## 3 Neurons and Neurochemistry of the Enteric Nervous System  
The oldest classification of enteric neurons was based on morphology [29]; this usually distinguished two morphological subpopulations: 'Dogiel type II' neurons with several long processes and 'Dogiel type I' neurons with numerous short processes. However, the restrictive dichotomy of the morphological data does not reflect the well-known chemical and functional diversity of enteric neurons. Electrophysiological classification of enteric neurons is also useful in defined circumstances. The first experiments to capture their electrical properties (which later provided the basis for classification) were recorded in the 1970s [30; 31; 32], where S (synaptic)- and AH (after-hyperpolarizing)-type neurons were identified. Despite the practicality of this classification, this kind of nomenclature has become obsolete now. Other functional subsets of enteric neurons can be inferred based on their axonal projection patterns, their synthesizing enzymes, calcium-binding proteins, cytoskeletal proteins and expression of primary transmitters [33; 34]. Nowadays, the enteric neurons are mostly divided into four main classes: motoneurons, intrinsic primary afferent neurons (IPANs), interneurons and intenstifougal neurons. Motoneurons directly innervate the smooth muscle, blood vessels or glands, while intrinsic primary afferent neurons process signals from the intestinal microenvironment and transmit them to interneurons. The intenstifougal neurons are a special group of neurons that synapse directly with neurons of the prevertebral sympathetic ganglia. Altogether, 14 different subtypes had been described previously (summarized in Table 2), but in a recent study, a new intrinsic afferent neuron subtype was also identified [35]. The substances that act as neurotransmitters in the central nervous system are also found in the enteric nervous system, so that in addition to the classical neurotransmitters, several neuropeptides, and other neurotransmitters (adenosine triphosphate (ATP) and nitric oxide (NO)) also play an important role. To date, 20 different enteric neuron types and projections have been isolated, each using a different neurotransmitter combination (chemical coding). \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Cell Type** & **DC\({}_{\text{LM}}\)** & **MP\({}_{\text{LM}}\)** & **MP\({}_{\text{SM}}\)** & **MP\({}_{\text{M}}\)** & **MP\({}_{\text{My}}\)** & **MP\({}_{\text{IG}}\)** \\ \hline \multirow{8}{*}{Markers} & CD45 & & & CD45 & & \\  & MHCII & CD45 & CD45 & & CD11b & MP\({}_{\text{M}}\) markers & \\  & CD11b & MHCII & MHCII & F4/80 & glycine-, & \\  & F4/80 & CD11b & CD11b & CX3 CR1 & nicotinic & \\  & CX3 CR1 \({}^{+}/^{-}\) & F4/80 & F4/80 & Retn1a & acetylcholine-, & \\  & CD11c & CX3 CR1\({}^{\text{low}}/^{+}\) & CX3 CR1\({}^{\text{low}}/^{+}\) & Mrc1 & adrenergic-, & CSF1R \\  & CD103 \({}^{+}/^{-}\) & connexin-43 & CD11c\({}^{\text{low}}/^{+}\) & CD163 & purine & \\  & connexin-43 & & & TRPV4 & receptors & \\  & & & & BMP2 & & \\ \hline \multirow{2}{*}{Morphology} & amoeboid & amoeboid & amoeboid & bipolar & & \\  & & & & star shaped & & star shaped \\ \hline \multirow{2}{*}{Presence} & Lamina propria & Lamina propria & & & & Within the \\  & mucosae & mucosae & Submucosa & Muscularis & Myenteric & ECM barrier of \\  & & & & & plexus & myenteric \\  & & & & & ganglions \\ \hline \multirow{2}{*}{Role} & antigen & & supporting of & & development of & \\  & presentation, & blood vessels and & & regulation of & the myenteric & phagocytosis of \\  & TC activation & phagocytosis & & gestrointestinal & plexus, & apoptotic \\ \hline \end{tabular}
\end{table} 
Table 1: The heterogeneity of intestinal macrophages according to surface markers, morphology, localization and function. The neurons of the mammalian enteric nervous system can be divided into three major groups based on their transmitter molecules: cholinergic, catecholaminergic and the so-called non-adrenergic cholinergic (NANC) groups of neurons.