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
1. Preganglionic 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
2. 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:
1) 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.
2) 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:
1) Synapse at the same level.
– In this case, the synapse is in the
same trunk ganglion.
2) Synapse at a higher or lower level.
– The preganglionic axon ascends or
descends the sympathetic trunk to
another trunk ganglion.
3) 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:
1. 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
2. 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—
1. Receptor
2. Visceral Sensory neuron
3. Integration center
4. Motor neuron
5. Effector
However, there are 2 key differences:
1) A visceral reflex arc has 2 consecutive
neurons in its motor component
2) 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