**2. Motor loops (motor and oculomotor)**

94 Neuroimaging for Clinicians – Combining Research and Practice

typical for the damage of a particular loop or cause typical symptoms for the damage of several of them (ibid.). The symptoms, being the consequence of, definitely localized in the brain, damages of the particular structures of cortico-subcortical loops, can overlap with the symptoms from different areas of encephalon connected with a specific functional system, which is not a part of this system (ibid.). The conceptual model of basalo-thalamo-cortical connections can be helpful in interpretations of the symptoms of mental disorders relating to basal ganglia pathologies, for example in Parkinson's disease (Tröster & Arnett, 2005). 

Fig. 3. The original conceptual model of the neuronal loop connecting the internal globus pallidus (GPI), subthalamic nucleus (STN) and thalamus with the cerebral cortex compiled

on the basis of the literature (Fix, 1997; Longstaff, 2003; Groenewegen, 2003).

Motor control of skeletal muscles relates to the motor loop (motor circuit) and the oculomotor loop (oculomotor circuit) (ibid.). The dorso-medial prefrontal loop, orbitofrontal loop and the anterior part of the cingular gyrus loop are associated with the control of cognitive and emotional functions (ibid.).

The motor circuit is responsible, inter alia, for automatic motor activity connected with maintenance of body posture and reflexes (Fix, 1997), as well as for the control of muscular tension. The motor loop plays an essential role in initiating and fluent performing of motor actions executed by skeletal muscles especially during will dependent movements. The disorders of this loop can cause muscular stiffness, bradykinesia, akinesia and hipokinesia (e.g. in Parkinson's disease and parkinsonian syndrome) or excessively large and uncontrolled movements of limbs (e.g. Huntington's chorea, balism) (Fix, 1997).

The oculomotor loop participates in the control of saccadic eyeball movements. Efferent connections to the superior colliculus (Sc) from the cortical areas of the brain and subcortical nuclei, especially the reticular part of substantia nigra (SNr) make it possible to control rapid eyeball movements through the inhibition of movements disturbing the execution of a task (Hikosaka, 2000). Pressumably the neurons of ventro-lateral part of substantia nigra pars reticularis and caudate nucleus play essential role in external eyeball muscles movements, both through the neurons in which information on previously executed movements is remembered (memory-guided saccades), as well as neurons reacting on currently incoming visual stimuli (visually-guided saccades) (ibid.). In the result of oculomotor loop damage visual fixation can be impaired, and unilateral neglect syndrome, as well as attention deficits can be observed especially in the tasks requiring rapid movements targeted at stimuli (Hikosaka, 2000). The shortage of functions of external eyeball muscles caused by damages of basal ganglia (e.g. in Parkinson's disease, Huntington's disease) can impair saccadic movements of eyeballs depending on previously remembered information. In persons with basal ganglia disorders dysfunctions in intentional inhibition of eye movements, triggered by visual stimuli (ibid.), were observed.
