MAINTENANCE OF
POSTURE AND
BALANCE
By
Dr. Khalid Ibrahim
Assistant Professor of Human Physiology
• At the end of the session the students should be able to:
• Describe the connections and functions of cerebellum.
• Explain the levels of control in body posture.
• Describe the body posture in spinal, decerebrate and
decorticate conditions.
Anatomical Divisions
 The cerebellum is attached to the brain stem by three pairs of tracts
called the cerebellar peduncles which consist of numerous pathways
connecting the cerebellum with several other centers in the nervous
system.
 The body of the cerebellum is divided into two “hemispheres”
connected at the midline by the “vermis”. Also, it is composed of an
outer cortical gray matter (cerebellar cortex) surrounding a core of white
matter which contains a group of nuclei (deep cerebellar nuclei).
 The cerebellar cortex is extensively
folded
by
transverse
fissures
to
increase its surface area.
 Two of these fissures ; the primary
fissure and the posterolateral fissure,
are deeper than others , dividing the
cerebellum into 3 anatomical lobes :
(i) The anterior lobe ,
(ii) The posterior lobe , and
(iii) the flocculonodular lobe.
Functional Divisions of the Cerebellum
 The cerebellum is functionally divided into longitudinal functional zones
(a) The vermal zone, which occupies the vermis and is the most medial
part of the cerebellar cortex.
(b) The intermediate (or paravermal) zone
of the cerebellar
hemisphere, lying on each side of the vermis, occupying the medial
regions of the cerebellar hemispheres.
(c) The lateral zone of the cerebellar
hemisphere, lying just
lateral
to the intermediate zone. In humans this region occupies the greater
portion of the cerebellar hemisphere.
 On
this
basis,
the
different
regions of the cerebellum are
functionally organized into three
major divisions:
(i) The vestibulocerebellum.
(ii) The spinocerebellum.
(iii) The cerebrocerebellum.
 An alternative classification of the descending motor tracts is based on the
sites of termination of these tracts in the spinal cord.
 On this basis , the descending motor pathways can be classified, with respect
to their spinal sites of termination , into:
Medial system
Lateral system
 composed of tracts which terminate composed of tracts which terminate
primarily on the ventromedial neurons (or primarily on the dorsolateral neurons
their associated interneurons)
(or their associated interneurons)
 innervate the trunk (axial) muscles and  innervate the distal muscles of the
the proximal (girdle) muscles of the limbs
limbs.
 more concerned with postural control.
 more concerned with controlling fine
voluntary
extremities.
movements
of
the
There are five important sets of descending motor tracts, named according to
the origin of their cell bodies and their final destination:
1) the corticobulbospinal tract,
(= Pyramidal tract)
2) the rubrospinal tract,
3) the reticulospinal tracts,
4) the vestibulospinal tracts, and
5) the tectospinal tract.
Extrapyramidal tracts
I) The Vestibulocerebellum
 This functional region of the cerebellum is composed of the
“flocculonodular lobe”
 has a close functional relationship with the vestibular system; so it has
been named the “vestibulocerebellum”.
vestibulocerebellum
Neural Connections
Efferents
Reticular
formation
EYE
Regulation of Equilibrium
Afferents
information about
position & movements
of the head.
Fastigial
nucleus
Vestibular
nuclei
Folloculonodular lobe
 These connections enable it:
1) To control the activity of the motor pathways (the vestibulospinal and
reticulospinal
tracts) concerned with regulation of the tone and
contractility of the antigravity muscles
in response to vestibular
sensory signals, and is therefore largely concerned with regulation of
equilibrium.
2) To regulates movements of the eyeballs during head movements to
maintain stable vision.
II) The Spinocerebellum
 corresponds to the “vermis” plus the “ intermediate zone” of the cerebellar
hemispheres on both sides of the vermis.
 Most of the sensory information reaching these regions of the cerebellum
arrives from the spinal cord,
“spinocerebellum”.
Spinocerebellum
thus they have been named the
B) Peripheral Receptors
Neural Connections
Afferents
A) Brain and Brainstem Centers
Information about the “plan”
of the movement ordered
by the higher motor centers .
Cerebral
Cortex
VSCT
Thalamus
DSCT
Red Nucleus
Reticular
Formation
Inferior
olivary
nucleus
1) The Dorsal Spinocerebellar Tract
 The “dorsal spinocerebellar tract” (DSCT) terminates ipsilaterally in the
vermis and intermediate zone of the cerebellar hemisphere.
 These tracts provide the cerebellum with sensory signals from muscle
spindles, Golgi tendon organs, joint receptors, and cutaneous touch and
pressure receptors. These signals inform the cerebellum about the present
state of muscle contraction, position of the body, and movements of its
different parts.
2) The Ventral Spinocerebellar Tract
 The ventral spinocerebellar tract (VSCT) terminates ipsilaterally in the
spinocerebellum.
 At its origin in the dorsal horn of the spinal gray matter, the ventral
spinocerebellar tract receives a few afferent fibers from muscle and joint
proprioceptors.
 In addition, it receives motor signals that reach it from::
(i) Collaterals of all the descending motor pathways
(ii) The Local motor neurons in the spinal motor neuron pools.
 The ventral tract quickly returns to the spinocerbellum copies of the
descending motor commands arriving at the spinal motor neuron pools.
This feedback information is called the “efference copy” of the motor
drive that finally reaches the spinal alpha and gamma motor neurons.
Efferents
A) From the Vermis
Reticular
Formation
control the “axial” body
musculature
Regulation of Posture
Vestibular
nuclei
Fastigial nucleus
A) From the Intermediate zone
Thalamus
Red
nucleus
Globose nucleus
control the “distal” body
musculature
Emboliform nucleus
“Comparator” and “Error-Correction” Mechanism
“Predictive” & “Damping” Mechanism
III) The Cerebrocerebellum
The cerebrocerebellum consists of the large “ lateral
zones ” of
the
cerebellar hemispheres, just lateral to the intermediate zones.
 These regions of the cerebellum have no sensory input from peripheral
tissues, but almost all of their afferent signals originate from the
contralateral cerebral cortex therefore this functional region of the
cerebellum is called the “cerebrocerebellum”.
 The cerebrocerebellum is the most recent
from a phylogenetic point of view, so it is
sometimes
referred
“neocerebellum”.
to
as
the
Neural Connections
Afferents
Efferents
“Planning” the “Sequence” and “Timing” of Movements
Cerebral
Cortex
Visual impulses
VL nucleus of
Thalamus
Red
nucleus
Pontine
nuclei
Dentatel nucleus
Afferents
 Almost all the afferent pathways to the cerebrocerebellum originate in
the cerebral cortex . Most of these afferents pass by way of the “pontine
nuclei”
and the middle cerebellar peduncle to the lateral zone of the
cerebellar hemisphere on the opposite side, via the “cortico - ponto cerebellar pathway”.
The cerebral cortical projections into the cerebrocerebellum provide it
with information from the motor areas concerning the motor command that
is about to be discharged.
 Sensory information from the somatic sensory areas about the present
postural state of the body , while that from the visual areas informs the
cerebellum about the spatial position of the body in relation to the
surrounding environment.
Efferents
 The cortex of the cerebrocerebellum projects its efferent fibers to the
“dentate” nucleus. The axons leaving the dentate nucleus pass through the
superior peduncle to terminate mainly in the ventral lateral (VL) nucleus of
the contralateral thalamus, which finally projects to the motor areas of the
cerebral cortex.
 A smaller number of dentate axons terminate in the contralateral red
nucleus.
 The “cerebello - dentato - cerebral” pathway mediates the role of the
cerebrocerebellum in adjusting the plan of the motor command before
being discharged from the cerebral cortical motor areas to the lower
motor centers.
Functions of the Cerebellum
I) Regulation of Equilibrium
 As one starts to move or lose equilibrium, it is immediately sensed by
the vestibular receptors.
 This is received and interpreted by the “vestibulocerebellum”, and
immediate corrective signals are sent to
(i)The vestibular nuclei, and reticular formation, to adjust the tone
and contractility of the axial and proximal limb muscles.
(ii) The superior colliculus and medial longitudinal fasciculus, to
coordinate eye movements, so that a stable image of the visual field can
be maintained on the retina. Clear vision is important for keeping
equilibrium during head movements.
II) Regulation of Posture
 The vermis is the principal region of the cerebellum concerned with
postural adjustment.
 “vermis” receives sensory information from muscle and joint
proprioceptors (particularly from the axial regions), concerning
“position” of the body.
 Its output controls the vestibulospinal and reticulospinal pathways that
principally regulate the tone and contraction of the axial and proximal
limb muscles.
III) Regulation (or Coordination) of Voluntary Movements:
 One of the most important features of normal motor activity is one’s
ability to proceed smoothly and precisely from one movement to the
next in proper succession.
 This is known as “coordination” of movements
 It is essentially one of the basic motor functions of the cerebellum .
 The cerebellar role in coordination of movements is carried on by a
number of mechanisms , which include :
1)“Comparator” and “Error -Correction” Mechanism
2) “Predictive”
and
“Damping” Mechanism
3) “Planning” the “Sequence” and “Timing” of Movements
1)“Comparator” and “Error-Correction” Mechanism:
 AS the motor areas of the cerebral cortex send motor commands to
muscles for performance of a voluntary movement, the spinocerebellum
(particularly the intermediate zone of the cerebellar hemisphere)
receives immediately:
1) an “efference copy” of the intended motor command
through
the
cortico – ponto - cerebellar pathway.
2) Via the ventral spinocerebellar tract transmits back to the
spinocerebellum another “efference copy” about the final motor signals that
reach the anterior motor neurons of the spinal cord .
3) Via the dorsal spinocerebellar tract, proprioceptive signals from the
moving parts of the body, especially from the distal parts of the limbs (as
movement proceedes) , that tell the cerebellum what is actually taking
place regarding movement and position of the limbs
 The intermediate zone of the spinocerebellum essentially acts as a
“comparator” that compares the motor intentions of the higher centers
with the actual performance of the involved muscles.
 When there is any “error”
in performance or “deviation”
from the
original plan of the intended voluntary motor act, then the intermediate
zone and the interposed nucleus send ‘corrective signals” back to the
motor areas of the cerebral cortex and the red nucleus, which give
origin to the descending motor tracts innervating mainly the lower motor
neurons of the distal limb muscles.
2) “Predictive” and “Damping” Mechanism
 The cerebellum receives information regarding the velocity and direction
of the intended movement, initially by means of the “efference copy” of
the descending motor command, which is very soon followed by
“sensory feed-back” signals from the muscles and joints of the moving
parts of the body .
 The cerebellum would predict from these parameters how far that part
of the body will move in a given time
 The cerebellum uses this information to determine the precise time to
damp the movement, then it sends its decision to the motor cortex to
stop the ongoing movement exactly at the intended position.
3) “Planning” the “Sequence” and “Timing” of Movements:
 The principal part of the cerebellum concerned with this coordination is
the “cerebrocerebellum”
(a) Planning the “Sequence” of Movements:
 the cerebrocerebellum uses the information provided from the cerebral
cortex and the basal ganglia for planning the sequence of contraction
of the different muscles involved in the voluntary motor act, to achieve
the goal of the movement .
 Then, the “plan” of the movement sequence is transmitted from the
cerebrocerebellum to the motor areas of the cerebral cortex, where it is
used to adjust the final motor command before it is discharged to the
lower motor centers.
(b) Timing of Movements
 Another important role of the cerebrocerebellum is to provide perfect
timing of voluntary movements.
 This is established by computing (calculating) the appropriate timing for
the “onset” and “termination” of contraction of each of the muscles
involved in the performance of the successive movements during
voluntary motor acts.
 Accordingly, the cerebrocerebellum assures the smooth progression
of the whole movement by providing accurate timing for proceeding
from one movement to the next in complex motor acts .
Neck Proprioceptors
 It gives CNS appropriate information about the orientation of the
head with respect to the body.
 This information is transmitted from the proprioceptors of the
neck and body directly to the vestibular and reticular nuclei in the
brain stem and indirectly by way of the cerebellum.
 When the head is leaned in one direction by bending the neck,
impulses from the neck proprioceptors keep the signals
originating in the vestibular apparatus from giving the person a
sense of dysequilibrium. They do this by transmitting signals that
exactly oppose the signals transmitted from the vestibular
apparatus.
 However, when the entire body leans in one direction, the
impulses from the vestibular apparatus are not opposed by
signals from the neck proprioceptors; therefore, in this case, the
person does perceive a change in equilibrium status of the entire
body.
Other important receptors:
1) Pressure receptors in the foot.
2) Visual receptors.
Some people with bilateral destruction of the vestibular
apparatus have almost normal equilibrium as long as their
eyes are open and all motions are performed slowly.
Decerebrate vs decorticate lesions
 Decorticate means lesions of CNS above the level of midbrain
(rubrospinal tract is intact)
 Decerebrate means lesions of CNS at a level between superior
colliculi and inferior colliculi (rubrospinal tract is not intact)
 Spinal animal:
Animals in which the connection between the spinal cord and
brain is disconnected.
Difference is due to:
Rubrospinal is intact in the decorticate lesion causing flexion in
the upper limb.
Whereas in decerebrate lesion
Rubrospinal is cut but reticulospinal and vestibulospinal are intact
causing extension of both upper and lower limbs.