L41 Muscle Innervation and Motor Unit

Question 1

where are the cell bodies of all muscle afferents?

Answer 1

DRG

Question 2

what are the two major sensory organs?

Answer 2

muscle spindle
golgi tendon organ
(also mechanoreceptors)

Question 3

what does the muscle spindle measure and what type of fiber does it use?

Answer 3

measures length and rate of change of length

Ia fiber

Question 4

what does the golgi tendon organ measure and what type of fiber does it use?

Answer 4

measure tension

Ib fiber

Question 5

what make up the additional mechanoreceptors?

Answer 5

thermo and nociceptors (temperature and pain)

Question 6

what is a muscle spindle

Answer 6

encapsulated structures found in most skeletal muscles used to monitor its length

Question 7

the number of spindles in the muscle is directly related to what?

Answer 7

the muscles function

Question 8

muscles involved in fine movement (hand) have ___ spindles than coarse movement (back)

Answer 8

more

Question 9

what are the components of the muscle spindle?

Answer 9

• small group of 2-12 intrafusal gamma fibers
• central regions are non contractile
• large diameter myelinated sensory axon (Ia axon) with sensory ending in the intrafusal belly
• small diameter myelinated motor axon that innervate the distal contractile regions (gamma motor ending)

Question 10

where do the intrafusal fibers lie?

Answer 10

within the spindle which is between/parallel to the extrafusal fibers and are attached to the muscles tendons

Question 11

when the muscle contracts, it shortens and ____ I a firing rate

Answer 11

decreases

Question 12

when the muscle stretches, Ia firing rate _____

Answer 12

increases

Question 13

what are the 3 types of intrafusal fibers?

Answer 13

dynamic nuclear bag fibers
static nuclear bag fibers
nuclear chain fibers

Question 14

what are dynamic nuclear bag fibers sensitive to?

Answer 14

length change and rate of change

Question 15

what are static nuclear bag fibers sensitive to?

Answer 15

length change

Question 16

what are nuclear chain fibers sensitive to?

Answer 16

length change

Question 17

what are the primary sensory ending and what does it wrap around

Answer 17

group Ia
each intrafusal nuclear bag and chain fiber

static and dynamic changes

Question 18

what does Group II sensory ending wrap around?

Answer 18

all nuclear chain and static nuclear bag fiber

only static changes

Question 19

describe A-Gamma fibers responsible for

Answer 19

small myelinated and low velocity axons carry motor innervation to the intrafusal fibers

Question 20

what are the A-Alpha fibers responsible for?

Answer 20

motor innervation of the extrafusal fibers

Question 21

during a sustained stretch reflex, as the spindle is continuously stretched, there is a _____ AP firing in the muscle spindle afferent

Answer 21

sustained

Question 22

what happens to the muscle spindle during alpha motor stimulation?

Answer 22

the muscle shortens and briefly alleviates the stretch on the spindle, at this time, there is no stimulation of the Ia axon = NO afferent discharge!

Question 23

what happens to the muscle spindle during simultaneous alpha and gamma stimulation?

Answer 23

the intrafusal and extrafusal fibers will contract together

because the intrafusal spindle consensually contracts, it continues to relay afferent information thus able to continually measure muscle length

Question 24

where are golgi tendon organs located?

Answer 24

in series between tendon and extrafusal fibers which are sensitive to tension

Question 25

what are each golgi tendon organ innervated by?

Answer 25

single Ib axon which is large myelinated and has high velocity

Question 26

once the Ib axon enters the tendon organ and intertwine in the collagen chains what happens?

Answer 26

the Ib axon will lose its myelin

Question 27

what happens to the Ib axon firing rate when the contraction of the muscle compresses the free, non-myelinated endings?

Answer 27

increase firing rate

Question 28

what happens to the alpha motor neuron when you increase Ib axon firing rates?

Answer 28

increase inhibition of alpha motor neurons - ensures even distribution of tension and contraction as well as avoiding excessive contraction to the point of damage

Question 29

what is the reverse myotactic reflex?

Answer 29

increasing Ib axon firing increases inhibition of alpha motor neurons - ensures even distribution of tension and contraction as well as avoiding excessive contraction to the point of damage

Question 30

what are the 2 types of motor neurons?

Answer 30

A-alpha | A-gamma

Question 31

describe A-alpha motor neurons?

Answer 31

large, myelinated, high velocity passing via ventral horn to limb muscle extrafusal fibers

Question 32

describe A-gamma motor neurons

Answer 32

small, myelinated, lower velocity passing via ventral horns to limb muscle intrafusal fibers

Question 33

where are the alpha and gamma motor neurons located in the spinal cord?

Answer 33

ventral horn

Question 34

what are renshaw cells?

Answer 34

inhibitory interneurons of the anterior horn that receive collaterals from the alpha motor neuron and work to avoid damage in the event of excessive contraction (like golgi)

Question 35

what makes up a motor unit?

Answer 35

motor neuron and the muscle it activates

Question 36

what is a motor nuclei?

Answer 36

100 motor neurons (average) to control a typical muscle | they congreate to create a nuclei in the anterior horn of the spinal cord as well as in the brainstem for motor CNs

Question 37

a motor axon branches at its muscle and each fiber receives ____ ending of the branching

Answer 37

one

Question 38

where is the NMJ located

Answer 38

in the center of the muscle fiber where APs are generated

Question 39

the number of muscle fibers in a motor unit ___ with increasing fine motor control

Answer 39

decreases

Question 40

what are the 3 types of motor units?

Answer 40

Type I Type IIA Type IIB

Question 41

describe Type IA motor units

Answer 41

slow twitch muscle fibers, low tension, fatigue resistant, aerobic, innervated by small motor neuron and axon

Question 42

describe Type IIA motor units

Answer 42

fast fatigue resistant, large tension, some aerobic capacity, innervated by large motor neuron and axon

Question 43

describe Type IIB motor units

Answer 43

fast fatigable, large tension, anaerobic, innverated by large motor neuron and axon

Question 44

the force developed by the muscle fibers increase with _____ firing rate of the motor neuron

Answer 44

increasing

Question 45

force will also increase as ____ motor units are recruited to the excited state

Answer 45

more

Question 46

describe hypotonia?

Answer 46

reduced muscle tone (atrophy) due to damage to either Ia afferent or the alpha motor neuron

Question 47

describe LMN syndrome

Answer 47

causes by destruction of the motor neuron in the anterior horn, the axons in the ventral roots or the peripheral nerves can cause - atrophy - loss of voluntary and reflex responses - hyporeflexia - fasciculations and fibrillations

Question 48

what does conduction velocity depend on

Answer 48

1. myelination | 2. axon diameter - the larger the axon diameter, the faster the conduction velocity

Question 49

what factors control contraction

Answer 49

1. firing rates of motor neurons | 2. number of motor units recruited