Colon and rectum

A tube of variable diameter which is approximately 150cm in length, extending from the ileocaecal junction to the anus

Embryology

- Primitive gut tube is formed from the endodermal roof of the yolk sac. In the third week, it divides into foregut, midgut, and hindgut.
  - Midgut - includes distal duodenum, small bowel, right colon, and proximal two-thirds of the transverse colon - supplied by SMA
  - Hindgut - distal third of transverse colon, descending and sigmoid colons, and rectum, down to upper anal canal - supplied by IMA
- Caecal bud appears at about the 6th week
  - Last part of the bowel to re-enter the abdomen after physiological herniation
- Appendix - mid-gut organ - first identified at 8-20 weeks as a small outpouching of the caecum. Becomes more elongated and tubular as the caecum rotates medially and becomes fixed in the RLQ.
- Rectum
  - Proximal - develops similar to the colon
  - Distal - develops from the terminal hindgut that enters into the cloaca (an endoderm-lined cavity in contact with the surface ectoderm at the cloacal membrane). The cloaca separates into an anterior urogenital sinus and a posterior distal rectum and anal sinus at about 5 weeks.
  - External anal sphincter formed by the posterior part of the cloacal sphincter, and the internal anal sphincter is formed from enlarging circular fibres of the rectum.
- Anal canal
  - See separate topic

Gross anatomy

Caecum

- - Saccular beginning of the colon
  - Average diameter 7.5cm and length of 10cm
  - No mesentery - usually completely covered by peritoneum, and is therefore considered an intra-peritoneal structure
  - Variably connected to the posterior abdominal wall by a peritoneal reflection
  - Variable mobility - hyper-mobile caecums can predispose to volvulus or bascule (intermittent anterior and superior folding of the caecum, associated with obstructive symptoms)
  - Thin wall compared to rest of colon, and large diameter, and therefore the site most likely to perforate (Laplace's law)
    - Acute dilation to >12cm is associated with risk of ischaemic necrosis and perforation and requires prompt treatment
  - Terminal ileum empties into the caecum along the posteromedial border through the ileocaecal valve
    - Two semilunar mucosal lips that fuse, regulating ileal emptying by preventing backward reflux of colonic contents
    - 20% incompetent
    - Can produce a closed loop obstruction if competent and LBO is present
    - Angulated entry maintained by superior and inferior ileocaecal folds

Vermiform appendix

- - Blind-ending tubular structure arising from posteromedial caecum approximately 3cm from ICV
    - 5-35cm in length, average length 8-10cm
    - Average diameter of 5mm
  - Position varies depending on fetal development:
    - Retrocaecal 65%
    - Pelvic 31%
    - Sub-caecal 2.3%
    - Pre-ileal 1%
    - Retro-ileal 0.4%
    - Base can be found by identifying the point of convergence of the taeniae coli
  - Locating the appendix
    - Follow the anterior taenia of the caecum to the convergence with the other two taeniae
    - Identify the bloodless fold of Treves extending from the antimesenteric border of TI to the base of the appendix, or the anterior surface of the mesoappendix, or to both areas. It contains no sizeable blood vessels. The TI is the only part of the ileum with a fold on the antimesenteric aspect.
  - Agenesis, duplication and triplication of the appendix have been reported
  - Mesoappendix is attached to caecum and proximal appendix, and contains the appendicular artery (branch of ileocolic) from SMA
    - Not a true mesentery, more of a peritoneal fold
    - There is also often a peritoneal fold in front of the appendix - bloodless fold of Treves

Right/ascending colon

- - Extends from ileocaecal junction and extends upward towards the hepatic flexure. Approximately 15cm in length.
  - Anterior and lateral surfaces covered in peritoneum (considered intra-peritoneal) and the posterior surface is fixed against the retroperitoneum by the fascia of Toldt
  - Can occasionally be bound down by an extension of peritoneum across the front of it, known as Jackson's veil
  - Either abnormal mobility or abnormal fixation both predispose to caecal volvulus
  - Best mobilised along the lateral peritoneal reflection by incising the white line of Toldt - this represents the fusion line of the peritoneum with the posterior fascia of the same name
    - Must be aware of the proximity of D2 near the hepatic flexure, which can be inadvertently injured here
  - Important posterior relations:
    - Quadratus lumborum/psoas muscles
    - Ureter
    - Upper part - lower pole of right kidney laterally, D2/D3 medially
  - Anteriorly it is overlain by liver/GB

Transverse colon

- - Approximately 45cm in length
  - Suspended between the hepatic and splenic flexures, which are fixed structures
  - Completely covered by visceral peritoneum
  - Connected to the posterior abdominal wall by the transverse mesocolon - made up of four layers (same as greater omentum)
  - Can be dissected free of greater omentum with relatively little bleeding due to embryonic tissue planes - if getting bleeding, likely in transverse mesocolon - easiest to establish correct plane at either flexure and work medially
  - 'U' shape in coronal plane which can extend as far inferiorly as the pelvis in some patients
  - Blood supply from middle colic artery

Splenic flexure

- - Where the transverse colon flexes downward
  - Usually higher and deeper than the hepatic flexure
  - Suspended by four mainly avascular ligaments:
    - Phrenicocolic ligament to the diaphragm (this is continuous with gastrosplenic ligament, but more laterally)
    - Splenocolic ligament to the lower pole of the spleen
    - Renocolic ligament to Gerota fascia
    - Pancreaticocolic ligament to the tail of the pancreas
  - It can be released without dividing any major blood vessels if one is in the correct plane
    - Commonly done by first dissecting along the white line of Toldt from below, then entering the lesser sac by lifting the omentum above the transverse colon, then taking down the splenic flexure with minimal traction

Descending colon

- - Begins at the splenic flexure where the intestine loses its mesentery, and extends downwards on the left side of the abdomen, until it transitions into the sigmoid colon
  - About 25cm long
  - Smaller in diameter than the ascending colon
  - Similar to right colon in terms of peritoneal attachments and dissection strategy
  - Posterior relations:
    - Lower pole of left kidney
    - Quadratus lumborum and psoas
    - Ureter
    - Gonadal vessels

Sigmoid colon

- - Begins at or below the level of the iliac crest, where the colon becomes completely intra-peritoneal
  - Covered on both sides with peritoneum and has a mesentery
    - Attached by the sigmoid mesocolon to the posterior abdominal wall and pelvis in an inverted V, creating the inter-sigmoid fossa
      - Left limb passes downwards along left edge of vertebral column
      - Right limb passes downwards towards the midline and terminates at around S3 level
    - This mesenteric fold is a good surgical landmark for the underlying left ureter and bifurcation of left CIA, as the ureter crosses over the iliac vessels at the apex
  - Thicker and more mobile compared to the descending colon
  - Varies in length from 15-50cm (average 38cm)
  - Ends at the rectosigmoid junction (see below)

Rectum

- - Begins at the rectosigmoid junction and ends at the anus - total length 15-20cm
    - Proximal landmarks of the rectum
      - Colonic taeniae confluence to form a complete longitudinal muscle layer (blending of lateral and anti-mesenteric taeniae to form a flat anterior muscular band)
      - Cessation of haustra
      - Colon loses its mesentery
      - Usually between the level of the sacral promontory and S3 (Jamieson's says at S3 level, 6cm distal to sacral promontory)
      - Can also be measured by stretching the rectum against the sacral promontory, then using a rigid sigmoidoscope to go 15cm from anal verge
    - Distal border defined differently by anatomists and surgeons
      - Anatomists - dentate (pectinate) line
      - Surgeons - proximal border of the anal sphincter complex at the level of levator ani, which is about 2cm above the dentate line. The muscle coat of the rectum becomes continuous with the sphincter mechanism here.
  - Divide into thirds, based on peritoneal covering:
    - Upper third - anteriorly and laterally - to about 10cm above dentate line
    - Middle third - only covered anteriorly, as the peritoneal covering is reflected onto bladder/uterine body - 5-10cm above dentate line. Widest part, lies on anterior sacrum, separated from it by the mesocolon.
    - Lower third - totally extra-peritoneal, no covering, from 1-5cm above the dentate line. Turns forward prior to penetrating levator ani, then turns backwards to penetrate the pelvic floor and ending at the anal canal.
  - Wide, easily distensible reservoir
  - No haustra, appendices epiploicae or taeniae coli
  - Surrounded by perirectal fat
  - Three curvatures corresponding to three intraluminal folds: left superior, right middle and left inferior folds ('valves of Houston')
    - Proximal and distal valves fold to the right and the middle valve folds to the left
    - These valves are lost after full surgical mobilisation of the rectum, which provides an extra 5cm of length
  - Relations
    - Anterior peritoneal reflection
      - Rectovesicular pouch (men) - 7-9cm from anal verge
      - Pouch of Douglas (women) - 5-7.5cm from anal verge
      - Metastases can accumulate here and be felt on DRE ("Blumer shelf")
    - Mesorectum
      - The visceral mesentery of the rectum - the direct continuation of the sigmoid mesocolon
      - Thick posteriorly, thinner along the sides, and very thin anteriorly
      - Thins out in the lower third, and virtually absent over the last 1cm
      - Contained within the mesorectal fascia (a.k.a. fascia propria or pelvic visceral fascia), which is fused posteriorly with the presacral fascia, and there is a bloodless plane between the two fasciae
        Dissection in this plane is relatively bloodless, and allows consistent excision of lymphatics
        
        Total mesorectal excision (TME) occurs in this plane
    - Presacral fascia (of Waldeyer)
      - Separated from rectum by thick posterior mesorectum, then thin layer of investing fascia (mesorectal fascia), then a bloodless plane, then presacral fascia
      - Mobilisation and dissection of the rectum posteriorly for cancer should proceed between the presacral fascia and mesorectal fascia
      - Separates the rectum and mesorectum from the sacrum and various nerves found posteriorly
      - Presacral space/retrorectal space
        The space between the posterior wall of the rectum and the anterior surface of the sacrum
        
        Boundaries
        Anterior - mesorectum and mesorectal fascia (depending on the context, this could instead be considered the posterior wall of rectum - e.g. radiologically)
        
        Posterior - sacrum
        
        Superior - peritoneal reflection
        
        Inferior - retro-sacral fascia
        
        Lateral - lateral ligaments (endopelvic fascia), ureters, internal iliac vessels
        
        Contents
        Loose connective tissue
        
        Lymphatics
        
        Middle sacral vessels
        
        Superior rectal vessels
        
        Branches of SNS and PNS
        
        Presacral venous plexus, lying on the anterior sacrum and coccyx. These veins drain into the sacral foramina. Dissecting deep to the presacral fascia can lead to severe bleeding here, as the vessels tend to withdraw into the sacral foramina.
    - Rectosacral fascia
      - A thick condensation of endopelvic fascia connecting the presacral fascia (3-5cm above the anorectal junction) to the mesorectal fascia at the level of S2-4
      - Note some authors describe this as Waldeyer's fascia too, but this might be erroneous
      - Divides the presacral space into superior and inferior compartments
      - Two layers - visceral and parietal
      - The 'holy plane' continues on from above between the visceral and parietal layers, which is the ideal posterior plane for TME and should be bloodless. Careful not to get behind the fascia entirely.
      - May be important in preventing rectal prolapse
      - Careful not to avulse, will get troublesome bleeding
      - Division will be necessary to complete the abdominal phase of TME in an APR
    - Endopelvic fascia
      - The lateral attachments of the pelvic organs to the pelvic side walls
      - Endopelvic fascia must be breached in an APR to get from pelvic floor to TME, however it is better to do this from above, to avoid risking getting behind the presacral fascia inadvertently.
    - Lateral stalks/ligaments of rectum
      - No ligaments really exist, but this is the complex of middle rectal vessels, splanchnic nerves entering the mesorectum, and their accompanying connective tissue
      - Extends from pelvic sidewall to the mesorectum
      - Common site of damage to inferior hypogastric plexus or splanchnic nerves, when diathermy is used to control bleeding from here
    - Denonvilliers fascia
      - Located anterior to the rectum
      - May be hard to find a clear ligament between mesorectal fascia and posterior vagina in women
      - A well-developed membranous layer that is an extension of the inferior peritoneal reflection and extends to the perineal body
      - Separates the rectum from anterior structures
      - Considered as the anterior border of a TME - some controversy as to which side is better to dissect along, perhaps oncologically sounder to dissect anteriorly, but perhaps higher risk of injury
      - Cavernous nerves run in neurovascular bundles anterior to the fascia at the posterolateral border of the apex and base of the prostate - probably where most parasympathetic nerve damage happens during ULAR
    - Relations to other organs
      - Men:
        Anteriorly: bladder, ureters, vas deferens, seminal vesicles, prostate
        
        Posterior: sacrum, sacral vessels, sacral nerve roots
      - Women
        Intraperitoneal: uterus, tubes, ovaries, upper part of posterior vaginal wall
        
        Extraperitoneal: uterine cervix, posterior vaginal wall
        
        Posterior: sacrum, sacral vessels, sacral nerve roots

- Anal canal
  - See separate topic

Differentiating the colon from small bowel:

- Larger calibre
- Less mobile
- Taeniae coli
  - Three bands of thickening in the longitudinal smooth muscle
  - Extend from base of appendix to rectosigmoid junction
  - Anti-mesenteric (taenia libera) - lies anterior in all parts of colon except transverse, where it lies inferior
  - Posterolateral (Taenia omentalis)
  - Posteromedial (taenia mesocolica)
- Haustra
  - Outpouchings of colon
  - Absent in caecum and relative sparse in ascending and proximal transverse colon, becoming more pronounced after that
- Plicae semilunaris
  - Circumferential folds of the bowel wall
- Appendices epiploicae
  - Fatty tags arising from the serosa of the colon
  - Covered in peritoneum
  - Unclear purpose

Blood supply

SMA

- - Arises at level of L1
  - Runs downwards posteriorly to pancreas, with SMV on its right; but then crosses anterior to left renal vein, uncinate process of pancreas, and D3 and enters root of mesentery of small intestine
  - Left side: gives off up to 20 branches to the small intestine
  - Right side: three major branches to the colon
    - Middle colic
      - Arises from SMA just below the pancreatic head
        Can arise as a common trunk with right colic artery (see below)
      - Enters the transverse mesocolon and, 5-7cm from the colon, divides into right and left branches, supplying right and left side of transverse colon
      - Can be identified by lifting up the transverse and palpating at the base of the mesentery just to the right of the ligament of Treitz, at the level of the inferior margin of the pancreas
      - Main blood supply to the splenic flexure in about a third of patients
    - Right colic
      - Most variable of all colic vessels - absent in 50%
      - May be a branch of middle colic or SMA
        Origin from middle colic (30%) - ascends towards hepatic flexure
        
        Origin SMA (20%) - runs transversely
    - Ileocolic
      - The most constant of these branches
      - Originates from SMA just caudal to D2, about 7cm from SMA origin
      - Last branch to arise from the right side of SMA, whereas the SMA runs more anteriorly and to the left within the mesentery
      - Runs towards the ICJ
      - Gives off ascending, anterior and posterior caecal arteries and the appendiceal artery (which can be double in African communities apparently)
      - Supplies TI, caecum and appendix

IMA

- - Arises at anterior/left aorta, at level of L2-L3, behind inferior border of D3, about 3cm above the aortic bifurcation
  - Can be identified about 2cm caudal to the ligament of Treitz; the origin is surrounded by the mesenteric and hypogastric nervous plexus
  - Descends inferiorly and to the left
  - Branches:
    - Left colic
      - Originates about 3cm distally to the origin of the IMA, which is 3cm below the inferior border of duodenum
      - Ascends obliquely and steeply to the left, then divides 3cm from colon
        Ascending branch (to splenic flexure)
        
        Descending branch (to descending colon)
      - Accessory left colic can come from SMA in some cases, which causes the entire descending colon to be tethered quite high
    - Sigmoid branches
      - Usually 2-4 in variable fashion
      - First branch usually the largest, called colosigmoid artery by some authors
    - Superior rectal (haemorrhoidal) artery
      - End branch of IMA - continues on down to pelvis after giving off left colic
      - It comes to lie in the sigmoid mesocolon, which shortens as the rectum is approached, so at the proximal rectum it lies immediately posterior
      - At around S3 (usually around proximal rectum), it divides into left and right branches, running posteriorly and downward. These give off branches which run close to rectal wall and supply the arteries which lie in the anorectal columns.
      - Excellent anastomosis between the superior rectal, lower sigmoid, and probably middle rectal arteries - so superior rectal can be interrupted at any level without risk of ischaemia to the rectal stump
- Internal iliac artery - blood supply to rectum
  - Strong anastomotic network, good collaterals
  - Middle rectal artery
    - Paired vessels derived from internal iliac artery, most often from internal pudendal branch
    - Runs superior to S3 nerve root
    - Supply the lower rectum's muscle coat, but anastomose with superior rectal artery
    - Found inconstantly and not considered a major blood supply to the rectum
    - Can be inadvertently injured when dissecting the 'lateral ligaments' during TME
  - Inferior rectal artery
    - Branches of the internal pudendal arteries
    - Cross through fat of the ischiorectal fossa to reach the anal region, where they supply the sphincter muscles
    - Doubtful whether there is significant anastomoses between these and the higher rectal arteries
    - Misnomer - generally supply the anus distal to the dentate line, not the rectum
- Two vascular anastomoses between SMA and IMA
  - Marginal artery of Drummond
    - Runs along the mesenteric margin of the colon from the caecocolic junction to the rectosigmoid junction
      - Sometimes deficient near splenic flexure and upper ascending colon
    - Vasa recta branch off at short intervals and supply the bowel wall directly
    - Important clinically when one of the larger arteries is obstructed, and the colon can receive collateral supply through this artery
  - Arc of Riolan ('meandering mesenteric artery')
    - Uncommon finding - Jamieson's says 10% have it
    - A thick tortuous vessel that runs a meandering course in the colonic mesentery between the main trunk of the middle colic artery, and the ascending branch of the left colic artery (so kind of parallel to the marginal artery but lower in the mesentery)
    - Collateral in case of SMA or IMA occlusion
    - Flow can be forward (IMA stenosis) or retrograde (SMA stenosis)
    - The presence of a large arc of Riolan suggests stenosis or occlusion of one of the major vessels
- Watershed areas are found at the edge of the region supplied by the SMA and IMA, which are frequently dependent on collaterals. They are vulnerable during systemic hypoperfusion because they are supplied by the most distal branches.
  - Splenic flexure (Griffiths point). Up to 50% of patients don't have a marginal artery here, so don't have very good supply. Avoid this area for anastomoses.
  - Rectosigmoid junction (Sudeck's point). Gets supply from distal sigmoid and superior haemorrhoidal branches, which are both far along the IMA, and hence can have poor supply.
  - Right colon can be susceptible too, as the ileocolic artery here is an end artery.

Venous drainage

- Follows the arterial supply
- SMV
  - Originates in the distal small bowel mesentery, where ileal and caecal branches join
  - Lies to the right of SMA
    - If not, may indicate intestinal malrotation
  - At the lower margin of the pancreas, the right gastrocolic vein (Henle trunk) and then the inferior pancreaticoduodenal veins enter from the right
  - Forms PV behind neck of pancreas with splenic vein
  - Trunk of Henle (right gastrocolic vein)
    - Confluence of RGEV with one or more colic veins +/- anterior superior pancreatic vein
    - Drains to SMV at a mean distance of 2.2cm from the inferior border of the pancreas
    - Present in 90%, but variable
- IMV
  - Formed as the continuation of the superior rectal vein as it crosses the pelvic brim
  - Identified at the level of the ligament of Treitz, just to the left of the DJ flexure
  - Continues along the base of the mesentery after the ligament of Treitz, and into the splenic vein
  - Can be divided to achieve extra colonic length for low pelvic anastomoses
  - Superior rectal vein
    - Drains the upper two thirds of the rectum
    - Internal venous plexus of anorectum - within the anorectal columns, draining into superior rectal veins (PV system)

- - - Can see varicosities in this system in portal hypertension

- Internal iliac veins
  - Middle and inferior rectal veins - drain to systemic circulation
    - Explains the higher rate of lung metastases seen in low rectal cancers

Nerve supply (see 'autonomic nervous system' for details)

- Has both sympathetic and parasympathetic components, which generally follow the blood supply
- SNS
  - Midgut: coeliac and superior mesenteric ganglia
  - Hindgut: lumbar part of sympathetic trunk
    - Superior hypogastric plexus
    - Hypogastric nerves
    - Inferior hypogastric plexus
- PNS
  - Midgut: vagus
  - Hindgut: pelvic splanchnic nerves (nervi erigentes)

Lymphatic drainage

- Generally follows the vascular supply
- Some communications between levels are found, especially at the level of the paracolic groups at the level of the marginal arteries
- There is some dual drainage from the distal transverse and splenic flexure into both superior and inferior mesenteric nodes, although some studies have shown drainage of splenic flexure cancers is mostly to IMA pedicle
- Lymph from upper two thirds of rectum drains upward to the inferior mesenteric and para-aortic nodes
- Lower third of rectum drains both laterally and downward to the internal iliac nodes, and upward to the inferior mesenteric nodes
- Below the dentate line, lymph drains to the inguinal nodes

Endoscopic anatomy

- Dentate line at 4cm or so
- Rectum extends to 15cm
  - Prominent vascularity
- Sigmoid
  - Circular folds of modest thickness, which can be thickened with hypertrophied musculature
- Descending
  - Relatively featureless and straight
- Transverse
  - Triangular due to three teniae
- Ascending
  - Can also have triangular folds
- Caecum
  - Fluid should puddle by ICV
  - Transverse caecal fold forms circumferentially around the caecum at level of the ICV. The triradiate fold forms at the base of the caecum and then joins the transverse fold.

Cellular physiology

- Appendix
  - Mucosa is of the colonic type
    - Columnar epithelium
    - Neuroendocrine cells
    - Mucin-producing goblet cells
  - Lymphoid tissue in the submucosa
    - May serve as a reservoir of 'good' bacteria, and aid in recolonising the colon
    - Patients with previous appendicectomy have worse outcomes after C diff infection - ?immunoprotective

Layers

- Serosa
  - Variable covering in peritoneum
- Muscularis propria
- Submucosa
- Mucosa

Physiology

- Absorption of fluid and electrolytes
  - The conversion of succus from the TI into formed stool that is stored in the rectal reservoir until it is ready to be excreted
  - The colon can absorb up to 5L of fluid per day, although normally only receives 1-2L per day from the TI, of which about 90% is reabsorbed
  - Sodium and chloride are actively absorbed via exchange, and water is passively absorbed along with the sodium
  - Potassium chloride and bicarbonate are actively secreted into the lumen
- Secretion
  - The colon can increase potassium secretion to compensate for dysfunctional kidneys, which is promoted by aldosterone and blocked by spironolactone
  - Increased potassium is secreted in IBD, cholera, and shigellosis
  - Some forms of colitis also promote chloride secretion
- Urea recycling
  - Colonic bacteria are rich in urease, which is an enzyme not produced by mammals
  - Ammonia produced in the colon is absorbed and transported to the liver
  - Antibiotics and lactulose decrease the amount of ammonia absorbed by lowering the concentration of bacteria and reducing colonic pH, respectively
- Recycling bile salts
  - The colon absorbs bile salts that escape the TI, by passive transport (nonionic diffusion)
  - When the reabsorptive capacity is exceeded, colonic bacteria deconjugate bile acids
  - Deconjugated bile acids can then interfere with sodium and water absorption, leading to secretory/choleretic diarrhoea. This is seen transiently after right hemicolectomy and more permanently after extensive ileal resection. This can be effectively treated by cholestyramine, which binds to bile acids.
- Colonic flora, fermentation, and short-chain fatty acids
  - Large bowel contents are about 50% bacteria by mass
  - Mostly anaerobic, and most common one is Bacteroides, but also Escerichia, Klebsiella, Proteus, Lactobacillus, Enterococci
  - Main source of energy for intestinal bacteria is (soluble) dietary fibre
  - Note that bulking agents such as psyllium and lignin are non-absorbable and non-fermentable by colonic baceria
  - Colonic flora produce short-chain fatty acids like butyrate, which is the principle form of nutrition for the colonocyte. This production is disrupted by antibiotics, leading to diarrhoea.
    - Gases (carbon dioxide, methane and hydrogen) are also produced in this process, and make up about 50% of flatus, along with the other half being swallowed air
  - Mucosal atrophy is also seen after diversion (diversion colitis) because no stool means no nutrition for colonocytes
- Probiotics and prebiotics
  - Probiotics - dietary supplements that contain live cultures of bacteria and/or yeast that are beficial to colonic and host function
  - Most commonly Lactobacillus and Bifidobacterium
  - Insufficient data to recommend probiotics for C diff prevention
  - Possible indications for use:
    - Necrotizing enterocolitis
    - UC
    - Pouchitis
    - Constipation
  - Prebiotics - nutrients that support the growth of probiotic bacteria. Little evidence to guide use.
- Colonic motility
  - Autonomic nervous system:
    - PNS: excitatory, and reaches the colon via the vagus nerve and the rectum via the sacral nerves (S2-S4) through the pelvic plexus.
    - SNS: inhibitory, from fibres at L2-L5, post-ganglionic hypogastric nerves, and the splanchnic nerves (T5-T12), which reach the colon and rectum through perivascular plexuses
  - Intrinsic nervous system:
    - Myenteric (Auerbach) plexus
    - Submucosal (Meissner) plexus
    - Interstitial cells of Cajal are the primary pacemaker cells
- Defaecation