Nervous System - Part 2 - The Cerebellum
Muscle Spindles:
     Input to the pilot human being within a human being comes largely from proprioceptive sensory organs
which include the neuromuscular spindles distributed throughout the skeletal muscles of the body. The focus
here will be on input from these spindles that feedback information on the relative position of the body's parts
with respect to the environment. They are generally small in relation to the muscle group they monitor and
simulate. They consist of a nuclear bag element and a nuclear chain element in parallel, as shown below.
Because of their dual structure they can be activated by gamma motor neurons independently of the parent
muscles and they can therefore simulate an anticipated action sequence prior to the parent muscles carrying it
out. It is via proprioceptive sensory feedback from muscle spindle simulations that we get a feel for doing an
action before we do it, often just an instant before.
    Since they are attached to the parent muscles the spindles also monitor the relative degree of flex or
relaxation during each action sequence in both the T5E and T5R physical action terms in Steps 2 and 4
respectively of each System 4 Cycle. In Step 3 of each Cycle there is a muscle spindle simulation. Part 1 shows
how they work in accord with System 4 at the spinal level and some further comments are relevant.  
    The nuclear chain element feeds back the amount of change in the related muscle and the nuclear bag
feeds back the rate of change. The nuclear bag element also receives motor input from a collateral branch of
the alpha motor neuron that activates the parent muscle, which keeps the spindle in adjustment with the parent
muscle. But it also receives two other gamma motor inputs which project to these proprioceptive organs from
the ventral horns of the spinal cord. (Some 30% of the motor neurons in the ventral horns of the cord are
gamma motor neurons.)
Diagram of Muscle Spindle redrawn from Cunningham's Textbook of Anatomy.  It does not distinguish between nuclear bag
and nucear chain fibers, nor between modes of motor innervation.
Diagram of Muscle Spindle redrawn from Ganong, W. F., Review of Medical Physiology. Typical nuclear bag and chain fibers
are shown, together with modes of innervation. The enclosing sheath is omitted.
Simulation:
     The motor simulation can originate in the brain stem or at various spinal levels via the reticular system.
The reticular system of the brain stem is a complex nerve net that is associated among other things with
conscious arousal and thus with the recall process. It is interconnected with both ascending and descending
reticular tracts that are multi-synaptic throughout the length of the spinal column. Since reticular tracts are
multi-synaptic and also closely associated with the hypothalamus, they can regulate autonomic energy
patterns at various spinal levels to adjust emotional energies that fuel modified action sequences at various
spinal levels accordingly.
    Since muscles spindles have two independent gamma motor supplies in addition to the alpha motor supply
to the parent muscles they can activate muscle spindles independently of the parent muscle. In general the
static gamma motor supply determines the amount of movement, and the dynamic gamma motor supply
determines the rate of movement. This duel organization allows for simulation since monitoring parent muscle
movement and maintaining muscle tone could be accomplished with a simpler arrangement.
    The parallel processes of the nervous system that synchronously mesh together precisely in accord with
the way that System 4 works are very complex. Before we begin a simplified review of the synchronous
System 4 transformations is in order, since they are quite complex as well.

Review of System 4 Transformations as Outlined in Part 1:
     As illustrated in Part 1, in System 4 there are three Particular Sets of active energies that transform
through a Six Step Sequence of different Terms, each set transforming through the same Term sequence
one Step apart. Particular Terms also have Expressive and Regenerative Modes that interact in the
synchronous matrix of transformations that is regulated and integrated by two Universal Sets in recurrent four
step Cycles. There are thus 12 Steps needed to complete both an expressive and a regenerative sequence.

Chart Summary of Twelve Step Sequence:
     The following chart from Part 1 is reproduced here for easier reference. Expressive and regenerative
particular terms, as well as universal terms, are shown for each sequential Step. Regenerative Terms are
shown in bold. For an explanation of how the Universal Terms work see Part 1.

Keep in mind that in general each System 4 Step corresponds to a synapse in the nervous system.

Step        Set 1        Set 2        Set 3        Set U1       Set U2        Cycle
1             T8E          T7R          T4E          T9             T3               #1
2             T5E          T1R          T2E          T9             T6
3             T7E          T4R          T8E          T8R           T6
4             T1E          T2R          T5R          T8R           T2E
5             T4E          T8E          T7R          T9             T3               #2
6             T2E          T5E          T1R          T9             T6
7             T8E          T7E          T4R          T8R           T6
8             T5R          T1E          T2R          T8R           T2E
9             T7R          T4E          T8E          T9             T3               #3
10           T1R          T2E          T5E          T9             T6
11           T4R          T8E          T7E          T8R           T6
12           T2R          T5R          T1E          T8R           T2E
Gross Anatomy of the Cerebellum:
     The hemispheres of the cerebellar cortex evolved in three major steps, analogous to the way the cerebral
hemispheres evolved. The most ancient is the vestibular cortex that appeared with fish and amphibians. The
spino-cerebellum developed mainly with the reptiles, and the neo-cerebellum expanded bilaterally into the
cerebellar hemispheres, mainly with the mammals.
    There are two homunculi topologically represented in the cerebellum, one being inverted upside down to
the other. One of them is represented in two halves, one for each half of the body. Their most central portions,
concerned with trunk movements, are represented in the middle portion, part of the spino-cerebellum. The
limbs of the homunculi extend bilaterally laterally through the intermediate zones into the cerebro-cerebellum.
This is especially important for the control of fine movements of the hands and fingers.
There are lateral ridges called folia that transverse the whole cortex, and their internal structure is similar
throughout. See the illustrations of the Cerebellum Cortex below. The cerebellum receives widespread input
from throughout the central nervous system. Inputs from different sources project preferentially to specific
areas of the cortex, with lesser projections to more widespread areas in some cases
General Cerebellar Inputs and Outputs(See Diagram of Cerebellar Cortex below):
     There are two general types of input to the cerebellar cortex, either by mossy fibers or climbing fibers.
Mossy fiber inputs arise from throughout the body and cerebral hemispheres. They have many collateral
branches that project to widely distributed areas of the cerebellar cortex. They always synapse with Granule
cells first, which synapse in turn with Purkinje cells which are the sole output from the Cerebellar Cortex.
    Climbing fibers originate only in the inferior olive, a major brain stem structure, and they have a very
specific pattern of projection directly to Purkinje cells. All Purkinje cells have an inhibitory output and they
project back either to four deep nuclei in the cerebellum or, in the case of the vestibular areas of the
cerebellar cortex, to four corresponding vestibular nuclei.
    The inferior Olive receives inputs from the cerebral cortex (indirectly via the red nucleus), nerves of the
face, proprioceptive sensory input from throughout the spinal cord, vestibular input from the vestibular nuclei
and inputs from various other sources. The inferior olive is a fairly complex structure in itself and climbing
fibers project in a highly specific way. Each climbing fiber projects to no more than 10 Purkinje cells in the
cerebellar cortex, and each Purkinje cell receives input from only one climbing fiber, albeit with a great many
synapses.
    Apart from sparse and diffuse inputs from two other minor sources ALL other projections to the cerebellum
arrive via widely branched mossy fibers. There are major projections from the Cerebral Cortex that are also
relayed via one synapse, in Pontine nuclei of the brain-stem, to the cerebellar cortex via mossy fibers.
    In general, both mossy and climbing fiber projections to the cortex of the cerebellum have collateral
projections to the deep cerebellar nuclei, or in the case of the vestibular system to the vestibular nuclei.

Cerebellum Cortex:
     The cortex of the cerebellum has an unusual organization. It has only three 3 layers containing 5 cell
types, and four of them are inhibitory, including output from Purkinje cells, which is the only output from the
cerebellar cortex. This strange situation has suggested to neural biologists that the cerebellum sculpts out a
result, rather than producing it directly by excitation, but it is not known how it works as a whole in a
meaningful way. In the diagram below note the following:
  • There are only five cell types organized in three cortical layers.
  • Spinal sensory and proprioceptive inputs come via widely branched Mossy fibers.
  • Sensory and motor centers of the brain also input via Mossy fibers through a relay in Pontine nuclei.
  • Vestibular inputs come via Mossy fibers.
  • There are also spinal, cerebral and vestibular inputs to the inferior olive.
  • Mossy fibers terminate in large endings that synapse with a number of Granule cells.
  • Granule cells have very long axons that run parallel through the Folia ridges of the cortex.
  • Granule cells synapse with hundreds of Purkinje cells.
  • Purkinje cells are inhibitory and are the sole output of the cerebellum cortex.
  • Climbing fibers originate in the inferior olive and project to no more than ten Purkinje cells.
  • Each Purkinje cell receives input from only one climbing fiber.
  • All cerebellar cortical cells are inhibitory except Granule cells.
  • Granule cells are the most populous in the human body (some 100 billion).

Note: Since the only possible output from the cerebellum is inhibitory it can not be considered as a System 4
Step. It either sculpts away from the pattern of collateral inputs to the vestibular and deep nuclei, modifying
their outputs in this way, or it remains neutral and does nothing. We shall see that this is very important.
    The cerebellar cortical output projects via Purkinje cells back to four deep nuclei within the cerebellum.
Inputs to the cerebellar cortex also send collateral branches to the deep nuclei so that the cortex itself is
bridged by inputs and outputs. The inputs are excitatory, and the outputs are inhibitory, so processes in the
cortex sculpt away conflicting input to the deep nuclei. They are very active centers that reconcile inputs from
parallel particular sets of System 4, Step by Step and synapse by synapse. But first let us review the actions
of each of the five cell types in the cerebellar cortex.

Purkinje Cells:
     Purkinje cells have huge dendritic trees that accommodate hundreds of thousands of synaptic inputs and
yet they have a single inhibitory output to targets in one of the four deep nuclei inside the cerebellum or to
corresponding vestibular nuclei. They also have collateral outputs to Golgi cells that inhibit Granule cells.
    Purkinje cell bodies are located in the middle layer of the cerebellar cortex and the dendritic tree of each
is arranged in a two dimensional plane that extends into the outer Molecular Layer of the cortex. The
dendritic trees are thus stacked like plates in rows. The only direct input they receive from outside the cortex
comes via climbing fibers from the inferior olive. All other synaptic inputs to the Purkinje cells require two
synapses within the cerebellar cortex, the first being mossy fiber projections to Granule cells. Except for
Granule cells, all synapses to Purkinje cells are inhibitory and so is there output.

Granule Cells:
     The most inner layer of the cortex is called the Granular Layer. Granule cells are the most populous in the
human body. The granular layer contains some 100 billion of them, about as many as the rest of the cells in
the entire brain. They are activated by a number of mossy fiber pathways which branch widely to many
Granule cells, even in adjacent folia.
    Granule cells have excitatory outputs that project to the outermost Molecular Layer of the cortex, where
their axons divide into parallel fibers running longitudinally in both directions for long distances across the
stacked rows of dendritic trees of the Purkinje cells. In this way one Granule cell can synapse with hundreds
of Purkinje cells. Also the parallel fibers from Granule cells overlap so one Purkinje cell can receive synapses
from as many as a couple hundred thousand parallel Granule axons. The axons of Granule cells project
longitudinally along the folia of the cerebellum, that is, along the folds that run laterally across the cerebellum.
Since the Purkinje cells are stacked like rows of two dimensional plates across the longitudinal axis of each
folia, this simple arrangement can accommodate an incredible wealth of diverse inputs.   

Stellate, and Basket Cells:
     The remaining three kinds of inter-neurons are all inhibitory, and they all receive inputs from Granule
cells. Stellate cells synapse on the dendrites of Purkinje cells. Basket cells synapse on the cell bodies of
Purkinje cells and thus have a stronger inhibitory influence on this the only cell that produces an output from
the cerebellar cortex. Since their inhibitory action occurs one Step after that of the Granule cell that results in
an inhibitory Purkinje cell output, the Purkinje cell tends to be neutralized in the following Step.

Golgi Cells:
     Golgi inter-neuron cells can receive synaptic input from three sources: (1) from collateral branches of
mossy fibers that project to Granule cells, (2) from collateral branches of Granule cells themselves, and (3)
from collateral outputs from Purkinje cells that synapse on their cell bodies.
    All Golgi cells project to inhibit Granule cells. Their dendrites branch tree-like to occupy a volume of the
outer Molecular Layer of the Cerebellar cortex where they receive input from many Granule parallel fibers.
Their axons also project tree-like into the Granular layer where they can synapse to inhibit  a volume of
Granule cells. Their axons do not synapse with the same pattern of Granule cells as their dendrites however,
since the parallel axon fibers of the Granule cells travel long distances as compared with their dendrites.
    Synapses on Golgi cells can have differently patterned effects, depending upon the source from which
they are activated or inhibited. When they are activated by mossy fibers in parallel with Granule cells, they
feedback most quickly to inhibit a volume of Granule cells, one System 4 Step later. Affected Granule cells in
this case get a chance to fire during only one System 4 synchronous Step, being inhibited in the next Step.
    This can have a synchronous reinforcing effect with basket and stellate inhibition of Purkinje cells as
described above. There will be overlap here in the pattern of Purkinje cells affected but the patterns will not
be identical. Since the stelate and basket cells are activated by parallel granule fibers their input can come
from broad areas on the cortex, whereas Golgi cell activation in this case comes directly from mossy fiber
input, which will be more specific for specific Golgi cells.    
    For example, if we are considering an initial mossy fiber input that originates from spinal proprioceptive
feedback that input may result in an inhibitory output from a Purkinje cell two Steps later. But in the following
Step, which may originate from a cerebral projection via the pontine nuclei the same spinal proprioceptive
input can feed forward, via synapses on Golgi cells, to inhibit the Granule cells involved in the Cerebral
System 4 sequence that follows one Step later. This feed forward from the spinal input will tend to prevent an
inhibitory output from the same or different Purkinje cells in the Cerebral System 4 sequence. This may or
may not reinforce the pattern of activity originating from the cerebral motor cortex, but it does contribute to
reconciliation between Spinal and Cerebral inputs in those areas of the cerebellar cortex where they overlap.
    When Golgi cells are activated by a volume of parallel fibers from Granule cells they feed back from broad
areas of the cortex to inhibit a related but smaller patterned volume of Granule cells one System 4 Step later.
(Because their dendritic trees synapse with parallel Granule fibers and their axon trees project to a more
limited volume of Granule cells.) In some affected Granule cells this can also have a synchronous reinforcing
effect with stellate and basket cells as above, but there can be mutual overlaps and exclusions in the pattern
of projections from different Golgi cells back to different Granule cells. These overlaps and exclusions can
further facilitate the mutual reconciliation of divergent inputs originating from the  Cerebral Hemispheres and
from sensory and proprioceptive spinal feedback. This feedback also functions as a feed forward mechanism,
since a Cerebral pattern of Granule cell activity can influence a Spinal pattern that follows one Step later, and
vice-versa.
    When Golgi cells receive a strong collateral inhibitory projection from Purkinje cells on their cell bodies,
they are strongly inhibited from acting, and the volume of Granule cells they synapse with are thus not
inhibited for that Step which is either one or two Steps after the initial pattern of synapses in the cerebellar
cortex depending whether that input came via mossy or climbing fibers. This inhibitory influence on Golgi cells
is more selective, since Purkinje output is more selective. This can fine tune the reconciliation between inputs
from the cerebral hemispheres with those from the spinal column and vestibular system both by mossy and
climbing fiber inputs.
    With these introductory formalities dispensed with we will proceed to the meat of the business.
Mossy Fiber Inputs:
(Adapted from Nieuwenhuys.Voogd.van Huijzen The Human Central Nervous System, Springer Verlag 1990)
Illustration adapted from J. H Martin, Neuroanatomy, Elsevier 1991.
Adapted from R.M. Berne, M.N. Levy, Physiology, Mosby-Year Book Inc., 1993.
(Adapted from Nieuwenhuys.Voogd.van Huijzen The Human Central Nervous System, Springer Verlag 1990)
INTRODUCTION:
     The cerebellum is widely interconnected with the central nervous system and is primarily concerned with the
integration of vestibular, spinal and cerebral inputs by a variety of pathways. While the synaptic
interconnections of these various pathways have been diligently mapped to a good degree, an understanding
of how they work together as an integrated whole has remained elusive. The pieces of the jigsaw puzzle are all
laid out on the table but without the picture on the cover of the box one hardly knows where to begin to put
them together. In what follows it will be shown that System 4 can provide an overall picture as to how they fit. It
can offer a solution to this vexing and complex challenge.
    First we will review from Part 1 some observations on how the proprioceptive nervous system works at the
spinal level via input from muscle spindles, since this is very relevant. Next we will explore the structure of the
cerebellum with some general observations on how it works within itself. Then we will explore the main pathways
to and from the cerebellum. In Part 1 it was shown how System 4 works at the spinal level. Here in Part 2 we will
see how spinal, vestibular and cerebral processes are mutually reconciled by the cerebellum.
    The cerebellum can be regarded as something of a pilot that navigates the immensely complex jungle of  
interconnected neural pathways within us according to our over-riding instructions from the control tower.

Note: A review of System 4 Terms will be very helpful in what follows.
The Human Nervous System - Part 2 - The Cerebellum
By Robert Campbell 2006
General Meanings of Each Term:
     For simplicity the Universal Terms will not be considered below. There are Cycles within Cycles and the
descriptions in Part 1 should be sufficient to give an overall idea. Note that meanings must be interpreted in
context. They are listed here as they apply to sensory and motor relationships at the spinal level. In their
projections to brain stem, cerebellar and cerebral levels they continue to represent these direct spinal
characteristics but obviously the representations do not relate directly to the environment. The
representations become neural abstractions of spinal processes. See Part 1 for more information on how
these meanings are generated by System 4.
    T1 - Perception of need in relation to response capacity.
    T4 - Ordered sensory input alternately from the environment and simulated.
    T2 - Creation of idea as a potential action response or creative concept.
    T8 - Balanced response to sensory input stimuli as a motor output to muscles.
    T5 - Action sequence of muscular activity with proprioceptive feedback.
    T7 - Sequence encoded as a unit memory simultaneous with recall to T1 and another sequence.
    With this background knowledge of System 4 clearly in mind, it is possible to follow through the neural
circuitry involved in the integration of behavior synapse by synapse. The main focus here is on how the
cerebellum works to reconcile spinal behavior with vestibular input and conscious cerebral intention. With
sufficient patient study the meaningful operation of the nervous system is rendered transparent. The
illustrations of neural anatomy are essential to understanding the text.
MAIN PATHWAYS TO AND FROM THE CEREBELLUM:

     The main pathways to and from the cerebellum will be explored in three main sections as listed below. The
first area to investigate is the vestibular system which can function directly via parallel routes that do not involve
the cerebellum, so we will investigate this first then proceed to routes that do involve the cerebellum.

  • The Vestibular System:
    We will begin with the vestibular system essential to balance which also implicates the visual sense and
    proprioceptive spinal input. We will consider it via the various pathways involved in the following:
  1.  Vestibular integration with eye, head and body movement.
  2.  Vestibular projections to the cerebral hemispheres.
  3.  Vestibular integration via the cerebellum.
  1. Direct projection by-passing the vestibular nuclei
  2. Projection via the vestibular nuclei.

  • Pathways to the Cerebellum via the Inferior Olive:
  1. Pathways originating in the spinal levels.
  2. Pathways originating in the vestibular system.

  • Cerebral pathways to and from the Cerebellum

THE VESTIBULAR SYSTEM

Primary Sensory Organs:
     There are two types of hair cells in the semi-circular canals of the ear that have active membrane potentials.
They discharge in the same way as a neuron but in response to fluid motion in the semi-circular canals. The
two types are similar in structure to the two types of cochlear hair cells involved in hearing. In these vestibular
sensory organs most of the output comes from Type 1 cells with intermittent output from Type 2 cells. Both
types of cells discharge across a synapse to primary neurons that project to the four vestibular nuclei with
collateral branches that project directly to the ancient vestibular areas of the cerebellum.
    The collateral branches of the primary neurons synapse directly with Granule cells in the vestibular areas of
the Cerebellar Cortex called the nodulus and flocculous. They also project to the Fastigial Nucleus of the
cerebellum and to the Nucleus Cuneatus. The latter nucleus relays spinal proprioceptive sensory feedback for
the upper body via a synapse in the thalamus to the Primary Sensory Cerebral Cortex, so direct vestibular
input can influence this. (Vestibular primary neurons correspond to spinal secondary neurons. Hair cells work the same as neurons.)  
     Both types of hair cells receive efferent projections via the eighth cranial nerve that arise in the reticular
formation. In Type 1 cells the efferent axons synapse with the primary afferent nerve endings that engulf the
body of each hair cell like a cup. In Type 2 cells the afferent nerve endings synapse directly on the body of the
hair cells and activate them directly. This direct activation indicates relative movement of some kind that is more
than an automatic reaction to fluid motion in the semicircular canals.
    The relative function of the two types of hair cells is not well understood and this structure suggests that
together they have a regenerative and an expressive role. There is evidence that efferent projections to the
afferent endings on Type 1 hair cells stalls  their activation while the opposite is true on Type 2 hair cells. This
can facilitate alternate expressive and regenerative sequences of System 4. We know from personal
experience, for example, that when we struggle to correct for sensations of vertigo or imbalance that there is an
anticipated, although tentative, sensation of proper balance that guides our efforts at corrective action. This
anticipated sense of balance is evidence of a regenerative input. Without it the body is left victim to blind
reactionary forces. (See the diagrams below:)
In both types of hair cells when the stereocilia are bent
towards the kinocilium the hair cell is depolarized and the
afferent fiber is excited. When the stereocilia are bent
away from the kinocilium the hair cell is hyper-polarized
and the afferent discharge slows or stops. These
structures together with the structure of the kinocilium
below suggest that the kinocilium of the Type 2 hair cells
can move with respect to the stereocilia independently of
fluid motion in the semi-circular canals to simulate a
regenerative sensory afferent output designated as T4R.
Illustrations adapted from R.M. Berne, M.N. Levy,
Physiology, Mosby-Year Book Inc., 1993.
The kinocilium:
Vestibular hair cells differ from auditory hair cells in that each one has one kinocilium in addition to a number of
stereocilia. The kinocilium is a true cilium made up of nine sets of two microtubues arranged in a circle, with an
additional pair in the center. Whether or not the hair cell depolarizes and fires depends upon the relative
position between the stereocilia and the kinocilium. The kinocilium is similar in structure to the motile cilia that
mobilize protists. It is thus reasonable to expect that it can move independently of the stereorcilia to regulate
expressive and regenerative modes of firing. It should further be noted that System 4 subsumes System 5,
which consists of two sets of nine terms, one regenerative and one expressive, with two primary universal terms
regulating the alternations between them. (For example the two divisions of the autonomic nervous system
correspond to an expressive sympathetic mode and a regenerative parasympathetic mode, each with
expressive and regenerative sequences reciprocating within them, consistent with System 5. System 5
elaborates on System 4.)
The above diagram illustrates the structure of a cilium. They occur widely in many applications from clearing
the lungs of inhaled debris to propelling single celled protists. Note that in this diagram of a cilium that propels
a one-celled animal the cilium corresponds to a System 5 structure while the basal body corresponds to the
three cycles of System 4 which subsume System 5. The basal body is similar in structure to the centrioles that
regulate cell division in eukaryotic cells.
Adapted from Life:The Science of Biology, W.K. Purves et al, Sinauer Associates,
W.H Freeman, 1998
The Regenerative & Expressive Sequences of System 4:
     Efferent projections to the hair cells initiates in the reticular formation that is concerned among other things
with conscious arousal. Arousal in this context can be synonymous either with the regenerative recall term T7R
or the expressive recall term T7E. The efferent regenerative projection in the following System 4 Step is T1R
which represents a pattern of motor simulation which projects to the hair cells. Type 1 cells may be
synchronously stalled by this projection while the synapse to Type 2 cells initiates a simulation of corrective
balance in a T4R term (analogous to the regenerative simulations in muscle spindles). The relationship
between the two hair cell projections represents the regenerative corrective balance. The efferent expressive
projection to hair cells T1E assesses the tonus and membrane potentials for a conditioned pattern of T4E
sensory input that follows. We thus have a situation where T4E terms alternate with T4R Terms in the
relationship between the two hair cell types, consistent with how System 4 works.
The Four Vestibular Nuclei:
    The four nuclei are called superior, lateral, medial and inferior. The nuclei receive sensory and proprioceptive
feedback from all spinal levels in addition to primary afferent input from hair cells and feedback from the
cerebellum. The afferent input from hair cells projects most densely to the superior, medial, and inferior nuclei,
which are reciprocally related by interneurons and also with their counterparts on the other side of the body.
    The lateral nucleus gives rise to the uncrossed lateral vestibular spinal tract that influences body movement at
all spinal levels. The lateral, medial and superior nuclei initiate the main projections via the thalamus to the primary
sensory cortex of the cerebrum to provide a conscious sensation of balance. The superior, medial and inferior
nuclei project bilaterally via the Medial Longitudinal Fasciculus (MLF) to the nerve centers that control eye
movement and the muscles of the neck and upper back. The muscles of the neck and upper back are also
supplied by the lateral vestibular spinal tract so there are inputs from two vestibular nuclei sources. The muscles of
the neck are also richly represented with muscle spindles for proprioceptive feedback to the vestibular nuclei.
    The vestibular nuclei also project directly to the cerebellum as well as via a collateral synapse in the inferior
olive to the cerebellum.  

Three Main Vestibular Outputs:
     The vestibular nuclei project to three main destinations:
    A.   The spinal cord for influencing body, head, and eye movement.
    B.   The sensory cerebral cortex via the thalamus.
    C.   The cerebellum, both directly and via the inferior olive.

       Some primary neurons excited by hair cells project directly to the vestibular areas of the cerebellum. These
mossy fiber inputs to the cerebellar cortex have collateral branches to the Fastigial nucleus, one of four deep
nuclei in the cerebellum that are analogous to the four vestibular nuclei. Two synapses later Purkinje cells in the
cortex project a pattern of inhibition back to the Fastigial nucleus. Vestibular inputs to the cerebellum thus bridge
the cerebellar cortex as do inputs from other sources.
    Purkinje cells of the vestibular cerebellum and vermal spinocerebellum project to the lateral vestibular nucleus.
They also project to the Fastigial nucleus which in turn projects to the lateral vestibular nucleus as well as to the
pontine reticular formation. These outputs thus influence axial and proximal limb muscles by way of the lateral
vestibulospinal and pontine reticulospinal tracts of the medial motor projection system.
A. Direct Vestibular Integration of Body, Head, and Eye Movements:
     The direct vestibular pathways that integrate head, body and eye movements do not involve the cerebellum.
The System 4 particular sequence terms correlate Step by Step with synapses as outlined below.

Term     Cycle             Neural pathway                                
T1E                      In Step 4 of this previous cycle there is an initial assessment of the preparedness of hair cell        
                           and muscle action potentials and tone. This can affect the selection of the pattern of cell               
                           activation that follows.
T4E     #1             Hair cells project to primary afferents of the vestibular system. Muscle spindles monitor parent     
                            muscle action and project to relay nuclei in the Central Nervous System (CNS).
T2E                      Primary afferents project from hair cells to vestibular nuclei as Idea Terms. Proprioceptive relay   
                            nuclei in the CNS also project as Idea Terms to the vestibular nuclei from all spinal levels.             
T8E                      Parallel motor outputs project to eye, neck, and spinal motor nuclei.        
T5R                      The motor output to eye & neck muscles is reconciled with other inputs to these motor nuclei.      
                            The eyes may move to compensate for head movement or move separately. This output is           
                             regenerative since it is governed by connections established through prior learning of the           
                             individual and the species. The motor output to the neck and eyes is via the Medial Longitudinal
                             Fasciculus (MLF). The motor output to the ventral motor horns of the spinal column is via the      
                             lateral vestibulospinal tract.                
T7R     #2             Memory of the sequence is stored in dendritic and synaptic protein synthesis of cells involved,     
                            as well as the Void. The synaptic connections evolve consistent with learning experience. At the   
                            same time related regenerative memories are spontaneously recalled in the reticular formation.   
                            This activates efferent projections to hair cells and muscle spindles.
T1R                      The regenerative efferent projections to hair cells and muscle spindles initiate a patterned motor
                             simulation. In the projection to muscle spindles this takes place in gamma motor neurons that      
                             project to the spindles.
T4R                       An actual simulation takes place in hair cells and in neck and body muscle spindles. The hair      
                            cell simulation involves the active movement of the kinocilium in the Type 2 hair cell with respect  
                            to the current state of the Type 1 hair cells. The muscle spindle simulation likewise simulates a     
                            pattern of movement with respect to the current position of muscles.
T2R                      The simulations project as regenerative vestibular input and likewise corrective proprioceptive     
                             feedback to vestibular nuclei as a regenerative idea.
T8E     #3             The simulations result in a synapse in the vestibular nuclei to a pattern of planned motor output   
                            to eye, neck and body muscles via the MLF and the lateral vestibulospinal tracts as in the T8E     
                            Term above. All Particular Set T8 Terms are expressive. (See Part 1)
T5E                      The output of vestibular nuclei to eye, neck and body spinal motor nuclei for corrective action is  
                             reconciled with other inputs to these nuclei as they project to muscles. This output is expressive  
                             since it is creatively planned by proprioceptive input. It is active electronic processes that direct   
                             cell processes whereas in the regenerative sequence it is conditioned cell interconnections         
                             established through experience that direct the electronic processes. (See Part 1)
T7E                       Memory of the action sequence is stored and expressive memories recalled similar to T7R          
                             above.
T1E                      There is a perception of the capacity to respond to ongoing vestibular needs. This involves the   
                             recovery of cells processes such as action potentials and the stabilization of fluid in                     
                             semi-circular canals, preparatory for the next action sequence.  

    Keep in mind that muscle action is organized in the Central Nervous System (CNS). But sensory input, motor
response in muscles and related proprioceptive feedback at the spinal level are functions of the Peripheral
Nervous System (PNS).  In the context of spinal reflexes the motor projection from the ventral cord is a T8E term
and the T5 term follows in the muscular actions that produce proprioceptive feedback in the process. The CNS
projection to the final motor centers such as the ventral horns of the cord is a T8E term in the CNS context and the
T5 term follows as the projection to muscles, not in the actions of muscles themselves. The projection to muscles
is the final T5 leg of the CNS action sequence, whereas muscle action itself is the final T5 term leg in local action
sequences, such as eye movements and spinal reflexes.
    This arrangement allows for the integration of multiple motor inputs in the final motor nuclei in the CNS that
project to muscles. If it was not this way reconciliation and integration of conflicting motor inputs would not be
possible. For example conscious motor projections from the primary motor cortex of the cerebral hemispheres
would not be able to override locally automated spinal responses in the cord that are not consciously determined.
As it happens the cerebral motor projection to the ventral spinal motor area is a T8E motor term that is one
sequence Step ahead of the spinal interneuron Idea Term T2 that also projects to the ventral motor horn. The
cerebral T8E term can thus activate designated motor neurons in the ventral horn preferentially to spinal input, but
it comes later. It takes one four Step Cycle to make a transition from one action sequence to another. The
structure of the nervous system has evolved to function in accord with System 4 in just this way.
    In conjunction with other inputs, body movement at all spinal levels is integrated via the lateral vestibular spinal
tract in parallel with neck and head movements as shown above. There are three Particular Sets involved in the
vestibular coordination of head, body and eye movements. They integrate past and future as usual. The universal
sets need not be considered in detail here since they work the same as at the spinal level illustrated in Part 1. It all
meshes together very neatly consistent with our phenomenal experience.
B. Vestibular Integration with the Sensory and Motor Areas of the Cerebral Cortex:
     Vestibular projections to the cerebral cortex are still imperfectly known, however scattered cells in the medial,
lateral, and superior vestibular nuclei project via a synapse in the thalamus to the primary sensory cortex of the
cerebral hemispheres and to a second adjacent sensory association area behind it in the parietal lobe. While the
projections in Section A above from the vestibular nuclei are motor projections T8E, it has been suggested in
some texts that these parallel projections to the sensory cortex should be considered sensory.
    They are projections of motor action patterns regulating balance that must be consciously integrated with
sensory input from all sources in working out an integrated planned System 4 Idea Term T2. This consciously
planned Idea is topologically formulated in the primary sensory cortex. It projects in turn to the primary motor
cortex where final motor instructions are topologically integrated in the T8E terms that project to the ventral horns
of the spinal column where they synapse with the T5 terms that project out to the peripheral skeletal muscles.
    In fact the primary sensory cortex has motor characteristics, just as the primary motor cortex has sensory
characteristics, as was emphasized by C.N. Woolsey, one of the early investigators of the motor and sensory
areas. It is known from electrical stimulation studies that there is a sensory core with a wider surround throughout
the sensory cortex, and a motor core with a wider surround throughout the motor cortex. Motor responses are
primary in motor areas and sensory responses in sensory areas, but each has characteristics of the other.
(Published in Cerebral Localization and Organization, G. Shaltenbrand & C.N. Woolsey, Eds., University of Wisconsin Press, 1964.)
     The vestibular motor characteristics that project to integrate head, body and eye movements thus relay this
information to the thalamus via the T8E term and on to the primary sensory cortex via the T5 term. The billions of
microscopic nerve cells of the cerebral cortex are arranged in six layers with complex interconnections between
them that are nevertheless suggestive of the six term sequence of the Particular Set transformations of Systems 4
and higher. It is these subsumed cortical processes that generate the coherent Idea plan T2 of the Sensory
Cortex, taking account of all inputs.
    The way System 4 works, T2 has a polar relationship with the resources of memory and recall, T7, which is the
next System 4 Term following the T5 vestibular projection to the Primary Sensory Cortex. The recall process is
obviously coupled to sensory input in a preceding sequence in order that we can subsequently respond
appropriately to circumstance as it is presented to us. This requires that the T7 synapse is synchronous with the
projection pattern of T4 synapses to the sensory cortex that results in the following T2 projection to the Primary
Motor Cortex. Terms 8, 7 & 4 always occur together but in different Particular Sets. This has implications in the
subsumed cortical processes where the vestibular T5 terms synapse. Idea development T2 is also tensionally
coupled with the physical action term T5 in the active matrix of System 4 transformations. Terms 1, 2 & 5 always
occur together in different Particular Sets.
    Later we will come back to the reciprocal relationships between the cerebral hemispheres and the cerebellum
that intermesh with these vestibular inputs. For the present it is relevant that the four vestibular nuclei play an
analogous role to the four deep nuclei of the cerebellum.
Adapted from Life:The Science of Biology, W.K. Purves et al, Sinauer Associates, W.H Freeman, 1998
C. Vestibular Projections to the Cerebellum:
     Vestibular inputs to the vestibular areas of the cerebellum follow two general routes as follows:
  1. Via primary afferents from hair cells, bypassing the vestibular nuclei, mossy fibers project directly to
    granule cells in the flocculo-nodular areas of the cerebellar cortex. Collateral branches project to the
    Fastigial cerebellar nucleus. This nucleus projects to the descending medial longitudinal motor tract
    via the reticular formation. It also projects via the thalamus to the premotor, primary motor, and
    parietal areas of the cerebral cortex as well as to the basal ganglia.
  2. Primary afferents from hair cells project to the lateral, superior, medial, and inferior vestibular nuclei.
    They project in turn via mossy fibers to granular cells in the vestibular areas of the cerebellum both
    directly and via a synapse in the inferior olive. The projections from the inferior olive are climbing
    fibers that synapse directly with Purkinje cells and these will be considered later.

C-1.) Vestibular Projections Bypassing Vestibular Nuclei:
     Since it is known that the cerebellum is concerned with integrating vestibular function and conscious motor
control with spinal proprioception, some parallels with spinal proprioception may be expected.
    Proprioceptive sensory feedback from muscle spindles projects into the dorsal sensory horns of the spinal
cord but these neurons also send collateral branches to the ventral motor horns, thus bridging interneuronal
connections within the spinal cord. System 4 indicates that the collateral projection to the motor horn designates
a motor target pattern that is anticipated. (See Part 1) It readies these motor neurons to fire when and if the
interneurons make the appropriate connections between sensory input from various sources and motor output.
They are also primed  to fire depending on a compatible motor pattern from higher centers including the cerebral
cortex. The degree to which the actual motor pattern that transmits to muscles matches the anticipated target
pattern thus depends upon synchronous sequences of System 4. These are transmitted as Idea Terms T2 via
interneurons in the cord to T8E motor neurons in the ventral cord, as well as by descending T8E motor patterns
from higher brain centers. All these motor inputs must be mutually reconciled in the ventral cord.
    The point here is that vestibular input from hairs cells that bridges both the vestibular nuclei and the
cerebellar cortex with collateral projections to the Fastigial Nucleus and vestibular nuclei can serve to prime a
pattern of neurons in readiness to fire depending upon other Fastigial or vestibular inputs. Some of these inputs
may come in System 4 Steps that precede or follow in synchronous sequences of System 4. We shall see below
that there is an inverse pattern to the way this works in the cerebellum, since Purkinje cells are inhibitory. The
Fastigial and vestibular nuclei play similar roles but with different patterns of projection.                 

Note: A very important point is that the cortex of the cerebellum can not be considered as entertaining System 4
Steps at all, since the only active output from it is inhibitory. This is an ingenious invention since it facilitates the
integration of inputs from many sources. It does not say what to do. It selects from its various inputs and from
them chooses what not to do. It either has no output at all from Purkinje cells, remaining neutral, or it has
inhibitory output.

     To simplify an otherwise complex description it will be assumed that the direct projection of primary vestibular
neurons to the cortex of the cerebellum is a regenerative sequence. This route by-passes the vestibular nuclei. It
has a collateral branch to the Fastigial nucleus, and of course there is a parallel pathway  that projects to the
vestibular nuclei. There are thus two parallel pathways to the cerebellum and both pathways can have expressive
and regenerative sequences. In the descriptions to follow in 3a) and 3b) the Fastigial and vestibular nuclei may
be interchangeable except that they project to different target patterns. The Fastigial nucleus will be used for the
regenerative sequence in 3a) and the vestibular nuclei will be used in the expressive sequence 3b). There will be
more on this later to clarify the expressive and regenerative pathways.  
    With these points in mind the System 4 sequence proceeds as follows: (Remember that there are three
Particular Sets involved and they follow the same sequence but starting in different Cycles. See Part 1)

  • T7R – In Step 1 there is a pattern of regenerative recall T7R in reticular formation arousal.
  • T1R – T7R initiates a motor simulation T1R in efferent neurons projecting to hair cells in Step 2
  • T4R – Type 2 hair cells simulate corrective action with respect to Type 1 hair cells, as T4R.  
  • T2R – Primary vestibular neurons project as T2R in Step 4 directly to Cerebellar Granule cells with           
    collaterals to the Fastigial Cerebellar nucleus.
  • T8E – The Fastigial nucleus reconciles inputs from other sources in motor projections ascending to the
    thalamus and descending to the reticular formation with a collateral projection back to vestibular nuclei.
    The Purkinje cells of the cerebellar cortex make inhibitory projections back to the Fastigial nucleus two
    Steps later. They thus integrate expressive and regenerative sequences from hair cells (since they
    alternate every other Step) along with other Fastigial inputs.
  • T5E – Motor projection proceeds from a synapse in the thalamus to motor and pre-motor areas of the
    cerebral cortex, as well as to the parietal lobe and the basal ganglia. Descending motor projections
    proceed from a synapse in the reticular formation to spinal levels via multi-synaptic tracts.
  • T7E – The vestibular and cerebellar sequence Steps are recorded in memory both in the Void and as
    protein synthesis that tends to modify the dendrite and axon connections of related cells accordingly, as an
    expressive sequence follows as outlined below.  

    Since the integrated pattern of motor output from the Cerebellar cortex to the Fastigial nucleus is only by
inhibitory Purkinje cells, this output effectively carves away from the pattern of collateral T2E inputs from the hair
cells two Steps later (assuming that both expressive and regenerative modes follow this pathway). The remaining
T2R inputs, that are not inhibited by an earlier T2E projection, initiate the modified T8E outputs of the Fastigial
nuclei to both descending spinal and ascending cerebral motor projections.
    The ascending projection is by a one synapse pathway to arrive at cerebral motor centers as a T5E motor
term. The descending projections are reticulated as a series of T8E motor projections until their final synapse on
the motor horn cells of the spinal column. There they synapse with motor neurons that represent the final leg of
T8E projection to the skeletal muscles. The descending T8E projections are a Step later in the System 4
sequence and tend to assume priority over the T2 spinal interneuron projections from the dorsal horn, but they
arrive in later sequences accounting for a delay in rectifying balance.  
    It should be noted that under normal circumstances that the influence of the vestibular regenerative mode
may be minimal unless one is a gymnast or jumping around a lot.
C-2.) Vestibular Projections via the Vestibular Nuclei:
     Consistent with comments above, for the sake of simplicity, for the time being let us assume that the primary
projections to the vestibular nuclei are an expressive mode of System 4 that alternates with the regenerative
mode in 3a) above. If there is a parallel regenerative projection to the vestibular nuclei it is easy enough to
follow by simply substituting the vestibular nuclei for the Fastigial nucleus in the description above. We will
come back to this later. As mentioned the general pattern is analogous to the way the expressive and
regenerative modes work via muscle spindles at the spinal level. Later we shall see that there are
complementary pathways via the inferior olive.
    Allowing alternating sequences in hair cells as well as for one synapse in the vestibular nuclei, this means
that there is an expressive sequence via the vestibular nuclei alternating with a direct regenerative sequence
projecting to the cerebellum every two Steps. Since the projection via granule cells to inhibitory Purkinje
feedback to the Fastigial nucleus takes two Steps this facilitates the reconciliation of the expressive and
regenerative modes. Purkinje inhibition from an expressive sequence modifies a regenerative pattern and vice
versa. It moderates the transitions between modes into smooth patterns of motor projection. In the case of a
parallel regenerative sequence via the vestibular nuclei it works in an identical way except that the Purkinje
projection is back to the vestibular nuclei rather than to the Fastigial nucleus.
    The vestibular nuclei play a similar role to the Fastigial nucleus in this expressive sequence as compared
with the regenerative sequence. There are various alternatives possible in some portions of the vestibular
system as opposed to other portions, affecting some motor projections in different ways than others, although
they all conform to the way System 4 works. This can direct experimental work to investigate the possible
expressive and regenerative alternatives. For now we will focus only on the expressive mode through the
vestibular nuclei as follows:    

  • T7E – Patterns of expressive recall T7E arouse in the reticular formation. (Step 3 of a Cycle)
  • T1E – Preparedness of cells to respond to needs by efferent projections to hair cells in Step 4.
  • T4E – Expressive sensory input from hair cells synapse with primary neurons in Step 1.
  • T2E – Primary neurons project mostly to the superior, medial and inferior vestibular nuclei, but also to
    the lateral nucleus in Step 2. The lateral nucleus also receives spinal T2 proprioceptive sensory inputs
    from the spinal dorsal horns.
  • T8E – Motor outputs from the lateral nucleus project via the lateral vestibulospinal tract to the motor
    areas at all spinal levels. The superior, medial and inferior nuclei project via the medial longitudinal
    fasciculus (MLF) to eye and neck motor nuclei. Motor outputs project to the inferior olive and also via
    mossy fibers to the cerebellar cortex. Both of these latter projections to the cerebellum result in a pattern
    of Purkinje inhibitory feedback two steps later. This reconciles expressive and regenerative motor
    projections from the vestibular nuclei,since they alternate every two System 4 Steps.
  • T5R – The motor projections T8E via the lateral vestibulospinal tract synapse in the spinal motor horns
    throughout the cord. As before the T8E reticulated motor pattern projections coincide with T2
    interneuron inputs to the motor cells in the ventral horns. These motor cells must reconcile these inputs
    from different sources  that project as T8E Terms in the final spinal motor projection to the skeletal
    muscles. This is the same as with the descending cerebral motor projection T8E described for the
    regenerative sequence above, but it follows two Steps later. Direct Motor projections from the Primary
    Motor Cortex of the cerebral hemispheres also project to the ventral horns as T8E Terms. Both have
    priority over the T2 spinal interneuron projection from the dorsal horn as pointed out above. We may
    stumble when walking but vestibular balance will tend to prevail over the next automated step and
    conscious intension quickly follows in a later sequence.  
  • T7R – The vestibular and cerebellar sequence Steps are recorded in memory both in the Void and as
    protein synthesis in the dendrite and axon connections of related sequence cells, as a regenerative
    sequence follows as outlined above. Learning through experience elaborates on synaptic connections
    that reflect our remembered ability to perform in specific patterns.

    The pattern of inhibitory output of Purkinje cells back to the vestibular nuclei is thus similar to the
regenerative sequence where they project back to the Fastigial nucleus. The inhibitory Purkinje output likewise
carves away from the T2E projections originating from hair cells and motor spindles alike to the vestibular
nuclei in the expressive sequence Steps just as it does in the regenerative sequence Steps to the Fastigial
nucleus. Purkinje cells in part of the Vermis area of the cerebellum project back to the lateral vestibular nucleus
that gives rise to the lateral vestibulospinal motor tract that projects as a T8E Term to all spinal levels. Purkinje
cells in the nodulus and floccuus areas of the cerebellar cortex project to back to the superior, medial and
inferior vestibular nuclei. Together these nuclei project as T8E terms to both the ascending and descending
medial longitudinal tracts (MLF) as outlined in Section A above.
THE INFERIOR OLIVE - CEREBELLUM CONNECTIONS:

Projections to the Inferior Olive:
The inferior olive is a major structure in the brain stem that receives inputs from the following two main sources:
  1. Collaterals of spinal somatic and proprioceptive sensory input from all levels. These all initiate in the PNS
    as T4 sensory input Terms of System 4 and project as T2 idea Terms to the inferior olive via a single
    synapse.
  2. Collaterals from the superior, medial and inferior vestibular nuclei arise as T4 sensory Terms from hair
    cells that project as T2 idea Terms to vestibular nuclei. They then project as T8E motor terms to the
    inferior olive.

Climbing Fiber Projections from Inferior Olive to Purkinje Cells & Deep Nuclei:
The inferior olive projects via climbing fibers directly to Purkinje cells in nearly all areas of the cerebellar cortex
with collateral projections to all four deep cerebellar nuclei. However it projects preferentially from specific olive
nuclei to the Vermis cortex, the Intermediate cortex, and the Cerebrocerebellar cortex. The respective collateral
branches project to the Fastigial nucleus, the Globus & Emboliform nuclei, and the Dentate nucleus. Since
Purkinje cells project back to these nuclei the cortex is bridged in a single Step as compared to two Steps via
mossy fibers.

1. Spinal System 4 Steps via Inferior Olive:
     Sensory input from the spinal level comes once in each System 4 Step of each Cycle from one of the three
Particular Sets. (See Chart above or Part 1.). Sensory input from the head does not use the spinal nerves but it
follows analogous pathways. In Step 1 of each Cycle it is always Somatic sensory input associated with touch,
pain, temperature. Proprioceptive feedback from muscle spindles comes in Steps 2, 3, & 4 of each spinal Cycle
and the nature of its origin is determined only by which Step it is in. For example proprioceptive simulation of an
action sequence always takes place in Step 3 of each Cycle. In Step 2 input from muscle spindles is generated
by an expressive muscle action sequence and in Step 4 by a regenerative muscle action sequence, but it still
comes from the same muscle spindles. These latter two are also sensory inputs and so far as higher centers are
concerned they are represented by T4 Terms, whereas at the spinal level they are represented by a specific
Relation Whole in the T5 Term providing direct sensory feedback that monitors muscle action in this spinal
context. (See Part 1) Although the specific expressive or regenerative nature of the proprioceptive sensory inputs
to the inferior olive may not seem very significant, in a later section we shall see that they are. In this section, it is
very significant with respect to the Fastigial nucleus as pointed out below. With these observations in mind the
sensory inputs to the inferior olive corresponds to the following sequence Steps.

  • T1 – Restoration of action potentials T1E occurs in Step 4 of each Cycle in readiness for a new sequence
    pattern at the beginning of the next Cycle. This is synchronous in both the spinal and vestibular pathways.
    In Step 2 of each Cycle in one Particular Set there is a Gamma T1R motor simulation projecting to muscles
    spindles (See Part 1). In the same Step there is also an efferent motor simulation projecting to hair cells.
    Both simulations arise as a pattern of spontaneous recall in the reticular system. We will look at the
    vestibular-olive projections in Section 2 below.   
  • T4 – In Step 1 of each Cycle primary somatic input from the PNS synapses in the CNS. In Steps 2, 3, & 4,
    proprioceptive sensory neuron project from muscle spindles in the PNS to a synapse in the CNS.
  • T2 – Collaterals of all secondary sensory neurons from the first CNS synapse, project to the inferior olive in
    each Step of each Cycle. Note that secondary neurons from spinal levels also project via mossy fibers
    directly to the cerebellar cortex with collaterals to the deep nuclei in each Step, but one Step ahead of
    projections to the cerebellar cortex via the Olive route. This is very significant, since Purkinje inhibitory
    feedback is very specific and comes in one Step via the climbing fiber route and in two Steps via the mossy
    fiber route which is more widely dispersed. This means that the respective patterns of Purkinje cell
    inhibition back to the deep nuclei are synchronous between mossy and climbing fibers. They arrive back at