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Chapter 10
that cerebellar neurons project to regions of cortex beyond the
sensorimotor areas, including those involved in memory and
attention. Patients with cerebellar disease have been observed
to display a fl atness of emotion, and a reduction of cerebellar
neurons has been suggested to underlie the abnormal sense of
self-awareness in some cases of
autism
.
Descending Pathways
The infl uence exerted by the various brain regions on posture
and movement occurs via descending pathways to the motor
neurons and the interneurons that affect them. The pathways
are of two types: the
corticospinal pathways,
which, as their
name implies, originate in the cerebral cortex; and a second
group we will refer to as the
brainstem pathways,
which
originate in the brainstem.
Fibers from both types of descending pathways end at syn-
apses on alpha and gamma motor neurons or on interneurons
that affect them. Sometimes these are the same interneurons that
function in local refl
ex arcs, thereby ensuring that the descend-
ing signals are fully integrated with local information before the
activity of the motor neurons is altered. In other cases, the inter-
neurons are part of neural networks involved in posture or loco-
motion. The ultimate effect of the descending pathways on the
alpha motor neurons may be excitatory or inhibitory.
Importantly, some of the descending fi bers affect
affer-
ent
systems. They do this via (1) presynaptic synapses on the
terminals of afferent neurons as these fi bers enter the central
nervous system, or (2) synapses on interneurons in the ascend-
ing pathways. The overall effect of this descending input to
afferent systems is to regulate their infl
uence on either the
local or brain motor control areas, thereby altering the impor-
tance of a particular bit of afferent information or sharpen-
ing its focus. This descending (motor) control over ascending
(sensory) information provides another example to show that
there is no real functional separation between the motor and
sensory systems.
Corticospinal Pathway
The nerve fi bers of the corticospinal pathways have their cell
bodies in the sensorimotor cortex and terminate in the spinal
cord. The corticospinal pathways are also called the
pyrami-
dal tracts
or
pyramidal system
because of their triangular
shape as they pass along the ventral surface of the medulla
oblongata. In the medulla oblongata near the junction of the
spinal cord and brainstem, most of the corticospinal fi bers
cross, or
decussate,
the spinal cord to descend on the opposite
side (
Figure 10–12
). Thus, the skeletal muscles on the left
side of the body are controlled largely by neurons in the right
half of the brain, and vice versa.
As the corticospinal fi bers descend through the brain
from the cerebral cortex, they are accompanied by fi bers of the
corticobulbar pathway
(“bulbar” means “pertaining to the
brainstem”), a pathway that begins in the sensorimotor cortex
and ends in the brainstem. The corticobulbar fi bers control,
directly or indirectly via interneurons, the motor neurons that
innervate muscles of the eye, face, tongue, and throat. These
fi bers provide the main source of control for voluntary move-
ment of the muscles of the head and neck, whereas the corti-
cospinal fi bers serve this function for the muscles of the rest of
the body. For convenience, we will include the corticobulbar
pathway in the general term
corticospinal pathways.
Convergence and divergence are hallmarks of the cor-
ticospinal pathway. For example, a great number of different
neuronal sources converge on neurons of the sensorimotor
cortex, which is not surprising when you consider the many
factors that can affect motor behavior. As for the descending
pathways, neurons from wide areas of the sensorimotor cor-
tex converge onto single motor neurons at the local level so
that multiple brain areas usually control single muscles. Also,
To skeletal
muscle
To skeletal
muscle
Brainstem
pathway
Basal
nuclei
Corticospinal
pathway
Sensorimotor
cortex
Thalamus
Brainstem
Cerebellum
Crossover of
corticospinal
pathway
Spinal cord
Spinal cord
Figure 10–12
The corticospinal and brainstem pathways. Most of the corticospinal
fi bers cross in the brainstem to descend in the opposite side of the
spinal cord, but the brainstem pathways are mostly uncrossed.
The descending neurons are shown synapsing directly onto motor
neurons in the spinal cord, but they commonly synapse onto local
interneurons.
Adapted from Gardner.
Figure 10–12
physiological
inquiry
If a blood clot blocked a cerebral blood vessel supplying a small
region of the right cerebral cortex, just in front of the central
sulcus in the deep groove between the hemispheres, what
symptoms might result?
Answer can be found at end of chapter.
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