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The ventricular mesocoelia, also known as the
cerebral aqueduct, the iter or the aqueduct of Sylvius,
connects the third and fourth ventricles of the
brain.
(d) Cerebral Penduncles
The cerebral peduncles are two tracts underneath
the tegmentum that are often considered part of the
mesencephalon. These tracts are bundles of nerve
fibers passing over the bottom of the brain that connect
the cerebral hemispheres to the spinal cord.
Hindbrain (Rhombencephalon)
The hindbrain includes the cerebellum, the pons
and the medulla oblongata, which function collectively
to support vital bodily processes (Fig. 5). Often
the midbrain, pons, and medulla are referred together
as the brainstem.
(a) Cerebellum
The cerebellum (small brain in Latin) is the part
of the brain that functions in movement, coordination,
motor control and sensory perception. It
is responsible for relaying messages about posture,
equilibrium, movement and fine motor skills such as
writing or catching a ball.
One of the main manifestations of cerebellar
dysfunction is problems with motor control. The
ability for motor activity remains, but it loses precision,
producing erratic, uncoordinated, or incorrectly
timed movements. One such common problem is
loss the of fine motor coordination called spasticity,
a distinctive way of walking in which first one foot
then the other is laboriously set forward. A standard
test of cerebellar function is to reach with the tip of
the finger for a target at arm's length: A healthy person
will move the fingertip in a rapid straight trajectory,
whereas a person with cerebellar damage will
reach slowly and erratically, with many mid-course
corrections. Hence, scientists concluded that the basic
function of the cerebellum is not to initiate movements,
or to decide which movements to execute,
but rather to control the detailed form of a movement.
The brain is composed of billions of neurons, but
the cerebellum has the most neurons compared to
any other part of the brain. Hence, when excessive
alcohol abuse affects the brain, it is usually the cerebellum
that is most affected. This is why alcohol
related brain damage can cause permanent slurred
speech, loss of balance or co-ordination.
Functional imaging studies have shown cerebellar
activation in language, attention, and mental imagery
activities. Other correlation studies have
shown interactions between the cerebellum and nonmotor
areas of the cerebral cortex. The cerebellum
also participates in error-correction and problem
solving of many different types, including screening
out incorrect responses by other brain systems.
Various non-motor symptoms have been recognized
in people with damage that appears to be confined to
the cerebellum.
Dr. K. Doya (5) proposed that the cerebellum is
involved in supervised learning, in contrast to the
basal ganglia, which perform reinforcement learning,
and the cerebral cortex, which performs unsupervised
learning.
(b) Pons
In Latin, the word pons means bridge. It connects
the cerebral cortex with the medulla and also
serves as a communications and coordination center
between the two hemispheres of the brain. The pons
is an important sensory relay system that provides
information to the cerebellum, cerebrum, and spinal
cord. It provides input to the cerebellar cortex
through the pontine nuclei, allowing the cerebellum
to coordinate much of its control.
The pons also functions as a motor relay center,
since many of the descending nerve fibers synapse
in the pons. Hence, any injury to the pons may
result in motor deficits.
The pons is also an important control center
for respiration. The apneustic center, located in the
lower pons, stimulates inspiration, while the pneumotaxic
center, located in the upper pons, inhibits
inspiration. Damage to the pneumotaxic center can
result in prolonged and so a decrease in the respiratory
rate.
The pons plays a role in Rapid Eye Move-
September—October 2012 Yang-Sheng (Nurturing Life) 25