Spinal Mechanisms in the Generation and Control of Movement*
Thomas M. Hamm, PhD
Division of Neurobiology, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona
*Courtesy of Thomas M. Hamm, PhD
The Hamm Laboratory: Tom Hamm received his PhD in physiology from the University of Tennessee, investigating the mechanical properties of skeletal muscle in the laboratory of Lloyd Partridge. He studied spinal motor function for several years with Douglas Stuart at the University of Arizona before joining Barrow Neurological Institute. Vladimir Turkin received his PhD from the Bogomolets Institute of Physiology in Kiev, Ukraine, analyzing auditory pathways of the cerebral cortex. He investigated the function of thalamic nuclei at Odessa State University before his arrival at Barrow. Derek O'Neill received a BS in zoology from Iowa State University. He was a supervisor in Laboratory Animal Care at Arizona State University before joining the Hamm Laboratory. Kalil Abdullah, a biotechnology student at Arizona State University, works part-time in the Hamm laboratory.
Spinal nerve cells are a critical factor in the generation and control of movement. This article considers the contribution of spinal nerve cells to movement at several levels of complexity. These include the intrinsic properties of motoneurons and their modulation by other spinal neurons, reciprocal activation of motoneurons that innervate muscles with opposing actions, and the organization of activity in different muscles to produce complex movements like locomotion. Each of these topics is briefly discussed in relation to ongoing research.
Key Words: central pattern generation, dendrite, locomotion, motoneuron, Renshaw cell, spinal cord
Most of us think little about how we move and what is required to perform each movement that we make, whether reaching for an object, walking, playing tennis, or dancing. If we were to examine this process as an engineer would, we would be confronted with complicated sets of equations and puzzled by the many problems (some seemingly insurmountable) that our nervous system solves quickly. Much of the task of controlling movement is accomplished below the level of conscious awareness, using networks and nerve cells of the cerebellum, brainstem, and spinal cord. In our laboratory we seek to understand the role of spinal neurons and networks in the process by which complicated patterns of muscle activity are created to perform the varied and complex movements of which we are capable. Knowledge and understanding of these neurons are also needed to develop treatments for repair and therapy after spinal injury and other injuries and diseases that result in dysfunction of the spinal cord.
The most intensively studied spinal neurons are motoneurons, which form the final pathway from the nervous system to muscles. In the human spinal cord, about 60,000 motoneurons innervate the muscles of each leg. Each motoneuron innervates a set of muscle fibers, each member of the set possessing the same metabolic, biochemical, and physiological properties.[9,14] These properties endow each motor unit (a motoneuron and the muscle fibers it innervates) with a capability adapted for particular types of activity, such as maintained activity at low forces, as needed for walking, distance running, and other aerobic activities, or brief, explosive movements that produce large forces for brief periods, as needed for sprinting and weight lifting. The complement of motor unit characteristics varies between muscles and from one individual to the next, depending on genetics, training, or patterns of use. The activity of each motoneuron determines the force produced by its innervated muscle fibers. The set of motoneurons selected for activation determines the characteristics of the contraction and the pattern of torques produced and, in interaction with the mechanics of the body and the environment, the movement an individual achieves. Within the group of motoneurons that innervate each muscle, the motoneuron pool, motoneurons normally are activated in an orderly sequence according to their size. Each larger motoneuron tends to activate a set of muscle fibers that produces a larger force than the motoneuron just activated.
To understand how the contraction is produced, how movement is controlled, and how control can become disordered after disease and injury, we must first understand how the motoneuron is activated and identify and understand the factors that determine its activity. We also must determine how spinal neurons, interacting with the descending nerve fibers of supraspinal neurons and of peripheral sensory and reflex pathways, generate the temporal patterns and combinations of muscle activity that yield well-controlled, purposeful movements.
The motoneuron is a cell of considerable complexity. One of the largest nerve cells in the nervous system, each motoneuron has 6 to 18 stem dendrites. Each dendrite branches repeatedly to form an extensive, complex dendritic arbor consisting of hundreds of segments that extend 3 to 4 mm across the spinal cord. This complex structure provides a huge area to receive synaptic inputs from sensory, supraspinal, and other spinal neurons.
Synaptic Physiology of Motoneurons
Figure 1. The profile of changes in impedance produced by changes in synaptic conductance as synapses move from the soma to pro- gressively more distant dentritic locations provides a means of identifying synaptic location by measuring neuronal impedance.
It was long thought that integration of the various inputs to motoneurons was a passive process. The simple electrical properties of the motoneuron (the resistance and capacitance of its dendritic membranes and its intracellular resistance) were thought to determine how synaptic currents depolarized the motoneuron to a threshold voltage, inducing action potentials and contraction of the motor unit. However, this concept of the motoneuron was inadequate. The measured strengths of synaptic inputs from sensory fibers and other spinal neurons are sufficient to produce only low levels of activity compared to the full functional range of motoneurons. Studies showing that motoneurons have a complement of conductance channels sensitive to membrane voltage and that some of these conductance channels make an important contribution to the excitability of motoneurons at and below the threshold for action potentials promise to resolve this difficulty.[1,2,15,16]