Extrapyramidal system: Basal Ganglia and Cerebellum
updated Sep 14, 2011
The more complex and varied motor activities of man and the monkey are more dependent on truly conscious effort, and are longer in being acquired, and longer in being reduced to automaticity. Yet we know that by constant and habitual repetition, modes of action which were acquired by long and painful education and conscious effort, ultimately become so easy as to be performed without attention... We have reason... to regard the corpora striata as the centers in which these habitual or automatic movements become organized. -- David Ferrier (1876) The Functions of the Brain

Cerebellar control of muscular movement employs practically all modalities of sense represented in the action system of the animal. The function of the cerebellum as the "head ganglion of the proprioceptive system" is not to pattern the muscular response (for these functions are localized elsewhere) but to facilitate its execution; and this facilitation employs all available sensory experience. -- C.J. Herrick (1948) The Brain of the Tiger Salamander

  1. Basal Ganglia
    1. Part of extrapyramidal motor system
    2. Structure (and not every connection)
      1. Caudate & putamen (Collectively known as striatum)
        1. Modular organization
          1. Striosomes
          2. Matrix
        2. Inputs
          1. cerebral cortex
            1. Topographically organized, e.g., frontal cortex to head, visual cortex to tail
            2. Common functional areas converge in striatum, e.g., frontal eye field and supplementary eye field
        3. Output to Globus pallidus and Substantia nigra
      2. Globus pallidus
        1. External segment (GPe)
        2. Internal segment (Gpi)
      3. Subthalamic nucleus (STN)
      4. Substantia nigra
        1. pars reticulata (SNpr)
        2. pars compacta (SNpc)
    3. Connectivity
      1. Loop: cortex basal ganglia thalamus cortex
        1. Cerebral cortex projects to caudate or putamen
        2. Caudate/putamen projects to globus pallidus or subsantia nigra pars reticulata
        3. Globus pallidus or subsantia nigra pars reticulata projects to thalamus
        4. Thalamus projects to cortex
      2. Four circuits
        1. Skeletal motor
          1. supplementary motor area (SMA) - putamen - globus pallidus (GP) - ventral lateral (VL) nucleus of the thalamus
          2. skeletal motor movements
        2. Ocular motor
          1. frontal eye field and supplementary eye field - caudate - SNpr - ventral anterior (VA) and medial dorsal (MD) thalamus and superior colliculus
          2. eye movements
        3. Dorsolateral prefrontal
          1. prefrontal cortex - caudate - GP - VA & MD thalamus
          2. working memory for guiding actions
        4. Lateral orbitofrontal
          1. orbital frontal cortex - caudate - GP - VA & MD thalamus
          2. behavioral set
        5. Anterior cingulate
          1. anterior cingulate cortex - ventral striatum - ventral GP - MD thalamus
          2. motivation and reward
      3. Neurotransmitters and connections (selective list)
        1. Cortex to striatum is excitatory, glutamate
        2. Striatum to GPe / GPi is inhibitory, GABA
        3. GPe to STN is inhibitory, GABA
        4. STN to GPe / GPi is excitatory, glutamate
        5. GPi / SNpr to thalamus is inhibitory, GABA
        6. Thalamus to cortex is excitatory, glutamate
    4. Function
      1. release of inhibition, for example from SNpr to superior colliculus to control saccadic eye movements.
  2. Cerebellum
    1. Evolved with vestibular system
    2. Structure
      1. Cortex
        1. Comprised of just 5 types of neurons in a neurocrystal organization
      2. Deep nuclei
        1. Output pathway for cerebellum
    3. Function
      1. Compares intention with performance and makes appropriate adjustments
      2. Circuits modified by experience; thus it plays a role in motor learning
    4. Three main divisions
      1. vestibulocerebellum
        1. phylogenetically oldest, maintain upright floating
      2. paleocerebellum, spinocerebellum
        1. next oldest, maintain upright posture
      3. neocerebellum, cerebrocerebellum
        1. most recently evolved, initiation, planning, timing of movement