Reflexes and descending control of reflexes

Question 1

What is a reflex?

Answer 1

-Reflex is an unlearned motor response to sensory stimulus
-reflex is mediated by spinal cord (+brainstem + cortex).

Question 2

What is the clinical importance of reflexes?

Answer 2

An abnormal reflex is a reflection of dysfunction of some component of the motor system.
-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 reflexes

Answer 3

-spinal cord does more than carry messages to and from the brain
-Spinal transection: lose sensation and voluntary movement below the cut. Spinal reflexes remain. There are dozens of spinal reflexes

Question 4

Spinal cord gray and white matter

Answer 4

-motoneurons –> ventral horn
-sensory neurons –> dorsal horn; pain
-interneurons (in between sensory and motor in function)
-ascending tracts
-descending tracts

Question 5

The Stretch reflex

Answer 5

-Stimulus: stretch of a muscle
-response: contraction of the same muscle
-clinically tested by tendon tap which stretches the muscle (deep tendon reflex)
-stretch reflex works to keep muscle length constant
-part of every physical exam

Question 6

Flex reflex circuit; muscle tone

Answer 6

-Afferent (sensory input): IA fibers from muscle spindle makes an excitatory synapse on the alpha motoneuron whose axon goes to the same muscle: one synapse, monosynaptic reflex
-monosynaptic reflex is a very short latency reflex (delay from stimulus to response)

Question 7

Circuitry of flex reflex

Answer 7

-Afferent (sensory input): IA fiber from muscle spindle makes an excitatory synapse on the alpha motoneuoron whose axon goes to the same muscle: one synapse, monosynaptic
-very short latency reflex –> means fast reflex. Very little synaptic delay

Question 8

What is muscle tone and what mediates it

Answer 8

Muscle tone is the resistance to passive stretch (i.e. from an external force) of a muscle
-stretch reflex mediates muscle tone
-abnormalities in muscle tone are found in various disorders of the motor system

Question 9

The circuitry for coordination present at the level of the spinal cord

Answer 9

-As agonist muscle contracts, the antagonist muscle relaxes. The circuitry includes an inhibitory interneuron to motoneurons to antagonist muscle. The pattern is called reciprocal innervation.
+ exciting agonist motoneurons
- inhibiting antagonist motoneurons

Question 10

Flexion-crossed extension reflex

Answer 10

-spinal cord reflex (not simple)
-has a protective component and a postural adjustment component
-stimulus: noxious (damaging, painful) stimulus to skin (A-delta or C-fibers). can be very small surface of skin
-Sensory fibers carrying pain information synapse on neurons in the spinal cord dorsal horn, pain information is then widely distributed in spinal cord
-response-flexion: contraction of flexors and relaxation of extensors at every joint in the limb.

-Response-crossed extension: posture is disturbed (think 4-legged animals); the contralateral leg extends, increasing postural support. SPINAL REFLEX (concious awareness of pain is not necessary)