Corticobulbar Tract

Introduction[edit | edit source]

Corticobulbar tract.png

The corticobulbar tract is composed of the upper motor neurons of the cranial nerves. The muscles of the face, head and neck are controlled by the corticobulbar system, which terminates on motor neurons within brainstem motor nuclei. Contrast this to the corticospinal tract were the cerebral cortex connects to spinal motor neurons, and controls movement of the torso, upper and lower limbs.[1].

Anatomy[edit | edit source]

Origin: Motor cortex (precentral gyrus and anterior part of the paracentral lobule) [1]

Course

The corticobulbar tracts leave the internal capsule and enter the basilar part of the pons as numerous bundles [1] The fibres leave the cerebral crus adjacent to the corticospinal tract.[1] The fibres can take several paths and have several different terminations:

  • Termine directly on alpha motor neurones or interneurones innervating alpha motorneurons in the brainstem. These control somatic motor acitivity in the head e.g. muscles that control mastication, expression and eye movement.[2]
  • Axons that innervate motor nerve cranial nuclei can decussate (cross) before they terminate, resulting in them innervating contralateral muscles. As some decussate and some descend ipsilaterally, it results in bilateral descending control. [2]
  • Directly innervate cranial nervces or through interneurone,si.e.,. via the corticospinal tract. [2]

Function[edit | edit source]

Innervates muscles of the face, tongue, jaw, and pharynx, via the cranial nerves.[3]

The corticobulbar tract directly innervates the nuclei for cranial nerves:

Cranial nerves motor regions of X (vagus nerve) in the nucleus ambiguus.[4]

Pathology[edit | edit source]

Unilateral leisons to the corticobular usually do not result in any clinical effect on the neck and head muscles as the lower motor neurons of the brain stem recieve bilateral corticobulbar innervation. [5] However there are two exceptions to these rules:

  1. Facial nucleus (VII): The muscles of the lower face recieve contralateral input from the opposite motor cortex. Therefore contralateral leisons to the motor cortex/internal capsule results in weakness to the face muscles in the opposite side of the face. However they would still be able to wrinkle their forehead as this is bilaterally innervated by the corticobulbar tract. [5][6]
  2. Hypoglossal nucleus (XII): The genioglossus muscle (muscle responsible for sticking out the tongue) receives innervation from the contralateral motor cortex. Therefore lesion involving the right motor cortex/ internal capsule would result in weakness in the left hypoglossal muscle. Therefore due to the weakness in the left side of the tongue, the strong muscle on the right side, pushes the tongue to the left. [5]

Terminology[edit | edit source]

The corticobulbar (or corticonuclear) tract, a white matter pathway connecting the cerebral cortex to the brainstem, gets the word 'bulb' from an archaic term for the medulla oblongata. In modern clinical usage, it sometimes includes the pons as well. The word 'bulbar' therefore refers to the nerves and tracts connected to the medulla, and also by association to the muscles thus innervated, those of the tongue, pharynx and larynx[7].

Resources[edit | edit source]

[8]


References
[edit | edit source]

  1. 1.0 1.1 1.2 1.3 Young, Paul. Med school Anatomy. CHAPTER 4. THE PYRAMIDAL SYSTEM. http://medschool.slu.edu/anatomy/guide_ms/chapter_4.html (accessed 6/5/2016)
  2. 2.0 2.1 2.2 Brown, AC. Physiology & Neuroscience Web Sites. Neuroscience: Motor Systems. http://www.acbrown.com/neuro/Lectures/Motr/NrMotrPrmr.htm (accessed 6/5/2016)
  3. The Brain from Top to bottom. THE AXONS ENTERING AND LEAVING THE MOTOR CORTEX. http://thebrain.mcgill.ca/flash/i/i_06/i_06_cl/i_06_cl_mou/i_06_cl_mou.html (accessed 6/6/2016)
  4. 4.0 4.1 Revolvy. Corticobulbar tract. http://broom02.revolvy.com/main/index.php?s=Corticobulbar%20tract&item_type=topic (Accessed 6/6/2016)
  5. 5.0 5.1 5.2 HyperBrain. A resource for learning Neuroanatomy. Chapter 10. Lower and Upper Motor Neurons and the Internal Capsule. http://library.med.utah.edu/kw/hyperbrain/syllabus/syllabus10.html (accessed 05/06/2016)
  6. Dong Z, Wang G, Lu S, Li J, Yan W, Wang SJ. Spontaneous facial expressions and micro-expressions coding: From brain to face. Frontiers in Psychology. 2022 Jan 4;12:784834. [1]
  7. Liquisearch Corticobulbar tract Available:https://www.liquisearch.com/corticobulbar_tract (accessed 27.4.2022)
  8. Soton Brain Hub. Corticobulbar tract. https://www.youtube.com/watch?v=GfTDIGoGiY8