Anatomy & physiology of the Autonomic nervous system

Provides a good description of the anatomy & physiology of the autonomic nervous system for undergraduate medical students. It goes over the parts & functions of the sympathetic & parasympathetic nervous system respectively & compares the differences between them.

1. Autonomic Nervous System Physiology Card-7 Item-8

2. ANS: Definition & Characteristics The autonomic nervous system is the portion of the nervous system that controls most visceral functions of the body involuntarily. This system helps to control arterial pressure, gastrointestinal motility, gastrointestinal secretion, urinary bladder emptying, sweating , body temperature, and many other activities. One of the most striking characteristics of the autonomic nervous system is the rapidity and intensity with which it can change visceral functions. For instance , within 3 to 5 seconds it can increase the heart rate to twice normal, within 10 to 15 seconds the arterial pressure can be doubled . the arterial pressure can be decreased low enough within 10 to 15 seconds to cause fainting. Sweating can begin within seconds, urinary bladder may empty involuntarily, also within seconds.

3. ANS: Classification Subdivisions of the Autonomic Nervous System: Sympathetic Nervous System The Sympathetic nerve fibers originate in the spinal cord along with spinal nerves between cord segments T1 and L2 (Thoracolumbar outflow) Parasympathetic Nervous System The Parasympathetic nerve fibers originate from the Cranial nerves III, VII, IX & X and the splanchnic nerves in the Sacral region of the spinal cord S2, S3, S4 (Craniosacral Outflow)

4. Sympathetic Nervous System

5. SNS Anatomy: Origin & Neurons Origin: The sympathetic nerve fibers originate in the spinal cord along with spinal nerves between cord segments T1 and L2 and pass first into the sympathetic chain and then to the tissues and organs that are stimulated by the sympathetic nerves . Types of Neurons: Each autonomic pathway from the spinal cord to the stimulated tissue is composed of 2 neurons, a P reganglionic neuron- Neurons of the intermediolateral column of the spinal cord , found within the levels T1-T12 and L1-L3. The axons of the preganglionic neurons leave the spinal cord through the anterior rami of spinal nerves and continue their path as  white rami communicantes Postganglionic neuron- The white rami communicantes synapse with the neuronal bodies of the sympathetic ganglia or pass through to form splanchnic nerves (preganglionic fibers) which synapse with collateral ganglia The axons of the ganglionic neurons are the postganglionic fibers . They leave the ganglia in the form of   gray rami communicantes  which join the rami of the spinal nerves

6. SNS Anatomy: Ganglia The ganglionic compartment is actually composed of the cell bodies of the postganglionic neurons. It consists of 2 types: Paravertebral ganglia  Location: Occur on either side of the  vertebral column  and are independently linked on either side, forming 2  sympathetic trunks (chains). Course: The paravertebral ganglia are the site where preganglionic fibers synapse with postganglionic neurons. The trunks extend the entire length of the vertebral column , from the base of the cranium to the coccyx where they converge anteriorly to the coccyx , forming the  ganglion impar. Connections: Each trunk is attached to the anterior rami of the T1-L2(3) spinal nerves. Prevertebral ganglia   Location: (splanchnic ganglia) are located in the abdominal cavity around the origin of the major branches of the abdominal aorta. Types: The prevertebral ganglia form aggregations around the abdominal prevertebral plexus and are referred to as the celiac, aorticorenal and superior and inferior mesenteric ganglia . Various nerve plexuses branch from these ganglia.

7. SNS Anatomy: Pathways of innervation Once a preganglionic axon reaches a trunk ganglion, one of 3 things can happen The preganglionic and postganglionic neurons can: Synapse at the same level. In this case, the synapse is in the same trunk ganglion. Synapse at a higher or lower level. The preganglionic axon ascends or descends the sympathetic trunk to another trunk ganglion. Synapse in a distant collateral ganglion. The preganglionic axon passes through the trunk ganglion and emerges from the sympathetic trunk without synapsing . These preganglionic fibers help form several splanchnic nerves and synapse in collateral/ prevertebral, ganglia located anterior to the vertebral column.

8. SNS: Functions The sympathetic system is involved in  energy-expending (catabolism), enabling the body to use energy appropriately to respond to stressful situations and emergencies, as in the  “fight or flight” response . 

9. Parasympathetic Nervous System

10. PSNS Anatomy Presynaptic Neurons The  presynaptic neurons of the parasympathetic system are located within the   Medulla Oblongota  and sacral spinal cord . They give off long axons (presynaptic fibers) that leave the CNS and travel towards the postsynaptic neurons. The presynaptic neurons are located within the 2 major parts of the central nervous system:    Cranial Part Preganglionic fibers run in the oculomotor, facial, glossopharyngeal , and vagus cranial nerves. Their cell bodies lie in the brain stem in motor nuclei of the associated cranial nerves Cranial nerves III, VII , and IX supply the entire parasympathetic innervation of the head , whereas the vagus nerves X supply thorax abdomen Sacral Part The sacral part serves the pelvic organs and the distal half of the large intestine . The sacral part arises from neurons located in the lateral gray matter of spinal cord segments S2–S4 . Axons of these neurons run in the ventral roots of the spinal nerves to the ventral rami and then branch off to form the pelvic splanchnic nerves. Postsynaptic Neurons The presynaptic fibers synapse with the bodies of the postsynaptic neurons in the parasympathetic ganglia , which typically lie near or within the target  organs. After receiving the impulse from the presynaptic neuron, the postsynaptic neuron conveys the neural impulse further down its axon (postsynaptic fiber). The postsynaptic fibers are significantly shorter than the presynaptic ones , given that the postsynaptic neuronal bodies lie in the close proximity of their target organs.

11. PSNS: Functions The activities of the parasympathetic part of the autonomic system are directed toward conserving and restoring energy.

12. PSNS & SNS : Summary

13. PSNS VS SNS: Anatomical differences

14. PSNS VS SNS: Differences

15. Visceral Reflexes Visceral reflex arcs have essentially the same components as somatic reflex arcs— Receptor Visceral Sensory neuron Integration center Motor neuron Effector However, there are 2 key differences : A visceral reflex arc has 2 consecutive neurons in its motor component The afferent fibers are visceral sensory neurons (not somatic sensory), which send information about chemical changes, stretch, and irritation of the viscera. Examples of visceral reflexes include micturition reflex , defecation reflex , gastrocolic reflex .

16. Neurotransmitters in ANS Neurotransmitters in Sympathetic division Preganglionic neurons are Cholinergic, secretes Acetylcholine (ACh) in synapses. Postganglionic neurons are Adrenergic, secretes Epinephrine (Epi) & Norepinephrin(NE) in synapses except in that of sweat glands, piloerector muscles and arterioles of skeletal muscles which are Cholinergic. Neurotransmitters in Parasympathetic division Preganglionic neurons are Cholinergic, secretes Acetylcholine (ACh) in synapses. Postganglionic neurons are Cholinergic , secretes Acetylcholine (ACh) in synapses

17. Receptors in ANS

18. Adrenergic Receptors