question: why do our knees shake when we get particularly scared?
comment: It's from the shot of adrenaline intended to help you fight or run away. In events that don't require either of those actions, you're stuck just standing there effectively in "Hulk mode" and it makes you fired up and twitchy until the effect goes away.
passages: Section::::Regine.
   Some of the hormones involved during the state of fight-or-flight include epinephrine, which regulates heart rate and metabolism as well as dilating blood vessels and air passages, norepinephrine increasing heart rate, blood flow to skeletal muscles and the release of glucose from energy stores, and cortisol which increases blood sugar, increases circulating neutrophilic leukocytes, calcium amongst other things.
   The Moro reflex is present at birth, peaks in the first month of life, and begins to integrate around 2 months of age. It is likely to occur if the infant's head suddenly shifts position, the temperature changes abruptly, or they are startled by a sudden noise. The legs and head extend while the arms jerk up and out with the palms up and thumbs flexed. Shortly afterward the arms are brought together and the hands clench into fists, and the infant cries loudly. 
   Early studies of methods to combat vasovagal fainting found that certain leg exercises and that individuals making themselves angry over imagined scenarios could increase blood pressure, thus elevating cerebral blood flow and preventing fainting upon exposure to a phobic trigger. A later study tested applied muscle tension as a way of preventing fainting when an individual with a fear of injuries was exposed to triggering visual stimuli. Lars-Göran Öst expanded upon this research, having BII-phobic individuals engage in applied muscle tension while shown blood stimuli. Those who were trained in the technique showed notable symptom improvement over the course of five one-hour treatment sessions. 
   The standard approach to treatment is the same as with other phobias - cognitive-behavioral therapy, desensitization, and possibly medications to help with the anxiety and discomfort.  In recent years, the technique known as "applied tension", applying tension to the muscles in an effort to increase blood pressure, has increasingly gained favor as an often effective treatment for blood phobia associated with drops in blood pressure and fainting.
   Concertina movement
  
  Concertina movement is the movement occurring in snakes and other legless organisms that consists of gripping or anchoring with portions of the body while pulling or pushing other sections in the direction of movement.
  
  Section::::Mechanism.
   The iliopsoas is the prime mover of hip flexion, and is the strongest of the hip flexors (others are rectus femoris, sartorius, and tensor fasciae latae). The iliopsoas is important for standing, walking, and running. The iliacus and psoas major perform different actions when postural changes occur.
   When disturbed, "D connectens" will either remain motionless or attempt to run away and if they need to defend themselves, they will raise their legs in a threatening posture and produce soft sounds. These sounds have been described as “soft and sibilant, rather like that produced by brushing together the heels of one's palms”. "D. connectens" has been described as an aggressive species, and will bite if provoked (although they do not appear to be strong enough to break skin).
   Section::::Biography.
  
  Section::::Biography.:"Odd How People Shake" (2002-2003).
   The B-side on the 7" was "That's What You Get (For Being Polite)".
  
  Section::::Live performances.
   The lateral grey column's connections mediate the functions of the sympathetic nervous system (SNS), which changes cardiac, pulmonary, hepatic (liver), and gastrointestinal activities to prepare the body for emergency situations (although the sympathetic system is always active to some extent, including in the absence of a stressful environment, to maintain the appropriate level of sympathetic function). Thus, when the brain responds to potential threats by sending signals to lateral grey column cells, the lateral column passes on the signals to initiate a variety of physiological changes that prepare the body for a “fight-or-flight” response. Hairs stand up to conserve heat. The gut relaxes and digestion slows so that more energy can be directed toward dealing with the threatening situation. The adrenal medulla is activated and releases epinephrine (adrenaline) into the bloodstream, where it mediates many changes, such as the preparation of muscles for emergency activity. A variety of smooth muscles relax; for example, the muscles wrapped around the bronchioles of the lungs relax, allowing more oxygen to enter the bloodstream. Heart rate increases to ensure all cells are supplied quickly with the substances they need. The liver produces glucose (sugar) to fuel the muscles. Blood vessels contract (vasoconstriction), which reduces bleeding and conserves body heat (the exception is blood vessels fueling large muscles that would be used in running or fighting). The pupils dilate, improving vision. Perspiration increases in certain areas of the body (the purpose of this is not yet fully understood, but there is some evidence that the odor produced by this sweat serves as a signal to other individuals).
   When the patellar tendon is tapped just below the knee, the tap initiates an action potential in a specialized structure known as a muscle spindle located within the quadriceps. This action potential travels to the L3 and L4 nerve roots of the spinal cord, via a sensory axon which chemically communicates by releasing glutamate onto a motor nerve. The result of this motor nerve activity is contraction of the quadriceps muscle, leading to extension of the lower leg at the knee (i.e. the lower leg kicks forward). Ultimately, an improper patellar reflex may indicate CNS injury.
   BULLET::::- Busher Jackson, Canadian NHL player and Hall of Fame enshrinee, in Toronto (d. 1966)
  
  BULLET::::- Ken Nelson, American record producer and Country Music Hall of Fame enshrinee, in Caledonia, Minnesota (d. 2008)
  
  Section::::January 20, 1911 (Friday).
  
  BULLET::::- Dr. Walter Bradford Cannon first had the insight of a connection between stress, increased secretions of adrenaline and higher levels of glucose in the blood, writing in his scientific diary, "Got idea that adrenals in excitement serve to affect muscular power and mobilize sugar for muscular use—thus in a wild state readiness for fight or run. flight or fight!"
   A German study from 2006 showed when anxiety-induced versus exercise-induced human sweat from a dozen people was pooled and offered to seven study participants, of five able to olfactorily distinguish exercise-induced sweat from room air, three could also distinguish exercise-induced sweat from anxiety induced sweat. The acoustic startle reflex response to a sound when sensing anxiety sweat was larger than when sensing exercise-induced sweat, as measured by electromyograph analysis of the orbital muscle, which is responsible for the eyeblink component. This showed for the first time that fear chemosignals can modulate the startle reflex in humans without emotional mediation; fear chemosignals primed the recipient's "defensive behavior" prior to the subjects' conscious attention on the acoustic startle reflex level.
   Section::::Treatment.:BICEPS.:Expectancy.
   Section::::Neuromuscular system.
  
  Section::::Neuromuscular system.:Intersegmental coordination.
   Every emotional response has a behavioral component, an autonomic component, and a hormonal component. The hormonal component includes the release of adrenaline, an adrenomedullary response that occurs in response to stress and that is controlled by the sympathetic nervous system. The major emotion studied in relation to adrenaline is fear. In an experiment, subjects who were injected with adrenaline expressed more negative and fewer positive facial expressions to fear films compared to a control group. These subjects also reported a more intense fear from the films and greater mean intensity of negative memories than control subjects. The findings from this study demonstrate that there are learned associations between negative feelings and levels of adrenaline. Overall, the greater amount of adrenaline is positively correlated with an arousal state of negative feelings. These findings can be an effect in part that adrenaline elicits physiological sympathetic responses including an increased heart rate and knee shaking, which can be attributed to the feeling of fear regardless of the actual level of fear elicited from the video. Although studies have found a definite relation between adrenaline and fear, other emotions have not had such results. In the same study, subjects did not express a greater amusement to an amusement film nor greater anger to an anger film. Similar findings were also supported in a study that involved rodent subjects that either were able or unable to produce adrenaline. Findings support the idea that adrenaline does have a role in facilitating the encoding of emotionally arousing events, contributing to higher levels of arousal due to fear.
   Section::::In humans.:Goose bumps.
  
  The pilomotor reflex, more commonly known as goose bumps, was originally a reflex that assured the raising of fur for additional insulation against cold. When scared, this response also made the frightened animal seem bigger and a more formidable enemy.
  
  Section::::In humans.:Hypnic jerk.
  
  The sudden startled arm-jerking response sometimes experienced when on the verge of sleeping is known as the hypnic jerk.
   Court records revealed that the pet shop owner was found pacing outside of the store with blood on his hands and wearing shorts by police who first arrived on the scene. Contrary to prior reports, the store owner told arriving police that the snake was still unaccounted for, leaving one to speculate whether the blood belonged to the boys or was his own (due to being bitten by the "enraged" snake or cut by debris from the collapsed ceiling).
   Iliopsoas
  
  The iliopsoas () refers to the joined psoas and the iliacus muscles. The two muscles are separate in the abdomen, but usually merge in the thigh. As such, they are usually given the common name "iliopsoas". The iliopsoas muscle joins to the femur at the lesser trochanter, and acts as the strongest flexor of the hip.
  
  The iliopsoas muscle is supplied by the lumbar spinal nerves L1-3 (psoas) and parts of the femoral nerve (iliacus).
  
  Section::::Structure.
   Certain physical maneuvers also have the capacity to temporarily boost blood pressure, alleviating symptoms of pre-syncope like lightheadedness by boosting blood flow to the brain. These include the phobic individual crossing their legs, making tight fists with both hands, or engaging muscles of the trunk or arms. Coughing, which can similarly increase cranial blood flow, can also be useful as a coping mechanism to avoid pre-syncope and syncope. 
  
  Symptoms of a phobic response are generally able to be fully alleviated within a few minutes simply by removing the phobic trigger.
  
  Section::::Related disorders.
  
  Section::::Related disorders.:Dental phobia.
   Physiological effects, similar to those in other anxiety disorders, are present in social phobics. In adults, it may be tears as well as excessive sweating, nausea, difficulty breathing, shaking, and palpitations as a result of the fight-or-flight response. The walk disturbance (where a person is so worried about how they walk that they may lose balance) may appear, especially when passing a group of people. Blushing is commonly exhibited by individuals suffering from social phobia. These visible symptoms further reinforce the anxiety in the presence of others. A 2006 study found that the area of the brain called the amygdala, part of the limbic system, is hyperactive when patients are shown threatening faces or confronted with frightening situations.
   It was not until 2011 that a link between severe pain, neuroinflammation and alarm pheromones release in rats was found: real time RT-PCR analysis of rat brain tissues indicated that shocking the footpad of a rat increased its production of proinflammatory cytokines in deep brain structures, namely of IL-1β, heteronuclear Corticotropin-releasing hormone and c-fos mRNA expressions in both the paraventricular nucleus and the bed nucleus of the stria terminalis, and it increased stress hormone levels in plasma (corticosterone).
   In trying to resist this position, the body will begin to shake in places such as the legs and hands. Several other things happen besides this. Muscles in the body contract, causing them to be tense and ready to attack. Second, "blood vessels in the extremities constrict". This can leave a person with the feeling of cold fingers, toes, nose, and ears. Constricted blood vessels also gives the body extra blood flow to the vital organs.
   Section::::Reactions.:Startle and orienting response.
  
  Startle response interrupts and disengages the organism from ongoing activity, directs attention to stimuli, and protects the organism from potential harmful stimuli. Orienting response is an organism’s innate reaction to a novel stimulus, and it is a defensive response. Heart rate increases after the onset of startle stimuli. The heart rate decreases during the orienting response.
  
  Section::::Measurement techniques for body reactivity.
  
  Section::::Measurement techniques for body reactivity.:Electroencephalography.
  