GI Nervous System Quiz

Gastrointestinal Nervous System and its Modulation

• Certain chemical substances can activate chemo receptors
• Stimulation of chemo receptors can lead to activation of submucosal plexus neurons
• Stimulation can lead to release of certain chemicals that stimulate specific glands
• Glands can release secretions rich in bicarbonate
• Stimulation can also lead to dilation of blood vessels, increasing absorption
• Fatty chyme can stimulate submucosal plexus to release substances
• Specific cells called I cells can be stimulated to release cholecystokinin (CCK)
• CCK can cause gall bladder contractions and enhance pancreatic secretion
• Glucose can stimulate K cells to release glucose insulinotropic peptide (GIP)
• GIP can stimulate insulin production and inhibit gastric motility
• Intrinsic nervous system can autonomously respond to chemical and stretch sensations
• Sympathetic nervous system opposes motility, secretions, absorption, and blood flow in the GI tract

Enteric Nervous System and GI Motility

• The enteric nervous system is sometimes not included as part of the autonomic nervous system, but it is a gut brain with the same number of neurons as the entire spinal cord.
• The enteric nervous system consists of the myenteric plexus and the submucosal plexus, which regulate motility and secretions intrinsically.
• The myenteric plexus, also known as the Auerbach plexus, is located within the muscularis externa and consists of ascending and descending fibers.
• The submucosal plexus, also known as the Meissner plexus, is located within the submucosa and consists of areolar and dense connective tissue with glands.
• The alimentary canal has four basic layers: mucosa, submucosa, muscularis externa, and serosa/adventitia, and is histologically composed of epithelial tissue, lamina propria, and muscularis mucosa.
• The myenteric plexus stimulates circular muscle contraction and longitudinal muscle relaxation in response to stretch receptors, facilitating propulsion of food bolus.
• In response to stretch receptors, the myenteric plexus releases acetylcholine and substance P to stimulate circular muscle and vasoactive intestinal peptide and nitric oxide to inhibit longitudinal muscle.
• Chemo receptors are also present in the alimentary canal and respond to chemicals like fatty acids, glucose, and acidic residues, influencing enteric nervous system activity.
• The enteric nervous system can be stimulated by stretch receptors and chemo receptors to regulate GI motility intrinsically.
• Short reflex arcs and long reflex arcs involve the extrinsic effects of the sympathetic and parasympathetic nervous systems in altering intrinsic enteric nervous system activity.
• The enteric nervous system operates in response to different stimuli, such as food bolus distension, by regulating muscle contraction and relaxation to facilitate movement through the GI tract.
• The enteric nervous system can also be influenced by the release of specific chemicals in response to chemo receptors, affecting its activity in the regulation of GI motility.

The Function of the Axon and Axonal Transport

• The primary function of the axon is to conduct action potentials, involving a depolarizing wave of positive charge followed by a repolarizing wave of negative charge.
• Microtubules in the axon carry motor proteins, such as kinesin and dynein, involved in axonal transport.
• Kinesin is a positive-directed motor protein, responsible for anterograde axonal transport from the cell body to the axon terminal.
• Anterograde axonal transport involves the movement of organelles, neurotransmitters, vesicles, and enzymes from the cell body to the axon terminal.
• Dynein is a minus-directed motor protein, responsible for retrograde axonal transport from the axon terminal to the cell body.
• Retrograde axonal transport involves transporting organelles, such as mitochondria, and bringing up growth factors to the cell body.
• Action potentials involve the opening of voltage-gated sodium channels, leading to a flow of positive charges down the axon, and the opening of voltage-gated potassium channels, leading to a flow of negative charges to cause the cell to rest.
• The involvement of kinesin in axonal transport includes transporting vesicles containing neurotransmitters, membrane proteins, enzymes, and mitochondria.
• Dynein is involved in transporting vesicles and mitochondria that need to be degraded in retrograde axonal transport.
• The axon conducts action potentials through the flow of positive and negative charges, involving voltage-gated sodium and potassium channels.
• Kinesin is responsible for transporting various substances from the cell body to the axon terminal, while dynein is involved in transporting substances from the axon terminal to the cell body.
• Axonal transport and the function of the axon are clinically relevant, as they play a crucial role in the transportation of essential substances within the neuron.

Axonal Transport and Axon Terminal Function

• Axonal transport is the process of transporting materials along the axon using motor proteins, such as dyneins and kinesins.
• Nerve growth factors are transported by dyneins from the axon terminal to the cell body to stimulate gene expression and protein production for repair and growth.
• Pathogens like polio, rabies, herpes simplex, and varicella zoster viruses can hijack axonal transport to reach the cell body and use the nuclear machinery for replication.
• The axon terminal is the secretory region where neurotransmitters are released and re-uptake of neurotransmitters occurs.
• Depolarization of the axon terminal stimulates voltage-gated calcium channels, leading to calcium influx into the axon terminal.
• The vesicles in the axon terminal contain snare proteins (v-snares and t-snares) that are involved in neurotransmitter release.
• Calcium acts as a bridge between v-snares and t-snares, causing vesicle fusion with the axon terminal membrane and neurotransmitter release into the synapse.
• After exerting its effect, neurotransmitters need to be degraded or taken out of the synapse through re-uptake or degradation.
• Re-uptake involves the retrieval of neurotransmitters by the presynaptic neuron, while degradation occurs through enzymes in the synapse.
• The axon terminal plays a crucial role in neurotransmission and synaptic communication between neurons.
• Understanding axonal transport and axon terminal function is essential for comprehending the mechanisms of nerve signaling and the pathogenesis of certain viral infections.
• The proper functioning of axonal transport and the axon terminal is critical for maintaining neural communication and overall nervous system function.