Spinal Reflexes

Question 1

describe a monosynaptic reflex vs polysynaptic reflex

Answer 1

• monosynaptic reflex
  
  • afferent and efferent limb are directly connected (e.g. deep tendon reflex)
• polysynaptic reflex
  
  • afferent and efferent limb are interconnected with one or more interneurons (pupillary light reflex, abdominal reflex)

Question 2

spinal segments for diaphragm + function?

Answer 2

C3, 4, 5

Question 3

spinal segments for deltoid + function?

Answer 3

C5, abduction

Question 4

spinal segment for biceps brachii/brachialis and function?

Answer 4

C5, 6; elbow flexion

Question 5

spinal segment for extensor carpi/radialis longus and brevis and function?

Answer 5

C6, 7; wrist extension

Question 6

spinal segments for triceps brachii reflex and function?

Answer 6

C6-8; elbow extension

Question 7

spinal segments for flexor digitorum superficialis and profundus and function?

Answer 7

C8; finger flexion

Question 8

spinal segments for interossei muscle and function?

Answer 8

C8, T1; finger ab- and adduction

Question 9

spinal segments for adductor longus and brevis muscles and function?

Answer 9

L2, 3, 4; thigh adduction

Question 10

spinal segments for quadriceps muscle and function?

Answer 10

L3, 4; knee extension

Question 11

spinal segments for extensor hallucis longus muscle and function?

Answer 11

L5, S1; great toe extension

Question 12

spinal segments for gastrocnemius and soleus muscle and function?

Answer 12

S1, 2; ankle plantar flexion

Question 13

spinal segments of sphincter ani externus and function?

Answer 13

S2, 3, 4; anal contraction

Question 14

describe the stretch reflex

Answer 14

• 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

Question 15

describe the function of the smaller gamma-motor neurons

Answer 15

• 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

Question 16

describe the inverse stretch reflex

Answer 16

• 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

Question 17

describe the flexion and crossed extension reflex

Answer 17

• 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

Question 18

describe an abnormal plantar response

Answer 18

• 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

Question 19

describe spinal shock

Answer 19

• 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

Question 20

describe why hyperreflexia is seen in UMN lesions

Answer 20

if the corticospinal tract (UMN) is lesioned, if can no longer modulate the stretch reflex —> hyperreflexia

Question 21

describe the inverse myotatic reflex

Answer 21