The Autonomic Nervous System: Survival Mechanisms Unveiled

1. BIO 132 Lecture 33 Autonomic Nervous System Neurophysiology

2. Lecture Goals: • Understanding the advantage of having an ANS • Understanding the differences between the Sympathetic and Parasympathetic nervous systems. • Understanding the structure and role of the Enteric NS.

3. There is no voluntary control of the ANS (its 'automatic') • The ANS has both somatic (body) and visceral (organs) sensory input to regulate autonomic motor output. • Autonomic motor output goes to the muscle of the heart and to smooth muscle (surrounds blood vessels, GI tract, airways, glands, etc).

4. The function of the ANS is to keep you alive by taking care of vital tasks so that you don't have to spend any conscious time, effort or thought on those tasks.

5. ANS Branches • The ANS has two branches that function to maintain your survival: • Sympathetic branch (SNS) - for short-term survival ('fight-or-flight') • The SNS branch is activated in stressful or emergency situations. • It activates some organs and inhibits others in order to prepare your body for taking immediate action (like fighting or running away from danger). • Parasympathetic branch (PNS) - for long-term survival ('rest and digest'). • The PNS branch is activated during low stress situations and functions to ensure the body has sufficient nutrients for normal growth. • It activates some organs and inhibits others to increase digestion and absorption.

6. SNS/PNS Duality • Because the two branches have somewhat opposite functions, when one is activated the other is inhibited. • The 'default' branch (activated at rest) is the PSN. • As stress levels increase SNS activation is increased and PNS activation is decreased. • The periventricular zone of the hypothalamus has direct control over both the SNS and PNS.

7. Sympathetic NS • Neurons of the SNS descend from the hypothalamus to make connections with SNS motor neurons located in the spinal cord. • The motor pathway from the spinal cord to the target organ involves two neurons. Hypothalamus spinal cord Target organ

8. Sympathetic NS • All SNS motor neurons synapsing with the target organ are long and have their cell bodies just outside the spinal cord. • The cell bodies of these neurons coming from the same level of the spinal cord (exiting the spinal cord between the same two vertebrae) are clustered together. • The cluster of these cell bodies is called a ganglion (pleural: ganglia).

9. Sympathetic NS • To make the referencing of the two SNS motor neurons easier, the first is called the preganglionic neuron and the second is called the postganglionic neuron. Hypothalamus Ganglion spinal cord Preganglionic Postganglionic Target organ

10. Sympathetic NS • The preganglionic neuron is short and releases acetylcholine (ACh) onto the postganglionic neuron. • The postganglionic neuron is long and releases norepinephrine (NE) onto the target organ. spinal cord ACh NE Preganglionic Postganglionic Target organ

11. Sympathetic NS • The NE binds to noradrenergic receptors on the target organ. • There are two types of noradrenergic receptors: alpha and beta. • Each of those has sub-types (i.e. alpha-1, alpha-2, beta-1, beta-2, etc.) • The actions of each activated receptor can be different, and many times opposite. • Because NE has different receptors with different actions, it can cause vasoconstriction in some blood vessels and vasodilation in others. • Having different actions on different organs allows for NE to coordinate all the body’s organs for one goal.

12. Sympathetic NS • Goal of SNS: Prepare the body for surviving a short-term emergency at the expense of long-term survival. • Physiological effects of SNS: • Increase MAP (increase heart rate & force of contraction) • Increase blood flow to the skeletal muscle and skin • Decrease blood flow to GI tract, kidneys, liver, etc. • Inhibit salivation and release of digestive enzymes • Increase diameter of airways • Dilate pupils • Increase release of sweat • Increase metabolism and glucose levels in the blood

13. Parasympathetic NS • Neurons of the PNS descend from the hypothalamus to make connections with SNS motor neurons located in the spinal cord. • The motor pathway from the spinal cord to the target organ involves two neurons. Hypothalamus spinal cord Target organ

14. Parasympathetic NS • All PNS motor neurons synapsing with the target organ are long and have their cell bodies just outside the target organ. • The cell bodies of these neurons going to the same target organ are clustered together. • The cluster of these cell bodies is called a ganglion (pleural: ganglia).

15. Parasympathetic NS • To make the referencing of the two PNS motor neurons easier, the first is called the preganglionic neuron and the second is called the postganglionic neuron. Hypothalamus Ganglion spinal cord Preganglionic Postganglionic Target organ

16. Parasympathetic NS • The preganglionic neuron is long and releases acetylcholine (ACh) onto the postganglionic neuron. • The postganglionic neuron is short and releases ACh onto the target organ which binds to cholinergic receptors of the muscarinic subtype. spinal cord ACh ACh Preganglionic Postganglionic Target organ

17. Parasympathetic NS • Goal of PNS: Prepare the body for long-term survival. • Physiological effects of PNS: • Decrease MAP (decrease heart rate) • Decrease blood flow to the skeletal muscle and skin • Increase blood flow to GI tract, kidneys, liver, etc. • Enhance salivation and release of digestive enzymes • Decrease diameter of airways • Constrict pupils • Decrease metabolism

18. The Enteric Nervous System • The Enteric NS is another branch of the ANS. • It is a large nervous system (network of neurons) involved in the digestion of food. • Has as many neurons as the entire spinal cord. • Has both sensory and motor neurons in the esophagus, stomach, intestines, pancreas, and gall bladder. • Operates with a large degree of independence • Doesn’t need the brain • Can be influenced by the SNS and PNS • Decrease metabolism

19. The Enteric NS Morphology • Most of the GI tract is hollow tubes surrounded by two layers of smooth muscle. • The smooth muscle layers coordinate the rhythmic contractions to move food through. • The inside of the tube (lumen) is lined with an epithelial layer, around which is a circular smooth muscle layer, around which is a transverse smooth muscle layer.

20. The Enteric NS Morphology Exploded cross section view of intestinal layers Epithelial layer Circular muscle layer Longitudinal muscle layer lumen

21. The Enteric NS Morphology • There are two network of neurons that comprise the enteric NS. • One network is called the Submucosal Plexus and is sandwiched between the epithelial layer and the circular muscle layer. • This plexus is primarily sensory: monitoring stretch of the walls of the GI tubes, chemical composition and pH of the lumen, and the hormone levels in the local blood supply. • The other network is called the Myenteric Plexus and lies between the circular and the longitudinal muscle layers. • This plexus is primarily motor: coordinating muscle contractions for moving food, controlling mucus and digestive secretions, and altering the diameter of local blood vessels.

22. The Enteric NS Morphology Exploded cross section view of intestinal layers Epithelial layer Circular muscle layer Longitudinal muscle layer lumen Submucosal Plexus Myenteric Plexus

23. The Enteric NS • The myenteric plexus receives sensory input from the submucosal plexus and makes the appropriate motor responses. • The Enteric NS can operate without any input from the brain. • The SNS tends to inhibit Enteric NS function while the PNS enhances it.