10 - Cortex

Cross references:         

10 - Cerebral Cortex

If you stop and think about it, it might seem a bit odd that the cortex evolved as part of the olfactory lobes.  After all, it's the superior colliculus that performs the highest level of sensory motor integration in the precortical brain.  So why didn't the cortex evolve over the superior colliculus?  

Perhaps it had something to do with memory.  

Before we go any further, we need to say something about the arrangement of the neurons in the precortical brain.  Your book describes the ventricles on K&W:44 but pretends that it doesn't know their function.  The heart, blood and blood vessels evolved slowly.  In the beginning, the hollow neural tube was an important part of the circulatory system, and the fluid which flowed through it provided nutriants and, most importantly, oxygen to the neurons.  In order to be as close as possible to this source of oxygen, the cell bodies of the neurons, which is where most of the cell's metabolism takes place, bunched up as close as possible to the central neural tube.  The axons and dendrites, where little metabolism takes place, were relegated to the periphery, away from the central neural tube and its supply of oxygen.  

The neuronal cell bodies are grey, while the axons are covered with a myelin sheath which is white.  Therefore, the bunched up neuronal cell bodies are called "grey matter" while the mass of axons and dendrites are called "white matter".  It was only with improvement in the delivery of oxygen by the circulatory system that it became possible for neurons to survive at a distance from the central canal.  These newly evolved neurons on the outside of the white matter became the grey matter of the cortex.  The axons of the cortical grey matter intermingle with the axons and dendrites of the precortical grey matter surrounding the central canal to form the white matter between the central and cortical grey matter of the human brain.  The improved delivery of oxygen by the circulatory system also allowed the enlargement of the subcortical grey matter.  There are some good pictures of cross sections of the human brain on K&W:4,44,58,331,499. 

Ontogeny (the development of the individual) in large part recapitulates phylogeny (the evolution of the species).  The neurons of the cortical grey matter originate among the subcortical grey matter and then migrate out through the white matter to their final place in the cortex.  Your book shows this quite clearly on K&W:246-247.  Although your book doesn't say so, it is my memory that, as they migrate outward, the neurons often (always?) leave behind a thin appendage.  This thin appendage, which stretches from their place of origin near the ventricle to their final position in the cortex, becomes an axon, and this can offer a hint as to the subcortical origin of the cortical neurons.  However, the neurons which migrate out to the cortex also often develop axon collaterals which reach other parts of the brain and nervous system, and these collaterals can become their more important contribution to the overall functioning of the nervous system.

The medial and lateral walls of the olfactory lobes face different circumstances when they enlarge with the addition of the cortex.  The lateral walls can expand outward, but this is precluded for the medial walls by the close proximity of the medial wall on the other side.  Since the medial walls cannot expand outward, they fold inward and form a rolled up structure in the lateral ventricle.  

A picture of brain evolution is given on K&W:38.  Unfortunately, the earliest brain it shows is the brain of a rat, and the rat has a fairly advanced cortex.   In particular, it has a corpus callosum, which neither monotremes nor marisupals have.  However, the rat brain cross sections in the book can still give some insight into how an early monotreme's brain might look.  

There are several cross sections of the rat brain in K&W.  The best is on K&W:173, but similar views are on K&W:79,104.  It is not easy to interpret what these pictures are showing, but comparing all three aids our understanding.  Looking at K&W:173, we see that the large lateral masses are the basal ganglia, which originate in the lateral wall of the olfactory lobes.  The lighter colored medial masses are the left and right septal nuclei which are pressed so closely together that the boundary between the two lobes is not visible.  The lateral ventricles are compressed into the thin lines between the darkly stained basal ganglia and the lightly stained septal nuclei.  A close up of the top portion of the compressed lateral ventricle is shown in the detail on K&W:79.  K&W:104 labels the rat corpus callosum.  
In a monotreme or marsupial, which do not have a corpus callosum, these cross sections would show a hippocampus.  In the rat, the corpus callosum has displaced the hippocampus farther to the rear.  The rat corpus callosum is relatively small, and its hippocampus, shown in the cross section on K&W:222, begins at the posterior edge of its corpus callosum.  My best guess is that all of the darkly stained areas in this picture are hippocampus.  The more medial lightly stained mass is probably the thalamus.  The gap between the lower portions of the hippocampus and the thalamus would not be there in a monotreme or marisupal.  It results from the fact that, in placental mammals, the cortex has expanded backwards to cover the upper portions of the brain stem.  The human hippocampus has been displaced still further by the development of our large corpus callosum which is pictured on K&W:44-45.  

Most of the neurons in the lateral cortex originate in, and provide output to, the basal ganglia.  They also send important output via collaterals to other parts of the neuroaxis, which we will discuss later.  Although the amygdala is in the lateral wall of the precortical olfactory lobe, most of its cortical input is from cingulate gyrus [C:636] which is now on the medial surface of the hemispheres.  However, it is my understanding that the cingulate gyrus was originally on the lateral convexity and has been rotated into its current medial position by the enormous expansion of the lateral surface of the hemispheres.  Thus the neurons of the now-medial cingulate gyrus may have originated with the lateral amygdala.  As will be seen below, the input to the cingulate cortex is also similar to the input to the other parts of the lateral cortex.  

Since the cortex of the hippocampus sends many axons back to what we now call the septal nuclei, it may be that what we now call the septal nuclei incorporate both the precortical septum and the precortical hippocampus.  However, if this is true, then I should have followed Herrick and called #3 in my diagram of the salamander's brain the "septum" and reserved the term "septal nuclei" for the medial subcortical nuclei of the cortical brain.  Since the medial position of the hippocampus prevented it from expanding outward, it has curled up into the lateral ventricle [C:618]. 

If, indeed, the cortex evolved on the outer surface of the dorsal pallium of the olfactory lobes, rather than on the outer surface of the superior colliculus, in response to a competitive advantage achieved by an improvement in or elaboration of memory, one would expect that at least the earliest cortices would prominently subserve memory.  As far as I know, this has not yet been established empirically.  However, I'm not sure that it's been looked for either.  

There is, indeed, a portion of the human cortex which seems to be related to memory.  The diagrams for explicit memory on K&W:503,508 and emotional memory on K&W:511 all show input-output from-to the temporal lobe.  When the temporal cortex of an awake human is stimulated, the person frequently reports experiencing a memory.  Unfortunately, this experiment is not possible with nonhuman animals due to the difficulty of interspecies communication.  

What has been established empirically is that the primitive lateral cortex is a very poorly differentiated version of the human lateral cortex.  In our discussion of the diencephalon, I postponed discussion of the thalamus because its main function is to provide input to the cortex.  It is now time to reconsider the thalamus.  

Almost all input to the lateral cortex passes through the thalamus.  Your book shows Brodmann's areas on K&W:56.  Each of these areas can be distinguished from the others by function, connections and subtle details of the neurons' arrangements.  The deep fissures in the cortex are called sulci, and the sulcus between areas 1 and 4 is called the "central sulcus".  All of the cortex posterior to the central sulcus is involved with processing sensory data, while all of the cortex anterior to the central sulcus is motor.  

Brodmann's areas 1,2&3 receive touch (somesthetic or somatosensory) input from the entire body via the ventral posterolateral thalamus (VPL) [C:508].  Brodmann's areas 41&42 receive auditory input from both the inferior colliculus and the medial geniculate body of the thalamus [C:508].  The sense of taste is transmitted to Brodmann's area 43 via the ventral posteromedial thalamus (VPM) [C:508].  Brodmann's area 17 receives visual input via the superior colliculus and the lateral geniculate body of the thalamus [C:508].  The small portion of area 17 shown on the
picture of the lateral cortex is misleading.  Most of area 17 is on the medial surface of the hemisphere, which is not shown.  In a manner similar to the lateral cortex, the cingulate cortex, which now lies on the medial surface of the hemisphere, receives its input from the anterior group of the thalamus.  As you can see on K&W:57, areas 4 and 6 comprise the motor cortex.  They also receive their input from the thalamus, in this case the ventral anterior nucleus.  

In the first mammalian cortex, as exemplified by the duck billed platypus, these areas comprise the entirety of the lateral cortex with a great deal of overlap between the motor and somatosensory areas.  As the cortex becomes larger, the motor and somatosensory areas become increasingly distinct and "association areas" evolve between the primary sensory areas.  Thus, areas 19,7,39 and 40 mediate associations between visual and somesthetic input, the "dorsal stream" [K&W:290].  Areas 37,21 and 22 mediate associations between visual and auditory input, the "ventral stream" [K&W:290].  Although there is some direct communication between these sensory and association areas, it is my impression that most of the communication between them takes place via a part of the thalamus called the pulvinar [K&W:289].  

Unlike the lateral cortex, which receives strong, direct subcortical input from the thalamus, the medial cortex, that is to say the hippocampus, receives only very moderate subcortical input, and that subcortical input is from the same septal nuclei from which the neurons of the cortical hippocampus originated.  K&W:502-503 lists only cortical input to the hippocampus, and those cortical areas which provide input to the hippocampus also themselves receive input only from other cortical areas.  

There is more to be said about the role of the frontal cortex in behavior, but first I want to talk about the very earliest human behavior, before and right after the infant is born.