Sensory nerves are vessels of the peripheral nervous system that carry signals toward the brain in response to stimuli. Such stimuli may be environmental, as in changes in temperature; deliver cues about touch such as texture; involve an injury, as in feeling pain; or relate to the status of an internal organ. They are differentiated from motor nerves, the vessels that carry impulses from the brain to the muscles and tell them to move. Sensory nerves transmit what are known as afferent signals, which the brain receives and responds to accordingly.
Composed of chains of nerve cells called neurons, sensory nerves are a part of the peripheral nervous system. Extending from the spinal cord, a component of the central nervous system, to receptor cells throughout the body, they penetrate nearly all of the body’s tissues. They also tend to be paired with motor nerves, which are referred to as efferent because they transmit nerve impulses away from the central nervous system. For example, upon touching a hot stove, sensory receptors in the fingertips quickly send a message along the nerves to the brain telling the brain that this stimulus is both hot and painful. In response, the brain sends an impulse along the motor nerves that innervate the relevant muscles of the arm and hand, causing them to quickly contract and thereby retracting the hand.
Within the sensory nerves, nerve cells are bundled in fibers and end in receptors that can be classified according to their function. Nociceptors, for instance, are those that notify the brain of injury and produce a pain response, while photoreceptors respond to light, and mechanoreceptors respond to touch and pressure. In addition, olfactory receptors in the nose detect odors, while taste receptors on the tongue detect the flavors in food.
The sensory nerves transmit messages from these receptors via an electrical signal known as a nerve impulse. Nerve impulses are conducted toward the nerve roots, which is the place where the sensory nerves enter the spinal cord, and from the spinal cord to their respective center in the brain. The brain is then responsible for interpreting these messages, such as whether an unstable surface that is prodded with the foot is safe to walk upon. Once the stimuli is received by and interpreted in the brain, it determines the appropriate response. It then sends a message back down the spinal column, out through the motor nerves, and to the relevant muscles, telling them to move accordingly.