Autonomic Nervous System

Question 1

Autonomic pathways involve how many ganglionic synapses?

Answer 1

• 1 ganglionic synapse; they are 2-neuron chain pathways (1st neuron’s cell body is in the CNS, 2nd’s is in the ganglion)
• EXCEPT for the adrenal medulla! in this case, there is no ganglionic synapse (only 1 neuron is involved)

Question 2

Where do sympathetic fibers originate? What about parasympathetic fibers?

Answer 2

• sympathetic: thoracolumbar (T1-L3); only these segments have lateral horns (projections from T1/T2 ascend to form the cervical ganglion and supply the head and neck –> Horner’s syndrome)
• parasympathetic: craniosacral (cranial nerves III, VII, IX, X, S2-S4)

Question 3

Are sympathetic preganglionic fibers short or long? What about sympathetic postganglionic fibers? Parasympathetic?

Answer 3

• sympathetic: short preganglionic (sympathetic trunk/sympathetic ganglion chain runs along each side of the spine), long postganglionic
• (some sympathetic preganglionic fibers pass through the sympathetic trunk WITHOUT synapsing; they synapse at a later sympathetic collateral ganglia; pre and post fibers are about equal in length)
• parasympathetic: long preganglionic (terminal ganglia are near the effector organ), short postganglionic

Question 4

Which neurotransmitter do sympathetic preganglionic fibers release? Parasympathetic preganglionic fibers? Postganglionic fibers?

Answer 4

• ALL preganglionic fibers release ACh
• parasympathetic postganglionic fibers release ACh
• (most) sympathetic postganglionic fibers release norepinephrine (those innervating thermoregulatory sweat glands release ACh instead)
• (therefore, all preganglionic and all parasympathetic postganglionic fiber are cholinergic fibers; most sympathetic postganglionic fibers are adrenergic fibers)

Question 5

Most visceral organs have a dual innervation (both sympathetic and parasympathetic tone), what are some of the exceptions?

Answer 5

• blood vessels only have sympathetic innervation (but those of the penis and clitoris receive both)
• thermoregulatory sweat glands only have sympathetic innervation (and these postganglionic fibers release ACh not NA)
• the liver, adipose tissue, and the kidneys are also all only innervated by the sympathetic branch
• (also, note that BOTH systems are stimulating in regards to salivary gland activity)

Question 6

What are the two types of cholinergic receptors? Where is each type found?

Answer 6

• nicotinic and muscarinic
• all autonomic postganglionic cell bodies have nicotinic receptors (including the chromaffin cells of the adrenal medulla, which are technically postganglionic)
• all effector cells innervated by the parasympathetic system have muscarinic receptors; these are GPCRs

Question 7

What are the four types of adrenergic receptors? Where is each type found? Which are excitatory?

Answer 7

• alpha 1: found in most tissues; excitatory (constriction)
• alpha 2: found in digestive organs; inhibitory (relaxation)
• beta 1: found mainly in heart; excitatory (constriction)
• beta 2: found in arterial and bronchiole smooth muscle; inhibitory (relaxation, dilation)
• (all 4 are GPCRs, just like muscarinic cholinergic receptors)

Question 8

Where are the cell bodies of the preganglionic sympathetic fibers? What about those of the preganglionic parasympathetic fibers?

Answer 8

• pre symp: in the lateral horns of the spinal cord (only the thoracic and upper lumbar segments have lateral horns)
• pre para: brain stem or sacral spinal cord

Question 9

What do adrenergic receptors respond to?

Answer 9

• norepinephrine and epinephrine

Question 10

What is the adrenal medulla? What does it secrete?

Answer 10

• the adrenal medulla is essentially a modified sympathetic ganglion that does not give rise to postganglionic fibers
• instead, it secretes neurotransmitters in a hormone form; 80% of its secretions is epinephrine, 20% is norepinephrine
• (the chromaffin cells here have the enzyme PNMT, which converts norepinephrine into epinephrine; regular nerve fibers do not have PNMT)
• (PNMT: phenylethanolamine-N-methyltransferase; requires cortisol for the reaction)

Question 11

What is the sympathetic trunk also known as? What about the collateral sympathetic ganglia? What do branches from the collateral ganglia innervate?

Answer 11

• sympathetic trunk is also known as the paravertebral ganglia
• collateral ganglia also known as prevertebral ganglia; 3 ganglia: celiac (stomach and SI), superior mesenteric (SI and LI), inferior mesenteric (LI, anus, bladder, genitalia)

Question 12

Follow the path of innervation of the adrenal medulla.

Answer 12

• roots originate from T5-T9 in the lateral horns, leave the spinal cord via the ventral roots (same as all efferents), pass through BOTH the sympathetic trunk and celiac collateral ganglion without synapsing, and travel to the adrenal medulla via the greater splanchnic nerve

Question 13

Which neurotransmitters are released by sympathetic postganglionic adrenergic nerves? What is each packaged in?

Answer 13

• 3 neurotransmitters
• the classic neurotransmitter NA, plus the non-classic neutrotransmitters ATP and neuropeptide Y
• NA and ATP are packaged in small dense-core vesicles, neuropeptide Y is packaged in large dense-core vesicles
• (small vesicles are released first, ATP actually binds before NA; further stimulation will release the large vesicles)

Question 14

What cell type does the greater splanchnic nerve synapse on when innervating the adrenal medulla? What receptor type is found here?

Answer 14

• chromaffin cells (these are the cells that secrete epinephrine (80%) and norepinephrine (20%) into the general circulation
• they have nicotinic receptors

Question 15

Which neurotransmitters are released by parasympathetic postganglionic nerves? What is each packaged in?

Answer 15

• the classic neurotransmitter ACh, plus the non-classic neurotransmitters VIP, NO, and others
• ACh is packaged in small clear vesicles, all others are in large dense-core vesicles

Question 16

Explain how pupil size is regulated.

Answer 16

• pupil size is controlled by the radial muscle (pupillary dilator) and the sphincter muscle (pupillary constrictor)
• the radial muscle is innervated by sympathetic fibers and has alpha 1 receptors; stimulation here results in activation/constriction of the pupillary dilator leading to pupil dilation
• the sphincter muscle is innervated by parasympathetic fibers (muscarinic receptors); stimulation here results in activation/constriction of the pupillary constrictor leading to pupil constriction

Question 17

How do nicotinic receptors respond to activation?

Answer 17

• these receptors are chemically gated Na+ and K+ channels that open when bound, leading to depolarization
• they respond to nicotine, ACh, and carbachol; inhibited by curare (hexamethonium inhibits only the ganglia nicotinic receptors, not the motor end plates)

Question 18

How do muscarinic receptors respond to activation?

Answer 18

• these are GPCRs
• some have a Gq protein, whose activation results in activating phospholipase C, which generates DAG and IP3/Ca2+, resulting in the activation of PKC (protein kinase C) (same mechanism as alpha 1 receptors)
• others have a G protein that directly brings about the response (so, without secondary messengers)

Question 19

How do alpha 1 receptors respond to activation?

Answer 19

• these are GPCRs that result in contraction/excitation when stimulated; they respond to large amounts of epinephrine and norepinephrine
• have a Gq protein, whose activation results in activating phospholipase C, which generates DAG and IP3/Ca2+, resulting in the activation of PKC (protein kinase C) (same mechanism as muscarinic receptors)

Question 20

How do alpha 2 receptors respond to activation?

Answer 20

• these are GPCRs that result in relaxation/inhibition when stimulated; they respond to large amounts of epinephrine and norepinephrine
• have a Gi protein, whose activation results in the inhibition of adenylyl cyclase, leading to decreased levels of cAMP

Question 21

How do beta 1 receptors respond to activation?

Answer 21

• these are GPCRs that result in contraction/excitation when stimulated; they respond to small amounts of epinephrine and norepinephrine
• have a Gs protein, whose activation results in the activation of adenylyl cyclase, leading to increased levels of cAMP (same mechanism as beta 2 receptors, but with a contractile result)

Question 22

How do beta 2 receptors respond to activation?

Answer 22

• these are GPCRs that result in relaxation/inhibition when stimulated; they respond only really to epinephrine
• have a Gs protein, whose activation results in the activation of adenylyl cyclase, leading to increased levels of cAMP (same mechanism as beta 1 receptors, but with a relaxing result)

Question 23

Explain the different sensitivities of the four adrenergic receptors and what this means for their activation.

Answer 23

• alpha 1 and 2: require large amounts of NE and E, meaning they only respond to nerves (the NE and E from the adrenal medulla is too diffuse and not enough)
• beta 1: require small amounts of NE and E, so they respond to nerves and/or adrenal medulla
• beta 2: mainly responds to E, so it really only responds to the adrenal medulla

Question 24

What is the highest center for autonomic function/control?

Answer 24

• the hypothalamus

- the anterior half deals with parasympathetic neurons, and the posterior half deals with sympathetic neurons

Question 25

White are the rami communicantes?

Answer 25

• these are bundles of sympathetic fibers
• the white ramus communicantes (myelinated) brings the preganglionic sympathetic nerves to the proper ganglion
• the grey ramus communicantes (unmyelinated) brings the postganglionic sympathetic nerves to the proper organ
• (preganglionic fibers are myelinated, postganglionic fibers are not)

Question 26

What is autonomic dysreflexia? Which patients are at an increased risk for developing this condition?

Answer 26

• autonomic dysreflexia is a condition of sudden high blood pressure that can eventually cause brain hemorrhage or fits
• common causes are bladder and/or bowel dysfunction; therefore, patients with spinal cord injuries are susceptible to developing this condition

Question 27

What do patients with autonomic dysreflexia present with? How do we treat it?

Answer 27

• patients develop a pounding headache that continues to get worse (as BP increases) and HTN
• also: blurred vision, flushing, sweating, chills, and bradycardia can occur
• treat the underlying cause (catheterization, manual bowel evacuation) and give GTN to decrease the BP

Question 28

What is Horner syndrome?

Answer 28

• sympathectomy of the face (loss of sympathetic input to the face); results in ptosis (droopy eyelid), anhidrosis (loss of sweating of face), and miosis (pupil constriction)
• results from lesions in the spinal cord occurring above T1 or in the lateral horns of T1