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Basic Anatomy for Trans-cranial Motor Evoked Potentials Monitoring

Basic Anatomy for Trans cranial Motor Evoked Potentials Monitoring

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Basic Anatomy for Trans-cranial Motor Evoked Potentials Monitoring

  1. 1. Transcranial Motor Evoked Potentials Anatomy Anurag Tewari MD UPPER MOTOR NEURON LOWER MOTOR NEURON MOTOR CORTEX MUSCLE
  2. 2. Anatomy and Physiology of Motor Systems • GENERAL ORGANIZATION OF THE SPINAL MOTOR SYSTEMS • DESCENDING SPINAL PATHWAYS • LOWER SPINAL MOTONEURON • MEDIAL SYSTEM • Miscellaneous Anurag Tewari MD
  3. 3. GENERAL ORGANIZATION OF THE SPINAL MOTOR SYSTEMS • The spinal motor system can be divided into ‘UPPER’ and ‘LOWER’ parts: The UPPER part consists of the CEREBRAL CORTEX BASAL GANGLIA CEREBELLUM The LOWER part consists of The SPINAL CORD, including the Alpha Motoneurons (“common final pathway”) Anurag Tewari MD
  4. 4. GENERAL ORGANIZATION Anurag Tewari MD
  5. 5. GENERAL ORGANIZATION CEREBRAL MOTOR CORTEX THALAMUS CEREBELLUM BASAL GANGLIA BRAIN STEM SPINAL CORD MOTOR NEURON Motor Control CorticalMotorPathway Anurag Tewari MD
  6. 6. GENERAL ORGANIZATION OF THE SPINAL MOTOR SYSTEMS The DESCENDING PATHWAYS start from the motor cortex and the motor nuclei in the brainstem and cerebellum, and terminate on the neurons in the spinal cord Anurag Tewari MD
  7. 7. GENERAL ORGANIZATION OF THE SPINAL MOTOR SYSTEMS • Traditionally, the descending motor pathways have been divided into Now known as Anurag Tewari MD
  8. 8. GENERAL ORGANIZATION OF THE SPINAL MOTOR SYSTEMS LATERAL SYSTEM MEDIAL SYSTEM The Lateral systems controls muscles in the distal limbs The Medial system controls proximal limb & trunk muscles Anurag Tewari MD
  9. 9. Motor Cortex • The primary cortex receives input from higher order cortical motor regions such as • PRIMARY MOTOR CORTICAL • SUPPLEMENTARY MOTOR AREA • PRE MOTOR CORTEX • The motor cortex also receives input from the somatosensory cortical areas Anurag Tewari MD
  10. 10. • The motor cortex generates motor commands • The primary motor cortex is somatotopically organized in a way similar to the somatosensory cortex • The HANDS and the FACE • comprise the largest parts of the motor cortex • located on lateral and dorsal surfaces of the brain • The TRUNK occupies a small part of the motor cortex • The DISTAL LEGS are represented by a region that is hidden between the two hemispheres Upper Motoneuron Anurag Tewari MD
  11. 11. The Motor Cortex • RECEIVES INFORMATION via the THALAMUS from • Brainstem • Cerebellum • Basal ganglia Upper Motoneuron Anurag Tewari MD
  12. 12. Upper Motoneuron The Motor Cortex • SENDS INFORMATION to the basal ganglia and brainstem Anurag Tewari MD
  13. 13. Upper Motoneuron • Main descending pathways from the motor cortex terminate in INTERNEURONS • INTERNEURONS are found in in different segments of the spinal cord and in nuclei of cranial motor nerves Interneurons create neural circuits, enabling communication between SENSORY or MOTOR neurons and the CNS Anurag Tewari MD
  14. 14. Basal Ganglia • Traditionally, the term “basal ganglia” is used to collectively describe • THE CAUDATE NUCLEUS • THE PUTAMEN • THE GLOBUS PALLIDUS However, various investigators have included substantia nigra, the subthalamic nucleus (STN), and the claustrum because they are related functionally to the other parts of the basal ganglia Anurag Tewari MD
  15. 15. Basal Ganglia • CAUDATE NUCLEUS and PUTAMEN are referred to as STRIATUM or NEOSTRIATUM • The PUTAMEN and the GLOBUS PALLIDUS are known as the LENTIFORM NUCLEUS Anurag Tewari MD
  16. 16. Basal Ganglia • The GLOBUS PALLIDUS consists of • an external segment (GLOBUS PALLIDUS EXTERNAL part [GPe]), and • an internal segment (GLOBUS PALLIDUS INTERNAL part [GPi]) • Parts of the SUBSTANTIA NIGRA are • PARS RETICULATA (SNr) • SUBSTANTIA NIGRA PARS COMPACTA (SNc) • These nuclei are of special interest in connection with movement disorders. Anurag Tewari MD
  17. 17. Basal Ganglia • The basal ganglia processes information from all parts of the cerebral cortex, including the motor cortex • And relay information to other subcortical structures and the thalamus Anurag Tewari MD
  18. 18. Basal Ganglia • The input to the basal ganglia from the primary motor cortex converges on the • CAUDATE NUCLEUS • PUTAMEN • CENTROMEDIAN NUCLEUS (CM) of the thalamus • SUBSTANTIA NIGRA Anurag Tewari MD
  19. 19. Basal Ganglia • Thus the basal ganglia receive input from the motor cortex and deliver its output back to the motor cortex • Therefore the descending pathways from the motor cortex (corticospinal tract) contain information from the basal ganglia Basal ganglia primarily helps in action selection –to decide which of several possible behaviors to execute at any given time Anurag Tewari MD
  20. 20. Basal Ganglia • All connections between the components of the basal ganglia are INHIBITORY EXCEPTION: the connections between the STN and the Gpi/SNr (excitatory) Anurag Tewari MD
  21. 21. Basal Ganglia • The output of GPi and SNr provides tonic inhibition on thalamocortical neurons • The direct dopaminergic nigrostriatal pathway from SNc might also modulate the activity in the two striato-pallidal pathways in two different ways: • one of which facilitates transmission in the “DIRECT” pathway • whereas the other is inhibiting transmission in the “INDIRECT” pathway Anurag Tewari MD
  22. 22. Basal Ganglia • The basal ganglia are associated with movement disorders such as • Parkinson’s disease (PD) • Huntington’s disease (HD) • Gilles de la Tourette syndrome Anurag Tewari MD
  23. 23. Thalamus (via Latin from Greek thalamos denoting the part of the brain at which a nerve originates) • The motor portion of the thalamus is involved in processing of movement information and it links the basal ganglia to the motor cortex • Lesions made in specific nuclei of the thalamus have been shown effective in treating movement disorders, as they have been in the treatment of sensory disorders and pain • Surgical lesions have now largely been replaced by implantation of electrodes for chronic electrical stimulation of specific nuclei (DBS). Anurag Tewari MD
  24. 24. Blood Supply to the Basal Ganglia & Thalamus Anurag Tewari MD
  25. 25. Blood Supply to the Internal Capsule Anurag Tewari MD
  26. 26. Break Questions?
  27. 27. DESCENDING SPINAL PATHWAYS The descending pathways of the spinal cord are organized anatomically together with ascending sensory pathways Anurag Tewari MD
  28. 28. DESCENDING SPINAL PATHWAYS The descending pathways are of two main groups: the MEDIAL systems the LATERAL systems Cerebrum Brainstem Spinal Cord Anurag Tewari MD
  29. 29. LATERAL PATHWAYS • The (dorso) lateral pathways include the • CORTICOSPINAL TRACTS • RUBROSPINAL TRACTS Anurag Tewari MD
  30. 30. DESCENDING SPINAL PATHWAYS Lateral Pathways • The corticospinal system is most developed in primates • Which makes studies of the motor system in other mammals less representative for humans • Many aspects of the corticospinal system in humans is incompletely known because of the limited number of studies of primates and the differences between humans and other primates. Anurag Tewari MD
  31. 31. MEDIAL SYSTEM • The medial system include the • VESTIBULOSPINAL TRACTS • RETICULOSPINAL TRACTS • TECTOSPINAL TRACTS Anurag Tewari MD
  32. 32. DESCENDING SPINAL PATHWAYS The fibers of the descending motor pathways terminate on cells in the VENTRAL HORN of the spinal cord Anurag Tewari MD
  33. 33. DESCENDING SPINAL PATHWAYS • The pathways of the lateral system provide • VOLUNTARY • sophisticated motor control for FINE MOVEMENTS • Mainly controlling muscles of distal limbs • It is almost exclusively the LATERAL TRACTS that are monitored in operations where the spinal cord is at risk of being injured Anurag Tewari MD
  34. 34. DESCENDING SPINAL PATHWAYS • The pathways of the medial system have general functions such as • Control of posture and • Control of basic function like walking • The medial group of pathways controls mainly • Trunk and • Proximal Limb Muscles • The medial system activates extensors more than flexors Anurag Tewari MD
  35. 35. DESCENDING SPINAL PATHWAYS Lateral Pathways • Corticospinal (Pyramidal) Tract connects cortical motor neurons with •ALPHA MOTONEURONS •PROPRIOSPINAL INTERNEURONS Anurag Tewari MD
  36. 36. DESCENDING SPINAL PATHWAYS Lateral Pathways • The corticospinal tract alone passes through the PYRAMIDS • Whereas other descending pathways pass through other parts of the medulla This is why the corticospinal tract have been known as the pyramidal tract and the other tracts were known as the extrapyramidal tracts: a distinction that is no longer valid Anurag Tewari MD
  37. 37. DESCENDING SPINAL PATHWAYS Lateral Pathways • Main parts of lateral pathways cross the midline at level of LOWER MEDULLA • 10% of the corticospinal fibers in humans do not cross the midline; there are large individual variations Anurag Tewari MD
  38. 38. DESCENDING SPINAL PATHWAYS Lateral Pathways • The corticospinal tract is asymmetric in about 75% of the population, • RIGHT SIDE often being larger than the left side • Some of the corticospinal fibers originate in the somatosensory cortex • which explains why motor responses can be obtained by stimulating the somatosensory cortex Lateral Corticospinal Tract Anurag Tewari MD
  39. 39. DESCENDING SPINAL PATHWAYS Lateral Pathways • Rubrospinal Tract • The rubrospinal tract originates in the nucleus ruber, which receives indirect input from the motor cortex Anurag Tewari MD
  40. 40. DESCENDING SPINAL PATHWAYS Lateral Pathways • Rubrospinal Tract • This pathway has very few fibers in humans • Estimated to be 1% of those of the corticospinal tract in monkey and man • The functional importance of the rubrospinal tract in humans has been questioned • The tract is responsible for large muscle movement as well as fine motor control, and it terminates primarily in the cervical spinal cord, suggesting that it functions in upper limb but not in lower limb control • It primarily facilitates flexion in the upper extremities • It is probably of little importance for monitoring purposes Anurag Tewari MD
  41. 41. DESCENDING SPINAL PATHWAYS Medial Pathways • The pathways of the medial system consist of the • RETICULOSPINAL • TECTOSPINAL • VESTIBULAR Anurag Tewari MD
  42. 42. DESCENDING SPINAL PATHWAYS Medial Pathways • The tracts of the medial system are, phylogenetically, the oldest motor pathways • The tracts of the medial system are less direct motor pathways than those of the lateral system, and the medial system comprises pathways with different origins • The motor tracts that belong to the medial system have both crossed and uncrossed tracts and their fibers terminate on neurons in the ventromedial zone of the spinal gray matter Anurag Tewari MD
  43. 43. DESCENDING SPINAL PATHWAYS Medial Pathways • These pathways mostly control propriospinal interneurons • The axons terminate on the motoneurons that control muscles on the • Trunk • Girdle • Proximal Limb muscles • The medial motor system mostly controls extensor muscles and muscles that are involved in posture (“antigravity” muscles). Anurag Tewari MD
  44. 44. DESCENDING SPINAL PATHWAYS Medial Pathways Reticulospinal, Tectospinal, and Vestibular Spinal Pathways The tectospinal and vestibulospinal fibers are mainly crossed but have small, uncrossed parts Anurag Tewari MD
  45. 45. DESCENDING SPINAL PATHWAYS Medial Pathways Reticulospinal, Tectospinal, and Vestibular Spinal Pathways • The reticulospinal tract contributes to maintaining posture and can orient the body in crude stereotyped movements • The importance in intraoperative monitoring of the reticulospinal tract is probably mostly related to its role of facilitating the alpha motoneuron • The reticulospinal tracts are suppressed by many forms of anesthesia Anurag Tewari MD
  46. 46. DESCENDING SPINAL PATHWAYS Nonspecific Descending Systems • The NORADRENALIN (NA)–SEROTONIN PATHWAYS belong to a nonspecific system originating in the raphe nuclei and they project to the spinal cord • One important function of these descending pathways is • ADJUSTING MUSCLE TONE, such as suppressing skeletal muscle activity, which occurs, for example, during rapid eye movement sleep • These faciliatory systems are sensitive to anesthetic agents • the reduction in the activity of these systems, caused by anesthetics, is likely to contribute to the decreased excitability of motor systems that is observed during surgical operations Anurag Tewari MD
  47. 47. Break Questions? Anurag Tewari MD
  48. 48. LOWER SPINAL MOTONEURON • Lower motor neurons consist of • INTERNEURONS in the spinal cord • ALPHA MOTONEURON • The motor nerves emerge and through which all spinal motor activity must pass (the “common final pathway”). Anurag Tewari MD
  49. 49. LOWER SPINAL MOTONEURON Alpha Motoneurons Alpha motoneurons are located in layer IX of the ventral horn of spinal cord Anurag Tewari MD
  50. 50. LOWER SPINAL MOTONEURON Alpha Motoneurons • Their axons form the ventral spinal roots and the motor portions of spinal nerves that innervate skeletal (extrafusal) muscles • The motor portion of peripheral nerves also contains the axons of gamma neurons that innervate the (intrafusal) muscles of muscle spindles Anurag Tewari MD
  51. 51. Extrafusal vs Intrafusal Muscle fibers • Extrafusal muscle fibers are the skeletal standard muscle fibers • Innervated by alpha motor neurons • Generate tension by contracting • Thereby allowing for skeletal movement • Intrafusal muscle fibers are skeletal muscle fibers • Serve as specialized sensory organs (proprioceptors) • Detect the amount and rate of change in length of a muscle Anurag Tewari MD
  52. 52. Blood Supply to the Spinal Cord The motor (ventral) portion of the spinal cord has a different blood supply than the dorsal portion of the spinal cord, where the sensory portion of the spinal cord is located Anurag Tewari MD
  53. 53. Blood Supply to the Spinal Cord Anurag Tewari MD
  54. 54. Blood Supply to the Spinal Cord • Compromises of the blood supply to the ventral portion of the spinal cord might therefore occur without the dorsal part of the spinal cord being affected and thus go unnoticed if only the SSEP is monitored • Monitoring of the function of the ventral portion of the spinal cord is important during operations in which there is risk of ischemia of the spinal cord Anurag Tewari MD
  55. 55. Muscles Monitored during TcMEPs • Upper Extremity • Trunk • Lower Extremity Anurag Tewari MD
  56. 56. Muscles Monitored during TcMEPs UPPER EXTREMITY • Trapezius (C3,C4) • Deltoid (C5, C6) • Biceps (C5, C6) • Triceps (C6, C7, C8) • Flexor Carpi Radialis (C6, C7) • Flexor Carpi Ulnaris (C6, C7, C8, T1) • Flexor Digiti Minimi Brevis (C8-T1) • Flexor Pollicis Brevis (C8-T1) Anurag Tewari MD
  57. 57. Muscles Monitored during TcMEPs TRUNK • Intercostal (T4 – T7) • External Oblique (T10) • Psoas (L1) Anurag Tewari MD
  58. 58. Muscles Monitored during TcMEPs LOWER EXTREMITY • Adductor Longus (L2, L3 & L4) • Rectus Femorus (L1, L2) • Vastus Lateralis (L2, L3, L4) • Tibialis Anterior (L4, L5) • Peroneus Longus (L5, S1) • Medial Gastrocnemius (S1, S2) • Flexor Hallucus Longus (L5, S1) • Extensor Hallucus Longus (L5, S1) • Flexor Digitorum Brevis (L5, S1) • ABDuctor Digiti Minimi Hallicus (S2, S3) • ADDuctor Digiti Minimi Hallicus (S2, S3) Anurag Tewari MD
  59. 59. Summary Anurag Tewari MD
  60. 60. Questions? Anurag Tewari MD

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Basic Anatomy for Trans cranial Motor Evoked Potentials Monitoring

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