07 - Diencephalon

Cross references: 

07 - Diencephalon

The precortical diencephalon responds to visual, or at least photic, input.  In addition to the eyes we still possess, sometimes referred to as the "lateral eyes", our ancestors had both pineal and parapineal eyes.  Since K&W devotes its entire Chapter 8 [K&W:276-318] to the "lateral eyes", I'll deal with them first.  

In the precortical brain, the primary projection of the lateral eyes is to the superior colliculus [K&W:289], which, as already mentioned, is often spoken of as the site of the highest level of sensory-motor integration in the precortical brain.  As far as I know, the only changes that the evolution of the cerebral cortex brought about in the subcortical structures related to the lateral eyes are the evolution, or at least the expansion, of the subcortical nuclei which relay the input of the lateral eyes to the cerebral cortex.  

Not counting the lateral eyes, the diencephalon has three parts: the thalamus, the hypothalamus, and the epithalamus.  In humans, the main function of the thalamus is to relay sensory data to the cerebral cortex.  In the precortical brain, the thalamus is very small, and I will not consider it further here.  

The pineal and parapineal eyes, in creatures which still possess them, are located in the epithalamus, at the top of the head.  The lamprey has both a pineal eye, which is on the right, and a parapineal eye, which is on the left.  Unlike the lateral eyes, the pineal and parapineal eyes have undergone enormous change over the course of evolution. My guess is that the pineal and parapineal eyes were originally identical, as our lateral eyes still are, but already in the lamprey, they differ.  

The neural structures just below the pineal and parapineal eyes are the right and left habenula, respectively.  In the lamprey, the pineal eye has become a well developed photoreceptor, does not function as a gland, as far as I know, and is strongly connected to a large right habenula.  The parapineal eye has already assumed a more glandular role, is a poor photoreceptor, and is only weakly connected to a small left habenula.  [The reference for this paragraph is The Biology of Lampreys, edited by Hardisty & Potter, Academic Press, pages 93 & 318.]  We will consider this again in the context of the left-right differences in the functions of the cerebral cortex.  
 
Our early ancestors, up through the amphibians, actually had a hole in the top of their skulls which allowed light to fall directly on their pineal and parapineal eyes.  With the evolution of the first mammals, that hole closed, and information about external photic conditions is now relayed to the epithalamus from the lateral eyes via the suprachiasmatic nucleus and the sympathetic nervous system.  

K&W has a picture of the human pineal on [K&W:195], and mentions it briefly on [K&W:9], [K&W:194-195], [K&W:454-455], and [K&W:458].  Although its photic input is now indirect, what we now call the "pineal" or "pineal complex" continues to perform the glandular function of the parapineal eye.  In humans there are now large lateral and small medial habenular nuclei on both the left and the right which continue to perform the nervous function of the large right habenula of the lamprey.  

K&W has a picture of the hypothalamus on [K&W:54], discusses it on [K&W:410-417], and mentions it briefly on [K&W:423-424] and [K&W:429-430].  As far as I know, the evolution of the cerebral cortex did not entail any major changes in the hypothalamus, so what K&W says about the human hypothalamus probably applies equally well to our precortical ancestors, such as the lamprey and the salamander.  

Both the pineal and the hypothalamus involve the endocrine system, and the endocrine system is extremely important in human behavior.  Unfortunately, I know much less about the endocrine system than about the nervous system.  So, in the interests of expediency, I will postpone discussion of the endocrine system until later.  K&W provides a good introduction on [K&W:226-230]. 








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