Enteric nervous system and gut hormones Flashcards
Summarise the 3 regulatory systems
Nervous stimulation: neurotransmitters released from neurones innervate target cells.
Paracrine : hormones released by cells in the vicinity of the target cell and reach target cell by diffusion.
Endocrine : hormones produced by endocrine cells, released into the blood where they reach their targets via the circulation.
What are the two types of nervous system found in the gut
Intrinsic (enteric)
Extrinsic (autonomic)
Describe the structure of the enteric nervous system
The wall of the GI tract contains huge numbers of neurons, totalling somewhere between 10 and 100 million nerve cells (compared with 300 billion in the brain). These neurons communicate with cells in the autonomic nervous system.
These neurons are arranged in rich plexuses (a dense local network of nerves and supporting cells) of ganglia (nerve cells which carry signals, and glial cells which provide insulate, protective, nutritional and structural support). These ganglia are interconnected by tracts of fine, unmyelinated nerve fibres
Describe the integrative function of the enteric nervous system
The most interesting feature of the enteric nervous system is as an integrating centre for coordinating function. This feature is similar to how the brain receives signals from different parts of the body (afferent signals), integrates them, and produces a response (efferent signals). This is why the enteric nervous system is sometimes referred to as the “second brain”. It can produce a coordinated response to specific stimuli independent of the central nervous system.
Describe the independent function of the enteric nervous system
Can function independently of central control.
If the sympathetic and parasympathetic nerves to the gut are cut many motor and secretory activities continue as controlled by the enteric nervous system
What can cause dysfunction to the enteric nervous system
Inflammation (ulcerative colitis; Crohn’s disease)
Post-operative injury
Irritable bowel syndrome- probably associated with enteric nervous system damage
Ageing (constipation)
List the functions that the enteric nervous system regulated
Motility Blood flow Water and electrolyte transport Secretion Absorption
Describe the 3 types of neurone found in the enteric nervous system
Sensory: respond to mechanical, thermal, osmotic and chemical stimuli.
Motor: axons terminate on smooth muscle cells of the circular or longitudinal layers, secretory cells of the gastrointestinal tract, or gastrointestinal blood vessels.
Interneurons: neurons between neurons integrate the sensory input and effector output.
Summarise the neurones found the enteric nervous system
Three types neuron
Most are multipolar (one axon,
multiple dendrites)
Describe the myenteric plexus
located between the circular and longitudinal smooth muscle layers. Controls activity of muscularis externa. Controls gut motor function.
This careful control of the entire activity of muscularis externa allows for coordinated control of motor function, and hence, motility.
Describe the submucosal plexus
Sensing environment within lumen
Blood flow, epithelial and endocrine cell function.
Describe the afferent and efferent functions of the submucosal plexus
This plexus has both afferent and efferent functions.
Afferent: Senses the environment within lumen using mechanoreceptors, chemoreceptors and osmoreceptors.
Efferent: Can fine tune local blood flow, epithelial transport and secretory/paracrine/endocrine cell function.
Describe the minor plexuses
including deep muscular plexus (inside circular muscle), and the ganglia supplying biliary system and pancreas
Summarise the autonomic nervous system
Regulates smooth muscle, cardiac muscle and glands. Not accessible to voluntary control. Two branches: i) Sympathetic ii) Parasympathetic
Why does the CNS need some control of the enteric nervous system
Needs to know pain, fulness, sickness and so has efferents to the gut to fine control its function
Describe the basic organisation of the sympathetic nervous system
Cell bodies of preganglionic neurons in the thoracic and lumbar spinal cord.
Cell bodies of postganglionic neurons in the pre- and para- vertebral ganglia.
Note the 3 main gut ganglions; the coeliac, superior mesenteric and inferior mesenteric ganglions.
Foregut = Coeliac ganglion
Midgut = SM ganglion
Hindgut = IM ganglion
Describe the sympathetic innervation of the gut
Thoracic splanchnic nerves carry innervation to fore and midgut.
Lumbar splanchnic nerves carry sympathetic innervation to the remainder of the gut
Describe the neurotransmitters in the SNS
SNS - The major neurotransmitter of the SNS is NOREPINEPHRINE (NE). Although synapses in the sympathetic chain use acetylcholine (ACh) to communicate, most synapses between the SNS and the enteric nervous system use NE.
What is the effect of the SNS in the gut
Activation of the sympathetic nerves usually inhibit the activities of the GI system.
Describe the organisation of the PNS
Cell bodies of preganglionic neurons in the brainstem and sacral spinal cord (cranio-sacral)
Cell bodies of postganglionic neurons close to target organs.
Preganglionic neurons synapse on ganglia close to gut wall or directly with enteric plexi
Describe the PSNS innervation of the gut
Most of the GI tract via branches of the vagus nerve (down to the level of the transverse colon).
Remainder of the colon, the rectum and the anus receive parasympathetic fibers from the pelvic nerves
§ Fore- to mid-gut stimulated by the VAGUS NERVE (CNX).
§ Hindgut to anus receive PNS stimulation from the PELVIC NERVES.
Describe the neurotransmitters in the PSNS
PNS - The major neurotransmitter of this branch of the ANS is acetylcholine.
