Neuronal integration and movement

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6 Neuronal integration and movement

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).

The hypothalamus also functions as a communication relay by funnelling information from the cortex via the cingulate gyrus, to the hippocampal formation, where the information is processed and reciprocally fed back to the cingulate gyrus via the mammillary bodies and anterior thalamic nuclei.

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).

Motivational drives produced in the limbic system appear to be the products of integrated sensory and emotional cues, which have been triaged into some order of importance that results in the activation of the appropriate areas of cortex to a readiness or activation mode. Thus, through motivational activation from the limbic system, the appropriate areas of cortex increase their central integrative state to a state of awareness, looking for something about to happen such as a change in posture or a change of emotional state.

The transition from motivation to the initiation of movement involves pathways from multiple premotor regions of the cortex to the motor regions of cortex.

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).

This type of processing suggests that motivational drives received from the limbic system are not blindly obeyed but are first presented to the association areas of premotor and parietal cortex, where a rudimentary form of judgment as to the appropriateness of the behaviour required is made.