Neuroanatomy, Extrapyramidal System

Excerpt

The extrapyramidal system (EPS) is an anatomical concept first developed by Johann Prus in 1898 when he discovered that the disturbance in pyramidal tracts failed to prevent epileptic motor activity. Prus postulated that, apart from pyramidal tracts, there must be alternative pathways, called the "extrapyramidal tracts," that "delivered epileptic activity" from the cerebral cortex to the spinal cord. Clinically, the term "extrapyramidal" was thus adopted to distinguish between the clinical effects produced by damage involving the basal ganglia and those of damage to the classic "pyramidal" pathway. Despite this distinction, however, the two systems have important anatomical and functional relationships.

The EPS serves an essential function in maintaining posture and regulating involuntary motor functions. In particular, the EPS provides:

1. Postural tone adjustment

2. Preparation of predisposing tonic attitudes for involuntary movements

3. Performing movements that make voluntary movements more natural and correct

4. Control of automatic modifications of tone and movements

5. Control of the reflexes that accompany the responses to affective and attentive situations (reactions)

6. Control of the movements that are originally voluntary but then become automatic through exercise and learning (e.g., in writing)

7. Inhibition of involuntary movements (hyperkinesias), which are particularly evident in extrapyramidal diseases.

The EPS, therefore, controls the automatic activities but also influences voluntary motility through a tonic function. These regulation mechanisms involve the processing of centers located in multiple brain regions, such as parts of the cerebral cortex, the cerebellum, the thalamus, the reticular substance, and several basal ganglia. The term basal ganglia or basal nuclei refers to a group of subcortical nuclei. Among these nuclei, the caudate nucleus and the putamen nuclei, which together constitute the neostriatum, plus the substantia nigra (SN), red nucleus (RN), and the subthalamic nucleus of Luys compose the nuclei of the EPS. From all these centers, numerous subcortical tracts, or the extrapyramidal tracts, stem out and terminate in the spinal cord. However, the majority of tracts travel through the basal ganglia. Thus, anatomically, the EPS can be defined as a set of nuclei and fiber tracts that receive projections from the cerebral cortex and send projections to the brainstem and spinal cord and, functionally, works as a complex motor-modulation system.

Alterations affecting the various circuits play a crucial role in the pathogenesis of extrapyramidal motor disorders. Classic examples of injury to the EPS are Parkinson disease (PD), Huntington chorea (HC) caused by degenerative processes in the striatum, Sydenham chorea, multiple systemic atrophy (MSA), and progressive supranuclear palsy. In 1995, the World Health Organization's International Classification of Diseases released a classification for extrapyramidal and movement disorders. This chapter encompasses PD, secondary parkinsonism, other degenerative diseases of the basal ganglia, and several clinical conditions featuring dystonia, dyskinesia, and tremors (e.g., essential tremor). The clinical aspects of these clinical conditions are manifold and are not only the effect of alterations of voluntary movements. Because EPS probably establishes connections with the motor cortex by regulating the process of movement from the first ideational stages, voluntary movement can also become impaired in extrapyramidal pathology. For instance, slowing of voluntary movements such as walking is usually observed.

Moreover, the alterations that lead to these extrapyramidal pathologies mainly concern neurodegenerative processes. Thus, depending on the specific disease, the main symptoms are alterations of involuntary movements such as tremors, spasms, impairment of voluntary movements as well as a decline in cognitive functions involving mainly memory tasks, and affective sphere disorders such as depression. Postural alterations are also detected. For instance, the so-called Pisa syndrome, which is an abnormal posture in which the body appears to be leaning to one side like the Tower of Pisa, is an atypical feature of MSA. Finally, autonomic alterations and several non-motor symptoms, such as pain, can be part of the clinical picture of these pathologies.