Neurology
Reflex Arc
Spinal reflexes — fast, automatic responses bypassing conscious control
A reflex arc is a neural pathway producing rapid, involuntary responses. Five components — sensory receptor, afferent (sensory) neuron, integration center (often in spinal cord), efferent (motor) neuron, effector (muscle or gland). Monosynaptic reflexes (one synapse) like the patellar (knee-jerk) tendon stretch reflex are fastest. Polysynaptic reflexes include the flexor withdrawal (pulling away from heat) with crossed extensor (contralateral leg supports body), and the inverse stretch (Golgi tendon organ). Clinical reflex testing localizes lesions: hyperreflexia and Babinski sign indicate upper motor neuron disease; hyporeflexia indicates lower motor neuron, peripheral nerve, or muscle disease. Deep tendon reflex grading 0 (absent) to 4+ (sustained clonus).
- ComponentsReceptor, afferent, center, efferent, effector
- MonosynapticPatellar (knee-jerk) reflex; ~50 ms
- PolysynapticFlexor withdrawal; crossed extensor
- BabinskiToes upgoing in UMN lesion (normal in infants)
- Spinal level (knee)L2-L4 (femoral nerve)
- Reflex grading0 absent, 1+ diminished, 2+ normal, 3+ brisk, 4+ clonus
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Why the reflex arc matters
- Neurologic exam. Reflex testing is a core localization tool.
- Stroke evaluation. Asymmetric reflexes and Babinski help identify hemispheric lesions.
- Spinal cord injury. Reflex level helps localize the lesion.
- Peripheral neuropathy. Distal hyporeflexia is an early sign.
- Brainstem assessment. Pupillary, corneal, gag reflexes guide coma evaluation.
- Pediatric milestones. Primitive reflexes appear and disappear on schedule.
- Anesthesia. Reflex monitoring guides depth and recovery.
Common misconceptions
- All reflexes go through the brain. Spinal reflexes complete in the cord; brain only modulates.
- Hyperreflexia means damage. Anxiety, hyperthyroidism, caffeine all cause brisk reflexes without pathology.
- Babinski in infants is bad. Normal until ~12 months; reflects immature corticospinal tract.
- Absent reflex always pathologic. Some healthy individuals have hard-to-elicit reflexes; reinforcement (Jendrassik) can help.
- Reflex strength shows muscle strength. Reflexes test arc integrity; strength tested separately.
- Reflexes are voluntary. Genuine reflexes occur faster than voluntary control (~50 ms vs ~250 ms reaction time).
Frequently asked questions
How does the patellar reflex work?
Tapping the patellar tendon stretches the quadriceps muscle. Muscle spindles (intrafusal fibers with sensory wrapping) detect stretch via Ia afferents. Ia fibers enter the spinal cord at L2-L4 and synapse directly on α-motor neurons that contract the quadriceps (extending the knee) and inhibit antagonist hamstrings via interneurons (reciprocal inhibition). Latency ~50 ms — too fast for cortical involvement. Tests integrity of L2-L4 spinal segments, femoral nerve, and the muscle. Diminished in lower motor neuron lesions; brisk in upper motor neuron lesions.
What is the Babinski sign?
Stroking the lateral sole of the foot from heel to toe. Normal adult response: plantar flexion of toes (downgoing). Abnormal (positive Babinski): great toe extends upward and other toes fan out — indicates upper motor neuron lesion above L4-S2 in adults. Babinski sign is normal in infants under ~12 months because corticospinal tract is incompletely myelinated. Joseph Babinski described it in 1896. Upgoing toes plus hyperreflexia, weakness, and spasticity suggest stroke, cord lesion, MS, ALS.
What's the difference between upper and lower motor neuron signs?
Upper motor neurons originate in motor cortex and descend through corticospinal tract; lower motor neurons originate in spinal cord ventral horn (or brainstem nuclei) and innervate muscles. UMN lesion: spasticity, hyperreflexia, Babinski, no significant atrophy, no fasciculations. LMN lesion: flaccid weakness, hyporeflexia, atrophy, fasciculations. Distinguishing localizes lesion — stroke (UMN), spinal cord (UMN below level, LMN at level), peripheral nerve (LMN), muscle (just weakness). ALS uniquely affects both UMN and LMN.
How does the flexor withdrawal reflex work?
Painful stimulus to the foot activates nociceptors. Aδ and C fibers enter the spinal cord, synapse on interneurons, which excite flexor motor neurons (withdrawing the limb) and inhibit extensors via reciprocal inhibition. Polysynaptic — involves multiple spinal segments (longer latency than monosynaptic). Crossed extensor reflex: simultaneously, contralateral leg extends to bear body weight as the affected leg lifts. Coordinated to prevent falling. Demonstrates spinal cord's intrinsic ability to generate complex motor patterns without brain input.
What is muscle spindle vs Golgi tendon organ?
Muscle spindles lie within muscle (intrafusal fibers parallel to extrafusal), detect length and rate of stretch via Ia and II afferents, drive stretch reflex. Golgi tendon organs lie in tendons in series with muscle fibers, detect tension via Ib afferents. Stretch reflex (muscle spindle) increases muscle tone in response to stretch — protects against tearing. Inverse stretch reflex (Golgi tendon organ) inhibits muscle when tension is excessive — protects against bone/tendon damage. γ-motor neurons set spindle sensitivity, allowing accurate detection across muscle lengths.
Why is reflex testing useful clinically?
Reflexes provide objective, reproducible localization. Asymmetry suggests lateralized lesion. Patellar (L2-L4), Achilles (S1-S2), biceps (C5-C6), brachioradialis (C5-C6), triceps (C7-C8). Spinal cord lesion shows hyperreflexia below the level and possibly flaccid (LMN) at the level. Cauda equina syndrome: bilateral leg weakness, saddle anesthesia, urinary retention, reduced reflexes. Diabetic neuropathy: ankle reflexes lost first. Hyperthyroidism: brisk reflexes. Hypothyroidism: hung-up (slow relaxation phase) reflexes.
What other reflexes are important?
Pupillary light reflex (CN II afferent, CN III efferent) — both pupils constrict to light in either eye; tests brainstem. Corneal reflex (CN V afferent, CN VII efferent). Gag reflex (CN IX afferent, CN X efferent). Cough reflex tests laryngeal/airway integrity. Plantar grasp in infants. Anal wink (S2-S4) tests sacral nerves in suspected cauda equina. Cremasteric reflex (L1-L2) tests genitofemoral nerve. Glabellar tap (Myerson's sign) extinguishes normally but persists in Parkinson's.