ABSTRACT
The basal ganglia are five separate nuclei of neuronal cell bodies (see Figure 1.7). The main pathways between the nuclei and their connections with other brain areas are as follows (Figure 11.2). The caudate nucleus and the putamen (together called the corpus striatum) have an input from the cerebral cortex (the higher centres of the brain) that involves the neurotransmitter glutamate. Two outputs from the caudate nucleus to the globus pallidus use GABA as the neurotransmitter; one is an excitatory pathway to the medial globus pallidus (GPm), the other is an inhibitory pathway to the lateral globus pallidus (GPl). The putamen has an output to part of the substantia nigra called the pars reticulata (abbreviated to SNr). The other part of the substantia nigra, the pars compacta (SNc), has dopaminergic feedback loops to both components of the corpus striatum, i.e. an excitatory loop back to the putamen, an excitatory loop back to the GPm caudate nucleus output, and an inhibitory loop back to the GPl caudate nucleus output. The output from GPl to the subthalamic nucleus (STN) utilises GABA, while the outputs from the STN to both the SNr and the GPm are facilitated via glutamate. Both the SNr and the GPm have GABA-mediated outputs to the thalamus (Bear et al. 1996). All these pathways can be visualised using Figure 11.2. From Chapter 1 we recognise that the substantia nigra is responsible for lowering muscle tone and it does this through its dopaminergic pathways to the corpus striatum. This statement is fundamental to our understanding of Parkinson’s disease.
