ANATOMY OF VISION IV¹

Inspired by Boileau Grant's Atlas of Anatomy (except for the mustache) {back to control panel}

Locating the nuclei for eye movements: The Colliculi

DIAGRAM: Nerves III, IV, VI scroll right for labels on dissection --> {back to control panel} scroll down for look at stump of nerve IV

{return to slide 10} Planes of dissection: X-X'

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CADAVER HEAD (for cranial nerve stumps in dura mater)

Note especially Nerves III, IV and VI. To reach the back of the orbit they have to pass through the cavernous (venous) sinus the tape man talks about. The paired cavernous sinuses are a subset of the dura mater; they seemingly straddle the sella turcica (in the middle cranial fossa) and are hidden from our perspective in this view. {back to control panel}

THE OCULOMOTOR NUCLEUS NOTE: Although schematic, this diagram is based upon microscopic examination of the Oculomotor complex in the section used for slide 19. {back to control panel}

EYE MOVEMENTS

Unlike certain reptiles, we primates don't normally move our eyes one at a time. To put it another way, if one of our eyes moves, its partner over on the other side of the head moves too (unless something's wrong in there). But those movements can be either yoked, the eyes moving parallel; or non-yoked (aka, vergences), where the vectors representing the axes of the two eyes will eventually cross. Yoked movements (considered below) are said to be conjugated; whereas the vergences -- con-vergence and di-vergence -- are sometimes describes as disjunctive. As far as the muscles are concerned, the vergences are much easier to talk about than the yoked movements. In convergence, if the medial rectus (MR) in one eye contracts, so does the MR over in the other eye. Likewise, when the eyes diverge back to the midline (both eye don't usually diverge to the corners of the orbit, incidentally ), both right and left lateral recti contract. What happens in vergences would never do for yoked movements. In yoked movements, if the LR contracts, MR contracts in the other. And in yoked movements, all of the muscles the can contract are obliged to do so, at some time during the shift from one gaze position to another. And the yoked pairs must pair up. The Yoke Table comes in very handy when figuring out which muscle are active (or inactive) in which eye. {back to control panel}

Yoke Table

Picture credit*

CORTICAL EYE FIELDS

Occipital Eye Fields (areas 18 and 19) provide the upper motor neurons (issue the orders for) unconscious, smooth tracking or pursuit movements of the eyes. The movements are yoked, or conjugate, meaning the yoked muscle of the two eyes work as a team to move the eyes in the same direction. Frontal Eye Fields (area 8) of the cerebral cortex come into play during saccades. In middle French, a saccade was when a knight yanked on the reins of his charger. In ocular motility, saccadic movements are stepwise excursions of the eyes. Saccades can be voluntary, as in reading or (if you're a protein-hungry monkey) grooming the hair of a tribe mate for a juicy louse with which to supplement the diet. Saccades can also be reflexive (as in certain kinds of nystagmus). Voluntary or involuntary, saccades are usually yoked (unless something's wrong in there). Temporal Eye Fields (area 22 ?). Convergence and divergence are disjunctive -- non-yoked eye movements. If the eyes were a team of oxen, they'd either bang into each other or pull away in opposite directions in vergence movements. Some investigators have elicited convergence in monkeys by stimulating a part of the temporo-occipital cortex homologous to the posterior segment of the human Brodmann area 22 (on the back end of superior temporal gyrus). Is this a temporal eye field dedicated to vergences? While there's no fundamental quarrel with that, evidence from the monkey cerebrum may or may not apply directly to humans. Because of the uncertainty, no temporal eye field has been identified on this map. {back to control panel}

NYSTAGMUS

To maintain a visual fix on a target, a person's gaze must compensate for movements of either the target, relative to the observe, or the observer in relation to the target. Two distinctly different nystagmus reactions relate to these activities. Medical dictionaries tell us that nystagmus is rhythmic oscillation of the eyes. The movements can be horizontal -- back and forth; vertical -- up and down; or even oblique. The eyes move first one way, then the other. Back and forth! To and fro! While a long list of interesting nystagmuses exist, our immediate interest is with two types: optokinetic nystagmus (or OKN) and vestibular (aka labyrinthine) nystagmus. The eye movements in both the latter types normally are yoked, meaning the eyes move in tandem like a cooperating team of horses or oxen. OKN reflects the attempt to maintain the gaze on a moving target; it is elicited by moving periodic or repetitive patterns, such stripes on a rotating drum (often called an OKN drum). The eyes pick up and try to follow the target; then, reaching some physiological maximum, ZAPPO! they bat back to the starting position, as though trying to pick up the next beat. A graph shows a biphasic plot -- a smooth pursuit in the direction of the motion followed by a quick saccade: OKN is sometimes called 'railroad' nystagmus from the reaction of eyes to the periodic passing of telegraph poles to an observer on a moving train. And it's easy to show that OKN depends on vision: spin the drum so fast that the stripes blur or turn off the lights and watch the subject in infrared and OKN quits. The input for OKN involves the visual pathways, obviously. The first phase  seems to depend on output from the occipital eye fields (areas 18 and19). Remember, the occipital eye fields issue signals for smooth, yoked pursuits of an unconscious nature. In OKN, signals from the occipital eye fields appear to be sent simultaneously down to cranial nerve nuclei III, IV and VI and also forward to the frontal fields in area 8 (on the back of the middle frontal gyrus) , the former down through the internal capsule and corticobulbar tracts, the latter via the superior longitudinal fasciculus. Neurons in the frontal eye fields, in their turn, issue signals to cranial nerve nuclei III, IV and VI for saccades -- the second, or corrective, phase of the OKN movements. Circumstantial (clinical) evidence also suggest that, concurrent with the signals down to the cranial nerve nuclei, the frontal eye fields also send inhibitory signals back to the occipital eye fields. The pathway for the latter seems to be a band of white matter called the inferior longitudinal fasciculus. The latter inhibitory signals from the frontal back to the occipital lobe would presumably set the stage for another round of OKN. (It's also worth noting that both hemispheres must intercommunicate during OKN. Transhemispheric signs, recall, are mediated via the corpus callosum -- the genu and head for the frontal lobe, the splenium for the occipital lobe. ) Vestibular nystagmus can stimulated in various ways, one of which is to rotate a person in what's called a Bárány chair (a merry-go-round also works). When the spinning begins, the eyes make a slow movement in the direction opposite the stimulation. The eyes try to correct for the shift in position of the head. Then the eyes bat back to the center; i. e., they execute the fast phase in the same direction as the movement, as though cocking for the next beat. But this is when the spin is in progress. When you stop the Bárány chair BOING! the nystagmus reverses: the fast phase is in the same direction as the spin had been and the slow phase -- opposite! Why the reversal? The stimulus for vestibular nystagmus is a function of the inertia of the fluid within the semicircular canals. Stopping the chair reverse the effect of starting it up in the first place. But that doesn't give us a very sophisticated explanation of what happens in the Bárány chair -- or in other methods of stimulating vestibular nystagmus (of which there are several). To dig further, though, we need to know a little about the vestibular apparatus in the inner ear, which we'll take up presently. But first, appreciate this point: OKN or vestibular -- or any other nystagmus -- is to eye movements what the knee jerk is to walking: The OKN drum and the Bárány chair are akin the rubber hammer. The elicited reflexes, in other words, should not be mistaken for the physiological adaptation, as some of the textbooks tell us. The big deal on that is that the threshold stimulus for the latter is orders of magnitude lower than for the reflexes. (According to Young** vestibular nystagmus thresholds are 5-10 times higher than for the subjective sensations of being spun.) {back to control panel} Now to the ear.

Lateral view of left inner ear:

Medial view of the left inner ear:

Recall that the human inner ear has two main parts: the familiar cochlea, the auditory (acoustic) or hearing part, and the vestibular apparatus -- the part used in balance (in which the eyes play a major role). Both senses depend on tiny organs in what's called the membranous labyrinth -- tiny tubes containing fluid (endolymph) and protected within what's known as the bony labyrinth -- a maze of hard-walled canals within the petrous portion of the temporal bone, inside the cranium. (The walls of the bony labyrinth are much denser than the surround bone, thus permitting the organ to be dissected with a chisel; or, if a skull is demineralized [soaked in vinegar], with a scalpel.) Now as unlikely as it may seem, changes in pressure translated to those little sense organs in the membranous labyrinth, via the endolymph, account for both vestibular functions and hearing. Our interest, though, is in the former.

Most of us have heard about the famous semicircular canals. We vertebrates own three pairs (lamprey eels have only two). Each bony canal contains a fluid-filled membranous semicircular duct, at the end of which is an ampule-like swelling or (aptly) an ampulla.

Each ampulla, in turn, contains a sense organs that can be stimulated by mechanical changes in the endolymph in the duct. One popular misconception is that the endolymph sloshes around in the semicircular duct like paint in a spun pail; but, , the physiologist tells us, the volume of the endolymph is way too miniscule for anything that. Instead most authorities tell us that changes in the angular momentum of the head either increases or decreases the pressure within the endolymph. The sense organ in question forms a crest (of neuroepithelium) within the ampulla, and is (wisely) called an ampullar crista. The crista is a mound of jelly (cupula) with 'hairs' projections up into it from sensory hair cells. (The hairs aren't like those in a mustache but are cilia -- thin extensions of the cell itself. Ampullar hair cells also have sensory nerve fibers wrapped around them, fibers that are branches of the vestibular division of cranial nerve VIII If those hairs get tickled, the hair cells either excite (depolarize) or inhibit (hyperpolarize) their nerve fibers. A given crista becomes excited when the endolymph pressure, like a shoved head, is directed toward the ampulla. But when the pressure is directed away, the ampulla, develops a case of the uptights and is actively prevented from sending signals to the brainstem. How's come? In a general way, hairs of a sensory hair cell aren't just stuck there like the coiffure of the Wild Man from Borneo (or Woodstock). The electronmicroscopist shows us that a hair cell sports two kinds of hairs: a bundle of ordinary hairs (stereocilia) and a single special hair (kinocilia) per cell.

When force is directed so as to push against the special hair, bingo! the hair cell excites (depolarizes) its nerve ending. When force is directed away, from the special hair, the nerve fiber becomes inhibited (hyperpolarized).
Okay, try to remember this principle of vestibular stimulation:

When pressure in the ampulla is against the crista, the nerves get excited and the eyes move in the opposite direction. Meanwhile over in the ampulla in the other ear, the pressure is away from the crista and the nerves are inhibited.

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Look again at the three semicircular ducts.

Those on either side of the head are paired up (lateral--lateral'; posterior--anterior'; anterior--posterior'), paired in such a way that when one side is excited the opposing side is inhibited.
If, say, the lateral member in the left ear, gets excited and starts firing salvos of impulses, its pal over on the right side becomes inhibited. The big deal about that (and it's mighty) is this: the six ampullae working in concert are ready to calculate the changes in angular momentum from any one point to any other point within the in 3-dimensional universe of your head.

But there's more, That name 'vestibular' comes from the vestibule-like chamber between the semicircular canals and the cochlea. Take another look at the saccule; in it there's a sense organ called the saccular macula or, better, the otolith organ. 'Otolith' means ear stones. The macula consists of a slab of jelly with the stones in question (calcium carbonate crystals) embedded in it. Also embedded in the jelly are -- as in the crista of the ampulla -- the hairs of sensory hair cells, replete with nerve ending wrapped around the base of each. Tilt your head and gravity forces those stones to shift. And, like the cristae, the direction of the force counts: If the hairs get pressed upon, bong! they get excited. If the shift is away from the stones, the nerve endings become hyperpolarized -- get the ionic uptights and become inhibited! What arrives at the vestibular nuclei in the medulla is an algebraic sum of the pluses and minuses in the slab of jelly. Which vestibular nuclear cells are turned on and which remain silent determines what kind of signal will ride up the MLF to the nuclei of III, IV and VI.
The big deal about the gravity-using otolith apparatus is that we have an organ to register lower order changes. They can tell us, statically, where the head is in space. They're responsible for the so-called 'Doll's Eye Reflex' where the eyes act as though they're free-floating in the orbit when the head is tipped.

Actually, there's nothing free-floating at all, not about our eyes. instead the otolith organs in the vestibular portion of the inner ear and sending signals to excite or inhibit the necessary neurons of ocular motility.

VESTIBULAR PATHWAYS (Optional Notes)

Recall that sensory nerves in general have their cell bodies in ganglia out in the periphery; the individual sensory neurons are said to be bipolar meaning they have one fiber attached to the sense organ and the other to nuclei in the spinal cord or brainstem (for cranial nerves). In the case of vestibular nerve fibers, those of the cristae and the maculae of otolith organs are served by (what else) the vestibular ganglia (of Scarpa). internal auditory meatus -- the hole in the petrous portion of the temporal bone that leads into brain case.

Four pairs of vestibular nuclei exist in the brainstem to receive and process the signals from the vestibular organs. It's easiest to locate the vestibular nuclei in the rhomboid fossa of the 4th ventricle.

The what? Recall that through most of its extent the 4th ventricle looks as though its open, covered only by cerebellum. Anyway, if we removed the cerebellum, the expanse that looks up at us is the rhomboid (diamond-shaped) fossa that curls up laterally toward the cerebellum. The vestibular nuclei occupy the lateral part of the floor of the rhomboid fossa and stretch all the way from there the medulla meets the spinal cord up to the midbrain (i.e., throughout the entire length of the hindbrain).

The four nuclei (lateral, medial, superior, inferior) sort out the various signals and distribute them not only to the nuclei for eye muscles, but down to the spinal cord (balance remember) and also to the cerebellum (via vestibulocerebellar tracts). (You can see cross sections of the vestibular nuclei in slides 6-10 ;click for slide control).

Recall again that the MLF is a main conduit for signals from the vestibular nuclei but up and down. In addition, there's a large vestibulospinal tract running down the anterolateral spinal cord with signals for the anterior gray horn and the somatic motor nerves of the body.

But, to summarize what's critical to remember for our purposes: Ampullar cristae + saccular maculae --> vestibular ganglia--> vestibular nuclei--> MLF--> nuclei III, IV and VI--> extraocular muscles

{mlf} = medial longitudinal fasciculus

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Brachium of the Superior Colliculus

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Superior Longitudinal Fasciculus

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IRIS AND PUPIL -- preliminary points

The iris contains the readout mechanisms for reflex changes in the pupil: the iris sphincter and the dilator of the pupil, two separate sets of smooth muscle cells whose names inform us of their actions. The sphincter is a contractil ring we readily see in sagittal sections through the iris (scroll right--->). The dilator, on the other hand, is a little tricky observe, partially because it is delicate, but also because of the optical properties of the milieu in the light microscope. Anyhow, the dilator consisting of thin myoepithelial strands on the front surface if what's called the posterior epithelium (shown below, but isn't on the quiz). The dilator, when it contracts, operates in a manner analogous to the strings of a radially configured Venetian blind. But, while it can actively widen the pupil, the dilator's moment-to-moment function is to prevent the sphincter from over-reacting (more about this later on).	***	***
A few points about the iris and pupil: Innervation: The sphincter of the iris is under parasympathetic control (preganglionic= EW III, postganglionic= ciliary ganglion in the orbit); the dilator is sympathetically innervated (preganglionic= ciliospinal center, postganglionic= superior cervical ganglion). Terms one encounters in the clinically oriented literature: miosis - contraction of the pupil mydriasis - dilation of the pupil (note a mydriatic drug is one that dilates the pupil)		***

PUPILLARY LIGHT REFLEX CIRCUIT

Pupillary light reflexes are usually tested in the dark (or at least reduced ambient illumination), using a penlight. One eye is stimulated and the other eye is shielded with an 'occluder.' The reaction of the pupil in the stimulated eye is called the direct reflex (or response). That of the other eye is called the consensual or indirect reflex (response). In a normal person the reactions of both pupils are virtually equal, the reason having to do with the underlying circuitry (which we'll talk about in due course). Recall that among reflexes in general the convention is to consider five principal elements: stimulus sensory or input arc association neurons motor or output arc effector organ (the readout mechanism -- a muscle or gland). In the pupillary light reflexes: The STIMULUS for both the direct and consensual reflexes is light. The SENSORY arc is the retino-tectal pathway (pretectal actually): begins in the stimulated retina and ends in both the ipsilateral and the contralateral pretectal areas (based on electrophysiological evidence) of the midbrain.. (Input can be summarized: hemi-retina , Nasal [crossed] and Temporal [uncrossed])--> Optic nerve-->optic chiasm-->optic tract-->brachium of superior colliculus (branch of optic tract, recall)-->pretectal neurons.) ASSOCIATION NEURONS: Pretectal nuclei--> EW (Edinger-Westphal) III and EW III' via posterior commissure (post. com.) Pretectal nuclei'--> EW III' and EW III via posterior commissure OUTPUT Parasympathetic part of N III (small diameter, visceral motor fibers): Preganglionic -- Edinger-Westphal (small, visceral motor neurons) Postganglionic --Ciliary ganglion EFFECTOR ORGAN (readout): Sphincter of the Iris

LESIONS IN THE CIRCUIT: A 'THINK' EXERCISE

WHAT HAPPENS WITH LESIONS AT VARIOUS PLACES IN THE CIRCUIT? The answers depend on where we stimulate--->

See if you can tell Why without peeking (-->) Doc. LESION NO. STIMULATED HEMI-RETINA* DIRECT REFLEX CONSENSUAL REFLEX WHY? 1 T+N spared spared lesion beyond branching of br. sup colliculus 2 T+N lost lost all photic input blocked 3 T+N spared spared circuit activated from opposite eye 4 T+N spared spared T signals P's on both sides, P' via post. com. 4 T spared spared ditto 4 N lost lost N, now blocked, would carry all input 5 T+N spared spared info can still cross via chiasm 5 T spared lost direct still open but no way to reach P' 5 N lost spared info can cross but can't return 6 T+N spastic paralysis spared upper motor neuron lost 7 T+N flaccid paralysis spared lower motor neuron lost *By placing a point of light in the visual field of the test hemi-retina. {back to stimulated hemi-retina}

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"DARK" REFLEX

The "dark" reflex (reaction) is the active (versus passive) dilation of the pupil in response to a sudden drop in the intensity of light reaching the retina and transitory inhibition of the EW nuclei (parasympathetic moiety of the oculomotor nerve). The dark reflex is sympathetically mediated and, therefore, is also known as sympathetic pupilodilation. The preganglionic neurons lie in the cilio-spinal center; their axons, without interruption, extend up the sympathetic chain to the superior cervical ganglion where they synapse with the postganglionic neurons of the sequence. The postganglionic sympathetic fibers travel on blood vessels, the long and short ciliary nerves and into uvea to reach the diliator muscle in the iris, whose contraction actively widens the pupil.
	NOTE: Budge's CILIO-SPINAL CENTER is part of the Intermediate (or lateral) Gray Horn of the spinal cord at Thoracic levels 1-3, incl.
Note: The sympathetic division of the autonomic nervous system is known in the popular media for its activity during 'flight-fight' reactions. However in a healthy person, under normal circumstances, sympathetic nerves everywhere in the body contribute to what the physiologist calls 'sympathetic tonus' -- just enough firing to keep, for example, the blood vessels from opening too far. In the case the iris, where constriction of its sphincter is a parasympathetic event, the dilator muscle, and the sympathetic 'dark circuit,' act as a brake to prevent the over-constriction of the pupil. This point is dramatically illustrated in unilateral neck injury (e.g., apical lung cancer) where the sympathetic trunk is damaged and, ipsilaterally, the pupillary light reflex becomes exaggerated.

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Slide 03:	plane of sectioning: Looks almost like spinal cord, doesn't it, Doc? You can actually see those decussations of the pyramids on the gross brain, if you look closely enough. click for slide control
Slide 04:	plane of sectioning: Note the gracile nucleus . The large, unlabeled arrowhead marks the boundary between the nucleus gracilis and the cuneate nucleus/ tract (fasciculus) ; the pink material scattered through the cuneate tract formation marks the beginnings of the cuneate nucleus. Gracilis mediates sensations from the lower and cuneatus from the upper the body. The spinal tract and nucleus of the trigeminal nerve (V) are tagged. Note also the crossing or decussation of the pyramids. [click for slide control]
Slide 05: The gracile and cuneate fasciculi [or tracti], collectively the dorsal funiculus of the spinal cord and lower medulla), terminate here in their respective nuclei. Those nuclei stain pinkish, while the fasciculi stain dark blue; the mixture of pink and blue indicates that a given nucleus and its corresponding tract extends vertically through the equivalence of several sections. The nerve fibers in those two tract formations belong to neurons of the dorsal root ganglia; they convey the following sensory information: a) discriminating and 2-point touch; b) deep sensations (from receptors in tendons); and c) proprioception (feedback signals from muscles).	plane of sectioning: Also take note of the spinal (or descending) nucleus and tract of the trigeminal (Vth cranial nerve); the spinal nucleus of V (pink) lies medial to the spinal tract of V. Fibers of the spinal tract of V synapse in the spinal nucleus of V. Axons from the spinal nucleus of V fibers cross to the opposite side, turn up and, baring experimental techniques, become indistinguishable from the rest of the medial lemniscus. Note though, some authors do refer to trigeminal lemnisci. click for slide control
Slide 06:	plane of sectioning: The MLF is off that white extension of the arrow. The internal arcuate fibers, tagged here but barely visible in the photograph, are the axons of cells in the cuneate and gracile nuclei (better seen in lower sections); those fibers arc ventrally, cross to the other of the medulla, as decussation of the medial lemniscus, then turn upwards, towards the thalamus. click for slide control
Slide 07: Here, as in 09, the medial lemniscus and the MLF touch and create the impression (false) that they're an integral part of the same formation. Recall, though, that at higher levels the medial lemniscus twists laterally semingly flopping on its back (in order to line up with the thalamus, where it will terminate). The MLF, because it is a fairly straight tract formation, lies in about the same relative position in all transverse sections through the medulla and midbrain. scroll right to continue caption-->	plane of sectioning: D is the dentate nucleus of the cerebellum; 4 marks the fourth ventricle. Note choroid plexus (in the fourth ventricle). Recall that cerebrospinal fluid is produce by choroid plexus. The section contains part of the vestibular division of the auditory (acoustic) nerve (cranial nerve VIII). Here the vestibular nerve can be seen entering the vestibular nuclei. The vestibular nuclei sort out the input from the vestibular apparatus in the inner ear and (among other things) relay the messages to nuclei of III, IV and VI, via the MLF. The small but conspicuous solitary tract can be seen here; the small pink mass just lateral to it is the solitary nucleus. The solitary tract and nucleus convey taste sensations carried by the facial (VII) glossopharyngeal (IX) and vagus (X) cranial nerves. click for slide control
Slide 09: (No 8)	plane of sectioning: Take note of the medial lemniscus. Notice how it seems almost continuous (above) with the MLF, the medial longitudinal fasciculus. Note also the inferior olive, lateral to the pyramid (pyr). [The inferior olives work with the cerebellum.] The section also shows two of the three pairs of cerebellar peduncles, here the middle (mid.) and inferior sets. The inferior cerebellar peduncle is also know as the restiform body. The nodulus, protruding into the 4th ventricle here, is a part of the cerebellum's vermis. ! click for slide control

Slide 10: The dentate nucleus belongs to the cerebellum. The facial colliculus is a bump in the floor of the 4th ventricle; it provides a surface marker for fibers of the facial nerve (VII); the latter fibers swoop dorsally from the nucleus of VII and form a genu (or knee bend) around the abducens nucleus (VI), the innervator of the lateral rectus muscle. (scroll to continue caption)-->	The arrow on the left points to fibers of the abducens nerve, en route to its exit just below the pons (as seen in the last section). The fibers of VII, having 'genued' around the abducens nucleus, leave this plane of sectioning, loop ventrally and exit at the inferior-lateral edge of the pons. Note also the relative positions of the medial lemniscus and the MLF, (not labeled in all copies of this section) as compared with the Slide 09. click for slide control
Slide 11:	plane of sectioning: The pyramids (output pathways) emerge on the surface and can be seen on the gross brain. Why? Because we're below the pons in this section (note plane of sectioning). The abducens nerve (cranial nerve VI) is on the reader's right. click for slide control
Slide 12:	plane of sectioning: click for slide control
Slide 13:	plane of sectioning: Note the mesencephalic tract and nucleus of the trigeminal (V). click for slide control
Slide 14:	plane of sectioning: Note the roots of the trigeminal nerve. Other tags as before. click for slide control
Slide 15:	plane of sectioning: First, note the medial lemniscus. Again, the MLF is the medial longitudinal fasciculus. The decussation of the trochlear nerve, mentioned in the caption for Slide 16 is conspicuous (and tagged) in the anterior medullary velum (not labeled). The cerebellum's midline vermis is tagged, as are the laterally situated cerebellar hemispheres. The cerebellar peduncles represent conduits to bring information into and out of the cerebellum. That feedback information forms a dynamic association between input and output. Other labels as in previous captions. click for slide control

Slide 16: First, take note of the medial lemniscus. You can see the lateral lemniscus just above (dorsal) to it, here. (The lateral lemniscus is on its way into the inferior colliculus.) This section just nicked the lower ends of the two inferior colliculi (or auditory tectum). The MLF is still the medial longitudinal fasciculus. Note again the sup. (superior) cerebellar peduncle. Note especially the trochlear nerve (IVth cranial nerve); the big arrow points to trochlear fibers destined to exit just below the inferior colliculi. The latter fibers arise at a higher level (slide 18), then extend down (to the level in slide 15) where they crossed (or decussate) before emerging. Do you see those two freckle-like dark dots at the end of the little arrow? Those dark dots are trochlear fibers in the anterior (or superior) medullary velum descending towards their decussation; after decussating, the trochlear fibers loop into the anterior medullary velum, execute their cross to the side opposite their origin and then come out onto the surface. The big deal is that the trochlear nerve innervates a muscle, the superior oblique of the eye, on the contralateral side of the body.

plane of sectioning: Note here the 4th ventricle. The latter space is directly in line with the cerebral aqueduct (iter). A ventricle, qua ventricle, must have some choroid plexus in it; that's also true of the lateral and third ventricles-- but not the cerebral aqueduct! (That's why the cerebral aqueduct is given a name but not a ventricle number. ) Note the mid. (middle) cerebellar peduncle. Also known as the brachium pontis, (can you guess why?), the middle cerebellar peduncle links the pons to the cerebellum. The superior cerebellar peduncle is also tagged in this section. Again the pyramids (output pathways) can be seen passing vertically through the pons. click for slide control

Slide 17:	plane of sectioning: This slide became available after the tape was in the can. It is not a serial with the other sections in the set. Notice the inferior colliculus (aka the auditory tectum). Observe the location of the medial lemniscus. There is a lateral lemniscus. While the medial lemniscus belongs to the somesthetic system (pain, touch, temperature), the lateral lemniscus is major conduit of the auditory system. Note the pyramidal tracts, or pyramids (also called the cortico-spinal and cortico-bulbar tracts). The pyramids are output (motor) pathways. Check out the sup. (superior) cerebellar peduncle in the tegmentum (aka, the brachium conjunctivum); the cerebellum is served by three pairs of peduncles (big tract formations): inferior, middle and the superior, tagged here. MLF, as before, is the medial longitudinal fasciculus. click for slide control
Slide 18:	click for slide control As with slide 19, this section is not true to the horizontal plane (and thus shows pons and superior colliculi in the same section. The big deal about that has to do with the trochlear nucleus (IV) which, in a true cross section, is at the level of the inferior colliculi (click for dissection). But in an approximate section through the midbrain (or mesencephalon) what appears to lie 'above' the cerebral aqueduct is the tectum (superior and inferior colliculi, collectively); 'below' is the territory of the midbrain tegmentum. [There's also a 'pontine tegmentum' -- dorsal to the fibers or 'pons proper' at the horizontal level of the pons.] Notice the MLF, medial longitudinal fasciculus and the medial lemniscus.
Slide 19:	click for slide control This section is somewhat oblique, rather than horizontal thus displaying the superior colliculus, dorsally, towards 12 o'clock (large arrow) and the upper edge of the PONS, ventrally. MLF is the medial longitudinal fasciculus; a recess of the interpeduncular fossa curls upwards at the superior border of the pons and here gives the false appearance of being a lake or a hole. Note especially the medial lemniscus and the heart-shaped oculomotor nuclear complex. The outline drawing of the oculomotor complex was based on a microscopic analysis of this very section. The pineal body seemingly floats above the tectum; it's delicate stalk is superior to this plane of sectioning. The superior cerebellar peduncle carries the cerebellum's output to the thalamus and from there to the output parts of the cerebral cortex . The big deal about the massive decussation of the sup. cerebellar peduncles is that voluntary control of the body directly depends on the cerebral hemisphere on the opposite side.

Slide 20: LGB, lateral geniculate body (nucleus); P, pulvinar (of thalamus) on reader's left; Nuc. III, oculomotor (or 3rd cranial nerve) nuclear complex; centr. gray is a cylinder of central gray matter surrounding the cerebral aqueduct or iter (not labeled). {scroll right for rest of caption}--> click for slide control

plane of sectioning: The posterior commissure (post com.) is the bridge of tissue over the central gray matter and cerebral aqueduct; it connects (associates) cells of the pretectal area (pre-text.) and the Edinger-Westphal portion of the oculomotor nucleus (N III) for pupillary reflexes; the pretectal areas (or nuclei) account for the name of this plane of sectioning; br. sup. col., brachium of the superior colliculus, a phylogenetically ancient branch of the optic tract (carrying the retino-tectal pathways, including the sensory input for internal ocular reflexes; (on the reader's left, the brachium of the superior colliculus appears as a dark triangle with its apex skewed to the left, while on the reader's right the brachium appears as dark tufts squeezed between the pulvinar and the medial geniculate body (nucleus),m; the apparent difference in the two sides is because the section is slightly (but fortuitously) cockeyed;sub. nigra, substantia nigra; F, interpeduncular fossa; U, uncus (contains the amygdala); the splenium of the corpus callosum is at 12 o'clock. [This section is just anterior to the tectum (superior colliculi); thus the term 'pre-tectal.'] {click to get back to pupillary light reflexes}

click for slide control

CN, head of caudate nucleus; LN (scroll left, if necessary), lenticular nucleus (putamen); 1 & 2, lateral ventricles; IC, internal capsule; sp's, septi pellucida

Slide 21:	plane of sectioning: CC (up at 12 o'clock), corpus callosum (body); LV, lateral ventricle; T, thalamus; 3, third ventricle; IR (toward 6 o'clock), infundibular recess of third ventricle; m mammillary body (nucleus); M-T, mammillo-thalamic tract. The arrow point just visible at 6 o'clock points to the infundibulum of hypothalamus. Notice, among other things, how the uncus (of the temporal lobe) presses the optic tract against the Internal Capsule. The Internal Capsule is just emerging onto the surface of the gross brain as the cerebral peduncle (aka, basis... pes... or crus cerebri). A lesion here (e.g., syphilitic gumma, glial tumor, hemorrhage in a branch of the middle cerebral artery) can simultaneously affect the opposite visual field (via the optic tract) and the voluntary control of muscles on the other side of the body (via the pyramidal tract). A foramen of Monro connects each lateral ventricle to the unpaired, narrow, third ventricle. click for slide control panel
Slide 22:	plane of sectioning: cp, choroid plexus; 3, third ventricle; LV, lateral ventricle click for slide control
Slide 23:	plane of sectioning:
Slide 24:	plane of sectioning: click for slide control IC, internal capsule (anterior limb) N, head of the caudate nucleus; black C, cingulate gyrus; white F, (above and below) longitudinal fissure; s.p.'s, septi pellucida; X, cavity of septum pellucidum; 1 & 2, lateral ventricles; since the plane is through the genu (knee or bend) of the corpus callosum, we see parts of the structure in the upper and lower parts of the section.

The Accommodation Triad

Next, the pupils constrict; i.e., exhibit what's called the near-point reaction. Near-point goes on independent of the papillary light reflexes. For, even though accommodative papillary constriction uses some of the EW part of cranial nerve III, and also the sphincter of the iris, the stimulus and input arc are different from those in the papillary light reflexes. In fact, this independence is the basis for an important clinical sign known as Argyll Robertson pupil: absence of the light reflexes but with near-point constriction still intact. Argyll Robertson pupil is exhibited by persons with tertiary syphilis. Anyhow, the stimulus for near-point is change in visual angle and the sensory input arc is via the visual pathways.

The third reaction in the triad goes on inside the eyeball. The readout is the elastic crystalline lens plus its operator, the ciliary muscle. The lens is to the eye what the fine adjustment knob is to the microscope: it sharpens (focuses) the image. The ciliary muscle is a ring (in the ciliary body) of at least three smooth muscle bundles, organized like the spokes in a bike tire), muscles that are innervated by the EW portions of the oculomotor nuclei. The lens is suspended in the space of the ciliary body by the appropriately named.
As the ciliary muscle contracts, the circumference of the ring's space narrows, tension is taken off the ligament and the lens rounds up, increases its curvature and concentrates the image-carrying beam of light onto a smaller area of retina (to wit, the fovea).

FOOTNOTES:

*Drawn by Dr. Linda Dejmek of the Appleton Eye Clinic in Wisconsin while she was a student earning her way through optometry school at Indiana University.

**L. R. Young in Medical Physiology, ed by V. B. Mountcastle, p. 714, Mosby, St. Louis, 1974.

***This item comes from an audiotutorial of Anthony J. Adams, OD, PhD, University of California at Berkeley, School of Optometry and produced by the School's Multimedia Center under an NIH grant to Dr. Adams. We are grateful to him for permission to use it.

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1An item in the electronic reserve collection of the Indiana University School of Optometry Library

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