The Brain and Cranial Nerves in Fish

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The Brain and Cranial Nerves in Fish
(This text is variously adapted from Walker(1980) and King and Custance (1982) with additional
explanation by RTL)
The nervous system of higher vertebrates is very complex but has developed from simple
beginnings. The nervous system of the dogfish should be studied carefully since the basic structure
of the nervous system can be seen exceptionally well. Not only can the system be easily exposed by
the removal of cartilage, but it is in a morphologically primitive and generalised stage. Various
nerves, including those emerging directly from the brain (which are thus known as the cranial
nerves) are fairly large and easy to see during dissection. The organisation of the cranial nerves is
often used to illustrate the theory that the vertebrate head, like the body, was fully segmented in the
ancestral condition. During later evolution, specific requirements made on the head have modified
the ancestral organisation, even in the lower vertebrates so that the original segmented condition is
hard to elucidate although still present.
The Dorsal View of the Brain:
The dissection of the nervous system should be done on the head of a dogfish (Squalus).
Remove the skin and underlying tissue from the dorsal surface of the chondrocranium and from
around the eye. Be careful not to cut a large dorsal nerve (the superficial ophthalmic nerve) that lies
dorsal to the orbit and lateral to the rostrum. Cut away the cartilaginous roof of the cranial cavity.
As you do so, look into the rostral part of the cavity and you may see a delicate, threadlike stalk
extending from a depressed area on the top of the brain (diencephalon) to the epiphyseal foramen in
the roof of the cranial cavity. This is the epiphysis, a homologue of the pineal eye of more primitive
vertebrates. The epiphyseal foramen permits more light to impinge on this organ than on adjacent
parts of the brain. The epiphysis contains photoreceptors and has been shown to be very sensitive to
light but its biological role in sharks is unknown (at least it was in 1980).
Next cut away the supraorbital crest and as much of the lateral walls of the cranial cavity as is
possible without damaging the nerves. Much of the ear on one side will have to be cut away. Be
particularly careful not to break the small trochlear nerve that leaves the brain dorsally and passes to
the dorsal oblique muscle of the eyeball (Fig. ?).
The brain should now be well exposed. Its surface is covered with a delicate, vascular connective
tissue, the meninx primitiva. Strands of connective tissue pass from the meninx to the connective
tissue lining the cranial cavity (the endochondrium) In life, cerebrospinal fluid lies in the apparently
empty perimeningeal space between the brain and the wall of the cranial cavity
The paired olfactory bulbs form the most cranial part of the brain (Figs.? and ?). They are the
lateral enlargements in contact with the olfactory sacs, and they receive the primary olfactory
neurons coming from the olfactory epithelium. Secondary olfactory neurons originate in the bulbs
and form the olfactory tracts that extend caudally to the cerebral hemispheres. These neurons
terminate in a ventrolateral portion of the hemisphere that is essentially homologous to the
mammalian piriform lobe. These parts of the brain constitute the telencephalon.
The diencephalon is the depressed area, often with a dark roof, situated caudal to the cerebral
hemispheres
The optic lobes develop in the roof, or tectum, of the mesencephalon and are the only part of the
mesencephalon apparent in a dorsal view (Fig. ?-3).
The metencephalon lies caudal to the mesencephalon and consists dorsally of the cerebellum. The
body of the cerebellum is the large, median, oval mass whose cranial end overhangs the optic lobes.
Note that it is partially subdivided into four parts by a longitudinal and transverse groove. The pair
of earlike flaps that lie on either side of the caudal part of the body of the cerebellum are the
auricular lobes of the cerebellum. The ventral part of the metencephalon contributes to the medulla
oblongata in fishes.
The Cranial and Occipital Nerves
The organisation of the cranial nerves is often used to illustrate the theory that the vertebrate head,
like the body, was fully segmented in the ancestral condition. During evolution, specific
requirements made on the head have modified the ancestral organisation, even in the lower
vertebrates. The developing embryo gives the best approximation to the ancestral condition of the
adult.
It seems that early in vertebrate evolution, the internal skeletal system and the muscular system
developed simultaneously. The organisation of these systems still exists in fish in broadly the same
form. The musculature is derived from the mesoderm of the embryo and is metameric. Each
myotome needs to be innervated, therefore the nervous tissue also takes on a segmental appearance.
Further innervation is required for the lateral plate musculature (associated with the visceral organs)
and the sense organs. Consequently, the dorsal nerve cord must give off fibres, in or between each
segment, to the segmental muscles (somatic motor) and the non-segmental muscles (visceral motor)
and sensory fibres receiving stimuli from the internal organs (visceral sensory) and the skin and
skeletal muscles (somatic sensory). These various fibres leave the dorsal nerve cord as nerve tracts,
or roots, which may be ventrally or dorsally placed. The dorsal root leaves the nerve cord
intersegmentally. It is probable that in the ancestral vertebrate, somatic motor fibres left in the
ventral root and all other fibres in the dorsal root. However, this pattern is modified in lower
vertebrates so that some of the visceral motor fibres also exit from the ventral root. Also, the two
roots join a short way from the nerve cord, to divide later on when they reach the various organs
that they supply. This arrangement of dorsal and ventral roots is found in the spinal nerves but the
arrangement of the cranial nerve roots is more complicated. However, it is still possible to work out
whether the nerves concerned are ventral or dorsal roots and what kind of fibres they contain. This
is done in the hope of demonstrating that the arrangement of the cranial nerves is only a-modified
form of the arrangement of the spinal nerves. If the cranial nerves also had a segmental background,
this would indicate that the head was once fully segmented.
The cranial and occipital nerves must be considered before the ventral and internal parts of the
brain can be examined. More of the lateral wall of the cranium will have to be removed as the
nerves are studied.
The Cranial Nerves
The olfactory nerve (I)
The optic nerve (II)
The oculomotor nerve (III)
The trochlear nerve (IV)
The trigeminal nerve (V)
The abducens (VI)
The facial nerve (VII)
The auditory (which can be called the statoacoustic or vestibulocochlear) nerve (VIII)
The glossopharyngeal nerve (IX)
The vagus (X)
The cranial nerves are ‘followed’ by paired spinal nerves although the arrangement of these at the
back of the cranium varies as the vertebrates advanced.
The nerves which can be considered to be ‘special’ nerves (supplying a specialised head structure)
are: the olfactory nerve (I), the optic nerve (II), the auditory (VIII), the glossopharyngeal (IX), and
the vagus (X). These last two supply the lateral line system.
The superficial opthalmic nerve contains fibres of both V and VII. Somatic sensory fibres in this
nerve supply the sense organs of the skin and snout. Another part of the nerve supplies the lateral
line system (the lateralis branch). Consequently, the superficial opthalmic nerve also falls into the
category of ‘special’ nerves because of its mixed nature. Other mixed nerves of this type are the
trigeminal, the facial, the glossopharyngeal and the vagus.
(A) NERVUS TERMINALIS
Fishes are usually described as having 10 pairs of cranial nerves, and these are both named and
numbered. However, an additional rostral nerve has been left out of this numbering system. The
nervus terminalis (Fig. ?) is a tiny nerve that lies along the medial surface of the olfactory tract and
extends between the olfactory sac and cerebral hemisphere. It is seen best at the medial angle
formed by the junction of the olfactory tract with the cerebral hemisphere, for it separates slightly
from the olfactory tract in this region. Although the terminalis is found in all vertebrates, except
cyclostomes and birds, its function is uncertain. The consensus is that it carries both somatic
sensory fibers (cutaneous, not olfactory fibers) from the nasal area, and visceral motor fibers of the
autonomic system. The latter are probably vasomotor. Some investigators have reported the
presence of ganglia along the nerve.
(B) OLFACTORY NERVE
The olfactory sense is very well developed in dogfish. The olfactory nerve (I) carries olfactory
impulses (somatic sensory) from the olfactory sac to the olfactory bulb of the brain. The ridges
inside the sac are covered by a sensory epithelium containing the olfactory receptor cells. They
differ from other vertebrate receptor cells by communicating directly with the brain; they send out
long fibres directly into the olfactory bulb. Since the sac and bulb are adjacent to each other in the
dogfish, the olfactory nerve is not compact but consists of a number of minute groups of neurons
passing between these structures. These may be seen by making a section through the olfactory sac
and bulb. The olfactory neurons are of the neurosensory type.
(C) OPTIC NERVE
The optic nerve (II) brings in optic impulses (somatic sensory) from the eye. Find it in the orbit
and trace it medially (Fig. ?). It is a thick nerve. Push the brain away from the cranial wall and note
that the optic nerve attaches to the ventral surface of the diencephalon. Since the retina of the eye
develops embryologically from an outgrowth of the brain, the optic nerve is really a brain tract
rather than a true peripheral nerve. The light receptive rods and cones terminate on bipolar neurons,
which are very short and lie within the retina. The bipolar neurons, in turn, synapse with ganglion
neurons, whose axons form the optic nerve. Nerve II can be put into the category of Special Nerve
since it supplies a specialised head structure.
(D) OCULOMOTOR NERVE
The oculomotor nerve (III) carries somatic motor impulses to most of the extrinsic ocular
muscles, receives proprioceptive impulses from these muscles (somatic sensory), and carries
autonomic fibers (visceral motor) to the eye. To see it, mobilise the eye on the intact side of the
head in the manner described in connection with the dissection of the eye (?), remove the gelatinous
connective tissue lying in the orbit, and look on the ventral surface of the eyeball. The branch of the
oculomotor going to the vertebral oblique muscle will be apparent (Fig. ??). Follow it caudally and
medially. It passes ventral to the ventral rectus, and at the caudal margin of this muscle crosses a
small, often whitish, blood vessel. This vessel, an artery, follows the margin of the ventral rectus
and enters the eyeball. The autonomic fibers of the oculomotor form a small ciliary nerve that
travels along this vessel, but these fibers can seldom be seen grossly. The branch of the oculomotor
to the ventral rectus lies between the ventral rectus and the small vessel. Turn your specimen over
and pick up the oculomotor nerve from the dorsal side. Cut the lateral rectus near its insertion on
the eyeball, and also the superficial ophthalmic nerve, and reflect them to see the oculomotor nerve
clearly. It extends dorsal to the origin of the dorsal rectus and enters the cranial cavity. Just before it
enters, it gives off one branch to the dorsal rectus and another to the medial rectus. (Do not confuse
the oculomotor with another nerve of the same size, the profundus, which crosses the base of the
oculomotor and extends along the medial surface of the eyeball.) Push the brain away from the
cranial wall and note the attachment of the oculomotor on the ventral surface of the mesencephalon.
(E) TROCHLEAR NERVE
The trochlear nerve (IV) is one of the nerves that innervates the eye-moving muscles. The trochlear
nerve (IV) has been noted crossing an optic lobe (?). Lift up the rostral end of the body of the
cerebellum and note where it attaches on the brain. The trochlear passes through the cranial wall,
goes ventral to, or perforates, the large superficial opthalmic nerve, and extends to the dorsal
oblique muscle. Like the oculomotor, it is primarily a somatic motor nerve, but it carries a few
proprioceptive fibers (somatic sensory). It is a ventral root.
(F) ABDUCENS NERVE
Skip the fifth nerve for a moment and consider the abducens (VI), which carries somatic motor
fibers to the lateral rectus and returns proprioceptive (somatic sensory) impulses from this muscle.
It can be seen on the ventral surface of the lateral rectus. Its attachment on the ventral surface of the
medulla will be seen later if the brain is examined after removal (Fig. ?).
(G) TRIGEMINAL NERVE
The trigeminal nerve (V) is a large nerve with three main branches which are easily visible. The
trigeminal was probably originally associated with an anterior gill slit which was lost as the mouth
extended backwards in the early vertebrates. The three main branches are : the profundus (V1),
which receives stimuli from the skin of the snout; the maxillary branch (V2) which innervates the
upper jaw and the mandibular branch which innervates the lower jaw. The trigeminal is a dorsal
root.
The trigeminal nerve (V), is the nerve of the mandibular arch and the general cutaneous sensory
nerve of the head (Figs. ? and ?). The trigeminal attaches more or less in common with the facial
(VII) and auditory (VIII) nerves on the dorsolateral surface of the medulla just caudal to the auricles
of the cerebellum. It is difficult to separate these nerves grossly at their attachments on the brain,
but they can be sorted out to some extent by their peripheral branches. The trigeminal has four
branches in fishes. A superficial ophthalmic branch, together with a comparable branch of the facial
nerve, forms the large superficial ophthalmic nerve that has been noted passing through the dorsal
region of the orbit and along the lateral surface of the rostrum. A deep opthalmic branch (or
profundus nerve) enters the orbit dorsal to the oculomotor, adheres to the connective tissue on the
medial surface (back) of the eyeball, and leaves the front of the orbit through a small foramen to
join the superficial ophthalmic nerve. The deep ophthalmic should be traced on the side of the head
in which the eyeball has been left. Both ophthalmic branches of the trigeminal return somatic
sensory impulses from general cutaneous sense organs (not lateral line organs) in the skin on the top
and side of the head. In addition, the deep ophthalmic has several minute and inconspicuous
branches to the eyeball. These are considered to be homologous to the long ciliary nerve of
mammals and, for the most part, return sensory fibers from parts of the eye other than the retina.
A mandibular branch of the trigeminal can be found by dissecting away the connective tissue on
the caudal wall of the orbit. It is a fairly thick nerve that lies caudal to the lateral rectus. The
mandibular carries special visceral motor fibers to the branchiomeric muscles of the first visceral
arch and returns some somatic sensory fibers from general cutaneous sense organs in the skin
overlying the lower jaw.
The maxillary branch of the trigeminal, together with the buccal branch of the facial, forms the
large infraorbital nerve that extends rostrally across the floor of the orbit. The infraorbital nerve is
fully as wide as any of the ocular muscles and is easily confused with a muscle. It divides near the
rostral border of the orbit and is distributed to the skin overlying the upper jaw and the underside of
the rostrum. The maxillary portion of this nerve returns somatic sensory fibers from general
cutaneous sense organs in this region.
Cut away enough cartilage and connective tissue from the caudomedial corner of the orbit to be
able to see where all of the branches of the trigeminal come together, and again note the attachment
of the trigeminal to the medulla. The main part of the trigeminal bears a slight enlargement, the
semilunar ganglion, that contains the cell bodies of the sensory neurons; however, it is unlikely that
you can distinguish this ganglion.
(H) FACIAL NERVE
The facial nerve (VII) has many branches, one being the hyomandibular branch which further
divides into the pre spiracular and post spiracular branches. The latter divides again into hyoidean
and mandibular branches. Nerve VII is a dorsal root.
The facial nerve (VII) is the nerve of the hyoid arch, spiracle, and the cranial lateral line organs.
As mentioned above, its superficial ophthalmic and buccal branches contribute to the superficial
ophthalmic and infraorbital nerves, respectively. They return somatic sensory impulses from the
lateral line canals, pit organs, and ampullae of Lorenzini. The attachment of certain fibers from
these trunks to the ampullae of Lorenzini can be seen. Cut away the skin adjacent to the
caudoventral corner of the spiracle on the intact side of the head, and pick away the underlying
connective tissue. The large nerve that will be seen is the hyomandibular nerve, a branch of the
facial (Fig. 6-6, p. 125). Follow it peripherally, noting that it is distributed to the hyoid muscles
(special visceral motor fibers), skin (somatic sensory fibers from lateral line organs), and lining of
the mouth (visceral sensory fibers of both general and taste nature). Follow it medially to its union
with the other branches and its attachment on the brain, which, as stated, is more or less in common
with that of the trigeminal and statoacoustic nerves. You will have to cut away some of the spiracle,
surrounding muscles, otic capsule, and ear as you go. About 1 centimeter from the brain the
hyomandibular bears a slight enlargement, the geniculate ganglion, that contains the cell bodies of
sensory neurons (Fig. ?-6). Another, and smaller, branch of the facial leaves from the rostroventral
surface of this ganglion. This is the palatine nerve, and it returns visceral sensory neurons from the
mouth lining.
(I) AUDITORY NERVE
A part of the auditory (VIII) (sometimes called the statoacoustic or vestibulocochlear) nerve**) may
have been noted coming from the ampullae of the anterior vertical and lateral semicircular ducts
during the dissection of the hyomandibular nerve. Continue to cut away the otic capsule and note
another, and longer, part of this nerve coming from the ampulla of the posterior vertical
semicircular duct, the sacculus, and parts of the utriculus. The statoacoustic contains somatic
sensory fibers from various parts of the inner ear.
(J) GLOSSOPHARYNGEAL NERVE
The glossopharyngeal nerve (IX) is quite small and innervates most of the tongue and the pharynx.
It is associated with the first gill slit. It is a dorsal root but falls int the special nerve category since
it innervates part of the lateral line system.
The glossopharyngeal nerve (IX) is the nerve of the third visceral arch and the first of the five
definitive gill pouches. It can be seen crossing the floor of the otic capsule caudal to the sacculus. It
passes ventral to the caudal branch of the statoacoustic and at first may be confused with this nerve.
Cut away this part of the statoacoustic and find the attachment of the glossopharyngeal on the side
of the medulla. Trace the glossopharyngeal laterally; this will be facilitated if you open the first gill
pouch by cutting through the skin and muscle dorsal and ventral to the first external gill slit. Cut all
the way to, but not through, the internal gill slit (the opening between the gill pouch and pharynx).
As the nerve leaves the otic capsule, it bears an oval-shaped swelling, the petrosal ganglion, that
contains the cell bodies of sensory neurons. Several branches leave from the petrosal ganglion. A
large posttrematic passes down the caudal face of the first gill pouch to carry special visceral motor
fibers to the branchial muscles and return visceral sensory fibers from this region. A smaller
pretrematic branch, which is entirely visceral sensory, passes down the cranial face of the first
pouch. A still smaller pharyngeal branch follows the pretrematic a short distance, then curves
around a tendon near the dorsal edge of the internal gill slit and is distributed to the wall of the
pharynx. It, too, is entirely visceral sensory. There is finally a small dorsal branch distributed to
lateral line organs, and often to the skin in the supratemporal region, but it is impractical to find.
(K) VAGUS NERVE
The vagus (X) is a large nerve which supplies various visceral organs and the lateral line along the
main part of the body. A series of branches from the vagus runs to all the gills slits (except the first
and the spiracle) innervating the pharynx and the gill muscles. The main branch of the vagus
continues posteriorly as the abdominal branch which relays sensations from the visceral organs to
the brain.
The vagus (X) is the nerve of the remaining visceral arches. Find its attachment on the
dorsolateral surface of the caudal end of the medulla and follow it caudally out of the otic capsule.
To see the rest of the nerve, you must cut open the remaining gill pouches in the manner described
for the first, If you cut as far as you should, you will cut into a-large blood space, the anterior
cardinal sinus, lying dorsal to the internal gill slits. This sinus must also be opened by a longitudinal
incision. A large branch of the vagus (visceral nerve) lies beneath the connective tissue on the
dorsomedial wall of the anterior cardinal sinus. It gives off four branchial branches that cross the
floor of the sinus and are distributed to the remaining four visceral arches and pouches. Each
branchial branch follows the pattern of the glossopharyngeal, having a sensory ganglion from which
posttrematic, pretrematic, and pharyngeal branches arise. Visceral sensory fibers in the posttrematic
of the last branchial branch are distributed to the caudal surface of the last gill pouch, but there are
no branchiomeric muscles here. Motor fibers in the last branchial branch form a small accessory
nerve that goes to the cucullaris. This nerve is difficult to find. After giving off the last branchial
branch, the visceral nerve continues as the intestinal nerve along the wall of the esophagus to the
viscera. It also sends a branch to the pericardial cavity. The intestinoaccessory nerve contains
visceral motor fibers of the autonomic system, visceral sensory fibers, and special visceral motor
fibers to the cucullaris.
Just before the visceral nerve enters the rostral end of the anterior cardinal sinus, the vagus gives off
a lateral (or dorsal) nerve. This branch lies medial to the visceral nerve and extends caudally
between the epaxial and hypaxial musculature. It receives somatic sensory fibers from the lateral
line canal proper, and in some elasmobranchs a few general cutaneous fibers from the skin in the
gill region. Cell bodies of these neurons lie in a ganglion near the proximal end of this branch.
(L) THE OCCIPITAL AND HYPOBRANCHIAL NERVES
Free the visceral branch of the vagus nerve from the wall of the anterior cardinal sinus. A
hypobranchial nerve emerges from the epibranchial musculature, crosses the visceral nerve at about
the level of its last branchial branch, and curves ventrally in the wall of the common cardinal vein
(Fig. ?-6). It carries somatic motor fibers to the hypobranchial musculature and returns a few
proprioceptive and cutaneous somatic sensory fibers. Trace the hypobranchial nerve medially and
cranially toward the vertebral column and chondrocranium. It becomes progressively narrower and
more difficult to trace through the musculature, but if you are successful you will see that it is
formed by the confluence of several spinal nerves and two or three occipital nerves. The first spinal
nerve emerges between the chondrocranium and the first vertebra. The last occipital nerve can be
seen between this point and the large root of the vagus. More rostral occipital nerves lie deep to the
root of the vagus. The occipital nerves resemble accessory rootlets of the vagus, for they appear to
join it. Actually, they only travel with the vagus a short distance as they leave the chondrocranium,
then they separate and, together with several spinal nerves, form the hypobranchial nerve.
The hypobranchial nerve, and its occipital and spinal components, are homologous to the amniote
twelfth cranial nerve, the hypoglossal. By convention the hypobranchial is not considered to be a
cranial nerve in fishes because its origin relative to the caudal end of the skull varies in different
groups. Frequently, as in Squalus, the origin is partly from the back of the brain (the occipital
nerves) and partly from the front of the spinal cord ( the spinal nerves). The occipital nerves are
serially homologous to the ventral roots of spinal nerves. The dorsal roots of these nerves are
believed to form the vagus.
FOOTNOTE ** This nerve (the auditory) is called the vestibulocochlear nerve in mammals. Lower
vertebrates lack a cochlea in their ear, so it is appropriate to retain the term statoacoustic for their
eighth nerve.
References
King and Custance (1982) A color atlas of vertebrate anatomy. Blackwell
Walker(1980) Vertebrate Dissection. Saunders College Publishing, USA.
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