The term “basal ganglia” (BG) is applied to several gray matter structures located at the base of the cerebral hemispheres. Nowadays, it is commonly used to refer to the striatum (caudate and putamen), the globus pallidus pars externa (GPe) and pars interna (GPi), the subthalamic nucleus (STN) and, the substantia nigra pars compacta (SNc) and pars reticulata (SNr). Traditionally, the BG have been associated with the control of movement and their dysfunction is the origin of movement disorders such as the parkinsonian syndrome, hemichorea-ballismus, and dystonia.1
Currently (see Chapter 4), the BG are functionally subdivided as motor, oculomotor, associative, limbic, and orbitofrontal according to the main cortical projection areas, which form loops through the posterior putamen, caudate nucleus, anterior, and ventral striatum.2–4 These connections engage in motor control and other functions such as explicit and implicit learning, reward-related behavior, habit formation and attention to stimuli.5–7 Accordingly, the traditional view that the BG are related only with movement and motor learning is no longer tenable.
This chapter reviews the main physiological features of the BG, common to all functional domains and, the pathophysiology of movement disorders according to their clinical relevance. More recently recognized clinical manifestations of BG dysfunction, i.e., behavioral and mood disorders, are considered in detail elsewhere.8,9
In this section, we discuss the main characteristics of neuronal activity in the normal BG and more recent functional studies on brain imaging in humans. We then conclude with a general appraisal of the operational mode of the BG. In order to achieve our objective, we start by providing a summary of the classic pathophysiological model, and also discuss the architectonic features of the BG. The latter are important inasmuch as anatomy is the substrate of function and provides constraints for any analysis regarding the physiological role of a structure. Details about connectivity and functional organization are in Chapter 4.
General Anatomophysiological Organization
Cortical inputs reach the BG through the striatum and rely back to the cortex via the thalamus, through the GPi and SNr, the output nuclei of the BG (Fig. 5–1A). Striatal medium spiny neuron (MSN) activity is conveyed to the GPi/SNr through a monosynaptic GABAergic projection (“direct” pathway), and polysynaptic (“indirect” pathway) connections which involve the external GPe and the STN.10,11,12 Neuronal activity in the GPi/SNr is mainly tonic and inhibitory onto their projection targets in the thalamus and brain stem; phasic reduction (“pausing”) of BG output is the key physiological feature underlying facilitation, i.e., movement or action, of cortical and brain stem motor regions.13,14 Dopamine modulates glutamatergic effects on corticostriatal inputs by exerting a dual effect on striatal neurons, exciting D1 neurons in the direct pathway and inhibiting D2 neurons in the indirect pathway.15 Accordingly, a general functional ...