Though there are over 10,000 specific types of neurons, the three general classifications are sensory neurons, motor neurons and interneurons. Sensory neurons send signals from outside the body into the central nervous system, motor neurons transmit signals to activate muscles and glands, and interneurons act as connectors between neurons.
The sensory neurons inform the central nervous system of outside stimuli through the senses of touch, sight, hearing, taste and smell. By causing muscles to contract and relax, motor neurons enable such involuntary muscle functions as the beating of the heart and the passage of food through the intestines, as well as voluntary skeletal muscle function. Interneurons form the connections that enable communication between the sensory neurons and the motor neurons and also perform many other complex signaling functions within the nervous system.
The brain is not always involved in signal transmission between neurons. In a monosynaptic reflex pathway, such as a knee-jerk reflex, sensory neurons pass messages directly to motor neurons, which stimulate the leg muscles to react automatically. As tasks increase in complexity, the pathways become more convoluted, and the brain comes into play. To process all this information, scientists estimate that the human brain contains roughly 200 billion neurons.