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Descending tracts

A detailed description of all the descending tracts in CNS. Initially a brief description of motor organization is given followed by a detailed description of pyramidal and extrapyramidal tracts. At the end a few slides on MLF, and other descending fibres is also given. This presentation is not a substitute to reading standard physiology books but is intended for aiding students to extract the main points and prepare their own notes.

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Descending tracts

  1. 1. DESCENDING TRACTS
  2. 2. Primary motor cortex lies in precentral gyrus which lies in front of the central sulcus. But Premotor cortex lies in front of the primary motor cortex.
  3. 3. Approaching ball from the opponent: The person uses visual cortex to judge size, direction and velocity of the ball.
  4. 4. Finally the motor command passes from Premotor area to Primary motor cortex and then via CST to spinal cord (directly) and via brain stem structures to spinal cord (indirectly). Amygdala alongwith other brain regions adjusts heart rate, respiration and other homeostatic mechanisms and Also activates hypothalamus to motivate the player to hit well.
  5. 5. A copy of motor command to be executed also passes to the cerebellum which supervises the entire motor plan being performed in real time. It sends corrective signals to the motor cortex directly if the movement deviates from its trajectory. Hippocampus stores the memory of the hit so that it can recollect if required in front of friends….!!!
  6. 6. PMA and SMA prepare the entire plan of motor action to be performed and also tells the Primary motor cortex • What posture needs to be adopted? (Lower limb, back and shoulder girdle) • What exact right hand trajectory to be executed and with what force? For this what should be the swing in arm, forearm and wrist.
  7. 7. • What posture needs to be adopted? (Lower limb, back and shoulder girdle) Group of muscles : Which..?? When to contract..?? To what amplitude?? Quadriceps femoris, gastrocnemius, Tibialis anterior etc, hamstrings, Quadratus lumborum, neck and shoulder muscles. Antigravity muscles are especially controlled to maintain upright and firm posture to hit well….!!
  8. 8. • What exact right hand trajectory to be executed and with what force? For this what should be the swing in arm, forearm and wrist. Group of muscles : Which..?? When to contract..?? To what amplitude?? Rt. Shoulder muscles- Pectoralis, deltoid, trapezius, biceps, triceps, brachioradialis and other forearm and hand muscles- for proper grip of racket. Very skilled and precise control for proper shot execution…!!!
  9. 9. Lesions of : • MI produce paralysis • PMA and SMA give difficulties with initiation of movements and with purposeful movements that require coordinated sequences of muscle contractions. • Posterior parietal areas ~ 3D map of the body • Prefrontal cortex are necessary for transforming sensory information into action, and for selection of actions that are behaviorally appropriate.
  10. 10. Originates from Cells of layer V of cerebral cortex Some cells in this layer in Primary motor cortex are very large and called as Betz cells.
  11. 11. Within CORTICOSPINAL TRACT – 2 divisions Corticonuclear/bulbar fibres Corticospinal fibres To motor cranial nerve nuclei To spinal motor neurons in brain stem ventral grey horn
  12. 12. Cranial nerve Nuclei
  13. 13. Giant cells or Betz cells or pyramidal cells in precentral gyrus of the motor cortex (area 4) Supplementary motor areas Premotor area (area 6) Somatosensory areas of parietal lobe ~ 30% ~ 30% ~ 40% ORIGIN
  14. 14. COURSE: Originate from cortex Descend by spreading in a fan shaped manner called corona radiata Converge and pass through posterior limb of internal capsule Pass downward through brain stem sequentially through Midbrain-form cerebral peduncles anteriorly Pons-descend through anterior part Medulla form pyramid shaped swellings
  15. 15. Crossing of 80% fibres in lower medulla constitute the pyramidal decussation Further descend as two separates parts- crossed and uncrossed Crossed- Lateral corticospinal tract ~ 80% Uncrossed- Anterior corticospinal tract ~20% Lateral corticospinal tract ~ 80% Anterior corticospinal tract ~20%
  16. 16. Spinal course of pyramidal tracts 80% Crossed pyramidal tract Lateral corticospinal tract Indirect corticospinal tract Descend in lateral white Funiculus Courses through the entire length of spinal cord 20% Uncrossed pyramidal tract Anterior corticospinal tract Direct corticospinal tract Descend in anterior white funiculus Well marked in cervical level, absent below mid thoracic level Majority cross to opposite side before termination
  17. 17. Termination of Pyramidal tracts Fibres of the pyramidal tract terminate on motor neurons of anterior gray horn either directly or through internuncial neurons. Axons of the motor neurons leave the spinal cord as spinal nerves through anterior nerve roots and supply the skeletal muscles. Neurons giving origin to the fibers of pyramidal tract are called the upper motor neurons. Anterior motor neurons in the spinal cord are called the lower motor neurons.
  18. 18. Axial muscles ventromedially; Limb muscles ventrolaterally Flexors ~ dorsally; Extensors ~ ventrally
  19. 19. Lateral CST Descend the full length of spinal cord through the posterior part of lateral white funiculus. Termination: Most fibers ~ terminate on interneurons in lamina VII & VIII. These subsequently synapse with α and γ MN. Some terminate on α MN monosynaptically. The termination is on neurons in ventral grey horn. Anterior CST Descend down through the anterior white funiculus of the same side. Fibres do not reach further than the mid-thoracic region. Termination: Fibres cross the midline (through the anterior white commissure) to reach grey matter on the opposite side of the cord and terminate in a manner similar to that of the fibres of the lateral corticospinal tract.
  20. 20. In accordance with the somatotopic pattern within MI fibers from the medial parts of the precentral gyrus (leg representation) end in the lumbosacral part of the cord, whereas fibers from more lateral parts (arm representation) end in the cervical and upper thoracic cord. Fibers from MI and SI also terminate differently in the cord. Thus, fibers from SI end predominantly in the dorsal horn, whereas fibers from MI end in the intermediate zone and the ventral horn (laminae VII–IX).
  21. 21. Direct monosynaptic connections of the Motor cortex provide for least automatic and highly skilled movements of great manual dexterity. Such Corticospinal connections are abundant in humans compared to monkeys and it is these movements that are lost following damage to pyramidal tract after stroke. Also these movements do not recover fully.
  22. 22. Thus, the corticospinal fibres of both the lateral as well as the anterior tracts ultimately connect the cerebral cortex of one side with ventral horn cells in opposite half of the spinal cord. By synapsing with γ MN indirectly CST controls the sensitivity of muscle spindle. Sensory signals can be controlled from the brain by CST originating from sensory cortex. This provides for suppression of certain kinds of sensory information for example, during movements.
  23. 23. Salient features: • Fibers of the corticospinal tract are unmyelinated at birth. Myelination begins in the second post-natal week and is completed by 2 years. • The large fibres of pyramidal tracts have the tendency to disappear at old age causing automatic shaking movements of old age. • Fibers are closely packed in their course through the internal capsule and brain stem, small lesions here can cause widespread paralysis.
  24. 24. • About 70% of the fibers are large myelinated fibers having a diameter of 4 to 22 micron.
  25. 25. Functions: 1. Controls voluntary fine skilled movements of the body through the corticospinal tracts. Interruption of the tract anywhere in its course leads to paralysis of the muscles concerned. 2. Pyramidal tract fibres also send collaterals to other areas of the motor control systems thus communicating motor command to the basal ganglia, cerebellum and the brain stem.
  26. 26. Collaterals from a single axon
  27. 27. 3. Fibers (corticonuclear fibres) terminate directly on the motor nuclei of cranial neurons controlling facial muscles. Since these fibres perform the same function as pyramidal tracts, they are also considered part of the pyramidal system. All cranial nerve nuclei except those supplying lower half of face has bilateral supply from these CST fibers. So usually in stroke lower half of face palsy is very obvious.
  28. 28. Symptoms of lesion involving Pyramidal system Upper motor neurons (from motor cortex to termination of CST) • Difficulty with tasks that require precise, fractionated finger movements, such as writing, tying shoelaces, buttoning a shirt, and picking up small objects (like a needle). • Less precise movements involving larger muscle groups are usually less severely affected. • In addition, lesions of the upper motor neurons in humans produce characteristic changes of muscle tone and reflexes (spasticity).
  29. 29. EXTRAPYRAMIDAL SYSTEM All those motor parts of brain apart from pyramidal system are included in the extrapyramidal system. Epileptic fits occurred in humans even after bilateral sectioning of pyramidal tracts. So something other than pyramidal system was able to transmit motor activity from brain to spinal cord. Those structures were accordingly named as Extrapyramidal system. Subsequently, many brain structures were added one after the other in this broad category.
  30. 30. Clinically diseases of pyramidal tract produced paralysis apart from other signs and symptoms. But there were those diseases characterized by nonparalytic abnormal involuntary movements and postures and this came to be grouped into EXTRAPYRAMIDAL DISORDERS.
  31. 31. EXTRAPYRAMIDAL SYSTEM Major contribution in the extrapyramidal motor system are the • Nuclei of the basal ganglia • Substantia nigra • Red nucleus • Subthalamic nucleus • Mesencephalic reticular formation • Cerebellum • Descending pathways running from the cerebrum, cerebellum and brainstem towards the spinal cord, without coursing through the pyramids of the medulla.
  32. 32. • Rubrospinal tracts • Tectospinal tracts • Reticulospinal tracts (lateral and medial) • Lateral and medial vestibulospinal tracts The extrapyramidal system is also often described as the motor-modulation system.
  33. 33. The term “extrapyramidal” is to distinguish between the effects of basal ganglia diseases and those of damage to the “pyramidal” system, even though there is an intertwine of a functional relationship between the two systems. Thus there are important anatomical and functional relationships between the two systems. Extrapyramidal system is polysynaptic in nature with many synapses within the brainstem.
  34. 34. The EPS serves an essential function in maintaining posture and regulating involuntary motor functions. E.g • Postural tone adjustment • Preparation of predisposing tonic attitudes for involuntary movements • Control of automatic modifications of tone and movements • Control of the reflexes
  35. 35. • Control of the movements originally voluntary then become automatic through exercise and learning (e.g., in writing) • Inhibition of involuntary movements (hyperkinesias), which are particularly evident in extrapyramidal diseases. • The indirect corticospinal pathways are instrumental in coordinating the various automatic responses initiated from the lower levels. This happens mainly by modulating the excitability of brain stem and spinal interneurons.
  36. 36. Some diseases associated with EXTRAPYRAMIDAL SYSTEM • Dyskinesia • Parkinson's disease • Non-spastic cerebral palsy • Huntington's disease
  37. 37. Red Nucleus Sup. Coll.
  38. 38. Reticular Nuclei
  39. 39. Vestibular Nuclei
  40. 40. Rubrospinal tract Origin: This tract arises from the large cells (nucleus magnocellularis) of red nucleus in the mid brain. The parvocellular part of the red nucleus has connections with cerebellum.
  41. 41. Location and course: Axons of neurons in nucleus magnocellularis Cross to opposite side (lower part of midbrain tegmentum) Ventral tegmental decussation Descends through lower part of pons and medulla In spinal cord descends similar to Lateral CST
  42. 42. Termination: The fibres terminate mainly on interneurons along with the corticospinal fibres. Functions: 1. Effect on regulating muscle tone Flexor muscles ~ facilitatory influence Extensor muscles ~ inhibitory influence
  43. 43. 2. Alternate route of pyramidal system to exert influence on the lower motor neurons
  44. 44. The rubrospinal tract is most important and much better developed in some animals than in human. In human beings, the red nucleus is relatively small and the rubrospinal tract reaches only the upper three cervical segments of the spinal cord. In the cat and monkey this pathway supplements the pyramidal tract in the control of voluntary movements (although it is of greater functional importance in the cat than in the monkey).
  45. 45. Reticulospinal tracts There are two reticulospinal tracts: Medial (pontine) reticulospinal tract Lateral (medullary) reticulospinal tract.
  46. 46. Medial (pontine) reticulospinal tract Origin: It arises in the medial pontine reticular formation. Course: The tract descends, mostly uncrossed, in the anterior funiculus of spinal cord. Termination: The fibres terminate in the laminae VII and VIII of spinal grey matter and through internuncial neurons influence alpha and gamma neurons of lamina IX. Reaches mostly motoneurons of axial muscles (neck, back, and abdomen).
  47. 47. Medial RST/Pontine RST: exert strong facilitatory influence via interneurons on both α and γ motoneurons. The neurons in this tract are spontaneously active and are unaffected by stimulation of motor cortex or internal capsule and not inhibitory to flexor reflex afferents.
  48. 48. Lateral (medullary) reticulospinal tract Origin: The fibres of this tract originate from the gigantocellular component of medullary reticular formation. Course: These fibres are mostly uncrossed and a few crossed. This tract descends in the lateral funiculus medial to the lateral corticospinal and rubrospinal tracts. Termination: The fibres terminate in the internuncial neurons of laminae VII, VIII and IX of the spinal cord. Has access to motoneurons innervating muscles of the extremities.
  49. 49. Lateral RST/Medullary RST: exert inhibitory influence on both α and γ motoneurons. Thus inhibitory to tone (stretch reflex). Receives corticoreticular projections from premotor and supplementary motor areas which activates inhibitory neurons of LRST. Cortical motor areas control tone through this centre.
  50. 50. Medial/Pontine RST Spontaneously active- no cortical control Supplies mostly muscles of back and neck (axial muscles) Strong facilitatory influence Lateral/Medullary RST Receives corticoreticular projections to exert inhibitory influence Supplies mostly muscles of extremities Strong inhibitory influence
  51. 51. Functions of reticulospinal tracts The reticular formation of the brain stem receives input mostly from the motor cortex through the corticoreticular fibres which accompany the corticospinal tracts. 1. Form additional polysynaptic pathways from the motor cortex to the spinal cord. 2. These tracts are concerned with control of movements and maintenance of muscle tone.
  52. 52. 3. The reticulospinal tracts, probably, also convey autonomic information from higher centres to the intermediate region of spinal grey matter and regulate respiration, circulation and sweating.
  53. 53. Together, reticulospinal fibers terminate in large parts of the ventral horn but primarily in medial parts, where the motoneurons to the axial muscles and the proximal muscles of the extremities are located. Connections are particularly ample to the motoneurons that innervate neck muscles, which are important for movements of the head. The medullary fibers terminate somewhat more laterally in the ventral horn (particularly in lamina VII but to some extent also in lamina IX) than the pontine fibers.
  54. 54. 4. The reticulospinal fibers indirectly act on both the α and γ motoneurons. The pontine and medullary reticular nuclei mostly function antagonistic to each other. Thus, the reticular formation controls the sensitivity of the muscle spindles, and in certain situations it is probable that it acts only on the γ motoneurons and not the α motoneurons.
  55. 55. 5. Reticulospinal fibers send collaterals to both cervical and lumbar segments of the spinal cord and often to the ventral horns of both sides. Thus, such neurons are capable of influencing numerous muscles at the same time. This may be functionally meaningful in relation to adjustment of posture and body balance.
  56. 56. Vestibulospinal tracts There are two vestibulospinal tracts: lateral and medial
  57. 57. Lateral vestibulospinal tract Origin: Fibres of this tract arise from the lateral vestibular (Deiters’) nucleus. These fibres are somatotopically arranged.
  58. 58. Location and course: Uncrossed tract, anterior funiculus of the spinal cord. Larger of the two. Termination: Extend up to caudal segments of the cord Terminate on interneurons of ventral grey column (laminae VII and VIII) Alpha and gamma neurons of lamina IX; some fibres directly reach the alpha neurons
  59. 59. Functions: Linear acceleratory displacement of the body Sensed by utricles (vestibular apparatus) Adjustment of postural muscles Extensor muscles ~ facilitated Maintenance of balance Flexor muscles ~ inhibited
  60. 60. The fibers exert an excitatory action on both α and γ motoneurons. The vestibulospinal ones act primarily on motoneurons in the medial parts of the ventral horn—that is, axial muscles and proximal muscles of the extremities. Thus, the lateral vestibulospinal tract can adjust the contraction of muscles that oppose the force of gravity (antigravity muscles).
  61. 61. Medial vestibulospinal tract Origin: fibres of this tract arise from the medial vestibular nucleus.
  62. 62. Location and course: This tract descends through the anterior funiculus (within the sulcomarginal fasciculus). The fibres are mostly uncrossed but some fibres are crossed. Termination: The fibres end in the anterior motor neurons directly or through internuncial neurons (laminae VII and VIII) of the cervical segments of spinal cord.
  63. 63. Functions: This part of the vestibulospinal tract receives signals from the vestibular apparatus mainly from the semicircular canals. Many of the neurons of the medial tract are inhibitory. Functionally, medial vestibulospinal tract is the donor connection of medial longitudinal fasciculus. This tract provides a reflex pathway for movements of head, neck and eyes in response to the visual and auditory stimuli.
  64. 64. The vestibulospinal neurons receive few cortical afferents unlike reticulospinal neurons. So they are more independent of the cerebral cortex than the reticulospinal neurons. The vestibular nuclei mediate primarily automatic, reflex movements and adjustments of muscle tone.
  65. 65. Medial VST Smaller Ends at cervical level Afferent impulses received from SCC Reflex pathway for head, neck and eyes movements in response to the visual and auditory stimuli Lateral VST Larger in size Descends through entire spinal cord length Afferent impulses received from Utricle Excitatory effect on axial and proximal antigravity muscles
  66. 66. Tectospinal tract Origin: Fibres of this tract arise from the superior colliculi. Course: The fibres cross the midline in the lower part of tegmentum of the mid brain forming dorsal tegmental decussation. Then the tract descends through the pons and medulla into the anterior white funiculus of the spinal cord .
  67. 67. Termination: The fibres terminate in upper cervical levels by synapsing on the anterior horn cells through internuncial neurons located in laminae V and VII of the spinal grey matter. Function: This tract forms the motor limb of the reflex pathway for turning the head and moving the arms in response to visual, hearing or other exteroceptive stimuli.
  68. 68. Olivospinal tract Origin: This tract originates from the inferior olivary nucleus. Course and termination: The tract fibres descend and terminate ipsilaterally in the anterior horn cells of the spinal cord. Functions: Inferior olivary nucleus receives afferent fibres from the cerebral cortex, corpus striatum, red nucleus and spinal cord. It influences muscle activity. Probably, it is involved in the reflex movements arising from the proprioceptors.
  69. 69. Concept of Medial motor descending system and Lateral motor descending system Medial motor systems controls the axial and proximal limb musculature required to maintain posture and balance. Involved in more automatic control of musclulature. The tracts include : • Anterior CST • All extrapyramidal tracts except rubrospinal tract
  70. 70. Lateral motor systems controls the musculature of distal extremity required to actually perform least automatic skillful movements. The tracts include : • Lateral CST • Rubrospinal tract
  71. 71. Medial longitudinal fasciculus Origin: The medial longitudinal fasciculus (MLF) extends from the mid brain downwards. The fibres of this tract take origin from different area of the brain stem namely: • Vestibular nuclei • Reticular formation • Superior colliculus • Interstitial nucleus of Cajal • Nucleus of posterior commissure
  72. 72. Course: Brain stem ~ closely related to the III, IV, VI & XII Cr. Nerve nuclei. Also related to the fibres of seventh nerve and to some fibres arising from the cochlear nuclei. Spinal cord ~ becomes continuous with the anterior intersegmental tract of spinal cord, which descends through the posterior part of anterior white funiculus.
  73. 73. This tract is well defined only in the upper cervical segments. Below this level, the fibres run along with the fibres of medial vestibulospinal tract. Termination: Along with the fibres of the medial vestibulospinal tract, the fibres of this tract make connections with ventral horn cells that innervate the muscles of neck.
  74. 74. Functions: MLF plays an important role in the pathway of ocular movements. Its function can be summarized as: • It ensures harmonious movements of the eyes and neck (head) in response to vestibular stimulation and auditory stimuli. • It facilitates simultaneous movements of the lips and tongue as in speech.
  75. 75. Cortico-ponto-cerebellar pathway Origin: This pathway consists of the fibres arising in the cerebral cortex of the frontal, temporal, parietal and occipital lobes. Course: After origin from the cerebral cortex, the fibres descend through the corona radiata and internal capsule to reach the crus cerebri. These fibres synapse with the pontine nuclei of the same side.
  76. 76. Axons of the neurons in the pontine nuclei form the transverse fibres of the pons. These fibres cross the mid line and pass into the middle cerebellar peduncle of the opposite side and reach the cerebellar cortex. Functions: This pathway forms the anatomical basis for control of cerebellar activity of cerebral cortex.
  77. 77. Other fibres ending in the brain stem Other fibres arising from the cerebral cortex end in the following masses of grey matter of brain stem: • Red nucleus (corticorubral fibres) • Tectum (corticotectal fibres) • Substantia nigra (corticonigral) • Inferior olivary nucleus (cortico-olivary fibres) • Reticular formation (corticoreticular fibres) The above fibres ultimately form part of extrapyramidal system.

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