Physiology of Reflexes
A reflex or reflex action is an involuntary activity that is usually carried out instantly in a response to a stimulus without the involvement of conscious effort. There are different types of reflexes some are swift while others being slow. A true reflex is a behavior that is mediated by a chain of only two or three neurons, known as reflex arc.
Different stimuli are constantly perceived by our body and our body responds to most of these stimuli without involvement of any conscious effort. A simple reflex is the blinking of your eyes when a sudden loud sound is produced near your ear or something flies swiftly before your eyes. Some reflexes are a constant phenomenon of our visceral organs. For example, the smooth muscles in the wall of distal part of our gut get relaxed when the presence of food is appreciated through the myenteric plexus.
The mechanism of conduction of a simple myotactic reflex involves two neurons only. These types of reflexes are usually protective and are carried out through a neural reflex arc. The stimulus incites a local sensory or afferent neuron and the impulse is carried out to spinal cord through distal horn. The impulse is conducted to the motor or efferent neuron via a single synapse between two neurons. This afferent impulse is carried to the effort organs and the response is produced. More complex reflexes usually involved addition neurons and even brain.
The time required for this autonomous reflex response once the stimuli are perceived is called the reaction time. It is different for different type of reflexes.
Different types of reflexes are classified and explained in the following paragraphs:
Deep Tendon Reflexes: Also known as stretch reflexes, provide valuable information about the functional integrity of the higher centers in the brain as well as the peripheral component of nervous system. Example of deep tendon reflex include
- Biceps reflex
- Brachioradialis reflex
- Extensor digitorum reflex
- Triceps reflex
- Patellar reflex or knee-jerk reflex
- Ankle jerk reflex or Achilles reflex
- Plantar reflex or Babinski reflex
Reflexes involving cranial nerves: Out of 12 cranial nerves only few nerves carry afferent fibers for these reflexes while the motor component is usually mediated through more than one cranial nerve. Examples of Cranial Nerve Reflexes include
- Pupillary light reflex
- Accomodation reflex
- Jaw jerk reflex
- Corneal reflex
- Vestibulo-ocular reflex
- Gag reflex
Primitive Reflexes: As the name indicates, these reflexes are present only in human infants. With the development of peripheral nervous system these reflexes start to regress and eventually disappear. These reflexes play important part in enabling infants to respond to environment.
- Asymmetrical tonic neck reflex
- Grasp reflex
- Hand-to-mouth reflex
- Moro reflex, also known as the startle reflex
- Rooting reflex
- Symmetrical tonic neck reflex
- Tonic labyrinthine reflex
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