2. The Autonomic Nervous System
Dr. Zeenat Zaidi

3. The Autonomic Nervous System
Concerned with the innervation and control of visceral organs, smooth muscle and glands
Regulates and coordinates visceral functions: heart rate, blood pressure, respiration, digestion, urination & reproduction
The majority of the activities of the autonomic system do not impinge on consciousness
The control exerted by the system is extremely rapid and widespread
Along with the endocrine system, its primary function is homeostasis of the internal environment

4. Organization of the Autonomic Nervous System
Like the somatic nervous system:
It is distributed both in the central and peripheral nervous system
It has both afferent & efferent components and contains afferent neurons, efferent neurons and interneurons
The visceral receptors include chemoreceptors, baroreceptors, and osmoreceptors. Ischemia or stretch can cause extreme pain

5. Visceral Sensory System
Like the somatic system:
Afferent impulses originate in the receptors, travel via afferent pathways to the CNS and terminate on the interneurons at different levels
Cell bodies of the afferent neurons are located in the sensory ganglia
Somatic
Autonomic

6. Visceral motor system is different from somatic motor system in many respects

7. Somatic motor system Autonomic motor system
Effector
Skeletal muscle
Cardiac muscle, smooth muscle, glands
Type of control
Voluntary
Involuntary
Neural pathway
One motor neuron extends from the CNS to skeletal muscle
Chain of two motor neurons: Preganglionic & Postganglionic neuron
Action on effectors
Always excitatory
May be excitatory or inhibitory
Neurotransmitter
Acetylcholine
Acetylcholine or norepinephrine
Rate of conduction
Rapid due to myelinated axons
Slower due to thinly myelinated or unmyelinated axons

8. Visceral motor system The efferent pathway is composed of two neurons:
Preganglionic neurons, whose cell bodies are located in the brain and spinal cord (CNS)
Postganglionic neurons whose cell bodies are located in the autonomic ganglia (PNS)
The axons of the preganglionic neurons synapse with the postganglionic neurons

9. Based on the anatomical, physiological and pharmacological characteristics, the autonomic nervous system is divided into:
Sympathetic: Activated during exercise, excitement, and emergencies. “fight, flight, or fright”
Parasympathetic: Concerned with conserving energy. “rest and digest”

10. Anatomical Differences in Sympathetic and Parasympathetic Divisions
Location within CNS
Sympathetic division lodges in all the thoracic & upper lumbar (L1,2) segments of spinal cord (thoracolumbar outflow)
Parasympathetic division lodges in brain & sacral (S2,3,4) segments of spinal cord (craniosacral outflow)

11. Location, number & size of ganglia
Sympathetic:
Fewer
Larger
Located nearer the CNS
Parasympathetic:
Many
Smaller
Located nearer the viscera, sometimes in the wall of the viscera

12. Pre- and post-ganglionic fibers
Sympathetic:
Shorter pre- & longer postganglionic fibers
Parasympathetic:
Longer pre- & shorter postganglionic fibers

13. Branching of axons Sympathetic axons: Highly branched.
Each preganglionic fiber synapses with many postganglionic neurons that pass to many visceral effectors
Influences many organs

14. Parasympathetic axons:
Few branches
Each preganglionic fiber usually synapses with four or five postganglionic neurons that pass to a single visceral effector
Localized effect

15. Neurotransmitter released by: preganglionic axons
Acetylcholine for both divisions (cholinergic)
postganglionic axons
Sympathetic: mostly norepinephrine
Parasympathetic: acetylcholine

16. Parasympathetic Division
Cranial Outflow
Emerges from brain
Preganglionic neurons located in cranial nerve nuclei (Edinger-Westphal, superior & inferior salivatory, lacrimal and dorsal motor nucleus of vagus nerve) in the brain stem
Preganglionic fibers are carried by Occulomotor, Facial, Glossopharyngeal and Vagus nerve and innervate organs of the head, neck, thorax, and abdomen

17. Sacral Outflow Emerges from S2-S4
Preganglionic neurons located in lateral horn of spinal gray matter
Preganglionic fibers carried by pelvic splanchnic nerves to innervates organs of the pelvis and lower abdomen

18. Parasympathetic Ganglia
Ganglia related to innervation of thoracic, abdominal & pelvic viscera
Preganglionic fibers carried by vagus nerve and pelvic splanchnic nerves

19. Ganglia related to innervation of Head & Neck
Ciliary ganglion:
Location: Orbit
Preganglionic fibers carried by occulomotor nerve
Postganglionic fibers carried by short ciliary nerve
Targets: Intraocular muscles
Otic ganglion:
Location: infratemporal fossa
Preganglionic fibers carried by glossopharyngeal nerve
Postganglionic fibers carried by auriculotemporal nerve
Target: Parotid gland

20. Pterygopalatine ganglion:
Location: Pterygopalatine fossa
Preganglionic fibers carried by facial nerve
Postganglionic fibers carried by maxillary nerve
Targets: Lacrimal gland, nasal & palatine mucosal gland
Submandibular ganglion:
Location: Submandibular region
Postganglionic fibers carried by lingual nerve
Targets: Submandibular & sublingual glands

21. Sympathetic Division Issues from T1-L2
Preganglionic neurons located in the lateral gray horn.
Preganglionic fibers run in the ventral roots of the spinal nerve
Supplies visceral organs and structures of superficial body regions

22. Sympathetic Ganglia Multiple, large in size
Located nearer the central nervous system
Based on their relation to the vertebral column, they are grouped into:
Paravertebral
Prevertebral

23. Paravertebral Ganglia
Consist of the right and left sympathetic chains or trunks.
The chains lie next to the vertebral column throughout its length, running across the necks of the ribs in the thorax and along the vertebral bodies in the abdomen.
There is approximately one ganglion associated with each spinal cord segment, except in the cervical and the sacral regions.
The chains end into a common ‘ganglion impar’ in front of coccyx

24. Adjacent ganglia of the sympathetic chain are connected to each other by interganglionic rami which contain fibers ascending or descending between ganglia.
Each ganglion in chain connected to ventral rami of the spinal nerves by white and/or gray rami communicantes

25. Prevertebral Ganglia Unpaired, not segmentally arranged
Located in abdomen, anterior to the vertebral column
Main ganglia
Celiac
Superior mesenteric
Inferior mesenteric
Aorticorenal

26. General Plan of the Sympathetic Nervous System
Preganglionic fibers arise from neurons in the lateral horn of the spinal cord between T1 and L2 levels of the cord
Fibers leave the spinal cord in ventral rootlets, ventral roots of spinal nerve
Travel through the spinal nerve, and then enter the first few millimeters of the ventral primary ramus
Join the sympathetic chain via the white rami communicantes. The white rami communicantes are so named because they are collections of myelinated axons

27. Each pregenglionic fiber is allowed to synapse once
Once the preganglionic fibers have arrived in the chain they do one of these three things:
May synapse immediately in the ganglion located at the level it entered.
May ascend or descend in the sympathetic trunk before synapsing in a ganglion located at a different spinal cord level.
May pass through the sympathetic chain ganglia without synapsing, run in splanchnic nerve to reach & synapse in a prevertebral ganglion. 2 1 3 2
Each pregenglionic fiber is allowed to synapse once

28. The postganglionic fibers reach their target structures by any one of these possible routes:
1. Many fibers re-enter the ventral primary rami of spinal nerves via gray rami communicantes and get distributed to the body, especially the blood vessels, sweat glands and arrector pili muscles, through the ventral and dorsal primary rami.

29. 2. Other fibers leave the ganglia and travel directly to their target organs. This is how postsynaptic sympathetic fibers reach the organs of the thorax.
3. Some fibers form peri-vascular plexuses along blood vessels to reach their targets e.g. fibers reaching organs in the head, and in the abdomen and pelvis.

30. T1 to L2 ventral rami are connected to the sympathetic chain via white rami communicantes, which carry preganglionic sympathetic fibers to the sympathetic chain
All the ventral rami receive postganglionic sympathetic fibers from sympathetic chain by a gray ramus
The sympathetic chains carry the preganglionic fibers from T1-L2 levels up to the head and neck and down into the lower abdomen and pelvis.

31. Sympathetic Pathways
Head & Neck
Thoracic Organs

32. Sympathetic Pathways
Abdominal Organs
Pelvic Organs

33. Distribution of Autonomic Fibers
Both divisions innervate mostly the same structures & operate in conjunction with one another (have antagonistic control over the viscus) to maintain a stable internal environment
The sympathetic system dominates at some sites and the parasympathetic system dominates at other sites

34. Exceptions in the sympathetic nervous system
Some viscera do not possess dual control e.g. arrector pili muscle is made to contract by the sympathetic activity, has no parasympathetic supply
Sweat glands:
Postganglionic neurons involved with stress-related excretion release norepinephrine (“sweaty palms”)
Postganglionic neurons involved with thermoregulation release acetylcholine
Kidneys:
Postganglionic neurons to the smooth muscle of the renal vascular bed release dopamine

35. The Role of the Adrenal Medulla
Major organ of the sympathetic nervous system
Secretes great amount of epinephrine & a little of norepinephrine
Stimulated by preganglionic sympathetic fibers

36. Enteric Nervous System
Intrinsic nervous system that directly controls the gastrointestinal system
Composed of two plexuses of nerve cells and fibers located in the wall of gastrointestinal tract from the esophagus to the anal canal
Submucous or Meisner’s plexus lies in the submucosa, is mainly concerned with the control of the glands in the mucous membrane
Myenteric or Aurbach’s plexus lies between the circular and the longitudinal muscle layer, controls the muscle and movements of the gut wall
Communicates with the CNS through the parasympathetic (eg, via the vagus nerve) and sympathetic (eg, via the prevertebral ganglia) nervous systems

37. Visceral Reflexes
Parasympathetic reflexes include: gastric and intestinal reflexes, defecation, micturition, direct light reflexes, swallowing reflex, coughing reflex, baroreceptor reflex and sexual arousal.
Sympathetic reflexes include: cardio-accelaratory reflex, vasomotor reflex, pupillary reflex and ejaculation (in males).

38. All visceral reflexes are polysynaptic.
The simplest visceral reflex arc consists of:
a receptor
a sensory neuron
an interneuron, &
Two motor neurons (pre- & postganglionic)
Long reflexes: processed in CNS, similar to polysynaptic somatic reflex
Short reflexes : bypass CNS entirely, processed in ganglia, e.g. enteric NS in walls of digestive tract

39. Central Control of the ANS
Cortical centers influence via connections with the limbic system
Hypothalamic integration centers interact with both higher and lower centers to regulate autonomic, somatic and endocrine systems to preserve body homeostasis
Reflex activity is mediated by spinal cord and brain stem (medullary centers).
Reticular formation exerts most direct influence

40. Disorders of the Autonomic Nervous System
Hypertension
Can result from overactive sympathetic vasoconstriction
Raynaud’s disease
Characterized by constriction of blood vessels
Provoked by exposure to cold or by emotional stress
Achalasia of the cardia & Congenital megacolon
Defect in the autonomic innervation of the esophagus and colon respectively

41. Primary autonomic failure: A chronic degenerative disease of the nervous system leading to fainting attacks, incontinence of urine and bowel, and impotence
Horner's syndrome: Due to damage or blockage of the path of the sympathetic fibers to the head & neck. The symptoms include:
Sunken eyeball (enophthalmos)
Constricted pupil (miosis) that does not react to light
Drooping upper eyelid (ptosis)
Pronounced lack of sweating (anhidrosis) on the forehead above the affected eye, face, and neck.
Facial flushing
normal
Horner’s syndrome

42. Sympathetic Parasympathetic Stress reaction Fight-or-flight
Primes body for intense skeletal muscle activity
Parasympathetic
Maintenance functions
Rest-and-repair
Counterbalances sympathetic function

43. Thank U