Motor systems

Abstract

The motoneurons in the spinal cord and the cranial nerve nuclei of the brain stem innervate the skeletal muscles. They form Sherrington’s “final common path” to the striated muscles for all signals that influence motor behaviour coming from peripheral sensory receptors and from motor centres in the brain stem and the cerebral cortex. The spinal and bulbar motoneurons with their respective premotor interneurons form the basic motor system. This system forms the basis for spinal and bulbar reflexes and for basic motor patterns such as walking, which in turn are modulated by descending supraspinal pathways. The interneuronal or propriospinal system is in large part responsible for the great versatility that characterizes motor behaviour. The descending pathways from the cerebral cortex and the brain stem to the interneurons and motoneurons of the spinal cord represent the instruments by which the brain steers movements of body and limbs. The descending supraspinal pathways consist of two parallel sets that are derived from the cerebral cortex and the brain stem, respectively. Likewise, corticonuclear or corticobulbar as well as brain stem projections innervate the interneurons and motoneurons of the trigeminal, facial, ambiguus and hypoglossal motor nuclei. Two other brain structures, the cerebellum and the basal ganglia, are crucially important for motor functions. The activity generated in the cerebellum and the basal ganglia is largely channelled through the brain stem descending pathways and the corticospinal tract. Several forebrain structures known for their close relation to emotional behaviour, such as parts of the amygdala and the hypothalamus, have direct access to brain stem areas controlling motoneurons. They form a highly integrated system which is especially concerned with emotional and motivational states. This is referred to as the emotional motor system. In this chapter, first the peripheral motor system, composed of spinal and bulbar motoneurons, will be discussed, including their role in reflex pathways and muscle tone (Sect. 9.2). These lower motoneurons are innervated by interneurons in the spinal cord and the brain stem through propriospinal and propriobulbar projections, respectively. Certain populations of interneurons form specialized pattern generators for rhythmic movements like locomotion. Subsequently, in Sect. 9.3 human walking, gait control and posture and in Sect. 9.4 the circuitry for central control of movement will be discussed. Disorders of the motor circuitry will be presented with emphasis on lesions of supraspinal motor structures and are abundantly illustrated with clinical cases. The English terms of the Terminologia Neuroanatomica are used throughout.