Lower Motor Neuron Systems

Question 1

What is the final common pathway?

Answer 1

the alpha-type lower motor neurons are the final common pathway for transmitting information to skeletal muscles

Question 2

Where are lower motor neurons located in spinal cord?

Answer 2

cell bodies of the lower motor neurons are located in ventral horn of spinal cord, in gray matter

Question 3

How are the lower motor neurons organized in the spinal cord?

Answer 3

lower motor neurons located in medial ventral horn synapse with more proximal muscles and in lateral ventral horn, synapse with more distal muscles

Question 4

Define a motor unit

Answer 4

an alpha-type lower motor neuron and its associated skeletal muscle fiber(s) it innervates

Question 5

difference between motor unit and motor

neuron pool

Answer 5

a motor unit is a single motor neuron and the muscle (s) it innervates

motor neuron pool is all the motor neurons that innervate a single muscle

Each individual muscle fiber is innervated by only one motor neuron, but one motor neuron may innervate several muscle fibers.

Question 6

Name and describe the 3 alpha-type motor neurons

Answer 6

1. fast fatigue ( FF type)-fatigue faster, generate more force
2. fast fatigue resistant (FR type)-fatigue slower, generate less force
3. slow (S type)-always on ( tonic); generate low force for postural muscles; do not fatigue

Question 7

What are the 4 key components/neurons of the myotatic/stretch reflex

Answer 7

1. sensory neuron=senses strecth in tendon and synpases in spinal cord
2. extensor lower motor neuron ( excited, extensor contracted)
3. inhibitory interneuron that synapse and inhibits the flexor motor neuron
4. inhibited flexor motor neuron ( flexor relaxed)

Question 8

How does the myotatic reflex detect and adjust muscle length?

Answer 8

the group IA afferent axons detect length/stretch in the muscle spindle and send info to gamma motor neurons; the gamma motor neurons that innervate muscle spindle adjust length of muscle

Question 9

What is a gamma-type motor neuron

Answer 9

lower motor neurons that innervate the intrafusal muscle spindle

Question 10

Compare and contrast alpha-type and gamma-type motor neurons

Answer 10

gamma and alpha type neurons are both motor neurons

gamma motor neurons, innervate the muscle spindle while the alpha motor neurons innervate the extrafusal muscle fiber.

There are more alpha motor neurons than gamma motor neurons

Question 11

what are group IA afferent axon

Answer 11

Sensory fibers that detect stretch and wrap around the nuclear bag fibers in muscle spindle

Question 12

what is muscle tone?

Answer 12

normal , ongoing tension in the muscle; measured by the resistance of muscle to passive stretching

Question 13

What are group Ib sensory axons?

Answer 13

sensory fibers found in the golgi tendon organ than sense muscle contraction

Question 14

Name and describe the components of the Golgi Tendon Organ

Answer 14

1. 1b afferent neurons sense tension in fibrils within golgi body and synapse with inhibitory and excitatory interneuron in spinal cord
2. The excitatory interneuron increases the activity of muscle that is to contract via an alpha motor neuron
3. The inhibitory interneuron decreases the activity of the antagonstic muscle via a alpha motor neuron

Question 15

What is meant by an increase in the gain of myotatic reflexes

Answer 15

the amount of muscle force generated in response to a given stretch of the intrafusal fibers

high gain of reflex=small stretch illicit a large recruitment of alpha motor neurons

low gain of reflex=large stretch illicit a small reccruitment of alpha neurons

Question 16

What are central pattern generators?

Answer 16

local neural circuity that encode rhythmic motions like walking. Once these motions are encoded ( ie learned), it does not require thinking ( i.e. processing by higher level brain centers) to happen

Question 17

Why are central pattern generators important?

Answer 17

These generators coordinate rhythmic motions on both sides of spinal cord allowing for

1) On side of organism, concurrrent inhibition and excitation of opposing muscle groups on one side ( for example, inhibit flex muscle and activate extend musc on L side of human while walking)
2) On the other side of organism, inhibition and excitation of opposing muscle groups is also occur by directly opposite to other side ( for example, excite flex muscle and inhibit extend musc on R side of human while walking)

Question 18

What is lower motor neuron syndrome?

Answer 18

Damage to lower motor neuron that results in neural and muscular deficits

Question 19

3 Signs of lower motor neuron damage

Answer 19

1. paralysis/paresis ( total or partial lost of voluntary movement)
2. areflexia-loss of myotatic reflex
3. decreased muscle tone

Question 20

2 signs of muscle fiber damage

Answer 20

1. muscle atrophy

2. Fibrillations/fasciculations