In order to reach the
muscles, motor commands generated in the central
nervous system must travel on upper motor neurons and
synapse with lower motor neurons.
motor neurons (UMN) are a type of first order
neuron. They are unable to leave the
central nervous system. The pyramidal tract is a very
important upper motor neuron tract. The extrapyramidal tract
also consists of upper motor neurons, and is multi synaptic.
As upper motor
neurons must remain inside the neuraxis, they synapse with
neurons of another type called lower motor neurons which can
carry messages to the muscles of the rest of the body. When children have neuromuscular problems due to UMN lesions that occur before, during, and shortly after birth they are said to have cerebral palsy.
motor neurons, or second order
neurons are cranial and spinal nerves. The
cell bodies of these neurons are located in the brain stem,
but their axons can leave the central nervous system and
synapse with the muscles of the body.
All lower motor
neurons are either spinal or cranial nerves. All spinal
nerves have a lower motor neuron component as they are mixed
nerves. However, not all cranial nerves have lower motor
neuron components. Some of the cranial nerves contain only
sensory fibers and therefore cannot be classified as lower
motor neurons. For example, CN I, the olfactory nerve, CN II
the optic nerve, and CN VIII, the auditory nerve, do not
have motor components.
group of fibers carries messages for voluntary motor
movement to the lower motor neurons in the brain stem and
of the cell bodies of the pyramidal tract are located on the
precentral gyrus of the frontal lobe, which is also known as
the motor strip. Particularly large cells located here whose
axons are part of the pyramidal tract are called pyramidal
cells. Approximately 20% of the pyramidal tract fibers also
originate in the postcentral gyrus of the parietal lobe, in
Brodmann's areas 1, 2, and 3. Regardless of the location of
their cell bodies, pyramidal tract fibers begin their
descent from the cortex as a corona radiata (radiating
crown) before forming the internal capsule.
This tract is
direct and monosynaptic, meaning that the axons of its
neurons do not synapse with other cells until they reach
their final destination in the brain stem or spinal cord.
These direct connections between the cortex and the lower
motor neurons allow messages to be transmitted very rapidly
from the central nervous system to the periphery.
The fibers that
synapse with cranial nerves form the cortico-bulbar tract.
Bulbar refers to the brain stem (midbrain, pons and
medulla). The ancients anatomists thought that the medulla
looked like a plant bulb.
The fibers of the
pyramidal tract that synapse with spinal nerves sending
information about voluntary movement to the skeletal muscles
form the cortico-spinal
tract. These axons are among the longest
in the central nervous system, as some of them travel all
the way from the cortex to the inferior part of the spinal
cord. As they descend through the brain, they form part of
the posterior limb of the internal capsule.
pyramids in the inferior part of
the medulla, eighty-five to ninety percent of cortico-spinal
fibers decussate, or cross to the other side of the brain.
The remaining ten to fifteen percent continue to descend
ipsilaterally. The fibers that decussate are called
lateral cortico-spinal tract or the lateral pyramidal
tract. Because they descend along the
sides of the spinal cord, the uncrossed or direct fibers that synapse with
spinal nerves on the ipsilateral side of the body are called
the direct pyramidal
tract. They may also be referred to as the
or the anterior cortico-spinal
since they travel down the ventral aspect of the spinal
The spinal nerves
receive only contralateral innervation from the
cortico-spinal tract. This means that unilateral pyramidal
tract lesions above the point of decussation in the pyramids
will cause paralysis of the muscles served by the spinal
nerves on the opposite side of the body. For example, a
lesion on the left pyramidal tract above the point of
decussation could cause paralysis on the right side of the
The fibers of the
pyramidal tract that synapse with cranial nerves located in
the brain stem form the corti-cobulbar
tract. Obviously, this is the part of the
pyramidal tract that carries the motor messages that are
most important for speech and swallowing. Corticobulbar
axons descend from the cortex within the
genu or bend of the internal capsule.
Almost all of the
cranial nerves receive bilateral innervation from the
fibers of the pyramidal tract. This means that both the left
and right members of a pair of cranial nerves are innervated
by the motor strip areas of both the left and right
This redundancy is
a safety mechanism. If there is a unilateral lesion on the
pyramidal tract, both sides of body areas connected to
cranial nerves will continue to receive motor messages from
the cortex. The message for movement may not be quite as
strong as it was previously but paralysis will not occur.
The two exceptions
to this pattern are the portion of CN XII that provides
innervation for tongue protrusion and the part of CN VII
that innervates the muscles of the lower face. These only
receive contralateral innervation from the
pyramidal tract. This means that they get information only
from fibers on the opposite side of the brain. Therefore, a
unilateral upper motor neuron lesion could cause a
unilateral facial droop or problems with tongue protrusion
on the opposite side of the body. For example, a lesion on
the left pyramidal tract fibers may cause the right side of
the lower face to droop and lead to difficulty in protruding
the right side of the tongue. The other cranial nerves
involved in speech and swallowing would continue to function
almost normally as both members of each pair of nuclei still
receives messages from the motor strip.
cranial nerves receive bilateral innervation, lesions of the
upper motor neurons of the pyramidal tract must be bilateral
in order to cause a serious speech problem. (The effects of
the inability to protrude the tongue and of paralysis of the
lower face on speech are negligible.)
On the other hand,
unilateral lesions of the lower motor neurons may cause
paralysis. This occurs because the lower motor neurons are
the final common pathway for neural messages traveling to
the muscles of the body. At the level of the lower motor
neurons, there is no alternative route which will allow
messages from the brain to reach the periphery. Muscles on
the same side of the body as the lesion will be affected.
Lesions on the
cranial nerve nuclei located in the brain stem are called
lesions. The paralysis that they produce is
called bulbar palsy.
Lesions to the
axons of the cranial nerves are called peripheral
As cranial nerves
are lower motor neurons, both bulbar and peripheral
lesions are lesions of the final common
lesions of the upper motor neurons of the pyramidal tract
occur, they produce a paralysis resembling that which occurs
in bulbar palsy. For this reason, the condition is known as
If a lesion occurs
in the brain stem and damages both the nucleus of a cranial
nerve and one side of the upper motor neurons of the
pyramidal tract, a condition known as alternating
hemiplegia may result. This involves paralysis
of different structures on each side of the body. The lesion
on the nucleus of the cranial nerve will cause a paralysis
of the structures served by that nerve on the same side of
the body as the injury. Because the pyramidal tract provides
only contralateral innervation to the spinal nerves, damage
to the upper motor neurons will meanwhile cause a paralysis
of different structures on the other side of the body. For
example, a lesion that affected the right nucleus of the
trigeminal cranial nerve and the right side of the pyramidal
tract would cause paralysis of the right side of the jaw and
the left arm or leg.
cortico-spinal and cortico-bulbar tracts send some axons to
the pontine nuclei as they descend to synapse with lower
motor neurons. These fibers that end in the pons form the
cortico-pontine tract. This pathway carries
information to the cerebellum (cortico-pontine-cerebellar)
about the type and strength of the motor impulses generated
in the cortex. While the cortico-pontine fibers actually end
in the pontine nuclei, second order neurons carry their
message to the cerebellum via the middle cerebellar
peduncle. This tract may be considered to be a part of the
system rather than a component of the
pyramidal tract since it does not synapse directly with
lower motor neurons.
system is involved in automatic motor movements, and in
gross rather than fine movement. It works with the autonomic
nervous system to help with posture and muscle tone and has
more influence over midline structures than those in the
periphery. Facial expression is one important communicative
behavior that is mediated by the extrapyramidal tract. This
is the reason that some Parkinson's patients have little
facial expression. In contrast to the pyramidal tract, the
extrapyramidal tract is an indirect, multisynaptic tract.
Components of the
extrapyramidal tract include the basal ganglia, the
nucleus, the substantia nigra, the reticular
formation and the cerebellum. All of these structures
send information to the lower motor neurons.
including the text by Love and Webb, 1992, consider the
basal ganglia to be the sole constituent of the
extrapyramidal system, saying that the other structures
listed above synapse with the extrapyramidal tract but are
not part of it.
The basal ganglia
acts to inhibit the release
phenomenon, or the rapid firing of motor
neurons. It is aided in this function by the substantia
nigra of the midbrain. The muscles most often affected by
this inhibitory functions are those controlling the head,
the hands, and the fingers.
neurotransmitters involved in the inhibitory function of the
basal ganglia include dopamine, which is produced by the
substantia nigra, acetylcholine, and GABA (gamma amino butyric acid),
which is a glutamate. Dopamine is an especially powerful
Projections to Lower Motor Neurons
tract has an important role in motor movement. It has
projections that carry autonomic motor impulses to voluntary
muscles in the body, including the muscles for speech and
swallowing. During speech, muscles are receiving input from
both the pyramidal and extrapyramidal systems. it is
involved in gross motor movement rather than fine. It is
responsible for facial expression such as sadness, irony and
passes through the red nucleus. The cerebellum sends
messages to the spinal nerves along this tract. Information
flows from the superior cerebellar peduncle to the red
nucleus and finally to the spinal nerves. This information
is very important for somatic motor, or skeletal muscle
control and the regulation of muscle tone for posture.
runs from the reticular nuclei of the pons and medulla to
the spinal nerves. It is involved in somatic motor control
like the rubrospinal tract and also plays an important role
in the control of autonomic functions.
has points of origin throughout the brain stem, but
especially in the midbrain area, and ends in the spinal
nerves. It is involved in the control of neck muscles.
runs from the vestibular nuclei located in the lower pons
and medulla to the spinal nerves. It is involved in balance.
(Note that all of
these tracts receive input from the cerebellum.)
Diseases and Syndromes Affecting Communication/Swallowing
Lesions in the extrapyramidal tract
cause various types of diskinesias or disorders of
problems most commonly affecting the extrapyramidal tract
include degenerative diseases, encephalitis, and tumors.
Disease, which is a degenerative disease, is
probably the most frequently occurring illness that results
from extrapyramidal tract lesions. It occurs when the
dopaminergic neurons of the substantia nigra are destroyed.
Its symptoms include:
movements, especially a festinating gait.
(Festinating movements are movements which become
increasingly rapid and uncontrolled).
associated specifically with lesions of the basal ganglia
Chorea and Sydenham's
Chorea. The term "chorea" comes from the
Greek "khoros" which means dance. Both of these diseases are
associated with jerky, uncontrolled movements of the limbs.
Sydenham's chorea was probably the cause of the malady that
was known as St. Vitus' Dance during the middle ages.
Huntington's Chorea is an inherited degenerative disease.
Sydenham's tends to clear up spontaneously.
Tremor Syndrome, which is associated with Spastic
Dysphonia may also be the result of basal ganglia lesions.
Lesions of the
basal ganglia will also cause hyperkinetic
Note that not only
is the definition of the extrapyramidal system
controversial, but also many sources say that it is very
difficult to make functional distinctions between the
extrapyramidal and pyramidal systems. When upper motor
neuron lesions occur, it is sometimes difficult to determine
which tract has been damaged.