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jerk(jurk) a sudden reflex or involuntary movement.jer´ky
jerkthe rate of change of acceleration with respect to time.
reflexinvoluntary and largely unconscious reaction in response to a peripheral stimulus; detected by an affector organ (e.g. tendon stretch receptors; heat-sensitive nerve endings within the skin), transmitted along afferent nerve fibres to central nervous system centres, and thence to an effector organ via efferent nerve fibres, e.g. skeletal muscle, causing distal limb movement, or sweat gland tissue, causing sweat flow and resultant cooling; see Table 1
Achilles reflex; ankle reflex deep tendon reflex triggered by percussion of Achilles tendon 5cm proximal to its insertion into posterior calcaneum; induces contraction of posterior muscle group (gastrocnemius and soleus) and brief plantarflexion of foot at ankle joint; demonstrates normal S1 function
deep tendon reflex; DTR involuntary contraction of skeletal muscle induced by sudden and brief lengthening or percussion of subserving tendon; magnitude increased in upper motor neurone lesions, and absent/greatly reduced in any type of lower motor neurone lesion, peripheral sensory neuropathy and the elderly; enhanced by isometric contraction (e.g. hand-clenching) at time of test
patellar reflex; knee-jerk reflex; quadriceps reflex sudden contraction of anterior-thigh muscle group (quadriceps) and resultant lower-limb ‘kick’ action due to brief knee joint extension; induced by transitory lengthening/percussion of patellar tendon at midpoint, 2 cm inferior to inferior border of patella when leg is relaxed and hanging pendulously from 90° flexed knee; demonstrates normal L4 function
pilomotor reflex contraction of smooth muscle within skin; caused by cold-induced contraction of arrector piles muscles; induces appearance of ‘gooseflesh’
plantar reflex plantar flexion of toes at metatarsophalangeal joints and foot withdrawal, away from stimulus; induced by a firmly moving stimulus across the plantar surface; normal response to plantar stimulation (contrast with response, extensor plantar)
spinal reflex response of longitudinal muscles surrounding the spine to sudden and imposed stretch; unexpected flexion (forward) or extension (backward) movements induce reflex contraction of the opposing spinal muscle group, maintaining the erect posture
|Tendon stretch excited by percussion||Spinal nerve roots stimulated||Reflex response elicited|
|Biceps brachialis||C5–C6||Flexion of forearm at elbow|
|Triceps brachialis||C7–C8||Extension of forearm at elbow|
|Patellar||L3–L4||Knee joint extension|
|Achilles||S1–S2||Ankle joint plantarflexion|