Nervous System Stimulation Patterns

 Nervous System Stimulation Patterns

Earlier in this online module we explored how neurons stimulate muscle cells to contract through a series of activities at the neuromuscular junction.  But how frequently do muscle cells receive those signals and how do cells change their contraction activities if they get multiple signals?
Stimulation Pattern 1: Summation

Most muscle cells do not fully contract themselves based on receiving just one, isolated stimulus from a neuron.  Rather, that single stimulation only causes a small, partial contraction.  To more completely shorten a muscle cell and maintain that contraction requires multiple consecutive stimuli over the neuromuscular junction.  The more consecutive stimuli, the shorter the muscle cell will become and the longer the contraction will last.

 Stimulation Pattern 2: Motor Units

Consider a single muscle, like the biceps brachii.  When the nervous system wishes for the biceps brachii to contract, it generally would not instruct every muscle cell in the biceps brachii to simultaneously contract.  If it did that, the contraction would be quite strong, but would not last very long.  The muscle cells would all tire relatively quickly at the same time and then the entire muscle would need to relax before it could contract again.

Instead, your nervous system would only instruct certain "motor units" to contract.  As the diagram below shows, a single motor unit consists of a selection of muscle cells within a larger muscle that always contract together.  Any given muscle would have lots of different motor units.  Your nervous system could have the motor units take turns, telling some to contract while others relax, so you could have a smooth and sustained contraction of the muscle over a long period of time.  The nervous system can also stimulate the proper number of motor units for the amount of force needed.  If you lift something extremely heavy, your nervous system might indeed stimulate nearly all motor units at once.  However, that sort of contraction would not be sustainable for very long.

 A Time to Think & Put the Pieces Together

Before moving on, take a moment to think about the above information in relation to our hypothetical friend's question.  Our friend started a new workout program where "she cycles quickly through various dumbbell exercises for a few minutes before resting and repeating later."

    How will the frequency of nervous system stimulation change as your friend initially lifts a weight, holds it up, and then relaxes her arms back down?
    What patterns of motor unit activity might you observe in your friend's arm muscles during the workout?  How would that vary depending on whether your friend is lifting light weights vs. extremely heavy weights?