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REPRESENTATIVE DESCENDING TRACTS Georgia Bishop PhD Department of Neuroscience The Ohio State University College of Medicine.

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Presentation on theme: "REPRESENTATIVE DESCENDING TRACTS Georgia Bishop PhD Department of Neuroscience The Ohio State University College of Medicine."— Presentation transcript:

1 REPRESENTATIVE DESCENDING TRACTS Georgia Bishop PhD Department of Neuroscience The Ohio State University College of Medicine

2 OBJECTIVES Trace the descending fiber tracts listed and discussed in the e-learning module. For each tract, you should be able to: a)Identify the location of the neurons that give rise to the tract. b)Define the primary motor function of the descending tracts c)Name the location and immediate anatomical relationships of each tract in different subdivisions of the brainstem and forebrain d)Describe the course of each tract from its origin to its ultimate site of termination including the location of any second and third order relays, whether it is ipsilateral or contralateral, and, if contralateral, identify the location where axons cross the midline (decussation) Be able to identify fiber tracts at all levels of the brainstem on cross sectional images from an atlas and on MRI/CT scans. Describe the location and relationships of relevant descending tracts in the CNS on gross brain specimens, cross sectional material, and radiographic images. At the end of this module you will learn to:

3 CORTICOSPINAL – Classically Defined As Mediating Voluntary Movement RUBROSPINAL – Alternate Route For Mediation/Modulation Of Movements MLF – 1) Input From Vestibular System For Postural Adjustments and Coordination of Head and Eye Movements. 2) Input From Nuclei Controlling Mucles of Eyes For Coordination Of Eye Movements (III, IV, VI) Motor neurons in the spinal cord are capable of activating muscles through various reflex loops (e.g., Extensor reflex, flexor withdrawal reflex). The function of descending tracts is to regulate the activity of motor neurons in the spinal cord. REPRESENTATIVE DESCENDING TRACTS Lateral Corticospinal Ventral (Anterior) Corticospinal Rubrospinal Medial Longitudinal Fasciculus Including Medial Vestibulospinal Lateral Vestibulospinal Lateral Reticulospinal Medial Reticulospinal Raphespinal Hypothalamospinal **

4 LOWER MOTOR NEURONS Motor neurons in spinal cord and cranial nerve nuclei that project to skeletal muscles. Final common pathway to innervate muscles. When lost – flaccid paralysis UPPER MOTOR NEURONS Neurons in cortex and other areas of brainstem that project to motor neurons in spinal cord or cranial nerve nuclei with somatic motor component. Damage results in spastic paralysis as reflexes go “unchecked”. Basal ganglia, cerebellum, association areas of motor cortex play vital role in the choice, design and monitoring of movement but have no direct effect on lower motor neurons. NEURON IN BRAIN MOTOR CONTROL COMPONENTS

5 CELLS OF ORIGIN: Pyramidal cells in motor regions of frontal lobe. Axons descend ipsilaterally through cerebral peduncle, basilar pons and rostral pyramidal tract in the medulla. Terminate either directly on motor neurons or on interneurons that contact motor neurons. PYRAMIDAL DECUSSATION LATERAL CORTICOSPINAL TRACT PRECENTRAL GYRUS MIDBRAIN PONS MEDULLA Axons course through posterior limb internal capsule (large fiber tract located lateral to thalamus). On reaching caudal medulla 85% cross to the contralateral side in the pyramidal decussation to form lateral corticospinal tract. 15% remain ipsilateral to form anterior corticospinal tract. INTERNAL CAPSULE CEREBRAL PEDUNCLE CEREBRAL CORTEX PYRAMID TO SKELETAL MUSCLE C8 L4 CORTICOSPINAL (PYRAMIDAL) TRACT

6 Neurons in the cerebral cortex also project to motor nuclei related to cranial nerves. Arise from face region of precentral gyrus. IC CP Motor Trigeminal (V) Although shown as bilateral input, V, X, XI and XII primarily receive crossed projection. Facial Motor Nucleus Ipsi: Upper Face Facial Motor Nucleus Contra: Upper Face And Lower Face INPUT TO VII IS COMPLEX. Cortical input is primarily crossed to innervate muscles of upper and lower face. Upper facial muscles receive a bilateral input from the cortex. These muscles may be spared when there is a stroke on 1 side of the cortex. This is important for distinguishing between cortical and peripheral injury to facial nucleus/nerve Hypoglossal Nucleus (Xii) Nucleus Ambiguus Muscles Of Larynx, Pharynx (IX, X) Accessory Motor Nucleus (Xi) UNABLE TO BARE TEETH ON LEFT SYMMETRIC ELEVATION OF FOREHEAD CORTICOBULBAR TRACT

7 IC ALL AFFERENTS AND EFFERENTS RELATED TO THE CORTEX TRAVERSE THE INTERNAL CAPSULE. Internal Capsule is a large fiber bundle located lateral to the thalamus and the caudate nuclues. INTERNAL CAPSULE: Descending Fibers Include: Corticospinal Corticobulbar Axons Corticopontine Internal Capsule: Ascending Fibers Include: Medial Lemniscus Spinothalamic Tract INTERNAL CAPSULE

8 THALAMUS TH PUT IC TH PUT IC MRI – PATIENT WITH DISEASE (ALS) LEADING TO DEGENERATION OF UPPER MOTOR NEURONS A M P L AL GENU PL Location Of Corticospinal/ Corticobulbar Tract In Posterior Limb Of Internal Capsule INTERNAL CAPSULE: CORTICOSPINAL TRACT AND INTERNAL CAPSULE

9 In The Midbrain, The CST and CBT Occupy the Middle Part Of The Cerebral Pecuncle (CP) SN TECTUM TEGMENTUM CP MRI – PATIENT WITH DEGENERATION OF UPPER MOTOR NEURONS RN ML STT SC CORTICOSPINAL TRACT - MIDBRAIN

10 In the pons the CST is broken up into fascicles that course between collections of neurons forming pontine nuclei Many cortical axons, not contributing to the corticospinal tract, project to brainstem nuclei in the midbrain, pons and medulla, especially cranial nerve nuclei with a motor component. In addition axons, terminate on neruons in the reticular formation. IV PN CST ML STT MLF SCP CORTICOSPINAL TRACT - PONS

11 COMPARISON OF SIZE OF CEREBRAL PEDUNCLE (BEFORE PONS) AND PYRAMID (AFTER PONS) TO SHOW HOW MANY AXONS LEAVE THE TRACT IN THE PONS AND ROSTRAL MEDULLA. THE IMAGES ARE AT THE SAME MAGNIFICATION. CP PY In the medulla, the fibers regroup to form the pyramidal tract (pyramid). However, there is a significant decrease in size of this fiber tract compared to that of the cerebral peduncle. CORTICOSPINAL TRACT – MIDBRAIN VS. MEDULLA

12 PYRAMIDAL DECUSSATION PY PY = PYRAMID; CORTICOSPINAL TRACT DX – PYRAMIDAL DECUSSATION DX At The Junction Of The Spinal Cord And Medulla, 85% of the Axons in the Corticospinal Tract Cross to the Contralateral Side. Note, They Also Move Dorsally And Laterally. This is Now Called the Lateral Corticospinal Tract. 15% Of The Fibers Remain on the Ipsilatral Side in the Same Ventral Position. This is Now Called the Anterior Corticospinal Tract. CORTICOSPINAL TRACT - DECUSSATION X X

13 LCST CERVICAL CORD THORACIC CORD LATERAL CST AXONS TERMINATE ON: Motor Neurons Located Laterally (Upper Limb And Digits) Interneurons That Project To Motor Neurons Neurons At Base Dorsal Horn ANTERIOR CST AXONS TERMINATE ON: Motor Neurons Located Medially (Axial Muscles). Some Cross To Contralateral Side End In Cervical And Thoracic Segments aCST LCST aCST PYRAMIDAL DECUSSATION LCSTaCST C8 L4 CORTICOSPINAL TRACT – Spinal Cord

14 CELLS OF ORIGIN: RED NUCLEUS IN THE MIDBRAIN Receives input from cerebral cortex and cerebellum and modulates activity of motor neurons in the spinal cord.. In spinal cord, axons are located just ventral to lateral corticospinal tract Axons cross midline to form the rubrospinal tract (RST) in midbrain and descend in lateral tegmentum Most terminate in cervical spinal cord. CERVICAL SPINAL CORD ROSTRAL MEDULLA PONS MIDBRAIN RED NUCLEUS RST (CTT) RUBROSPINAL TRACT

15 TO SPINAL CORD RST TEG RST RUBROSPINAL TRACT

16 LCST In the spinal cord the rubrospinal tract is located adjacent to LCST RST CERVICAL LCST RST THORACIC RUBROSPINAL TRACT

17 Fiber Tract Extending from the Midbrain to the Spinal Cord that Connects the Three Extraoculuar Nuclei that are Responsible for Coordinated Movements of the Eye: Oculomotor, Trochlear And Abducens. Also Contains Input from the Vestibular Nuclei to Coordinate Movement of the Head and Eyes. VESTIBULAR NUCLEUS (VIII) ABDUCENS NUCLEUS (VI) TROCHLEAR NUCLEUS (IV) OCULOMOTOR NUCLEUS (III) MLF TO CERVICAL SPINAL CORD MIDBRAIN MEDIAL LONGITUDINAL FASCICULUS

18 VN VI IV III MLF – ASCENDING LIMB MLF – DESCENDING LIMB Ascending and descending limb of the MLF begin in the vestibular nuclei Ascending limb links vestibular nuclei and nuclei that control movements of the eye (III, IV, VI) Descending limb links vestibular nuclei with interneurons and motor neurons in the cervical spinal cord. MEDIAL LONGITUDINAL FASCICULUS - VESTIBULAR INPUT

19 Medial Vestibulospinal Tract = descending MLF MEDIAL VESTIBULOSPINAL TRACT MLF Descending part of MLF (a.k.a. Medial Vestibulospinal Tract) is bilateral. Most axons stay ipsilateral but many cross. Remain immediately adjacent to midline in medulla in tract located immediately dorsal to medial lemniscus Axons enter ventral funiculus of spinal cord and terminate primarily on motor neurons that innervate axial musculature of neck. Medial Vestibulospinal Tract is not found below cervical levels. DESCENDING LIMB MLF ARISES FROM THE VESTIBULAR NUCLEI AND CONTROLS NECK MUSCLES

20 ML VI MLF ST VN IN THE PONS, THE MLF RETAINS ITS POSITION DORSALLY AND MEDIALLY AS THE MEDIAL LEMNISCUS RE-ORIENTS FROM VERTICAL TO HORIZONTAL AS THE BASILAR PONS EXPANDS. MLF (MVST) VI ST ML MLF IN THE MEDULLA AND PONS

21 MLF (MVST) ML ST DSCP IV MLF IN THE ROSTRAL PONS

22 IV ML ST RN CC SN MLF III MLF IN THE MIDBRAIN

23 TRACTORIGINDECUSSATIONTARGETFUNCTION Lateral Corticospinal Pyramidal cells in Prefrontal Gyrus (Motor related areas especially those related to limbs). In caudal medullaAlpha motor neurons especially those related to hand and digits or interneurons Initiation of movement Anterior Corticospinal Pyramidal cells in Prefrontal Gyrus (Motor related areas; especially those related to trunk muscles). Uncrossed. Some cross in spinal cord. Motor neurons related to trunk muscles. Input is bilateral Initiation of movement in trunk muscles. RubrospinalRed NucleusCross at level of origin in midbrain Alpha motor neurons related to proximal muscles especially flexors Supraspinal control of flexor motor neurons and proximal limb muscles Medial Vestibulospinal a.k.a MLF - descending Medial Vestibular Nucleus Bilateral from origin. Does not extend beyond cervical region Alpha and gamma motor neurons; especially for extensor muscles Supraspinal control of extensor muscles. MLF - AscendingVestibular nuclei, abducens, trochlear, and oculomotor nuclei BilateralLinks vestibular and nuclei related to moving eye. Coordinates head and eye movement SUMMARY OF DESCENDING TRACTS

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25 Thank you for completing this module  If you have any questions, please contact me: 

26 Survey We would appreciate your feedback on this module. Click on the button below to complete a brief survey. Your responses and comments will be shared with the module’s author, the LSI EdTech team, and LSI curriculum leaders. We will use your feedback to improve future versions of the module. The survey is both optional and anonymous and should take less than 5 minutes to complete. Survey


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