P. VNormal Atrioventricular septum defect
17.2.2 The parasympathetic system (craniosacral outfl ow)
As Figure 17.2 illustrates, parasympathetic preganglionic neurons originate from specialized cranial nerve nuclei and from the inter- mediolateral horns of the sacral segments of the spinal cord, hence the alternative name, the craniosacral outfl ow . At fi rst sight, these origins suggest the parasympathetic system has a rather limited dis- tribution. Furthermore, it seems that there is a huge gap in the distri- bution of parasympathetic nerves with no innervations between the head and the pelvis where most of the important viscera of the body are located. These organs clearly require a parasympathetic nerve supply if control of visceral function and homeostasis is to be main- tained. This gap is fi lled by the distribution of parasympathetic pre- ganglionic nerves to the thoracic and most of the abdominal organs through the vagus nerves, the tenth cranial nerves. The Latin word
‘vagus’ means ‘wanderer’—appropriate for a nerve that originates in the medulla, branches in the head, and then wanders through the trunk.
As you can see in Figures 17.1 and 17.2, the preganglionic parasym- pathetic neurons are considerably longer than the post-ganglionic neurons and travel with the cranial or sacral nerves with which they originate to reach the ganglia. There is no equivalent to the sympathetic trunks in the parasympathetic division of the ANS; instead, the ganglia are individual structures close to or in the organs being supplied. The post-ganglionic axons are usually short.
Parasympathetic stimulation produces secretion from visceral glands and motility in the gastrointestinal tract among other activities; because of these actions, the neurons are often referred to as secretomotor neurons.
The cranial parasympathetic nerves
The parasympathetic preganglionic neurons in the cranial region begin in motor nuclei of the brainstem equivalent to the lateral horn of the spinal cord. They leave in cranial nerves III ( oculomotor ), VII ( facial ), IX ( glossopharyngeal ), and X ( vagus ). A fuller account of
Box 17.2 Is your patient going to faint?
As mentioned in the main text, the sympathetic nervous system may be stimulated entirely when people are terrifi ed or angry—
the ‘fl ight or fi ght reaction’. Even if you have the best interpersonal skills in the world, most patients attending your dental surgery will be somewhat apprehensive and will already have some adrenalin in their circulation due to the fl ight or fi ght reaction. Dental local anaesthetics usually contain noradrenalin as well as the active anaesthetic agents. The noradrenalin causes vasoconstriction in the area of administration and, therefore, slows the bloodborne removal of anaesthetic from the site, prolonging working time for the den- tist. Sometimes, administration of local anaesthetic can add enough noradrenalin to cause the patient to faint.
You are not in a position to monitor heart rate and many other warning signs that accompany the fl ight or fi ght reaction and
precede a faint, but you can see the warning signs that occur in the patient’s face. Their skin will become very pallid as blood is directed away from skin to muscles and their pupils will dilate. If you see these signs, you should stop the procedure immediately, take preventative action, and reassure the patient. Simply being aware of their nervousness and manipulating the dental chair so that their feet are above the level of their head is often suffi cient to prevent a faint.
The patient’s medical history should always be checked before starting any dental procedure requiring local anaesthetic, especially for the presence of cardiovascular or respiratory disease. Anaesthet- ics without noradrenalin should be used for compromised patients to prevent any potential adverse eff ects on the cardiovascular or respiratory systems from the noradrenalin.
Visceral sensory neurons 157
the parasympathetic components of the cranial nerves is given in Chapter 18 .
Briefl y summarized, the parasympathetic components in the:
• Oculomotor nerves supply the ciliary body that controls lens thick-
ness for focusing and the sphincter pupillae muscle of the eyeball to decrease pupillary diameter and thus regulate the amount of light falling on the retina;
• Facial nerves stimulate the secretion from salivary glands in and
around the oral cavity and the tear-producing lacrimal glands in the orbit;
• Glossopharyngeal nerves stimulate the secretion from the parotid
salivary glands;
• Vagus nerves innervate the respiratory and cardiovascular system
and the gastrointestinal tract as far as the left fl exure of the large intestine.
The preganglionic axons in the:
• Oculomotor nerve relay in the ciliary ganglion ;
• Facial nerve in the pterygopalatine and submandibular ganglia ;
• Glossopharyngeal nerve in the otic ganglion .
The anatomy and connections of these ganglia will be described in Section 4 . The corresponding synaptic sites between pre- and post-gan- glionic neurons in the vagus nerve are found in the autonomic plexuses and ganglia in or close to the thoracic or abdominal organ being supplied.
The sacral parasympathetic nerves
The gastrointestinal tract below the left colonic fl exure and the remain- ing pelvic viscera receive their parasympathetic innervation through preganglionic neurons which leave the spinal cord in the second to fourth sacral spinal nerves. These connect with the post-ganglionic neurons in ganglia close to the viscera.
17.3 Visceral sensory neurons
Visceral sensory neurons may be divided into two main groups, gen- eral visceral sensory neurons from the viscera and blood vessels and special visceral sensory neurons concerned with taste. The pathways for taste travel through cranial nerves and are, therefore, described in Chapter 18 .
General visceral sensory neurons convey information which does not usually reach consciousness. They are the peripheral processes of cells located in the dorsal root ganglia of spinal nerves ( T1 to L3 ) which contribute visceral motor neurons to the sympathetic nervous system and in the sensory ganglia of the glossopharyngeal (ninth) and vagus (tenth cranial) nerves. These processes vary in diameter and may be myelinated or unmyelinated.
The peripheral processes pass from the dorsal root ganglia into the spinal nerves, then via the white rami communicantes to the sympa- thetic ganglia. They do not synapse in the ganglia, but pass straight through to be distributed through visceral and vascular branches to unencapsulated sensory endings in their targets. The central process- es enter the spinal cord through the dorsal roots and synapse in the
grey matter of the thoracic and upper lumbar segments and ascend in the dorsal columns (see Section 16.2.2 ). Collateral branches at the spinal level connect either directly or through interneurons with the preganglionic sympathetic neurons in the lateral horn, thus establish- ing pathways for refl ex visceral action. Some general visceral sensory neurons convey nociceptive information to the pain matrix (see p. 142);
the resulting visceral pain is outlined in Box 17.3.
The vagus nerves carry a large number of general visceral sen- sory neurons. The cell bodies are located in the inferior vagal gan- glia. Their peripheral processes are widely distributed through the branches of the vagus nerves, the area of supply corresponding broadly with that of the parasympathetic secretomotor components of the vagus. Their central processes end in the inferior part of the nucleus of the tractus solitarius (see Section 18.10 ). The glossopha- ryngeal nerves convey general visceral sensory neurons from the carotid sinuses and carotid bodies at the bifurcation of the com- mon carotid arteries. These terminate in the cardiovascular centres in the medulla.
Box 17.3 Visceral pain and diseases aff ecting the autonomic nervous system Noxious sensations from the viscera may reach consciousness
under certain pathological conditions. Possibly the best known example is ‘stomach ache’. Unlike pain caused by peripheral stimulation, e.g. a kick on the shin, the pain is diff use and poorly localized. It may be felt in the region of the organ aff ected or may be referred to the area of skin which receives somatic sensory innervation from the same segments of the spinal cord. The central pathways followed by these nociceptive visceral sensory neurons are not well understood.
Diseases of the autonomic nervous system are not common; it is more usual for parts of the system to be aff ected by lesions in adjacent organs.
The cervical sympathetic trunk may be interrupted by compression of the upper part of the thoracic sympathetic trunk or the cervical part of the trunk itself. One cause of such compression in the thorax is enlarge- ment of mediastinal lymph nodes as a result of the spread of bronchial carcinoma from the lung. Similarly, cancerous lesions in the neck (laryn- geal cancer, for example) can cause similar compression of the cervical sympathetic trunk. Interruption of the trunk produces ipsilateral:
158 The autonomic nervous system
• Constriction of the pupil ( miosis ) due to paralysis of the dila-
tor pupillae and unopposed action of the sphincter pupillae muscle;
• Drooping of the upper eyelid ( ptosis ) because of paralysis of the
part of levator palpebrae superioris composed of smooth muscle (see Section 23.1.5 );
• Flushed dry facial skin due to vasodilatation and absence of sweating ( anhydrosis );
• And possibly slight retrusion of the eyeball ( enophthalamos ).
This group of clinical signs is known as Horner’s syndrome and may be the fi rst clinical manifestation of the primary disease.
18
The cranial nerves
Chapter contents
18.1 Introduction 160
18.2 General anatomy of the cranial nerves 160
18.3 The olfactory nerves (CN I) 164
18.4 The optic nerves (CN II) 164
18.5 The oculomotor (CN III), trochlear (CN IV),
and abducens nerves (CN VI) 165
18.6 The trigeminal nerves (CN V) 168
18.7 The facial nerves (CN VII) 172
18.8 The vestibulocochlear nerves (CN VIII), auditory,
and vestibular pathways 175
18.9 The glossopharyngeal nerves (CN IX) 177
18.10 The vagus nerves (CN X) 178
18.11 The accessory nerves (CN XI) 179
18.12 The hypoglossal nerves (CN XII) 179
160 The cranial nerves
The cranial nerves are the most important neural structures relevant to dental students and practitioners. The cranial nerves are the nerve supply to all the structures in the head and neck and underpin of the anatomy and function of these regions—the head and neck will not work without them. In a wider context, correct functioning of the cranial nerves is a very good indicator of the health or otherwise of the CNS; it may be necessary to test the function of some, or even all, of the cranial nerves at times to assess neural function. In addition, many of the cra- nial nerves may be involved in various diseases of the head and neck.
As outlined in Chapter 3 , 12 pairs of cranial nerves arising from the brain form one major component of the peripheral nervous system, the 31 pairs of spinal nerves forming the other. Each pair of cranial nerves has a name and number. Conventionally, they are numbered using the Roman numerals I to XII. The nerves are numbered from one to 12, according to their origin from the brain; nerves with the lowest numbers arise from the most anterior aspect of the brain (the forebrain) whereas those with highest numbers arise from the lowest part (the medulla).
Several aspects of any nerve anywhere in the body are required to describe its anatomy and function in complete detail:
• Its origins and terminations in the CNS;
• Its neuronal components—are they motor, sensory, or autonomic?
• Its course to and from its target tissues;
• Its distribution to specifi c areas and structures through specifi c
branches;
• Its overall functions and specifi c functions of its component parts.
In addition, if the clinical signifi cance is going to be appreciated, we will also need to consider:
• The eff ects of damage or disease on the nerve;
• Its important relationships to other structures;
• How to test whether the nerve is functioning correctly.
Given that there are 12 pairs of nerves, does a competent dentist need to know everything in the two lists about every cranial nerve?
The answer, you will be relieved to hear, is ‘no’. To paraphrase George Orwell—‘all cranial nerves are important, but some are more impor- tant than others’. You should be able to list, for all 12 pairs of cranial nerves:
• Their component types of neurons;
• Their function;
• How to test their function;
• And outline their distribution.
More detail than this bald outline is required in some of the dif- ferent headings for some nerves ( III, IX, X, and XII ), but precise detail is required for a few ( V and VII ). Why have certain nerves been emphasized? The nerves that require detailed knowledge are those nerves that dentists will be working on everyday, either by anaes- thetizing them or trying to avoid damage to them during surgical procedures.