Fifty One

Question 1

What is the highest center for regulation of autonomic responses? What autonomic centers are in the midbrain? Pons? Medulla?

Answer 1

Many types of autonomic phenomena have been elicited from various parts of the

cerebral hemispheres, e.g., frontal lobe, cingulate gyrus, orbital-insular-temporal cortex,

hippocampus, amygdala, caudate nucleus, etc. Most of the visceral responses are diffuse

and tend to overlap somatic reactions. The autonomic or visceral responses elicited by

stimulation in the cerebral hemispheres are funneled through the hypothalamus which is

considered the highest center for the regulation of autonomic responses.

In addition to the hypothalamic nuclei, other groups of neurons at various levels also

strongly influence autonomic activities. At the level of the midbrain, centers for

pupillary and accommodation responses are located in the pretectal area and superior

colliculus. In the pons, a micturition center rostrally governs the initiation of urination

and pneumotaxic and apneustic centers more caudally influence respiration. Within the

medulla are other respiratory and the cardiovascular centers.

Question 2

What are the two efferent branches of ANS? Name 5 differences between the two.

Answer 2

Sympathetic
thoraco-lumbar (preganglionic neuron location)
paravertebral and prevertebral ganglia
relatively long, thus a more diffuse action (postgang. fibers)
larger (e.g. 1:17) (pre to post gang. ratio)
prepares organism for “fight or flight” (Function)

Parasymp

cranio-sacral (pregang neuron location)

terminal ganglia (ganglion location)

relatively short, thus a more discrete action (postgang fibers)

smaller (e.g. 1:2) (pre to post gang. ratio)

prepares organism for “rest and digest”

Question 3

Where does activity in parasymp nerve fibers originate? What are the 4 preganglionic parasymp neurons that are located in the brainstem?

Answer 3

All activity in parasympathetic nerve fibers originates in the brainstem or spinal cord.

The brainstem preganglionic parasympathetic neurons are found in 1) the Edinger- Westphal nucleus, the visceral component of the oculomotor nuclear complex, 2) the superior salivatory nucleus, the visceral component of the facial nuclear complex, 3) the inferior salivatory nucleus, found near the rostral part of the nucleus ambiguus and contributing fibers to the glossopharyngeal nerve, and 4) the dorsal nucleus of the vagus as well as neurons scattered near the caudal part of the nucleus ambiguus whose axons emerge in the vagus nerve.

Question 4

Where are the parasymp postganglionic neurons located that receive pregang fibers from the oculomotor nerve, the facial nerve, and the glossopharyngeal nerve, and the vagus nerve? From what brainstem center do the nerves originate?

Answer 4

The parasympathetic postganglionic neurons receiving the preganglionic fibers from the

oculomotor nerve (Edinger westphal) are in the ciliary ganglion, from the facial nerve (superior salivatory) in the

pterygopalatine and submandibular ganglia, from the glossopharyngeal nerve (inferior salivatory) in the

otic ganglion, and from the vagus (dorsal nucleus of the vagus) in terminal ganglia both extrinsic and intrinsic to the thoracic, abdominal, and pelvic viscera that are vagally innervated.

Question 5

Where are the sacral pregang parasymp neurons located? Where do they emerge from the spinal cord? What do they synapse? Which organs do they innervate and what is their function?

Answer 5

The sacral preganglionic parasympathetic neurons are in and near the

intermediolateral nucleus in segments S.2, 3, 4.

The preganglionic fibers emerge from the spinal cord in the ventral roots and pass to the

terminal ganglia of the descending colon and rectum, the urinary bladder, and the

erectile tissues. Hence, the sacral parasympathetics influence defecation, urination, and

erection.

Question 6

Where are the pregang symp nerons located? Where do they emerge from the spinal cord? What happens from there? Where do they synapse? Where do they postgangl fibers then run? What do they innervate? What is their distribution like? Where are the paravertebral ganglia located? What is another name for them? Where are the prevertebral ganglia located? What is another name for them?

Answer 6

All activity in sympathetic nerve fibers originates in the spinal cord. The preganglionic

sympathetic neurons are found in several columns extending from about C.8 to L.2 or

3. They are located in the intermediolateral cell column of the lateral horn. The axons

of these neurons leave the spinal cord in the ventral root and initially run together in the

spinal nerve. They then separate from the somatic motor axons, enter the white

communicating rami and project to various sympathetic ganglia. Postsynaptic

sympathetic axons are found in virtually all spinal nerves and around major arteries

(periarterial plexuses) from which they extend to and innervate all the body’s blood

vessels, sweat glands, arrector muscles of hairs and visceral structures. Unlike the

parasympathetic distribution, which is more restricted, the sympathetic nervous system

innervates tissues throughout the entire body.

The postganglionic sympathetic neurons are in the paravertebral (sympathetic trunk

ganglia) and in the prevertebral (collateral or autonomic plexus) ganglia. The

sympathetic trunk ganglia are comprised of 20-25 pairs along the vertebral column, while

the autonomic plexus ganglia are found along the abdominal aorta, especially around the

origins of the celiac, superior mesenteric and inferior mesenteric arteries.

Question 7

Which branch of the autonomic afferents are mainly in charge of reflexes? Which branch of the autonomic afferents are mainly in charge of sensations? What are these sensations like? What is an exception?

Answer 7

The importance of impulses arising from visceral organs and blood vessels is mainly the

initiation of visceral reflexes; most do not reach the level of consciousness. Those

autonomic afferent impulses that do reach levels of awareness result in sensations that are

vague and poorly localized, e.g., hunger, nausea, fullness of urinary bladder and rectum,

etc. In certain conditions visceral sensations become painful.

In general, those fibers associated with reflex control of visceral activity accompany the

parasympathetic nerves, whereas those that convey visceral pain sensations

accompany the sympathetic nerves. An exception to this is visceral pain fibers from

certain pelvic viscera (sigmoid colon, rectum, neck of bladder, prostate gland, and cervix

of uterus) that accompany the pelvic parasympathetic nerves.

Question 8

What are the only conspicuous structures within the brainstem identified with the visceral afferent system? Where are they located? Where do the fibers come from? What is their output? Where do they synapse? What information is located in the rostral part of this nucleus? Which nerves synapse there? Same questions for intermediate part? Same questions for caudal part.

Answer 8

The solitary tract and solitary nucleus are the only conspicuous structures within the

brainstem that can be identified with the visceral afferent system. The solitary tract

extends from the lower part of the pons to the obex and is closely related throughout its

course to the solitary nucleus. The primary GVA fibers in the solitary tract arise chiefly

from the glossopharyngeal and vagus nerves. They synapse in the solitary nucleus from

whence secondary fibers enter the reticular formation through which connections are

made with the respiratory and cardiovascular centers, visceral and somatic motor nuclei,

and higher centers.

Along the solitary nucleus, taste information, from special visceral afferents in the facial, glossopharyngeal and vagus nerves, is represented most cranially,
gastrointestinal information, from vagal afferents, is represented in an intermediate position and cardiovascular and respiratory inputs, from vagal and glossopharyngeal afferents, are located in the caudal part of the nucleus.

Question 9

Describe the pathway for gustation including which nerves innervate which parts of the tongue, what the receptors are like for taste, in which ganglia these receptors synapse, where the postganglionic fibers synapse, and what is the output of the gustatory nucleus. What happens in damage to this pathway?

Answer 9

Taste buds in the tongue and epiglottis are chemoreceptors responsible for gustatory

impulses. The taste impulses are carried to the brainstem via primary neurons in the

geniculate (Facial, chordi tympani, nervus intermedius/anterior 2/3) petrosal (glossopharyngeal/posterior 1/3), and nodose ganglia (vagus/epiglottis).

The central branches of these ganglion cells enter the brainstem in the sensory root of CN

VII, the nervous intermedius, and in CN IX and X. In each case the axons enter the

solitary tract and synapse in the rostral part of the solitary nucleus, sometimes called

the gustatory nucleus.

Secondary taste fibers from the gustatory nucleus ascend contralaterally to the ventral

posteromedial nucleus of the thalamus. Clinical and experimental data show that the

cortical gustatory area is located chiefly in area 43 of the parietal operculum. It

extends into the adjacent insular cortex also. Interruption of the gustatory pathway either

peripherally or centrally results in a loss of taste (ageusia).

Question 10

Describe the pathway of visceral spinal autonomic afferents.

Answer 10

Besides the vagal route to the brainstem, the thoracic and abdominal viscera send afferent

fibers to the spinal cord via sympathetic trunks. From the heart, coronary vessels,

bronchial tree, and lungs visceral afferent fibers travel in the cardiac and pulmonary

nerves to the sympathetic trunk. From the abdominal viscera, afferent fibers travel

through the mesenteric and celiac plexuses, and the thoracic and lumbar splanchnic

nerves to the sympathetic trunk. After an uninterrupted course these afferent fibers

enter the thoracic and upper lumbar spinal nerves through the white communicating rami.

Their cell bodies are located in the dorsal root ganglia of T.1-L.2 and their first

synapse is in the spinal cord at these segments.

Visceral afferent impulses from the pelvic viscera also travel centrally by two routes.

Receptors in the sigmoid colon, rectum, urinary bladder, proximal part of the urethra, and

cervix of the uterus transduce stimuli into visceral afferent impulses subserving reflexes

and sensations. Some of these fibers course in the pelvic splanchnic nerves and have

their cell bodies located in the dorsal root ganglia of the second, third, and fourth

sacral spinal nerves. Others travel through the various hypogastric plexuses, lumbar

splanchnic nerves, and sympathetic trunk and its white communicating rami to reach their

cells of origin in the dorsal root ganglia of the lower thoracic and upper lumbar

spinal nerves.

Regardless of their route from the thoracic, abdominal, or pelvic cavities, or from blood

vessels, autonomic afferent fibers make their first synapse in the spinal cord.

Question 11

What are true visceral sensations like? Why? What are they insensitive to? What can cause visceral pain? What is true visceral pain? What is referred pain?

Answer 11

Although viscera are relatively insensitive, exaggerated stretching or contraction of

smooth muscle and certain pathological conditions will elicit awareness, discomfort, or

pain. True visceral sensations, e.g., heartburn, nausea, hunger, fullness of bladder or

bowels, etc., tend to be vague and poorly localized. This is due to such characteristics as

their multisynaptic central pathways and the meager representation of viscera in the

sensory areas of the cerebral cortex.

Visceral organs including the brain and spinal cord are insensitive to ordinary mechanical

and thermal stimuli. Thus, even though handling, cutting, crushing, or burning of viscera

occurs during surgical procedures, sensations are not elicited. Painful sensations do

result from excessive stretch, violent or spastic contractions, or decreased blood supply. In such conditions the pain may be felt in the region of the organ itself (truevisceral pain) or in a region of skin or other somatic tissue innervated by the same spinal cord or brainstem level that receives the visceral afferent impulses (referred pain).

Question 12

Describe the different ocular reflexes

Answer 12

Postganglionic sympathetic fibers from the superior cervical ganglion innervate the

dilator muscles of the iris, while the postganglionic parasympathetic fibers from the

ciliary ganglion innervate the sphincter muscles of the iris that constrict the pupil.

Pupillary size is controlled by the balanced activity of the sympathetic and

parasympathetic systems, however, under conditions of excitement or alarm there is a

shift in this balance, in which increasing sympathetic tone results in pupillary dilation.

The parasympathetic system also controls focusing of the lens during accommodation by

innervation of the ciliary muscles via postganglionic fibers. Muller’s muscle, which

retracts the upper eyelid, is under sympathetic control.

Question 13

Describe the pathways of the baroreceptor reflex.

Answer 13

The regulation of arterial blood pressure is accomplished by the baroreceptor reflex.

Arterial baroreceptors are located in the carotid and aortic sinuses. The carotid sinus is innervated by afferents in the glossopharyngeal nerve whereas the aortic sinus baroreceptors are innervated by afferents in the vagus nerve. These impulses synapse in the caudal part of the solitary nucleus which then modulates the adjacent cardiovascular center in the medulla. From there impulses travel to preganglionic parasympathetic neurons in the dorsal nucleus of the vagus or descend the spinal cord to innervate preganglionic sympathetics in upper thoracic levels. Increases in blood pressure elicit a vagal response, resulting in bradycardia, and decreases in blood pressure elicit sympathetic responses, resulting in tachycardia.

Question 14

Explain the parasympathetic control of the detrusor muscle and internal sphincter.

Answer 14

The excitatory input to the wall of the bladder that causes contraction of the

detrusor muscle and subsequent emptying is entirely parasympathetic.

Preganglionic parasympathetic fibers from sacral levels, S.2, 3, 4, travel in the pelvic

splanchnic nerves and activate postganglionic parasympathetic neurons in the pelvic

ganglion plexus within the wall of the bladder. There activation results in contraction of

the bladder’s detrusor muscle. These postganglionic neurons are inhibited when the

bladder begins to fill but are activated by visceral afferents in pelvic splanchnic nerves

when the bladder is distended.

It also inhibits the internal sphincter.

Question 15

Explain the sympathetic control of the detrusor muscle and internal sphincter muscle.

Answer 15

The action of the sympathetics is to relax the detrusor muscle. Axons of

preganglionic sympathetic neurons project from lower thoracic and upper lumbar spinal

cord (T.12-L.2) to the inferior mesenteric ganglion. From there, postganglionic fibers

travel to the bladder in the hypogastric nerves. When the sympathetic system is activated

the detrusor is relaxed and the internal sphincter muscle is activated maintaining

contraction of the bladder outlet allowing for filling of urine.

Question 16

Explain the somatic input concerning urination.

Answer 16

Somatic motor neurons in the sacral spinal cord (referred collectively as Onuf’s

nucleus) innervate striated muscle in the external urethral sphincter, causing it to
contract. Axons from Onuf’s nucleus project to the external sphincter muscle in the

pudendal nerve. When these neurons are activated the sphincter is in the contracted state

and the bladder outlet is closed.

Question 17

Explain supraspinal center involvement in bladder and urethral function.

Answer 17

Supraspinal centers involved in bladder and urethral function are found in various

cortical and subcortical areas. A frontal lobe micturition center located on the medial

surface near the anterior cingulated gyrus functions to tonically inhibit signals to the

detrusor muscle to prevent the bladder from emptying until a socially acceptable time and

place to urinate is available. A pontine micturition center (PMC) initiate’s micturition.

Ascending spinal afferents relay information to the PMC of bladder fullness. Descending

impulses from the PMC cause coordinated inhibition of sympathetic and somatic

systems, relaxing both sphincters, and cause excitation of parasympathetic impulses

resulting in emptying of the bladder.

Question 18

What is neurogenic bladder? What is uninhibited reflex bladder? Where is the lesion? What do pontine lesions result in? What is a non reflex bladder? Where is the lesion? What is an autonomic reflex bladder? Where is the lesion?

Answer 18

Lesions of the CNS or PNS may result in loss of bladder function, referred to as

neurogenic bladder. A bilateral frontal lobe lesion destroying the frontal micturition

center in the supplementary motor cortex causes a complete loss of voiding control

(unihibited reflex bladder). The spinal reflexes for voiding remain intact however and

affected individuals show signs of urge incontinence and spastic bladder (also known as

detrusor hyperreflexia). The bladder is disinihibitied by lack of cortical control and

empties too quickly and too often, with relatively low quantities, and storing urine in the bladder is difficult.

Pontine lesions result in voiding difficulty and incontinence. However, these are often masked by more serious symptoms such as unconsciousness

and respiratory arrest due to interruption of adjacent brainstem centers.

Bilateral lesions of the spinal cord above sacral levels result in an autonomic reflex bladder which is spastic and overactive. Affected individuals experience urge incontinence and the bladder empties too quickly and too frequently.

Bilateral lesion of the sacral cord involving sacral levels S.2, 3, 4 or a peripheral lesion involving spinal nerves S.2, 3, 4 (i.e. the cauda equina), interrupts the spinal reflex resulting in a non-reflex bladder that is under active and flaccid (also known as detrusor areflexia). Affected individuals may not be able to sense when the bladder is full and have difficulty eliminating urine and experience overflow incontinence.

Question 19

Waht is a neurogenic bowel? What is an LMN or flaccid bowel? Where is the lesion? What is an UMN or reflex bowel? Where is the lesion?

Answer 19

Emptying of the bowel (defecation) is similar to that of the bladder in that both

autonomic and somatic efferents are involved and are controlled by surpaspinal centers in

the brainstem and frontal lobe. A neurogenic bowel may result from interruption of the

reflex either in the sacral cord or sacral nerves (S2, 3, 4) or with injury to brainstem or

frontal lobes. A spinal cord injury involving the sacral cord or sacral nerves (most

commonly the pudendal nerve in females during childbirth) may damage the defecation

reflex and relax the anal sphincter muscle resulting in fecal incontinence. This is known

as a lower motor neuron or flaccid bowel. Management of this type of bowel problem

may require more frequent attempts to empty the bowel or manual removal of stool.

If the spinal cord injury is above the sacral cord bowel movements will occur on a reflex basis. This means that when the rectum is full, the defecation reflex will occur, emptying the bowel. This results in an upper motor neuron or reflex bowel.

Question 20

What inputs play an important role in the sexual response? What is the parasympathetic pathway? The result? Sympathetic? Somatic? Supraspinal?

Answer 20

Sexual reflexes are organized in a pattern that is analogous to those controlling bladder

function in that both spinal and supraspinal inputs play an important role in the sexual

response. Mechanoreceptors in the genitalia increase the rate of firing when stimulated.

Signals from the mechanoreceptors enter the sacral spinal cord, leading to stimulation of

parasympathetic neurons in segments S.2, 3, 4. Parasympathetic stimulation causes

arterioles in the genitalia to dilate, leading to engorgement of the genitalia. Genital

responses during orgasm are mediated by the sympathetic innervation from the lower

thoracic and upper lumbar cord and somatic innervation of the pelvic floor from the

pudendal nerve. Little is known about the role of supraspinal control centers for sexual

function although it is thought that cerebral processing determines libido and desire. A

familiar formula to remember the control of sexual reflexes is the mnemonic

Parasympathetic = Point; Sympathetic and Somatic = Shoot.