A reflex, commonly known as a reflex action is the quick, instant response given by the body to an external or an internal stimulus. This action is made possible because of certain neural pathways arising from our spinal cord called the reflex arcs. These are generated from the spinal cord because they are so quick in nature that before the information even reaches the brain to be processed and reacted upon; a decision has already been taken. A reflex is basically an automatic response without any conscious thought.
The knee reflex
The most common reflex action we’re familiar about is the knee reflex. This is the one that we observe when a doctor taps below our knee and our leg moves as a reaction to that tapping. This is a rather simple process to understand. What happens here is, the doctor uses a tool to tap the patellar tendon that’s right below out knee. This tapping (external stimulus) sends a signal towards our brain which gets first intercepted by the spinal cord. The cell bodies of motor neurons in the spinal take decision themselves without relaying it all the way to the brain and they send this decision back to the target (here: the leg) to react. This information received by the muscles in the leg cause a sudden contraction and thereby a jerk which is viewed as a reflex to the tapping.
Various types of reflexes
Based on the number of neurons involved, reflexes can be classified as monosynaptic (involving a single motor neuron and single sensory neuron) and polysynaptic (involving multiple motor and sensory neurons). Furthermore, they are also classified based on their processing site i.e the spinal cord or the brain. Reflexes generated from the brain are usually called cranial reflexes and include Gag reflex, pupillary light reflex, or jaw jerk reflex to name a few. Reflexes can also be somatic or autonomic in nature. Somatic reflexes affect the muscles and are controlled more by the brain than the spinal cord. Whereas, the autonomic reflexes which affect the inner organs are mediated by the spinal cord.
Sneeze: A reflex?
We all are aware what a sneeze is. By definition, sneezing is the forceful expulsion of air via the nose and the mouth as a reaction to an external, irritable stimulus. People generally sneeze as a response to some dirt or a strong smell entering their nasal pathway, or to a sudden clod breeze, or to sudden exposure to bright light, or even sometimes to a full stomach apart from the regular symptoms for viral infections or common colds.
The basic function of a sneeze is expelling mucus which contains dirt or other irritants in order to clear the nasal cavity. The neurons controlling the sneeze reflex are located in the brainstem in the region that controls the pharynx, larynx and the respiratory muscles. These muscles work together to generate the sneezing activity.
Genetically speaking, sneezing reflexes are observed in 18-35% individuals and are acquired as an autosomal dominant trait, meaning it is not associated to the sex chromosomes and is expressed as the prevailing trait to its non-sneezing counterpart.