Number 4 (四; accounting 肆; pinyin sì) is considered an unlucky number in Chinese because it is nearly homophonous to the word “death” (死 pinyin sǐ). Due to that, many numbered product lines skip the “4”: e.g., Nokia cell phones (there is no series beginning with a 4), Palm PDAs, Canon PowerShot G’s series
Sensory neuron. Sensory neurons are nerve cells within the nervous system responsible for converting external stimuli from the organism’s environment into internal electrical impulses. For example, some sensory neurons respond to tactile stimuli and can activate motor neurons in order to achieve muscle contraction.