Online Lesson

SPH405

SPH405 : The Class : Efferent Systems : Descending Tracts : Online Lesson 1

Neurological Foundations of Speech, Language and Hearing

Online Lesson
The Corticobulbar Tracts of the Pyramidal System

GOAL: TO PRESENT THE CORTICOBULBAR TRACTS OF PYRAMIDAL SYSTEM

OBJECTIVES:

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

Associate the functions of the corticobulbar tracts with communication and swallowing.

Trace the corticobulbar tracts from the frontal lobe to the bulbar musculature.

The PYRAMIDAL SYSTEM Is the neurological structure by which voluntary, conscious, motor impulses travel from the frontal lobe to the muscle the individual intents to contract.

There are three distinct groupings of nerve fibers in the pyramidal system: the two cortocobulbar tracts and the corticospinal tract.

Corticobulbar #1 is for the voluntary eye field. It feeds input into cranial nerves III; IV; VI.

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

Corticospinal tracts are for spinal motor nerves. spinal motor nerves emerge from the ventral aspect of the cord. with final common synapses in the anterior gray horns.

Corticobulbar #1 pathway begins in the cerebral cortex.

This is Brodmann's area #6, located in the inferior part of the frontal lobe.

Upper Motor Neuron cell bodies lie here.

Axons go to INTERNAL CAPSULE.

They form a compact bundle and enter at the genu, which looks like the knee of a little leg.

They are bound for the motor nuclei of to III IV & VI.

III fibers have mixed decussation at this point.

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

This gives bilateral innervation for the muscles served by III, including the superior rectus; medial rectus; inferior rectus and inferior oblique; and the levator palpebrae superioris.

Cranial nerve III leaves the brainstem at the upper midbrain level.

IV fibers aren't crossed, and descend ipsilaterally to the nucleus of the trochlear nerves.

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..

The anatomical arrangement of IV is distinctive: it is the only Cranial nerve to leave the brainstem from the dorsal aspect.

IV supplies motor impulses to the superior oblique muscles to allow eye gaze temporally and interiorly.

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

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

Leave the brainstem at the level of the lower pons above the junction of the pons medulla junction

To reach the lateral rectus muscles which rotates the eye to the temporal aspect.

Cortocobulbar #2 fibers provide motor innervation to the motor nuclei of cranial nerves V, VII, IX, X-XI and XII.

They originate in the lateral 1/3 of the precentral gyrus of the frontal lobe: area #6.

Axons course from here to the genu of the internal capsule.

Corticobulbar #2 fibers innervates cranial nerves nuclei BILATERALLY with three exceptions.

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

Those that go to the portion of X-XI for innervation of the levator Veli Palatini muscle

Those that supply the XII for muscles tongue.

This means that lesions of the right Upper Motor Neurons will result in

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

Paralysis of the left levator Veli Palatini (velum will move up and right).

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

Once you have finished you should:

Go on to Online lesson 2
or
Go back to Major Descending Tracts

E-mail Bill Culbertson at bill.culbertson@nau.edu
Call Bill Culbertson at (520) 523-7440