Basal Ganglia PDF

BASAL GANGLIA Presenter : Dr. Mohamed Hassan - Basal ganglia: A group of nuclei, act as a unified functional unit. - The basal ganglia, like the cerebellum, constitute another accessory motor system that functions usually not by itself but in close association with the cerebral cortex and...

BASAL GANGLIA Presenter : Dr. Mohamed Hassan - Basal ganglia: A group of nuclei, act as a unified functional unit. - The basal ganglia, like the cerebellum, constitute another accessory motor system that functions usually not by itself but in close association with the cerebral cortex and corticospinal motor control system. - the basal ganglia receive most of their input signals from the cerebral cortex itself and also return almost all their output signals back to the cortex.  Basal Ganglia – Consists of Four Principal Nuclei 1. Striatum 2. Globus pallidus 3. Subthalamic nucleus, 4. Substantia nigra  Striatum The striatum is the input nucleus of the basal ganglia, receiving afferent projections from the cerebral cortex. Three subnuclei comprise the striatum: 1. The caudate nucleus, which participates in eye movement control and cognition; 2. The putamen, which participates in control of limb and trunk movements; and 3. The nucleus accumbens, which participates in emotions.  Globus pallidus The Globus pallidus consists of two separate nuclei, the external and internal segments. The internal segment is one of the major output structures of the basal ganglia, whereas the external segment is part of their intrinsic circuitry.  Substantia nigra includes two separate nuclei: the pars compacta and pars reticulata. the pars compacta contains dopaminergic cells that project heavily to the striatum. The pars reticulate of the substantia nigra and the internal segment of the globus pallidus can be viewed as a single output structure divided by the internal capsule.  Subthalamic nucleus is a small nucleus situated between the thalamus and the substantia nigra. It receives projections from the external segment of the globus pallidus, the cerebral cortex, thalamus, and brain stem, and sends output to both segments of the globus pallidus and to the substantia nigra pars reticulata. Basal ganglia Input 1. Parietal cortex (primary and secondary somatosensory information, secondary visual information), 2. Temporal cortex (secondary visual and auditory information), 3. Cingulate cortex (limbic and emotional status information), 4. Frontal cortex (primary and secondary motor information), 5. Prefrontal cortex.  Cortico–Basal Ganglia– Thalamocortical Circuits There are two important pathways through which striatal information reaches GP(internal) - the direct pathway and the indirect pathway. In the direct pathway, striatal cells project directly to GP(internal) while indirect pathway project to GP(external). The consequence of this pathway is to increase the excitatory drive from thalamus to cortex.  Cortex Direct pathway Striatum Excitation (glutamate) Inhibition (GABA) VA/VL * GPe STN * GPi/SNr  Cortex Direct pathway pathway: facilitates movement Striatum Excitation (glutamate) Inhibition (GABA) VA/VL * GPe Disinhibition STN * GPi/SNr Brain stem/ Spinal cord  Cortex Striatum pathway: Indirect pathway inhibits movement VA/VL * GPe Excitation (glutamate) Disinhibition Inhibition (GABA) STN * GPi/SNr Brain stem/ Spinal cord  Cortex Direct pathway: facilitates movement Striatum D2 D1 Indirect pathway: inhibits movement SNc VA/VL * GPe Excitation (glutamate) Inhibition (GABA) STN * GPi/SNr Brain stem/ Spinal cord Direct and indirect pathways Neurotransmitters in the basal ganglia Dopaminergic And Cholinergic Modulation Of Direct And Indirect Pathways the effect of the dopaminergic nigrostriatal projection is to increase motor activity. the effect of the cholinergic striatal interneurons is to decrease motor activity by inhibit striatal cells of the direct pathway and excite striatal cells of the indirect pathway.  Functions of basal ganglia. 1. Voluntary movement Initiation of movement Control of ramp movement Change from one pattern to other Programming and correcting movement while in progress (thalamocortical circuts) 2. Postural control Righting reflex Automatic associated movement (walking) 3. Control of muscle tone 4. Role in arousal mechanism 5. Speech , lesion of left caudate results in disturbed speech dysarthria  Control of voluntary motor activity. Cognitive control of motor activity. Neural discharge in Basal Ganglia begins well before the movement begins. Most of the motor actions occur as a consequence of thought process in mind. So basal ganglia is involved in planning & programming of movements Timing & scaling of intensity of movements. How rapidly & how much large the movement should be. Subconscious execution of some movements. Swinging of arm while walking. Crude movements of facial expressions with emotions. Movements of limbs while swimming. Lesions of Basal Ganglia globuspallidus – athetosis - spontaneous writing movements of the hand, arm, neck, and face. putamen – chorea - flicking movements of the hands, face, and shoulders substantianigra – Parkinson's disease - rigidity, tremor and akinesia – loss of dopaminergic input from substantianigra to the caudate and putamen  Lesions of Basal Ganglia subthalamus hemiballismus - sudden flailing movements of the entire limb. caudate nucleus and putamen huntington’s chorea - loss of GABA containing neurons to globus pallidus and substantia nigra.