General Principles of Gastrointestinal Function

Alimentary tract

Intestinal wallLayers:

1. Serosa

2. Smooth muscle layer

1. Longitudinal

2. Circular

3. Submucosa

4. Mucosa

1. Mucosal muscle

2. Epithelial cells

Electrical activity of GI smooth muscleContinual slow, intrinsic electrical activity:

•Slow waves (Δ5-15 mV) • Determine the frequency of

contraction

• Interstitial cells of Cajal (electrical pacemakers)

• Do not themselves cause contraction (except stomach!), but induce spike potentials

•Spike potentials • Action potentials

• Slow voltage-gated calcium channels

•Changes of resting membrane potential• Depolarization

• Hyperpolarization

Control of GI Function

• Endocrine• Neurocrine• Paracrine

Neural control of GI function1.Enteric nervous system1.Myenteric plexus

Increaed tonic contraction. Increased intensity and rate of

rhythmic contractions. Increased velocity of wave

propagation. Inhibitory neurons – VIP, inhibition of

sphyncters.

2. Submucosal plexus Local intestinal secretion, absorption

and contraction of muscularis mucosae.

2.Autonomic nervous system

Enteric nervous system

Autonomic control of GI tract•Parasympathetic

stimulation – increased activity of ENS.

•Cranial nerve fibers (vagus): esophagus, stomach and pancreas.

•Sacral (S2-S4, pelvic nerves): distal half of colon (especially sigmoidal, rectal i anal)

•Sympathetic stimulation – GI inhibition

•T5-L2

•Celiac and mesenteric ganglia

•Innervates all parts of GI system

•Acts mainly through affecting ENS.

Sensory innervation of GI tract

• Neuronal bodies: ENS or dorsal root ganglia.

• Stimuli: • Irritation of mucosa

• Stretching of gut wall

• Presence of specific chemical substance in the gut

• GI reflexes:

• Local: integrated entirely within the gut wall by the ENS

• Distant: • Gut – prevertebral sympathetic ganglia – gut

• gastrocolic, enterogastric, colonoilealal reflexes

• Gut- spinal cord or brain stem – gut

1. Control of gastric motor and secretory activity by vagus

2. Pain reflexes – inhibition of entire GI tract

3. Defecation reflex

Hormonal control of GI motility

Functional Movements in GI tract1. Propulsive movements - Peristalsis - stimuli: distension of the gut chemical/physical irritation

of epithelial lining parasympathetic stimulation - direction – towards the anus

- peristaltic (myenteric) reflex

2. Mixing movements - often peristalsis alone - local intermittent constrictive contractions

Splanchic circulationIn liver sinusoids:

Reticuloendothelial cells remove bacteria and

particulate matter from GI tract

Hepatocytes Process ½ to ¾ of

nonfat, water-soluble nutrients

Anatomy of GI blood supply

Intestinal villus“Countercurrent” blood flow

Control of GI blood flow

• ↑ local activity increased blood flow

• Autoregualtory mechanisms:• Release of peptide vasodilatory hormones (CCK, VIP,

gastrin, secretin).

• Release of local vasodilatory kinins in gut wall – kallidin, bradykinin.

• Decreased oxygen concentration.

• Nervous control: parasympathetic and sympathetic nerves.

Propulsion and mixing of food

Ingestion of food

• Chewing (Mastication)• Grinding food

• Increasing in total surface area of food particles

• Makes food passage easier

• Chewing reflex

• Food bolus- Reflex inhibition of mastication musclesdroping of lower jaw

• Rebound contraction

Swallowing (Deglutition)1. Voluntary stage

2. Pharyngeal stage (2 s)- reflex• Upwards pulling of soft palate

• Palatopharyngeal fold – medial movement medijalno

• Approximation of vocal cords

• Moving larynx upward and anteriorly – epiglotis

• Opening of upper esophageal sphyncter and peristaltic wave

3. Esophageal stage• Primary and secondary peristalsis

• Lower esophageal sphyncter- receptive relaxation (achalasia)

Esophageal stage

Motor functions of the stomach

1. Storage of food• Capacity (relaxed) of 0.8 - 1.5 l

2. Forming chyme (semifluid mixture)• Mixing food: mixing waves (weak, upper portions) become strong

peristaltic waves towards antrum. • Emptying :

• constricting ring in antrum moves several ml of chyme through pylorus (also constricts – pyloric pump)

• the rest of chyme returns upstream – retropulsion (mixing function)

• Hunger contractions• Start in the body of the stomach (12-24 h after last meal)

Regulation of stomach emptying

1. Gastric factors – promote emptying Streching of stomach wall promotes emptying

Directly, via increased activity of myenteric plexus Indirectly, via gastrin release from antrum

2. Duodenal factors (more potent) – inhibit emptying

Neural: enterogastric reflex (local and distant component)

Influencing factors: 1. Duodenal distension

2. Acitity of duodenal chyme (<3.5-4 pH blocks pyloric pump)

3. Irritation of duodenal mucosa

4. Osmolality of the chyme

5. Breakdown products of proteins and < fats

Hormonal: cholecystokinin i secretin

Movements of the small intestine1. Kontrakcije miješanja (segmentacijske

kontrakcije) prati ritam sporih valova Maksimalna frekvencija 13/min u jejunumu, 8-9/min ileum

2. Propulzivne kretnje (peristaltički valovi) kretanje himusa 1 cm/min (3-5 sati do ileocekalnog

spoja) Pod kontrolom lokalnih (rastezanje), živčanih

(gastroenterični i gastroilealni refleksi) i hormonskih signala (gastrin, kolecistokinin, inzulin, motilin, sekretin)

Peristaltička navala - kod jakog nadražaja crijevne sluznice

Ileocekalni zalistakSprječava povratak sadržaja

iz kolona u ileum.Proksimalno od IC zaliska –

ileocekalni sfinkter.Aktivnost IC sfinktera ovisi o

lokalnim uvjetima u cekumu i ileumu, te o udaljenim refleksima (gastroilealni refleks).

Debelo crijevo

Kretnje debelog crijeva1. Kretnje miješanja (haustracije) U cekumu kreću se u smjeru anusa - pridonose potiskivanju sadržaja 8-15 sati za potiskivanje himusa do poprečnog kolona

2. Kretnje potiskivanja (masovne kretnje) Od poprečnog kolona do sigmoida 1-3 x dnevno, trajanje 10-30 min (svaka kontrakcija 2-3 min) Gube se haustracije u dužini od 20 cm Prazni čitav dio kolona distalno od početka kontrakcijskog prstena i

dovodi fekalni sadržaj u rektum Nakon obroka ih inicira gastrokolični i duodenokolični refleksi (pod

kontrolom autonomnog živčanog sustava)

Defekacija Potreba za defekacijom započinje dolaskom fekalnog sadržaja u rektum.

Vanjski i unutarnji analni sfinkter sprečavaju nekontrolirano izlaženje fekalnog sadržaja.

Rastezanje rektuma započinje defekacijske reflekse (vlastiti –posredovan mijenteričkim refleksom, i parasimpatički refleks) kojima se započinje defekacija.