IVMS Autonomic Nervous System Notes

Autonomic Nervous System Notes
Companion PowerPoint Presentations

IVMS-ANS Pharmacology -Intro to the Autonomic Nervous System

IVMS-ANS Pharmacology- Adrenergic Drugs

IVMS- ANS Pharmacology- Cholinergic Agents

• The Autonomic Nervous System is divided into sympathetic and parasympathetic divisions.
• Basic plan: Nerve impulses propagate from autonomic sensory receptors (and in some cases:
somatic sensory neurons and special sensory neurons – fight-n-flight mechanism) in visceral
organs and blood vessels to integrating centers in CNS. Then, impulses from the autonomic
motor neurons propagate back to the effector tissues regulating smooth (eg., intestinal walls,
blood vessel walls), cardiac, and many glands.
• Autonomic Nervous System operates without conscious control.
• Autonomic Nervous System is controlled by centers in the hypothalamus, limbic system, brain
stem, spinal cord, and to some extent, by the cerebral cortex.
• Examples of autonomic sensory receptors: chemoreceptors, mechanoreceptors, etc.
• Impulses from these receptors are not consciously sensed, except in severe cases like angina
pectoris.
• Autonomic motor neurons work by increasing or decreasing activities of effector tissues.
• Tissues powered by autonomic motor neurons function to some extent even if their nerve
supply is damaged.
• The structure of Autonomic Nervous System is basically two types of neurons connected in
series. The nucleus of the first neuron is located in the spinal cord or brain; and its axons, after
traveling as part of cranial or spinal nerve, are located in the autonomic ganglion. The nucleus
of the second neuron is located in the autonomic ganglion, and its axons are located in effector
tissue.
• The two types of giant neurons communicate only through Ach, but the second neuron
communicates with effector tissue using Ach or norepinephrine (NE).
• The two types of neurons send their impulses through sympathetic and parasympathetic
divisions.
• Impulses from both the divisions oppose each other.
• The hypothalamus is connected to both the systems.

Autonomic Nervous System Notes Marc Imhotep Cray, M.D.

Anatomy of Autonomic Nervous System (Illustration)

• The axon of the first neuron is a small-diameter, myelinated type B fiber that extends to an
autonomic ganglion.
• The axon of the second neuron is a small-diameter, unmyelinated type C fiber that extends to
visceral effector tissue.

Sympathetic Division of the Autonomic Nervous System (Illustration)
• Preganglionic neurons have their cell bodies in lateral horns of the gray matter in the 12
thoracic segments and the first two lumbar segments of the spinal cord.
• Also known as thoracolumbar division.
• There are two types of autonomic ganglia: sympathetic trunk ganglia and prevertebral ganglia.
• Sympathetic trunk ganglia extend from the base of the skull to coccyx.
• Postsympathetic axons from sympathetic trunk ganglia innervate organs above the diaphragm.
• Prevertebral ganglia lie anterior to the vertebral column and innervate organs below the
diaphragm.
• Three major types: celiac ganglion, superior mesenteric ganglion, inferior mesenteric ganglion.
• Once axons of sympathetic preganglionic neurons pass to sympathetic trunk ganglia, they
connect to (sometimes up to 20) postganglionic neurons in three different ways.
• This way, many sympathetic responses affect almost the entire body simultaneously.
• Path of a sumpathetic preganglionic neuron: Anterior root of spinal nerve > intervertebral
foramina > white ramus> sympathetic trunk ganglion > postganglionic neurons.
• Adrenal medulla is an organ that is directly hooked up to sumpathetic preganglionic neurons.
• Upon stimulation, adrenal medulla releases epinephrine, norepinephrine, and dopamine
directly into blood. These hormones linger in the blood prolonging the effect of sympathetic
stimulation.
• All preganglionic neurons are cholinergic.
• All postganglionic neurons that innervate most sweat glands are cholinergic.
• Most postganglionic neurons are adrenergic.


Parasympathetic Division of the Autonomic Nervous System (Illustration)
• Preganglionic neurons have their cell bodies in the nuclei of four cranial nerves in the brain
stem (III, VII, IX, and X) and in lateral gray horns of the second through fourth sacral segments.
• Also known as craniosacral division.
• Axons of preganglionic neurons emerge as part of a cranial nerve or as part of the anterior root
of a spinal nerve.
• There are two types of preganglionic neuron divisions: cranial parasympathetic outflow (vagus
n; CN X makes up 80% of this) and sacral parasympathetic outflow (anterior roots of S2-S4;
pelvic splanchnic nerves).
• The preganglionic and postganglionic axons synapse in terminal ganglia, which are located in
effector tissues.
• Most of these ganglia are actually located in the visceral effector tissue, so parasympathetic
preganglionic neurons have much longer axons.
• Preganglionic neurons synapse with only 4-5 postganglionic neurons, all of which supply a
single visceral effector.
• Parasympathetic responses can be localized to a single effector.
• All preganglionic neurons are cholinergic.
• All postganglionic neurons are cholinergic.

Autonomic Nervous System Neurotransmitters and Receptors
• There are two types of autonomic neurons: cholinergic or adrenergic
• Receptors for neurotransmitters are integral plasma proteins what are located in the plasma
membrane of the postsynaptic neuron or effector cell.

Cholinergic Neurons and Receptors
• They release Ach.
• They are all the sympathetic and parasympathetic neurons, sympathetic postganglionic
neurons that innervate most sweat glands, and all parasympathetic postganglionic neurons.
• ACh from synaptic clefts in presynaptic neurons > binds specific cholinergic receptors in
postsynaptic neurons.
• There are two types of cholinergic receptors: nicotinic receptors and muscarinic receptors.
• ACh causes depolarization in tissues with nicotinic receptors.
• ACh causes depolarization or hyperpolarization in tissues with muscarinic receptors.
• ACh is deactivated by acetylcholinesterase (AChE).


Adrenergic Neurons and Receptors
• Adrenergic neurons release norepinephrine (NE), also known as noradrenalin.
• NE in synaptic vesicles > diffusion across synaptic cleft > bind to adrenergic receptors.
• Adrenergic receptors bind both hormones norepinephrine and epinephrine.
• There are two types of receptors: alpha and beta.
• They are further classified as alpha1 (excitation), alpha2 (inhibition), beta1 (excitation), beta2
(inhibition), and beta3 (present on brown fat; activation causes heat produciton).
• NE action is terminated either when it is reabsorbed back into the releasing axon, or by
enzymatic inactivation by catechol-O-methyltransferase (COMT) also known as monoamine
oxidase (MAO).
• NE creates long lasting effects because its reuptake is slower than that of ACh.

Agonists and Antagonists
• Agonists activate receptors, mimicking the effect of a natural neurotransmitter or hormone.
• Antagonists deactivate receptors.

Physiologic effects of the Autonomic Nervous System
• Sympathetic and parasympathetic systems are in opposition to each other; mediated by the
hypothalamus.
• A few structures receive only sympathetic innervation.

Sympathetic Responses
• It dominates during mental or physical stress.
• 4E’s : exercise, excitement, emergency, and embarrassment.
• Activates the fight-or-fight response.
• Effects are longer lasting than parasympathetic system.


Parasympathetic Responses
• It dominates during periods of relaxation.
• Activates the rest-and-digest response.
• SLUDD: salivation, lacrimation, urination, digestion, and defecation.
• Also causes 3D: decrease in heart rate, diameter of airways, and diameter of pupils.

Control of Autonomic Functions
• Receptor: located at the distal end of a sensory neuron.
• Sensory neuron: conducts impulses from receptors to CNS.
• Integrating center: located in the hypothalamus and brain stem (thus, we are not aware of their
responses), these centers relay signals from sensory neurons to motor neurons. There are no
integration centers for urination and defecation.
• Motor neurons: relay signals from integrating centers to effectors.
• Effector: they are the smooth muscle, cardiac muscle, or glands.

Questions
1. What are the principal components of the autonomic nervous system (Autonomic
Nervous System)?
Sympathetic and Parasympathetic systems.
2. What is the general function of the Autonomic Nervous System. Contrast this to the
function of the somatic branch of the peripheral nervous system.
Control autonomic processes such as breathing, heart-rate, etc. Somatic branch deals with the
(voluntary) nervous system.
3. How are alpha and beta-receptors related to noradrenaline?
Noradrelin binds to alpha and beta receptors.
4. What are the autonomic effects on the SA node?
Sympathetic: increases heart rate; Parasympathetic: decreases heart-rate.
5. What are the autonomic effects on the ventricular muscle?
Sympathetic: increases heart rate (more powerful contractions); Parasympathetic: decreases
heart-rate (less powerful contractions).
6. Describe the components of the “alarm reaction" and their physiological basis.
Fight-or-flight response


7. What is the physiologic purpose for the tachycardia in the alarm reaction?
This helps to increase blood flow to skeletal muscles, in preparation for fight or flight
8. Which organ produces large amounts of catecholamines in the body?
Epinephrine and norepinephrine are produced by adrenal medulla.
9. What is the effect of increased sympathetic discharge on skin blood flow? On skeletal
muscle blood flow?
Blood is diverted from skin (less priority) to skeletal muscles (high priority) during fight-or-flight
response.
10. A certain category of poisons works by inhibiting acetylcholinesterase – this results
in increased amounts of acetylcholine binding to receptors. Knowing what you know of
the effects of acetylcholine, what signs and symptoms would you expect in a patient who
had ingested this sort of poison?

The autonomic neurons are predominantly responsible for the involuntary regulation of the
heart, smooth muscle and glands. This control is essential to maintain homeostasis and life.

Divisions: Neurons are divided into Preganglionic and postganglionic neurons. Is under
involuntary control. Preganglionic fibres are sited in the midbrain and hindbrain, and T1-S4 of
the spinal cord.
1. Sympathetic system: Short pre - and long postganglionic fibres (See Atlas).
2. Parasympathetic system: Long pre - and short postganglionic fibres.
3. Main innervation: Smooth muscle, heart muscle and glands.
4. Autonomic ganglia: are sited in the head, neck, and abdomen.
5. Sympathetic chain: lies on both sides of the thoracic and lumbar vertebra.

General Functions:
1. Functions at a subconscious level, but is strongly influenced by higher integrative levels i.e.
hypothalamus.
2. Integration of cardiac function.
3. Control of GUT movement i.e. smooth (visceral) muscle.
4. Integrates secretion of various glands (i.e. enteric, endocrine)
5. Regulation of visceral organs and blood vessels.


Regulation/control of the Autonomic Nervous System (Autonomic Nervous System):
The control of the sympathetic and parasympathetic divisions is by higher centers sited in the
hypothalamus. Homeostasis is thereby maintained by hypothalamic integration of the
Autonomic Nervous System and Neuroendocrine systems. This is possible by the secretion of
releasing factors and release-inhibiting factors. The cerebral cortex, hypothalamus and limbic
systems work in concert to affect homeostasis.

Parasympathetic System: Brainstem and sacral outflow of spinal cord.
Divisions:
1. Cranial outflow: Visceral structures of the head are supplied by the cranial nerves
(oculomotor, facial, glossopharyngeal and vagus nerves) relaying through the ciliary,
sphenopalatine, submaxillary and otic ganglion.
2. Sacral outflow (pelvic nerve - S2, 3, 4).
3. Length of fibres: Long preganglionic fibres relay on short postganglionic neurons located
adjacent or near the visceral structures.
4. Association with parasympathetic extracranial ganglia (ciliary, pterygopalatine, submandular
and otic ganglia).

Mechanism of Action: Stimulation results in -
1. Heart: Bradycardia (slowing of the heart).
2. Lungs (bronchi): Bronchoconstriction.
3. Pupil: Constriction.
4. GIT: Increased peristalsis, sphincters relaxed.
5. Blood vessels: Dilatation of visceral blood vessels.
6. Sweat glands and hair follicles: Regulated by sympathetic system.
7. Responses 1-6 are due to the release of acetylcholine from postganglionic fibres. The results
just the opposite (antagonistic) to the effects of sympathetic stimulation.

Innervation:
1. Cranial nerves, eye, lacrimal gland, submandibular gland, sublingual gland and parotid gland.
Vagus innervates widely (heart, larynx, trachea, bronchi, lung, small bowel, abdominal vessels,
liver, gallbladder, bile ducts, pancreas, kidney and ascending colon.
2. Sacral outflow (S1 - 4). Descending colon, sigmoid and rectum, bladder, sex organs and
external genitalia.


Sympathetic System: - Thoraco-lumbar section of spinal cord.
Divisions:
1. Spinal component (axons). Segments T1 - L3 (thoraco-lumbar outflow).
2. Superior, middle, inferior, cervical ganglia.
3. Sympathetic ganglionated (paravertebral) chain, white and gray rami communicantes.
4. Greater, lesser and small splanchnic nerves.
5. Ganglia: Coeliac, superior mesenteric and inferior mesenteric (termed collateral or
Prevertebral ganglia).
6. Long postganglionic fibres.

Thoracic splanchnic nerves:
1. Chief source of sympathetic nerves for the abdomen.
2. Greater, lesser and lowest are important (visible, once the lung is removed during thoracic
dissection of the cadaver).
3. The nerves contain preganglionic fibres and originate from the 5th to the 12th thoracic
sympathetic ganglia.
4. Twigs are sent to the Coeliac ganglia and renal plexus.

Sympathetic chain and ganglia:
1. Are visible on both sides of the thoracic and lumbar vertebra. Union of the two chains occurs
at the ganglion impar sited on the coccyx. Therefore the chains pass through the posterior
aspect of the abdominal cavity into the pelvis.
2. The cervical, thoracic and lumbar sympathetic chains are surgically important, laparoscopic
sympathectomy may be indicated for hyperhidrosis (excessive sweating in the regions of the
Axilla, hands and feet).

Rami Communicantes: Arrangement:
1. Preganglionic neuron fibres pass from the spinal cord via the anterior roots (T1 - T3) and the
white rami comunicantes to the paravertebral sympathetic ganglion chain. Fibres either continue
to the viscera or re-enter the spinal nerve via the grey rami communicantes.


Mechanism of Action: Stimulation results in -
1. Heart: Tachycardia.
2. Lungs: Bronchodilation.
3. Pupil: Dilation.
4. GIT: Inhibited peristalsis, sphincters contracted.
5. Blood vessels: Vasoconstriction.
6. Sweat glands: Increased sweating.
7. Hair follicles: Hair stands on end via m. arrectores pilorum.
8. Responses 1-7 are due to release of epinephrine from the adrenal medulla.

Innervation:
1. Superior cervical ganglion: Fibres pass via the carotid artery plexus and distributed to the
eye, lacrimal, submandibular, sublingual and parotid gland.
2. Middle and inferior cervical ganglion innervate the heart and respiratory tree.
3. T1 - T4: innervate the heart and respiratory tree.
4. T5 - T9: Enter the Coeliac ganglion and then are distributed to the stomach, gut, abdominal
vessels, liver biliary system and pancreas.
5. T10 - T12: via the small splanchnic nerve which relay in the Coeliac and superior mesenteric
ganglia and innervate the adrenal medulla and kidney.
6. L1 - L3 via the inferior mesenteric ganglion innervate the transverse colon downwards,
rectum, bladder and sex organs.
7. The Coeliac ganglion, superior and inferior ganglion are sited near to the viscera and are
termed collateral ganglia.

Summary: Autonomic System:

Division: Sympathetic (for “flight and fright” reaction) and parasympathetic systems.


Sympathetic System features:
1. Short preganglionic fibres.
2. Ganglionated sympathetic trunk visible in the paravertebral zone. Superior, middle and
inferior ganglia important in the neck (paravertebral ganglia). Occasionally the inferior cervical
ganglion fuses with T1 and forms the Stellate ganglion which overlies the neck of the first rib.
3. Two trunks fuse inferiorly near the coccyx - ganglion impar.
4. Outflow fibres: white rami.
5. Postganglionated fibres: grey rami communicantes.
6. Cranial sympathetic outflow is relayed on the internal carotid artery.
7. Other zones:
a. Cervical sympathetic.
b. Thoracic sympathetic (pulmonary, cardiac plexus: splanchnic nerves - greater, lower, lowest.
c. Lumbar sympathetic - nerves.
d. Pelvic sympathetic - presacral and inferior hypogastric plexus.

Parasympathetic System or Craniosacral System:
1. Cranial parasympathetic outflow: via cranial nerves (3, 6, 9, 10, and 11). Relay in otic, ciliary,
submandibular and sphenopalatine ganglia).
2. Branches of Vagus nerve. Distributed to
a. Cardiac branches.
b. Esophageal branches.
c. Cardiac plexus.
d. Pulmonary plexus.
e. Anterior and posterior gastric nerves.
3. Sacral outflow (S2, 3, 4) - supplies rectum, bladder, uterus, erectile tissue of penis and
clitoris, distal colon.
4, Visceral efferent pathways: consist of three sets of neurons:
a. Upper motor neuron (grey matter of brain).
b. Preganglionic neuron (lower motor neuron i.e. sited in grey matter of brainstem or spinal
cord).
c. Postganglionic neuron (sited in autonomic ganglion).

Also see: IVMS Autonomic Nervous System Summary


IVMS Autonomic Nervous System Notes

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IVMS Autonomic Nervous System Notes