OS 202 Neuroanatomy of the Corticobulbar & Corticospinal Tracts PDF

OS 202: HUMAN BODY AND MIND 1: INTEGRATION AND CONTROL SYSTEMS NEUROANATOMY OF THE CORTICOBULBAR & CORTICOSPINAL TRACT UPCM 2029 | Dr. Jose Leonard Pascual | LU3 A.Y. 2024-2025 OUTLINE...

OS 202: HUMAN BODY AND MIND 1: INTEGRATION AND CONTROL SYSTEMS NEUROANATOMY OF THE CORTICOBULBAR & CORTICOSPINAL TRACT UPCM 2029 | Dr. Jose Leonard Pascual | LU3 A.Y. 2024-2025 OUTLINE I.​ Overview of the E.​ Analogies and Clinical Anatomy of the Motor Correlation Cortex III.​ Lower Motor Neurons A.​ Motor-Hand Area A.​ Lower Motor Neurons B.​ Premotor Cortex & in the Brainstem Axonal Tracts B.​ Lower Motor Neurons C.​ Premotor Cortex & of the Spinal Cord Axonal Tracts IV.​ Lesions II.​ Corticospinal and A.​ Analogies Corticobulbar Tracts B.​ Upper Motor Control of A.​ Tract Orientation Cranial Nerves B.​ Corticobulbar Tract C.​ Upper Motor Control of Made Simple Spinal Nerves C.​ Corticobulbar Fiber V.​ Review Decussation VI.​ References D.​ Corticospinal Tract VII.​ Appendix Decussation Figure 2. Superior Dorsal View of the Precentral Gyrus (Red); Central sulcus (yellow) LEARNING OBJECTIVES B.​ PREMOTOR CORTEX & AXONAL TRACTS 1.​ Trace the corticobulbar and corticospinal pathways from the ​ “Upside Down Omega” precentral gyri through the projection fibers in the subcortical ○​ Important landmark to remember the position of the motor-hand white matter into the brainstem and spinal cord. area of the PCG 2.​Differentiate upper motor neuron vs lower motor neuron. ○​ Round part of the omega sign ​ Observed in the PCG as the motor-hand area ABBREVIATION MEANING ​Represented as a “knob”[2026 trans], similar to a bump ​Part of the brain that moves your hands PCG Precentral gyrus ​ Constant and always present, may just look different ​ Anatomical variants of the PCG are normal and differences may CSF Cerebrospinal fluid vary from one individual’s brain to another ○​ Some brains have the upside down omega only in one side (refer PMC Primary motor cortex to Fig. 4) ​ Identifying the omega sign guides neurosurgeons to be careful not to destroy the motor-hand area I.​ OVERVIEW OF THE ANATOMY OF THE MOTOR CORTEX ○​ Some brains have PCG look like an epsilon instead of an upside A.​ MOTOR-HAND AREA down omega (refer to Fig. 5) ○​ Because of the anatomical differences, using constants (such as ​ Fibers of the corticospinal & corticobulbar tracts come from the the frontal gyri) in the brain anatomy is important in properly sensorimotor cortex at the banks of the central sulcus [2025 trans] identifying the PCG ○​ 55% from the frontal lobe ○​ 35% from the parietal lobe ​ Precentral Gyrus (PCG) ○​ 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 Sylvian fissure [2026 trans] ○​ The central sulcus can be used as a guide in identifying the Figure 3. Precentral gyrus represented as an inverted Omega sign precentral gyrus[2026 trans] ○​ Figure 1. Lateral View of the Precentral Gyrus (Red); Central sulcus (yellow) Figure 4. Upside-down Omega seen only on one side in yellow) Trans 4 TG30: Villanueva, Villanueva, Vitug, Wee, Yadao, Yu, Yu TH: Marquez 1 of 12  Figure 5. PCGs in an inverted omega and an inverted epsilon shape ​ The surrounding sulci can help in identifying the motor-hand area (refer to Fig. 6) ○​ In front of the PCG is the precentral sulcus (pink line) Figure 8. Lesions (red circles) in the motor-hand area which causes weakness of the hand on the opposite side ○​ In front of the precentral sulcus are three frontal gyri (superior, ​ Area for the legs and feet in PCG middle, and inferior frontal gyri) ○​ Located near the motor-hand area towards the vertex ​ Superior frontal gyrus ○​ Supplied by the anterior vertebral artery ​Runs parallel to the interhemispheric fissure ((runs anterior ○​ Motor-hand area is supplied by the middle cerebral artery to posterior)[2026 trans] ​ In between the hand and the foot area is the trunk ​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 9. Motor-hand area and area for the legs and feet in PCG Clinical Correlation ​ 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 Figure 6. Central sulcus (yellow line), precentral sulcus (pink line), superior frontal sulcus patient’s neuroplasticity (red line) ​ Locating the PCG (refer to Fig. 7) C.​ PREMOTOR CORTEX & AXONAL TRACTS ○​ 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 ○​ 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 10. Precentral Gyrus (Face Motor Control Area) ​ Find the gyrus that is oriented from the vertex to the Sylvian fissure ​ Face area ○​ Lateral to the motor-hand area ○​ Goes all the way inside the Sylvian fissure to the deeper recesses of the mouth and tongue ○​ Large part of the PCG is dedicated to the face 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 11. Precentral Gyrus (Mouth and Tongue Motor Control Area) ​ Black CSF in T1 becomes white ​ Gray matter becomes black ​ Mouth and tongue area ○​ Speech ○​ Ventrolateral to the motor-hand area at the banks of the Sylvian fissure OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 2 of 12  thalamus ​ 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 ○​ Course of axons: centrum semiovale → corona radiata → internal capsule ​ Face and tongue motor fibers ○​ Also converges with leg and hand motor fibers ○​ Targets the brainstem’s cranial nerves instead of spinal cord ○​ Has to arch around the insula to join the hand and foot fibers in the internal capsule Figure 12. Precentral Gyrus (Mouth and Tongue Motor Control Area) ​ Frontal eye field ○​ As bipedal animals, our gaze is allotted a significant portion of the precentral gyrus ○​ At the junction of the precentral sulcus and superior frontal sulcus ○​ Directs eye gaze to the other side of the body ○​ Where initiation of eye movements originates from Figure 16. Structures Traversed by the Motor Fibers ​ Caudate nucleus ○​ Forms lateral wall of the lateral ventricle ○​ Forms a dolphin-like structure along with putamen separated by the anterior limb of the internal capsule ​ Putamen ○​ Elongated piece of gray matter ​ Globus pallidus ​ Internal capsule ○​ Indicated in white in Fig. 16 ​ Partitioned into ​Genu ○​ Knee-like structure Figure 13. Diagram of the Locations of the DIfferent Axonal Tracts on the Precentral ​Anterior limb Gyrus ​Posterior limb ​ Every motor function is assigned an area and depending on the ○​ Where most fibers converge importance of the function, the larger the assigned region is. ○​ The blood vessels in this area are tiny and prone to rupture ​ The allotment of areas for different motor functions is different when blood pressure gets high, which may result in among people bleeding and paralysis of the arm, legs, and face ○​ ○​ Some might have a larger area allotted for their mouth etc. Figure 17. Reorientation of Motor Fibers in the Internal Capsule Figure 14. Contrasting Paths for the Axonal Tracts of the Hand and Foot ​ The fibers twist as they reach the posterior limb of the internal capsule (Fig. 17) ○​ Leg area fibers (green): most posterior ○​ Hand area fibers (yellow): middle ○​ 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) ​ 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 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. Figure 15. Axonal Paths for Different Motor Functions ​ Leg motor fibers ○​ Closest to the lateral ventricle ​ Seen in fig. 14, the fibers need to curve around (jump over) the ventricle and arch over the caudate nucleus ​Enlarged lateral ventricle (due to blockage of CSF pathways like in hydrocephalus) causes stretching of leg motor fibers, resulting in unsteadiness/weakness of stance and gait ​ In children, it can be a congenital blockage of ventricular system or tuberculosis in the brain ​ The moment the fibers from the PCG arrives at the lateral ventricle, it is called the corona radiata ​ Course of axons: centrum semiovale → corona radiata → lateral Figure 18. Axial cut of the brain from “Big Brain Project” with the Internal ventricle → caudate nucleus → putamen & globus pallidus → Capsule of the Diencephalon projecting caudally as the Crus Cerebri in the Midbrain OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 3 of 12  ○​ Voluntary are closer to the midline ​ Corticobulbar tract fibers will cross the midline as they approach II.​ CORTICOSPINAL AND CORTICOBULBAR TRACTS their target cranial nerve nuclei (lower motor neurons) in the ​ Corticospinal tracts contain axons of the upper motor neurons brainstem. ○​ Its arm and leg fibers can be found at the middle third of the ○​ Decussation is important since it allows for the localization of cerebral peduncles and end at the spinal cord. lesions. ​ The fibers only account for 1 million out of 20 million fibers ​ Corticobulbar fibers originate in the region of the sensorimotor The Corticobulbar Tract complex where the face is represented and terminates on motor ​ Everything is controlled contralaterally neurons within the brainstem motor nuclei ○​ Axons of the right corticobulbar neuron will end up on the left ○​ Bulbar means towards the head and throat side, moving the left face, eyeball, throat, and tongue, and vice-versa A.​ TRACT ORIENTATION ○​ Decussation of the fibers must occur just before/above 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 resides 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 ​ 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 C.​ CORTICOBULBAR FIBER DECUSSATION ​ Only 50% of corticobulbar fibers will cross or decussate to synapse Figure 19. Slice of a Supratentorial Cerebral Hemisphere (left) and a slice of the on the lower motor neurons in the brainstem Midbrain (right) ○​ The other half will synapse on the lower motor neurons in the CORTICOSPINAL TRACT FIBERS brainstem ipsilateral, on the same side ​ Only half decussates because there should be redundancy of ​ The orientation of the corticospinal tract fibers in the internal innervation or back-ups capsule is preserved in the crus cerebri of the midbrain ​ This tract controls muscles that are essential for life (chewing, ​ In Fig. X, the arms fibers (yellow) are anterior to the leg fibers swallowing, etc.) and are difficult to paralyze (green) D.​ CORTICOSPINAL TRACT DECUSSATION ​ The corticospinal tract crosses 90% of its fibers within the cervicomedullary junction and descend as the lateral corticospinal tract ○​ Lateral = Limbs ​ Some corticospinal fibers (10%) descend uncrossed as the anterior corticospinal tract ○​ This 10% can be harnessed to allow a paralyzed patient to sit, for example ○​ Anterior = Axial E.​ ANALOGIES AND CLINICAL CORRELATION ​ Check if the problem has to do with muscles or a problem with innervation (upper or lower motor neuron?) Figure 20. 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] ​ Tractography is an MRI-based method used to delineate white matter tract in the brain by tracking the diffusion of water in neural tissue ​ The arm and leg fibers join together at the internal capsule, towards the midbrain and then to the pons where they will be dispersed. (Fig. X, B) ​ As the corticospinal tract goes down from the internal capsule the same pattern remains unchanged. ○​ In the internal capsule, the fibers can be found at the posterior limb. ○​ On the midbrain, the fibers can be found at the middle third of the cerebral peduncles. (Fig X, D) B.​ CORTICOBULBAR TRACT MADE SIMPLE Figure 22. Analogy for localizing motor neuron lesions[Pascual, 2025] Clinical Correlation ​ Stroke ○​ 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. Figure 21. Decussation of the corticobulbar fibers ​Depending on the severity, the anterior corticospinal tract ​ Corticobulbar tract is located medially to the corticospinal tract. may be left undamaged. ○​ It is closer to the genu of the internal capsule ​Undamaged ACT is what physical therapists rely on during ○​ Targets everything from the head and neck therapy of patients. ​ Corticobulbar fibers are the axons of the pyramidal neurons in the ​Neuroplasticity will remap the function of the LCT to ACT face, eye, mouth, and tongue areas of the frontal lobe OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 4 of 12  III.​ LOWER MOTOR NEURONS ○​ Fibers have to loop around the 6th nerve first before they go out[2027 Trans] ​ Any corticobulbar or corticospinal tract targets a lower motor ​ Ambiguus nucleus neuron as the final neurological control for muscles ○​ “Common nucleus of the efferent fibers for glossopharyngeal A.​ LOWER MOTOR NEURONS IN THE BRAINSTEM (CN 9) and vagus (CN 10) nerves” [NIH, 2023] ​ The lower motor neurons are not randomly and haphazardly ○​ Control swallowing muscles scattered throughout the brainstem [2027 Trans] ​ Accessory nucleus (CN 11) ○​ Need to have control from the cerebral cortex ○​ Control neck and shoulder muscles ​ Oculomotor Nucleus Complex (CN 3) ​ Despite some of these actions being automatic, one is still able to ○​ Prominent; have some form of voluntary control over these movements ○​ Controls almost all eye muscles such as iris, medial rectus, superior rectus, inferior rectus, inferior oblique, levator palpebrae B.​ LOWER MOTOR NEURONS OF THE SPINAL CORD superioris, superior oblique ​ Located in the anterior horn of the spinal cord’s “butterfly” ○​ Must cross just above the midbrain ​ The more neurons are required for more skillful muscles ○​ Knocking out this nucleus complex can affect eye muscles ​ Arrangement: The more external, the more dextrous/agile as it ​ May cause ptosis; may lead to eye pointing laterally and ​ evolved later [2027 Trans] downward ​ Medial Columns (near midline) ○​ Origin of oculomotor nerves [2027 Trans] ○​ Needed for keeping an upright posture ○​ Combination of several oculomotor subnuclei [2027 Trans] ○​ Control the postural/antigravity muscles [2027 Trans] ​ Trochlear Nucleus (CN 4) ​ Torso muscles (trunk) and axial muscles with less laterality in ○​ Controls the superior oblique (*mnemonic: SO4) terms of control (neck, back, chest) [2026 Trans] ○​ Must cross just above the midbrain ○​ Receives bilateral innervation [2026 Trans] ○​ Important for when you’re reading something ○​ Hard to knock out with a stroke [2027 Trans] ​ Abducens Nucleus (CN 6) ​ Some stroke patients can still sit up or stand ○​ Controls the lateral rectus (*mnemonic: LR6) ○​ Also missed in ALS, multiple sclerosis, or Guillain-Barre syndrome ○​ Must cross just above the pons [2026 Trans] ​ Hypoglossal Nucleus (CN 12) ​ Lateral Columns ○​ Controls all tongue muscles except palatoglossus ○​ Upper limb (arms, hands, fingers) and lower limb muscles (legs, ​ Palatoglossus muscle innervated by CN 9 (Glossopharyngeal) feet, toes) [2026 Trans] ○​ Must cross just above the medulla ○​ Needed for finer motions [2027 Trans] ​ They do not occur simultaneously, resulting in differences among ○​ Evolved later than medial columns [2027 Trans] these weaknesses ○​ Serves as a clue that the problem lies in the brainstem Figure 25. Lower Motor Neurons of the Spinal Cord. IV.​ LESIONS A.​ ANALOGIES Figure 23. Lower Motor Neurons of the brainstem. 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 Figure 26. Broken electric fan analogy. ○​ Hypoglossal nucleus (CN 12) ​ Problem: The electric fan propeller won’t spin. ​ Also contains the fourth ventricle ​ Systematically look for the cause of the problem (lesion) ○​ Check the propeller (muscle) and other external parts for COLUMNS OF THE MOTOR NUCLEI brokenness or deformities. ​ Destroying the lower motor neuron leads to atrophy of the muscle ​ E.g., No deformities, doesn’t spin, but makes a sound when it controls turned on. ○​ Flaccid paralysis ○​ Check the power supply (nerves) ​ Check the electric socket (lower motor neuron) ​E.g., other electrical appliances and lights in the room are still on, so it must be isolated to this “lower motor neuron” ​ Check the powerline (upper motor neuron) ​Other houses in the neighborhood have electricity, so it must be isolated to your “house” ○​ The problem is probably mechanical and the fan itself needs fixing. (The lesion is in the muscle and not in the lower or upper motor neurons) ​ Example: Patient comes in with weakness of the hands and keeps dropping their cell phone. ○​ Identifying whether the problem is in the upper or lower motor neuron will save the physician and patient time and resources. Figure 24. Cranial Nerve Nuclei and the corresponding muscles they control. Diamonds represent the CSF pathway. Dark blue cylinders represent the nuclei. ​ Trigeminal Motor Nucleus (CN 5) ○​ Control jaw muscles ​ Facial nucleus (CN 7) ○​ Control facial muscles OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 5 of 12  Figure 27. House analogy. ​ Problem: Assume the powerlines are crossed. The light in one half Figure 29. Facial nerve nucleus and hypoglossal nerve of the house goes off. ​ Tongue muscles are controlled by the hypoglossal nerve ○​ Check the light bulb (specific body muscle) contralaterally (e.g., right nerve controlled by left corticobulbar ​ Muscle strength and reflex testing [2024 Trans] tract ​ If other household appliances still work, then the light bulb is ​ The tongue is more agile or dextrous; hence more contralateral broken. Otherwise; innervation[2024 Trans] ○​ Check the power socket (lower motor neuron) ​ Motor neurons for the arms, legs, thumb, face, jaw, etc. [2024 Trans] SAMPLE CASES ​ If a working household appliance won’t turn on when plugged into the socket being examined, then the problem is the power socket. Otherwise; ○​ Check the powerline (upper motor neuron) ​ If half the house lost power, then the transformer may have short-circuited ​ Check for signs that may narrow down the source of the problem. ○​ E.g., hyperreflexia is seen only in upper motor neuron problems ​ Location can only be determined through findings from history taking and neurological examinations B.​ UPPER MOTOR CONTROL OF CRANIAL NERVES Figure 30. Lesion on the left corticobulbar tract ​ Lesion: Left corticobulbar tract ○​ Affects the right facial nerve nucleus and right hypoglossal ○​ Due to ipsilateral back-up supplies, the muscles will only experience partial weakness Figure 31. Lesion on the left facial nerve nucleus ​ Lesion: Left facial nerve nucleus ○​ Left face is completely paralyzed ○​ Right tongue is partially weak ​ Lesion is before the decussation for the hypoglossal nerve. A lesion at and after the lesion will have ipsilateral effects Figure 28. Facial nerve nucleus ​ Something must have disconnected the right hypoglossal ​ The facial nerve has two parts: nucleus from the tract at the level of the indicated facial nerve ○​ Upper part: Comes from left and right hemispheres nucleus ○​ Lower part: Purely comes from the contralateral side ​ If the lower motor neuron is already damaged, that would be a ​ The left facial nerve nucleus has control from above from the clue to which side of the brain stem has a problem right precentral gyrus via the right corticobulbar tract. ​ The hypoglossal muscle has been damaged prior to its ​ The left corticobulbar tract also gives some backup to the decussation to have involved the left facial nerve[2024 Trans] left facial nerve nucleus ​ It cannot be easily isolated but there will be clues [2024 Trans] ○​ 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. ​ The face will not be completely paralyzed unless the nerve itself has been cut or the facial nerve nucleus has been destroyed[2024 Trans] OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 6 of 12  Figure 35. C5 anterior horn lesion Figure 32. Lesion on the left medulla ​ Lesion: Cervical anterior horn ​ Lesion: Left medulla ○​ Ipsilateral damage to the arms since it is after the decussation ○​ Unaffected facial muscles ○​ Legs are unaffected ○​ Tongue cannot protrude towards the left and deviates to the right (ipsilateral damage) ​ 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[2024 Trans] C.​ UPPER MOTOR CONTROL OF SPINIAL NERVES Figure 36. Lesion at the corticospinal tract above the decussation ​ Lesion: Stroke above the spinal cord (e.g., cerebral hemisphere) ○​ Contralateral weakness in the arms and legs ​ If the lesion is on the left tract, then the weakness is on the right side of the body. ​ If there is also weakness on the left side then either the neuro exam is wrong or there is something else wrong with the patient. There may be more than one problem. Figure 33. Corticospinal tract at the level of the cervical spine (C5) and lumbar spine (L1) ​ Corticospinal tract ○​ 10% of fibers will NOT decussate at the cervicomedullary junction ○​ C5 root innervates the arms and L1 root innervates the legs ○​ Whichever goes out first, decussates first [2024 Trans] ​ 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 ​ If the shoulder is weak and there is no atrophy of the muscles [2024 Trans] Figure 37. Lesion at the corticospinal tract below the decussation ○​ Could it be a peripheral nerve if there is shrinkage of the muscles there? ​ Lesion: Corticospinal tract, after the decussation ○​ Or is it in the anterior horn, which would mean there are other ○​ Ipsilateral weakness to both arms and legs anterior horns affected as well as they are all clustered there ○​ Or is the problem in the corticospinal tract? V.​ REVIEW ○​ These are the things that the neurologic exam will tell you ​ Localization 101 when the reflexes are hyporeflexia, hyporeflexia, or areflexia ○​ Is there a (neurological) lesion? (absence of the reflexes) ​ Consciousness ​ Movement SAMPLE CASES ​ Senses ​ Where is the lesion? ○​ Is it primarily a nervous system lesion or secondary to a systemic condition? ○​ Is the lesion focal or diffusely involving the nervous system? ○​ Is there a single lesion that will explain all the neurological signs and symptoms or are there multiple lesions? ○​ Occam’s Razor ​ “Plurality is not to be posited without necessity” - William Ockham ​ The solution has the simplest explanation. ○​ Criteria on where you can find the lesion: ​ Level of consciousness ​ High cerebral functions ​ Motor Function of neck & extremities ​ Pain and light touch ​ Vision Figure 34. C5 Peripheral nerve lesion ​ Pupils and other motor cranial nerves ​ Lesion: Peripheral nerve ​ Balance and coordination ○​ Only that specific body part innervated by the peripheral nerve ​ Autonomic functions will be affected (e.g., biceps in this case) ○​ Will exhibit normal reflexes in unaffected body parts but weak reflexes in the biceps OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 7 of 12 Table 1. Guide in knowing where the lesion is Part of Nervous Sensor- Proprio- Autono Vision Motor Pain Balance System ium ception -mics Cerebral Cortex ✔ ✔ ✔ ✔ ✔ Subcortical ✔ ✔ ✔ ✔ ✔ Diencephalon ✔ ✔ ✔ ✔ ✔ ✔ Midbrain ✔ ✔ ✔ ✔ ✔ ✔ Pons ✔ ✔ ✔ ✔ ✔ ✔ Medulla ✔ ✔ ✔ ✔ ✔ Cervical ✔ ✔ ✔ ✔ ✔ Thoracis ✔ ✔ ✔ ✔ ✔ Lumbar ✔ ✔ ✔ ✔ ✔ Conus Medullaris ✔ ✔ ✔ ✔ ​ Key Facts ○​ Connections to and from the brain will cross over (decussate) to the other side at some point. ​ Decussation Localization Brain Hacks ○​ Remember where the long tracts decussate ○​ Be mindful if the lesion interrupt long tracts before or after they have crossed ○​ Occam’s razor applies, most of the time. ​ Triangulating Neurological Lesions ○​ Exclusively intracranial ​ Special senses ​ Extraocular movements ​ Facial expressions and jaw movements ​ Tongue movements ​ Swallowing and phonation ​ Language and other higher cerebral function ​ Localization Approach to the Unconscious ○​ Check the pupils ○​ Examine the extraocular movements ○​ Examine responsiveness to painful stimuli as well as motor response ○​ Observe for abnormal respiratory patterns VI.​ REFERENCES AND CITATION ​ 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., (2025). Neuroanatomy of the Corticobulbar & Corticospinal Tracts. Powerpoint Presentation. ​ UPCM 2024 Trans. (January 29, 2024). Neuroanatomy of the Corticobulbar & Corticospinal Tracts. ​ UPCM 2027 Trans. (November 4, 2022). Neuroanatomy of the Corticobulbar & Corticospinal Tracts. ​ UPCM 2026 Trans. (December 7, 2021). Neuroanatomy of the Corticobulbar & Corticospinal Tracts. OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 8 of 12  APPENDIX Figure 16. Structures Traversed by the Motor Fibers Figure 17. Reorientation of Motor Fibers in the Internal Capsule Figure 18. Axial cut of the brain from “Big Brain Project” with the Internal Capsule of the Diencephalon projecting caudally as the Crus Cerebri in the Midbrain OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 9 of 12  Figure 23. Lower Motor Neurons of the brainstem. Figure 24. Cranial Nerve Nuclei and the corresponding muscles they control. Diamonds represent the CSF pathway. Dark blue cylinders represent the nuclei. Figure 25. Lower Motor Neurons of the Spinal Cord. OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 10 of 12  Figure 28. Facial nerve nucleus Figure 29. Facial nerve nucleus and hypoglossal nerve OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 11 of 12  Figure 33. Corticospinal tract at the level of the cervical spine (C5) and lumbar spine (L1) OS 202 Neuroanatomy of the Corticobulbar & corticospinal tracts 12 of 12