2. Reflexes and Descending Control of Reflexes

Question 1

definition of reflex

Answer 1

unlearned motor response to sensory stimulus

Question 2

what is reflex mediated by

Answer 2

spinal cord, brainstem, cortex
(circuitry for a reflex may be present in the spinal cord or brainstem but reflex strength or gain may be modulated by descending pathways - stronger, weaker, totally suppressed; descending axons can be excitatory or inhibitory)

Question 3

spinal transection

Answer 3

lose sensation and voluntary movement below the cut (axons carrying sensory info to brain and descending axons carrying commands for voluntary movement are cut)

Question 4

neurons and tracts in spinal cord

Answer 4

• motoneurons (ventral horn)
• sensory neurons (dorsal horn)
• interneurons (btw motor and sensory)
• ascending tract (carry info TO brain)
• descending tract (carry info FROM brain)

Question 4

neurons and tracts in spinal cord

Answer 4

• motoneurons (ventral horn)
• sensory neurons (dorsal horn)
• interneurons (btw motor and sensory)
• ascending tract (carry info TO brain)
• descending tract (carry info FROM brain)

Question 5

stretch reflex stimulus, response, and clinical test

Answer 5

• stimulus: stretch of a muscle
• response: contraction of same muscle
• clinical test: tendon tap which stretches the muscle (deep tendon reflex)
• stretch reflex works to keep muscle length constant

Question 6

afferent (sensory input)

Answer 6

• IA fiber from muscle spindle makes an excitatory synapse on the alpha mn whose axon goes to the same muscle: one synapse, monosynaptic reflex
• very short latency reflex (fast reflex, little synaptic delay, short delay from stimulus to response)

Question 7

muscle tone definition

Answer 7

resistance to passive stretch of a muscle (stretch reflex mediates muscle tone)
- abnormalities in muscle tone found in various disorders of motor system

Question 8

stretch reflex and coordination

Answer 8

• as one muscle, the agonist, contracts, its antagonist relaxes
• circuitry includes an inhibitory interneuron to mn to antagonist muscle (this is called reciprocal innervation)
• agonist mn are excitatory
• antagonist mn are inhibitory
• circuitry for coordination present at the level of the spinal cord

Question 9

flexion-crossed extension reflex

Answer 9

• protective reflex (ex: pulling hand away from hot stove)
• stimulus: noxious (damaging, painful) stimulus to skin (may be a very small area)
• sensory fibers carrying pain info synapse on neurons in the spinal cord dorsal horn, pain info is then widely distributed in spinal cord

Question 10

response-flexion

Answer 10

contraction of flexors and relaxation of extensors at every joint in the limb

Question 11

response-crossed extension

Answer 11

posture is disturbed (esp in 4 legged animal); the contralateral leg extends, increasing postural support

Question 12

spinal reflex

Answer 12

• conscious awareness of pain not necessary
• occurs below the cut after spinal transection