UNIT 9 - Postural Control System
A. Structure
The postural control system, in total, is comprised of the archicerebellum/vestibulocerebellum, the reticular formation, and the structures of the vestibular system. The vestibular system consists of four pairs of vestibular nuclei, the vestibular portion of the eighth cranial nerve, the utricle, saccula, and semicricular canals, the medial and lateral vestibulospinal tracts (MVS, LVS), and the ascending medial longitudinal fasciculus. The descending portion of the medial longitudinal fasciculus is also referred to as the medial vestibulospinal tract. The four pairs of vestibular nuclei are the superior, inferior, medial, and lateral vestibular nuclei. The vestibular nuclei are located in the floor of the fourth ventricle (or the rhomboid fossa) at the junction of the pons and the medulla, between the metencephalon and the myelencephalon. In addition, the medial and lateral reticulospinal tracts also deal with postural control. Afferently this system is primarily dependent upon vestibular, proprioceptive, and visual input.
B. Function
The postural control system has many functions.
It is concerned with:
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Learned reflex activity which is compensatory in nature, to regain our balance and equilibrium when lost.
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Learned muscular responses, which are automatic in nature, used to prevent loss of balance or stability. This is the proactive component of the postural control mechanism.
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Occular control to allow for visual fixation within our environment so as to maintain or regain a vertical position in space.
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The ability to influence muscle tone in response to vestibular input during a high risk activity to ensure maximal stability.
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It has the capacity to influence sympathetic and parasympathetic control over the autonomic nervous system via vestibular input through the connections of the vestibular nuclei, the reticular formation, and the hypothalamus.
NOTE: As covered in chapter 8, the paleo-spinocerebellum can also influence autonomic activity specifically; Cardiorespiratory function and urinary bladder responses through its connections with the reticular formation and hypothalamus, as well as pupillary changes through its connection with the superior colliculi and area 8.
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C. Afferent Connections for Vestibular Input
Specific tonic and phasic receptors (Chapter 5) found in the vestibular membranous labyrinths in the inner ear (utricle, saccule, and semicircular canals) which are supplied by the vestibular portion of the eighth cranial nerve carry vestibular input into the C.N.S. These receptors are bipolar neurons and their cell bodies form the vestibular (Scarpa’s) ganglion, which is located at the base of the internal auditory meatus. Central processes of these neurons course inward between the pons and medulla and go to one of the three following areas:
1. Archicerebellum/Vestibulocerebellum
A small percentage of fibers from the vestibular portion of the eighth cranial nerve go directly to the ipsilateral archicerebellum/vestibulocerebellum via the restiform body
2. Fastigial Nuclei
Both the fastigial nuclei also receive vestibular input via the restiform body. This information is then relayed by both fastigial nucleie to the cerebellar cortex they serve.
3. Vestibular Nuclei
Vestibular fibers of the eighth cranial nerve unilaterally supply each of the vestibular nuclei on that side.
Fig. 1 –
D. Interconnections
The vestibular nuclei, archicerebellum/vestibulocerebellum, and reticular formation all have reciprocal connections with each other. Also, the structure and arrangement of the cerebellar cortex allows information which is sent specifically to the archicerebellum/vestibulocerebellum to be relayed to other parts of the cerebellum. This allows for a constant sharing of information to occur between all lobes of the cerebellum and structures comprising the postural-control mechanism.
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E. Efferent Connections
The efferent information sent from the postural-control system is a combination of information sent from the vestibular nuclei, the archicerebellum/ vestibulocerebellum, and the reticular formation.
1. Reticulospinal Tract - Medial and Lateral
The medial and lateral reticulospinal tracts are used by the postural-control mechanism for the following functions: 1.) reactive control of our non-antigravity muscles outside of the cone of stability 2.) proactive control of our body musculature to prevent a loss of balance or stability, 3.) to influence muscle tone based upon vestibular input in our non-antigravity muscles during high risk activities to provide for a higher level of stability. 4.) To influence tone at rest in all body musculature. In all cases, control is ipsilateral except reactive control of our neck non-antigravity muscles, which is bilateral.
In addition, vestibular input sent to the reticular formation via the vestibular nuclei can influence both the sympathetic and parasympathetic systems via the reticular formation’s connection with the hypothalamus. Excessive vestibular input to the reticular formation via the vestibular nuclei when the surface we are on is moving can facilitate the vomiting centers within this structure to produce motion sickness.
2. Lateral Vestibulospinal Tract
Information from the archicerebellum/vestibulocerebellum, via the fastigial nuclei, is sent to the lateral vestibular nuclei and relayed to the spinal cord by the uncrossed lateral vestibulospinal tract. These pathways function to control our axial and limb musculature so as to reflexively regain our balance and equilibrium through control of the antigravity muscles. In addition, this tract via input from the vestibular nuclei can influence muscle tone in our antigravity muscles during high-risk activities unilaterally.
3. Medial Longitudinal Fasciculus (MLF)
The medial longitudinal fasciculus is a fibrous tract that runs from the midbrain to the level of C7. It has an ascending portion and a descending portion.
a. Ascending - Medial Longitudinal Fasciculus
Information from all four vestibular nuclei, which receive information from the archicerebellum/vestibulocerebellum, ascends to the motor nuclei of cranial nerves 3, 4, and 6 bilaterally as the ascending MLF, to control occular movements of the eyes in coordination with movements of the head and trunk to maintain visual fixation within our environment. This, in turn, allows us to regain a vertical position in space. Thus, the ascending MLF is both a crossed and an uncrossed tract.
b. Descending Medial Longitudinal Fasciculus a.k.a. Medial Vestibulospinal Tract
Fig. 2 –
Information from the medial and inferior vestibular nuclei, which receive information from the archicerebellum/vestibulocerebellum, descends to the cervical area of the spinal cord as the descending MLF to reflexively control bilateral movements of the head in space regarding anti-gravity muscles when responding reactively. These responses are referred to as the labyrinthine righting responses. Thus the descending MLF is both a crossed and an uncrossed tract. In addition, information received from the vestibular nuclei based on vestibular input will influence tone in the antigravity neck musculature during high risk activities unilaterally.
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