We begin our discussion with how the brain controls intermuscular coordination. Skeletal muscles cause two bones to move around their joint. Muscles on one side of the joint contract while muscles on the other side relax, and this involves intermuscular coordination. As you watch the contraction of the biceps muscle here, notice how the elbow joint changes. When the joint narrows, the action is joint flexion. And when the triceps contract, the biceps relax and the joint angle becomes larger. This action is referred to as joint extension. When the biceps contract, the triceps must relax and when the triceps contract the biceps must relax. If they both contract or both relax no movement will occur. Now remember that a whole muscle is made up of many muscle cells or muscle fibers as they're also called. Each of these muscle fibers is activated by a neuron. And nerves are clusters of neurons bundled together into a connective sheath. Now it's not necessary for us to go into any great detail about the anatomy of nerves and neurons but it is helpful to know the difference between them. Here's a motor neuron here on the screen. It has dendrites attached to its cell body, it has an axon where the signal is transmitted to the axon terminal. And the terminal end of the axon is split into strands, each of which enervates a single muscle fiber. This illustration shows the spinal nerves, consisting of bundles of neurons exiting from the spinal cord. And the nerve highlighted in green here is the vagus nerve. And we're going to come back and talk about this nerve quite a bit in upcoming lectures. It's a really really important nerve. It controls the athlete's body during rest. Now within the nerve, there are three types of neurons, each performing just a slightly different function. Sensory nerves transmits sensory information about the environment, what you're looking at. The position of joints and muscles, etc., and it sends that information back to the brain. The interneurons relay information between the sensory neurons and the motor neuron that in turn transmits information to the cells or muscles, telling them what they should do. Motor neurons have highly branch dendrites. In fact, dendrite is Greek meaning tree. Dendrites are analogous to an antenna that gather signals and pass the information on. Dendrites receive signals from other neurons. And these signals are integrated and passed on to the cell body and then down the axon to the axon terminal. And here are the muscle fibers. And here is the axon of the neuron. The cell bodies, and the dendrites, off motor neurons are clustered in a column shape nuclei called the motor neuron pool. And here's another graphic showing where the motor neuron pools are located in the spinal cord. Groups of motor units within the motor neuron pool work together to coordinate the actions of a single muscle. Highly trained athletes have complex and integrated dendritic trees from many neurons within the motor neuron pool. And this enables them to activate more motor units within the motor unit pool and this in turn contributes to their ability to use a higher level of force than a lesser trained athlete can produce. Their motor unit pool is one of the important concepts for you to understand. The second important concept is that a muscle fiber is innervated but only one motor neuron. The split ending of the motor neuron axon, each activates one muscle fiber. And this means that one neuron can innervate many muscle fibers within a muscle. Note here on this diagram how a single motor neuron innervates many muscle fibers. The number of muscle fibers innervated by a single motor neuron is called its innervation ratio. The number of muscle fibers in a motor unit varies. Motor units of a muscle controlling fine movements such as muscles of the finger or hand contain less than a 100 muscle fibers. Motor units of the eye only have around ten muscle fibers. Eye movements are very, very concisely controlled. And if a muscle required only for coarse movements such as the quadriceps muscle the motor units will have a high innovation ratio. Each motor unit innovates a thousand or more muscle fibers in the quadriceps. And in the case of the quadricep muscle, it's not necessary for individual muscle fibers to produce a fine movement. The quadricep produces gross movements involving very, very high force and power.
