Friday, November 17, 2006

An imaginary journey up Jack’s ventricles

As I attempt to continue my armchair explorations into neurology, I am slowly realizing that unless I have a good idea of the various neural substrates and their spatial existence within our heads, it may be very difficult for me to understand where the structures are in the first place and secondly, how they are interconnected. With this in mind, I modified an elegantly laid out thought experiment in one of the books that I was reading (Neuroanatomy through Clinical Cases by Hal Blumental from Yale) and thought that you might enjoy this piece.

(It might be useful to use this diagram as a guide as you go through your journey)

It is basically a thought experiment where I am a scuba diver with a powerful flashlight on a mission to find the hippocampus (that elusive organ that is supposed to orchestrate our short and long term memories. The same thought experiment allows me to miniaturize myself such that I would be able to pass myself though a syringe used for a lumbar puncture on one of my close friend Jack who has volunteered to sit through my explorations. Included in my survival kit is also a miniature copy of Grey’s anatomy that will help me navigate my way through some of those tough sounding Latin named structures that I am going to encounter along the way – hey, we all need a Rand McNally’s when we go on a long exciting trip, don’t we? So here goes…

OK, that minitaturization hurt a little, but I was happy to pass easily through the lumbar puncture syringe into the subarachnoid space of Jack’s lumbar cistern passing through the skin, subcutaneous tissues, the hard interspinous ligament, through the tough dura mater and finally into the cerebrospinal fluid in the subarachnoid space. As soon as I was released into the cerebrospinal fluid, I stretched my legs and hands and looked around me to try and learn what was going on… Instinctively I notice that I was placed above the vertebral body S1 between L3 and L4. I also notice that I am bounded internally by the pia mater and externally by the arachnoid. As I start to swim in the cerebrospinal fluid, I start to notice a lot of wispy spiderweb like protrusions called the arachnoid trabecula from the outer arachnoid wall that seem to swim and sway in the cerebrospinal fluid and some of them even reach out and touch the pia mater. Carefully avoiding the spiderweb like protrusions, I use my flippers to swim upwards a little bit. I look up and I see wondrous rope like filaments descending all around I am somehow reminded of a horse’s tail. The tail seems to sway in the cerebrospinal fluid all the way into the lumbar cistern. I understand that I am looking at the cauda equina. Looking upwards and in front of me, I see a gleaming whitish pink tube like organ through the translucent pia and look at the conus medullaris portion of the spinal cord. I swim around the cord a little bit trying my best to avoid all of the horse tail like strings and see some of the nerve roots entering and exiting the spinal cord. The ones entering the spinal cord seem to be sensory nerves from the dorsal side (Jack’s back side) and motor nerves seem to be exiting on the ventral side of the whitish cord.

I make my way up, swimming against the flow of the cerebrospinal fluid for quite a while until I see a large opening up above me. It seems to be about the level of Jack’s mouth and realize that I am looking at the foramen magnum (a large ring shaped entrance to the cranial cavity). It is a bit foreboding as I seem to be heading into a large chamber of some sort and realize that this is the Cisterna Magna. I look up and see the ventral aspects of the cerebellum that looks like grayish pink structure with lots of little goose bumps all over… Below me I see that the spinal cord has given way to become the medulla. I also realize that the reticular formation forms some part of the floor beneath me if I stand here… I swim around the whitish pink cord and am fascinated by looking at the pons on the opposite side. I also seem to have fallen into the Pontine cistern on the other side. I quickly extricate myself from the Pontine cistern and come back to the cisterna magna and notice that there is another ventricular structure around me to either side. On either side of me, I can see the lateral foramina of Luschka. In fact I can slide down the walls of this foramen, but decide against it as I have more interesting things up there and let’s not forget, I am in search of the hippocampus. I decide to swim up against the dorsal side of the medulla and as soon as I come up a little above the cerebellum I notice the midline foramen of Megendie. The cerebellum still stretches on above me… a pulsating greyish mass that seems to be busy calculating coordination, gait and other higher motor functions. Without stopping at the foramen of Megendie, I decide to keep swimming upstairs… The force of the cerebrospinal fluid seems to be especially strong now and I force myself upward and notice that I have entered a large cavity and realize that this is the fourth ventricle that I am in…. I stop swimming and slowly land on the ventral floor of the fourth ventricle. Rostrally, I see the pons up ahead and caudally, I see the medulla. You must realize that I am standing perpendicular to Jack’s upright posture. (if I wanted to shoot out of his body, I would come head first out of the lower portion of his head at this time – just to give you a visual of where I was) Dorsally, the roof seems to be the cerebellum, with the large cerebellar peduncles on either side. I look at the miniature copy of the Grey’s anatomy that I am carrying and notice that the floor is also called the rhomboid fossa (containing important structures like the facial colliculus and the sulcus limitans). Parts of the floor rostrally also seem to be colored bluish grey and on looking it up is the locus ceruleus, which owes its color to an underlying patch of deeply pigmented nerve cells, termed the substantia ferruginea. I turn my headlight again rostrally and put it on high beam and realize that now I have come to a very narrow passage that I doubt that I will be able to cross. I start swimming upwards and with great difficulty clamber my way through a narrow tunnel called the cerebral aqueduct of Sylvius. The rush of the cerebrospinal fluid at this portion is really strong and I had to use all of my energy in staying my course at this point in time. I swam some of the way and walked some of the way as the tunnel seems to gradually slope with an upward trajectory. I also notice that there is a substantial grey matter at this point in time and slowly understand that I am crossing the periaqueductal grey portion of the midbrain (an important descending pathway which can be instrumental in inhibiting pain signals from the spinal cord). Clambering out of the tunnel, I thought that I could relax for a bit, but I seem to be immediately pulled down and start to sink to the depths of another cavity – the third ventricle. As I am traveling down the third ventricle, I look to my left and right and notice that I first pass the walls of the thalamus and then the walls of the hypothalamus. The third ventricle seem be bounded by the thalamus and hypothalamus on the left and the right. I also notice that the two thalami seem to be joined tighter at the interthalamic adhesion midway along the third ventricle.

At this point I stop swimming, look up and notice two parallel white arches running along the roof of the ventricle and realize that I am looking at parts of the fornix running over me. This is really exciting! I also notice that there is a profusion of capillary like vessels separated from the subarachnoid space by pia mater. Liquid seems to be filtering through ependymal cells (a type of neuroglia) from blood to become the cerebrospinal fluid that I am swimming in. I now realize that I was seeing this all along my way except along the narrow walls of the cerebral aqueduct, but only now did I begin to notice this. Looks like the cerebrospinal fluid is being made all along the ventricular system. I look behind me and notice the pineal and the suprapineal recesses on the caudal end of the third ventricle. I decide not to go the caudal end, but I seem to have two ways to go forward or rostrally, I can either swim upwards entering one of the two narrow tunnels that I see there or I can swim forward (rostrally) the third ventricle and see if there is anything out there that will lead me to the hippocampus. I first decide to swim straight ahead (rostrally). No luck. I seem to have come up against a dead end of some sort, but I notice that I can touch the supra optic recess (above the optic chiasma – where Jack’s optic nerves birfucate), and the infundibular recess (above the pituitary stalk). I know that the hypophysis is somewhere at the end of the infundibular recess, but I have other things to do… I also notice that walls of the hypothalamus extend all its way down here also. Well, now that I have had no luck, I decide to swim back up and squeeze myself through the tunnel to my right. On consulting Grey’s I find that I am entering the right foramen of Monro. Just as I am entering this new tunnel, I pause and look around me and I notice that I am standing on the anterior commissure (a bundle of white fibers, connecting the two cerebral hemispheres across the middle line) with my left hand up on the fornix and my right hand on the walls of the thalamus. I swim my way up this passage and find myself in a larger chamber – in fact one of the largest chamber that I have been in so far. Well, this is it – it is the right lateral ventricle. I start to swim forward in this cavity to find my way around a little better and I reach the rostral or anterior end of the lateral ventricle. This is called the anterior horn. Looking up, I see a bunch of white fibers running in close formation and recognize that as the corpus callosum – that great highway of fibers that connect the two hemispheres. I also realize that I am deep in the frontal lobes of Jack’s head. The floor here seems to be the head of the caudate nucleus.

I then decide to turn all the way around and swim to the other ends of this great chamber and realize that in reality this big cavity is made of three large horns, the anterior or the frontal horn that I just ran into, the posterior or the occipital horn and the inferior or temporal horn located lower down in the temporal lobes. I am sure that I will be able to find the hippocampal structures somewhere here. As I swim back I feel like I am being sucked into the foramen on Monro and have to swim quite strong against the current of the cerebrospinal fluid that is draining into the third ventricle. I seem to be in the body of the lateral ventricle now… I look to my right and see a translucent wall of membrane called the septum pellucidum. The septum pellucidum is located in the midline of the brain, between the two cerebral hemispheres. It is attached superiorly (above), anteriorly (in front), and inferiorly (below) to the corpus callosum, the large collection of nerve fibers that connect the two hemispheres. Inferiorly and posteriorly (in back), it is attached to the anterior part of the fornix. I shine my headlight through the septum pellucidum and look into the further reaches of Jack’s left lateral ventricle. To my left, I see a huge grey mass bulging into the walls of the lateral ventricle and realize that this is the body of the caudate nucleus. I also notice that I am able to make out the outlines of part of the thalamus, the choroids plexus and the fornix from my vantage point in the body of the lateral ventricle. I decide to keep swimming forward and presently find myself in the posterior horn of the lateral ventricle. Now I am in close proximity of the occipital lobe but still no sign of the hippocampus. I still notice that on looking up, I can see the great fibers of the corpus callosum running silently overhead carrying all of that important information between the hemispheres. I reach a dead end here also and then turn right around and am planning on heading back home when I lose my step and fall headlong into a long curvy descending passageway down and slowly realize that I am falling down the steep slope of the temporal horn of the lateral ventricle.

Luckily I do not hurt myself and manage to gather myself and stand up and look around. Upwards I see nothing but the white of the sky and realize that I am staring at Jack’s right cerebral hemisphere inside the temporal lobe – the seat of his higher thinking and memory. Anteriorly, along the median, I see the stria terminalis. The stria terminalis extends from the region of the interventricular foramen to the temporal horn of the lateral ventricle, carrying fibers from the amygdala to the septal, hypothalamic, and thalamic areas of the brain. It also carries fibers projecting from these areas back to the amygdala. It participates in anxiety and stress responses. I also notice that I can see the tail of the caudate nucleus upfront. I turn my head caudally and notice the amydaliod nucleus towards the terminal end of the inferior horn. OK, now where is the hippocampus. I look down and cannot seem to believe my eye.. There in a row I recognize the fimbria (a prominent band of white fibers along the medial edge of the hippocampus) the hippocampus itself and the collateral eminence (elevation along the floor of the posterior part of the temporal horn of the lateral ventricle, lateral to the hippocampus caused by the deep collateral sulcus). I realize at this point that I am standing on the hippocampus!!!!

Now I need to find the shortest way out – but that is another story.

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