Muscle tension, the force generated by a muscle, is commanded directly by the nervous system. The nervous system controls muscle force with precision, allowing for everything from delicate movements to immense power. This control uses strategies that manipulate the fundamental functional unit linking the nerve and the muscle.
Defining the Motor Unit
The foundational element of this control system is the motor unit: a single motor neuron and all the muscle fibers it innervates. The motor neuron’s cell body is in the spinal cord, and its axon branches to stimulate multiple muscle fibers. When the motor neuron sends an electrical signal (an action potential), all connected fibers contract simultaneously in an all-or-nothing manner.
The size of a motor unit varies significantly depending on the muscle’s function. Muscles requiring fine control, like those in the eye, may innervate only a handful of fibers. Conversely, large power-generating muscles, such as those in the thigh, can have a single motor neuron controlling hundreds or thousands of fibers. A larger motor unit consequently generates a greater amount of force when activated than a smaller one.
Spatial Summation: Activating More Muscle Fibers
The primary method the nervous system uses to increase muscle tension is spatial summation, or motor unit recruitment, which involves activating an increasing number of motor units. This process adheres to the Size Principle, dictating a fixed, orderly sequence. Smaller motor units, which have lower activation thresholds and innervate fatigue-resistant fibers, are always recruited first for minimal force generation.
As the demand for force increases, the nervous system progressively recruits larger motor units with higher activation thresholds. These larger units control more muscle fibers, which are typically faster-twitch and more powerful, contributing significantly more tension. This orderly recruitment ensures movement uses fatigue-resistant units first, only engaging high-force, more fatigable units when necessary for maximum effort.
By continuously adding more active motor units, the nervous system spatially distributes the electrical signal to generate a cumulative, greater force. This allows for smooth grading of muscle force until all motor units have been recruited, achieving maximum capacity.
Temporal Summation: Increasing Signal Frequency
The second strategy for increasing tension is temporal summation, or rate coding, which increases the frequency of action potentials sent to active fibers. A single action potential causes a brief, weak contraction (a muscle twitch). If the motor neuron sends a second signal before the muscle fibers fully relax, the resulting force is added to the residual tension (wave summation).
As the firing frequency increases, successive contractions blend together. At a high enough frequency, the individual twitches become indistinguishable, creating a smooth, sustained, maximal contraction known as fused tetanus. This rapid signaling maintains a high concentration of calcium in the muscle fibers, driving sustained force production.
Temporal summation is important for generating force at intermediate and high levels of effort. While recruitment (spatial summation) dominates at low forces, the nervous system relies on increasing the firing rate of activated units to produce maximum sustained tension. Firing rates typically range from 8 to 25 pulses per second.
Higher Level Control and Reflexive Modulation
The brain and spinal cord exert regulatory oversight over both spatial and temporal summation to fine-tune muscle tension. Descending pathways from the cerebral cortex provide the voluntary command, sending signals that determine the level of excitation to the motor neuron pools. This input initiates the orderly recruitment of motor units and the adjustment of their firing rates.
The spinal cord also contains reflex arcs that provide immediate, involuntary modulation of tension based on sensory feedback. The stretch reflex, mediated by muscle spindles, instantly activates motor neurons to contract a muscle if it is unexpectedly lengthened, helping maintain posture. This quick loop adjusts muscle tone without direct brain intervention.
Sensory organs within the tendons, called Golgi tendon organs, monitor muscle tension and can trigger an inhibitory reflex to prevent damage. The central nervous system can consciously override this protective inhibition during maximal effort, allowing a temporary surge in motor unit output. This interplay ensures muscle tension is constantly adjusted to meet intention and physical demands.