03 - Reticular Formation

Cross references: 

I will occasionally use a hard copy reference:

    Human Neuroanatomy, 8th Edition
    Malcom B. Carpenter & Jerome Sutin
    Williams & Wilkins, 1983

It is a standard text and available in many libraries.  The format for the references will be [C:page number].  

[K&W:50-55] gives a review and some good pictures of the brainstem .  It divides the brainstem into three components, the hindbrain, the midbrain and the diencephalon, and further divides the hindbrain into the medulla, the reticular formation and the pons.  However, as you can see from the picture, the reticular formation actually extends from the medulla, through the pons and into the midbrain.  Since it is central to the brainstem, I will start by discussing the reticular formation.  

K&W gives a very brief overview of the reticular formation (RF) on [K&W:52] and a much more detailed discussion of its role in activating the cerebral cortex on [K&W:476-477].  My contribution will be to provide a few more details.  

The input to the RF is from all the sensory modalities [C:441].  All modalities affect the RF in much the same way, so that output from the RF depends on the total quantity of the sensory input rather than on the specific modality from which the input is received.  

The axons which provide RF output divide into two branches, one of which goes forward toward the nose while the other goes back, toward the tail [C: 332,393].  It is assumed that both branches carry the same output.  Thus, stimulatory output from the RF which activates more forward structures such as the cerebral cortex also activates structures more toward the tail, such as the spinal locomotor generator.  

It should also be noted that, while the preponderance of the RF output is stimulatory, a small amount is inhibitory.  While the inhibitory neurons tend to be more toward the tail, the two types of neurons are always at least somewhat intermixed, so it is not possible to clearly differentiate between stimulatory and inhibitory sections [C:334-335].

There is a very interesting discussion of the "four major nonspecific ascending systems" on [K&W:174].  This material is entirely new to me and may reflect new knowledge gained since my neuroanatomy book was published in 1983.  It is my impression that these nuclei are imbedded in the RF and therefore probably strongly influenced by it, but I know none of the details.  One of these days, when I have the time, I intend to learn more about this.