Unique properties of proximal and distal colon reflect distinct motor functions

Abstract

The gastrointestinal (GI) tract is made up of specialized organs that work in tandem to facilitate digestion. The colon regulates the final steps in this process where complex motor patterns in proximal regions facilitate formation of fecal pellets that are propelled along the distal colon via self-sustaining neural peristalsis and temporarily stored prior to defecation. Historically, our understanding of colonic motility has focused primarily on distal regions, and the intrinsic reflex circuits of the enteric nervous system (ENS) involved in neural peristalsis have been defined, but we do not yet have a clear grasp on the mechanisms orchestrating motor function in proximal regions. New approaches have brought to the forefront the unique structural, neurochemical, and functional characteristics that exist in distinct regions of the mouse and human colon. In this mini-review, we highlight key differences along the proximal-distal colonic axis and discuss how these differences relate to region-specific motor function.

Graphical Abstract

Created in BioRender. Morales-Soto, W. (2025) https://BioRender.com/n49x053

Introduction

The gastrointestinal (GI) tract exemplifies the intricate relationship that exists in physiology between structure and function, where individual parts are uniquely adapted to perform specific tasks essential for homeostasis. Nuances in physical, cellular, and molecular properties equip the GI tract with the proper machinery to handle luminal contents ranging in composition while simultaneously protecting against region-specific threats. For instance, the stomach harbors specialized parietal cells that contribute to acidic conditions required for digestion, but the thick mucus layer produced by goblet cells shields the stomach lining from damage (1, 2). Different regions within an individual organ can even be responsible for distinct functions. In the colon, for example, proximal regions promote water and electrolyte reabsorption and form luminal contents into solid, discrete fecal pellets; distal regions are responsible for moving formed pellets aborally, where they are temporarily stored until defecation (3). To perform these tasks, a diverse set of motor patterns must be engaged along the proximal-distal axis of the colon to facilitate mixing and segmentation in proximal regions and peristalsis and expulsion in distal regions (3–5).

The colorful palette of motor behaviors in the GI tract arises from the numerous spatiotemporal permutations of smooth muscle contractions and relaxations driven by activity in myenteric neurons of the enteric nervous system (ENS) and/or interstitial cells of Cajal (ICC) and the neurogenic and myogenic motor patterns they generate, respectively. In addition to intrinsic control by the ENS and ICC, there are sources of sensory-motor control via extrinsic nerve pathways from prevertebral ganglia, the spinal cord, and brainstem (6, 7). Intrinsic ENS and extrinsic neural networks interact with one another and several other cell-types, including ICC (8, 9), PDGFRα (7, 10), enteric glia (11–13), and enteroendocrine cells (EECs) (14), to shape various aspects of GI motility. Specifically in the colon, our understanding of the mechanisms underlying motility has focused primarily on distal regions and defining the ENS-circuits for neural peristalsis. However, there are likely different mechanisms for engaging the complex motor behaviors required to process and shape contents in proximal colon regions. In recent years, new technologies and innovative approaches have brought to the forefront the unique characteristics in proximal and distal colon that underscore their distinct roles in digestion (4, 7, 14–18). In this mini-review, we summarize key differences in the neural mechanisms utilized by proximal and distal colon regions for motility, discuss their impact on motor function, and highlight remaining questions that represent new and exciting areas of research waiting to be explored.

Regional tuning of colonic motor patterns

The colon facilitates removal of digestive waste by transforming amorphous luminal material coming from the cecum into solid, discretely formed pellets and shuttling them towards the distal colon to be eliminated upon defecation. To accomplish this, each part of the colon invokes motor patterns that are tuned to serve region-specific tasks (Table 1). In fact, a functional transition exists at the “colonic flexure” naturally dividing the colon into proximal and distal regions. In herbivores (e.g. rabbits, guinea pigs), it is a clear anatomical distinction, but in omnivores (e.g., mice, humans), the flexure represents the location where contents are transformed from a contiguous, semi-liquid material to solid pellets. As such, predominant motor patterns in regions proximal of the flexure are designed to promote fecal pellet formation, whereas motor patterns in regions distal of the flexure primarily drive fecal pellet propulsion, and the mechanisms driving these motor behaviors are likely distinct.

Table 1.

Definitions of the anatomical and functional terminology discussed in this review.

Terminology and Definitions
Colonic flexure	Functional transition (and anatomical distinction in herbivores) in the colon where amorphous fecal contents become solid, discrete pellets
Proximal colon	Segment of colon proximal to the colonic flexure
Distal colon	Segment of colon distal to the colonic flexure
Colon motor complex*	Spontaneous bouts of motor activity observed as clusters of rhythmic contractions that are regularly initiated in proximal colon and migrate varying distances
Colon migrating motor complex*	CMC that migrates along the distal colon
Neural peristalsis	Self-sustaining and continuous propagation of fecal pellets from the colonic flexure to the anus initiated and maintained by distension
Ripples	Myogenic contractions generated by submucosal ICC

^*Depending on experimental conditions, some CMCs and CMMCs originate in distal colon and migrate orally (i.e., retrograde)

Different types of motor patterns, both spontaneous and stimulus-evoked, have been recorded in humans and animal models with in vivo and ex vivo experimental approaches (5). One of the most widely measured motor patterns in freshly excised isolated colons is the colonic motor complex (CMC; also known as colonic migrating motor complexes or CMMCs) that consists of regular bouts of neurally-mediated motor activity (3–5, 19–21) correlated with propulsion of fecal content (22). Generated every 2–5 minutes, CMCs can be observed as clusters of rhythmic contractions that typically originate in proximal regions and migrate aborally toward the distal colon. CMCs in proximal and distal colon exhibit remarkable differences in propagation length and speed, how smoothly contractions advance along the colon, and how they are initiated (3–5, 19–21). In fact, recordings from rabbit, guinea pig, and mouse colon suggest that these motility behaviors represent at least two distinct motor patterns (and possibly a third) that serve unique functions, predominate in different colon regions and, although both are dependent on neural activity, they are likely driven by distinct mechanisms. CMCs in proximal colon, also known as “proximal CMCs” or “incomplete CMCs” propagate slowly across short distances and exhibit intermittent advancement of circular contractions capable of pinching off pellet-sized boluses from contiguous contents at regular intervals (4, 21). Some proximal CMCs continue to propagate along the entire colon length, usually in the presence of luminal pressure, fecal contents, or some other applied sensory stimulus. These “complete CMCs” (or CMMCs) can initiate discontinuous propulsion of pellets at the flexure, but do not facilitate sustained propulsion, oftentimes passing over solid fecal pellets in distal areas (21). “Neural peristalsis” represents the other major motor pattern that is predominantly in distal regions of the colon and characterized by faster, continuous, and self-sustaining propagation of fecal pellets all the way to the anus (19, 21, 23).

Extensive efforts have gone towards defining the intrinsic ENS circuits underlying neural peristalsis in distal colon. According to the current ‘neuromechanical loop’ hypothesis, peristalsis is initiated and sustained by a solid bolus that distends the colon wall, sequentially activating the polarized enteric neural pathways responsible for propelling it forward (19, 24, 25). Distension activates intrinsic primary afferent neurons (IPANs) that engage ascending excitatory and descending inhibitory enteric neural networks to induce oral contraction and aboral relaxation of smooth muscle, respectively. This propels the bolus forward, leading to distension of the adjacent area and activation of a new set of polarized reflexes that repeat the process, thus ensuring that the self-sustaining motor pattern reaches the end of the colon (6, 19). Because the mechanisms are better defined, neural peristalsis is often solely used to describe colon motility but does not offer a comprehensive understanding of motor patterns along the entire colon, particularly in proximal regions. The defining feature of neural peristalsis is the sensory activation of ENS circuits by fecal pellet-induced distension (23, 26), but CMCs in proximal colon are regularly generated, even in the absence of luminal contents, and do not exhibit self-sustaining propagation (21, 27, 28). Furthermore, CMCs continue to occur rhythmically in the proximal colon after it has been separated from the distal end, but CMCs in isolated distal colon can only be evoked by stretching the tissue (28). Therefore, the mechanisms for CMC initiation and propagation in proximal colon appear to be rhythmic in nature, independent of luminal contents, and thus, distinct from the distension-activated mechanisms utilized in distal colon for neural peristalsis.

The spontaneous and repetitive generation of CMCs at regular intervals is a striking, yet perplexing, phenomenon. The fact that CMCs regularly originate in proximal colon seemingly without sensory input (e.g., luminal contents, distension) suggests that an intrinsic pattern generator (or pacemaker) exists in proximal regions capable of orchestrating activity in CMC motor networks in a self-regenerating and rhythmic manner. The identity of this pacemaker remains unknown, but recent studies using in situ calcium imaging and optogenetics have provided insights into key players. It is well established that CMCs require ENS activity, and episodes of synchronous activity across ENS networks firing at a rhythm of ~2 Hz have been observed during CMCs in the mouse distal colon (20, 29). However, it is unknown if the same activity pattern also exists in proximal regions where CMCs are initiated, and more importantly, it is not clear what triggers these regular bouts of coordinated activity in the ENS. Several neurochemical classes of enteric neurons have been targeted using optogenetic approaches to determine how they influence CMCs. Activation of calretinin- (30), choline acetyltransferase (ChAT)- (31), or nitric oxide synthase (NOS)-expressing neurons (in the proximal colon) (32) can elicit anterograde CMCs, and stimulation of Cdh6 IPANs in the distal colon can evoke retrograde CMCs (33). However, the exact mechanism driving CMC rhythmicity remains to be determined. Rather than a single cell-type representing the pacemaker, it may be that the intrinsic pattern generator for CMCs consists of complex interactions among multiple cell-types.

New evidence has emerged suggesting that the mechanisms for CMC initiation and propagation may involve neurogenic and myogenic interactions between the ENS and ICC, the cells traditionally known as ‘pacemakers’ of the gut (8). In the colon, the ICCs near the submucosal border (ICC-SM) generate electrical oscillations that propagate via gap junctions into smooth muscle cells of the circular muscle as ‘slow waves’ and cause ripple contractions, likely involved in mixing fecal material and fluid absorption but not in the propulsion of fecal contents (34). A separate network of ICCs located near the myenteric plexus (ICC-MY) also generates electrical oscillations at higher frequencies, but this activity is normally under tonic nitric oxide (NO)-dependent inhibition (5). When released from inhibition, interactions between the two ICC pacemaker systems can occur and produce slow phasic contractions in proximal regions near the colonic flexure, and similar oscillations have been observed in motor activity leading up to CMCs (35, 36). This suggests that dynamic interactions between nitrergic neurons and myogenic processes may be involved in the generation of CMCs, but the use of genetic knock-out mouse models and pharmacological approaches have yielded conflicting results (32, 34, 37–39). Whereas some studies report no changes in CMCs, others have reported that CMCs become uncoordinated and no longer migrate properly, and yet other groups report that CMCs are abolished but myogenic phasic contractions are increased. These discrepant results seem to be, in part, due to different interpretations regarding the nature of the remaining contractions but also suggest that other approaches with better spatial and temporal control are required to untangle this highly complex and dynamic system. Indeed, when Koh et al. incorporated optogenetics with pharmacology and knock-out mouse models, they found evidence that post-stimulus excitation following nitrergic responses in ICC are responsible for CMC initiation (32). In any case, it is well accepted that the colon is typically under tonic inhibitory control during intervals between CMCs and that release from inhibition must occur for the generation of CMCs. However, like neural peristalsis, post-stimulus excitation and disinhibition both rely on an inciting factor to trigger the cascade of events leading to a CMC, and neither fully explains the intrinsic CMC rhythmicity observed in the proximal colon.

Preliminary work from our group suggests that the rhythmicity of proximal CMCs may be associated with cyclical interactions among ICC-generated ripples, IPANs, and enteric motor neurons (28). Although further experimental testing is required, the rhythmic nature of CMCs may be due to cyclical, self-regenerating interactions between myogenic and neurogenic processes. We have observed repeated and predictable changes in the organization of activity across ICC-SM networks that coincide with CMC timing and directly correlate to the magnitude of the ripple contractions they produce. Leading up to a CMC, ICC-SM become organized and ripples reach peak amplitude just before the onset of robust ENS activity during a CMC, whereas ICC-SM are disorganized and ripples are weakest immediately following the CMC. Based on our observations, we propose an enticing new idea that ICC-generated ripple contractions are sensed by a unique subset of IPANs that recruit activity in ENS motor networks to drive CMC contractions (40), and during the CMC, local neuronal feedback onto ICC-SM networks transiently disrupts their ability to organize (28), potentially via muscarinic receptor and IP3-dependent mechanisms (41), thus resetting the system. We have demonstrated in a proof-of-concept computational model that ICC-IPAN-ENS interactions can drive rhythmic CMCs (42) but experiments that test the pharmacological basis and functional interactions among these cellular components are still required.

Nevertheless, the proximal and distal regions of the colon appear to utilize distinct mechanisms to facilitate motility. Motor patterns in proximal colon are rhythmic and intrinsically generated, most likely to ensure the timely and continuous shaping of colonic contents. Once solid and discrete pellets have reached the colonic flexure, distension-activated neural peristalsis takes over as the predominant mechanism utilized by the distal colon for propulsion. Thus, there is an intricate interplay between proximal and distal motor patterns that highlight how regional specialization within the colon facilitates its overall function.

Regional differences in intrinsic innervation of the colon

The myenteric and submucosal plexuses of the ENS exhibit remarkable regional differences in organization and ‘neurochemical code’ along the proximal-distal colonic axis that reflect their ability to perform specific tasks (43). This review aims to highlight the differences in regional colon motility; thus, our discussion will focus on the myenteric plexus.

The proximal colon is characterized by a higher density of myenteric neurons and larger ganglia within the myenteric plexus that is tasked with mixing, processing, and forming fecal contents into solid and discrete pellets (16, 44). This segment encompasses a unique subpopulation of CGRPa+ myenteric neurons that co-express calretinin, calbindin, ChAT, and SP, form extensive and complex synaptic contacts with numerous other neurons within the same ganglion, and may represent a unique IPAN subtype only found in proximal regions of the colon (16). We have recently shown that activity in CGRPa+ IPANs correlates to ripple contractions that increase in intensity leading up to CMCs, and optogenetic activation can re-entrain CMC rhythms (40), making this IPAN population a prime candidate for mediating the ICC-ENS interactions (described above) proposed to be involved in CMC initiation. Furthermore, the proximal ENS exhibits greater and longer-lasting responses to electrical and cholinergic input and appears to make more synaptic contacts compared to distal regions (16, 44), likely supporting the gradual summation (or build-up) of neuronal activity that precedes spontaneous and evoked CMCs in proximal colon (16).

The distal colon has sparser distribution of smaller ENS ganglia, reflecting a simpler, spread-out architecture adapted for propagating contents across distances (16, 43, 44). Although the distal colon is more enriched with inhibitory motor neurons (NOS+ and VIP+) compared to proximal colon (16, 43), the primary neurotransmitter mediating inhibitory junction potentials changes from NO-predominant in more proximal regions to ATP-predominant in more distal regions. Accordingly, NO has been associated with sustained relaxation ideal for receiving contents in proximal colon, while the fast, transient ATP-mediated relaxations are optimal for neural peristalsis along the distal colon (45). Whereas proximal ENS exhibits stronger responses to acetylcholine, the distal colon ENS responds more robustly to serotonin (5-HT) (16) and is more enriched with the excitatory 5-HT receptor HTR3A (18). Accordingly, specialized EECs that release 5-HT in response to stimulation, i.e., enterochromaffin cells (46), are transcriptionally distinct along the proximal-distal gut axis. Distal EECs are enriched in genes for mechanosensitive channels like Piezo2 (47), supporting the argument that distal colon regions may be preferentially tuned to activate motor patterns in response to luminal, mechanosensory activation of the epithelium (14).

Extrinsic innervation of proximal versus distal colon

Recent advancements in the anatomical, molecular, and functional characteristics of the colon’s extrinsic neural innervation have shed light on bidirectional communication with the central nervous system and how distinct pathways control motor function. Notably, the colon receives neural input from three extrinsic sources: the vagus, splanchnic, and pelvic nerves each contain afferent fibers from nodose or dorsal root ganglia (DRG) sensory neurons, as well as autonomic motor efferent fibers originating from the brainstem, thoracolumbar or lumbosacral spinal cord (48). Matching the high degree of heterogeneity in intrinsic innervation between the proximal and distal colon, distinct structural and functional properties of extrinsic innervation to different colon regions have further emphasized their diverse roles.

The vagus nerve has several branches running along the length of the GI tract with projections that can extend to the colon (49–52) but because of the technical challenges introduced by its long, “wandering” path, determining the structural and/or functional differences in vagal input to proximal and distal regions of the colon requires further investigation. The degree of vagal innervation is generally believed to decrease from proximal to distal regions of the GI tract with no vagal innervation reaching the rectum. However, studies using distinct neuroanatomical tracing approaches (e.g., retro- versus antero-grade tracers) have yielded slightly different results, with one showing similar innervation of proximal and distal regions (50), another reporting less extensive innervation of distal regions compared to proximal (49), and yet another finding no innervation of distal regions (52). Functional studies have shown that vagal stimulation elicits contractions from proximal and distal colon, suggesting innervation of both regions, but responses may be due to direct or indirect connections (51). Information from the vagus nerve converges within the nucleus tractus solitarius (NTS) of the brainstem and projects to brain structures involved in autonomic control, metabolism, satiety, and affect, thus integrating crucial information from the colon for overall intestinal digestion and GI motility. Future research should investigate the types of sensory modalities vagal afferents in the colon respond to and whether effector functions are mediated by vago-vagal reflexes or via descending spinal cord pathways.

The two spinal pathways associated with the pelvic and splanchnic nerves play distinct roles in regulating colon motility with significant differences in their influence on proximal and distal colon function (48). Sensory neurons located in lumbosacral DRG preferentially innervate distal regions of the colon (50, 53) and express the mechanosensitive ion channel Piezo2 (50, 54), and studies have shown that these afferents respond to a wide-range of mechanical stimuli (e.g., mucosal brushing, stretch) (53, 55). Electrical stimulation along the lumbosacral-pelvic nerve pathway increases ENS activity and induces contractions (56). Therefore, sensory-motor circuits at the level of the lumbosacral spinal cord and pelvic nerve appear to be involved in sensing and integrating mechanical cues from the distal colon to augment propulsive contractions and mediate defecation (48).

The other sensory-motor spinal circuit resides at the level of the thoracolumbar spinal cord, but this pathway is associated with the splanchnic nerves and sympathetic prevertebral ganglia. Sensory neurons in thoracolumbar DRG innervate both proximal and distal regions of the colon, express neuropeptides associated with neurogenic inflammation, and typically have higher thresholds for activation, indicating their likely involvement in transmitting inflammatory pain signals (50). The motor efferents of this pathway have distinct effects on colon motility in proximal and distal regions, partly due to their influence on myogenic versus neurogenic motor patterns. For example, electrical stimulation of postganglionic motor neurons in prevertebral ganglia decreases propulsion in the distal colon by suppressing ENS activity involved in neural peristalsis, but the same stimulus promotes myogenic ripple contractions in proximal regions by increasing the frequency of ICC oscillations (7). Thus, the thoracolumbar-splanchnic nerve pathway likely plays a pivotal role in coordinating interactions between neurogenic and myogenic motor patterns, adjusting CMC rhythmicity and regulating the timing of propulsion of contents from proximal to distal colon (48, 50). The distinctions in spinal pathways underscore their specialized functions for maintaining colon physiology, and although their individual contributions are still being defined, future studies should also focus on how vagal and spinal pathways interact with one another to regulate colon homeostasis and determine the neuroplastic changes that occur in disorders of gut-brain interaction.

Key questions for future investigation

Until recently, most of what we know about colon physiology has come from studies of distal regions, but emerging evidence using new approaches indicates that significant differences exist between the proximal and distal colon regarding motor activity, ENS organization and neurochemical make-up, and neural innervation from the central nervous system. In this review, we have compiled existing evidence that highlight these distinct properties of the proximal and distal colon and bring forth neuronal and non-neuronal cell types that may contribute to the region-specific mechanisms underlying these differences (Figure 1). Some of the evidence we present here is based on observational findings, and as such, further investigation is required to address the functional contributions of these cell types to motor networks in the colon and how they vary between regions. Recent advances in single-cell sequencing have uncovered a wealth of molecular targets for both intrinsic (18) and extrinsic (50) neural networks that provide a promising framework for unraveling the mechanisms driving proximal motor patterns and how they differ from those regulating neural peristalsis in the distal colon. Additionally, these technologies raise important questions about the unique sensory and motor functions of the extrinsic nerve pathways that innervate the colon. Cutting-edge approaches, such as optogenetics (7, 14, 56) and spatial transcriptomics (57), offer unprecedented spatial and temporal resolution that provide the ability to probe these questions with greater precision and uncover the intricate mechanisms underlying regional differences in colonic function. We highlight below some key questions for future investigation:

Figure 1.

Summary schematic highlighting differences between proximal and distal colon structure and function. Created in BioRender. Morales-Soto, W. (2025) https://BioRender.com/c12w146.

• What are the mechanisms underlying proximal CMCs, and how are they different than the mechanisms driving neural peristalsis in distal colon?

• Are there unique sensory and motor functions for extrinsic nerve pathways that innervate the colon, and do they affect proximal and distal regions differently?

• How does activity in one region of the colon modulate activity in the other region?

• What are the regional differences in pathophysiological mechanisms underlying diseases of the colon?

In summary, this review emphasizes the critical need to re-evaluate our thinking of the colon as the sum of two distinct parts rather than a single uniform organ. The distinct structural and functional properties of proximal and distal regions contribute to normal colon physiology but also introduce the potential for regional differences in pathophysiological mechanisms underlying GI disease that oftentimes present in a region-specific manner, e.g., irritable bowel syndrome, inflammatory bowel disease, colon cancer. Because current research related to proximal colon is far behind that of its distal counterpart, there are numerous opportunities for investigators, both established and new to the field, to dive into this uncharted territory for a comprehensive understanding of colon function, from one end to the other.