Introduction to the Nervous System 5
Cranial nerves. In the domestic mammals, there are twelve pairs of cranial
nerves: olfactory, optic, oculomotor, trochlear, trigeminal, abducent, facial
(sometimes designated intermediofacial), vestibulocochlear,
glossopharyngeal, vagus, accessory, and hypoglossal. From rostral to
caudal, these nerves are numbered I – XII.
Olfactory Nerve. The olfactory nerve is Cranial Nerve I and is composed
of the many olfactory filaments that pass from the olfactory mucosa to the
olfactory bulb (some descriptions account each filament as an olfactory
nerve). The filaments are small bundles of unmyelinated axons that pass
through the foramina of the cribriform plate to reach the olfactory bulb.
Each axon is the efferent process of a modified neuron, a bipolar
neuroepithelial cell, whose dendrite, as a single
cilia
process, reaches the mucosal surface where it exhibits
8 – 20 sensory cilia. The membrane of each cilium
bears receptor proteins that bind a limited group of
dendrite
specific odorant molecules. Attachment of molecules at
a cilium results in depolarization of the neuron, the
excitatory state then passing by its axon to synapsis
with mitral and tufted cells at a glomerulus of the
cell body
olfactory bulb. The glomerulI, each a tangle of
synapsing processes of tufted, mitral, and
neuroepithelial cells, form a peripheral layer of the
olfactory bulb. In the olfactory mucosa, there are
axon
millions of randomly distributed sensory neuroepithelial
cells, each of which functions as a receptor for a
limited number of odorant molecules. It is the
Fig. 1. Neuroepithelial
combination of specific stimulation, neuronal
cell. From Lehrbuch der
HIstologie und
excitation, and a path that leads to the sensory,
Vergleichenden
primary, olfactory cortex and associated areas of
Mikroskopischen Anatomie
the brain that results in awareness of odors, the
der Haustiere, Otto Krölling
generation of olfactory responses and olfactory
and Hugo Grau, 1960:
reflexes.
Verlag Paul Parey, Berlin
The olfactory mucosa is caudodorsal in the nasal cavity, applied to the
more dorsal of the ethmoturbinate bones and the neighboring nasal
septum. It is pigmented in the horse, yellow-brown in color. The epithelium
of the mucosa is composed of 1) neuroepithelial cells, 2) sustentacular
cells that support the neuroepithelium, and 3) basal cells that generate new
neuroepithelial cells every 30 – 60 days. The epithelium rests on
a basement membrane and a lamina propria of loose connective
tissue that is fused with the
periosteum of the bone to which the Fig. 2. Sustentacular
mucosa is applied. The axons of the cell. Its sides are
neuroepithelial cells pass caudally
compressed by the
in the lamina propria, gathering into
bodies of the
small bundles, the filaments, to
neuroepithelial cells
reach the foramina of the cribriform
and its “foot” is adapted
plate of the ethmoid bone. They
to the basal cells. From
Lehrbuch der HIstologie
pass through the foramina and
und Vergleichenden
enter the olfactory bulb. Serous
Mikroskopischen Anatomie
oflfactory glands of the mucosa
der Haustiere, Otto Krölling
elaborate a fluid that bathes the
and Hugo Grau, 1960:
cilia, providing a solution in which
the odorant molecules dissolve to
reach the receptor proteins.
Fig. 3. The olfactory mucosa. high power. From:
http://astro.temple.edu/%7Esodicm/labs/RespWeb/sld004.htm .
Oval nuclei of the
sustentacular cells.
Bodies (w/round nuclei)
of neuro-epithelial cells.
Basal cells.
Serous acinus of
olfactory gland.
The part of the cerebral cortex whose excitation gives rise to the sensation
of smell is the primary olfactory cortex. In animals, it is variously described
as being the olfactory tubercle, the cortical grey matter of the piriform lobe,
or the rostro-ventral end of the parahippocampal gyrus. In humans, it is
thought be located in the uncus (curved rostroventral end of the
parahippocampal gyrus) or the orbitofrontal cortex (cortex of the frontal
lobe that overlies the bony orbit).
hippocampal
sulcus
choroid fissure
parahippocampal gyrus
dentate gyrus
Fig. 4. Canine right cerebral hemisphere,
medial view. Color is added to the larger figure
to emphasize the dentate (yellow) and
parahippocampal (pink) gyri. The white
alongside the dentate gyrus represents the
fimbrial fibers (see NervousSystem 4). Figure
is from Handbuch der Vergleichenden
Anatomie der Haustiere, Zietzschmann,
Ackerknecht, and Grau, 1943: Springer Verlag
Berlin.
olfactory bulb
Fig. 5. Canine brain,
ventral view. The
pituitary gland is cut
from the infundibular
stalk.
olfactory
peduncle
olfactory
tubercle
piriform
lobe
rostroventral end of
parahippocampal
gyrus
bulb
peduncle
tubercle
pyrifm. lobe
parahip. gyrus
Fig. 6. Medial
view, left
cerebral
hemisphere,
showing position
of pyriform lobe
relative to the
parahippocampal
gyrus.
pyriform
lobe
parahippocampal
gyrus
Vomeronasal nerves and the terminal nerve are topographically related
to the olfactory nerve but are generally not accounted a part of it.
Vomeronasal nerves are axons of the bipolar cells of the olfactory mucosa
of the vomeronasal organ. They pass upon the nasal septum, through the
foramina of the cribriform plate, to reach the accessory olfactory bulb
(AOB). The AOB, in mammals, is a defined area in the caudodorsal part of
the olfactory bulb. From the AOB, the neural pathway results in striated
motor activity (in some species, the flehmen response and associated
striated motor activity) and autonomic reflexes by way of the amygdala and
the bed nucleus of the stria terminalis. The latter reflexes are mediated in
the hypothalamus.
The terminal nerve, is generally described as arising like the vomeronasal
nerves as axons of mucosal neuroepithelial cells of the vomeronasal organ.
The nerve is a slender plexiform filament that proceeds with the
vomeronasal nerves from the vomeronasal organ. (Scattered ganglia are in
the plexus and some work on this nerve in lower vertebrates (fish) suggests
that it may be an efferent nerve to the organ and the olfactory bulb.) The
terminal nerve passes with the vomeronasal nerves through the cribriform
plate but is distinguished from them by the fact that it bypasses the
olfactory bulb (and accessory olfactory bulb) to proceed alongside the
medial olfactory tract, which it joins. It is distributed to the medial and lateral
septal nuclei and the preoptic area of the hypothalamus. Function and
precise anatomy are not surely known. The nerve is thought (key word) to
mediate functions associated with mating behavior.
Fig. 7. A rough drawing
that shows the position
of the olfactory tracts.
The amygdala is a large
nucleus (shown by
shading) that is chiefly
responsible for the
prominence of the
piriform lobe.
intermediate
olfactory tract
olfactory bulb
olfactory
peduncle
lateral olfactory
tract
piriform lobe (external
prominence of grey matter)
amygdala (nucleus, shaded
area deep to piriform cortex)
parahippocampal gyrus
Some other views of the anatomy:
medial
olfactory tract
olfactory
tubercle
Fig.
surface
of the
bone,
horse;
semidiagrammatic.
Fig.8.7.Nasal
Cerebral
surface
of ethmoid
the canine
ethmoid
bone,
showing the The
drawing
shows
manner
attachment
of the
bones
cribriform
platethe
in which
theofolfactory
bulbs
rest.ethmoturbinate
The smaller foramina
(this
includes
the dorsal
nasal
concha,
which
is the
largest
and most
admit
the filaments
of the
olfactory
nerve.
The
larger
foramina,
which
dorsal
of
the
ethmoturbinates)
in
the
solid
areas
of
the
plate.
The
form a paramedian line on either side, are, at least in part, for theright
cribriform
platenerves
is shown
outline
only.nerve, which pass caudally applied
vomeronasal
andinthe
terminal
to the median nasal septum. The scroll-like ethmoturbinate bones attach
to the nasal surface of the cribriform plate in the solid areas between the
foramina. Figure is from Atlas of Canine Anatomy, Anderson and
Anderson, 1994: Lea & Febiger, Philadelphia.
turbinate attachment
cribriform plate
perpendicular plate
Optic Nerve. The optic nerve is Cranial Nerve II. Different from all other
nerves, the right and left optic nerves are formed from a bilateral extension
of the diencephalon of the neural tube. Well before the diencephalon is fully
distinguished from the hemispheres and the midbrain, a depression
appears internally on either side of the diencephalic neural tube where the
extension of the optic stalk will appear. The retina, and the optic nerve
which proceeds from it, have their origin in the optic cup and optic stalk of
the embryo. The retina will be considered with the anatomy of the eyeball.
The optic nerve is composed of the axons of the ganglion cells of the
retina, which, unmyelinated in the retina, gain a myelin sheath on passing
from the eyeball through the lamina cribrosa of the sclera.
The optic nerve is a strictly sensory nerve. Receptors are the rod and
cone cells of the retina. The neuroreceptor synapse of rod and cone cells is
with bipolar neurons whose axons synapse with a ganglion cell.
The axons of the ganglion cells come together to form the optic nerve at
the optic disc. Gathered into large bundles, they penetrate the sieve-like
openings of the lamina cribrosa of the sclera and, as the optic nerve,
ensheathed by extensions of the meningeal coverings of the CNS pass to
the optic chiasm at the rostroventral end of the hypothalamus. The chiasm
is ventral to the rostral part of the hypothalamus and is continuous dorsally
with the lamina terminalis.
Fig. 9. Section through the eyeball of the goat at the level of the optic disc. The
lamina cribrosa is identified as the pigmented connective tissue, seen as short
transverse strips in relation to the origin of the optic nerve at the level of the
sclera. Figure is from Lehrbuch der Histologie und Vergleichenden
Mikroskopischen Anatomie der Haustiere, Krölling, O. and Grau, H.: 1960,
Verlag Paul Parey, Berlin and Hamburg.
optic disc
choroidea
lamina cribrosa
retina
sclera
optic nerve
blood vessel
dura-arachnoidea
subarachnoid space
pia mater
Fig. 10. Median section at the level of the third ventricle, brain, dog.
anterior commissure
lamina terminalis
hypothalamus
right optic nerve
optic chiasm (left
optic nerve and left
half of chiasm cut
away)
The right and left optic nerves meet at the chiasm where decussation of
fibers occurs. Decussation is the crossing of optic nerve fibers from that
part of the retina that is excluded from the visual field of the opposite eye.
Such fibers cross at the chiasm to the opposite hemisphere. In an animal
whose vision is strictly monocular, decussation is complete and binocular
vision does not take place; all fibers of the optic nerve pass to the
hemisphere of the opposite side. This takes place in many lower
vertebrates and probably in some mammals (whales, for example) that lack
binocular vision. Most mammals have some degree of binocular vision in
which there is overlap of the visual field of each eye and the extent of
decussation is correspondingly lessened. In this case, an animal such as
the dog with a total field of vision of about 250 degrees and a binocular field
of 75 – 85 degrees (www.dog.com/dog-articles/dog-eye-facts/2052/) would
be expected to have about 60 to 68 per cent of its optic nerve fibers
crossing at the chiasm.
From the optic chiasm, the right and left optic tracts, each (in an animal
with binocular vision) bearing fibers of right and left optic nerves, pass to
the lateral geniculate body of the diencephalon, to the pretectal area, and
to the rostral colliculus of the midbrain tectum. At the chiasm, fibers pass to
the suprachiasmatic nucleus of the hypothalamus. Presumably, the
majority or all of the fibers to the lateral geniculate body are original;
whereas, the fibers to the pretectal area, the rostral colliculus, and the
suprachiasmatic nucleus are collateral branches of original fibers. To the
writer’s knowledge, whether or not the fibers are collateral or original has
not been established. Synapse of the geniculate fibers occurs in the lateral
geniculate nucleus from which fibers pass in the optic radiation to the optic
(visual) cortex of the occipital lobe of the brain. Awareness of the retinal
image takes place in the optic cortex. Fibers that pass to the pretectal area
mediate pupillary reflexes that result in constriction of the pupil and those
fibers that pass to the rostral colliculus mediate reflex saccadic movements
of the eyeball. The suprachiasmatic fibers function in a pathway that
stimulates melatonin secretion by the pineal gland, which regulates the
circadian, daily, rhythms of the body.
Fig. 11. Canine brain with left hemisphere removed at its
junction with the diencephalon. The stump of the left optic
nerve can be seen. The left optic tract, lateral and medial
geniculate bodies, rostral and caudal colliculi are labeled.
rostral colliculus
caudal colliculus
optic chiasm
Fig. 12. Dorsal view, canine brain, with much of the corpus callosum
and dorsal and caudal parts of the cerebral hemispheres removed.
The cerebellum and most optic
of thetract
anterior and posterior medullary vela
lateral geniculate body
are also removed.
medial geniculate body
left optic nerve (cut)
pineal gland
pretectal area
lateral geniculate body
rostral colliculus
medial geniculate
body
caudal colliculus
Internal capsule. The internal capsule is the name given to the large mass
of thalamocortical fibers that pass from the thalamus and metathalamus to
the sensory cortex and descending motor fibers from the cortex and
subcortical nuclei to lower motor centers in the brain and cord. It is usually
accounted as including shorter fibers that pass between the subcortical
nuclei (caudate nucleus, lentiform nucleus) and other fibers that utilize the
capsule pathway to connect with the cortex; but the main constituents are
the two groups of fibers first mentioned. The optic radiation passes from the
lateral geniculate body to the optic cortex as a caudal part of the internal
capsule. It is largely fibers of the internal capsule that are severed in a
section that separates the hemisphere from the diencephalon (Fig. 11).
Thalamocortical fibers pass laterodorsally in the space between the
caudate and lentiform nuclei and, emerging, meet the transverse fibers of
the corpus callosum to form the corona radiata. In this way the two groups
of fibers, and the descending fibers from the cerebral cortex, form a
common pathway that reaches all areas of the hemisphere.
Fig. 13. Canine brain. Hemisphere in lateral view, as if transparent,
showing the subcortical nuclei, amygdala, and hippocampus. The
space traversed by internal capsule fibers is shown in bold outline.
hippocampus
caudate nucleus
Fig. 14. As for Fig. 13, above, with the nuclei colored and the space
lentiform occupied by the internal capsule shown in yellow.
nucleus
amygdala
(amygdaloid nucleus)
Fig. 15. Canine brain, left half with dorsal part of hemisphere removed to
show internal capsule fibers emerging lateral to caudate nucleus. The fibers
are passing between the caudate nucleus and the lentiform njucleus.
caudate nucleus, body
thalamus
internal capsule fibers
lat geniculate body
caudate nucleus, tail
rostral colliculus
The corona radiata:
Fig. 16. Cross-section, myelin-stained sheep’s brain. Callosal
fibers join internal capsule fibers at the corona radiate forming a
common bundle of fibers that pass to and from the cortex.
calllosal fibers
corona radiata
internal capsule fibers
Fig. 17.
Fig. 18. Canine brain. The open arrow shows the path of the fibers of
the optic radiation, which form a caudal part of the internal capsule. As
myelinated axons, the optic radiation fibers pass from the lateral
geniculate nucleus to the optic cortex of the occipital lobe of the brain.
Fig. 19. Optic cortex of the canine brain, lateral view. From Lehrbuch der
Anatomie der Haustiere, Band IV, G. Böhme, Ed., Nickel, Schummer,
Seiferle, 1992: Verlag Paul Parey, Berlin and Hamburg.
Fig. 20. Canine left cerebral hemisphere, medial view, showing the
optic area. From Lehrbuch der Anatomie der Haustiere, Band IV, G.
Böhme, Ed., Nickel, Schummer, Seiferle, 1992: Verlag Paul Parey,
Berlin and Hamburg.
Pretectal nucleus. Optic nerve fibers synapse in the pretectal nucleus.
Axons of the pretectal neurons pass to the parasympathetic nucleus
(accessory optic or Edinger-Westphal nucleus) of the oculomotor nucleus
of the ipsilateral and contralateral side. The fibers that pass to the
contralateral side cross in the posterior commissure, which is in the roof of
the mesencephalic aqueduct caudal to the pineal gland (Fig. 24). Axons of
neurons in the R and L parasympathetic nuclei of the oculomotor nerve
pass in the corresponding oculomotor nerve and synapse in the ciliary
ganglion, a small ganglion, which lies beside the ventral branch of the
oculomotor nerve and is connected to it by a short parasympathetic “root.”
From the ciliary gangion, numerous short ciliary nerves penetrate the sclera
and proceed in the choroid to the ciliary smooth muscle and the smooth
muscle of the iris constrictor muscle. In the animal with binocular vision, all
of these events take place bilaterally; that is, from fibers of one optic nerve
there will be excitation of both the right and the left pretectal nuclei.
The size of the pupil, and its change in size, can be examined in the
conscious and unconscious patient and is useful in every physical
examination in which brain injury is a consideration. Pupillary diameter is a
function of the balance of impulses from the iris dilatator muscle, which is
innervated by sympathetic nerves of the cranial (superior) cervical
ganglion, and the iris constrictor muscle, which is innervated by
parasympathetic nerves from the oculomotor’s parasympathetic nucleus. In
the normal animal, when a bright light is shone into the eye, the pupil of the
eye receiving stimulation constricts, the direct reflex, and also on the
opposite side, the consensual reflex. This is mediated by a path that begins
with stimulation of the receptor cells of the retina and ends in the smooth
muscle of the iris constrictor. Its path follows the optic nerve fibers to the
pretectal nucleus, to the parsympathetic nucleus of the R and L oculomotor
nerves to the ciliary ganglion that is within the orbit beside the ventral
branch of each oculomotor nerve. Short ciliary nerves of the ganglion
innervate the iris constrictor muscle. The reflex takes place irrespective of
cortical stimulation; that is, it takes place in the conscious and unconscious
animal (or human) and the animal with injury to the optic area of the cortex.
An animal that is blind owing to failure to excite the cortical optic area, will
nevertheless exhibit the pupillary reflex if the pathway to the iris constrictor
is intact. In humans, if the person is unconscious and the pupillary reflex is
absent, the probability is that the midbrain is damaged. Under like
circumstances, this conclusion would hold for animals.
In general, if the pupillary reflexes, direct and consensual, are present in
both eyes, it indicates that the optic and oculomotor nerves are intact. If the
direct is present and the consensual is absent, the probability (not certainty;
the examiner evaluates all signs) is that the oculomotor of the opposite side
is interfered with; depending on other signs, this could indicate a midbrain
lesion or a problem in the peripheral course of the oculomotor nerve. If the
direct is absent and the consensual is present, the probability is injury to
the oculomotor parasympathetic nucleus or nerve of the same side. But the
animal may be wholly or partially blind when the pupillary reflex is intact.
Blindness would then probably be due to injury or defect of the visual
cortex.
Fig. 21. Canine brain, dorsal view with much of the hemispheres and all
of the cerebellum removed. The path of left optic tract fibers )or ) to the
lateral geniculare nucleus, pretectal nucleus, and rostral colliculus is
shown in blue. The path from the pretectal nucleus to the R and L
parasympathetic nuclei of thee oculomotor nerves is shown in red. Only
left optic tract fibers are shown,
2. pretectal nucleus
fibers to 1., 2., 3.
fibers from the left
pretectal nucleus to 4.
1. lateral geniculate body
3. rostral
colliculus
4. R and L parasympathetic
nuclei of the oculomotor
nerves
The motor nucleus of the oculomotor nerve is in the midbrain at the level of
the rostral colliculus. Its parasympathetic nucleus rests immediately dorsal.
The myelin stained section below, taken from an internet site
(www.msu.edu/~brains/brains/redkangaroo/sections/redroo_sec1103.jpg),
shows the relation quite well in the red kangaroo. Note that the oculomotor
nuclei are at the level of the red nucleus (nucleus ruber), a midbrain motor
nucleus, and that the oculomotor nerve fibers pass ventrally in relation to
the nucleus ruber. Oculomotor nerve fibers emerge from the midbrain
medial to the descending fibers of the crus cerebri, which are fibers
descending from the motor cortex and subcortical nuclei to lower motor
centers.
Fig. 22. Myelin-stained section at the level of rostral midbrain of the red kangaroo
(from: Brain Biodiversity Bank, Michigan State University:
www.msu.edu/~brains/brains/redkangaroo/sections/redroo_sec1103.jpg).
rostral colliculus
parasympathetic nucleus,
oculomotor nerve
motor nucleus,
oculomotor
nerve
red nucleus
oculomotor nerve
crus cerebri
Fig. 23. Brain, canine, ventral
view.
optic nerves
optic chiasm
optic tract
oculomotor nerve
crus cerebri
Fig. 24. Brain, canine, median section at diencephalic and midbrain levels,
showing the posterior commissure, where pretectal fibers cross to reach the
contralateral parasympathetic nucleus of the oculomotor nerve.
pineal gland
pretectal area
rostral colliculus
mesencephalic aqueduct
posterior commissure
Rostral colliculus. The rostral colliculus is a visual reflex center. It
integrates information from the visual cortex and from the retina (by way of
the optic tract), from auditory stimuli, and from somatosensory receptors
(pain, touch/pressure, temperature, proprioception). From this input, it
directs reflex movements of the eyes, head, and neck toward the visual
stimulus. These motor functions are effected by way of the medial
longitudinal fasciculus to the motor nuclei of the oculomotor, trochlear, and
abducent nerves and by the tectobulbar and tectospinal tracts to the
striated muscles that act to move the head and neck. Tectopontine fibers,
probably as collaterals of the descending tectobulbar and tectospinal fibers,
pass to the cerebellum, which functions in the coordination of these
movements. Rostral and caudal colliculi, designated collectively the
corpora quadrigemina, form the roof, or tectum, of the midbrain, which is
the origin of the tectopontine, tectobulbar, and tectospinal tracts.
Suprachiasmatic nucleus. The suprachiasmatic nucleus is the first
nucleus in a neural pathway that begins with the gangion cell layer of the
retina and ends at the pineal gland (Fig. 25). Retinohypothalamic fibers, a
specific neural pathway
Fig. 25
within the optic nerve,
pass at the chiasm to the
suprachiasmatic nucleus
of the hypothalamus.
From suprachiasmatic
neurons, an efferent
pathway passes caudally
in the wall of the third
ventricle to synapse at the
paraventricular nucleus
(there are rostral and
caudal paraventricular
nuclei in the dog), whose axons then descend to synapse on first neurons
in the intermediolateral gray column of the cranial thoracic spinal cord;
these first neurons are neurons of origin of the two-neuron sympathetic
efferent pathway. Axons of these first neurons pass from the spinal cord in
the ventral roots of the cranial thoracic spinal nerves. They continue
cranially in the cervical sympathetic trunk to the cranial cervical ganglion.
Postganglionic fibers enter the cranial cavity with the internal carotid nerves
and probably (key word) follow a vascular path to reach the pineal gland.
There is general agreement that the sympathetic innervation stimulates the
pinealocytes to secrete melatonin, a hormone whose molecular structure is
derived from tryptophane and is similar to that of the neurotransmitter,
serotonin. Hormone is secreted in response to darkness mediated by
increased sympathetic stimulation. During the day, the suprachiasmatic
nucleus inhibits the paraventricular nucleus, which otherwise maintains
sympathetic stimulation to the pineal. Light thus acts to decrease melatonin
secretion. Melatonin acts to depress gonadal function and to promote
sleep. In animals, this pathway brings about seasonal changes in gonadal
function and hibernation and other circadian rhythms. In humans the
hormone has been shown to effect the daily, circadian, cycle of sleepwakefulness. Inhibition of suprachiasmatic nuclear activity is thought to be
humoral, by melatonin concentration in the blood and cerebrospinal fluid.
The pineal gland also receives parasympathetic fibers from the otic and
pterygopalatine ganglia and sensory fibers from the trigeminal ganglion.
The role of parasympathetic innervation is unclear. See: Hormones, Brain
and Behavior, Vol. 5, Edited by Donald W. Pfaff, Arthur R. Arnold, Anne M.
Entgen, Susan T. Fahrbach, and Robert T. Rubin; 2002: Elsevier Science.
See also: Suprachiasmatic Nucleus, The Mind’s Clock, Edited by Klein, David
C.; Moore, Robert Y.; and Reppert, Steven M., Oxford University Press, 1991.
Figs. 26a and 26b. Median section of the
canine brain at the level of the third ventricle,
showing the position of some of the
hypothalamic nuclei.
26a
preoptic nucleus
suprachiasmatic
nucleus
rostral and caudal
paraventricular nuclei
26b
lateral nucleus
ventromedial nucleus
Fig. 27. Cross-section through the canine brain at the level of the
hypophysis. In the inset, the caudal paraventricular (PVN), lateral (LN),
and ventromedial (VMN) nuclei are identified.
Accessory optic system (AOS). The accessory optic system receives
retinofugal fibers of the optic nerve. It is a system of cerebral nuclei and
their connections, which, receiving input from the retina, function to
stabilize the image on the retina when the global surround is moving and
the head is relatively stationary. The reflex is initiated, for example, in
humans when a person gazes out the window while sitting in a moving car;
or, in animals, when a horse is running. Such reflexes, which constitute a
series of short, rapid to-fro eye movements that serve to maintain an image
on the retina, require integration of the changing visual image with
contraction of ocular muscles and their integration with cerebellar circuitry.
The series of movements are designated optokinetic reflexes (OKR) and
are thought to involve also vestibulo-ocular reflexes (VOR). Although not all
vertebrates have been studied, the AOS appears to be uniformly present in
the subphylum. The system is present in humans.
The accessory optic system is composed of the accessory optic tract and
its dorsal (superior) and ventral (inferior) fasciculi, dorsal, lateral, and
medial terminal nuclei (DTN, LTN, MTN) on which the tract fibers end, and
the efferent and afferent connections of the nuclei, which bring about the
optokinetic reflex. The accessory optic tract is a slender bundle of optic
nerve fibers that lies alongside the caudal margin of the optic tract and
ends by dividing into two branches lateral to the chiasm. Its dorsal
fasciculus consists of fibers that continue with the collicular branch of the
optic tract. Reaching the brachium of the rostral colliculus, the fasciculus
turns ventrally on the superficial aspect of the cerebral peduncle, then
continues medially onto the peduncle’s ventral aspect as the transverse
peduncular tract. The dorsal fasciculus ends at the MTN, which is
superficial on the medioventral aspect of the peduncle. The ventral
fasciculus leaves the accessory optic tract at its division and proceeds
caudally on the medial aspect of the peduncle to end in the MTN.
Both the DTN and the LTN are small nuclei embedded in the fibers of the
dorsal fasciculus. The DTN is located at the bend of the dorsal fasciculus,
where it leaves the collicular branch of the optic tract. Here it is in close
association with the nucleus of the optic tract (NOT). The LTN is lateral on
the peduncle.
The topography of these fasciculi and nuclei, in the rat, are shown below.
Note that, in this species, the dorsal fasciculus gives off slender bundles of
fibers as it ascends with the optic tract to reach the brachium of the rostral
colliculus. These fibers are lacking in the domestic ungulates and
carnivores. However, the transverse peduncular tract is well developed in
these species.
Fig. 28. Rat brain, semidiagrammatic, ventrolateral and ventral views
showing the superior and inferior fasciculi of the accessory optic tract
and associated nuclei. OT, optic tract; AOT-SF, superior (dorsal)
fasciculus of the accessory optic tract, (passes with the optic tract, giving
off small fascicles along the way and ending as the more substantial
transverse peduncular tract); AOT-IF, inferior fasciculus of the accessory
optic tract. Figure is from eScholarship, University of California: The
accessory optic system: basic organization with an update on
connectivity, neurochemistry, and function. RA Gioli, RH Blanks, and F
Lui.
ventrolateral view
tj
ventral view
In the domestic mammals the portion of the system most apparent grossly
is the transverse peduncular tract.
Fig. 29. Equine brain,
ventral view, showing
the transverse
peduncular tract.
Figure is from Sisson.
transverse
peduncular tract
The topography of the main features of the aos in the dog are shown in
semidiagrammatic form below. The medial terminal nucleus rests alongside
and a little dorsal to the medial border of the crus cerebri. I have not tried to
dissect these. The nuclei are only microscopically apparent and I have not
identified a transverse peduncular tract in the dog grossly.
Fig. 30. Canine brain, lateral view with left hemisphere removed. The
approximate topography of the dorsal (superior) ventral (inferior) fasciculi of the
assessor optic tract, the dorsal and lateral terminal nuclei, and the nucleus of
the optic tract are shown. The small fascicles dispatched by the dorsal
fasciculus as it ascends with the optic tract are lacking in the carnivores studied
to date. A transverse peduncular tract is present. The medial terminal nucleus is
present but is not seen in this view. It is where the ventral fasciculus meets the
transverse peduncular tract medial to the crus cerebri.
nucleus of the optic tract
dorsal fasciculus
dorsal terminal
nucleus
accessory optic tract
ventral fasciculus
lateral terminal nucleus
transverse
peduncular tract
The accessory optic tract is composed of crossed fibers only. The nucleus
of the optic tract (NOT) receives contralateral and ipsilateral optic nerve
fibers and is not designated a terminal nucleus. With the terminal nuclei, it
projects to the inferior olive from which olivocerebellar fibers then pass to
the cerebellum. The terminal nuclei and the nucleus of the optic tract and
an associated area dorsal to the MTN designated the ventral tegmental
reflex zone are joined to the contralateral nuclei by fibers that pass in the
posterior commissure. The pathway(s) reaching the ocular motor nuclei III,
IV, and VI, remain(s) uncertain (The Human Central Nervous System,
Nieuwenhuys, R., Voogd, J., and Van Huijzen, C., 3rd Ed.; 2008: Springer
Verlag, Berlin, Heidelberg, New York). Excellent demonstrations of
optokinetic, vestibulo-ocular, and different forms of nystagmus and some
associated reflexes in humans can be seen at:
http://www.youtube.com/watch?v=U3KHgkZHuzc. An excellent early work
on the accessory optic nucleus, the transverse peduncular (posterior
accessory) tract, and the medial terminal nucleus in various mammals,
including the cat (THE CONNECTIONS OF THE BASAL OPTIC ROOT
[POSTERIOR ACCESSORY OPTIC TRACT] AND ITS NUCLEUS IN VARIOUS
MAMMALS, LOIS A.GILLILAN, Department of Anatomy. University of Michigan.
Ann Arbor) is provided at (http://)
deepblue.lib.umich.edu/bitstream/2027.42/49926/1/900740303_ftp.pdf.
Summary: The optic nerve is composed of retinofugal fibers that are
myelinated axons of the ganglion cell layer. These fibers project to the
following:
1. Lateral geniculate body nuclei, whose axons project to the
visual cortex. Conscious perception of the retinal image
takes place in the visual (optic) cortex.
2. Pretectal nucleus, which projects bilaterally to the
parasympathetic nuclei of the oculomotor nerve. This
pathway mediates the pupillary (constrictor smooth muscle)
reflex (and smooth muscle reflexes involving
accommodation).
3. Rostral colliculus, which mediates visual reflexes that are
saccadic movements of the eyeball utilizing striated muscle.
4. Suprachiasmatic nucleus, which is the beginning of the
pathway to the pineal gland and the secretion of melatonin.
Melatonin mediates the effect of light in conditioning
circadian rhythms: sleep and wakefulness, gonadal
development and estrus cycles in animals, hibernation.
5. Terminal nuclei and the nucleus of the optic tract. The
accessory optic tract is composed of optic nerve fibers which
end on the nucleus of the optic tract and the terminal nuclei
of the accessory optic system. The accessory optic system
initiates optokinetic reflexes, which cooperate with vestibulo-
ocular reflexes in stabilizing the retinal image when the
visual surround is moving relative to the head.