The somatic, or voluntary, nervous system is the part of the nervous system that is under conscious control. The other functional component of the peripheral nervous system, the autonomic system, participates in the preparation and facilitation of movement, but the somatic nervous system is where exercise actually happens. Consisting mainly of motor neurons, it is the bridge between the brain's intention to move something and the muscles that actually do it.
Structure
The somatic nervous system enables you to react consciously to environmental stimuli. It includes 31 pairs of spinal nerves and 12 pairs of cranial nerves and controls the movements of your skeletal muscles. These nerves include sensory, or afferent, fibers that transmit information from the outside world to your brain; during exercise, it is the motor, or efferent, fibers that carry instructions from the brain to the muscles via motor neurons.
Motor Neurons
The neuron is the functional unit of the nervous system; as the name implies, motor neurons are responsible for movement. They receive input from the spinal cord at one end and transmit this input along axons to muscles at the opposite end. This transmission involves a complex electrochemical process. Some motor neurons are more than 3 feet long. As described in "Exercise Physiology for Health, Fitness, and Performance" by Dr. Sharon Plowman, motor neurons effect muscle contraction during exercise via acetylcholine, a neurotransmitter.
Adaptations in Exercise
Exercise results in greater strength and endurance in muscles; this is usually evident from increases in muscle size and tone. The nervous system also undergoes adaptations from exercise, although they may not be visible to the naked eye. These changes include: alterations at the neuromuscular junction, where the neuron meets the muscle that it innervates, or connects, an adaptation seen most often in endurance-type training; a higher frequency of nerve stimulation and a greater number of motor neurons recruited for a single movement, characteristic of resistance training; and greater coordination of primary muscles with the synergistic muscles that support them, such as the various quadriceps muscles of the thigh.
Motor Units
A motor unit is a single motor neuron and all the muscle fibers that it innervates. According to Dr. Peter McGinnis in "Biomechanics of Sport and Exercise," a single motor neuron may connect to 20 or fewer muscle fibers but up to 1,000 or more. The greater the ratio of neurons to muscle fibers innervated, the greater voluntary control that you have over a given muscle during exercise. Muscles such as hamstrings, which require strong contraction but little coordination, have motor units with many muscle fibers per neuron; fingers, on the other hand, have comparatively few fibers per neuron.
References
- "Exercise Physiology for Health, Fitness, and Performance"; Sharon A. Plowman; 2007
- University of Pennsylvania Health System; Body Guide; Nervous System
- "Biomechanics of Sport and Exercise"; P. McGinnis; 2005
