Section VIII—Brain and Cranial Nerves

The brain and cranial nerves (Figures 3.41-3.45; Table 3.1) should be exposed by removing thin slices of car­tilage from the roof of the chondrocranium, as was done

TABLE 3.1 The cranial nerves and their branches of the shark.

FIGURE 3.42 Brain of the shark in (a) dorsal and (b) ventral views, with regions of the brain color-coded.

for the ear. Earlier dissections of the ear and eye prob­ably destroyed some of the cranial nerves that must still be identified, so look for these structures mainly on the intact side of the head. Work carefully in removing the cartilage forming the dorsal roof of the orbit to avoid injuring the superficial ophthalmic nerve. The nerve was noted on page 63, but its connections to the rest of the nervous system may now be followed.

The brain sits in the large cranial cavity within the pos­terior part of the chondrocranium. Begin exposing the brain posteriorly, between the otic capsules. As you work your way forward, peer into the anterior part of the cranial cavity and you may be able to see the thin, strand-like epiphysis (Figures 3.43-3.45) extending from the brain dorsally to the epiphyseal foramen (see page 28) in the roof of the chrondrodranium. Continue to shave the roof and as much of the lateral walls of the cavity as you can without injuring the nerves. When you have finished opening the cranial cavity, the dorsal surface of the brain will be revealed. Work carefully, as the brain is easily scrambled by poking with sharp instruments. Also remove thin frontal slices, mainly of muscle tissue, posterior to the chondrocranium until you expose the vertebral column.

Brain

The brain is subdivided into the following five regions, in anterior to posterior order: telencephalon, dien­cephalon, mesencephalon, metencephalon, and myelen­cephalon (Figure 3.42). The telencephalon includes the olfactory bulbs anteriorly (Figure 3.43). The olfactory sacs (see page 63) lie anterior to the bulbs. The bulbs narrow into the olfactory tracts, which in turn extend into the cerebral hemispheres. Together these constitute the cerebrum. The diencephalon lies posterior to the

FIGURE 3.43 Brain and cranial nerves of the shark in (a) dorsal and (b) ventral views.

cerebral hemispheres. Its roof is the epithalamus, its sides the thalamus, and its floor the hypothalamus. The epiphysis attaches posteriorly on the epithalamus. Most of the epithalamaus is formed by a thin, vascular tela choroidea. Anteriorly the tela choroidea attaches to the cerebrum. Carefully remove the tela choroidea to reveal the third ventricle (Figures 3.43 and 3.45), the cavity of the diencephalon that contains various vascular struc­tures such as the choroid plexus, which is an extension of the tela choroidea and is involved in production of cerebrospinal fluid. Anteriorly the third ventricle leads, through the foramen of Monro, into left and right lateral ventricles, which lie respectively within the left and right cerebral hemispheres.

The mesencephalon includes a pair of rounded optic lobes. The main structure of the metencephalon is the large cerebellum, which partially overhangs the optic lobes anteriorly and the myelencephalon posteriorly. The auricles of the cerebellum project anterolaterally from the posterior end of the metencephalon. The myel­encephalon includes nearly all of the medulla oblon­gata, the posterior part of the brain than narrows into the spinal cord.

FIGURE 3.44 Brain and cranial nerves of the shark in left lateral view.

Figure 3.45 Right half of the brain of the shark, showing sagittal surface.

These are the visceral sensory columns. Deep within the depression (thus between the somatic motor column and visceral sensory column) is a much smaller column, the visceral motor column. Finally, there is a large longitu­dinal ridge dorsal to each visceral sensory column. This is the somatic sensory column, and its surface has the form of small, bead-like swellings. Anteriorly the so­matic sensory column becomes enlarged and forms the acousticolateral area.

Cranial Nerves (CNN 0, I-X, and Lateral

Line Nerves)

This section mainly examines the cranial nerves, but the occipital and the anterior spinal nerves must also be con­sidered to properly comprehend the pattern and distrib­ution of the nerves arising from the brain. Many of the nerves attach to the ventral surface of the brain, but their proximal ends will be examined shortly. Thus, unless otherwise instructed, do not jump ahead and study the other structures of the brain. Remove as much cartilage as possible from the lateral wall of the cranial cavity and of the neural arch without damaging the nerves.

The cranial nerves of vertebrates were first studied in humans, and the sequence and names of the nerves were based on the pattern in humans. Twelve nerves were initially recognized and they were given both name and Roman numeral designations (see Table 7.5). A small anterior nerve, the terminal nerve, was discovered later to be present in almost all vertebrates (but not birds). As it is anterior to CN I, it was designated as “0” (zero—which is not a Roman numeral!). These cranial nerves and their designations became commonly accepted, as their pattern is generally applicable to all amniotes.

This sequence, however, does not apply equally well to anamniotes, in which the final two nerves, the accessory (CN XI) and hypoglossal nerves (CN XII), are not defin­itively recognized. Further, many anamniotes also have a set of six additional cranial nerves associated with the lateral line and electroreceptor organs. As these six nerves, termed lateral line nerves, are lacking in amniotes, they were never numbered and were consid­ered to be components of some of the conventionally recognized cranial nerves. However, the lateral line nerves have their own roots and they are currently con­sidered separate nerves, even though their separate emergence from the brain is difficult to identify grossly. They enter the brain very near cranial nerves VII and X and so may be referred to as either preotic or postotic lateral line nerves. More distally, the fibers of some do accompany those of other cranial nerves (see Table 3.1). For these reasons, they will not, with one exception, be considered in detail.

The most anterior cranial nerve is the terminal nerve (CN 0), a thin nerve passing along the medial side of the olfactory tract. It is often difficult to find, but look for it where the olfactory tract meets the cerebrum. The olfac­tory nerve (CN I) is formed by many fine fibers that pass from the olfactory sac into the olfactory bulb, but they will not be seen grossly.

The oculomotor nerve (CN III) arises from the ventral surface of the mesencephalon and branches out to inner­vate four eye muscles (ventral oblique, and dorsal, ventral, and medial rectus muscles). Examine the ventral oblique and find the branch of the oculomotor that passes to it, and then trace the nerve posteriorly (see Figure 3.37). The nerve passes ventral to the ventral rectus muscle, then curves dorsally and gives off a branch to the ventral rectus muscle. It continues dor- sally, crosses anterior to the base of the lateral rectus muscle and dorsal to the base of the dorsal rectus muscle, and passes medially. It gives off branches to the dorsal and medial rectus muscles and then enters the chondrocranium. The oculomotor nerve can be seen leaving the brain by pushing the brain gently to the side. The slender trochlear nerve (CN IV) extends anterolat- erally from the dorsal surface of the mesencephalon. It crosses over the optic lobe, and passes to innervate the dorsal oblique muscle.

The abducens nerve (CN VI; this is discussed here, out of sequence, so that the nerves discussed in the next paragraph can be treated together) arises from the ventral surface of the medulla. Its origin will be seen later. The aducens passes anterolaterally to innervate the lateral rectus muscle. It can be found on the ventral surface of this muscle.

The next three nerves, the trigeminal (CN V), facial (CN VII), and statoacoustic (CN VIII) nerves share a close origin from the surface of the brain. In addition, the roots of the preotic lateral line nerves arise from between the roots of the trigeminal and facial nerves. All of these nerves arise in this sequence from the ante­rior part of the medulla, beginning just behind the auri­cles of the cerebellum. However, they emerge so closely together that it is difficult to identify them separately.

Some branches of the trigeminal, facial, and lateral line nerves merge to form larger nerves or trunks. Although some branches of the trigeminal, facia, and lateral line nerves were observed during dissection of the orbit, use the following description to review them carefully and ascertain their pattern of distribution.

The trigeminal[*] is a large nerve that divides into four branches on emerging from the cranial cavity into the orbit. These are the superficial ophthalmic, deep oph­thalmic, mandibular, and maxillary branches. The first branch is accompanied by the superficial ophthalmic branch of the ADLLN, the second branch by the buccal branch of the ADLLN (Table 3.1). The superficial oph­thalmic branch is the most dorsal and passes just ventral to the dorsal margin of the orbit. Anteriorly, the infra­orbital divides into separate maxillary (medially) and buccal (laterally) nerves. The deep ophthalmic branch passes through the orbit but adheres to the dorsomedial surface of the eyeball. The mandibular branch extends laterally along the posterior wall of the orbit, almost directly posterior to the lateral rectus muscle. The max­illary branch contributes to the formation of the infra­orbital nerve, the large nerve passing along the floor of the orbit and crossing the preorbitalis muscle.

The facial nerve has two branches. One is the hyomandibular branch, which forms the hyomandibu­lar nerve with the AVLLN. The hyomandibular nerve was observed as it passed toward the spiracle on the external surface of the levator hyomandibulae muscle (page 41). Trace it now from the brain, carefully cutting away portions of the musculature, ear, and spiracle. Near its origin, the hyomandibular branch bears a swelling, the geniculate ganglion. The second branch of the facial is the palatine branch, which participates in the innervation of the lining of the oral cavity.

The Statoacoustic nerve is a short nerve that innervates the ear. Remove cartilage of the otic capsule to see branches passing, in particular, to the ampullae of the semicircular ducts and the sacculus.

The glossopharyngeal nerve (CN IX) arises posterior to the statoacoustic nerve. It extends posterolaterally through the floor of the otic capsule to the first pha­ryngeal pouch. Pick away the cartilage of the otic capsule to follow the nerve. Note the swelling, the pe­trosal ganglion, along the nerve just before it emerges from the capsule. Very near the dorsal margin of the pharyngeal pouch, the glossopharyngeal divides into pretrematic and posttrematic branches (remember that trema refers to the slit, or opening, of the pouch).

The three postotic lateral line nerves are the middle lateral line nerve (MLLN), the supratemporal lateral line nerve (STLLN), and the posterior lateral line nerve (PLLN). Their roots emerge from the brain between the glossopharyngeal (CN IX) and vagus (CN X) nerves. The peripheral distribution of the PLLN will be traced below.

The vagus nerve (CN X) mainly innervates the remaining pharyngeal pouches and the viscera. Note the series of fan-like rootlets emerging from the medulla just poste­rior to the glossopharyngeal nerve. These include the roots of the lateral line nerves, more anteriorly, and of the vagus, more posteriorly. However, it is not practical to attempt to separate them. The roots of the vagus and PLLN merge and pass posterolaterally through the otic capsule. Follow them posteriorly, removing cartilage and soft tissue as required. The vagus and PLLN separate just medial to the first pharyngeal pouch. The PLLN lies medial to the vagus and curves posteromedially as it extends between the epaxial and hypaxial musculature to innervate the lateral line canal in the trunk.

The vagus has two main parts, the visceral and intesti­nal branches. The visceral branch may be observed as it passes over the pharyngeal pouches and gives rise to four branchial branches, one each to all but the most anterior pharyngeal pouch (which is innervated by the glossopharyngeal nerve). Reflect the visceral branch of the vagus and follow the branchial branches laterally (Figure 3.4). They lie along the floor of the anterior cardinal sinus (see pages 43 and 58). Like the glosso­pharyngeal, each branchial branch sudivides into pretrematic, posttrematic, and pharyngeal branches. The intestinal branch continues posteriorly after the last of the branchial branches. Medial to the last branchial pouch, it turns sharply medially. Dissect carefully here, as other nerves cross it dorsally (see below). The intesti­nal then continues ventrally into the pleuroperitoneal cavity, passing initially along the esophagus. This portion may also be seen by turning your specimen on its dorsal surface, slitting open the posterior cardinal sinus, and examining the dorsomedial wall of the sinus.

Return to the nerves crossing the intestinal branch of the vagus, noted in the preceding paragraph. The large nerve is the hypobranchial nerve, which passes ventrally to innervate the hypobranchial musculature. A smaller nerve, a spinal nerve, passes just posterior to the hypo- branchial in this region. Trace the hypobranchial antero- medially as it passes deep to the PLLN. Note that the hypobranchial becomes gradually thinner as you trace it toward the brain. It initially arises from the brain as two or three occipital roots (see below).

The nerves immediately posterior to the vagus of the shark, and most anamniotes, are not directly compara­ble to those of more derived vertebrates. In the shark these nerves merge from roots that arise from the tran­sition between the medulla and spinal cord. They are thus formed from roots that are occipital and spinal. As a result, they are not entirely within the chondrocra­nium and thus are not “cranial.” There is ambiguity because the posterior end of the cranium is phyogenet- ically variable among anamniotes. However, they are in part homologous with cranial nerves of higher verte­brates and so are considered here.

Usually, the first two slender roots that emerge poste­rior to the vagus unite to form the occipital nerve, which then partially merges with the vagus nerve. The occipi­tal nerve continues posteriorly and receives contribu­tions from the first few spinal nerves, which arise posterior to the occipital nerve. The union of the occip­ital and spinal nerves is the hypobranchial nerve, as noted above. Each spinal nerve results from the union of a dorsal root and a spinal root that arise from the spinal cord. The roots unite a short distance from the spinal cord, and the dorsal root bears a swelling, or gan­glion. In amniotes the transition between the head and the trunk becomes fixed and the occipital nerves clearly arise from the brain within the skull as a “cranial” nerve termed the hypoglossal nerve (see page 222). Thus, the hypobranchial nerve of the shark (and other anam- niotes) is homologous with the hypoglossal (CN XII) of amniotes. As noted above, the accessory nerve (CN XI) of higher vertebrates (mainly derived from the vagus), is not represented as a distinct nerve in the shark.

Ventral View of the Brain

When you are familiar with the cranial nerves, the brain may be removed from the chondrocranium. Cut the olfactory tracts and then across the spinal cord just pos­terior to the medulla. Lift the anterior part of the brain, and locate and cut the optic nerves. Continue to cut each of the cranial nerves, leaving as long a stump as possible. Lift the brain laterally and note the ventral extension, the hypophysis, just behind the optic nerves. Cartilage pos­terior to the hypophysis will have to be removed in order to free the brain entirely without damage.

Examine the ventral surface of the brain, noting its regions as described above (page 70) and the cranial nerves, particularly those that arise from ventral surface—the optic, oculomotor, and abducens. The optic nerves converge toward the anterior part of the hypothalamus and form the optic chiasm, where the optic nerves cross over to the opposite side of the brain (Figures 3.43 and 3.44). The rest of the hypo­thalamus is formed mainly by the infundibulum, which bears several important structures. The paired inferior lobes of the infundibulum lie just posterior to the optic chiasm. Posterior to each inferior lobe, the infundibu­lum continues as a vascular sac. Between the left and right inferior lobes and vascular sacs lies the hypoph­ysis, noted above. The hypophysis is usually torn during removal of the brain. If this occurs in your specimen, examine another student’s specimen.

Sagittal Section of the Brain

The brain and spinal cord are hollow, with various con­nected cavities or ventricles in the brain itself, and a narrow central canal in the spinal cord. In life, the ven­tricles and central canal are filled with cerebrospinal fluid. Some of the ventricles have already been noted (see page 71), but to examine all the ventricles and their relationships to each other, cut the brain in half by making a midsagittal section using a new scalpel blade. Observe one of the halves in sagittal view and briefly review the regions of the brain before studying the ventricles (Figure 3.45).

The third ventricle, noted above (page 71), is the cavity of the diencephalon and communicates anteriorly with each of the lateral ventricles (one each in the cerebral hemispheres) through the foramen of Munro. The third ventricle extends into the optic lobes as the optic ven­tricle, as well as into the hypophysis. The fourth ven­tricle is the cavity of the medulla, and continues into the cerebellum as the cerebellar ventricle. The third and fourth ventricles are connected by a narrow canal, the cerebral aqueduct.

Key Terms: Brain and Cranial Nerves

abducens nerve (CN VI) accessory nerve (CN XI, not recognized in shark) (spinal accessory)

acousticolateral area branchial branches of visceral branch of vagus nerve buccal branch of facial nerve central canal cerebellar ventricle cerebellum cerebral aqueduct

anterodorsal lateral line

nerve (ADLLN) anteroventral lateral line

nerve (AVLLN) auricles of cerebellum

(aqueduct of Sylvius) cerebral hemispheres cerebrum choroid plexus deep ophthalmic branch of facial nerve (profundus nerve) diencephalon epiphysis epithalamus facial nerve (CN VII) foramen of Monro

(interventricular foramen)

fourth ventricle geniculate ganglion glossopharyngeal nerve (CN IX) hyomandibular branch of facial nerve hypobranchial nerve hypoglossal nerve (CN

XII, not recognized in shark)

hypophysis hypothalamus infraorbital nerve infundibulum lateral ventricles middle lateral line nerve

(MLLN) medulla oblongata mesencephalon metencephalon myelencephalon occipital nerve

oculomotor nerve (CN III)

olfactory bulbs olfactory nerve (CN I) olfactory tracts optic chiasm optic lobes optic nerve (CN II) optic ventricle otic lateral line nerve (OLLN)

palatine branch of facial nerve

petrosal ganglion posterior lateral line nerve (PLLN) somatic motor columns somatic sensory column spinal nerves statoacoustic nerve

(CN VIII)

(vestibulocochlear, octaval)

superficial ophthalmic branch of ADLLN

superficial ophthalmic branch of trigeminal nerve

superficial ophthalmic nerve

supratemporal lateral line nerve (STLLN)

tela choroidea telencephalon terminal nerve thalamus third ventricle trigeminal nerve (CN V) trochlear nerve (CN IV) vagus nerve (CN X) vascular sac visceral branch of vagus nerve

visceral motor column visceral sensory columns