D. THE RETICULAR FORMATION
Over the millions of years of man's evolution from its early stages to the present, many of his bony
structures have been remodeled in both size and shape, from the skull to the metatarsal bones. This
remodeling can safely be presumed to have been an adaptive response to the pressures of the surrounding
environment, diet, or other compelling factors. Pre-human fossils, which have been discovered on various
continents, are testimonials to these evolutionary changes (see Figure 1-6).
The central nervous system has also followed an important line of evolution. Some of the brain structures
that characterize humans in our day and age were absent from the hominoid brain three million years ago.
The size of the brain of the Australopithecus (400-600 cc.) was much smaller to that of Modern Man
(1000-1400 cc.). The position and relative size of the brain lobes was also different, and some of the
nervous pathways, including the cerebral cortex, were present in a more rudimentary form of development.
Two particular components in the central nervous system, which have remained practically unchanged over
time, are the reticular formation and the subcortical motor areas. Other neurological systems have
developed in and around that system to allow the body to adjust to the continuous evolutionary changes,
perhaps first and foremost to bipedalism. The gradual adaptation by pre-humans to full bipedalism required
a great degree of remodeling within the central nervous system, especially in the system of locomotion and
equilibrium. As man was becoming a biped by reducing his base of support to a few square inches, his
cerebral cortex and cranial capacity began to expend.
The development of the cerebral cortex and of the cranial bones is directly related to bipedalism, as we
will see in a subsequent chapter. The restructuration of the foot (and in particular that of the calcaneus
and talus) has in fact been an essential driving force behind the restructuration of cranial bones.
The reticular formation itself is, as mentioned above, a phylogenetically very old and primitive system
of nuclei and nervous pathways located at the core of the brain stem. It consists of two parts, an
ascending and a descending formation. Oftentimes, the ascending reticular formation is defined as the
reticular activating system; this definition, however, is too restrictive since it is often only associated
with degrees of consciousness or alertness.
The reticular formation extends upward into the medulla, pons, mesencephalon, and diencephalon (hypothalamus,
thalamus) and downward into the spinal cord where the reticular formation consists of internuncial cells
and their multisynaptic pathways.
The descending reticular formation plays a major role in the neurological modulation of visceral and
somatic activities (see Figure 1-7). From the hypothalamus by way of the dorsal longitudinal fasciculus
and the mamillotegmental tract, it relays impulses to the autonomic nervous system and various organs.
From the subcortical motor areas, it relays impulses to voluntary muscles.
Posture and general muscle tone are directly related to the descending reticular formation. In fact, it
appears that posture and equilibrium control are two of the most important motor functions of the reticular
formation. It would also seem that the evolutionary process toward bipedalism was highly dependent upon
it. Autonomic functions such as gastrointestinal peristalsis, glandular secretion, and urinary bladder
are controlled by the reticular formation of the medulla, pons, and mid-brain. The respiratory system,
the cardiovascular system, swallowing, mastication, and vomiting reflexes are all equally controlled by
the reticular formation at the level of the medulla obloganta.
The ascending reticular formation arises from cells of the posterior horn (Lamina V). Spinoreticular
fibers ascend in the ventrolateral part of the spinal cord. Collaterals are distributed to large areas
of the brain stem reticular formation in the medulla, pons, and to a less extensive area in the mid-brain.
The spino-reticular fibers which terminate in the medullary reticular nuclei (nuclei reticularis
gigantocellularis) are mostly uncrossed fibers. The presence of these uncrossed fibers and of diffuse
nuclei gives the system a more primitive and phylogenetically older appearance. The system is also more
undifferentiated because it is concerned with both autonomic and motor control.
Some spinoreticular fibers terminate in the pontine reticular nuclei with a bilateral distribution.
Some other fibers reach the mid-brain reticular nuclei. A certain number of spinoreticular fibers
terminate in the rostral intralaminar nuclei of the thalamus. From there, the rostral intralaminar
nuclei relay impulses to the cerebral cortex. Ascending projections from the mesencephalic reticular
formation also send direct impulses to the hypothalamus through the afferent fibers of the dorsal
longitudinal fasciculus and the mammillary peduncle.
The main function of the ascending reticular formation is to convey sensori stimuli (proprioception,
touch, vibratory sense, pain, temperature, visual and auditory stimuli) from all the sensory pathways
through the ascending collateral fibers. As previously mentioned, these sensory stimuli, through
multisynaptic chains of neurons, terminate in a group of nuclei located in the thalamus which then relays
the sensory impulses to the cerebral cortex.
Standing posture can be maintained because the reticular formation sends impulses to the extensor
muscles of the upper and lower limbs to stiffen and control the position of the body's center of
gravity and to maintain the gravity line within the base of support. Other associated nuclei such
as the vestibular nuclei also play an important role in the control of equilibrium.
In a natural static, standing position, in which no or few cortical (pyramidal) impulses interact,
that is, in the absence of voluntary motion or pre-motion, the upper limbs are aligned with the lower
limbs in such a way that the olecranon is oriented in a semi-frontal or semi-prone position.
To support the body against gravity and maintain that position over an extended period of time with
minimal muscular effort, the gravity line will fall between the feet in front of the talus bone. Any
slight departure from that specific position will increase the general tone in the extensor muscles.
The excitatory impulses of the upper portion of the reticular formation from the vestibular nuclei and
above simultaneously stimulate the extensors of the associated muscles of the thigh, leg, arm, and neck
on the same side, namely, the quadriceps femoris, triceps brachii, and the extensors of the leg, forearm,
and neck.
Similar findings of association between distal segments of the body have been clinically identified in
individuals having suffered a trauma to one particular area of the body. The corresponding areas exhibit
the same type of dysfunction (see Chapter IV). For example, an injury to the front of the left leg will
generate a similar imbalance in the extensors of the forearm and neck on the side of the injury.
We have to look at the body as an entity in which all the components, muscles, organs, bones, and joints
are integrated not only as individual units controlled by the central nervous system but also in their
relationship or interaction with other muscles, organs, bones, and joints.
Posture could not be maintained without proper coordination of the neck muscles and leg muscles.
Similarly, the iliopsoas muscle must synchronize its activities with the sternocleidomastoid muscle to
control balance. It is important to remember that, even though cortical and subcortical nuclei and the
reticular formation are interconnected, a standing position is mostly monitored by the lower level, i.e.
the reticular formation, and that at that level, the segmental associations are predominantly ipsilateral.
Ipsilateral association between muscles and muscles, bones and bones, joints and joints, and organs and
organs refers to the presence of common nuclei and common nervous pathways in the central nervous system,
more precisely in the reticular formation. These nuclei and pathways synchronize ipsilateral functions,
postural reflexes, common activation or relaxation of structures, and in general, coordinate the response
to both internal and external stimuli.
The cerebral cortex becomes activated during pre-motion (ready-to-move position) and motion. The gradient
of involvement of the central nervous system, which may extend from the reticular formation and the
internuncial cells in the spine to all subcortical nuclei and the cortex, is proportional to the number
and complexity of internal and external stimuli. An interesting feature in the process emerges as a
gradual switch in the association between muscles, joints, and other structures arises. In addition to
the ipsilateral connection which has been previously described, there exists a contralateral supervision
by the cortical motor areas.
A good example of the contralateral connection between muscles is the association between the extensors
of the lower limbs and the flexors of the upper limbs. For instance, the quadriceps of one lower limb
is associated to the biceps of the opposite upper limb and the hamstring of the opposite lower limb, and
vice-versa. The combined ipsilateral and contralateral control over the upper limbs will force the arms
in a semi-flexed position and the forearms in a semi-pronated position so that the radius and the ulna
naturally cross. This crossing of the radius and the ulna must have been, at one time, a key factor in
the evolution of man and primates. From the time tree dwellers or brachiators, the common ancestors of
man and primates, acquired this ability to cross the radius and the ulna, they developed a higher degree
of mobility and dexterity in the hands and wrists. At a much later date, pre-humans started to engage
in the direction of contralateral bipedalism and therefore increased, in at least 1 or 2% increments,
the number of motor and sensory fibers that would decussate in the medulla oblongata, thus giving the
opposite cerebral hemisphere additional nervous control over the extremities and allowing the hand and
fingers to become even more dexterous. Although the ankle and the foot are closely related to the wrist
and hand, as we will see later on, their functional use during human evolution was of a different nature.
The ankle and the foot were simply structured to stand, walk, or run.
| 
