Cranial nerves are more specialized than spinal nerves, since they have more differentiated tasks to perform, yet they bear an unmistakable family resemblance to their spinal relatives.
In some respects the cranial nerves may represent the ancestral type, as exhibited by all of the segmental nerves of amphioxus and the lamprey eels. It will be recalled that in these lower chordates the dorsal and ventral roots of all nerves remain separate and that the dorsal roots include visceral motor fibers which supply the internal organs. In all vertebrates the cranial nerve roots also remain separate, some cranial nerves (III, IV, VI, and in amniotes XII) representing the motor ventral roots, while others (V, VII, IX and X) are mixed dorsal root nerves which include not only sensory fibers but also visceral motor components. Two other cranial nerves are also dorsal-root nerves but are not mixed, the auditory (VIII) including only sensory fibers while the spinal accessory (XI) of amniotes is made up solely of visceral motor elements. The olfactory (I) and optic (II) “nerves” belong to entirely different categories from the others as will be shown when they are specifically considered.
Some factors that have influenced the modifications of the cranial nerves are as follows: (1) the presence of sense organs located in the head; (2) the elaboration in water-dwelling vertebrates of the splanchnocranium; and (3) the degeneration of most of the embryonic myotomes in the head region.
Although the dorsal sensory roots of the spinal nerves arise solely from dorsal ganglia that have formed from the neural crest of the early embryo, some of the neurons of certain of the cranial nerves (V, VII, IX, and X) have their origin in patches of thickened ectoderm, the epibranchial placodes.
No two pairs of cranial nerves have the same uniform make-up. For example, in addition to the four general functional types of neurons characteristic of spinal nerves, namely, somatic afferent and visceral afferent neurons of the dorsal root, and somatic efferent and visceral efferent neurons of the ventral root, there are present three other types, namely, special somatic afferent and special visceral afferent neurons associated with the superimposed cranial sense organs of sight, hearing, smell, and taste; and special visceral efferent neurons in connection with the branchiomeric musculature of the primitive gill arches and their derivatives.
No single pair of cranial nerves possesses all of these seven functional types of elements, and rarely do two pairs have the same composition, as will be evident from the inspection of Table XI.
It will be seen that the sensory elements of the cranial nerves (column 5), as in spinal nerves, are associated with ganglia located outside the brain itself. The efferent motor elements, on the other hand, have their headquarters in nuclei within the gray substance of the brain.
The twelve pairs of cranial nerves were first identified in man and are named with reference to the parts which they supply, being customarily designated by the Roman numerals I to XII. The first two pairs, or olfactory and optic, are in a class by themselves. The eight cranial nerves from III to X inclusive have been called “spinal-cranial nerves,” because they can be interpreted as modified spinal nerves. The XIth and XIIth cranial nerves, occurring only in amniote vertebrates, show most of all the spinal character, although the hypoglossal (XII) has lost its dorsal sensory root, which was present in the embryo.
In lower vertebrates there is a group of from one to five pairs of small transitional spino-occipital nerves in the region where the medulla passes over into the spinal cord. Although entirely postcranial in cyclostomes, the more anterior ones become enclosed within the skull in fishes and amphibians. These nerves, which usually lack dorsal roots and are therefore purely motor, supply muscles associated with the gill region. In amniotes the persistent portions of these nerves participate in the formation of the XIth and XIIth cranial nerves which are added to the ten already present in an amniotes. The Xlth arises chiefly from roots split off from the Xth nerve of lower forms but with the addition of a few twigs from the most anterior spino-occipital nerves. The hypoglossal (XII) nerve, which supplies the tongue, is the result of the fusion of several of the spino-occipital nerves the central connections of which shift forward to the medulla region.
Since the identification of the twelve classical pairs of cranial nerves, the somewhat embarrassing discovery of an additional pair has been made. This extra pair, called the nervus terminalis, was first found in the lungfishes and is now known to be present, at least embryonically, in all classes of vertebrates including man. As the nervus terminalis is anterior to all the other cranial nerves, it remains without a Roman numeral, since its logical designation as the first pair would upset the well-established and generally accepted succession of the other twelve pairs. Consequently it is numbered 0.
It will be seen from Table XI (column 3) that most of the cranial nerves take their apparent origin from the myelencephalon, or medulla. The exceptions are the nervus terminalis (0) and the olfactory (I), which connect with the telencephalon; the optic (II) from the diencephalon; the oculomotor (III) from the mesencephalon; and the trochlear (IV) from the intermediate region between the mesencephalon and the metencephalon.
Furthermore, as far as function is concerned, I, II, and VIII are entirely sensory in character, lacking motor roots, while XI and possibly XII, having lost their sensory components, are entirely motor. Although formerly considered to be purely motor because of the absence of typical sensory roots, III, IV, and VI are now known to be mixed for they include proprioceptive fibers. The nervus terminalis (0) is of doubtful function. The remaining pairs, namely, V, VII, IX, and X, are mixed nerves, although some of them have branches that are either entirely sensory or entirely motor in character.
With regard to neuronic components, the distribution is shown in Table XII.
A typical mixed cranial nerve, like the spinal nerves, divides into dorsal and ventral branches, distal to its ganglion. The short dorsal branch in fishes, containing only afferent neurons, brings the sensations to the brain from the skin and the lateral-line organs. It becomes much reduced in land forms and eventually disappears entirely.
The ventral branch usually forks into a pretrematic twig and a post-trematic twig, that extend on either side of the splanchnocranial openings, such as the mouth, spiracle, and gill slits. These twigs of the ventral branch may carry either efferent or afferent neurons and in some cases both. Usually there is also a sensory pharyngeal twig to the pharyngeal lining.
There is evidence that the trigeminal nerve (V) is compounded of two original nerves. The facial (VII) probably represents the dorsal root of a nerve of which the abducens (VI) corresponds to the ventral root.
The acoustic nerve (VIII), now entirely sensory and independent, was probably a part of the Vllth nerve formerly, while the vagus (X) is no doubt a complex of multiple origin concerned with several branchial arches, having the spinal accessory (XI) split off from it as an independent nerve.
Thus the homologies of the cranial nerve become a complicated problem, calling for much patient and searching investigation on the part of comparative anatomists.
The following brief survey of the separate cranial nerves supplements the information buried in Table XI.
Terminal Nerve (0)
The terminal nerve arises from the cerebral hemispheres in the region of the medial olfactory tract and extends to the snout region in close association with, but independent of, the olfactory nerves.
It possesses one or more ganglia and is of doubtful function, being described as sensory by some investigators, as part of the autonomic system by others.
Olfactory Nerve (I)
The olfactory nerve is peculiar in that its fibers arise from cells of the olfactory epithelium. The dendrites of these cells have thickened endings which are exposed on the surface of the epithelium. The unmyelinated neurites making up the nerve have only a short distance to run to reach the olfactory bulb of the brain. In cyclostomes, which have only a single nasal sac, the olfactory nerves are paired as usual, indicating that the single organ represents a fusion of what was formerly two sacs.
In the skull of most vertebrates the olfactory nerve passes through a single foramen on either side to reach the brain, but in many of the higher animals it consists of a brush-like bundle of non-medullated fibers, fila olfactoria, that penetrate the skull separately through the pepperbox-like pores of the cribriform plate of the ethmoid bone. Thus, in man, instead of a single pair of olfactory nerves, it would not be entirely incorrect to say that there are at least twenty pairs.
Optic Nerve (II)
The optic nerve is not a true nerve but rather a tract of the brain both in development and structure. In development an evagination from the diencephalon forms the retinal layer of the eyeball. Then some of the retinal cells send their neurites back to the brain as the optic nerve, which therefore connects two parts derived from the embryonic central nervous system, the retina and the diencephalon. The fibers which make up this nerve, like most fibers in nerve tracts, are myelinated but without any neurolemma.
Upon reaching the ventral side of the diencephalon the fibers decussate, in the optic chiasma, before passing into the brain. The crossing of the fibers is complete except in the higher mammals in which the fibers from the nasal half of the eyeball cross while those from the temporal half do not. In man, therefore, the fibers from the left halves of the two eyes go to the left side of the brain, those from the right halves to the right side of the brain (Fig. 656). This arrangement develops in association with improvement in binocular vision. After passing through the chiasma region, the fibers continue as the optic tracts each of which, after running over the outer surface of a cerebral peduncle, sends fibers into three parts of the brain, the lateral geniculate body, the anterior colliculus, and the region just anterior to the latter.
In these three pairs of regions the fibers of the optic tract synapse with neurons which relay the impulses to several different parts of the brain. From the lateral geniculate bodies the relaying fibers run chiefly to areas of visual sensation in the occipital cerebral cortex. The anterior colliculi, which receive fibers from the occipital cortex as well as the optic tracts, relay messages to the nuclei of the several cranial nerves which control the movements of the eyeballs and also of the head. Fibers from the area just in front of each anterior colliculus carry impulses which eventually reach the intrinsic muscles of the eyeball.
The intricate optic pathways of mammals have evolved from a relatively simple plan in lower vertebrates. In fishes most of the optic-tract fibers run into the optic lobes (Fig. 657). In reptiles optic lobes continue to be highly important relaying centers in the optic pathways, but some optic fibers terminate in the diencephalon where they synapse with neurons leading to the cerebral cortex, which is definitely present in these animals. In mammals relatively few primary optic fibers go to the metencephalon, most of them terminating in the lateral geniculate bodies of the diencephalon.
Eye-Muscle Nerves (III, IV, VI)
The nerves of the eyeball muscles have much in common, being efferent somatic nerves associated with the three pairs of myotomes that remain in the head region. The oculomotor (III) and the trochlear (IV) arise from the mesencephalon, while the abducens (VI) comes from the myelencephalon. No cranial nerves take their origin from the metencephalon.
The most important nerve of the eye muscles is the oculomotor, which supplies not only the inferior oblique and three of the rectus muscles, namely, superior, inferior, and internal, but also the levator palpebrae that lifts the upper eyelid, as well as the intrinsic iris and ciliary muscles of accommodation within the eyeball itself.
The trochlear supplies the superior oblique muscle. The two trochlears decussate in the anterior medullary velum.
The abducens actuates the lateral, or external, rectus muscle. It also sends a branch to the retractor bulbi, a muscle derived from the external rectus, which pulls back the eyeball as the name implies. This muscle is present in all tetrapods except ophidians and primates. The nictitating membrane, or third eyelid of reptiles, birds, and some mammals, is also supplied by the abducens nerve.
All three of these nerves are now believed to include proprioceptive sensory fibers from the structures which they supply.
Trigeminal Nerve (V)
The trigeminal nerve comes off dorso-laterally from the anterior region of the medulla in close association with the VIIth and the VIIIth cranial nerves, constituting together a most important group for the supply of the head. It has a strong dorsal root and a lesser ventral root and is one of the largest of the cranial nerves. It is the great sensory nerve of the head carrying nearly all of the general somatic sensory fibers from the surface of the head to the brain. The cell bodies of these fibers are located in the Gasserian ganglion of the dorsal root, one of the largest of all the ganglia associated with cranial nerves.
The trigeminal nerve is so called because of its three component branches, the ophthalmic, maxillary, and mandibular. It was probably formed by the joining of two original dorsal root nerves, the ophthalmic being the remains of one, and the maxillary and mandibular of the other. According to this interpretation the corresponding ventral root nerves are the oculomotor and the trochlear.
In fishes the ophthalmic, which is sensory in all animals, supplies the skin of the snout and dorsal part of the head. In mammals it is distributed to the nose, orbit, forehead, and top of the head. The maxillary, which is also sensory, goes to the teeth of the upper jaw, the upper lips, and the cheeks. The mandibular is mixed, being composed of: (1) sensory fibers from the teeth and skin of the lower jaw and also the skin of the side of the head; and (2) motor fibers which innervate the mandibular group of branchiomeric muscles (see Chapter XVIII).
The mandibular branch also includes proprioceptive sensory fibers coming from the muscles of mastication which it supplies. These proprioceptive neurons have their cell bodies in the mesencephalic nucleus of this nerve. In this respect they are unique, for the sensory’ neurons of all other vertebrate nerves have their cell bodies in ganglia, outside of the central nervous system. It will be recalled, however, that amphioxus has no ganglia, all of the cell bodies being within the central nervous system.
The maxillary branch may be regarded as the pretrematic nerve anterior to the mouth, and the mandibular branch, the corresponding posttrematic nerve posterior to the mouth. This pre- and posttrematic arrangement on either side of a pharyngeal opening, like mouth, spiracle, or gill slit, is repeated in other cranial nerves.
Facial Nerve (VII)
The VIIth nerve, or facialis, undergoes much modification as the result of evolutionary emergence from water to land life, owing to the loss of the lateral-line organs and, in higher mammals, to the development of the mimetic musculature of the face. The cell bodies of its sensory fibers are in the geniculate ganglion which in the lower vertebrates is often fused with the Gasserian ganglion of the Vth nerve.
In fishes and urodeles several dorsal branches of this nerve serve chiefly as sensory pathways from the lateral-line organs. These branches are lacking in anurans and higher vertebrates.
The ventral branch of the VIIth nerve forks into two trunks in fishes, the palatine and hyomandibular, which are respectively the pretrematic and posttrematic nerves of the spiracular opening.
In mammals the pretrematic branch is probably represented by the chorda tympani, a small nerve which runs through the middle-ear cavity. It will be recalled that this cavity corresponds to part of the spiracular passageway of fishes. The chorda carries fibers from the taste-buds on the anterior two-thirds of the tongue. It has also been joined by autonomic fibers associated with the sublingual and submaxillary glands. The posttrematic portion becomes the group of nerves which supply the hyoid-arch group of branchiomeric muscles, including the extensive muscles of facial expression in man.
Acoustic Nerve (VIII)
The acoustic, or auditory, nerve is a very short sensory nerve which does not emerge from the skull. It divides into two branches, the vestibular and cochlear, that go respectively to the semicircular canals and to the auditory mechanism (cochlea) of the internal ear. Each branch has a ganglion of similar name. As the semicircular canals aid in maintaining equilibrium, some of the fibers of the vestibular nerve run directly to the cerebellum, the main equilibrium center of the brain. Impulses from the cochlea, like those from the eye, are distributed through various parts of the brain. After being carried to the restiform body they are relayed to: (1) the auditory center in the cortex of the temporal lobe of the cerebrum; (2) the posterior colliculus, a center for reflexes initiated by sound; and (3) other centers in the brain including those of the several motor nerves.
Glossopharyngeal Nerve (IX)
The glossopharyngeal nerve, primarily associated with the third splanchnocranial arch, forks around the first functional gill slits of fishes and sends a branch into the pharynx. Frequently it also has a small branch associated with the lateral-line system. Its sensory fibers have their cell-bodies located in the petrosal ganglion.
In mammals the sensory components are connected with the taste buds of the posterior third of the tongue, and general sense organs of this region and the neighboring portions of the pharynx. Most of its efferent fibers supply pharyngeal muscles derived from the group originally associated with the third visceral arch; a few synapse with autonomic fibers running to the parotid gland.
Vagus Nerve (X)
The vagus nerve, as the name implies, “wanders” to many parts of the body. It is apparently a composite of several segmental nerves. In fishes it splits into two main trunks, lateral and visceral. The lateral trunk, which is sensory, supplies some of the lateral-line organs of the head and extends along the side of the body as the nerve of the lateral-line canal. The visceral trunk, after giving off mixed branchial nerves to the remaining gill arches, continues posteriorly carrying visceral afferent and efferent fibers to the heart, the blood vessels, and the digestive tract and its derivatives as far as the posterior end of the small intestine. The cell-bodies of the visceral afferent fibers are in the nodosal ganglion.
In strictly land vertebrates, with the loss of the lateral line system and the gills, most of the nerves connected with these structures disappear. Motor fibers of the branchial nerves persist, however, to innervate striated muscles of the pharynx and larynx. The main visceral trunk, which runs to the internal organs, retains its importance, carrying both sensory fibers and efferent fibers which connect with the autonomic nervous system.
In cyclostomes the vagus includes many somatic sensory fibers from the skin of the posterior part of the head. In fishes and amphibians these cutaneous fibers are limited to the dorsal portion of this region, while in higher forms they are associated with the skin of the external ear.
Spinal Accessory Nerve (XI)
The spinal accessory nerve, found only in amniotes, is composed entirely of visceral efferent fibers. Part of these fibers, arising from the posterior portion of the medulla and running to join the vagus, are probably represented in lower vertebrates by posterior rootlets of the vagus. The others, arising by a series of rootlets from the anterior cervical region of the spinal cord, are thought to correspond to spino-occipital nerves of lower forms. These fibers of spinal origin innervate the trapezius and sternocleidomastoid muscles. The fibers which join with the vagus are distributed with it to the striated muscles of the pharynx and larynx and to the autonomic nervous system.
Hypoglossal Nerve (XII)
The hypoglossal nerve is made up of somatic motor fibers which innervate the muscles of the tongue. Although it is found only in amniotes, it probably corresponds to some of the spino-occipital nerves of fishes. The functions of the cranial nerves are cartooned in Figure 658.