Colonic Absortive and Motor Functions including Defecation

What are the main functions of the colon and rectum?

Absorption of water and ions; bacterial fermentation of unabsorbed nutrients; storage of waste and undigested material; elimination of waste

List the anatomical divisions of the large intestine.

Cecum; ascending colon; transverse colon; descending colon; sigmoid colon; rectum; anal canal

What are taeniae coli?

Three longitudinal bands of muscle fibers in the large intestine running from cecum to rectum

What happens to taeniae coli at the rectum?

They spread out to form a continuous longitudinal muscle layer

What forms the internal anal sphincter?

Thickened circular smooth muscle layer of the anal canal

What forms the external anal sphincter?

Striated (skeletal) muscle surrounding and distal to the internal sphincter

Which nerves provide parasympathetic innervation to the colon?

Vagus nerve (proximal colon) and pelvic nerves (distal colon)

Which nerve controls the external anal sphincter?

Pudendal nerve (somatic control)

What is the main component of colonic secretion?

Mucus with some bicarbonate

What stimulates colonic secretion?

Local reflexes and sacral parasympathetic nerves

Functions of colonic mucus.

Protects mucosa from excoriation, bacteria, and acids; provides lubrication and adherence for feces

What causes diarrhea in terms of colonic absorption?

Impaired absorption when >4–5 L fluid/day or presence of impermeant solutes → isotonic fluid passes through anus

How is colonic transit time measured?

In days rather than hours

Typical mouth-to-anus transit time.

Approximately 60 hours

Typical mouth-to-cecum transit time.

2–4 hours

Average time between bowel movements.

About 27 hours (range: 3/day to 1 every 3 days)

Average stool weight.

100–200 grams

What is the function of the right colon?

Absorption of water and electrolytes

What is the function of the left colon?

Formation and storage of feces

What occurs in the rectum regarding sphincters?

Internal and external sphincters regulate defecation and continence

What are the three types of colonic contraction patterns?

Mixing movements; haustral contractions (migration); mass movements

Describe mixing movements.

Non-propulsive localized contractions in haustra that mix contents

Function of mixing movements.

Enhance digestion and absorption by mixing chyme with fluids

Describe haustral contractions.

Rhythmic contractions of haustra that segment contents

Function of haustral contractions.

Promote water absorption and feces formation

Describe mass movements.

Powerful propulsive contractions moving contents long distances

Trigger for mass movements.

Gastrocolic reflex or gastroileal reflex after meals

Function of mass movements.

Move fecal material from transverse colon to rectum

What are the three types of spike activity in colon?

Short spike bursts; long spike bursts; migrating spike bursts

Characteristics of short spike bursts.

Frequent single spikes; slow waves do not propagate; associated with haustral contractions

Characteristics of long spike bursts.

Last 5–15 minutes; occur over several slow waves; separated by ~30 min; associated with haustral migration

Characteristics of migrating spike bursts.

Rapid oscillations; not related to slow waves; associated with mass movements

Effect of tetrodotoxin on colon.

Causes sustained contraction → indicates tonic inhibition by enteric nervous system

Effect of parasympathetic stimulation on colon.

Stimulates colonic motility

Effect of sympathetic stimulation on colon.

Inhibits motility and decreases spike frequency

Clinical relevance of sympathetic inhibition.

Important in postoperative ileus

What initiates colonic response to a meal?

Signals from stomach and intestine via neural and hormonal pathways (gastrocolic reflex)

What is the anal sphincter reflex?

Rectal distension causes internal sphincter relaxation and external sphincter contraction → allows continence

What triggers the defecation reflex?

Distension of rectum activating mechanoreceptors

Afferent pathway of defecation reflex.

Signals from rectum to spinal cord via pelvic nerves

Efferent pathway of defecation reflex.

Parasympathetic signals increase peristalsis and relax internal sphincter

Role of external anal sphincter in defecation.

Voluntary control via pudendal nerve; prevents defecation until consciously relaxed

What strengthens the intrinsic defecation reflex?

Parasympathetic reflex via pelvic nerves

Additional actions during defecation.

Deep breath, glottis closure, contraction of abdominal muscles → increased intra-abdominal pressure

What is Hirschsprung’s disease?

Congenital absence of enteric nervous system in distal colon → severe constipation

Effect of dietary fiber on colon transit.

Increases bulk and enhances transit

What influences colonic motility from CNS?

Emotional and higher brain center inputs

Composition of feces.

~75% water, 25% solids (30% bacteria, 10–20% inorganic, 10–20% fat, 2–3% protein, 30% roughage)

What gives feces its brown color?

Stercobilin and urobilin (bilirubin derivatives)

Cause of fecal odor.

Bacterial products (indole, skatole, mercaptans, hydrogen sulfide)

Main gases in GI tract.

H2, CO2, CH4, N2, O2

Sources of intestinal gas.

Swallowed air, bacterial fermentation, acid neutralization