The document provides an overview of the sympathetic and parasympathetic nervous systems. It discusses the structural organization and functions of each system. The sympathetic system is activated during fight or flight responses and increases heart rate, blood pressure, respiration and mobilizes energy stores. It is organized with cell bodies in the spinal cord that project to ganglia. The parasympathetic system counteracts the sympathetic responses and is organized with cell bodies in the brainstem and sacral cord that project to target organs. Both systems use acetylcholine as a neurotransmitter but target different receptor types to produce their effects.

1. Dr G N SHASHIREKHA
ASSOCIATE PROFESSOR

2. FUNCTIONAL OVERVIEW
The Sympathetic System
 When the body requires support to meet emergency conditions and stressful
situations, sympathetic system provides the desirable assistance.
Sympathetic system also helps to control normal functions of the body such
as regulation of blood pressure, respiration, metabolism etc.
 The cell bodies of sympathetic system are located in the thoracic and
abdominal segments of the spinal cord and sympathetic ganglia are present
as a chain close to the vertebral column.
 This system is also termed as the thoracolumbar division of the ANS for the
location of the ganglia and cell bodies of neurons at thoracic and lumbar
segments of spinal cord

3. The Parasympathetic System
 The component of ANS that works by the side of the sympathetic system is the
parasympathetic division of ANS. It has a reciprocal influence on organ functions
to that of sympathetic influence.
 The cell bodies are found in the brainstem cranial nerve nuclei (cranial
component), and in the most caudal part of the spinal cord (spinal or sacral
component).
 Hence, it is also termed as craniosacral division of ANS.
The Enteric Nervous System
 The enteric nervous system (ENS) is the local neural network in GI system which
has strong anatomical and physiological link with ANS. Therefore, ENS is
considered as the ‘Third division of ANS’.

5. The Sympathetic System
Sympathetic Neuron
Preganglionic Neurons
 Cell bodies of preganglionic neurons of the sympathetic division are located in the
intermediolateral horn of the thoracic (T1 to T12) and upper lumbar (L1 to L3)
segments of spinal cord.
 The preganglionic neurons come out of the spinal cord via ventral roots. After the
merger of dorsal and ventral roots, spinal nerve emerges.
 Sympathetic preganglionic axons leave the spinal nerve via the white rami
communicantes and enter the paravertebral sympathetic ganglia, which is an
interconnected chain located on both sides of the vertebral column.

6. Postganglionic Neurons
 Postganglionic neurons for somatic structures such as sweat glands, piloerector
muscles, cutaneous blood vessels and blood vessels of skeletal muscles leave the
paravertebral ganglion in the gray rami communicantes and reenter the spinal
nerve to supply the target tissues.
 Postganglionic neurons to head, heart and lungs originate in the cervical or
upper thoracic paravertebral ganglia and proceed to the organs as separate
nerves, for example the cardiac nerve to the heart.

7. Paravertebral Ganglia: Cervical Ganglia:
SUPERIOR CERVICAL GANGLION: The superior cervical ganglion provides
sympathetic fibers that innervate the structures in the head.
 These sympathetic fibers travel in the perivascular plexus along the carotid arteries
and innervate radial muscle of the iris that causes dilation of the pupil, supply
Muller’s muscle that assists in elevating the eyelid, and innervate lacrimal and
salivary glands.
MIDDLE & INFERIOR CERVICAL GANGLIA: The middle and inferior cervical
ganglia innervate structures in the chest, including the trachea, esophagus, heart
and lungs.

9. Thoracic Ganglia
 There are about 12 thoracic ganglia.
 Preganglionic fibers from T1 and T2 supply structures in head and neck, from
T3 and T4 supply thoracic viscera, from T5 to T9 supply structures in upper
limb, and from T6 to T12 supply upper abdominal viscera.
Lumbar & Sacral Ganglia
 There are three lumbar ganglia for three lumbar segments. However, there
are two additional lumbar and at least four sacral ganglia that are present
below the lumbar segments. Preganglionic fibers from T10 to L2 supply
structures in lower limbs, and from L1 and L2 supply lower abdominal viscera

11. Prevertebral Ganglia
Postsynaptic neurons for the abdominal and pelvic visceral organs arise from the
prevertebral ganglia. They are also called collateral ganglia.
1. Celiac ganglion: The preganglionic axons for celiac ganglion originate in the T5 to
T12 spinal levels and provide innervation to the stomach, small intestine, liver,
pancreas, gallbladder, spleen & kidneys.
2. Superior mesenteric ganglion: The preganglionic fibers for superior mesenteric
ganglion originate primarily in T10 to T12 and innervate the small & large intestines.
3. Inferior mesenteric ganglion: The preganglionic fibers for inferior mesenteric
ganglion originate from L1 to L3 and innervate the lower part of colon, rectum,
urinary bladder & reproductive organs.

12. Terminal Ganglia
 These are located in the organ innervated by sympathetic fibers. Examples are
adrenal medulla, heart, pancreas and urinary bladder.
Adrenal Medulla
 Adrenal medulla is a neuroendocrine structure. It forms the inner core of the
adrenal gland.
 Cells of the adrenal medulla are innervated by preganglionic sympathetic fibers
originating in the lower thoracic spinal segments that travel in lesser splanchnic
nerve.
 Preganglionic fibers terminate on the chromaffin cells that represent modified
ganglion cells. Chromaffin cells synthesize both epinephrine and norepinephrine.

13. EFFECTS OF SYMPATHETIC STIMULATION
Effects are mediated by release of noradrenaline from sympathetic nerve endings
and adrenaline from adrenal medulla.
Effects via Adrenergic Receptors
 Catecholamines elicit their effects by acting on adrenergic receptors. Adrenergic
receptors are broadly divided into two types:  and β.
 The  receptor has two subtypes: 1 and 2.
 The β receptor has three subtypes: β1, β 2 and β3.
 Generally, β receptors are more sensitive to adrenaline and  receptors to
noradrenaline.

14. Effects of  receptor Stimulation
Effects of 1 Stimulation
 The 1 receptors are present in vascular smooth muscles of cutaneous and
splanchnic circulation, sphincters of bladder and GI tract and radial muscles of iris.
Stimulation of these receptors causes contraction or constriction of the structures
in which they are present.
 Effects of 2 Stimulation
 2 receptors are present in presynaptic nerve endings, wall of GI tract, platelets
and adipocytes. Stimulation of these receptors often causes relaxation or inhibition
of the structure.

15. Effects of β receptor Stimulation
Effects of β1 Stimulation
 β1 receptors are present in SA node, AV node and ventricular muscle. Stimulation
of these receptors causes excitation of these structures.
 Effects of β2 Stimulation
 β2 receptors are present in blood vessels of skeletal muscles, bronchial smooth
muscles & wall of GI tract. Stimulation of these receptors causes relaxation of
these structures. They are more sensitive to adrenaline than noradrenaline.
Effects of β3 Stimulation
 β3 receptors are present in adipose tissues. Stimulation of these receptors causes
lipolysis.

19. Fight-or-Flight Response
It is a typical widespread response of sympathetic activation. This occurs in critical
situations of life when one has to either fight the situation or flee from the situation.
 The effects are as follows:
1.Sympathetic stimulation of CVS increases blood pressure due to increased
cardiac output and vasoconstriction. Also, redistribution of the blood flow occurs to
skeletal muscles and heart from splanchnic and cutaneous territories so that
performance enhances.
2. In lungs, increased exchange of blood gases occurs due to stimulation of the
respiratory rate and dilation of bronchiolar tree. This increases supply of oxygen to
the tissues.

20. 3. Sympathetic stimulation to salivary gland decreases salivary secretion.
4. Supply of metabolic substrates increases, which is an essential component of
effective stress reaction. The demand for increased supply of substrates like glucose
and fatty acids is met by the actions of circulating epinephrine on hepatocytes and
adipocytes.
5. Sympathetic stimulation to sweat glands causes secretion of a watery fluid, and
evaporation of body heat.
6. Cutaneous vasoconstriction with concurrent sweat gland activation causes cold,
clammy skin of a frightened individual.

21. 7. Activation of piloerector muscles of hair follicles causes hair-standing-on the
skin.
8. Pupillary dilation enhances visual acuity and increases visual perception to make
the individual environmentally more alert.
9. Stimulation of brainstem reticular system makes the individual maximally alert
and mentally conscious to take appropriate decisions in quick successions.
10. Activity of bowel and bladder temporarily ceases due to constriction of
sphincters.

22. The Parasympathetic System
STRUCTURAL ORGANIZATION
 The parasympathetic system is the craniosacral outflow of autonomic nervous
system.
 The cranial component emanates from the brainstem, and the sacral component
originates from intermediolateral gray column of sacral segments of spinal cord.
 Parasympathetic ganglia are located either close to the organ or embedded in
the organ. Therefore, preganglionic neurons are much longer than
postganglionic neurons.

23. Cranial Component
 Cell bodies of preganglionic neurons of cranial component of parasympathetic
system are located in the brainstem.
 Brainstem parasympathetic neurons innervate structures in the head, neck, thorax
and abdomen.
 Parasympathetic axons from brainstem travel in III, VII, IX, and X cranial nerves.
 Nuclei of these cranial nerves are present in the midbrain in tectum, pons and
medulla.
 Therefore, these nuclei serve as the centers for the integration of autonomic
reflexes for the organ systems they innervate.

25. Sacral Component
 Sacral parasympathetic neurons innervate structures in the pelvis.
 Preganglionic fibers originate in the intermediolateral gray column of the sacral
segments S2, S3, and S4 of spinal cord.
 The preganglionic fibers terminate in ganglia in or near the viscera that include
descending colon, sigmoid colon, rectum, internal anal sphincter, urinary bladder
and the reproductive organs.

26. PARASYMPATHETIC FUNCTIONS
 Parasympathetic system restores body’s energy reserve.
 Except on cardiovascular system, most parasympathetic effects are
stimulatory, especially for the processes that facilitate energy storage and
growth.
 They stimulate intestinal motility, secretion, digestion and absorption.
 They promote reproductive functions

27. Neurotransmitters and Receptors
Muscarinic Receptors
 Cholinergic muscarinic receptors are present in heart, smooth muscles and
glands. These receptors are activated by acetylcholine and muscarine.
 Activation of these receptors produce inhibitory effects on heart, for example,
decreased heart rate, and excitatory effects on smooth muscle and glands, for
example, increased GI motility and secretion etc.
 In smooth muscles and glandular tissues, effects are mediated by intracellular
IP3 and Ca++.
 Muscarinic receptors are blocked by atropine.

28. Nicotinic Receptors
 Cholinergic nicotinic receptors are present in autonomic ganglia (both
sympathetic and parasympathetic), neuromuscular junctions and adrenal
medulla. These receptors are activated by acetylcholine and nicotine.
 Activation of these receptors produces excitatory effects on target tissue. Effects
are mediated by direct binding of acetylcholine to  subunits of the
receptors.
 Receptors also contain Na+ and K+ channels.
 Nicotinic effects are blocked by ganglion blockers such as hexamethonium.

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