a deep tendon reflex mediated by the fifth cervical *spinal nerve (C5). The examiner’s thumb or index finger is placed over the patient’s biceps tendon in the elbow crease and struck sharply with a tendon hammer; the normal response is a reflex contraction of the biceps and flexion of the elbow. The jerk is exaggerated in upper *motor neuron lesions, such as a stroke, and reduced or absent in lower motor neuron lesions, such as a disc herniation, peripheral nerve injury, or peripheral neuropathy (e.g. diabetes, alcoholism).