Muscle Receptors and Spinal Reflexes

Question 1

What do muscle spindles respond to?

Answer 1

muscle stretch

Question 2

What are the components of the muscle spindles?

Answer 2

a connective tissue capsule

intrafusal fibers (with a noncontractile center and contractile ends)

Question 3

The core of the intrafusal fiber is noncontractile. WHat two types of afferent receptor does it have?

Answer 3

1a = annulospiral endings

2 = flow spray endings

Question 4

The contractile ends of the muscle spindles themselves are innervated by what motoneurons?

Answer 4

gamma motoneurons

Question 5

What receptor responds to stretch within the extrafusal fibers between the muscle and tendon junction?

Answer 5

golgi tendon organs (with type 1b afferent fibers)

Question 6

The reflex are the muscle spindle respondible for?

Answer 6

the stretch reflex

Question 7

Describe the stretch reflex?

Answer 7

Muscle spindles will respond to stretch of the muscle

they will send info through the 1a fibers to the gamma motor neurons and alpha motoneurons.

This causes contraction in both the intrafusal fibers and the extrafusal fibers

so moderate stretch of the muscle elicits contraction of the muscle

Question 8

What reflex are the golgi tendon organs responsible for?

Answer 8

inverse stretch

Question 9

What happens in the inverse stretch reflex?

Answer 9

muscle contraction deforms the type 1b fibers

they depolarize

this activates an inhibitory interneurons

this inhibits the alpha motoneurons

muscle relaxes

Question 10

True or false…muscle spindles are inactive when the muscle is relaxed.

Answer 10

False!

the muscle spindles are NEVER completely inactive

they are less active when the muscle is relaxed though.

this is how we maintain tone!

Question 11

Stretch of the muscle results in depolarization or hyperpolarization of the receptor?

Answer 11

depolarization

Question 12

How do Type1a and Type2 receptors respond differently to stretch?

Answer 12

type 1a responds to dynamic movement of the muscle - the velocity

type 2 repond to amplitude of the stretch (more of a static thing)

Question 13

Which of the receptors is more phasic in nature and which is more tonic?

Answer 13

type 1a is phasic

type 2 are more tonic

Question 14

What is the difference between monosynaptic reflexes and polysynaptic reflexes? What does this mean for coordinated reflex movements?

Answer 14

in a monosynaptic, the afferent synapses directly on the efferent, while in polysynaptic reflexes involve indirect pathways.

Reflexes tend to come in combinations of these, so a reflex can have “waves” of different combined motions since it takes longer for the signal to reach the muscle if it goes through a complex polysynaptic pathway

Question 15

How do motoneurons essentially turn off themselves?

Answer 15

through recurrent inhibition

they activate inhibitory interneurons called renshaw cells, which will then turn off the nearby motoneurons

Question 16

What are two ways motor reflexes can be modified?

Answer 16

1. habituated - decline in amplitude after repeated exposure to the reflex-stimuli
2. Sensitized - they can be increased after the stimulus is paired with a noxious stimulus

Question 17

The stretch reflex and inverse stretch reflex are normally combine seamlessly into what ?

Answer 17

the lengthening reaction

Question 18

Describe the lengthening reaction in terms of weight lifting?

Answer 18

In response to a light weight, the muscle stretches and then contracts through the stretch reflex

but if you life a weight that is super heavy, the force required to life the weight will lead to relaxation - and you drop the weight

Question 19

The lengthening reaction is usually inhibited by descending corticospinal tracts, so what clinical finding do you see in upper motor neuron damage?

Answer 19

clasp knife sign

it’s an exaggeration of the lengthening reaction

the physician pushes ont he patient’s leg. There will be a “catch” as the stretch reflex fires, and then with continued pressure the inverse stretch reaction occurs and you get rapid relaxation

the leg looks like a clasp knife.

Question 20

In the withdrawal reflex, what muscles need to contract and what muscles need to relax? How does the system do this?

Answer 20

the flexors need to contract and the extensors need to relax

It’s done through reciprocal inhibition

collaterals off the 1a fibers activate inhibitory interneurons to inhibit the alpha motoneurons of the extensors

Question 21

If the withdrawal reflex is triggered in a limb, what will occur in the opposite limb?

Answer 21

is will extend

this is the crosed extensor response

Question 22

lower motor neuron lesions result in ___ paralysis while upper motor neuron lesions result in _____.

Answer 22

flaccid paralysis

spasticity

Question 23

What are alpha rigidity and gamma rigitiy and how are they different?

Answer 23

alpha rigidity is when you have increased excitatory signals or decreased inhbitory signals on the alpha MNs, causing contraction of the extrafusal fibers

gamma rigidity is the same thing only on the gamma MNs, resulting in contraction of the intrafusal fibers

they are different because alpha rigitidy is independent of the stretch reflex pathway , while gamma ridigity is dependent on it

Question 24

When will gamma rigidity be particularly apparent?

Answer 24

When the physician moves the patient’s limb in a velocity-specific way

move slowly: little resistance to movement

Move quickly: resistance due to an inappropriately strong stretch reflex

(followed by the clasp knife reflex is the resistance is strong enough)

Question 25

After spinal cord transection, spinal reflexes are absent due to what? In what order do they return?

Answer 25

spinal shock

they return simpliest first