In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action[1] and nearly instantaneous response to a stimulus.[2][3]
The simplest reflex is initiated by a stimulus, which activates an afferent nerve. The signal is then passed to a response neuron, which generates a response.
Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs via neural pathways in the nervous system called reflex arcs. A stimulus initiates a neural signal, which is carried to a synapse. The signal is then transferred across the synapse to a motor neuron, which evokes a target response. These neural signals do not always travel to the brain,[4] so many reflexes are an automatic response to a stimulus that does not receive or need conscious thought.[5]
Many reflexes are fine-tuned to increase organism survival and self-defense.[6] This is observed in reflexes such as the startle reflex, which provides an automatic response to an unexpected stimulus, and the feline righting reflex, which reorients a cat's body when falling to ensure safe landing. The simplest type of reflex, a short-latency reflex, has a single synapse, or junction, in the signaling pathway.[7] Long-latency reflexes produce nerve signals that are transduced across multiple synapses before generating the reflex response.
Types of human reflexes
Myotatic reflexes
The myotatic or muscle stretch reflexes (sometimes known as deep tendon reflexes) provide information on the integrity of the central nervous system and peripheral nervous system. This information can be detected using electromyography (EMG).[8] Generally, decreased reflexes indicate a peripheral problem, and lively or exaggerated reflexes a central one.[8] A stretch reflex is the contraction of a muscle in response to its lengthwise stretch.
While the reflexes above are stimulated mechanically, the term H-reflex refers to the analogous reflex stimulated electrically, and tonic vibration reflex for those stimulated to vibration.
Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive reflexes. These automatic reactions to stimuli enable infants to respond to the environment before any learning has taken place. They include:
Many of these reflexes are quite complex, requiring a number of synapses in a number of different nuclei in the central nervous system (e.g., the escape reflex). Others of these involve just a couple of synapses to function (e.g., the withdrawal reflex).
Processes such as breathing, digestion, and the maintenance of the heartbeat can also be regarded as reflex actions, according to some definitions of the term.
In medicine, reflexes are often used to assess the health of the nervous system. Doctors will typically grade the activity of a reflex on a scale from 0 to 4. While 2+ is considered normal, some healthy individuals are hypo-reflexive and register all reflexes at 1+, while others are hyper-reflexive and register all reflexes at 3+.
Grade
Description
0
Absent ("mute")
1+ or +
Hypoactive
2+ or ++
"Normal"
3+ or +++
Hyperactive withoutclonus, with spread to adjacent muscle groups
Depending on where you are, another way of grading is from –4 (absent) to +4 (clonus), where 0 is "normal".
Reflex modulation
An example of reflex reversal is depicted. Activating the same spinal reflex pathway can cause limb flexion while standing, and extension while walking.
Some might imagine that reflexes are immutable. In reality, however, most reflexes are flexible and can be substantially modified to match the requirements of the behavior in both vertebrates and invertebrates.[9][10][11]
A good example of reflex modulation is the stretch reflex.[12][13][14][15] When a muscle is stretched at rest, the stretch reflex leads to contraction of the muscle, thereby opposing stretch (resistance reflex). This helps to stabilize posture. During voluntary movements, however, the intensity (gain) of the reflex is reduced or its sign is even reversed. This prevents resistance reflexes from impeding movements.
The underlying sites and mechanisms of reflex modulation are not fully understood. There is evidence that the output of sensory neurons is directly modulated during behavior—for example, through presynaptic inhibition.[16][17] The effect of sensory input upon motor neurons is also influenced by interneurons in the spinal cord or ventral nerve cord[15] and by descending signals from the brain.[18][19][20]
^Pierrot-Deseilligny E (2005). The Circuitry of the Human Spinal Cord: Its Role in Motor Control and Movement Disorders. Cambridge University Press. ISBN978-0-511-54504-7.
