Learning about spinal and brain control of movement (chapter 13 and 14) brought up questions about embodied cognition and mirror neurons, especially as both relate to music. Before getting to those, the smaller interconnected systems that populate motor systems should be outlined.
At the base level there are different types of motor neurons; upper motor neurons exist in the brain and innervate the spinal cord while lower motor neurons exist in the ventral horn of the spinal cord and innervate somatic musculature. Lower motor neurons directly control muscle contraction and are therefore called the “final common pathway” for the control of behavior. There is a “segmental organization” of lower motor neurons related to their wiring throughout the segments of the spine; axons group together to form ventral roots which then join with dorsal roots to create spinal nerves. Lower motor neurons are further grouped into alpha and gamma motor neurons which work in different specialized circuits. A motor unit is comprised of a motor neuron and the muscle fibers it innervates, whereas a motor pool contains multiple motor units.
The wiring and synaptic transmission of motor neurons create programatic circuits that then lead to movement and proprioception. These circuits include the graded control of muscle contraction by alpha motor neurons, excitation-contraction coupling between motor neurons, and proprioception from muscle spindles, golgi tendon organs, and joints. Spinal interneurons mediate these processes and form networked relationships that lead to spinal motor programs, such as that used for walking.
One of the fascinating things about the spinal control of movement is that neural circuits generate rhythmic patterns of activity. This can be seen in the spinal circuitry that direct the movement of everything from the ancient sea creatures called lampreys to bipedal and quadrupedal animals such as humans and dogs. The intrinsic timing produced through interconnected spinal circuits means that rhythm is a fundamental part of motor system function. Extending this thought to more behavioral and cultural outcomes, it makes me wonder about embodied music cognition – which led me to then stumble upon a paper of the same title. I don’t have the mental bandwidth to read and digest this paper prior to publishing this blog, but I look forward to learning more when I have the time.
Another thing that made me think about musical processes was the discussion in chapter 14 regarding mirror neurons. Apparently this theory is not well studied in humans, but there was an example using an animal model where motor neurons in monkeys were tracked using implanted electrodes. When a monkey reached for a peanut on a table a primary motor area neuron fired, and the same neuron fired when the monkey watched a human pick up a peanut. When a person used a tool (forceps) to pick up the peanut the same monkey motor neuron did not fire. This made me think back to the finding I cited a few blogs ago, where motor neurons are activated in people listening to music – could this be due to mirror neurons? The book mentions that these surgically implanted neuron measuring techniques have been used in consenting humans (as a side note, how do researchers get consent from monkeys and what are the ethical implications if they don’t get consent?) so it seems that similar experiments could be done in humans when looking into mirror neurons as related to music. My major question is: how linked are music perception/cognition and the motor system?