Circuitry of the Basal Ganglia

Circuitry of the basal ganglia    The inputs from the cortex to the basal ganglia are excitatory. The outputs from the GPi and reticular part of the substantia nigra are inhibitory. Basal ganglia is organised somatotopically with the motor cortex (e.g. part of the motor cortex for the arm projects to the arm part of the basal ganglia and its components). o However, some overlap occur so it is not entirely distinct.  The inputs of basal ganglia come not only from the cortex, but also from the substantia nigra pars compacta (nigro-striatal pathway). o This connection is affected by dopamine. o Direct pathway (D1 and 5) facilitates movement, whilst indirect pathway (D2, 3 and 4) inhibits movement. o Direct pathway has a direct inhibitory link to the GPi, hence disinhibiting GPi activity on the thalamus. o Indirect pathway has an inhibitory link to the GPe, which itself inhibits GPi and STN. Inhibition of STN increases excitation to GPi, which inhibits activity on thalamus. Basal ganglia movement disorders  Hypokinetic disorders – characterised by akinesia (lack of movement) and bradykinesia (slowness of movement).  Hyperkinetic disorders: characterised by dyskinesia (disruption of movement), athetosis (writhing), chorea, ballism and dystonia. Clinical correlation: Parkinson’s disease  As a result of disruption to the substantia nigra pars compacta.  Done in more detail in PBL 10. Explaining some symptoms of diseases of basal ganglia  Hemiballismus (involuntary flinging motions of the extremities). o Classified by a decreased activity of the STN. o Consequently, there is less excitation of the GPi  deinhibition of thalamus  increased drive to motor cortex.  Chorea (quick movements of the feet or hands). o Classified by a decreased activity of the putamen, such that it inhibits the indirect pathway. o Consequently, there is deinhibition of GPe  inhibition of STN  inhibition of GPi  deinhibition of thalamus  increased drive to motor cortex. Role of basal ganglia in cognition, mood and behaviour  Dorsolateral prefrontal circuit. o Associated with disorders in organising behavioural responses, using verbal skills in problem solving (executive functions).  Lateral orbitofrontal circuit. o Damage associated with irritability, emotional lability, failure to respond to social cues and lack of empathy.  Prefrontal circuit. o Intensified action (impulsivity) and flattened action (apathy), rather than of perception or sensation.  Limbic circuit. Summary of basal ganglia  Basal ganglia consists of a number of separate nuclei.  Major role is motor control – output modulates activity in thalamic nuclei.  Two main pathways that run through the BG, direct and indirect, both use dopamine, but have opposing effects.  Damage to components lead to motor deficits, both hypokinetic and hyperkinetic.  There are also non-motor roles through widespread connections with cerebral cortex. Cerebellum      Receives extensive sensory input, but is not involved in sensory discrimination or interpretation. Profoundly influences motor function, however resection of large parts of the cerebellar cortex does not result in lasting paralysis. Plays a major role in motor learning and higher mental function. Comprises ~10% of brain by volume, but contains >50% of the number of neurons. Neurons are arranged in a basic circuit that is repeated throughout the cerebellum. o Divided into distinct regions that receive signals from different parts of the brain and spinal cord and project to different motor systems. o It is thought that the different regions perform the same function, but use different inputs and affect different motor systems. Structure  Located on the dorsal side of the brainstem.  Connected to brainstem by inferior, superior and middle (largest) cerebellar peduncles.     Arranged in folia and divided into two hemispheres. External grey matter: cerebellar cortex. Internal white matter: tracts and three pairs of deep nuclei. o Fastigial nucleus. o Globose and emboliform nuclei. o Dentate nucleus (largest). Almost all of the output leaves through the three deep nuclei. o Some outputs leave via the vestibular nerve to the vestibular nuclei. Adjustment of motor output  Evaluates disparities between intention and execution of movements and adjusts the motor output accordingly.   Input: o Receives input from the cerebral cortex (intention information). o Input from sensory receptors provides information about execution of desired movement. Output: o Through cerebellar nuclei to motor and premotor regions.