Processes and mechanisms in neuropsychiatry: Motor-executive processes

Introduction “The irritating historical division between neurology and psychiatry is at its most arbitrary in the field of movement disorders” (Lennox & Lennox, 2002, p. 28). The introduction and differentiation of “extrapyramidal motor disorders” from “pyramidal disorders,” by Wilson in 1912, heralded a major paradigm shift (Rogers, 1992). Wilson conceptualized disorders that had traditionally been regarded as “psychiatric” or “functional,” such as Parkinson's disease, as extrapyramidal. In addition, he also described a group of patients with motor symptoms, the majority of whom also experienced psychiatric symptoms with diagnoses of hysteria or schizophrenia. Although neurology and psychiatry have continued to develop along separate lines, disorders such as Parkinson's disease, Huntington's disease and Gilles de la Tourette syndrome (GTS), which straddle the neurology and psychiatry boundary, highlight the importance of understanding both motor and psychological processes in these and other conditions. This may offer insight into the neural correlates and clinical management of these disorders. This chapter will discuss the relevance of the neuromotor circuitry and recent theoretical advances in motor theories that relate to the underlying neuropathophysiology of these disorders. Neuromotor circuitry The basal ganglia and cerebellum are key neural structures in the brain's motor circuitry. The basal ganglia are comprised of the caudate, putamen, globus pallidus and substantia nigra. The caudate and putamen form the striatum, and the putamen and globus pallidus are referred to as the lentiform nucleus. Generally speaking the striatum is the input layer of the basal ganglia and lentiform nucleus the output layer. Basal ganglia efferents are inhibitory to the thalamus. The cerebellar hemispheres are functionally analogous input layers of the cerebellum; however, unlike the striatum which receives afferents directly from the cortices, the cerebellum receives cortical input via pontine nuclei that then project via mossy fibers to the cortex of the cerebellum.