06 - Pons & Cerebellum

Cross references:       

06 - Pons & Cerebellum

I will consider the pons and cerebellum together since the pons is essentially a collection of relay nuclei for input from the cerebral cortex to the cerebellum.  Before the evolution of the cerebral cortex, the pons was almost nonexistent.  In accordance with the statement on [K&W:50] that "The brainstem, then, both produces movements and creates a sensory world.", I want to provide a picture of how our brains worked before we evolved a cerebral cortex.  So, at this point, I will only consider the cerebellum's subcortical connections.  Although the cerebellum receives input from all sense modalities, it is my impression that its primary inputs are from the spinal cord and the vestibular nuclei.  

Although I don't have a reference for this, my impression is that the cerebellum evolved in order to allow our aquatic ancestors to swim in a straight line.  In order to swim in a straight line, the head must maintain its orientation in space while the tail moves back and forth.  This is accomplished by integrating input from the vestibular system, which responds to motion of the head, with output to the spinal locomotor generator and the muscles of the neck.  Input from the eyes and nose allows reorientation of the head and direction of motion in response to sights and smells.  

In addition to N.V, which we've already considered, the cranial nerves leaving the brainstem at the level of the pons are:

N.VII, the Facial Nerve, which mediates facial movement and sensation [C:385-389].  This is the only cranial nerve leaving the brainstem at the level of the pons, besides N.V, which has a sensory component, and, therefore, the only one with input to the reticular formation.  As far as I know, it is stimulatory.  

N.VI, the Abductans [C:389-391], N.IV, the Trochlear [C:415] and N.III, the Oculomotor [C:426-431], are all involved in eye movement.  They are strictly motor and have no sensory component at all.  Their input comes via secondary neurons from the various sensory nuclei.  These secondary neurons, of course, would provide input, via collaterals, to the RF, but, as far as I know, the motor neurons themselves do not provide RF input.