Over a lifetime, the various parts of the brain that control the body's movements can suffer damage from various causes, such as head injuries, strokes, and degenerative brain diseases.
One typical example is what happens when one side of the motor cortex is completely destroyed by a stroke. Because of crossed control, this centre normally controls the voluntary movements on the opposite side of the body. With this centre destroyed, no further motion is possible; that side of the body becomes completely paralyzed.
THE MOTOR CORTEX
All of the body's voluntary movements are controlled by the brain.
One of the brain areas most involved in controlling these voluntary
movements is the motor cortex.
The motor cortex
is located in the rear portion of the frontal lobe, just before
the central sulcus (furrow) that separates the frontal lobe
from the parietal lobe. The motor cortex is divided into two
main areas, Area 4 and Area 6. Area 4, also known as the primary
motor cortex, forms a thin band along the central
sulcus. Area 6 lies immediately forward of Area 4. Area 6 is
wider and is
further subdivided into two distinct sub-areas.
To carry out goal-directed movements,
your motor cortex must first receive various kinds of information
from the various
lobes of the brain: information about the body's position in
space, from the the parietal lobe; about the goal to be attained
and an appropriate strategy for attaining it, from the anterior
portion of the frontal lobe; about memories of past strategies,
from the temporal lobe; and so on.
In 1870, Hitzig and
Fritsch electrically stimulated various parts of a dog's
motor cortex. They observed that depending on what part
of the cortex they stimulated, a different part of the
body contracted. Then they found that if they destroyed
this same small area of the cortex, the corresponding
part of the body became paralyzed. This is how it was
discovered that every part of the body has a particular
region of the primary motor cortex that controls its
movement.
But what is remarkable about this "motor
map" is that certain parts of the body—those
that can make the finest movements—take up much more
space than others. These parts of the body are shown
larger than the others in the illustration here.
The putamen and the
caudate nucleus are traversed by the myelinated axons of
the internal capsule. These bundles of white matter form
stripes that distinguish them from the grey matter of the
nuclei that they traverse. That is why this group of neural
structures taken together is often referred to as the corpus
striatum, or striped body.
Likewise, the shape of the putamen and the globus pallidus
recall that of a lens, which is why these two nuclei together
are known as the lenticular nucleus.
THE BASAL GANGLIA
As
their name suggests, the basal ganglia consist
of a set of neural structures buried deep inside the
cortex. The main basal ganglia are the caudate nucleus,
the putamen, and the globus pallidus.
These ganglia, or clusters of nerve cells, are tightly interconnected.
They also receive information from several different regions
of the cerebral cortex. Once the basal ganglia have processed
this information, they return it to the motor cortex via the
thalamus.
This role of the basal ganglia in initiating and regulating motor commands becomes
clearly apparent in people whose basal ganglia have been damaged, such as patients
with Parkinson's disease. These patients display difficulty in starting the movements
they have planned, as well as trembling and slowness once they do begin them.
The cerebellum is
composed of a number of lobes and lobules which,
like the convolutions in the cerebral cortex, increase
the surface area of the cerebellar cortex considerably.
This large surface area of grey matter provides the cerebellum
with a very high density of neurons—so high that the
cerebellum, which accounts for only about 10% of the brain's
total volume, accounts for over 50% of its neurons!
The cerebellum's
anatomical location helps us to better understand its functions.
It lies parallel to two main neural pathways:
one that carries sensory messages to the part of the brain
that analyzes them, and another that emerges from the cortex
and descends to the muscles to make them contract.
The cerebellum thus receives a copy of all the information
that is sent up from the sensory organs to the sensory cortex
and all the information that is sent down from the motor
cortex to the spinal cord. The cerebellum also receives information
from many other areas of the cerebral cortex and the sub-cortical
regions of the brain.
THE CEREBELLUM
For
you to perform even so simple a gesture as touching
the tip of your nose, it is not enough for your brain
to simply command your hand and arm muscles to contract.
To make the various segments of your hand and arm deploy
smoothly, you need an internal "clock" that
can precisely regulate the sequence and duration of
the elementary movements of each of these segments.
That clock is the cerebellum.
As so often in neurobiology, to understand
exactly what the cerebellum does, we can observe patients in
whom part of this structure has been destroyed (by a tumour or
a stroke, for example). When these patients try to grasp an object,
their hands start moving late, advance unsteadily, and either
stop before reaching their target, or, often, accelerate past
it. In terms of posture, people with damaged cerebellums characteristically
display balance problems similar to those found in people who
are drunk. In fact, the clumsiness that accompanies excess consumption
of alcohol is
directly related to its depressive effects on the activity of
the cerebellum.
In a healthy
person, the cerebellum first receives information about the
intended movement from the sensory and motor cortexes. Then
it sends information back to the motor cortex about the required
direction, force, and duration of this movement Thus this
loop involving the cerebellum operates in addition to the
loop involving the basal ganglia to regulate the details
of motor control.
Another metaphor summarizes the role of your
cerebellum rather well: it acts like an air traffic controller
who gathers an unbelievable amount of information at every moment,
including (to return to our original example) the position of your
hand, your arm, and your nose, the speed of their movements, and
the effects of potential obstacles in their path, so that your
finger can achieve a "soft landing" on the tip of your
nose.
The production of
movement is organized into various levels of control.
At the highest level, the cortex controls voluntary movements.
These are all the movements that require co-ordination
and precision to adapt to particular situations on the
basis of information provided by the senses.
At the most basic level, movement is controlled by the spinal
cord alone, with no help from the brain. The neurons of the
spinal cord thus take charge of reflex
movements as well as the rhythmic movements involved
in walking.
Between these two levels, there are all other kinds of movements.
For instance, like the movements involved in walking, the
movements involved in breathing have an automatic component
but can also be altered voluntarily (for example, if you
want, you can hold your breath, just as you can run instead
of walk).
THE ACTIVATION SEQUENCE FOR THE
MOTOR AREAS
The basic function
of the brain is to produce
behaviours, which are, first and foremost, movements.
Several different regions of the cerebral cortex are involved
in controlling the body's movements.
These regions are organized into a hierarchy like the crew of a ship. On an ancient
galley, for example, the captain determined the destination for a voyage by assessing
the various factors that might make such a trip worthwhile. Then his lieutenants
calculated the direction that the ship had to travel to reach that destination,
based on weather conditions. Finally, the lieutenants transmitted their orders
to the crew manning the oars, who used their muscles to move the ship in the
desired direction.
Similarly, in the human
brain, planning for any given movement is
done mainly in the forward portion of the frontal lobe. This
part of the cortex receives information about the individual's
current position from several other parts. Then, like the ship's
captain, it issues its commands, to Area 6. Area 6 acts like
the ship's lieutenants. It decides which set of muscles
to contract to achieve the required movement, then
issues the corresponding orders to the "rowers"—the
primary motor cortex, also known as Area 4. This area in
turn activates specific muscles or groups of muscles via
the motor neurons in the spinal cord.
Even for a movement as simple as picking up a glass of water,
one can scarcely imagine trying to consciously specify the sequence,
force, amplitude, and speed of the contractions of every muscle
concerned. And yet, if we are healthy, we all make such movements
all the time without even thinking of them.
The decision to pick up a glass of water is accompanied by increased electrical
activity in the frontal region of the cortex. The neurons in the frontal cortex
then send impulses down their axons to activate the motor cortex itself. Using
the information supplied by the visual cortex, the motor cortex plans the ideal
path for the hand to follow to reach the glass. The motor cortex then calls on
other parts of the brain, such as the central
grey nuclei and the cerebellum, which help to initiate and co-ordinate the
activation of the muscles in sequence.
The axons of the neurons of the primary motor cortex descend
all the way into the spinal cord, where they make the final relay of information
to the motor neurons of the spinal cord. These neurons are connected directly
to the muscles and cause them to contract. Finally, by contracting and by thus
pulling on the bones of the arm and hand, the muscles execute the movement that
enables the glass to be picked up.
In addition, to ensure that all of these movements are fast, precise, and co-ordinated,
the nervous system must constantly receive sensory information from the outside
world and use this information to adjust and correct the hand's trajectory. The
nervous system achieves these adjustments chiefly by means of the cerebellum,
which receives information about the positions in space of the joints and the
body from the proprioceptors.
