Final answer:
The C5-C6 nerve roots are responsible for the biceps reflex, which is tested by striking the biceps tendon and observing contraction due to activation of a muscle spindle and subsequent neural pathways.
Step-by-step explanation:
The nerve root responsible for the biceps reflex is typically the C5-C6 nerve roots. During a reflex test, the biceps tendon is struck with a reflex hammer, which quickly stretches the muscle and activates a muscle spindle. This sensory input sends a signal through the dorsal root into the spinal cord, where it synapses directly on the ventral horn motor neuron. This neuron, in turn, activates the biceps brachii muscle, causing contraction. The biceps reflex tests the integrity of the lower motor neuron and the nerve pathways involved.