OPTO 242 - Ocular Neuroanatomy and Physiology by nuhman10


									OPTO 242                 Neuroanatomy and Physiology

                          COURSE OUTLINE

Part A

1) Gross anatomy of the cerebrum and brain stem.
2) Anatomy of the Visual Pathway:
    i)   Anatomy of the optic nerve.
    ii) Anatomy of the optic chiasm.
    iii) Anatomy of the optic radiations.
    iv) Anatomy of the Visual Cortex

3) Visual information processing (physiology of vision),

Part B

1) Nuclear organization and innervational control of the cranial
   Nerves which subserve the oculovisual system.

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Divisions of the Brain

        There are many terminologies used to describe the different parts of the brain. In
the most widely used terminology in medical circles, the brain is divided into six separate
parts. They are:

1)   The Cerebrum
2)   The Diencephalon
3)   The Mesencephalon (Midbrain)
4)   The Cerebellum
5)   The Pons
6)   The Medulla Oblongata (usually called simply “medulla”).

        Together with the Diencephalon, the Cerebrum forms the Prosencephalon or
forebrain. The forebrain is the large portion of the brain filling the anterior and superior
three-fourths of the cranial cavity.

        The Mesencephalon is a tiny potion of the brain located at the base of the
forebrain. Despite its small size, it is the only connection between the forebrain and the
lower portions of the brain (this includes the Cerebellum, Pons, and Medulla), and the
spinal cord.

       The Cerebellum, Pons, and Medulla form the Rhombencephalon or Hindbrain.


From figure 2, it is clear that the Cerebrum comprises of two halves (Cerebral
hemispheres). These halves are connected (and hence communicate) by many groups of
fibres, two of the most important of which are:

1)     The Corpus Callosum          This is shown in figure 3 and comprises about 20
       million fibres.

2)     The Anterior Commissure This is also shown in figure 3. It is a much smaller
       bundle comprising only I million fibres. It connects mostly the anterior and medial
       portions of the 2 temporal lobes.

Corresponding parts on the two halves of the brain connect with each other across
these two bundles of nerve fibres. Therefore, when the Corpus Callosum and the Anterior
Commissure are destroyed, each hemisphere behaves as a separate brain and body

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functions are uncoordinated, such that the left and right portions of the body behave
completely differently from each other.


Looking at figure 1 again, we can see that the Cerebrum has many folds. These
folds are referred to as Cerebral Convolutions and each single fold is called a gyrus. The
grooves (line depressions) between the gyri are called Fissures or Sulci.

Generally, fissures and sulci separate functional parts of the Cerebrum.


As can be seen in figures I and 2, the Cerebrum is divided into 4 lobes. They are:

1)     Frontal Lobe
2)     Parietal Lobe
3)     Occipital Lobe
4)     Temporal Lobe

There is also a fifth (minor) lobe called the insula.

From figure 1, it is evident that the central sulcus separates the frontal lobe from
the parietal lobe, and the Sylvian (lateral) fissure separates the frontal lobe from the
anterior part of the temporal lobe.


1)     The frontal lobe is concerned mainly with control of muscle movement and also,
       with certain types of thinking processes.

2)     The parietal lobe receives sensory information from all over the body.

3)     The temporal lobe receives auditory information (i.e. hearing information).

4)     The occipital lobe receives visual signals.

Little is known about the insula except that it probably functions as part of the
limbic system, helping to control behaviour.

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Department of Optometry, King Saud University          5
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Looking at figure 4, the Cerebrum is composed of areas that appear grey to the
naked eye - grey matter - and other areas that appear white - white matter.

The grey areas are actually collections of great numbers of neuronal cell bodies.

The white areas are composed of many bundles of nerve fibres leading to or from
the neuronal cells in the grey matter. These areas appear white because of the myelin
sheets covering the nerve fibres.

The grey matter covers the whole surface of the Cerebrum and it numerous folds
(fissures and sulci). The advantage of these folds is that the total area of the brain is 3X
its exposed surface area.

The Cerebral cortex is associated (in connection with other parts of the brain)
with the thinking process. It is 6mm thick in most areas, and it contains some 50 to 80
billion cell bodies.


The functional areas of the Cerebral cortex are shown in figure 5. They are as

A)     The Motor Area                  This area is divided into;

i)     The motor cortex      This controls specific muscles in the body,
       especially those responsible for small movements e.g. thumb motions, lip and mouth
       motions for talking and eating.

ii)    The premotor cortex Once a person has learned certain skilled
       movements (for instance how to play squash or tennis), it is in this area that such
       knowledge is stored for later use.

iii)   Broca's area         Controls the coordinate movements of the larynx
       and mouth to produce words of speech.

B)     Somatosensory Area              This is divided into & primary area and a secondary

       The primary area directly receives sensations of pain, touch etc. from the body.

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Department of Optometry, King Saud University          8
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The secondary area receives signals that have first been processed in other parts
of the brain, or in the primary somatosensory area.

The somatosensory area occupies the entire parietal lobe.


In almost all areas of the cerebrum apart from the cerebral cortex and basal ganglia, we will find
White Matter. This white matter consists almost exclusively of fibres that are organized into
specific bundles called Fibre Tracts.

Three of the principal fibre tracts, each of which contains millions of fibres are as follows:

The Corpus Callosum           Connects corresponding areas in the cerebral cortices of both
                              hemispheres with each other.

The Optic Radiations          By now we should all be familiar with these as the final pathway in
                              the relay of visual signals from the eyes to the visual cortex. They
                              pass backwards from the lateral geniculate nucleus of the
                              thalamus, to the calcarine sulcus area of the occipital lobe.

The Internal Capsule          Found in the areas between the thalamus, the caudate nucleus, and
                              the Putamen. Through this capsule, most signals travel between the
                              cerebral cortex on the one hand, and the lower brain parts and the
                              spinal cord on the other.

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C)    The Visual Area        This area occupies the entire occipital lobe. It is divided
      into the primary visual area (Brodmann‟s area 17), and the secondary visual areas
      (Brodmann‟s areas 18 and 19).

      The primary area detects specific light and dark spots, as well as orientation of
      lines and borders in the visual field.

      The secondary areas (which occupy most of the occipital lobe) function to
      interpret and coordinate visual information. It is in these areas that the meanings of
      written words are interpreted (explain).

D)    The Auditory Area This area is located in the upper anterior two-thirds of the
      temporal lobe. It is divided into primary and secondary auditory areas.

      The primary auditory area detects specific tones, loudness, and other qualities of

      The meanings of spoken words are interpreted in the secondary area.

E)    Wernicke’s Area for sensory integration This area is extremely important,
      because it is here that sensory information from all three sensory lobes (the temporal,
      occipital, and parietal) come together and are interpreted and integrated (i.e. it is here that
      the final meanings of sentences or thoughts are integrated to get "the big picture").

      Wernicke's area is fully developed only in one half of the brain (usually in the left
      cerebral hemisphere). This is so that the brain does not suffer from confusion of thought
      processes between both cerebral hemispheres

F)    Short-term memory Area              The lower half of the temporal lobe is responsible for
      the storage of short-term memories which can last from a few minutes to several weeks.

G)    The Prefrontal Area                    To date, the function of this area has not been well
      defined. It has been removed in many psychotic patients to bring them out of depressive
      states. Although such patients can still function, they lose their ability to concentrate for
      long periods, and to think through problems, among other things.

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The Basal Ganglia
The grey matter is made up of nuclei (a nucleus is a collection of nerve cell bodies
packed into a cohesive area). There are 2 groups of nuclei

1)     Basal ganglia
2)     The Thalamus

The three most important basal ganglia are:

a)     The Caudate nucleus
b)     The Putamen
c)     The Globus Pallidus

The basal ganglia have different functions in different mammals. In humans, they
function to control background movements (as opposed to specific small movements -
like movement of the fingers - which are controlled by the cerebral cortex).

The Thalamus will be discussed later in this lecture.

In addition to these three, the Claustrum. and the Amygdala are considered to be
basal ganglia. The function of the claustrum is unknown, and the amygdala functions as
part of the limbic system.

To achieve the high degree of coordination necessary for body movements, there is a
complex connection of fibres between:

1)     The cerebral cortex and the basal ganglia of the cerebellum.
2)     The thalamus and the subthalamus in the diencephalon.
3)     The red nucleus and the substantia nigra in the mesencephalon
4)     The cerebellum in the hindbrain.

The Diencephalon
This is also called the between brain. It provides a linkage between the cerebrum
and the lower parts of the brain. Anatomically, the diencephalon is so tightly fused to the
basal portion of the cerebrum that it is difficult to demarcate its boundaries with the

The important structures of the diencephalon are:

a)     The Thalamus
b)     The Hypothalamus

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       The Thalamus

       The thalamus rests directly on top of the midbrain. It is effectively a relay station
       for signals between the brain stem & the spinal cord on the one hand, and the cerebral
       cortex on the other. It also has numerous connections to and from the cerebral cortex.
       Finally, the thalamus relays many signals from the brain stem & spinal cord, to the basal
       ganglia. It also provides direct connections from the basal ganglia to the cerebral cortex.

       Some examples of the signals relayed through the thalamus are:

i)     Somatosensory signals - from the body (i.e. touch, pain, temperature etc.) to the
       somatosensory cortex of the parietal lobe.

ii)    Visual signals - To the calcarine sulcus area of the occipital lobe.

iii)   Muscle control signals from the cerebellum, mesencephalon, and other parts of
       the lower brain stem, to the motor cortex and basal ganglia.

Note           The thalamus is a much more primitive portion of the brain than the
               cerebrum. Therefore, pain sensation (one of the most primitive of man's
               sensations) is still retained even when much of the cerebral cortex is destroyed.

It is also important to note that the lateral geniculate body (in addition to the
Pulvinar) forms the posterior portion of the thalamus.

Having discussed the relay function of the thalamus, one might ask what the importance
of the thalamus really is? Now, look at the cerebral cortex to be mainly an external part of
the thalamus, because, without the thalamus, the cerebral cortex is useless. The cerebral
cortex has a large capacity for storing information, but, access to that information, and
the usage of the information to control body functions is subject to the control of centres
in the thalamus. Therefore, it is the thalamus that drives cerebral activity.

The Hypothalamus

This is a very small structure at the middle of the base of the brain. Although
small, it is a very important part of the brain for it is the major centre in the brain for
controlling internal body functions. The position of the hypothalamus in the brain can be
seen in figures 2 and 3. Figure 7 shows an enlarged internal view of the hypothalamus.

Some functions of nuclei found in the hypothalamus are listed below:

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The Preoptic nucleus          This is primarily concerned with body temperature

The Supraoptic nucleus        This controls the secretion of anti-diuretic hormone
                              which helps to control the concentration of electrolytes (define) in
                              body fluids.

The Medial nuclei             When stimulated, they give an individual the sense
                              that he is satisfied (especially satisfied for food). On the other hand,
                              stimulation of the most lateral part of the hypothalamus causes a
                              person to be very hungry, while anterior stimulation of the
                              hypothalamus causes a person to be very thirsty.

Also, stimulation of different areas of the hypothalamus causes its neurons to secrete
many hormones (define) called releasing hormones which are then carried in the blood
directly to the anterior pituitary gland, and here, they cause the secretion of anterior
pituitary hormones. Anterior pituitary hormone controls various activities such as
metabolism of proteins and fats, as well as functions of the sex glands.

The hypothalamus has many other functions and thus, despite its small size, it is clearly a
vital portion of the brain.

The Limbic System

The word limbic means border. The limbic system (illustrated in figure 8)
comprises the border structures of the cerebrum and the diencephalon that mainly
surround the hypothalamus. The limbic system functions mainly to control our emotional
and behavioral activities.

Some important parts of the limbic system are:

a)     The Amygdala                   It is considered to be part of the basal ganglia, although, it
       functions closely with the hypothalamus to help control the appropriate behavior of a
       person in each type of situation.

b)     The Hippocampus                 There is one hippocampus in each half of the brain. It is a
       primitive area of the cerebral cortex that interprets the importance of a person's sensory
       experiences for the brain. If the hippocampus thinks a particular event is important, this
       event is stored as memory in the cerebral cortex. If however, the hippocampus is
       destroyed, a person's ability to store memories is severely impaired.

c)     The Mammillary Bodies         They function closely with the thalamus,
       hypothalamus and the brain stem to control many behavioral functions such as how
       awake a person is, and also, how well, ill, or content a person feels.

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d)     The Septum Pellucidum Stimulation of different parts of this septum can
       cause many behavioral effects such as anger (rage).

Signals from the limbic system to the hypothalamus can modify any one or all of the
many internal functions controlled by the hypothalamus. The same is true for signals
from the limbic system leading into the midbrain, which can control such behavior as
sleep, excitement, or rage. Yet, the precise manner in which the limbic system functions
together to control all these emotional and behavioral functions of the body is still only
slightly understood.

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The brain stem as has been mentioned connects the forebrain to the spinal cord. It's major
divisions are:

1)     The Mesencephalon or midbrain.
2)     The Pons.
3)     The Medulla

Some anatomists also consider the diencephalon part of the brain stem because this too
connects the forebrain with the spinal cord.

Many of the fibres in the brain stem carry sensory signals from the spinal cord mainly to
the thalamus, and, motor signals from the cerebral cortex to the spinal cord.

The brain stem also contains many important centres, which control physiological
variables like respiration and arterial pressure. More importantly, it contains centres that
determine the level of cerebral activity and cause the wake-sleep cycle of the nervous

Finally, the brain stem connects the cerebellum and the cerebrum superiorly, and the
cerebellum and the spinal cord inferiorly.


The surface anatomy of the midbrain is shown in figure 9. It consists of 2 major sections:

1)     The Cerebral Peduncles which occupy most of the front of the midbrain.

2)     The Tectum which consists of structures near the posterior (back) surface of the

Cerebral Peduncles

Each cerebral peduncle is divided into 3 separate areas:

a)     A surface layer of corticospinal and corticopontine fibres that conduct motor
       signals from the cortex to the spinal cord and pons.

b)     A deeper layer of darkly pigmented cell bodies - Substantia Nigra - which
       function as part of the basal ganglial system to control subconscious muscle
       activities of the body. Destruction of these neurons causes Parkinson's disease in

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which there is uncontrolled shaking of muscles in part or all of the body. This
shaking is so bad sometimes that muscle functions become useless.

c)     The Tegmentum which lies behind and medially to the substantia nigra and
       contains many important fibre tracts (explain) and nuclei that provide specific
       functions as follows:

i)     The Medial Lemniscus           This sends sensory signals from the body to the

ii)    The Medial Longitudinal Fasciculus This connects many of the nuclei of
       the brain stem with each other and also with the diencephalon (the obvious advantage of
       this is that each nucleus, or group of nuclei, can modify the signals of other nuclei -
       much like the horizontal cells of the retina which interconnect rods and cones with each
       other so that their signals can be modified before being sent along to the brain via the
       ganglion cells).

iii)   The Red Nucleus     Functions with the basal ganglia and cerebellum to coordinate muscle
       movements of the body.

iv)    Nuclei of the Oculomotor and Trochlear Nerves Small collections of nerve
       cells on each side of the midbrain that control most of the muscles for eye movement.

v)     Periaqueductal Gray Seems to play a major role in the analysis of the reaction to

vi)    The Reticular Formation These nuclei extend from the top of the spinal cord
       all the way to the diencephalon passing through; the medulla, pons, and midbrain, and
       even extending to the middle of the thalamus. The reticular formation contains many
       important centres in the brain. Its most important function however, is as a major centre
       for controlling the brain's level of activity. It is therefore one of the most important
       structures of the brain.

All these fibre tracts and nuclei (as well as the tectum and cerebral peduncles) are shown
in figure 10.

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The Tectum

This is the posterior one-fifth (demonstrate) of the mesencephalon. It consists principally of
two superior colliculi and two inferior colliculi.

a)     Superior Colliculi       In lower animals (especially fish) they act more like the
                                visual cortex. However, in humans, they have lost their visual
                                functions but cause eye and body movements to sudden visual signals
                                in a person's visual field.

b)     Inferior Colliculi       They serve as relay stations for auditory signals from the
                                ears to the cerebrum. They also cause a person to turn his head in
                                response to sounds coming from different directions.


This is divided into 2 parts:

i)     The ventral part
ii)    The dorsal part (which is also called the tegmentum of the pons).

i)     Ventral part of the Pons      This is the large part of the pons (to the left of the
       diagram) in figure 9. The ventral part of the pons also contains the same corticospinal and
       corticopontine fibres found in the cerebral peduncles of the mesencephalon. The
       corticospinal fibres continue to the spinal cord, but the corticopontine fibres terminate

ii)    Tegmentum of the Pons        It contains the following 3 structures that are
       continuous with those of the midbrain:

a)     Medial Lemniscus
b)     Medial Longitudinal Fasciculus
c)     The Reticular Formation

In addition, the tegmentum of the pons contains the nuclei of several cranial nerves:

1)     The Abducens Nerve                      This helps to control eye movements

2)     The Facial Nerve                        This control the muscles of facial expression (i.e.
                                               muscles for smiling, frowning etc.).

3)     Trigeminal Nerve                        This controls the muscles of mastication and transmits
                                               sensory signals from the face to the brain.

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4)     Vestibulocochlear Nerve                Transmits sensory signals from the structures of the


The distinguishing structures of the medulla are:

1)     The Pyramids            These contain the same corticospinal fibres that have
                               already been mentioned.

2)     Two Olives              These protrude, one from each anterior lateral surface, from the

The medulla generally contains many of the same fibres and nuclei as the midbrain and

It also contains the nuclei for cranial nerves 9,10,11, and 12.

The reticular formation in the medulla and pons control 2 very important control centres:

a)   The Vasomotor Centre              This controls heart pumping activity and blood
                                       vessel size and therefore can greatly alter blood pressure.

b)   Respiratory Centre                This controls the muscles for breathing.

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When light falls on the retina, the photoreceptor cells are excited and transmit these light
impulses to the brain via the following pathway (the general pathway of transmission to
the brain is shown in figure 11):

1)     After converting the light energy (from an object in the visual field) to electrical
       energy, the rods and cones transmit these signals to the bipolar, amacrine,
       horizontal, and interplexiform cells.

       a)      The bipolar cells transmit rod and cone impulses directly to the ganglion cell (see
               figure 12 for a diagram of the retinal layers).

       b)      Horizontal cells form synapses with rods and cones, and bipolar cells. Amacrine
               cells form synapses with bipolar and ganglion cells. Both these cells seem to
               control transmission of the impulses from rods and cones to the ganglion cells.

       c)      Interplexiform cells have synapses with rods, cones, and bipolar cells on the one
               hand, and with bipolar and ganglion cells on the other. Some of these cells operate
               a feedback mechanism (i.e. back from the ganglion cells to the rods and cones) to
               control or regulate the signals coming from the rods and cones.

2)     After the light signals have passed through these 4 types of cells, they reach the
       ganglion cells. These cells occupy most of the optic nerve and carry the light
       signals (now electric signals) towards the brain.

       Ganglion cells are made up of 2 types:

       i)     A-Cells (Magno cells)           These are large cells and are sensitive to
                                              contrast, and they also detect movement of objects
                                              within the visual field (i.e. they transmit rod signals).
                                              The information from these cells are conducted very
                                              rapidly to the visual cortex.

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        ii)    B-Cells (Parvo cells)            These are smaller cells and they have less
                                                dendrites and axons. They are concerned with sharp
                                                vision as well as colour vision (i.e. they transmit cone
                                                signals). The information from these cells are
                                                conducted very slowly to the visual cortes.

Optic nerve anatomy

The optic nerve (as has been mentioned) is comprised of axons of the retinal ganglion cells.

These nerves leave the retina at the optic disc and commence their odyssey to the brain. Within the
retina, the nerves are not covered by myelin sheaths, but after passing through the sclera, they take
up these sheaths which provide physical support and nutrition for the nerves, as well as insulate the
nerves to prevent a discharge of the electrical signals which they carry to the visual cortex (and to
ensure a faster conduction of these signals). The added myelin sheaths increase the individual nerve
thickness from about 1.5 mm to 3 mm.

There are about one million nerves fibres within the optic nerve which extends from 40 to 55mm
between the optic disc (papilla) and the optic chiasma.
It is important to note that though most of the fibres in the optic nerve are afferent visual fibres,
there are also afferent papillary fibres, and efferent visual fibres within the nerve.

The optic disc on the retina is where the optic nerve meets the retina. It is also referred to as the
optic nerve head or papilla. It is round or vertically oval (with a 1.5 mm diameter), lies about 3mm
nasally from the fovea, and it is lighter than the rest of the fundus because it lacks photoreceptors.
As a result, it is a „blind‟ area of the retina, and its projection into the visual field is referred to as
the Blind Spot.

Normally the optic disc if filled with ganglion cells‟ axons.

Nerve fibres from the macula enter the disc temporally but other fibres from the temporal retina
arrive at the optic disc either by arching above or below the papillomacular fibre bundle (as shown
in figure 13).

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      Figure 13             Schematic illustrating temporal nerve fibres’
                            Paths to the Optic Disc

      Macular fibres occupy the central positions within the optic nerve while peripheral fibres
      take up outer positions within the optic nerve.

      The part of the optic nerve within the orbit (intraorbital part) is 25 to 30 mm long and is
      curved in an s-shape. This makes the optic nerve considerably longer than the distance from
      the back of the eye to the optic canal, and permits the rotation of the eye.

      The CRA enters the optic nerve about 15 mm behind the globe and the CRV leaves the
      optic nerve a bit further back.

      The intracranial part of the nerve is 10 – 12 mm long and here the nerve loses its outer
      meninges, covered only by pia mater and it is in company with many large blood vessels –
      the anterior cerebral artery superiorly, the internal carotid artery laterally, and the
      ophthalmic artery inferiorly.

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      This is where the nasal ganglion cell axons of each eye crosses over to travel with the
      temporal fibres of the opposing eye toward the visual cortex.

      The chiasma has transverse diameter (maximum width if you like) of about 12 mm, and it is
      8 mm in length.

      If we assume that a vertical line through the fovea separates the retina into nasal and
      temporal halves, then, the nasal retinal fibres cross of the chiasma while the temporal fibres
      do not cross.

      Fibres which cross are more numerous than those which do not cross simply because, from
      our assumption above, the nasal retinal fibres are much more numerous than the temporal
      retinal fibres.

      Partial crossing of the visual fibres enables stimulation of corresponding retinal points in
      both retinas which cause simultaneous messages to be sent to the visual centres of one side
      of the brain. This makes the fusion of the images from both eyes possible.

      As we see from figure 14a, nasal retinal fibres do not cross in a straight-forward manner.
      Inferior medial fibres travel in the anterior part of the chiasma as if attempting to enter the
      opposite optic nerve, and then loop downward into the opposite optic tract.

      Superior medial fibres travel downward at first as if they intend to continue traveling with
      the ipsilateral temporal fibres and then turn sharply to travel in the posterior part of the
      chiasma and eventually join the opposite optic tract.


      This path carries nerve impulses to the dorsal lateral geniculate nucleus (LGN), which is
      located at the dorsal end of, and is partly associated with, the thalamus

      In its journey, each optic tract bends around the brain stem.

      Before the tract reaches the LGN, about 10% of the fibres leave the tract (these are mostly
      pupillomotor fibres) to end in the pretectal nucleus, which is situated inferiorly to the
      superior colliculus in the mid-brain.

      We can divide the visual pathways into an old system (which travels to the mid-brain and
      the base of the forebrain) and a new system (which travels straight to the visual cortex).

      The old system sends fibres to the following pre-cortical brain areas:

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      The Suprachiasmatic nucleus of the hypothalamus:                Presumably for controlling
      circadian rhythms.

      The Pretectal nuclei:           For eliciting some reflex movements of the eyes, and for
      activating the papillary light reflex.

      The Superior Colliculus:        For control of rapid directional movements of the two eyes.

      The ventral lateral geniculate nucleus of the thalamus: From here the visual fibres also
      travel to the surrounding basal regions of the brain , presumably to help control some of the
      body‟s behavioral functions.

      The new system is responsible in man for the perception of form, color and other conscious
      vision. On the other hand, in many lower animals, even form vision is detected by the older
      system, where the superior colliculus is used in the same manner as the visual cortex is used
      in mammals.


      This serves two principal functions:

      First, it serves to relay visual information from the optic tract to the visual cortex.

      This relay function is so accurate that we have what is called a retinotopical arrangement of
      LGN cells, so that the fibres coming from the retina connect in specific regions of the LGN
      in accordance to the region of the retina from which they originate.

      Like the rest of the cortex, the dorsal lateral geniculate nucleus is divided into 6 layers. The
      crossed nasal fibres go to layers (from ventral to dorsal) 1,4, and 6 while the uncrossed
      temporal fibres go to layers 2,3, and 5.

      Fibres from the macula are so distributed that they terminate either in the middle or in the
      dorsal portions of the LGN.

      Peripheral retinal fibres end in the anterior part of the LGN, with the „upper quadrants‟
      represented medially, and the lower ones represented laterally.

      There are two main cell types in the dorsal LGN. In the most ventral layers (1 and 2), called
      the magnocellular layers, signals from the A-type ganglion cells are received. In layers 3-6,
      called the parvocellular layers, signals from the B- type ganglion cells are received.
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      The second important function of the dorsal LGN is to “gate” the transmission of signals to
      the visual cortex. It accomplishes this task by receiving gating control signals from two

             i)      Corticofugal Fibres running in a backward direction from the visual cortex
                     to the LGN.

             ii)     The Reticular Formation of the midbrain

      Both these signals are inhibitory and when turned on, can completely cut off visual signals
      through selected portions of the dorsal LGN.

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      LECTURE 5


      These are the geniculocalcarine paths carrying visual impulses to the higher cortical centers.

      Fibres serving the superior aspect of the retina travel steadily backward, pass outside the
      lateral ventricle to terminate at the top of the calcarine sulcus. Those fibres serving the
      inferior aspect of the retina pass underneath the posterior horn of the lateral ventricle, and
      then bend backward to terminate on the lower part of the calcarine sulcus.

      Macular fibres travel between these two sets of fibres.

      We can see therefore, that the retinotopic fibre arrangement is maintained all the way to the
      visual cortex.


      As we can see from figures 15 and 16, the visual cortices are located primarily in the
      occipital lobes.

      The visual cortex is divided into the primary visual cortex and the secondary visual areas.

      The Primary Visual Cortex

      This area (also called area 17, visual area I – V1, or the striate cortex) lies in the calcarine
      fissure area, and it extends to the occipital pole along the medial aspect of each occipital
      cortex. This area of the visual cortex receives the most direct visual signal input from the

      Signals from the macula terminate near the occipital pole while signals from the peripheral
      retinal areas terminate in concentric circles, anterior to the pole and along the calcarine

      The upper part of the retina is represented superiorly, and the lower part, inferiorly.

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      A particularly large area at the occipital pole is reserved for fibres coming from the
      macular. Considering the actual size of the macula area, it is more than 100 times better
      represented in the primary visual cortex than more peripheral visual areas.

      The Secondary Visual Areas

      The secondary visual areas, also called visual association areas, lie superiorly, inferiorly
      and anteriorly to the primary visual cortex. For example, area 18 completely surrounds area
      17 and is referred to as V2 because virtually all the visual signals from area 17 pass next
      into area 18. More distally located visual areas are designated V3, V4 and so on.

      Areas 18 and 19 also receive signals from other visual areas (4, 6, and
      8), from the thalamus, and from the superior colliculus. The visual signals from area 17 are
      combined with the impulses from these other regions of the brain, and then analyzed and
      interpreted to form perceptual images and conscious impressions (elaborate).

      In addition, area 18 has interconnecting fibres with the corpus callosum to communicate
      with visual centres in the other half of the brain. This enables coordination of binocular
      information from the 2 eyes to take place.

      The Layered Structure of the Primary Visual Cortex

      Like other parts of the cerebral cortex, the visual cortex is divided into six layers (layers
      I, II, III, IV, V, VI). Layer IV is further divide into a, b, cα and C.

      Signals from A-ganglion cells (also called Y-ganglion cells) terminate in layer IVc and
      from here, these signals are distributed to other parts of the cerebral cortex. Visual signals
      from B-ganglion cells (sometimes called X-ganglion cells) are relayed to layers IVa and

      Remember that the B-ganglion cells carry cone signals and are responsible for sharp
      clear vision, as well as colour vision.

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Vertical Neuronal Columns of the Visual Cortex

The cells of the primary visual cortex are organized into millions of vertical
columns. Each column is 30 – 50 m wide and contains about 1,000 neurons. These
columns analyze different parts of the retinal image.

After the visual signals terminate in layer IV of the visual cortex, they are then conducted both
outward and inward along the vertical column unit for further processing. Those signals that are
transmitted outward to layers I, II, and III eventually transmit higher orders of for short distances
laterally in the cortex.

The signals that move deeper to layers V and VI are re-transmitted to much further parts of the

‘Color Blobs’ in the visual cortex

These are special column-like areas among the primary visual columns that respond specifically to
color signals.

They are also found in certain secondary areas.

Bilateral visual signal interaction

We will recall that the visual signals from the two eyes are separated in the lateral geniculate
nucleus. This separation is retained up to the visual cortex.

There are also horizontal neuronal columns (sort of like zebra-crossing lines) about 0.5mm wide.
The signals from one eye enters the horizontal column of every other (zebra) line, in effect
alternating with visual signals from the opposite eye.

As the signals are sent for further processing to the outer, and deeper layers of the primary vertical
visual columns, this compartmentalization of bilateral visual signals is gradually lost.

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The fast ‘Position’ and ‘Motion’ Pathway

Looking back at figure 15 and following the black arrows, we trace the path by which visual
signals from the fast - rod-information dominated (Y –ganglion cell) – pathway are processed.
After leaving area 17, they synapse in area 18, and then from there move into the parieto-occipital
cortex area to be analyzed and integrated.

These signals tell the position of every object in the visual field and whether and how fast it is

The signals from this pathway are transmitted very rapidly, but only in black and white. The reason
for this should be quite clear, information from objects moving in the visual field require to be
interpreted quickly by the brain incase, for instance, evasive action has to be taken by the

The Slow ‘visual detail’ and ‘color’ pathway

The path of analysis of the signals from this pathway is traced by the red arrows in figure 15.

These signals pass from area 17 into area 18 and then from there to the inferior ventral and medial
portions of the occipito-temporal cortex.

This pathway is concerned with the recognition of letters, reading, determining the texture of
surfaces, determining the detailed colors of objects, and then, deciding from all this information,
what the „meaning‟ of the object is.

It should then become rather obvious why this is the „slow‟ pathway. Decisions to be made by the
brain as far as this pathway is concerned are not time-critical but detail critical. Infact, a fast
processing of the signals from this pathway, will result in loss of information or incomplete and/or
incorrect analysis of the information.

This pathway carries information from the cones and the X-ganglion cells which subserve them.
Most of this information must thus come from the macula area.

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Let us assume, a person looks at a blank white wall. What happens is that very few retinal ganglion
cells are stimulated.

According to the theory of „Ganglion cell receptive fields‟, equally stimulated retinal areas
mutually inhibit each other. Significant retinal stimulation occurs when there are light and dark
areas within the object of regard. The higher the contrast, the higher the retinal stimulation.

It therefore becomes clear that in the initial processing of the details of a centrally-regarded object
in the visual field, contrast is a crucial factor.

                 Retinal Image                                    Cortical Stimulation

       Figure 17        Cortical excitation in response to retinal cross image

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Taking figure 17 for example, the cross object in the visual field results in a similar cross image on
the retina (all be it inverted and laterally reversed).

However, the first response of the cortex is to detect the edges of the cross where the contrast is
maximal. It is only later that the details of the surface of the cross are filled-in.

Detection of Lines and Borders

Not only does the visual cortex detect the presence of lines and borders in the retinal image, it also
detects the orientations of these lines, detects the line orientation when it is laterally or vertically
displaced in the visual field, and it detects lines of specific lengths angles or other shapes.

1)     Detection and Orientation of lines and borders - This function is carried out
       by neurons called simple cells.

2)     Detection of line orientation when the line is displaced laterally or vertically in
       the visual field - This function is carried out by complex cells.

3)     Detection of lines of specific lengths, angles, or other shapes (like circles or
       triangles) - This function is carried out by hyper complex cells.

Detection of Color

This is done in very much the same way as detection of lines and borders. In color, detection is by
means of color contrast.

The color contrasts may be between different cones that lie adjacent to each other or cones that lie
far apart.

For example a red area is frequently contrasted against a green area, and a blue area, against a red.
All colors however are contrasted against white.

The mechanism of color contrasting depends on the fact that contrasting colors (called opponent
colors) mutually excite certain of the neuronal cells.

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Serial versus Parallel Visual Image Analysis

It should have become clear by now that there are two ways in which the visual image is analyzed.

The serial (sequential) pathway analyzes the images in sequential steps (i.e. step 1 to step 2 to step
3) and is represented by the simple, complex and hypercomplex cells.

The parallel (simultaneous) pathway analyzes several parts of the visual image at the same time.
This pathway is represented by the „fast‟ and „slow‟ processed pathways.

It is the combination of both types of analyses that gives one the full picture of what is actually
going on in the visual scene.

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The brain and the spinal cord make of the central nervous system (CNS), which communicates with
the body through the peripheral nervous system (PNS).

The peripheral nervous system consists of: 12 pairs of cranial nerves; a system of other nerves
which branch out throughout the body and originate from the spinal cord (Spinal Nerves); and the
autonomic nervous system (which regulates vital body functions that are not under conscious
control, such as, the activities of the heart and smooth muscles, and of the glands.

The 12 pairs of cranial nerves are numbered in their order of origin from the basal surface of the

In rank order, the 12 pairs of cranial nerves are as follows:

   1.      Olfactory nerves
   2.      Optic nerves
   3.      Oculomotor nerves
   4.      Trochlear nerves
   5.      Trigeminal nerves
   6.      Abducens nerves
   7.      Facial nerves
   8.      Vestibulocochlear (Auditory) nerves
   9.      Glossopharyngeal nerves
   10.     Vagus nerves
   11.     Accessory nerves
   12.     Hypoglossal nerves

The olfactory nerves arise from the cerebrum, and the optic nerves arise from the diencephalon. All
the all 10 pairs of cranial nerves arise from the brain stem. The exceptions are the accessory nerves
which simultaneously arise from the brain stem and the spinal cord.

Some of the cranial nerves are entirely sensory, others are entirely motor, and some have both
sensory and motor components.

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The Motor Nuclei of the Brain Stem

Figures 7.1 and 7.2 depict the cranial nerves‟ origins in the brain stem, and the posterior view of
the motor and sensory cranial nerve nuclei in the brain stem.

The important motor nuclei for the cranial nerves are as follows:

a)     The Oculomotor, Trochlear and Abducens nuclei (nerves III, IV, and VI)

       These send nerve fibres to the different muscles of the orbit to cause movement of the eye.

       The upper portion of the Oculomotor nucleus is called the Edinger-Westphal nucleus,
       and it controls the muscles that regulate the size of the pupil.

b)     The Trigeminal motor nucleus (nerve V)

       This controls many of the muscles of mastication (muscles for chewing).

c)     The Facial nucleus (nerve VII)

       This controls many of the muscles for facial expression.

d)     The Dorsal Vagal nucleus (nerve X)

       This is the most important nucleus in the parasympathetic system. It controls muscle
       activities in many of the viscera (notably the heart, where it causes a slowing of the heart
       rate), and the upper digestive tract (where it results in increased peristalsis of the stomach
       and small intestine, and increased secretion).

e)     The nucleus Ambigus

       This sends signals through three different nerves: The Glossopharyngeal (IX); The
       Vagus (X); and The Accessory (XI) nerves.

       This nucleus controls such muscles as those for swallowing and speech.

       The lower end of the nucleus ambigus is continuous with the anterior horn of the spinal
       cord, from where signals are transmitted through the spinal roots of the Accessory nerve,
       to control portions of the Trapezius and Sternocleidomastoid muscles.

Assignment: Briefly read up the positions and functions of the Trapezius and
            Sternocleidomastoid muscles.

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f)     The Hypoglossal nucleus (nerve XII)

       This controls the primary movements of the tongue.

The Sensory Nuclei of the Brain Stem

To the right of figure 7.2 are the sensory nuclei of the brain stem. From top to bottom, they are:

a)     The Trigeminal nuclei (nerve V)

       These extend all the way from the mesencephalon, downward into the upper part of the
       spinal cord. The three major divisions of these nuclei are:

       i)      Main Sensory nucleus:          Located in the pons, and subserves mainly the
                                              touch sensations of the face, mouth and scalp.

       ii)     Mesencephalic nucleus:         This receives signals mainly from the muscles and
                                              other deep structures in the head.

       iii)    Spinal nucleus:                This is the principal nucleus for the receipt of pain
                                              signals from the face, mouth, and scalp.

b)     The Cochlear nucleus (part of nerve VIII)

       This is the receptor area for sound signals from the ear.

c)     The Vestibular nucleus (other part of nerve VIII)

       This receives signals from the vestibular apparatus, which is the sensory organ for

d)     The nucleus of the Tractus Solitarius

       This is the principal nucleus for the receipt of visceral sensory signals from such organs
       as the heart, stomach, special blood pressure receptors (baroreceptors), and taste buds of
       the mouth.
       The signals that this nucleus receive come through the Facial, Glossopharyngeal, and
       Vagus nerves (nerves VII, IX, and X).

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The External Distributions of the Cranial Nerves

The Olfactory tract and Olfactory nerves (nerve I)

The olfactory tracts and nerves are the sensory pathway for smell.

Figure 7.3 depicts the neural connections of the olfactory system.

There are 20 olfactory nerves, each 1 – 2 cm in length. They arise from the olfactory epithelium,
which is located in the upper part of the nasal cavity. These nerves pass through small holes in the
cribriform plate of the ethmoid bone (this plate forms the superior boundary separating the nasal
cavity below, from the cranial cavity above), to enter the olfactory bulb in the cranial cavity.

The olfactory lobe is located just above the cribriform plate and below the inferior surface of the
frontal lobe of the cerebrum.

Leading backward from the olfactory bulb is the olfactory tract, which terminates in the olfactory
areas of the cerebrum. These areas are located in and between the anterior medial portions of the
two temporal lobes.


Brief Overview of the Olfactory System

The olfactory structures in the brain are among its oldest structures, and much of the rest of the
brain developed around these ancient beginnings.

The part of the brain that subserved olfaction (in the early stages of evolution) eventually
evolved into what we now know as the Limbic System.

The olfactory bulb is actually an anterior outgrowth of cranial tissue, which originates at the
base of the brain. It lies just over the cribriform plate (the connective tissue that separates the
cranial cavity from the upper reaches of the nasal cavity).

The olfactory tract enters the brain at the junction between the mesencephalon and the
cerebrum. The tract then divides into two pathways: one goes to the medial olfactory area; the
other goes to the lateral olfactory area.

The medial olfactory area represents the Very Old Olfactory System, which is responsible for
primitive responses to olfaction such as salivation or primitive emotional drives associated with

The lateral olfactory area is composed mainly of the prepyriform and pyriform cortex, plus the
cortical portion of the amygdaloid nuclei.

Department of Optometry, King Saud University                                                   47
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From these areas signals pass into almost all portions of the limbic system (especially the
hippocampus), and therefore control changes in human behavior in response to smell.

The system just described above is the Old Olfactory System.

The New Olfactory Pathway has been found to pass through the thalamus and eventually ends
up in the orbitofrontal cortex. It appears that this new system helps especially in the conscious
analysis of odor.

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The Optic Nerves (II)

Figure 7.1 shows the entire extent of the optic nerve. After leaving the eye, this nerve passes
through the optic foramen of the sphenoid bone, and finally reaches the basal brain surface at the
posteromedial limit of the frontal lobes.

This is the point (optic chiasma) at which the nasal fibres in each nerve decussate into the opposite
optic tract, there to travel to the LGN (LGB) on the opposite side of the brain, passing posteriorly,
along the lateral surface of the hypothalamus.

The LGN is located in the posterior thalamus.

The Oculomotor Nerves (III), Trochlear Nerves (IV), and Abducens Nerve (VI)

These nerves will be considered together because they are the cranial nerves which control eye

The Oculomotor nerve leaves the brain stem near the mid-line of the anterior surface of the

The Trochlear nerve arises from the lower posterolateral surface of the mesencephalon, and then
wraps around its side onto its anterior aspect.

The Abducens nerve arises from the pons, near its junction with the medulla.

All of these nerves then pass through the superior orbital fissure to innervate the extraocular and
intraocular muscles which eye and iris movement respectively.

The paths of these nerves inside the orbit are depicted in figure 8.1 below.

The Trigeminal Nerves (V)

Depicted in figure 7.1, each of these nerves arises from the anterolateral surface of the mid-pons. It
immediately enlarges for a distance of 1 cm, to a diameter about twice its original diameter. This
enlarged area is the Trigeminal Ganglion (also known as the Gasserian Ganglion or Semilunar

From the trigeminal ganglion, the three major branches of the trigeminal nerve arise. They are: The
Ophthalmic nerve; The Maxillary nerve; The Mandibular nerve.

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The Ophthalmic nerve

As shown in figure 8.2 above, it sends out three main branches: The uppermost of which
courses along the roof of the orbit and goes on to supply the face, scalp, and upper part of the
nasal cavity; The lower two branches innervate the eye, the lower part of the nasal cavity, and
the air sinuses.

The Maxillary nerve

This leaves the cranial cavity through the Foramen Rotundum, passes into the inferior orbit,
and eventually passes through a bony canal under the eye to supply the anterior and lateral sides
of the face. It also supplies sensation to the upper teeth, upper portions of the oral mucosa, the
mucosa of the nasal cavity, and the nasopharynx.

The Pterygopalatine (Sphenopalatine) Ganglion connects with the maxillary nerve.
Vasodilatory and secretory fibres for the lacrimal gland from the facial nerve, synapse in this

The Mandibular nerve

This nerve passes through the Foramen Ovale into the space anterior and inferior to the
temporal bone. It is the only branch of the trigeminal nerve that does not course through the

The sensory division of the Mandibular nerve supplies the most lateral portions of the face, outer
surfaces of the lower jaw and chin, the lower teeth, and the lower portions of the oral mucosa
(including the anterior two-thirds of the tongue).

The motor division innervates the muscles for chewing. These are: The Masseter; and the
Medial and Lateral Pterygoid muscles.

The Facial nerves (VII)

Each one arises from the brain stem at the posterolateral junction of the pons and medulla (see
figure 7.1). It then passes through the internal auditory meatus and enters the facial canal in the
temporal bone (figure 8.2). From here, it enters the posterior facial region anterior and inferior to
the ear.

From here, it spreads through the superficial layers of the entire lateral and anterior facial regions to
innervate all the muscles of facial expression and the Buccinator muscles of the cheek (figure 8.3).

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Early in its course, when it is still just anterior to the ear, the facial nerve passes through (or
adjacent) to the parotid gland (one of the glands for the secretion of saliva). The parotid gland
occasionally becomes cancerous and when this happens, the facial nerve is destroyed by the cancer,
or by the surgery required to remove the cancerous tissue. Therefore, such a patient losses almost
all (or all) capability of emotional expression on that side of the face, his cheeks bulge outward
with food while eating, and he cannot close his eyes or lips completely on that side of the face.

Worthy of note is that a branch of the facial nerve (Chorda Tympani) combines with the lingual
nerve (a sensory branch of the Mandibular nerve). The chorda tympani fibres eventually terminate:

a)     In the submandibular ganglion    From here nerves are sent out to innervate the
       submandibular and sublingual glands, to control salivary secretion.

b)     In the anterior two-thirds of the tongue to provide the taste sensation.

The sensory ganglion of the facial nerve is located in the facial canal, and it is called the geniculate

Vestibulocochlear nerve (VIII)

This arises from the pons-medullary junction, just lateral to the facial nerve (figure 7.1). It is a short
nerve that immediately enters the internal auditory meatus to innervate both the vestibular
apparatus (the organ of equilibrium), and the cochlear (the organ of hearing).

Glossopharyngeal nerve (IX)

This arises from the upper lateral border of the medulla. It passes from the cranial vault via the
jugular foramen, into the posterior pharyngeal region (figure 8.4).

It provides sensory innervation to the mucous membrane of the pharynx, and the posterior third of
the tongue, including sensory and taste sensations from this area.

A motor branch of this nerve innervates the superior pharyngeal muscles that are important for

Vagus nerve (X)

This arises from the lateral border of the medulla, close to the Glossopharyngeal nerve. It enters the
cervical (neck) region - via the jugular foramen – with the Glossopharyngeal and accessory nerves.

It passes inferiorly to the thorax region traveling along the path of the internal carotid artery, and
then the common carotid artery and internal jugular vein.

Department of Optometry, King Saud University                                                      52
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Branches of the vagus nerve in the neck and upper thorax supply the muscles of the larynx for
control of speech.

Above the heart, branches of the vagus nerve combine with sympathetic branches from the thoracic
sympathetic chains, to form the cardiac plexus, from which nerves go on to innervate the heart.

 The vagus nerve then travels with the esophagus to reach the stomach area. Here it forms the
anterior and posterior gastric nerves, which give parasympathetic innervation to the stomach, entire
small intestine, proximal colon, and other viscera in the abdominal cavity.

We see then that the vagus nerve is responsible for most of the parasympathetic innervation for the
internal organs of the body.

Note   The distal colon and pelvic organs receive parasympathetic innervation from the sacral
       spinal nerves.

The Accessory nerves (XI)

Each one arises from the lateral border of the inferior medulla as well as from the anterolateral
surface of the upper five segments of the spinal cord.

It also leaves the forehead via the jugular foramen and some of its fibres then join the vagus nerve
to innervate the muscles of the larynx and pharynx.

However, all fibres of these nerves with spinal roots pass downward along the posterolateral
portion of the neck to supply the Sternocleidomastoid and trapezius muscles. These muscles also
receive fibres from the cervical plexus in the neck region.

The Hypoglossal nerves (XII)

Each one arises from the lateral border of the lower medulla anterior to the vagus and accessory
nerve origins.

Each leaves the skull via the hypoglossal foramen and enters into the inferior Mandibular region of
the neck and is distributed to all the tongue muscles including the hypoglossus, genioglossus,
styloglossus, and intrinsic tongue muscles.

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