Chapter 10. Upper Motor Neuronal Tracts
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.
Upper 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.
Lower 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.
The Pyramidal Tract
This group of fibers carries messages for voluntary motor movement to the lower motor neurons in the brain stem and spinal cord.
Approximately 80% 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.
At the 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 the 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 ventral pyramidal tract or the anterior cortico-spinal tract since they travel down the ventral aspect of the spinal cord.
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 body.
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 hemispheres.
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.
Because most 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 bulbar lesions. The paralysis that they produce is called bulbar palsy.
Lesions to the axons of the cranial nerves are called peripheral lesions.
As cranial nerves are lower motor neurons, both bulbar and peripheral lesions are lesions of the final common pathway.
When bilateral 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 pseudo-bulbar palsy.
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.
Both the 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 extrapyramidal system rather than a component of the pyramidal tract since it does not synapse directly with lower motor neurons.
This 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 red nucleus, the substantia nigra, the reticular formation and the cerebellum. All of these structures send information to the lower motor neurons.
Some sources, 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.
The 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 inhibitor.
Extrapyramidal Projections to Lower Motor Neurons
The extrapyramidal 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 happiness.
The rubrospinal tract 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.
The reticulospinal tract 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.
The tectospinal tract 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.
The vestibulospinal tract 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.)
Extrapyramidal Diseases and Syndromes Affecting Communication/Swallowing
Lesions in the extrapyramidal tract cause various types of diskinesias or disorders of involuntary movement.
The problems most commonly affecting the extrapyramidal tract include degenerative diseases, encephalitis, and tumors.
Parkinson's 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:
Tremor
Festinating movements, especially a festinating gait. (Festinating movements are movements which become increasingly rapid and uncontrolled).
Hypokinetic dysarthria
Weak Voice
Mask-like facial expression
Diseases associated specifically with lesions of the basal ganglia include Huntington's 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.
Essential 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 dysarthria.
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.