal connective tissue. In the human colon, the myenteric plexus consists of irregularly spaced stellate ganglia joined by interganglionic connectives that resemble thick bands. Each ganglion contains, on average, 70–80 nerve cell bodies and 6–7 times as many glial cells. Together, the myenteric ganglia and connectives comprise the primary plexus. A secondary plexus (non-ganglionated) consists of nerve trunks aligned with circular muscle bundles that innervate the muscle layer and penetrate through it, en route to the submucosa. A tertiary plexus is associated with the longitudinal muscle layer and contains axons of motor neurons that innervate it.

The submucous plexus in human colon actually consists of several distinct, although interconnected, layers. These are referred to as Meissner’s plexus (referring to the innermost layer, closest to the mucosa) and Henle’s plexus or Schabadasch’s plexus (referring to the outermost layer nearest the circular muscle). Some authors have distinguished an intermediate plexus. These plexuses contain different populations of nerve cells that can be distinguished on the basis of their size and immunohistochemical coding and projection. In humans and larger animals, it is apparent that the outer submucous plexus contains motor neurons that project to the external muscle layers and presumably control motility. In contrast, the inner submucosal plexus contains more neurons that project to the mucosa and likely regulates secretomotor and vasomotor activity.

To understand how the ENS controls colonic motility, it is necessary to understand the classes of neurons, their properties, connections, and activity during different behaviors. A brief account follows; for more details, the reader is referred to extensive reviews and the present volume. Before providing this outline, it should be noted that our current understanding of the intrinsic innervation of the gut originates from a few models, mostly in the upper gut of laboratory animals; our understanding of colonic innervation in humans is limited. Moreover, few studies have used identical techniques to compare innervation of two regions of the gut in a single species and even fewer studies have compared innervation of the same region of gut between two species. Our ability to relate events at the cellular level to the real-time behavior of the whole organ is in its infancy and somewhat speculative; computational modeling will play a valuable role in the future.

Enteric neurons belong to a number of different functional classes. It is a long-standing tradition to divide nerve cells into three types: sensory neurons, interneurons, and motor neurons. However, this classification serves us poorly when applied to enteric neurons. For example, a major class of neuron, characterized by the presence of several long axons arising from the cell body, is called the Dogiel Type II neuron. These cells show mechanosensitivity and chemosensitivity and have therefore been classified as “intrinsic primary afferent neurons” (IPANs) — a subset of enteric sensory neurons. However, these neurons also function as interneurons, since they also receive, and can be excited by, prominent slow synaptic potentials. These inputs initiate long trains of action potentials. Moreover, because they partly mediate cholinergic secretomotor input to the mucosa, they could also be considered as motor (i.e., secretomotor) neurons. Indeed, many neurons that hitherto would have been classified as interneurons or motor neurons have direct mechanosensitivity, and this may be a widespread characteristic of enteric neurons. This raises questions about the names that we should use for different classes of enteric neurons; but the significance of these findings go well beyond nomenclature.

Enteric neurons have been subdivided into different classes by a variety of characteristics, including morphology, histochemical or immunohistochemical coding, electrophysiological characteristics, and so forth. Classifications that integrate these approaches have been most useful for distinguishing different functional classes of enteric neurons and describing their unique characteristics.