The Autonomic Nervous System

Autonomic versus Somatic NS

• preganglionic and postganglionic neurons

Basic Anatomy of ANS

Preganglionic neuron

- cell body in brain or spinal cord
- axon is myelinated type B fiber that extends to autonomic ganglion

Postganglionic neuron

- cell body lies outside the CNS in an autonomic ganglion
-axon is unmyelinated type C fiber that terminates in a visceral effector

Divisions of the ANS

2 major divisions – parasympathetic – sympathetic Dual innervation – one speeds up organ – one slows down organ – Sympathetic NS increases heart rate – Parasympathetic NS decreases heart rate

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Sources of Dual Innervation

• Sympathetic (thoracolumbar) division

– preganglionic  cell bodies in thoracic and first 2 lumbar segments of spinal cord

• Parasympathetic (craniosacral) division

– preganglionic cell bodies in nuclei of 4 cranial nerves and the sacral spinal cord    

Locations of Autonomic Ganglia

• Sympathetic Ganglia

– trunk (chain) ganglia near vertebral bodies
– prevertebral ganglia near large blood vessel in gut

• celiac
• superior mesenteric
• inferior mesenteric

• Parasympathetic Ganglia

– terminal ganglia in wall of organ

Autonomic Plexuses

• Cardiac plexus
• Pulmonary plexus
• Celiac (solar) plexus
• Superior mesenteric
• Inferior mesenteric
• Hypogastric

Organs Innervated by Sympathetic NS

• Structures innervated by each spinal nerve

– sweat glands, arrector pili mm., blood vessels to skin & skeletal mm.

• Thoracic & cranial plexuses supply:

– heart, lungs,esophagus & thoracic blood vessels
– plexus around carotid artery to head structures

• Splanchnic nerves to prevertebral ganglia supply:

– GI tract from stomach to rectum, urinary & reproductive organs

Circuitry of Sympathetic NS

• Divergence = each preganglionic cell synapses on many postganglionic cells

• Mass activation due to divergence

– multiple target organs
– fight or flight response explained

• Adrenal gland

– modified cluster of postganglionic cell bodies that release epinephrine & norepinephrine into blood

Anatomy of Parasympathetic NS

• Preganglionic cell bodies found in

–  4 cranial nerve nuclei in brainstem
–  S2 to S4 spinal cord

• Postganglionic cell bodies very near  or in the wall of the target organ in a  terminal ganglia

Parasympathetic Sacral Nerve Fibers

• Form pelvic splanchnic nerves
• Preganglionic fibers end on terminal ganglia in walls of target organs
• Innervate smooth muscle and glands in colon, ureters, bladder & reproductive organs

ANS Neurotransmitters

Adrenergic

Cholinergic

Cholinergic Neurons and Receptors

• Cholinergic neurons release acetylcholine from  preganglionic neurons & from parasympathetic postganglionic neurons

• Excites or inhibits depending upon receptor type and organ involved

• Nicotinic receptors are found on dendrites & cell bodies of autonomic NS cells and at NMJ

• Muscarinic receptors are found on plasma membranes of all parasympathetic effectors

Adrenergic Neurons and Receptors

• Adrenergic neurons release norepinephrine (NE) )

– from postganglionic sympathetic neurons only
– Excites or inhibits organs depending on receptors
– Alpha1 and Beta1 receptors produce excitation
– Alpha2 and Beta2 receptors cause inhibition
– Beta3 receptors(brown fat) increase thermogenesis

• NE lingers at the synapse until enzymatically inactivated by monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT)

Physiological Effects of the ANS

• Most body organs receive dual innervation

– innervation by both sympathetic & parasympathetic

• Hypothalamus regulates balance (tone) between  sympathetic and parasympathetic activity levels

• Some organs have only sympathetic innervation

– sweat glands, adrenal medulla, arrector pili mm & many blood vessels
– controlled by regulation of the “tone” of the sympathetic system

Sympathetic Responses
Dominance by the sympathetic system is caused by physical or emotional stress -- “E situations”

– emergency, embarrassment, excitement, exercise

• Alarm reaction = flight or fight response

– dilation of pupils
– increase of heart rate, force of contraction & BP
– decrease in blood flow to nonessential organs
– increase in blood flow to skeletal & cardiac muscle
– airways dilate & respiratory rate increases
– blood glucose level increase

• Long lasting due to lingering of NE in synaptic gap and release of norepinephrine by the adrenal gland

Parasympathetic Responses

• Enhance “rest-and-digest” activities

• Mechanisms that help conserve and restore body energy during times of rest

• Normally dominate over sympathetic impulses

• SLUDD type responses = salivation, lacrimation, urination, digestion & defecation and 3 “decreases”--- decreased HR, diameter of airways and diameter of pupil

• Paradoxical fear when there is no escape route or no way to win

– causes massive activation of parasympathetic division
– loss of control over urination and defecation

Autonomic or Visceral Reflexes

• Autonomic reflexes occur over autonomic reflex arcs. Components of that reflex arc:

– sensory receptor
– sensory neuron
– integrating center
– pre & postganglionic motor neurons
– visceral effectors

• Unconscious sensations and responses

– changes in blood pressure, digestive functions etc
– filling & emptying of bladder or defecation

            Control of Autonomic NS

• Not aware of autonomic responses because control center is in lower regions of the brain

• Hypothalamus is major control center

– input: emotions and visceral sensory information

• smell, taste, temperature, osmolarity of blood, etc

– output: to nuclei in brainstem and spinal cord
– posterior & lateral portions control sympathetic NS

• increase heart rate, inhibition GI tract, increase temperature

– anterior & medial portions control parasympathetic NS

• decrease in heart rate, lower blood pressure, increased GI tract secretion and mobility