GOAL: TO PRESENT THE Corticobulbar TRACTS OF PYRAMIDAL SYSTEM

OBJECTIVES:

1. Differentiate the structures and functions of the divisions of the pyramidal tracts.

2. Associate the functions of the Corticobulbar tracts with communication and swallowing.

3. Trace the Corticobulbar tracts from the frontal lobe to the bulbar musculature.

  1. The PYRAMIDAL SYSTEM is the neurological pathway by which voluntary, conscious, motor impulses travel from the frontal lobe, through the Central Nervous System, to the peripheral nerves and, finally, to the muscle the individual intends to contract.


  2. There are four distinct groupings of nerve fibers in the pyramidal system: two Corticobulbar tracts and two Corticospinal tracts.


    1. Corticobulbar #1 tract is for voluntary eye movement.


      1. It feeds input into the motor nuclei of cranial nerves III, IV & VI, allowing the individual to voluntarily direct the movements of his or her eyes to a given object of interest.


      2. Note that many eye movements are automatic, and, therefore, not effected by the Corticobulbar #1 tracts.


    2. Corticobulbar tract #2 feeds into the motor nuclei of cranial nerves V, VII, IX, X-XI, & XII.


      1. This tract enables voluntary, conscious movement of the muscles of the face, mouth and pharynx.


      2. Note that some movements of the face, mouth and pharynx are involuntary, and not effected by Corticobulbar #2 tract.


    3. Corticospinal tracts project to spinal motor nerves. Spinal motor nerves emerge from the ventral aspect of the cord, with final common synapses in the anterior gray horns.


      1. There are two Corticospinal tracts, coursing along the dorsolateral and anterior (ventral) aspects of the white spinal cord columns.


      2. The Lateral Corticospinal tracts provide voluntary movement of the appendicular musculature.


      3. The Anterior Corticospinal tracts provide voluntary movement of the axial musculature.


  3. THE CORTICOBULBAR TRACTS


    1. Now that we have had an overview of the Pyramidal Tracts in general, let's focus on the Corticobulbar tracts. These, arguably, have the greatest relevance to the practice of Speech-Language Pathology.


    2. Students should NOT infer that, just because the Corticospinal tracts are essential to voluntary speech and eye movements, the Corticospinal tracts are irrelevant to communication. Without the Corticospinal tracts, there would be no world exploring, no writing, no hand gesturing and no sign language.


  4. Corticobulbar #1 pathway begins in Brodmann's area #6 of the Cerebral Cortex.


    1. For those of us who haven't completely memorized Broadmann's areas, this is the Paracentral part of the frontal lobe, in the middle of the lateral surface, just anterior to the Precentral Gyrus


    2. Here, we find the cell bodies of the Multipolar Upper Motor Neurons. Their axons descend through the cortical layers.


  5. Axons go to INTERNAL CAPSULE.


    1. They form a compact bundle and enter at the genu, which looks like the knee of a little leg. The fibers enter the Internal Capsule (and, yes, there is an "External Capsule," but we're leaving it alone right now.), just below the knee. Anatomists call this part of the Internal Capsule the "Posterior Limb."


    2. The axons are bound for the motor nuclei of to III IV & VI.


      1. Axons bound for III have mixed decussation in the Brainstem.


        1. Some fibers cross in the brainstem, some do not. The fibers that cross do so before the synapse with the motor nucleus of III.


        2. This gives bilateral innervation for the m.m. served by III, including the Superior Rectus; Medial Rectus; Inferior Rectus and Inferior Oblique; and the Levator Palpebrae Superioris.


        3. Cranial nerve III leaves the brainstem at the upper Midbrain level.


      2. IV fibers aren't crossed while they're in the CNS, and descend ipsilaterally to the nucleus of the Trochlear n.n.


        1. IV then crosses after it leaves the Brainstem at Midbrain level. The cranial nerve leaves the brainstem at the level of the of the Inferior Colliculus near the junction with the Pons.


        2. The anatomical arrangement of IV is distinctive.


          1. It is the only Cranial nerve to leave the brainstem from the dorsal aspect.


          2. It is also the only Cranial Nerve (the only nerve, for that matter) to cross AFTER it leaves the Brainstem.


        3. IV supplies motor impulses to the Superior Oblique m.m. to allow eye gaze temporally and interiorly.


        4. There are ABERRANT fibers of IV that join up with VI for coordination of eye m.m..


    3. Fibers of VI are all crossed before they reach the nucleus of VI.


      1. Leave the brainstem at the level of the lower pons above the junction of the Pons Medulla junction


      2. To reach the Lateral Rectus m.m. which rotates the eye to the temporal aspect.


  6. Corticobulbar #2 fibers provide motor innervation to the motor nuclei of Cranial Nerves V, VII, IX, X-XI and XII.


    1. They originate in the lateral 1/3 of the Precentral Gyrus of the Frontal Lobe, near the Sylvian (Lateral) Fissure: Broadmann's area #4. Note that this is the same part of the Cerebral Cortex that is the origin of the Corticospinal tracts.


    2. Axons course from here to the Genu of the Internal Capsule.


    1. Corticobulbar #2 fibers innervate cranial n.n. nuclei BILATERALLY with three exceptions.


      1. Tracts that innervate motor nucleus of VII for the lower m.m. facial expression.


      2. Those that go to the portion of X-XI for innervation of the pharyngeal m.m., including Levator Veli Palatini and intrinsic laryngeal muscles.


      3. Those that supply the XII for m.m. tongue.


    2. This means that lesions of the right upper motor neurons will result in


      1. Paralysis of the left lower m.m. facial expression. The patient will smile emotionally, but not voluntarily.


      2. Paralysis of the pharyngeal musculature. Symptoms of such paralysis are quite variable, but may include hypernasal speech (velum will move up and right), dysphagia and dysphonia, depending upon the extent of the lesion.


      3. Paralysis of the left tongue m.m. (the tongue will protrude to right).


    3. Because of the affected labial, lingual and pharyngeal musculature, speech may be affected by spastic Dysarthria. However, many patients can compensate for the motor speech disorder. Speech symptoms may range from "Barely noticeable" to "Barely Intelligible and will change as the patient recovers.


    4. Patients with Dysphagia secondary to the Upper Motor Neuron lesion will also have widely variable symptoms. The skills of the Speech-Language Pathologists are required to evaluate and monitor any danger of aspiration during the recovery from disease or injury of the Nervous System

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