6 Neuronal integration and movement
Clinical cases for thoughtCase 6.1
Questions
• 6.1.1 What neurological structures are involved in the creation of motivation to move?
• 6.1.2 How is this thought process transformed into mechanical movement of muscles?
• 6.1.3 Based on neural pathways and integrative circuits, what theory can be formed connecting his declining activity level and his lack of motivation?
Postures and movements are controlled by a hierarchy of systems
Postures and movements are controlled simultaneously by different levels of nervous organisation including the cortex (cognitive control), the sensory system (sensory control), and the emotional system (emotive control). These levels of the organisation first suggested by Jackson are classified into a vague three-tier system (Jackson 1882).
The highest levels of cognition are concerned with the relevance and importance of the task to the present situation. This analysis seems to occur prior to communicating with the lower levels of the hierarchy. The ‘cognitive ‘component is composed of sensory, motor, and associative systems and the ‘emotive’ component is largely composed of limbic circuits (Fig. 6.1).
Limbic and hypothalamic involvement in movement
The limbic system has been traditionally described as involving a complex network of neural circuitry composed of the parahippocampal gyrus, the cingulated gyrus, the subcallosal gyrus (which is the anterior and inferior continuation of the cingulate gyrus), the hippocampal formation (which includes the hippocampus proper, the dentate gyrus, and the subiculum), various nuclei of the septal region, the nucleus accumbens (which is an extension of the caudate nucleus), neocortical areas such as the orbital frontal cortex, subcortical structures such as the amygdala, and various nuclei of the hypothalamus (Iversen et al. 2000) (Fig. 6.2).
The hypothalamus contributes to limbic system function primarily through controlling influences on the pituitary gland. Neurons in the medial basal region of the hypothalamus release peptide neurohormones that act as stimulators or releasing factors that act on the cells of the anterior pituitary gland or adenohypophysis. The pituitary cells then release a variety of hormones including luteinizing hormone (LH), the growth hormone (GH) somatotrophin, adrenocorticotrophic hormone (ACTH), thyroid stimulating hormone (TSH), follicle-stimulating hormone (FSH), and prolactin. Axons of neurons in the supraoptic and paraventricular nuclei release the neurohormone oxytocin and the antidiuretic hormone vasopressin (Fig. 6.3).
Neurons in a variety of hypothalamic nuclei also project to the intramedial lateral (IML) cell columns of the spinal cord grey regions where they modulate the activity of the preganglionic neurons of the sympathetic nervous system, which control a variety of functions including blood flow to virtually all areas of the body. This pathway is important in modulating blood flow to various muscle groups and organs including the brain, prior to and during movement. The control of the blood flow to the hypothalamus arises from postganglionic sympathetic neurons located in the superior cervical ganglion, which are under the influence of the hypothalamus itself (Fig. 6.4).
The development of motivational drives
The limbic system is deeply involved in the creation of motivational states or drives that modulate the central integrative states of neurons in wide-ranging areas of the central nervous system (CNS) that produce a variety of behavioural responses such as movement, temperature regulation, active procurement of food, sexual drive, emotional context, and curiosity (Swanson & Mogenson 1981; Brooks 1984; Kupfermann et al. 2000).
The majority of the neurons in the inferior and posterior regions of the intraparietal sulcus (Brodmann area 7) show an early response to sensory cues that relate to the execution of movement (Mountcastle et al. 1975). Smaller numbers of neurons in area 7 exhibited more complex response patterns, where activation only occurred in specific situations where a number of variables were met simultaneously, e.g. sight of food and the presence of hunger (Fig. 6.5).
Corticoneostriatal and corticopontine projections
The judgement system seems to consist of a series of complex gate systems of ‘and’ or ‘or’ gates that are both involved directly in gating inputs and are indirectly gated themselves by being involved in the more complex array of interactions involved in the complete execution command required. This complex array of gated pathways in the association cortex projects directly to the motor cortex via association fibres as well as to the striatum and pontine nuclei. These projections to the striatum and pontine nuclei further project to other areas of the nervous system and form indirect pathway projection loops from the association cortex to the motor cortex (Rolls 1983; deZeeuw et al. 1997).
The first indirect projection loop involves the striatum (caudate nucleus and putamen), whose output nuclei the globus pallidus pars interna projects through the anterior division of the thalamic fasciculus to the pars oralis area of the ventral lateral and ventral anterior thalamic nuclei. Neurons of the ventral lateral thalamic nuclei project their axons to the ipsilateral motor, premotor, and parietal cortical areas (Fig. 6.6).
