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 In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

reaction. The startle reflex is a

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...

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, 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 In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses fr ...

, 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 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 nucleus ...

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 An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...

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

. 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 The Moro reflex is an infantile reflex that develops between 28 and 32 weeks of gestation and disappears at 3–6 months of age. It is a response to a sudden loss of support and involves three distinct components: # spreading out the arms (abd ...

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 Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...

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

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 ver ...

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 syste ...

, bed nucleus of the

stria terminalis The stria terminalis (or terminal stria) is a structure in the brain consisting of a band of fibers running along the lateral margin of the ventricular surface of the thalamus. Serving as a major output pathway of the amygdala, the stria term ...

(BNST) and

anterior cingulate cortex In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involve ...

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 (LLN) from where it activates a motor centre in the

reticular formation The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formatio ...

. 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 o ...

) →

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...

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 Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, though ...

. 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 In psychology, self-efficacy is an individual's belief in their capacity to act in the ways necessary to reach specific goals. The concept was originally proposed by the psychologist Albert Bandura. Self-efficacy affects every area of human end ...

References