startle reaction
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In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative affect.Rammirez-Moreno, David. "A computational model for the modulation of the prepulse inhibition of the acoustic startle reflex". ''Biological Cybernetics'', 2012, p. 169 Usually the onset of the startle response is a startle reflex reaction. The startle reflex is a brainstem reflectory reaction (reflex) that serves to protect vulnerable parts, such as the back of the neck (whole-body startle) and the eyes (eyeblink) and facilitates escape from sudden stimuli. It is found across many different species, throughout all stages of life. A variety of responses may occur depending on the affected individual's
emotional state Emotions are mental states brought on by neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is currently no scientific consensus on a definition. ...
,
body posture Human positions refer to the different physical configurations that the human body can take. There are several synonyms that refer to human positioning, often used interchangeably, but having specific nuances of meaning. *''Position'' is a gen ...
, preparation for execution of a motor task, or other activities. The startle response is implicated in the formation of
specific phobias Specific phobia is an anxiety disorder, characterized by an extreme, unreasonable, and irrational fear associated with a specific object, situation, or concept which poses little or no actual danger. Specific phobia can lead to avoidance of the o ...
.


Startle reflex


Neurophysiology

A startle reflex can occur in the body through a combination of actions. A reflex from hearing a sudden loud noise will happen in the primary acoustic startle reflex pathway consisting of three main central synapses, or signals that travel through the brain. First, there is a synapse from the
auditory nerve The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
fibers in the ear to the cochlear root neurons (CRN). These are the first acoustic neurons of the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
. Studies have shown a direct correlation to the amount of decrease of the startle to the number of CRNs that were killed. Second, there is a synapse from the CRN axons to the cells in the nucleus reticularis pontis caudalis (PnC) of the brain. These are neurons that are located in the
pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...
of the brainstem. A study done to disrupt this portion of the pathway by the injection of PnC inhibitory chemicals has shown a dramatic decrease in the amount of startle by about 80 to 90 percent. Third, a synapse occurs from the PnC axons to the
motor neurons A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly ...
in the facial motor nucleus or the spinal cord that will directly or indirectly control the movement of muscles. The activation of the facial motor nucleus causes a jerk of the head while an activation in the spinal cord causes the whole body to startle.Davis, M. (2007). "Neural systems involved in fear and anxiety based on the fear-potentiated startle test." ''Neurobiology of Learning and Memory'' (pp. 381–425). Elsevier Incorporated. During neuromotor examinations of newborns, it is noted that, for a number of techniques, the patterns of the startle reaction and the Moro reflex may significantly overlap, the notable distinction being the absence of arm abduction (spreading) during startle responses.


Reflexes

There are many various reflexes that can occur simultaneously during a startle response. The fastest reflex recorded in humans happens within the
masseter muscle In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...
or jaw muscle. The reflex was measured by electromyography which records the electrical activity during movement of the muscles. This also showed the latency response or the delay between the stimulus and the response recorded was found to be about 14 milliseconds. The blink of the eye which is the reflex of the
orbicularis oculi muscle The orbicularis oculi is a muscle in the face that closes the eyelids. It arises from the nasal part of the frontal bone, from the frontal process of the maxilla in front of the lacrimal groove, and from the anterior surface and borders of a short ...
was found to have a latency of about 20 to 40 milliseconds. Out of larger body parts, the head is quickest in a movement latency in a range from 60 to 120 milliseconds. The neck then moves almost simultaneously with a latency of 75 to 121 milliseconds. Next, the shoulder jerks at 100 to 121 milliseconds along with the arms at 125 to 195 milliseconds. Lastly the legs responds with a latency of 145 to 395 milliseconds. This type of cascading response correlates to how the synapses travel from the brain and down the spinal cord to activate each motor neuron.Davis, M. (1984). "The mammalian startle response". In R. Eaton (Ed.), ''Neural stop of Startle Behavior'' (pp. 287–351). Plenum Publishing Corporation.


Acoustic startle reflex

The acoustic startle reflex is thought to be caused by an auditory stimulus greater than 80 decibels. The reflex is typically measured by electromyography,
brain imaging Neuroimaging is the use of quantitative (computational) techniques to study the structure and function of the central nervous system, developed as an objective way of scientifically studying the healthy human brain in a non-invasive manner. Incre ...
or sometimes positron electron tomography. There are many brain structures and pathways thought to be involved in the reflex. The
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verte ...
,
hippocampus The hippocampus (via Latin from Greek , ' seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, ...
, bed nucleus of the stria terminalis (BNST) and
anterior cingulate cortex In the human brain The human brain is the central organ (anatomy), organ of the human nervous system, and with the spinal cord makes up the central nervous system. The brain consists of the cerebrum, the brainstem and the cerebellum. It c ...
are all thought to play a role in modulating the reflex.Medford, Nick. "Conjoint Activity of Anterior Insular and Anterior Cingulate Cortex:Awareness and Response". ''Brain Structure and Function'', 2010, p. 535Lee, Younglim. "Role of the Hippocampus, the Bed Nucleus of the Stria Terminalis, and the Amygdala in the Excitatory Effect of Corticotropin-Releasing Hormone on the Acoustic Startle Reflex". ''
The Journal of Neuroscience ''The Journal of Neuroscience'' is a weekly peer-reviewed scientific journal published by the Society for Neuroscience. It covers empirical research on all aspects of neuroscience. Its editor-in-chief is Marina Picciotto (Yale University). According ...
'', 1997, p. 6434
The anterior cingulate cortex in the brain is largely thought to be the main area associated with emotional response and awareness, which can contribute to the way an individual reacts to startle-inducing stimuli. Along with the anterior cingulate cortex, the amygdala and the hippocampus are known to have implications in this reflex. The amygdala is known to have a role in the "
fight-or-flight response The fight-or-flight or the fight-flight-or-freeze response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It was first des ...
", and the hippocampus functions to form memories of the stimulus and the emotions associated with it. The role of the BNST in the acoustic startle reflex may be attributed to specific areas within the nucleus responsible for stress and anxiety responses. Activation of the BNST by certain hormones is thought to promote a startle response The auditory pathway for this response was largely elucidated in rats in the 1980s. The basic pathway follows the auditory pathway from the ear up to the nucleus of the
lateral lemniscus The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral inferior colliculus of the midbrain. Three distinct, primarily ...
(LLN) from where it activates a motor centre in the reticular formation. This centre sends descending projections to lower motor neurones of the limbs. In slightly more detail this corresponds to ear (
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory or ...
) → cranial nerve VIII (auditory) →
cochlear nucleus The cochlear nuclear (CN) complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN). The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucle ...
(ventral/inferior) → LLN →
caudal pontine reticular nucleus The caudal pontine reticular nucleus or nucleus reticularis pontis caudalis is a portion of the reticular formation, composed of gigantocellular neurons. In rabbits and cats it is exclusively giant cells, however in humans there are normally size ...
(PnC). The whole process has a less than 10ms latency. There is no involvement of the superior/rostral or inferior/caudal colliculus in the reaction that "twitches" the hindlimbs, but these may be important for adjustment of
pinnae The auricle or auricula is the visible part of the ear that is outside the head. It is also called the pinna (Latin for "wing" or " fin", plural pinnae), a term that is used more in zoology. Structure The diagram shows the shape and location ...
and gaze towards the direction of the sound, or for the associated blink.


Application in occupational settings

In a 2005 study by Martin et al., at the Department of Aviation and Logistics,
University of Southern Queensland The University of Southern Queensland (branded as UniSQ and formerly branded as USQ) is a medium-sized, regional university based in Toowoomba, Queensland, Australia, with three university campuses at Toowoomba, Springfield and Ipswich. It offe ...
, the performance of aircraft pilots following unexpected critical events, in relation to recent aircraft accidents, is examined. The deleterious effects of the startle response was identified as causal or contributory in these events. The authors argue that fear resulting from threat (especially if life-threatening) potentiates startle effects and has significant deleterious effects on cognition. This could contribute to poor performance following an unexpected critical event in aviation. They discuss training strategies for improved performance, which would expose pilots to unexpected critical events more often, and develop greater self-efficacy.


See also

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Escape response Escape response, escape reaction, or escape behavior is a mechanism by which animals avoid potential predation. It consists of a rapid sequence of movements, or lack of movement, that position the animal in such a way that allows it to hide, freeze ...
*
Jumping Frenchmen of Maine The Jumping Frenchmen of Maine were a group of 19th-century lumberjacks who exhibited a rare disorder of unknown origin. The syndrome entails an exaggerated startle reflex which may be described as an uncontrollable "jump"; individuals with this ...
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Jump scare A jump scare (often shortened to jumpscare) is a technique often used in horror films and video games, intended to scare the audience by surprising them with an abrupt change in image or event, usually co-occurring with a loud, jarring sound. The j ...
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Prepulse inhibition Prepulse inhibition (PPI) is a neurological phenomenon in which a weaker prestimulus (prepulse) inhibits the reaction of an organism to a subsequent strong reflex-eliciting stimulus (pulse), often using the startle reflex. The stimuli are usually ...
– attenuation of the startle response after a weaker preceding ''prepulse'' stimulus *
Surprise (emotion) Surprise () is a brief mental and physiological state, a startle response experienced by animals and humans as the result of an unexpected event. Surprise can have any valence; that is, it can be neutral/moderate, pleasant, unpleasant, positive, ...


References

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* * * {{Authority control Ethology Reflexes