Innervation of gastro intestinal tract

INNERVATION OF GASTRO-

INTESTINAL TRACT

DIVISIONS OF ANS

. ENTERIC NERVOUS SYSTEM- [MINI BRAIN OF GUT]

. PARA SYMPATHETIC NERVOUS SYSTEM

. SYMPATHETIC NERVOUS SYSTEM

1.SUBMUCOSAL/ MEISSNER’S PLEXUS

2.MYENTERIC/ AUERBACH’S PLEXUS

ENTERIC NERVOUS SYSTEM

ENTERIC NERVOUS SYSTEM

NEUROTRANSMITTERS

Excitatory motor neurons -ACH -Substance P

Secreto motor neurons -ACH -VIP -Histamine

Inhibitory motor neurons -ATP -Nitric oxide -VIP

Others -Norepinephrine -Seratonin -GABA -Dopamine -Somatostatin -Leu-enkephaline -Met-enkephaline -Bombesin

TYPES OF ENTERIC NEURONSAH-TYPE S-TYPE

-Multiple long processes

-long lasting hyper polarizing potential

-largest population of neurons in Myenteric Plexus

-Single long axon with multiple short dendrites

-Exhibit nicotinic fast EPSPs-Low RM Conductance and greater excitability

-Max in Submucosal Plexus

Application of TETRODOTOXIN abolishes APs in S-type but not in AH-type

PARASYMPATHETIC INNERVATION

S2, S3, S4

DORSAL VAGAL COMPLEX OF MEDULLA

SYMPATHETIC INNERVATION

T5

L2

SENSORY AFFERENTS SPLANCHNIC NERVES VAGAL FIBERS

-Mixed nerves-Sensory afferents from gut to spinal cord-Sympathetic efferents to gut-Cell bodies of sensory afferents are in dorsal root spinal ganglia

-80% of vagal fibres are sensory afferents-transmit information from gut to CNS for processing

SENSORY RECEPTORS

1.Mechano Receptors

2.Chemo Receptors

3.Pain Receptors/ Nociceptors

4.Mesenteric Receptors

5.Thermoreceptors

SYNAPTIC TRANSMISSION IN ENS

1.Slow EPSPs

2.Fast EPSPs

3.Slow IPSPs

4.Presynaptic Inhibition

5.Presynaptic Facilitation

SLOW EPSPs

-In AH-type neurons receptor is Adenylyl Cyclase & 2nd messenger is cAMP.

-In S-type neurons receptor is Phospholipase C & 2nd messenger is free intra-neuronal Calcium.

Significance- Long-lasting responses of the gut effectors during physiological stimuli.

FAST EPSPs

Mediated by Acetylcholine acting on ionotropic nicotinic receptors

Significance- rapid transfer of information between elements of enteric microcircuits.

SLOW IPSPs

Eukephalins, Dymorphine and Morphine are slow IPSP mimetics

NE-alpha 2 adrenoreceptor , Galanine, Adenosine, ATP

Significance- shunting of blood by symp. stimulation during exercise

PRESYNAPTIC INHIBITION

-Mechanism for selective shutdown of a microcircuit

-Release of Histamine in response to food allergens & infectious agents

NE-α2 adrenoreceptor- supress fast & slow EPSPs

Seratonin supresses both fast & slow EPSPs

Opiates supress some fast EPSPs

ACH supresses some fast EPSPs

PRESYNAPTIC FACILITATION

-Action of CCK on Gall Bladder

-Mechanism of action of Prokinetic drugs like TEGASEROID & CISAPRIDE

NEURO-MUSCULAR JUNCTION IN GIT

-Site where NTs released from axons of motor neurons act on smooth muscle fibres & interstitial cells of Cajal.

-Simpler structers than the motor end plates of Skeletal muscles.

-NTs are released from multiple varicosities of motor axons which spread out along the axon.

-This structural organization is an adaptation for simultaneous application of a chemical NT to a large number of muscle fibers from a small number of motor axons.

HIERARCHY OF NEURAL REGULATION

GASTRO INTESTINAL REFLEXES1.Local Reflexes -control much of GI secretion, peristalisis, mixing contractions and local inhibitory effects. 2.Short Reflexes -Gastro-colic reflex -Entero-gastric reflex -Colono-ileal reflex -Ileo-gastric reflex -Intestino-intestinal reflex3.Long Reflexes -Vago-vagal reflex -Swallowing reflex -Pain reflexes -Vomiting reflex -Chewing reflex -Defecation reflex

APPLIED ASPECTS1.Hirschprung disease/ congenital megacolon/ congenital aganglionosis2.Cardiac Achalasia3.Abolition of Deglutition reflex4.Dumping syndrome after vagotomy & gastric antrectomy for peptic ulcer5.Idiopathic gastric stasis6.Paralytic Ileus7.Incontinence8.Dysphagia- Nut cracker’s oesophagus9.Emesis/ Vomiting

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