Topics and Speakers John H. Martin, PhD, and John W. Krakauer, MD
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Synopsis
In this introductory lecture to motor systems, Dr John H. Martin discusses the hierarchy of the motor system the and multiple origins of the descending motor pathways and their parallel organization; how the cortical motor paths have both a direct and indirect route to the spinal cord; and the "medialateral rule," which states that the medial pathways exert principal control over proximal muscles whereas lateral pathways exert principal control over distal muscles.
A descending motor pathway is a nerve pathway that goes down the spinal cord and allows the brain to control body movement below the head. The hierarchy of the motor system (from lowest to highest) ranks as follows: spinal cord, brainstem, cortical motor circuits, and premotor areas. The spinal cord is involved in limb and trunk reflexes. Moving up to the brainstem, there are some new functions (e.g. complex postural reflexes) and shared functions with the spinal cord (e.g., especially the cranial reflexes like the jaw-jerk reflex). Next in line are the cortical motor circuits, which are involved in skilled movements. Finally, we arrive at the premotor areas of the cortex, which are involved in planning and organization of movements.
It is up at the level of the cortex that we have our main three origins of descending motor pathways: as was mentioned, the cortex; the primary motor cortex; and the brainstem motor control nuclei. Each of these pathways are characterized by two factors: the type of synaptic connections used (mono- or polysynaptic) and the types of neurons involved (intersegmental or segmental neurons). For instance, while the primary motor cortex seems to contain only monosynaptic connections to motor neurons, the brainstem seems to contain both mono- and polysynaptic. As their names imply, intersegmental neurons reside in between spinal cord segments, whereas segmental neurons reside within a specific segment of the spinal cord. It is believed that segmental interneurons may be more important in integrating descending control signals with sensory information from a limb's local areas. In the brainstem, there are four further major origins of descending pathways: the red nucleus, the superior colliculus, the reticular formation, and the vestibular nuclei.
All of these pathways exhibit parallel organization, much like the parallel organization of the dorsal column-lemniscal system and the anterolateral system. Each parallel path is thought to serve a distinct motor control function, since they produce different inputs and outputs. Other origins of descending motor pathways would include the basal ganglia and the cerebellum. These are discussed in further detail.
Dr Martin then moves on with a discussion of the medialateral rule, wherein pathways that are located medially in the spinal cord influence proximal motor control preferentially and those located laterally, influence distal motor control preferentially. There are both excitatory and inhibitory interneurons that help regulate the lateral or medial pathways according to the desired muscle movement. For instance, the brainstem has both lateral and medial pathways. The only major lateral brainstem pathway is the rubrospinal tract, which is thought be involved in the control of both the flexor and extensor muscles. The medial brainstem pathways, such as the tectospinal tract coming from the superior colliculus, is involved in coordinating eye and head movements. The lateral and medial pathways of the cortex are a bit more complex. The medial paths have a bilateral projection to the spinal cord whereas the lateral paths are crossed. Both kinds of paths have direct and indirect routes. For instance, there are two indirect cortical pathways that exert influence over motor neurons: cortical projections directly to spinal interneurons, and then from interneurons to motor neurons; and cortical projections to brainstem motor control circuits, and then from there to interneuronal circuits and then to motor neurons. Dr Martin quickly notes that the cortical areas devoted to different body parts are not related to the size of that body part but to the degree of control we have over it (e.g., the hand, a small area, has a large cortical brain area whereas the trunk, a very big area, has small cortical representation). The lecture concludes with a brief conversation on the importance of the diversity of motor pathways and their roles in healing spinal cord injuries.
