Neuroanatomy of the Corticobulbar and Corticospinal Tracts PDF

OS 202: integration & control systems Neuroanatomy of the Corticobulbar & corticospinal tracts Jose Leonard Pascual, MD | January 29, 2024 OUTLINE I. Overview of the Anatomy III. Lower Motor Neurons...

OS 202: integration & control systems Neuroanatomy of the Corticobulbar & corticospinal tracts Jose Leonard Pascual, MD | January 29, 2024 OUTLINE I. Overview of the Anatomy III. Lower Motor Neurons of the Motor Cortex A. Lower Motor Neurons in A. Motor-Hand Area the Brainstem B. Premotor Cortex & B. Lower Motor Neurons Axonal Tracts of the Spinal Cord II. Corticospinal and IV. Lesions Corticobulbar Tracts A. Analogies A. Tract Orientation B. Upper Motor Control B. Corticobulbar Tract of Cranial Nerves Made Simple C. Upper Motor Control C. Corticobulbar Fiber of Spinal Nerves Decussation D. Neurological Atlas of D. Corticospinal Tract the Brainstem Decussation V. Appendix E. Clinical Correlation Abbreviation Meaning Figure 2. Superior Dorsal View of the Precentral Gyrus (Red); Central sulcus PCG Precentral gyrus (yellow) CSF Cerebrospinal fluid PMC Primary motor cortex B. MOTOR-HAND AREA “Upside Down Omega” I. OVERVIEW OF THE ANATOMY OF THE MOTOR CORTEX → Important landmark to remember the position of the Fibers of the corticospinal & corticobulbar tracts come from the motor-hand area of the PCG sensorimotor cortex at the banks of the central sulcus [2025 trans] → Round part of the omega sign → 55% from the frontal lobe ▪ Observed in the PCG as the motor-hand area → 35% from the parietal lobe − Represented as a “knob”[2026 trans], similar to a bump Precentral Gyrus (PCG) − Part of the brain that moves your hands ▪ Constant and always present, may just look different Anatomical variants of the PCG are normal and differences may vary from one individual’s brain to another → Some brains have the upside down omega only in one side (refer to Fig. 4) ▪ Identifying the omega sign guides neurosurgeons to be careful not to destroy the motor-hand area → Some brains have PCG look like an epsilon instead of an upside down omega (refer to Fig. 5) → Because of the anatomical differences, using constants (such as the frontal gyri) in the brain anatomy is important in properly identifying the PCG Figure 1. Lateral View of the Precentral Gyrus (Red); Central sulcus (yellow) → Contains neurons with a 1:1 representation of the muscles on the body ▪ The more fine the movement, the more neurons are involved and present. Gross movements will have fewer neurons. ▪ More important movements will have more neurons assigned to that muscle → Part of the frontal lobe of the telencephalon which functions in the voluntary muscle activation and is connected to the corticospinal tract[2026 trans] → Runs transversely from the brain’s apex to the ear towards the Figure 3. Precentral gyrus represented as an inverted Omega sign Sylvian fissure)[2026 trans] → The central sulcus can be used as a guide in identifying the precentral gyrus[2026 trans] Exam 01 Trans 02 TG10: Decena, Dealca, Demerey, De Perio, Dela Cruz, Del Rosario TH: Cansino 1 of 10  → Through superior frontal sulcus: ▪ Locate the superior frontal sulcus (going front to back) and locate the sulcus that intersects with it (precentral sulcus) ▪ Forms an arrow or anchor that points to the motor-hand area of the PCG Figure 4. Upside-down Omega seen only on one side in yellow) Figure 7. Brain sample with intact arachnoid mater (UPCM). PCG (yellow); Central Sulcus (red line); Note the bent area of the PCG which is the motor-hand area Infarction of the “hand knob” area (refer to Fig. 9)[2026 trans] → (A) is a T1 sequence → (B) is a T2 sequence ▪ Negative of T1 Figure 5. PCGs in an inverted omega and an inverted epsilon shape ▪ Black CSF in T1 becomes white The surrounding sulci can help in identifying the motor-hand area ▪ Gray matter becomes black (refer to Fig. 6) → In front of the PCG is the precentral sulcus (pink line) → In front of the precentral sulcus are three frontal gyri (superior, middle, and inferior frontal gyri) ▪ Superior frontal gyrus − Runs parallel to the interhemispheric fissure ((runs anterior to posterior)[2026 trans] − Directed anteriorly ▪ Superior frontal sulcus (red line) − Lateral to and bounding the superior frontal gyrus and intersects with the precentral sulcus (pink [2026 trans] → Superior frontal sulcus (red) and precentral sulcus (pink) becomes an imaginary arrow that points to the bend of the upside down omega (motor-hand area) Figure 8. Lesions (red circles) in the motor-hand area which causes weakness of the hand on the opposite side Area for the legs and feet in PCG → Located near the motor-hand area towards the vertex → Supplied by the anterior vertebral artery → Motor-hand area is supplied by the middle cerebral artery In between the hand and the foot area is the trunk Figure 6. Central sulcus (yellow line), precentral sulcus (pink line), superior frontal sulcus (red line) Locating the PCG (refer to Fig. 7) → Through superior frontal gyrus: ▪ Locate the superior frontal gyrus which goes from front to back ▪ Locate where it hits a gyrus which goes from the vertex Figure 9. Motor-hand area and area for the legs and feet in PCG OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts 2 of 10  → Caudate nucleus Clinical correlation → Space between putamen and globus pallidus → Stroke involving the anterior cerebral artery will present weakness in the legs and not so much on the hands → Middle cerebral artery strokes will affect the face and hands but not so much on the legs and feet → These patients can still walk after the stroke ▪ Weakness tends to persist over time depending on the patient’s neuroplasticity C. PREMOTOR CORTEX & AXONAL TRACTS Figure 12. Schematic diagram of obstacles traversed by PCG (leftmost structure). Figure 10. Primary Motor Complex (PMC) and associated motor controls in the Precentral gyrus (red) Face area → Goes all the way inside the Sylvian fissure to the deeper recesses of the mouth and tongue Figure 13. Orientation of the passing axons from the PCG containing the → Large part of the PCG is dedicated to the face[2026 trans] Primary Motor Area Mouth and tongue area In Fig.13 : → Speech area → Leg motor fibers → Ventrolateral to the motor-hand area at the banks of the ▪ Closest to the lateral ventricle Sylvian fissure[2025 trans] − Fibers need to curve around (jump over) the ventricle and Frontal eye field arch over the caudate nucleus [2026 trans] → At the intersection of the precentral sulcus and superior frontal − Enlarged lateral ventricle (due to blockage of CSF sulcus [2026 trans] pathways) causes stretching of leg motor fibers, resulting → Directs eye gaze to the other side of the body in unsteadiness/weakness of stance and gait → Where initiation of eye movements originates from − In children, it can be a congenital blockage of ventricular system or tuberculosis in the brain [2026 trans] ▪ Most of the arching fibers of the corona radiata will be coming from here ▪ Course of axons: centrum semiovale → corona radiata → lateral ventricle → caudate nucleus → putamen & globus pallidus → thalamus [2025 trans] → Hand motor fibers ▪ Converges with the leg motor fibers to form a large oval piece of white matter called the centrum semiovale − Arranges in a logical order such that it forms a small model of your body inside (homunculus) ▪ More direct path to the internal capsule, with no obstacle course going to the internal capsule [2026 trans] ▪ Course of axons: centrum semiovale → corona radiata → Figure 11. Schematic Diagram of the Axonal Connections from the Cortex internal capsule [2026 trans] to Various Body Parts (upper axons closer to vertex; lower axons closer to → Face and tongue motor fibers Sylvian fissure). ▪ Also converges with leg and hand motor fibers ORIENTATION OF AXONS OF PCG / PRIMARY MOTOR AREA ▪ Targets the brainstem’s cranial nerves instead of spinal cord Cortical fibers need to traverse the following structures to get to ▪ Has to arch around the insula to join the hand and foot the internal capsule fibers in the internal capsule [2026 trans] → Lateral ventricle (only for the leg fibers) OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts 3 of 10  → Face and tongue fibers (red): most anterior, closest to the genu The fibers for the arms and legs (yellow and green) are the corticospinal tract and the fibers for the head (red) are the corticobulbar tract. The layout of the fibers in the internal capsule continues in the midbrain cerebral peduncle (Fig. 17) → If we flip the specimen on the other side, the internal capsule becomes the cerebral peduncles (crus cerebri in the midbrain) [2025 trans] ▪ Still the same arrangement (corticobulbar tract still more anterior / medial to the corticospinal tract) ▪ A certain part of the cerebral peduncle is the same as the Figure 14. Comparison between paths of Leg Motor Fibers and Motor-hand Fibers internal capsule (red circle in Figure 17) → Going down the midbrain, the arm and leg fibers are located in the middle third of the cerebral peduncles [2026 trans] Figure 15. Schematic diagram of structures traversed by the fibers Caudate nucleus → Forms lateral wall of the lateral ventricle Figure 17. Axial cut of the brain from “Big Brain Project” with the Internal → Forms a dolphin-like structure along with putamen separated Capsule of the Diencephalon projecting caudally as the Crus Cerebri in by the anterior limb of the internal capsule the Midbrain Putamen II. CORTICOSPINAL AND CORTICOBULBAR TRACTS → Elongated piece of gray matter Corticospinal tracts contain axons of the upper motor Globus pallidus neurons Internal capsule → Its arm and leg fibers can be found at the middle third of the → Indicated in white in Fig. 15 cerebral peduncles and end at the spinal cord. → Partitioned into ▪ The fibers only account for 1 million out of 20 million fibers ▪ Genu Corticobulbar fibers originate in the region of the sensorimotor − Knee-like structure complex where the face is represented and terminates on motor ▪ Anterior limb neurons within the brainstem motor nuclei ▪ Posterior limb → Bulbar means towards the head and throat − Where most fibers converge − The blood vessels in this area are tiny and prone to A. TRACT ORIENTATION rupture when blood pressure gets high, which may result in bleeding and paralysis of the arm, legs, and face Figure 18. Slice of a Supratentorial Cerebral Hemisphere (left) and a slice of the Midbrain (right) Corticospinal Tract Fibers The orientation of the corticospinal tract fibers in the internal Figure 16. Reorientation of the fibers of the PMA towards the Posterior Limb of Internal Capsule capsule is preserved in the crus cerebri of the midbrain In Fig. 18, the arms fibers (yellow) are anterior to the leg fibers The fibers twist as they reach the posterior limb of the internal (green) capsule (Fig. 16) → Leg area fibers (green): most posterior → Hand area fibers (yellow): middle OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts 4 of 10  As the corticospinal tract goes down from the internal capsule the The Corticobulbar Tract [2027 Trans] same pattern remains unchanged. On the midbrain, the fibers can Everything is controlled contralaterally be found at the middle third of the cerebral peduncles. → Axons of the right corticobulbar neuron will end up on the left side, moving the left face, eyeball, throat, and tongue, and vice-versa → Decussation of the fibers must occur just before their target nuclei Examples: → Eyeballs: midbrain and pons → Jaw: motor nucleus of the trigeminal nerve → Face: facial nucleus in the lower pons → Tongue: hypoglossal nucleus reside in the medulla Weakness on one side of the face requires finding the lesion just before the crossing of the fibers to their target nuclei C. CORTICOBULBAR FIBER DECUSSATION Only half of corticobulbar fibers will cross to synapse on the lower motor neurons in the brainstem (contralaterally) The other half of corticobulbar fibers will synapse on the lower motor neurons in the brainstem on the same side (ipsilaterally) → Head and neck muscles are innervated ipsilateral and contralateral to allow “back-ups” as these muscles are essential for life (chewing, swallowing, etc.) making them Figure 19. Tractography of the brain, Precentral knob for the hand is colored blue, difficult to paralyze and mediodorsal part of the primary cortex for the leg is colored red[Kwon et al., 2011] Tractography is an MRI-based method used to delineate white matter tract in the brain by tracking the diffusion of water in neural tissue In Fig. 19 (B), the arm and leg fibers join together at the internal capsule, towards the midbrain and then to the pons where they will be dispersed. Corticobulbar Tract Fibers The corticobulbar tract is located medial to the corticospinal tract (red dot in Fig. 18) → It is closer to the genu of the internal capsule → Targets everything from the head and neck Although the tract occupies a tiny space, any relatively small lesions in the cerebral peduncle of the posterior limb of the internal capsule can cause paralysis B. CORTICOBULBAR TRACT MADE SIMPLE Figure 21. Ipsilateral and Contralateral Crossing of the Corticobulbar Fibers D. CORTICOSPINAL TRACT DECUSSATION The corticospinal tract crosses 90% of its fibers within the cervicomedullary junction (Figure 22) → Descends as the lateral corticospinal tract (LCT) → Lateral = Limbs The corticospinal tract has a similar pattern of crossing is similar to the corticobulbar tract however the target is the arms and the legs → Arms controlled by the neurons in cervical spinal cord → Legs controlled by neurons in the thoracic and lumbar spinal cord Figure 20. Decussation of the corticobulbar fibers) ▪ All neurons will cross above the junction (e.g. neurons for Corticobulbar fibers are the axons of the pyramidal neurons in the the arms cross before the cervicomedullary junction) face, eye, mouth, and tongue areas of the frontal lobe ▪ Arm neurons will cross first, then leg neurons right after Corticobulbar tract fibers will cross the midline as they approach ▪ Decussation takes place within the pyramids their target cranial nerve nuclei (lower motor neurons) in the Some corticospinal tract fibers (10%) descend uncrossed as the brainstem. anterior corticospinal tract (ACT) (Figure 23) → Anterior = Axial OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts 5 of 10  → Prominent; controls almost all eye muscles: iris, medial rectus, superior rectus, inferior rectus, inferior oblique, levator palpebrae superioris, superior oblique → Knocking out this nucleus complex can affect eye muscles ▪ may cause ptosis; may lead to eye pointing laterally and downward → Origin of oculomotor nerves [2027 Trans] → Combination of several oculomotor subnuclei [2027 Trans] Trochlear nucleus (CN 4) → Controls the superior oblique (*mnemonic: SO4) → Important for when you’re reading something Abducens nucleus (CN 6) → Controls the lateral rectus (*mnemonic: LR6) Hypoglossal nucleus (CN 12) Figure 22. Diagram of the lateral corticospinal tract (anterior horn only visible after → Controls all tongue muscles except palatoglossus arm fibers have crossed) ▪ Palatoglossus muscle innervated by CN 9 (Glossopharyngeal) Figure 23. Diagram of the anterior corticospinal tract (purple) Figure 25. Cranial Nerve Nuclei and the corresponding muscles they control. Diamonds represent the CSF pathway. Dark blue cylinders represent the nuclei. Locating the Cranial Nerve Nuclei [2027 Trans] Midbrain Area → Oculomotor nucleus complex (CN 3) → Trochlear nucleus (CN 4) ▪ Located just beneath the aqueduct of Sylvius Pons → Abducens nucleus (CN 6) ▪ Diamond enlarges denoting the fourth ventricle at the level of the pons and upper medulla Medulla → Hypoglossal nucleus (CN 12) ▪ Also contains the fourth ventricle Figure 24. Pyramids of upper medulla (red circle), Pyramids of the caudal medulla Columns of the Motor Nuclei (blue circle), and the dorsal horn of the caudal medulla (green circle) Destroying the lower motor neuron leads to atrophy of the muscle E. CLINICAL CORRELATION it controls Stroke → Flaccid paralysis → The uncrossed fibers or “back-ups” of the corticospinal and corticobulbar tracts can serve as a possibility for patients to regain limited motor functions through physical or speech therapy. ▪ E.g. The lateral corticospinal tract is often the one damaged. − Depending on the severity, the anterior corticospinal tract may be left undamaged. − Undamaged ACT is what physical therapists rely on during therapy of patients. − Neuroplasticity will remap the function of the LCT to ACT IV. LOWER MOTOR NEURONS A. LOWER MOTOR NEURONS IN THE BRAINSTEM The lower motor neurons are not randomly and haphazardly scattered throughout the brainstem [2027 Trans] Figure 26. Cranial Nerve Nuclei and the corresponding muscles they control. → Need to have control from the cerebral cortex Diamonds represent the CSF pathway. Dark blue cylinders represent the nuclei. Oculomotor Nucleus Complex (CN 3) Trigeminal motor nucleus (CN 5) OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts 6 of 10  → Control jaw muscles ▪ If it continues spinning → muscle affectation Facial nucleus (CN 7) Check if plugged (lower motor neuron) → Control facial muscles → Supplies power to the propellers (muscle) → Fibers have to loop around the 6th nerve first before they go Check Meralco/power box (upper motor neuron) out [2027 Trans] → Supplies power to the plug (lower motor neuron) Ambiguus nucleus → If you connect to a generator or car battery and the fan works, → “Common nucleus of the efferent fibers for glossopharyngeal then the problem is with Meralco (CN 9) and vagus (CN 10) nerves” [NIH, 2023] Note the “in-betweens” → Control swallowing muscles → Can be in between the plug and the power box (electric pole, Accessory nucleus (CN 11) wires) → Control neck and shoulder muscles Light Bulb B. LOWER MOTOR NEURONS OF THE SPINAL CORD Located in the anterior horn of the spinal cord’s “butterfly” The more neurons are required for more skillful muscles Arrangement: The more external, the more dextrous/agile as it evolved later [2027 Trans] Medial Columns (near midline) → Needed for keeping an upright posture → Control the postural/antigravity muscles [2027 Trans] ▪ Torso muscles (trunk) and axial muscles with less laterality in terms of control (neck, back, chest) [2026 Trans] → Receives bilateral innervation [2026 Trans] → Hard to knock out with a stroke [2027 Trans] ▪ Some stroke patients can still sit up or stand → Also missed in ALS, multiple sclerosis, or Guillain-Barre Figure 29. Light bulb analogy syndrome [2026 Trans] In this analogy, the light bulbs in the house are powered Lateral Columns contralaterally → Upper limb (arms, hands, fingers) and lower limb muscles Light bulbs = muscles (legs, feet, toes) [2026 Trans] Plug = lower motor neuron → Needed for finer motions [2027 Trans] Powerlines = tracts → Evolved later than medial columns [2027 Trans] Transformer = upper motor neuron If a light bulb dies (say the left side) → Check if the light bulb has a problem ▪ Real-life counterparts: Muscle strength testing and reflex testing → Check the plug/power socket ▪ Real-life counterparts − Motor neurons in the spinal cord (arms and legs) − Motor neurons in the medulla (thumb) − Motor neurons in the pons (face, jaw) − CN III, IV, IX (extraocular muscles) → Check the wires → Check for tendon reflexes [2026 Trans] Figure 27. Lower Motor Neurons of the Spinal Cord. → Check transformer IV. LESIONS [2027 Trans] Location can be anywhere from the origin of power line, before Lesion = region in an organ that has suffered damage the crossing junction, after the crossing junction Location can only be determined through findings from history A. ANALOGIES taking and neurological examinations Electric Fan B. UPPER MOTOR CONTROL OF CRANIAL NERVES Figure 28. Electric fan analogy How to fix a fan that is buzzing but not spinning? Figure 30. Facial nerve nucleus → Spin the propellers (muscle) OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts 7 of 10  Figure X represents a lower motor neuron (in this case, for the face) → If for example, the face was paralyzed, where is the lesion: nerve, lower motor neuron, cranial nerve nucleus, or corticobulbar tract? → If the left side is paralyzed, is it the left facial nerve or the upper motor neuron? → It cannot be isolated but there will be clues ▪ If CN VI is affected, then the problem might be at the level of the pons ▪ If CN III, then the problem is at that level. Figure 33. Damaged motor nuclei in which decussations are spared In Figure 33 wherein the motor nucleus is damaged due to tumor, stroke, or inflammation in the brain stem → Figure shows the affected nuclei are the right facial nucleus and right hypoglossal nucleus → Abnormalities in the examination would be in the same side as these are after the decussations → In this case, the decussations are spared Figure 31. Facial nerve nucleus with the left and right corticobulbar tracts → Each Facial nerve nucleus moves the ipsilateral side of the face[2027 trans] ▪ E.g. Left facial nerve nucleus moves the left side of the face → Right corticobulbar tract crosses/contralateral to the left facial nerve nucleus (i.e. the power line from the earlier analogy) [2027 trans] → For cranial nerves, they are also supplied by the contralateral (major) and ipsilateral (minor) tracts [2027 trans] ▪ Important as these movements are essential ▪ They are bilaterally innervated Figure 34. Lesion at the level of the facial nucleus prior to decussation → The face will not be completely paralyzed unless the nerve itself has been cut or the facial nerve nucleus has been In Figure 34, the weakness is on the left face and right tongue destroyed. → Something must have disconnected the right hypoglossal nucleus from the tract at the level of the indicated facial nerve nucleus → If the lower motor neuron is already damaged, that would be a clue to which side of the brain stem has a problem → The hypoglossal muscle has been damaged prior to its decussation to have involved the left facial nerve Figure 32. Facial nerve nucleus with the hypoglossal nucleus → In the Hypoglossal nucleus, the same bilateral innervation pattern is seen ▪ Except that the muscle (tongue) that is innervated is more agile or dextrous; hence, there would be even more contralateral innervation ▪ Tells us that which is more versatile: the face or the tongue Figure 35. Weakness only on the left side of the tongue o Definitive answer was not provided In Figure 35, there is weakness on the left side of the tongue, and everything else is normal → To have ONLY affected the left side of the tongue means the problem must be in the ff. possibilities: ▪ Nerve itself ▪ Its lower motor neuron ▪ In the muscle ▪ Neurovascular junction of the peripheral nerve OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts 8 of 10  C. UPPER MOTOR CONTROL OF SPINAL NERVES In upper motor control of spinal nerves, it now involves the corticospinal tract → Refers to control of arms and legs → Exhibits contralaterality (e.g. Left corticospinal tract innervating the right arm and right leg) → Whichever goes out first, decussates first ▪ Nerves of the arm sprout first from the cervical spinal cord, hence, it decussates first than nerve of the leg from lumbar spinal cord → Goes to the anterior horn cell for both arms and legs’ lower motor neurons Figure 37. Lesions if the Peripheral nerve, anterior horn, and corticospinal tract V. REFERENCES Kwon, H., Hong, J., & Jang, S. (2011). Anatomic Location and Somatotopic Arrangement of the Corticospinal Tract at the Cerebral Peduncle in the Human Brain. American Journal of Neuroradiology, 32(11), 2116-2119. Pascual, J.L., (2024). Neuroanatomy of the Corticobulbar & Corticospinal Tracts. Powerpoint Presentation. UPCM 2027 Trans. (November 4, 2022). Neuroanatomy of the Corticobulbar & Corticospinal Tracts. UPCM 2026 Trans. (December 7, 2021). Neuroanatomy of the Corticobulbar & Corticospinal Tracts. Figure 36. Upper motor control of spinal nerves In Fig. 36, the shoulder is weak and there is no atrophy of the muscles → Could it be a peripheral nerve if there is shrinkage of the muscles there? → Or is it in the anterior horn, which would mean there are other anterior horns affected as well as they are all clustered there → Or is the problem in the corticospinal tract? → These are the things that the neurologic exam will tell you when the reflexes are hyporeflexia, hyporeflexia, or areflexia (absence of the reflexes) OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts 9 of 10  APPENDIX Figure 19. Tractography of the brain, Precentral knob for the hand is colored blue and mediodorsal part of the primary cortex for the leg is colored red[Kwon et al., 2011] OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts 10 of 10