Our
study of the nervous system in this lab will be concerned with an examination
of the brain of the Sheep, and comparison with the Dogfish and Mudpuppy to show
the main evolutionary changes. In the embryo, a three-part brain develops, but
adult vertebrates are born with a five-part brain. Examine these five parts in
the three animals dissected and note that their structures remain the same. We
will also be looking at the ear in the dogfish and a generalized spinal cord.
The
brain in encased in membranes called meninges. In dogfish
there is only a single meninx, which holds cerebrospinal fluid between it
and the brain. The cerebrospinal fluid also circulates within the cavities of
the brain and spinal cord. In mudpuppies, there is an outer dura matter and an
inner secondary meninx. In mammals, there are three meninges, an outer dura
matter, a middle arachnoid membrane and an
inner, pia matter.
Dogfish Brain
and Ear
Remove
the skin from the dorsal side of the head of your specimen. The cut should be
0.5cm to the left of the mid line to 1.5cm behind the spiracles. Remove the
muscles of the head, and in thin horizontal slices, cut away the roof of the
chondrocranium. In the area of the eye and inner ear dissect the RIGHT SIDE
ONLY. Be careful in this dissection not to cut the cranial nerves, some of
which issue dorso-laterally from the brain. As you slice through the hyaline
(glassy) cartilage, you will slice through the inner ear. When you have removed
the cartilage from the roof and the right side of the head, study the brain
in situ.
The
ear of fishes is an organ of equilibrium, and is specialized for collecting
information concerning the position and movements of the head. Although many
fish are able to hear, only a very few of the bony fish have a sense of hearing
comparable with that of man. The mechanism by which sound vibration is detected
is very different in the two groups. In the dogfish the sense of hearing is
very poorly developed, and there is no specialization in the structure of the
ear for reception of sound, it is primarily done in the lateral line.
To
expose the ear requires careful dissection to avoid damaging its delicate
structure. To save time, we will slice through the ear in layers while exposing
the brain. Look at the injected plastic models first to get an idea of its size
and shape.
The
ear of the dogfish lies embedded in the hyaline cartilage of the skull in the
otic or auditory capsule Locate the spiracle and remove
the skin between it and the opening of the endolymphatic duct. Then, with
a sharp scalpel, slice fine horizontal slices from the cartilage watching
carefully for the first signs of the colourless, worm-like, semi-circular
canals. Note the endolymphatic duct, which connects the sacculus of the ear
to the outside. Continue cautiously, removing the cartilage around the canals
until the whole ear is exposed. The first structures to appear are the two
vertical semicircular canals, the anterior and the
posterior. They join at an angle of slightly more than ninety degrees. The
horizontal canal projects laterally and horizontally from the ventral ends of
the other two.
Examine
the figure and note the two ends of the posterior vertical canal are united by
the posterior utriculus. The dorsal ends of the anterior vertical and
horizontal canals unite to form the anterior utriculus. The latter
runs anteriorly and ventrally, and divides to form the ventral ends of the same
two canals. Each canal bears a swelling near its ventral end. These are the ampulla, which
contains sensitive hair cells connecting with a branch of the auditory nerve (cranial
nerve VIII).
The
main part of the inner ear consists of a large sac-like structure, triangular
in lateral view. This is the sacculus. It is connected to each of the
utriculi through a pore, and opens to the surface of the head dorsally through
the endolymphatic duct. It is usually dark in colour as a result of
sand grains, which enter through the endolymphatic duct. Several sensory spots,
or maculae, are located in the walls of the sacculus and utriculi. These
contain sensory hair cells (too small to see), which are connected with
branches of the auditory nerve (cranial nerve VIII). The nerve
branches will be seen later. At the posterior end of the ventral margin of the
sacculus is a small outpocketing or depression, the lagena. This is
best detected using a blunt probe. In higher vertebrates, this pocket is elaborated
to form the cochlea, the part of the ear responsible for the acute
sense of hearing.
The
semicircular canals, utriculi, and sacculus, which together are known as the
membranous labyrinth, contain a fluid, the endolymph that
circulates within them. Surrounding the membranous labyrinth, and occupying the
space between its walls and the cartilage of the chondrocranium is another
fluid, the perilymph that prevents the membranous labyrinth from
adhering to the chondrocranium.
The
sense of equilibrium may be divided into the ability to detect movement of the
head, and the ability to detect its position in space.
These senses are localized in different parts of the ear. The semicircular
canals, ampulla, and utriculi are concerned with the detection of motion.
Because of its inertia, the endolymph will tend to circulate around the canals
when the head is moved. As the endolymph moves through the ampulla, it bends
the delicate hair cells in their walls, and thus initiates nerve impulses to
the brain. The plane in which motion of the head occurs will determine which of
the six ampullae are stimulated. This is, therefore, an extremely accurate
system for providing information concerning the animal’s movement.
The
sense of position is centered largely in the sacculus and lagena, which contain
sand grains (or, in most vertebrates tiny concretions of calcium carbonate).
These, known collectively as the otolith, lie free in the endolymph
and will alter their position in response to gravity when the position of the
head is changed. Depending on their position, they will rest on hair cells in
one or other of the maculae, thereby triggering nerve impulses to the brain,
and thus providing information concerning the position of the
head.
In
the human, the sense of equilibrium depends on structures basically similar to
those of the inner ear of the dogfish. In addition, modification of the
lagena to form the cochlea, and the development of the middle
and external ears, has provided man with an extremely sensitive mechanism for
detecting sound vibrations.
The
sense of hearing in the ear is not well developed in the dogfish, and little is
known about its mechanism. Possibly sound vibrations in the water impinge on
the endolymphatic duct and initiate corresponding vibrations of the endolymph.
This in turn might stimulate the maculae.
As
you slice slivers off the hyaline cartilage, you are slicing through the chondrocranium
(neurocranium), a box around the brain. You will first see the cerebellum,
which sticks up; the rest of the brain is deeper. The brain and nerves are
cream coloured. Be especially careful of the trochlear nerve, which is
delicate. Nerves should be teased out of the cartilage with
dissecting needles and forceps as the scalpel will cut them off. The cartilaginous
peg between the optic lobe and the eye is the orbital process of the
palatoquadrate (upper jaw). It helps keep the jaw in place. Using your
drawings, identify the following brain parts and their nerves.
|
Brain Part |
Structures |
Nerves |
|
Telencephalon |
olfactory lobes |
olfactory I |
|
Diencephalons |
pineal body |
optic II |
|
Mesencephalon
|
optic lobe |
oculomotor III |
|
Metencephalon
|
cerebellum |
|
|
Myelencephalon
|
medulla oblongata |
trigeminal V |
Table: Review of the divisions of the dogfish brain
|
3 Part Brain |
5 Part Brain |
Structures |
|
Prosencephalon |
Telencephalon |
Pallium, olfactory bulbs,
olfactory lobes, cerebral hemispheres, ventricles I and II, part of Anterior
Choroid Plexus |
|
Diencephalons |
pineal, thalamus, pituitary,
optic chiasma, part of Anterior Choroid Plexus, Ventricle III |
|
|
Mesencephalon |
Mesencephalon |
cerebral aqueduct, optic lobes,
nerves III and IV |
|
Rhombencephalon |
Metencephalon |
cerebellum, part of the fourth
ventricle |
|
Myelencephalon |
the posterior part of the fourth
ventricle, Posterior Choroid Plexus, and Medulla Oblongata, nerves V to X |
Spinal Cord
The
spinal cord is enclosed by the vertebral column and contained in the neural canal;
it is continuous with the brain at the foramen magnum. Spinal nerves of the
peripheral nervous system can be seen metamerically arranged along the length
of the cord. They exit as dorsal and ventral roots which merge briefly and
cross over to emerge as a visceral root to the organs, glands and skin and in a
somatic root to the trunk muscles. The visceral (autonomic) nervous system is
shown in the diagram. The somatic nerves operate the skeletal and trunk muscles
consciously or in a reflex - arc and are responsible for locomotion and
movement of the body. Examine the anterior part of the dogfish spinal cord and
the models in the lab.
The
Telencephalon
has olfactory bulbs with an olfactory nerve, and elongate cerebral
hemispheres.
The Diencephalons
only shows as a small paraphysis between the posterior part of the cerebral
hemispheres and a dorsally projecting epiphysis. The optic nerve II
leads to the eye. The Mesencephalon has two rounded optic lobes and occulomotor nerve
III and trochlear IV, which will not be seen. In the Metencephalon, the cerebellum is poorly developed and
is only a narrow transverse band of tissue behind the optic lobes. The Mylencephalon has a large medulla
oblongata
continuous with the spinal cord. Its parts and nerves are similar to those of
the shark.
Mammal Brain
We
will be examining whole and bisected Sheep brains. There are models, keys and
photographs on display. Identify the five parts of the brain and their
associated structures.
In the Telencephalon, the two large symmetrical structures overlying, and just anterior to, the cerebellum, are the cerebral hemispheres. They are convoluted with hills (gyri) and furrows (sulci). Within the hemispheres are two cavities; the first and second ventricles joined by the lateral ventricle, which is roofed by the corpus callosum, a commissure to relay information. The olfactory lobes represent the anterior area of the telencephalon. They are two small lobes ventral to the cerebral hemispheres.
The Diencephalons lies between the cerebral hemispheres and the midbrain and is covered by the former. The roof of the diencephalon anteriorly, forms part of the anterior choroid plexus. The cavity of the diencephalon is the third ventricle. The posterior dorsal projection of this part of the brain is the epiphysis or pineal body. This pineal complex is light sensitive; it affects pigmentation, circadian rhythms, reproductive rhythms etc., and synchronizes them with the external environment. The lateral walls are thalami (thalamus sing.), which are relay and integrating centers for nerve tracts connecting the cerebral hemispheres with the posterior parts of the brain. Thc hypothalamus or floor, is ventral, visible externally, and coloured blue in the models. Note the optic chiasma anterior to the hypothalamus. The projection of the hypothalamus is the infundibulum, which, with the neuro part of the hypophysis forms the posterior pituitary from the brain (neural ectoderm). The larger adeno part of the hypophysis, or anterior lobe of the pituitary gland, comes from the somatic ectoderm lining of the mouth. This portion often falls off in our preserved sheep brains. The thalamus and pituitary glands affect smooth muscle contraction, membrane permeability, controls reproduction, growth etc., through other glands.
The Mesencephalon is not very visible dorsally, being concealed by the cerebellum, and cerebral hemispheres. The midbrain is painted orange on the models on demonstration. The cavity of the Midbrain is the cerebral aqueduct, or aqueduct of Sylvius, it connects the third ventricle with the fourth ventricle. Locate the optic lobes and auditory lobes sometimes called the corpora quadragemmina.
The dorsal portion of the Metencephalon overlies the medulla and is the cerebellum. Its surface is formed into folds. It is also called arbor vitae (tree of life). The pons relays information to the cerebral hemispheres.
The
most posterior portion of the brain is the large, triangular medulla
oblongata of the Mylencephalon. The posterior choroids plexus lies
on its dorsal surface. Its posterior limit is the point at which the first pair
of spinal nerves emerge. Its cavity is the 4th ventricle. The
same spinal nerves emerge as in the shark.
Comparative Brain Models
Having
reviewed the structure of the Dogfish Brain, the Mudpuppy brain and the
dissected Sheep Brain, examine the series of brain models of Dogfish, Perch,
Frog, Alligator, Pigeon, Rabbit and Cat, to review the evolutionary story.
Note, in particular, that there is little change in the Rhombencephalon and
Mesencephalon, but great changes in the Prosencephalon.
|
Assignment: Identify the various areas in each of these models and note how they have changed from the 5-part lamprey brain by filling in the following table.
|
Table: Comparison of brain functions in fish and mammals
|
LOBE |
DOGFISH |
MAMMAL |
|
Telencephalon |
Olfaction and Instinctive
Behaviour |
Intelligence, memory, instinct
(cerebral hemispheres); olfaction. |
|
Diencephalons |
Integration of sensory and motor
|
Involuntary actions (emotions,
sleep, breathing, temperature), endocrine secretions |
|
Mesencephalon |
Vision |
Conscious optic in cerebrum,
only reflex optic (i.e. focus) and reflex auditory remain. Motor path to cerebellum
via pons. Nerves III & IV. |
|
Metencephalon |
Integration of Balance, Hearing
and Vision (cerebellum) |
Integrating locomotion and motor
function. Info. to cerebellum goes to cerebrum for interpretation, (decision)
then back to cerebellum for correlation, relayed by the pons. |
|
Myelencephalon |
Otic (balance) exit of nerves
V-X to gills, jaws, heart. |
Otic (hearing and balance) exit
of nerves V-X to lungs, heart, digestive tract. |
Updated
by Sandra Millen, December, 2003