Understanding Neurology

164characteristic for that condition. For example, althougha rest tremor is characteristic for PD, actioncomponents to the tremor can coexist.Specialist anatomy and physiologyThe basal ganglion is a term which refers to four mainsubcortical nuclei that regulate and coordinatemovement: the striatum (comprising the putamen andcaudate), globus pallidus, subthalamic nucleus, andthe substantia nigra (129, 130). These grey matternuclei maintain muscle tone needed to stabilize jointpositions or inhibit muscle tone during initiation ofmovement. PD is a neurodegenerative disease ofunknown aetiology, in which degeneration ofdopaminergic neurones projecting from the substantianigra pars compacta to the striatum occurs, resultingin deficiency of dopamine within the striatum. Thedegeneration of the neurones within the substantianigra causes loss of the characteristic black (‘nigra’)pigmentation. Clinical symptoms occur when there isdepletion of 50–70% of striatal dopamine.129CortexStriatumGlobuspallidusThalamusSubthalamicnucleusCaudatePutamenExternaInternaSubstantia nigra129 Neuroanatomical schematic diagram of extrapyramidalstructures.130CORTEXGlutamateTHALAMUSEncephalin/GABASTRIATUMGABAGlutamateSUBTHALAMIC NUCLEUSInhibitory neuroneExcitatory neuroneGABAInterna ExternaGlobus pallidusSUBSTANTIANIGRAParkinson’s diseaseDisruption here ofdopaminergic fibresfrom the substantianigra to the striatumDopamineAcetylcholineHuntington’s diseaseDegeneration of fibresfrom the striatum to theglobus pallidus externa.(loss of neurones in thecaudate nucleus)130 Schematic representation of the interaction between basal ganglia, thalamus, and cortex. GABA: gamma amino-butyric acid.