The Autonomic Nervous System Dr. Zeenat Zaidi

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

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

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

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

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

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

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”

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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.

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.

Sympathetic Pathways Head & Neck Thoracic Organs

Sympathetic Pathways Abdominal Organs Pelvic Organs

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

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

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

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

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).

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

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

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

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

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

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