Brain Dissections with Peggy Mason
On and off for years and years, I taught neuroanatomy, solo or with my friend and colleague Cliff Ragsdale, to medical, graduate and undergraduate students at The University of Chicago. The last time that Cliff and I did this was Winter 2020, with the final week of class – exams – shut down by the pandemic. Consequently, UChicago undergraduates who began their neuroscience classes in the pandemic hit academic year of 2020-21 have not been taught neuroanatomy. The NEURO club, a student-run organization dedicated to “Neuroscience, Education, University Research, & Outreach” then approached me to lead them in brain dissections.
To give an overview of neuroanatomy, I happily recorded an hour of dissections designed to provide a neuroanatomical framework. To start with, it is critical to understand the parts of the brain and for this we need to go back to development. By about day 28 of gestation (in humans), the neural tube consists of four parts, a spinal cord and three vesicles or swellings with Latin names where the head will develop. They are from caudal, towards the tail, to rostral, towards the snout:
- spinal cord
The spinal cord at day 28 is destined to develop into the spinal cord. The Latin named vesicles will become, again from caudal to rostral, the hindbrain, midbrain and forebrain. The prosencephalon splits once more to form two daughter vesicles: the diencephalon and the telencephalon. There are no English terms for these that are currently in use. At one point, the terms interbrain (di’) and endbrain (tel’) were used but sadly, no more. So, I will refer to the structures that develop from these two vesicles by their vesicle names.
We are going to leave aside the spinal cord and restrict our focus to the brain. What are the structures that develop from each vesicle?
- rhombencephalon (hindbrain) -> medulla, pons, cerebellum
- mesencephalon (midbrain) -> midbrain
- prosencephalon (forebrain)
- diencephalon -> thalamus or dorsal thalamus, hypothalamus (and epithalamus if you want to be really complete)
- telencephalon -> cerebral cortex including neocortex (also called pallium), striatum and pallidum (the core structures of the basal ganglia), amygdala; and if you want to be complete, the claustrum
Now a word about the telencephalon. The hungry beast of the brain lurks within the telencephalon and in the dorsal telencephalon to be specific. There are a crowd of subcortical structures such as the striatum and amygdala with normal appetites for neural real estate, appetites not appreciably different from that of the hindbrain or midbrain.
Neocortex is different. This structure which forms the rind of the telencephalon is only found in mammals and its appetite is voracious. The neocortex expands every which way – up, forward, toward the midline, laterally and most prominently, the neocortex expands caudally. It does what I call a “comb-over” atop the rest of the brain. The upshot is that as you look down on a mammalian brain, you see telencephalon (cortex) and you do not see diencephalon or midbrain at all. Depending on the mammal and the view, you may see a touch of hindbrain – cerebellum to be specific.
So the basic messages are:
The four parts of the brain (hindbrain, midbrain, diencephalon, telencephalon) are lined up all nice in box car fashion until you get to the derivatives of the dorsal telencephalon: the cortex.
Cortex expands every which way and most prominently does a comb-over atop the rest of the brain. This fold-up of the brain is how the brain fits in the unforgiving bony vault of the cranium (the space in the skull where the brain sits).
Between the expanded cortex and the underlying hindbrain, midbrain, and diencephalon are spaces which are not part of the brain. They are outside of the brain albeit inside the cranium. One such space, the velum interpositum, is present between the diencephalon and telencephalon.
Although I did not discuss it here, note that the cerebellum also has a cortex and also does a mini-comb-over. As superficial stuctures that depend on surface area, cortices are hungry beasts.
There are additional points made in this hour-long video (too long, I know). I introduce the retina and optic nerve, internal capsule, corpus callosum, pineal gland, superior colliculus, substantia nigra where the dopamine cells that die in Parkinson’s disease are located, lateral ventricle, third ventricle, cerebral aqueduct, fourth ventricle, dura and a few more structures. If you want more of this, leave a detailed comment about what you are interested in and I will try to oblige.