Reflexes

Question 1

What reflexes are cortical?

Answer 1

placing reaction

hopping reaction

Question 2

What reflexes are spinal?

Answer 2

stretch (myotatic)

golgi tendon reflex

crossed extensor

Question 3

What reflexes are brainstem/midbrain?

Answer 3

vestibular

righting reflex

suckle

yawn

eye/head movements

Question 4

What can make reflexes look like volitional movement?

Answer 4

the preciseness of reflexes

but reflexes are faster

Question 5

What is the difference in level of organization for reflexes and volitional mvt?

Answer 5

reflexes at any CNS level

volitional requires cortical and subcortical involvement

Question 6

What is the differency in circuitry btw reflexes and volitional mvt?

Answer 6

reflexes = fixed

volitional = variable depending on motions

Question 7

What is a muscle spindle

Answer 7

within skeletal muscle, embedded in fusiform capsule

parallel to muscle fibers

has both afferent and efferent components

Question 8

What are the parts of intrafusal fibers?

Answer 8

sensory: nuclear bag fiber, nuclear chain fiber; not contractile - sensitive to length

motor portion

Question 9

What does the 1a fiber do?

Answer 9

innervates both the nuclear bag and nuclear chain

large, myelinated

sensitive to both length and how fast length is changing

Question 10

How does the firing of the 1a fiber change with stretch?

Answer 10

increases in frequency

gets even faster when speed of stretch increases

Question 11

What is the secondary afferent nerve?

Answer 11

group 2 fiber = smaller, myelinated

innervates only the nuclear chain fiber

sensitive only to length

Question 12

What innervates the motor portion of the intrafusal contractile element?

Answer 12

gamma motor neuron

controls the length of the sensory portion

Question 13

How does is the sensitivity of the intrafusal fiber complex increased?

Answer 13

contractile intrafusal fibers contract –> stretches the sensory portion –> increases sensitivity of 1a and II fibers

Question 14

In myotactic reflex, what does the 1a afferent fiber synapse on and what NT is used?

Answer 14

alpha motor neuron

EAA

Question 15

How is antagonist muscle relaxed during a reflex?

Answer 15

1a afferent synapse on an interneuron –> releases inhibitory GABA on alpha motoneuron –> relaxes antagonist muscle

Question 16

What does a golgi tendon reflex look like?

Answer 16

sudden relaxation of a contracted muscle –> protects from damage due to excessive force

Question 17

How does a golgi tendon work?

Answer 17

bare nerve ending w/ lots of branches –> AP increases w/ tension –> 1b fiber to SC interneuron –> releases GABA on alpha motoneuron –> relaxes contracting muscle

*high threshold = need a lot of gaba to overcome muscle contracting

Question 18

What turns the golgi tendon discharge rate back to normal?

Answer 18

abrupt relaxation of a muscle

Question 19

How do higher centers of the brain affect reflexes?

Answer 19

often seen to be inhibitory

certain regions can be excitatory

Question 20

What occurs in spinal shock?

Answer 20

transection of the spinal cord –> even though neurons producing reflexes below level of transection are intact, reflex fails to occur

recovery later bc of axonal sprouting and expression of receptors that are self-activating

Question 21

What is decerebrate posturing?

Answer 21

results from loss of all structures rostral to pons, but caudal to the red nucleus

see regidity bc of continual activation of alpha-motoneurons (not clear why)

spacicity bc of activation of gamma-motoneurons

extended arms and legs

Question 22

Why does cortical damage cause spasticity?

Answer 22

brainstem facilitory region activates gamma motoneurons –> makes m spindle more sensitive –> spontaneously active

need cortical activation –> brainstem inhibitory region –> stops reflexes

w/ loss of corex –> inhibitory region not activated –> + region dominates –> spasticity

Question 23

Why does flexion of upper limb occur in decorticate posturing?

Answer 23

disinhibition of red nucleus and its control of UE flexors

Question 24

Why does extension of the lower limbs occur in decorticate posturing?

Answer 24

disinhibition of reticulospinal and vestibulospinal pathways

Question 25

What can cause unilateral decorticate posturing?

Answer 25

stroke in vecinity of internal capsule affecting one side