The extrapyramidal system is composed of motor fibers
which do not pass through the medullary pyramids but which
nevertheless exert a measure of control over bodily movements.
The system is difficult to describe, partly because of the
complexity of pathways and feedback loops which compose it.
Nevertheless, the extrapyramidal system can be divided into
three controlling systems: the cortically originating indirect
pathways, the feedback loops, and the auditory-visual-vestibular
Cortically Originating Indirect Descending Pathways At the same
time signals are being transmitted over the pyramidal system to
produce a specific movement, additional signals relative to the
movement are also relayed to the basal nuclei, red nucleus, and
brainstem reticular formation. The basal nuclei evaluate the
command signal sent down the pyramidal pathways and may
contribute to the establishment of needed background muscle
tone for the movement. The nuclei are able to do this in part
by projecting to the red nuclei, which influence spinal cord
alpha and gamma motor neurons via rubrospinal tracts. Similar
indirect routing to the spinal cord is achieved through
corticoreticulospinal and corticorubrospinal pathways (Fig.
The function of these indirect pathways to the spinal cord motor
neurons may include more than providing background muscle tone
for movements directed by the motor cortex. Recall from Chap. 6
that ablation studies in which the rubrospinal tracts are
experimentally cut have shown that the corticospinal and
rubrospinal tracts have somewhat similar effects on spinal motor
neurons. When the rubrospinal tracts of monkeys were damaged
along with earlier pyramidal tract sections, the loss of skilled
control in distal muscles became even more severe and yet there
was little or no loss of proximal muscle control. Even so,
because the red nucleus receives input from the basal and
cerebellar nuclei as well as direct input from the cerebral
cortex, its function may include modifying or "fine tuning" the
motor neurons which innervate the muscles involved in a given
movement.Feedback Loops The feedback loops described here
include neural circuits in which a signal sample is fed back to
a "comparator," which is in a position to compare the signal
with some desired condition and subsequently take steps to
"adjust" or "modify" it. The extrapyramidal system includes two
such feedback systems: the cortically originating extrapyramidal
system feedback loops
(COEPS feedback loops)
and the proprioceptor originating extrapyramidal system
(POEPS feedback loops).
The CO EPS feedback loops are composed of fibers originating in
the motor cortex which synapse in subcortical centers. After
integrating and evaluating the signals, the centers project
fibers back to the cortical source for modification. Three such
loops are illustrated in Fig. 16-6. In loop A the signal is
"tapped off" to the corpus striatum (caudate and putamen), which
in turn project to the globus pallidus. Pallidothalamic fibers
then project to the thalamus, which completes the loop by
projecting back to the cortical source. Somewhere in this loop
the original signal sent down the pyramidal tracts is compared
and evaluated with other input relative to the movement. After
appropriate integration, modifying feedback signals are
returned to the cortex via the thalamocortical fibers. In loop
B the sample signal is sent to pontine nuclei for subsequent
referral to the cerebellum, where it is probably compared to
proprioceptive input coming from muscles, tendons, and joints
involved in the movement. This input probably includes such
things as the current state of muscle tone and the relative
position and movement of the limb involved. Following
integration of this input, the cerebellum then projects its
output to the thalamus (via dentatothalamic tracts) which then
completes the loop by sending fibers back to the cortical source
through thalamocortical projections. In loop C. the sample
signal is sent to the substantia nigra. which projects in turn
to the corpus striatum. From here the feedback circuit is
identical to that illustrated in loop A. The importance of
these feedback loops to normal motor control can be most clearly
seen by an examination of the clinical signs associated with
dysfunction of the basal nuclei and their related brain stem
areas, which we will examine later.
The other feedback loop system which is included in the
extrapyramidal system is composed of the POEPS feedback loops.
In this system the modification is not directed back toward the
cortical source (as are the COEPS loops), but to the spinal cord
motor neurons instead. The principal loop involves the relay of
muscle, tendon. and joint proprioceptive information to the
cerebellum via the spinocerebellar tracts. The signals are
integrated in the cerebellum and probably compared with the
intended signals sampled by corticopontocerebellar pathways. In
this way the cerebellum might compare the intended movement with
the instantaneous performance of that movement as sampled by the
proprioceptors of the spinocerebellar tracts. It could then
direct modification through its projections to the vestibular.
reticular, and rubral nuclei and their respective descending
tracts to the appropriate motor neurons of the spinal cord.
Auditory Visual Vestibular Descending Pathways Postural
adjustments in response to auditory, visual. and vestibular
signals is an additional way to regulate the activity of spinal
motor neurons. Auditory and visual input to the tectal nuclei of
the midbrain may be responsible for producing reflex movements
of the body in response to a sudden sound or bright light.
Similarly. input from the vestibular apparatus to the vestibular
nuclei and cerebellum no doubt plays a role in reflex postural
adjustments through the vestibulospinal and other tracts.
It should be pointed out here that because of the complex nature
of the neural circuits which effect motor control through routes
other than the pyramidal system, a precise and universally
agreed upon definition of the extrapyramidal pathways has never
Clinical Signs of
Basal Nuclei and Related Brainstem Dysfunction
Certain disease conditions relating to motor control appear to
be positively linked to dysfunction of the basal nuclei and
those structures functionally related to them including the
thalamus, subthalamus, and substantia nigra.
Chorea is a condition characterized by uncontrolled random
movements of the body often accompanied by facial grimaces.
Evidence indicates that the condition is often associated with
dysfunction of the corpus striatum. It is often seen as a
complication of rheumatic fever in children. Recovery from this
childhood form of the disease, Sydenham's chorea, is
usually complete with no subsequent lingering effects. A more
severe form, Huntington's chorea, is a hereditary
disease which becomes progressively worse and often leads to
severe mental debilitation. A thetosis is a condition
characterized by slow wormlike movements of the peripheral
appendages, and is also associated with damage to the corpus
striatum and lateral parts of the globus pallidus. Voluntary
movements in the affected appendages are often impaired.
Violent flinging of a limb or limbs is a rare condition called
ballismus. If one limb is involved the condition is
called monoballismus, and if both limbs on a single side
are affected the term is hemiballismus. It is generally
associated with damage to the subthalamus and can occur
spontaneously or be brought on by the initiation of a voluntary
movement involving the affected limb.
Perhaps the most familiar disease condition in this group is
Parkinson's disease (paralysis agitans). It is characterized
by an increasing tremor during rest. Also observed are a
"pill-rolling" action of the fingers, a poverty of movement
expressed by difficulty in initiating voluntary movements such
as getting up from a chair and walking, a plastic or deathlike
rigidity often demonstrated by a "cog-wheeling" phenomenon when
a limb is passively moved, and an increasing masklike fixed
expression to the face.
The cog-wheeling phenomenon that occurs as a limb is passively
moved is tentatively explained by the following mechanism.
Initial resistance is due to muscle tone as the limb is moved.
Release comes when group Ib afferents from Golgi tendon organs
inhibit homonymous alpha motor neurons. Then as the passive
movement of the limb continues, tension again develops until the
threshold of the Golgi tendon organs is once again reached,
causing a second release. This rachetlike movement continues as
the limb is passively moved. Parkinson's disease is usually
associated with dysfunction of the basal nuclei and the
Feedback loops in electronic systems must be finely tuned in
order to prevent oscillations. In physiological systems the
feedback loops must also be working properly in order to prevent
oscillations in muscle systems. In Parkinson's disease, the
fine tuning is lost and oscillating signals to motor neurons
produce tremors. It appears that the principal, site of
malfunction lies in the dopamine-releasing fibers of the
nigrostriatal pathway. There are both excitatory cholinergic
nigrostriatal fibers and inhibitory doparninergic nigrostriatal
fibers. Fine tuning seems to require the complete integrity of
both types. In Parkinson's disease, the feedback system becomes
"untuned" by the inability of the inhibitory dopaminergic
neurons to produce and release dopamine. Some success has been
achieved in the treatment of this condition by the
adminstration of i.-dopa, a dopamine precursor which is taken
up by dopaminergic nigrostriatal fibers and converted to
dopamine. With this subsequent "replacement" of the missing
transmitter, some degree of fine tuning is restored and the
severity of symptoms is often reduced
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