1) Describe cellular structures that support movement
2) Characterize the neural firing properties that modulate contraction force
3) Differentiate between spinal interneurons and their role in movement
4) Evaluate how the spinal cord produces simple and complex movements
This chapter will explore how the spinal cord is organized to produce complex movements. There is an orchestrated, precise balance between the type of afferent information entering the cord and how it is integrated by neurons in the gray matter of the spinal cord to activate muscles. The difference between a simple movement like the patellar tendon reflex and running on a treadmill can be as little as adding a set of interneurons to the monosynaptic circuit. We will consider how the spinal cord is designed to support multi-joint movements that adapt to the changing environment and unexpected perturbations within milliseconds. This chapter will show you that the spinal cord is organized in remarkable ways so that even a few neurons can mediate and sustain intricate, complex movements without input from the brain. Probably the most exciting feature we will consider is that the spinal cord can learn tasks independent of the brain. With this functional capacity existing in the cord, the brain can provide descending input and create even more complex, skilled movements.
Structure and Organization of the Spinal Cord
The human spinal cord extends from the base of the skull to the vertebral level between L1 and L2. It is made up of cervical, thoracic, lumbar, and sacral regions, which receive afferent information and send motor signals to nearby muscles – cervical supplies the arms and hands; thoracic supplies the trunk; lumbar supplies the legs and feet; and sacral supplies the bowel, bladder, and genitalia. The spinal cord has unique features in each region based on the shape and size of gray matter and the amount of surrounding white matter, comprised of myelinated axons that ascend to the brain, descend from the brain, and propriospinal axons connecting different regions of the cord (Figure 4-1).
Organization of the gray and white matter of the spinal cord. (Reproduced with permission from Kandel ER, Schwartz JH, et al., Principles of Neural Science, 5th ed. New York, NY: McGraw-Hill; 2013.)
In regions that supply many muscles as in the upper and lower extremities, the gray matter will be larger in the cervical and lumbar cord. Specifically, greater motor neurons reside in the ventral horn of the gray matter in these regions. Smaller gray matter and thin ventral horns appear in the thoracic cord, which primarily supplies intercostal muscles. The white matter is also larger in cervical and lumbar enlargements than in the thoracic cord due to increased myelinated axons to innervate motor neurons and sensory axons ...