05 - The Medulla

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05 - Medulla

I want to discuss the input of the cranial nerves to the reticular formation (RF), but this is actually a shorthand.  Much of the input of the cranial nerves to the RF is not from the cranial nerves directly but rather from collaterals of "secondary" neurons which have their origin in the sensory nuclei in which the sensory portion of the cranial nerves end.  These secondary neurons radiate to many portions of the nervous system, and while the "primary" neurons of the cranial nerves do send some collaterals to the RF, the preponderance of input to the RF from the cranial nerves is via the secondary neurons.  Since the various sensory nuclei receive the input of secondary neurons from other sensory nuclei as well as from other structures such as the cerebral cortex, the input the RF receives from the secondary neurons of a particular sensory nucleus is dependent on more than just the input of the primary sensory neurons in the cranial nerves to that sensory nucleus.  However, the output carried by the secondary neurons of a particular sensory nucleus are dominated by the input of the primary neurons, so, to simplify things, I will just talk about the input from a particular cranial nerve to the RF even though the underlying situation is somewhat more complex.  

I will organize my discussion of the medulla by considering the cranial nerves in sequence from tail to head (what would be known in a neuroanatomy text as "caudal to rostral".  

[K&W:51] shows the cranial nerves which they number 1-12.  It is more traditional to use the Roman numerals I-XII.  As you can see N.VIII through N.XII arise from the medulla.  

N.XII & N.XI

N.XII, the Hypoglossal [C:340], and N.XI, the Spinal Accessory [C:341], innervate the tongue, lower jaw and upper neck.  They are overwhelmingly, and perhaps even solely, motor.  As far as I know, motor nerves do not send collaterals to the reticular formation (RF). 

N.X & N.IX

N.X, the Vagus, and N.IX, the Glossopharyngeal, and their associated nuclei comprise the Parasympathetic Nervous System, which is half of the Autonomic Nervous System (ANS).  K&W discusses the ANS on [K&W:50-51], [K&W:62-64] and [K&W:171-172].  The ANS is central to many of the phenomena studied in the field of psychology.  Most, perhaps even all, of the sensations which we associate with our various emotional states are mediated by the ANS.  There is an ongoing debate, named after two early psychologists, Cannon and Bard, about whether the physiological changes mediated by the ANS are themselves the emotion or whether the emotion is a mental experience which causes the physiological changes.  The compromise position, of course, is that emotion is both mental and physical.  

The two branches of the ANS have opposing physiological effects, and their connections with the central nervous system are also very different as shown on [K&W:64].  

N.X and N.IX originate from nuclei in the medulla, and they conduct both ways.  They are not only motor but also sensory.  This provides the medullary parasympathetic nuclei with direct input from the same organs to which they provide output, and coordination among the internal organs is the special perview of the parasympathetic system.  The sacral parasympathetic neurons, which innervate the internal organs of the pelvis, are similar to N.X and N.IX but arise from the Autonomic Nucleus of the Intermediate Gray in cord segments S2 to S4.  

Since the Parasympathetic System is associated with restful, quiet behavior, the collaterals of the parasympathetic sensory neurons may activate the inhibitory neurons of the RF, but this is pure speculation.  As far as I know, this issue awaits further research. 

In contrast with N.X and N.IX, the sympathetic nerves are strictly motor.  They contain no sensory component at all.  These sympathetic motor nerves originate in the intermediolateral cell column (IML) which extends the length of the spinal cord.  In the photograph on [K&W:367], in which the normally white conducting elements on the periphery are stained dark, the IML is the small, sharp point of gray extending into the dark stained part at the level of the commissure joining the two halves.  Although some of the input to the IML originates in the medulla, much of its input originates in other parts of the hindbrain, especially the hypothalamus, which we will discuss when we get to the diencephalon.  Since these are strictly motor neurons, they don't have collaterals to the RF, as far as I know.  

N.VIII

Although N.VIII, the Auditory Vestibular, does not have the psychological significance of N.X, it is very important to behavior.  It mediates both hearing and balance.  "Balance" is spoken of by neuroanatomists as "vestibular" because the organs which mediate our sense of balance are located in a protected "vestibule" in the skull.  N.VIII carries information about both hearing and balance because both senses evolved from the lateral line system of fish.  The auditory information in N.VIII's output goes to the auditory sensory nuclei, whose secondary neurons project to the reticular formation, the midbrain, the thalamus and the cerebral cortex.  The vestibular input is to the vestibular (balance) sensory nuclei whose secondary neurons project mostly to the cerebellum.  

N.V

Although N.V, the Trigeminal Nerve, leaves the brainstem at the level of the pons, its large sensory nucleus runs the length of the medulla [C:338], so I will discuss it here.  N.V is both motor and sensory.  Its sensory input is from all parts of the head which do not provide sensory input to the dorsal column nuclei.  The sensory portion of N.V is "a particularly potent source of tonic influence to the reticular activating system, exceeding in importance the contributions by other sensory cranial nerves" [C:442].  












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