Spinal Reflexes Flashcards
describe a monosynaptic reflex vs polysynaptic reflex
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monosynaptic reflex
- afferent and efferent limb are directly connected (e.g. deep tendon reflex)
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polysynaptic reflex
- afferent and efferent limb are interconnected with one or more interneurons (pupillary light reflex, abdominal reflex)
spinal segments for diaphragm + function?
C3, 4, 5
spinal segments for deltoid + function?
C5, abduction
spinal segment for biceps brachii/brachialis and function?
C5, 6; elbow flexion
spinal segment for extensor carpi/radialis longus and brevis and function?
C6, 7; wrist extension
spinal segments for triceps brachii reflex and function?
C6-8; elbow extension
spinal segments for flexor digitorum superficialis and profundus and function?
C8; finger flexion
spinal segments for interossei muscle and function?
C8, T1; finger ab- and adduction
spinal segments for adductor longus and brevis muscles and function?
L2, 3, 4; thigh adduction
spinal segments for quadriceps muscle and function?
L3, 4; knee extension
spinal segments for extensor hallucis longus muscle and function?
L5, S1; great toe extension
spinal segments for gastrocnemius and soleus muscle and function?
S1, 2; ankle plantar flexion
spinal segments of sphincter ani externus and function?
S2, 3, 4; anal contraction
describe the stretch reflex
- the basic reflex arc mediates muscular contraction in response to a stretch
- the sensory signal for the stretch reflex arises in the muscle spindles
- the primary afferent directly excites the LMNs innervating the stretched muscle
- the tapping of a tendon with a reflex hammer stretches the muscle spindles, activating the associated Ia afferents
- the signals are transmitted to the spinal alpha-motor neurons and then directly to the muscle
describe the function of the smaller gamma-motor neurons
- smaller gama motor neurons modulate the excitability of muscle spindles
- when the muscle contracts, the tension of the muscle spindle is reduced and the spindle is no longer able to measure changes of the length of the muscle
- the gamma-motor neurons therefore activate the contractile ends of the muscle spindle fibers to maintain the tension of the middle regions, where the sensory axons are located
- this co-activation of alpha and gamma motor neurons enables the spindles to fxn at all muscle lengths, thereby regulating both movement and posture
describe the inverse stretch reflex
- the inverse stretch reflex (inverse myotatic or Golgi tendon reflex) is slower than the myotatic reflex for 2 reasons:
- the Ib fibers that constitute the afferent limb of the inverse myotatic reflex may be of marginally smaller diameter, therefore, lower conduction velocity than Ia
- the inverse myotatic reflex is polysynaptic (not mono) with an added synapse necessarily delaying response
- function: protection of the muscle and tendon
describe the flexion and crossed extension reflex
- noxious stimulation of the skin induces a polysynaptic reflex which produces ipsilateral excitation of flexor muscles (and inhibition of extensors) and contralateral excitation of extensor muscles (and inhibition of flexors)
- afferent: nociceptive fibers (A delta and C)
- CNS units: a number of excitatory and inhibitory interneurons over many spinal segments
- efferent limbs: alpha motor neurons, ipsilateral flexors and contralateral extensors
describe an abnormal plantar response
- normally, newborn babies have an extensor plantar response because their pyramidal (corticospinal) tract is immature (incompletely myelinated)
- in adults, extensor plantar response indicates a lesion in the corticospinal tract
describe spinal shock
- extensive bilateral spinal damage commonly yields spinal shock
- transient areflexia occurs caudal to the site of the lesion
- disrupted reflexes will typically re-emerge after days-to-weeks
describe why hyperreflexia is seen in UMN lesions
if the corticospinal tract (UMN) is lesioned, if can no longer modulate the stretch reflex —> hyperreflexia
describe the inverse myotatic reflex
