The autonomic nervous system consists of sympathetic and parasympathetic divisions that regulate organs through two-neuron chains. The sympathetic division originates in the spinal cord and targets organs like the heart and blood vessels. The parasympathetic division originates in the brainstem and sacral spinal cord and targets the digestive system. Sensory fibers carry visceral sensation to the CNS while the autonomic plexuses distribute efferent fibers to target organs.

2. AUTONOMIC NERVOUS SYSTEM

4. INTRODUCTION
• Concerned with control of target tissues: the
cardiac muscle, the smooth muscle in blood
vessels and viscera, and the glands.
• It helps in homeostasis
• Consists of efferent pathways, afferent
pathways, and groups of neurons in the brain
and spinal cord that regulate the system's
functions.

5. • Autonomic reflex activity in the spinal cord
accounts for some aspects of autonomic
regulation and homeostasis.
• Modulated by supraspinal centers such as
brain stem nuclei and the hypothalamus-
hierarchical organization

6. Divisions
• anatomically distinct
• largely opposing actions
• sympathetic (thoracolumbar)
• parasympathetic (craniosacral) division

8. Critical point
• Many commonly used medications (e.g.
medications for treating high blood pressure,
for regulating gastrointestinal function, or for
maintaining a regular heart beat) have their
major actions on neurons within these
systems.

9. AUTONOMIC OUTFLOW/EFFERENTS
• efferent components arise from preganglionic
cell bodies in different locations.
• organized more diffusely than the somatic
motor system

10. Comparison with somatic nervous
system
• In the SNS, LMN project directly from the
spinal cord or brain, without an interposed
synapse, to innervate a relatively small group
of target cells (somatic muscle cells).
• A more slowly conducting two-neuron chain
characterizes the autonomic outflow.

11. • The cell body of the primary neuron (the
presynaptic, or preganglionic, neuron) within
the central nervous system is located in the
intermediolateral gray column of the spinal
cord or in the brain stem nuclei.

12. • It sends its axon, which is usually a small-
diameter, myelinated B fiber out to synapse
with the secondary neuron (the postsynaptic,
or postganglionic, neuron) located in one of
the autonomic ganglia

14. • From there, the postganglionic axon passes to
its terminal distribution in a target organ.
• Most postganglionic autonomic axons are
unmyelinated C fibers.

15. • The autonomic outflow system projects widely
to most target tissues and is not as highly
focused as the somatic motor system.
• Postganglionic fibers outnumber the
preganglionic neurons by a ratio of about
32:1, a single preganglionic neuron may
control the autonomic functions of a rather
extensive terminal area.

16. Sympathetic Division
• Arises from preganglionic cell bodies located
in the intermediolateral cell columns of the 12
thoracic and upper 2 lumbar segments of the
spinal cord

17. PREGANGLIONIC SYMPATHETIC
EFFERENT FIBER SYSTEM
• Mostly myelinated
• Coursing with the ventral roots, they form the
white communicating rami of the thoracic
and lumbar nerves, through which they reach
the ganglia of the sympathetic chains or
trunks

18. • These trunk ganglia/paravertebral ganglia lie
on the lateral sides of the bodies of the
thoracic and lumbar vertebrae.
• On entering the ganglia, the fibers may
synapse with a number of ganglion cells,

19.  pass up or down the sympathetic trunk to
synapse with ganglion cells at a higher or
lower level, or
pass through the trunk/paravertebral ganglia
and out to one of the collateral
(intermediary)/prevertebral sympathetic
ganglia (eg, the celiac and mesenteric
ganglia).

20. Splanchnic nerves
• Arise from the lower seven thoracic segments
• pass through the paravertebral ganglia to the
celiac and superior mesenteric ganglia.
• Synaptic connections occur with ganglion
cells-postganglionic axons then pass to the
abdominal viscera via the celiac plexus.

22. Splanchnic nerves
• arising from spinal cord segments in the
lowest thoracic and upper lumbar region
convey fibers to synaptic stations in the
inferior mesenteric ganglion and to small
ganglia associated with the hypogastric
plexus, through which postsynaptic fibers are
distributed to the lower abdominal and pelvic
viscera.

23. THE ADRENAL GLAND
• Preganglionic sympathetic axons in the
splanchnic nerves also project to the adrenal
gland, where they synapse on chromaffin cells
in the adrenal medulla.
• The adrenal chromaffin cells, which receive
direct synaptic input from preganglionic
sympathetic axons, are derived from neural
crest and can be considered to be modified
postganglionic cells that have lost their axons

24. POSTGANGLIONIC EFFERENT FIBER
SYSTEM
• mostly unmyelinated postganglionic fibers
form the gray communicating rami.
• fibers may course with the spinal nerve for
some distance or go directly to their target
tissues.

25. • Gray communicating rami join each of the
spinal nerves and distribute the vasomotor,
pilomotor, and sweat gland innervation
throughout the somatic areas
• Branches of the superior cervical sympathetic
ganglion enter into the formation of the
sympathetic carotid plexuses around the
internal and external carotid arteries for
distribution of sympathetic fibers to the head

26. • After exiting from the carotid plexus, these
postganglionic sympathetic axons project to
the salivary and lacrimal glands, the muscles
that dilate the pupil and raise the eyelid, and
sweat glands and blood vessels of the face
and head.

27. • superior cardiac nerves from three pairs of
cervical sympathetic ganglia→ cardiac plexus
→ cardioaccelerator fibers

28. • Vasomotor branches from the upper five
thoracic ganglia →thoracic aorta → posterior
pulmonary plexus→bronchi[dilatation]

31. Parasympathetic Division
from preganglionic cell bodies in
A. the gray matter of the brain stem
 medial part of the oculomotor nucleus,
Edinger–Westphal nucleus,
 superior and inferior salivatory nuclei)
B. the middle three segments of the sacral cord
(S2–4)

32. • Most of the preganglionic fibers from S2, S3,
and S4 run without interruption from their
central origin to
• either the wall of the viscus they supply or
• the site where they synapse with terminal
ganglion cells associated with the plexuses of
Meissner and Auerbach in the wall of the
intestinal tract

34. • the parasympathetic postganglionic neurons
are located close to the tissues they supply-
relatively short axons.
• parasympathetic distribution confined entirely
to visceral structures.

35. Cranial nerves conveying
preganglionic parasympathetic fibers
• CN III, VII, and IX distribute parasympathetic
or visceral efferent fibers to the head
• axons in these nerves synapse with
postganglionic neurons in the ciliary,
sphenopalatine, submaxillary, and otic
ganglia, respectively

37. • The vagus nerve (cranial nerve X) →thoracic
and abdominal viscera via the prevertebral
plexuses.
• The pelvic nerve (nervus erigentes) →most of
the large intestine and to the pelvic viscera
and genitals via the hypogastric plexus.

38. Autonomic Plexuses
• conduit for the distribution of the sympathetic
and parasympathetic (and afferent) fibers that
enter into their formation

39. Cardiac plexus
• Formed from the cardiac sympathetic nerves
and cardiac branches of the vagus nerve
• Supplies the myocardium and the vessels
leaving the heart.

40. Pulmonary plexuses
• joined with the cardiac plexus
• formed from the vagus and the upper thoracic
sympathetic nerves
• distributed to the vessels and bronchi of the
lung.

41. Celiac (solar) plexus
• located in the epigastric region over the
abdominal aorta
• formed from vagal fibers reaching it via the
esophageal plexus, sympathetic fibers arising
from celiac ganglia, and sympathetic fibers
coursing down from the thoracic aortic plexus

42. • projects to most of the abdominal viscera,
which it reaches by way of numerous
subplexuses, including phrenic, hepatic,
splenic, superior gastric, suprarenal, renal,
spermatic or ovarian, abdominal aortic, and
superior and inferior mesenteric plexuses.

43. Hypogastric plexus
• located in front of the fifth lumbar vertebra
and the promontory of the sacrum.
• receives sympathetic fibers from the aortic
plexus and lumbar trunk ganglia and
parasympathetic fibers from the pelvic nerve.
• Its two lateral portions, the pelvic plexuses, lie
on either side of the rectum.

44. • Supplies pelvic viscera and genitals via
subplexuses that extend along the visceral
branches of the hypogastric artery.
• middle hemorrhoidal plexus, to the rectum;
• the vesical plexus, to the bladder, seminal
vesicles, and ductus deferens;

45. • the prostatic plexus, to the prostate, seminal
vesicles, and penis;
• the vaginal plexus, to the vagina and clitoris;
• uterine plexus, to the uterus and uterine
tubes.

46. VISCERAL AFFERENT PATHWAYS
• Visceral afferent fibers have their cell bodies in
sensory ganglia of some of the cranial and
spinal nerves.
• Most are unmyelinated and have slow
conduction velocities

47. Pathways to the Spinal Cord
• Via middle sacral, thoracic, and upper lumbar
nerves.
• The sacral nerves carry sensory stimuli from
the pelvic organs-nerve fibers are involved in
reflexes of the sacral parasympathetic outflow
that control various sexual responses,
micturition, and defecation.

48. • Axons carrying visceral pain impulses from the
heart, upper digestive tract, kidney, and
gallbladder travel with the thoracic and upper
lumbar nerves.
• Pathways are associated with sensations such
as hunger, nausea, and poorly localized
visceral pain

49. • Pain impulses from a viscus may converge
with pain impulses arising in a particular
region of the skin, causing referred pain.
• Examples:
• shoulder pains associated with gallstone
attacks
• the pains of the left arm or throat associated
with myocardial ischemia

50. Examples
• shoulder pains associated with gallstone
attacks
• the pains of the left arm or throat associated
with myocardial ischemia

51. Pathways to the Brain Stem
• Visceral afferent axons in CN IX and especially
CN Xcarry a variety of sensations to the brain
stem from the heart, great vessels, and
respiratory and gastrointestinal tracts.
• The ganglia involved are the inferior
glossopharyngeal and the inferior vagal

52. • The afferent fibers are also involved in reflexes
that regulate blood pressure, respiratory rate
and depth, and heart rate through specialized
receptors or receptor areas.
• Baroreceptors- aortic arch and carotid sinus .
• Chemoreceptors -aorta and carotid bodies.

53. • Chemosensitive zone –medulla
• Chemoreceptor neurons -alter their firing
patterns in response to alterations of pH and
pCO2 within the cerebrospinal fluid.