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THE MAJOR NEURONAL CIRCUITS

THE MAJOR NEURONAL CIRCUITS. THE MOTOR CORTEX The anatomical region of the brain known as Area 4 is the primary motor cortex (PMC) Focal stimulations elicited muscle contractions Is organized somatotopically The surface area varies. Is proportional to the precision of the movements.

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THE MAJOR NEURONAL CIRCUITS

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  1. THE MAJOR NEURONAL CIRCUITS

  2. THE MOTOR CORTEX The anatomical region of the brain known as Area 4 is the primary motor cortex (PMC) • Focal stimulations elicited muscle contractions • Is organized somatotopically • The surface area varies. • Is proportional to the precision of the movements

  3. The motor cortex also includes Area 6, • Which lies rostrally to Area 4 Divided into • Premotor area or premotor cortex(PMA) • Supplementary motor area. (SMA) • The PMA is believed to help • Regulates posture by dictating an optimal position to the motor cortex for any given movement. • The SMA, for its part, seems to influence • The planning and initiation of movements • On the basis of past experience • The mere anticipation of a movement triggers neural transmissions in the SMA

  4. Somatotopic representations in the motor cortex

  5. The posterior parietal cortex (PPC) Also plays a role in voluntary movements • The parietal cortex receives: • Somatosensory • Proprioreceptive • Visual inputs • Assesses the context in which they are being made. Based on which determines such things as • The positions of the body • And the target in space. • Produces internal models of the mov’ts to be made. • Before the premotor & motor cortices are involved

  6. Within the PPC, two particular areas • Area 5 & Area 7 • Area 5 receives information from • Somatosensory areas 1, 2, & 3 of the cortex. • Area 7 further integrates • The already highly integrated signals from the visual areas of the cortex, such as MT and V5.

  7. The parietal lobes are themselves closely • interconnected with the prefrontal areas. • The two regions represent the highest level of integration in the motor control hierarchy. • Decisions are made about what action to take.

  8. The PP & PFA send their axons to Area 6 • Which, once it has been informed about the kind of action to take • Helps to determine the characteristics of the appropriate movement for this purpose

  9. These two elongated regions face each other • Have the same somatotopic organization • MC & in the SSC, the scale of this map is not constant.

  10. In the somatosensory cortex, it varies with: • Each body part's sensitivity to sensory information. In the motor cortex, it varies with: • The precision of the movements controlled in the body part in question.

  11. AXONS ENTERING AND LEAVING THE MOTOR CORTEX • Receive inputs from PMA &SMA • The pyramidal neurons of the PMC receive information directly from: • Somatosensory areas 3, 1, & 2. • Axons mainly from the thalamus, Caudal ventrolateralnucleus ( VLc) • Relays information from the cerebellum.

  12. The PMC projects its axons mainly to the Corticospinal tract, which is composed of the • Lateral • Ventromedial systems.

  13. The lateral system has two main neural pathways • The Lateral Corticospinal tract. (The larger) • Arising mainly in Areas 4 and 6 of the frontal lobe, which together constitute the motor cortex, • Is the longest neural pathway • One of the largest in terms of the number of axons containing 1 milion • The other axons in this tract arise mainly in the somatosensory areas and the parietal lobe.

  14. Axons cross internal capsule, midbrain& pons • They join together at the medulla oblongata to form a dense bundle of nerve fibers. • Shaped like a pyramid (pyramidal tract) • Extends along the ventral surface of the medulla, • Just before entering the spinal cord, the pyramidal tract decussates. • They ultimately synapse on the • motor neurons and interneurons of the dorsolateral portion of the ventral horn of the spinal cord.

  15. The lateral system

  16. Rubrospinal tract the 2nd in the lateral system. • Arising from the neurons of the rednucleus, in the midbrain. • This nucleus receives information from the frontal cortex. • Which also projects massively to the corticospinal tract.

  17. The other major descending pathway, is the Ventromedial system • Is composed of four tracts that originate in various areas of the brainstem • Contribute chiefly to: • postural control • certain reflex movements.

  18. The originating neurons of these tracts receive sensory information related to: • Balance • Body position • And the visual environment

  19. The Vestibulospinal tract • Originates in the vestibular nuclei. • Maintains the head in position in relation to the shoulder • Which is essential for continuing to look in a given direction while the body is moving

  20. 2. The Tectospinal tract • Arises in the superior colliculus in the midbrain. • The superior colliculus receives: • some visual information directly from the retina • somatosensory and auditory information • the tectospinal tract contributes to visual orientation

  21. 3. Pontine (medial) and 4. Medullary (lateral) Reticulospinal tracts • Arise from the reticular formation nuclei in two main parts of the brainstem: • The pons & the medulla oblongata. • The reticular formation receives inputs from many sources • Extends the entire length of the brainstem, from the pons to the medulla.

  22. These two tracts help to maintain posture. • Pontine axons enhance the spinal antigravity reflexes. • The medullary have the opposite effect, releasing the muscles • Thus facilitating other movements

  23. ventromedial system

  24. THE BASAL GANGLIA

  25. The basal ganglia and its connections • No input from ascending sensory path way • No direct spinal projection • Cortico-striato-pallido-thalamo-cortical loop • Cortical projections of to the basal ganglia are directed mainly to the striatum and subthalamic nucleus (STN)

  26. The striatum receives inputs from all areas of the cerebral neocortex • Cortical inputs to the subthalamus come mainly from the frontal lobe • The striatum sends GABAergic inhibitory projections: • Globus pallidus externa GPe • Globus pallidus interna GPi • Substantia nigra pars reticulata SNr

  27. GPe sends GABAergic inhibitory projections to: • GPi / SNr & STN • STN sends excitatory Glutamergic fibers to GPe and GPi/ SNr • Thus there are to types of path ways • Direct : a. Striatum-Gpi/SNr • Indirect: a. Striatum-GPe-STN-GPi / SNr b. Striatum-Gpe-Gpi/SNr

  28. G L O B U S P A L L I D U S (E) DORSAL G L O B U S P A L L I D U S (I) S U B S T A N T I A N I G R A ® ASS VA MD Anterior putamen Most of caudate ASSOCIATIVE ASSOCIATIVE ROSTRO MEDIAL MD LIMBIC N. Accumbens Olfactory tubercle Ventral putamen Ventral caudate THALAMUS LIMBIC CORTEX LIMBIC STRIATUM VL/VA SEN- MOTOR VENTRO LATERAL P.Comm.Putamen D.lat. head of caudate S.MOTOR SENSORY MOTOR

  29. LIMBIC Accumbens Olfactory tubercle Ventral putamen Ventral caudate ASSOCIATIVE Anterior putamen Most of caudate STRIATUM SENSORY MOTOR Postcommissural DorsolateralHead of caudate DORSAL ROSTROMEDIAL VENTROLATERAL INTERNAL AND EXTERNA L PALLIDAL SEGMENTS

  30. SUBSTANTIA NIGRA R. OCCULO MOTOR SUP. COLLICULUS GLOBUS PALLIDUS INTERNA Thalamic Nucleii LIMBIC ASSOCIATIVE MD VA MD SENSORY MOTOR VL VA

  31. CEREBR CORTEX GLUTAMATE Excitatory MD STRIATUM D1 GABA/P D2GABA/E D1GABA/P RET CM/PF DIRECT PATH GABA VA/VL INDIRECTPATH GABA SNc DA Globus Pallidus Extrna INDIRECT PATH 2 GABA INDIRECTPATH 1 GABA Glu Gpi/SNr GABA STN Glu PPN/MEA Ach/GlU

  32. Parkinsonism • Rest tremor upper or lower extrimities • Rigidity,akinesia and bradykinesia • Incapacitating type of action tremor UL Loss of dopamine in the nigrostriatal pathway • Facilitates the direct path way • Inhibits the indirect path way • Net result is increased and abnormally phasic activity in STN and GPi/SNr

  33. Tremor • Multiple sites neuronal oscillations have been observed • Neurons in the cortex,thalamus (VL),Gpi, STN • No clear cut specific source of tremor • Rather collective oscillation Akinesia • Large cholinergic interneurons b/n striosomes &matrisomes • Give burst-pause-burst signal could be “GO” signal for mov’t • Its absence in Dopa depleted neurons.? reason for akinesia • Loss Dopa & NA inputs in the cortex Start hesitation and freezing • Inhibition of the brainstem via GPi/SNr to PPN/MEA

  34. Chorea • Destructive lesions of STN • Gives hemiballismus and hemichorea • Slow writhing mov’t of athetosis- in conjunction with chorea • Is probably due to reduced STN inhibitory output on GPi/SNr • But pallidal lesion doesn’t cause hemiballismus or chorea IN HUNTINGTON DISEASE • Loss of GABAE earlier than GABAP • Loss of inhibition of the indirect pathway • Chorea appears In addition the striatal striosomes(caudate & putamen) • Loss of inhibitory input to SNc • Gives enhanced dopaminergic state leading to chorea

  35. Dystonia An abnormal cocontraction of antagonistic muscles • In reduced inhibition of VL thalamic neurons by Gpi • Hence dystonia is a hyplerkinetic disorder • Hence thalmotomy for treatment Tics Brief coordinated sterotyped movements and vocalizations • Most are suppressed for short period • Which might cause anxiety • The limbic pathway in the BG play a role in pathogenesis • Effectiveness of D2 antagonists

  36. THE CEREBELLUM The pathologies of the cerebellum have long revealed that this part of the brain is involved in motor co-ordination The cerebellum is divided into three regions, each of which is connected to a specific structure in the brain and involved in a specific function

  37. 1,The archicerebellum (vestibulocerebellum) • First appeared in fish. • It is connected to the vestibule of the inner ear • And is involved in balance.

  38. 2,ThePalaeocerebellum(Spinocerebellum) • Consists mainly of the vermis. • Is connected to the spinal cord • Controls postural muscle activity by influencing muscle tonus. • The cerebellum therefore controls muscle tension at all times • while releasing those muscles required to execute movements.

  39. 3,Nneocerebellum(Cerebrocerebellum) • It is more voluminous in primates and especially so in humans. • It consists of the Cerebellar hemispheres, • Is connected to the cortex, and contributes to the co-ordination of voluntary movements.

  40. Cerebellum schema

  41. The cerebellar grey matter organized like the cerebral grey matter • A cortex forming the grey matter at the surface, • Deep nuclei that serve as relays for the efferent pathways leaving this cortex .

  42. There are four cerebellar nuclei on either side of the median line: • Fastigial nuclei for the archicerebellum • Emboliform for the palaeocerebellum • Globose nuclei for thepalaeocerebellu • Dentate nuclei for the neocerebellum

  43. Provides control over the timing of the body's movements. • Via loop circuit that connects it to the MC • Modulates the signals that the motor cortex sends to the motor neurons • Analyzes the visual signals associated with movement

  44. Signals from the movement of objects • Or from moving body segments themselves. • Calculate the speed of these movements and adjust the motor commands accordingly. • Errors in such calculations largely account for the poor motor control • Participates in language, attention, memory, and emotions

  45. The Purkinje cells are the most characteristic type of neurons in the cerebellum. • The axons of the Purkinje cells synapse on the neurons of the Dentate Nuclei of the Cerebellum. • These nuclei relay the information to the thalamus • Which then projects to the cortex and the Striatum.

  46. Cellular organization of the cerebellum

  47. The dendrite branches of each Purkinje cell receive synapses from of a single afferent Climbing Fiber. • This fiber is the axon of a neuron in the inferior olive, a nucleus in the medulla oblongata. • The inferior olive integrates the information from the muscle proprioceptors.

  48. Each climbing fiber winds closely around the dendrites of its corresponding Purkinje cell • So that the activation of this fiber will cause a massive excitation of this cell.

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