In humans, the quadriceps muscle is a skeletal leg muscle under voluntary control. As such, it contracts in response to signals from the motor cortex in the cerebrum. If such a muscle contracts involuntarily, that indicates:

The correct answer is - operation of reflex arc.

Explanation:

The reflex arc is a unique and special type of neural pathway that starts with a sensory neuron and ends with a motor neuron. In this operation sensory neuron acts as the receptor and motor neuron is effector. This type of neural circuit helps in faster response.

Voluntary muscles like quadriceps muscle are normally in control voluntarily however, such muscle may contract involuntary due to reflex arc. This type of contraction takes place before the signal has even reached the brain.

In this response the sensory neuron takes the signal to the spinal cord and then motor neuron to the effector muscle for faster response.

Thus, the correct answer is - operation of reflex arc.

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the three components of a nucleotide are the nitrogenous base, the phosphate group and the five-carbon sugar.