Thirty Eight Flashcards
What are the visual receptors of the eye? Where are the receptors located? Where are the cell bodies located? What are the cell bodies of neurons located in the retina? Which layers? What forms the optic nerve?
The visual receptors are the rods and cones located in layer 2 of the retina. Their cell
bodies are located in layer 4. The rods and cones synapse with bipolar neurons which in
turn synapse with ganglion cells. The cell bodies of the bipolar neurons are in layer 6 and
the cell bodies of the ganglion cells are in layer 8. The axons of the ganglion cells radiate
parallel to and near the inner surface of the retina and converge at the optic disc to form the
optic nerve.
What happens to the optic nerves once they enter the cranial cavity? What are optic tracts? What information do they carry (info from contralateral visual field)? Where do they synapse?
After entering the cranial cavity the two optic nerves unite to form the optic chiasm where a
partial decussation occurs. Fibers from the nasal halves of the retinae cross to the opposite
side, whereas those from the temporal halves remain uncrossed.
The optic tracts emerge from the chiasm and skirt the hypothalamus and adjacent portions of
the cerebral crus. Most optic tract fibers synapse in the lateral geniculate nucleus.
What is the significance of the six layers of the lateral geniculate nucleus? What tract do they form? Where does it synapse?
The lateral geniculate nucleus consists of six layers of neurons. The two ventral layers (1
and 2) consist of large neurons. These magnocellular layers receive their input primarily
from rods and deal with the location and movement of objects in the visual field. The four
dorsal layers (3-6) consist of small neurons. These parvicellular layers receive their input
primarily from cones and deal with color and form. The lateral geniculate nucleus gives rise
to the optic radiation or geniculocalcarine tract which projects to the primary visual
cortex.
Where is the primary visual cortex located? What is it referred to as? Why? Where does the processing of most visual information occur? Where is info concerning located and movement of objects in the visual field located? Where is color and form information processed? How is the primary visual cortex organized?
The primary visual cortex is located in the walls of the calcarine sulcus, with the cuneus
above and the lingual gyrus below. This cortical area is also referred to as the striate area.
The latter term is due to the presence of a conspicuous intracortical white band, the line of
Gennari. Although the processing of visual information occurs chiefly in the occipital lobe,
other cortical regions are also involved. For example, the posterior parietal cortex receives
information concerning the location and movement of objects in the visual field, while the
inferior temporal cortex receives the color and form information.
Retinotopic localization occurs in the primary visual cortex with the macula being
represented in the posterior half and the paramacular and peripheral parts in the
anterior half. Within the anterior half, the peripheral part is more anterior.
What happens to the visual field due to the lens? What happens to the visual field in the optic chiasm? Where does the dorsal part of the optic radiation go? Where is its input from? SAme quesitons for the ventral part of the optic radiation. How is information arranged in the lateral geniculate nucleus?
Through the action of the lens the four quadrants of the visual field (upper right, upper left,
lower right, lower left) are projected onto the retina in a reversed and inverted pattern.
Through the partial decussation of optic nerve fibers in the optic chiasm, where fibers from
the nasal halves of the retinae cross while those from the temporal halves do not, distal to
the chiasm each half of the visual field is represented in the contralateral visual path.
Moreover, the dorsal part of the optic radiation, which arches from the lateral geniculate
nucleus (upper visual field laterally, lower visual field medially) initially into the parietal lobe and ends in the cuneus, carries the input from the
contralateral lower quadrant, while the ventral part, which sweeps initially into the
temporal lobe and terminates in the lingual gyrus, carries input from the contralateral
upper quadrant of the visual field. Hence, visual field defects may result from lesions in
the parietal, temporal, or occipital lobes.
Where will blindness occur is a lesion of the optic nerve? What does heteronymous mean? Where will the lesion be in a heteronymous visual field loss? Homonymous? Where will the lesion be in a homonymous visual field loss? What is bitemporal hemianopia? Where is the lesion? What will usually cause the lesion?
Lesions of the optic nerve result in blindness in the ipsilateral eye. Lesions of the optic chiasm, or the visual path distal to it, result in defects described according to the part of the visual field that is lost in each eye. Defects are heteronymous when the visual field loss is different in each eye. Conversely, defects are homonymous when the visual field loss is the same in each eye. A homonymous defect will result from any lesion distal to the optic chiasm. For example, total destruction of the entire optic tract (or lateral geniculate nucleus or optic radiation or primary visual cortex) will result in the loss of vision in the contralateral half of the visual field in each eye, a condition called contralateral homonymous hemianopia. On the other hand, lesions of the optic chiasm usually result in heteronymous defects. When the fibers crossing in the middle of the chiasm are interrupted
(most commonly due to a pituitary tumor), the temporal or lateral half of the visual field is lost in each eye, a condition called bitemporal hemianopia. Since the right half of the visual field is lost in the right eye and the left half is lost in the left eye, this is an heteronymous hemianopia because the fields are different in each eye.
What happens in the direct light reflex? In the consensual light reflex? What is the pathway of the afferent limb of the direct path? Of the Indirect pahways? What is the pathway for the efferent limb? Which nuclei and/or ganglions are part of the pathway? What 3 parts of the CNS does the light reflex test?
When light entering one eye increases or becomes brighter, pupillary constriction (miosis)
occurs in both eyes. The pupillary constriction of the eye stimulated is due to the direct
light reflex. The constriction in the opposite eye is due to the consensual light reflex.
The afferent limb of the light reflex involves the receptor and neuronal elements of the
retina, the optic nerve, the optic chiasm, and the optic tract. From the optic tract, the
impulses enter the brachium of the superior colliculus which carries them to the light
reflex center in the pretectal region. These neurons in the pretectal region give axons that
terminate on visceromotor neurons of the oculomotor nuclear complex, commonly referred
to as the Edinger-Westphal nucleus. For the consensual response, crossing occurs in the
optic chiasm or in the posterior commissure, thus involving the contralateral
Edinger-Westphal nucleus.
The efferent limb of the light reflex involves preganglionic parasympathetic axons from
the Edinger-Westphal nucleus that travel in the oculomotor nerve and its branches to the
ciliary ganglion. Postganglionic fibers from this ganglion course through the short ciliary
nerves and terminate on the constrictor muscle of the iris. Hence, shining a light in one eye
tests the integrity of the ipsilateral optic nerve, both oculomotor nerves, and the pretectal
area of the midbrain.
Describe the various steps of the pupillary dilation pathway?
Pupillary dilation (mydriasis) is usually a result of emotional expressions (fear, rage, etc.)
or pain. Impulses descending from the hypothalamus travel through the brainstem reticular
formation to the ciliospinal center, comprised of preganglionic sympathetic neurons
located at about C.8 and T.1.
These preganglionic sympathetic neurons give axons that emerge with the ventral roots of
T.1 and T.2, traverse the white communicating rami, ascend in the sympathetic trunk,
and terminate in the superior cervical ganglion. Postganglionic sympathetics then travel
with the carotid plexuses and via the nasociliary and long ciliary nerves to the dilator
muscle of the iris.
What is the near triad? Describe the steps in the afferent pathway for accomodation. And the efferent pathway.
The mechanism for accommodation is based on an inherently elastic lens that is suspended from the ciliary body. Upon contraction of its muscles the ciliary body moves closer to the
lens thereby decreasing the tension on the suspensory ligaments. This allows the lens to
increase its curvature by bulging. To facilitate visual acuity further, convergence of the eyes
and constriction of the pupils are combined with the accommodation of the lens. These three
components of accommodation are commonly referred to as the near triad.
The stimulus for accommodation is the perception of an object and the reflexes are
dependent upon the occipital cortex. The afferent limb in the reflex is represented by the
visual pathway from the retina to the striate cortex. Corticotectal projections from the
occipital lobe then descend to the accommodation center located at the levels of the
superior colliculus and pretectal region.
From the accommodation center impulses pass to the parasympathetic Edinger-Westphal
neurons for changes in the lens and pupil, and to alpha motoneurons for convergence. The
efferent limb is the oculomotor nerve with a synapse in the ciliary ganglion for the
parasympathetic impulses responsible for the involuntary accommodation and
constriction. The alpha motoneuron impulses for convergence pass uninterruptedly to the
medial recti muscles.
