Chapter 12: The Autonomic Nervous System

Question 1

Describe the basic organisation of the ANS.

Answer 1

Afferent, connector, efferent.

Question 2

Where do the afferent impulses originate? Where do they then travel?

Answer 2

Originate in visceral receptors.
Travel to afferent pathways to CNS.
Integrated through connector neurons at different levels.
Leave via efferent pathways to visceral effector organs.

Question 3

What are efferent pathways made up of?

Answer 3

Efferent pre-ganglionic and post-ganglionic neurons.

Question 4

Where are the cell bodies of the efferent PREganglionic neurons situated?

Answer 4

• Lateral gray columns (horns) of the spinal - cord

- Motor nuclei of the 3rd, 7th, 9th, 10th cranial nerves

Question 5

The axons of the PREganglionic neuron cell bodies synapse where?

Answer 5

On the cell bodies of the POSTganglionic neurons that are collected together to form ganglia outside of the CNS.

Question 6

The ANS is concerned with the innervation of which involuntary structures?

Answer 6

Heart, smooth muscle, glands

Question 7

Is the ANS distributed throughout the central or peripheral nervous systems.

Answer 7

Both

Question 8

Name the divisions of the ANS. Which part is larger?

Answer 8

Sympathetic (larger) and parasympathetic.

Question 9

The SNS innervated which structures? Name 5.

Answer 9

Heart and lungs, muscle in the walls of many blood vessels, hair follicles, sweat glands, many abdominal pelvic viscera.

Question 10

Describe the Direct and Consensual Light Reflex.

Answer 10

• Afferent nervous impulses travel from the retina, through the optic nerve, optic chiasma, and optic tract.
• A small number of fibers leave the optic tract and synapse on nerve cells in the pretectal nucleus (lies close to superior colliculus)
• Impulses are passed by axons of the pretectal nerve cells to the parasympathetic nuclei (Edinger-Westphal nuclei) of the oculomotor nerve on both sides.
• Here the fibers synapse and travel through the oculomotor nerve to the ciliary ganglion in the orbit.
• Finally, postgnaglionic parasympathetic fibers pass through the short cilliary nerves to the eyeball and the constrictor pupillae muscle of the iris.
• Both pupils constrict in the consensual light reflex, because the pretectal nucleus sends fibers to the parasympathetic nuclei on both sides of the midbrain.

Question 11

Why do both pupils constrict in the consensual light reflex?

Answer 11

Because the pretectal nucleus sends fibers to the parasympathetic nuclei (Edinger-Westphal nuclei) on both sides of the midbrain.

Question 12

Describe the 3 components of the Accommodation Reflex.

Answer 12

When focussing on a near object:

(1) Medial recti muscles contract -> convergence of the ocular axes
(2) Lens thickens by contraction of the ciliary muscle -> increases refractive power
(3) Pupils constrict to limit light waves to the thickest central part of the lens.

Question 13

Describe the Accommodation Reflex pathway.

Answer 13

Afferent impulses travel through optic nerve -> optic chiasma -> optic tract -> lateral geniculate body -> optic radiation -> visual cortex -> eyefield in frontal cortex.

Cortical fibers descend via internal capsule-> oculomotor nuclei in midbrain -> oculomotor nerve to medial rectus muscles.

Some descending cortical fibers synapse with the parasympathetic nuclei (Edinger-Westphal nuclei) of the oculomotor nerve on both sides -> synapse here -> parasympathetic preganglionic fibers travel through oculomotor nerve to the cilliary ganglion of the orbit. -> postganglionic parasympathetic fibers pass through the short ciliary nerves to the ciliary muscle and the constrictor pupillae muscle of the iris.

Question 14

Describe the carotid and aortic arch baroreceptor reflex.

Answer 14

Blood pressure rises -> nerve endings situated in vessel walls stimulated:

• > afferent fibers from carotid sinus ascend in the glossopharyngeal nerve and terminate in the nucleus solitarius.
• > afferent fibers from the aortic arch ascend in the vagus nerve

Connector neurons in the medulla oblongata activate the parasympathetic nucleus (dorsal nucleus) of the vagus -> slows HR

At the same time, reticulospinal fibers descending to the spinal cord inhibit the preganglionic sympathetic outflow to the heart and cutaneous arterioles.

The combined effect of stimulation of the parasympathetic action on the heart and inhibition of the sympathetic action on the heart and peripheral blood vessels reduces the rate and force of contraction of the heart and peripheral resistance.

-> BP falls.

Question 15

What is the Bainbridge right atrial reflex?

Answer 15

Nerve endings in the wall of the right atrium and venae carvae stimulated by a rise of venous pressure -> afferent fibers ascend in the vagus to medulla oblongata -> terminate on nucleus of the tractus solitarius -> connector neurons inhibit the parasympathetic (dorsal) nucleus of the vagus -> reticulospinal fibers stimulate the thoracic sympathetic outflow to the heart -> cardiac accelleration.

Question 16

Describe the 3 features of Horner’s Syndrome.

Answer 16

d

Question 17

a 43-year-old man was standing on a ladder and sawing off the limb of a tree when he lost his footing and slipped. As he began to fall, he grabbed at a branch with his left hand and held on until he was rescued some minutes later. Careful examination of his left upper limb showed paralysis of his flexor carpi ulnaris and flexor digitorum profundus and weakness of the palmar and dorsi interossei and the thenal and hypothenar muscles. There was also some loss of sensation on the medial side of the forearm and hand. It was noted that the pupil of his left eye was constricted and that he had ptosis of the left upper lid. A slight degree of enophthalamus was also present. The skin of his left cheek felt warmer and drier than the right cheek. How can you account for these widespread physical signs?

Answer 17

• Sustained a severe traction injury of the C8/T1 nerve roots of the brachial plexus on the L side
• Various paralysed muscles and the sensory loss were characteristic of Klumpke’s paralysis.
• The white ramus communicans passing from the 1st thoracic nerve to the stellate ganglion was torn, cutting off the preganglionic sympathetic fibers to the head and neck and producing left-sided Horner’s syndrome.

Question 18

A 19-year-old woman with a history of experiencing severe attacks of painful discolouration of the 4th and 5th fingers of both hands, especially during cold weather, visited her physician. A diagnosis of Raynaud’s disease was made. In view of the severity of her symptoms and the possibility of gangrene of the fingertips, it was decided to recommend a cervicodorsal preganglionic sympathectomy. What is the innervation of the arteries of the upper limb?

Answer 18

• The arteries of the upper limb are innervated by sympathetic nerves.
• The preganglionic fibers originate from the nerve cells in the lateral gray columns of the 2nd to the 8th thoracic segments of the spinal cord.
• They reach the sympathetic trunk via the white rami communicantes and ascend in the trunk to synapse in the middle cervical, inferior cervical and first throacic or stellate ganglia
• The post-ganglionic fibers join the spinal nerves that form the brachial plexus travelling in the grey rami communicantes
• The sympathetic fibers are distributed to the digital arteries within the branches of the brachial plexus.

Question 19

Explain the presence of sympathetic nerve fibers in a parasympathetic ganglion.

Answer 19

• It is not uncommon to find sympathetic preganglionic and postganglionic fibers passing through a parasympathetic ganglion without interruption.
• The nerve fibers are merely using the ganglion as a conduit en route to their destination.
• Visceral sensory nerve fibers travel in a similar manner.

Question 20

Explain the large intestine’s innervation by the vagus nerves down as far as the splenic flexure, although the vagal nerve trunks apparently come to an end very soon after piercing the diaphragm with the oesophagus.

Answer 20

• The vagal nerve trunks, on reaching the abdominal cavity, split up after a short course on the oesophagus into their terminal branches.
• The posterior vagal trunk (R vagus) gives off an important branch that passess to the celiac and superior mesenteric plexuses.
• The terminal fibers are distributed with the branches of the celiac and superior mesenteric arteries to the small and large intestine as far as the splenic flexure.

Question 21

Explain how the sacral parasympathetic outflow reaches the splenic flexure of the colon.

Answer 21

• The sacral parasympathetic outflow (S2, S3, S4) leaves the anterior rami of the sacral nerves as the pelvic splanchnic nerve.
• There preganglionic fibers pass through the hypogastric and aortic plexuses to reach the inferior mesenteric plexus.
• The fibers are then distributed to the splenic flexure and descending colon, along with branches of the mesenteric artery.

Question 22

A 4-year-old boy with a history of chronic constipation and abdominal distention was taken to a paediatrician. The child’s mother said that the constipation was getting progressively worse. It was not responding to laxatives, and she was finding it necessary to give her son an enema once a week to relieve his abdominal distension. On physical examination, the child’s abdomen was obviously distended and a dough-like mass could be palpated along the course of the descending colon in the left iliac fossa. Examination of the rectum showed it to be empty and not dilated. After an enema and repeat colonic irrigation with saline solution, the patient was given a barium enema followed by radiographic examination. The radiograph showed a grossly dilated descending colon and an abrupt change in lumen diameter where the descending colon joined the sigmoid colon. It was interesting to note that the child failed to empty the colon of the barium enema. Using your knowledge of the ANS to the colon, you would find what diagnosis? How would you treat this patient?

Answer 22

• This child has Hirschsprung’s disease, a congenital condition in which there is a failure of development of the myenteric plexus (Auerbach’s plexus) in the distal part of the colon.
• The proximal part of the colon is normal but becomes greatly distended because of accumulation of faeces
• In this patient, the lower pelvic colon, later at operation, was shown to have no parasympathetic ganglion cells
• Thus, this segment of the bowel has no peristalsis and effectively blocked the passage of faeces
• Once the diagnosis has been confirmed by taking a biopsy of the distal segment of the bowel, the treatment was to remove the aganglionic segment of the bowel by surgical resection.

Question 23

Examination of a patient with neurosyphilis indicated that the pupil of her left eye was small and fixed and did not react to light, but contracted when she was asked to look at a near object. Using your knowledge of neuroanatomy, state where you believe the neurologic lesion to be situated to account for these defects.

Answer 23

• The patient has an Argyll-Robertson pupil, which is a small, fixed pupil that does not react to light but contracts with accommodation.
• The condition usually is due to a syphilitic lesion.
• The neurologic lesion in this patient interrupted the fibers running from the pretectal nucleus to the parasympathetic nuclei of the oculomotor nerve on both sides.

Question 24

What transmitter substances are liberated at the following nerve endings?

(a) Preganglionic sympathetic?
(b) Postganglionic parasympathetic?
(c) Postganglionic parasympathetic?
(d) Postganglionic sympathetic fibers to the heart muscle
(e) Postganglionic sympathetic fibers to the sweat glands of the hand

Answer 24

(a) Ach
(b) Ach
(c) Ach
(d) Noradrenaline
(e) Ach

Question 25

During a neurologic examination of a patient aged 18 years, the resident noted that her right pupil failed to react to the direct and consensual light reflexes. Moreover, the same pupil contracted very slowly when the patient was asked to focus on a near object. The pupillary reflexes were normal in her left eye.

(1) Explain this finding.
(2) What test could you perform to confirm the diagnosis?
(3) Explain the possible underlying anatomic defect in this condition.
(4) Can the condition become bilateral?
(5) How does this condition differ from an Argyll-Robertson pupil?

Answer 25

(1) Adie’s tonic pupil syndrome.
(2) The syndrome can be confirmed by looking for hypersensitivity to cholinergic agents such as 2.5% methacoline or 0.1% pilocarpine. The Adie’s pupil should constrict when the drops are put in the eye. These cholinergic agents do not cause pupillary constriction in mydriasis caused by oculomotor lesions or in drug-related mydriasis.
(3) Adie’s syndrome is a benign disorder that is probably caused by a lesion of the parasympathetic innervation of the constrictor pupillae muscle.
(4) The condition can become bilateral although initially it is uniocular.
(5) The Argyl-Robertson pupil is caused by neurosyphilis, the lesion interrupting the nerve fibers that run from the pretectal nucleusto the parasympathetic nuclei of the oculomotor nerve on both sides. It is characterised by the pupil being small and fixed; the pupil does not react to light but contracts with accommodation.

Question 26

A 5-year-old girl with right-sided medial strabismus was undergoing surgery under general anaesthesia. During dissection of the medial rectus muscle, the resident placed gentle traction on the muscle. The child’s HR immediately plummeted to 20bpm but recovered within a minute when the resident stopped manipulating the muscle. What reflex was triggered by this procedure?

Answer 26

• The oculo-cardiac reflex can produce severe bradycardia.
• Even gentle manipulation of the eyeball, the extraocular muscles, or the orbital fascia can trigger afferent sensory impulses that travel to the CNS via the ophthalmic division of the trigeminal nerve.
• These impulses reach the parasympathetic dorsal nucleus of the vagus nerve and the bradycardia is initiated.
• The reflex action is generally of short duration and often responds to atropine.

Question 27

What is acetylcholinesterase responsible for?

Answer 27

Hydrolysing and limiting the action of acetylcholine at nerve endings. It can be blocked by certain drugs.

Question 28

Name some acetylcholinesterase inhibitors. (5)

Answer 28

Physostigmine, neostigmine, pyridostigmine, carbamate and organophosphate insecticides

Question 29

What is the effect of an acetylcholinesterase inhibitor?

Answer 29

Use results in excessive stimulation of the cholinergic receptors, producing the “SLUD” syndrome - salivation, lacrimation, urination, defecation.

Question 30

What is “SLUD” syndrome?

Answer 30

Salivation, lacrimation, urination, defecation.. Results from excessive stimulation of the cholinergic receptors from acetylcholinesterase inhibitor use.

Question 31

What is the mechanism of action of Black Widow spider venom?

Answer 31

The venom causes a brief release of Ach at the nerve endings, followed by permanent blockade.

Question 32

Describe the action of the botulinium toxin.

Answer 32

• A very small amount of this toxin binds irreversibly to the nerve plasma membranes and prevents the release of Ach at cholinergic synapses and neuromuscular junctions
• Produces an atropine-like syndrome with skeletal muscle weakness

Question 33

At which anatomical sites can sympathetic blockade theoretically be achieved?

Answer 33

1) Subarachnoid or epidural block - Where the sympathetic preganglionic fibers leave the spinal cord (T1 - L2) in the anterior roots of the spinal nerves.
(2) Ganglion block - Where there is a synapse between the preganglionic fibers and the ganglionic neurons
(3) Peripheral nerve block - In a peripheral nerve, where the post-ganglionic fibers travel to their destination.
(4) Perivascular block - In a perivascular plexus
(5) Where there is a pharmacological block at the ganglion or the terminal receptor sites