Much of the work of the nervous system depends upon established pathways between neurons. These pathways are called neural circuits. The simplest kind of neural circuit is a reflex. A reflex is an automatic and involuntary response to a stimulus. Simple reflexes do not involve the brain. Impulses in a reflex action follow a simple pathway that goes through the spinal cord and links a receptor and an effector.
One of the simplest reflexes is the knee jerk reflex. A tap on the tendon below the kneecap stretches the muscle there, and this action stimulates special receptors to produce an impulse. The impulse travels through a sensory neuron, over an axon to the spinal cord, and through a synapse to a motor neuron. There, a second impulse is generated. The second impulse travels over the axon of a motor neuron back to the muscle that was stretched. The impulse causes the muscle cells to contract and the leg to jerk.
Many reflexes are more complicated. They involve at least one association neuron between the sensory and motor neurons. The association neuron, in turn, may be connected with many complex neural pathways, some of which may lead to the brain. A common but complex reflex is involved in withdrawing from a painful stimulus. For example, if a person steps on a sharp object, the immediate response is to lift the foot. In addition, association neurons stimulate the muscles of the other leg to adjust and maintain balance. Interconnecting neural pathways to the brain also may be stimulated so that the person becomes aware of what has happened.
Reflexes alone cannot account for the vast number and variety of actions performed by human beings. People, as well as some other kinds of animals, can learn new behaviour patterns. Voluntary muscular movements necessary for learning new skills travel along complex nerve pathways that extend from the brain throughout the body. Complicated actions, such as riding a bicycle or walking, eventually can be performed without constant, conscious control after they have been learned.