A reflex arc is a nerve pathway in the body of humans and some animals that connects certain muscle groups to others, without involving the brain. These sorts of pathways primarily control involuntary movements in response to some sort of stimulus. Rapidly blinking the eyes in response to dust or dirt in the air is one example; coughing when food is lodged in the windpipe and kicking the leg out when whacked in the center of the knee are others. Reflex arcs are wholly independent of the pathways that most nervous impulses travel on. The messages carried over them are no less important, though, and problems with reflexes often signal bigger issues with nerve control and muscular support.
The key difference between a reflex action and any other nervous system action is the involvement of the brain. In the case of most nerve signals, stimulated cells send a message, called a nerve impulse, to the brain. The brain receives the message and then sends back another message in response to this initial stimulation that basically tells the body what action to take. This takes place very quickly, but it is not automatic like the response seen with reflexes.
Reflexes are basically any of a number of automatic muscular movements that happen because of messages that are transmitted from one place to another in response to a particular external stimulation, and this transmission happens along the arc itself. Arcs are typically based in the spine or the brain, though there is no conscious control involved in either location. The biggest difference is location and proximity. Reflex arcs are usually shorter than longer nerve pathways, and as a result those in the spine typically control responses in the larger muscles of the arms and legs, while those in the brain usually relate to reactions in the face.
Nervous Pathways Involved
Nerve impulses in reflex situation travel along sensory neurons from the site of stimulation to the spinal cord or brain and then back to the area of the response along motor neurons. In some arcs, the sensory neurons are connected to the motor neurons by connector neurons, but either way, there is no control by the brain.
Knee-Jerk Reaction Example
Most people are familiar with the “knee-jerk” reflex, which causes the leg to kick out involuntarily when the knee is struck with a blunt object. Using this reflex as an example can provide a good illustration of how the arcs work. The pathway for this reflex arc starts at a stretch receptor within the tendon. Hitting this receptor stimulates it, which causes it to send a nerve impulse along a sensory neuron to the spinal cord.
Within the spinal cord, the nerve impulse passes from the sensory neuron to a motor neuron and travels back to the thigh muscle. When the impulse arrives at the thigh muscle, it causes it to contract and jerk the lower part of the leg upward. The person is usually aware that this is happening, so sensory impulses do travel from the spinal cord to the brain, but there is nothing a person can do to stop the movement from happening.
Reflexes are an important part of muscular protection and communication. People who have abnormal or unpredictable reflexes may also have other, larger problems with their nervous systems more generally, and should usually get evaluated by a medical provider. This is often particularly true of newborns and young infants. Babies often have a number of instinctive reflexes, often known as “primitive” reflexes, that help them adapt to the world around them. These include sucking and rooting, which are really important when it comes to feeding. Doctors often look for evidence of strong reflexes within the first few days of life, and scan for neurological disorders if these aren’t present. When caught early enough, disorders can often be solved or effectively treated with medication and lifestyle changes.