Unit 3: Intro to motor systems & spinal cord reflexes pg 85 - 92

Question 1

What is the function of the lower motor neurons (LMNS)?

Answer 1

Innervate contractile elements in the skeletal muscle

Serve as final common pathway neurons: final link between CNS and skeletal muscle cells

Question 2

Where are lower motor neurons located?

Answer 2

• spinal cord ventral horn (lamina IX)

- brainstem motor nuclei of cranial nerves

Question 3

Where do lower motor neurons project their axons?

Answer 3

• via ventral rootlets of all spinal nerves
• via (most) cranial nerves: III, IV, V, VI, VII, IX, X, XI, XII
(all except I, II, VIII)

Question 4

Where are upper motor neurons located?

Answer 4

Supranuclear motor control areas

Question 5

The axons that project from upper motor neurons form what pathway and synapse where?

Answer 5

axons form descending motor control

pathways and synapse on lower motor neurons

Question 6

What kind of control of lower motor neurons do upper motor neurons provide?

Answer 6

Voluntary and involuntary control

Question 7

Lower motor neurons are also affected by direct sensory input at segmental level in _____ and _____.

Answer 7

• Spinal cord

- Brain regions receiving input from cranial nerves

Question 8

Direct sensory input from lower motor neurons in the spinal cord mediate what?

Answer 8

Spinal cord reflexes

Question 9

Direct sensory input form lower motor neurons in the brain regions receiving input from cranial
nerves mediate what?

Answer 9

reflex responses

Question 10

There are two types of lower motor neurons. Where are they both located?

Answer 10

lamina IX of

spinal cord or cranial nerve nuclei within brainstem

Question 11

There are two lower motor neuron cell types. What are they?

Answer 11

Alpha motor neurons

Gamma motor neurons

Question 12

Axons of alpha motor neurons are comprised of what group type of fibers?

Answer 12

A alpha

Question 13

Axons of gamma motor neurons are comprised of what group type of fibers?

Answer 13

A gamma

Question 14

Both alpha and gamma motor neurons are multipolar neurons. Which are larger?

Answer 14

Alpha motor neurons

Question 15

Both alpha and gamma motor neurons exit the CNS with ______ and _______ nerves.

Answer 15

• Spinal

- Cranial

Question 16

What kind of skeletal muscle fibers do alpha motor neurons (a type of LMNS) innervate?

Answer 16

Extrafusal muscle fibers

Question 17

What makes up a motor unit?

Answer 17

a single α motor neuron and all extrafusal muscle fibers it innervates

Question 18

In a small motor unit, 1 α motor neuron may innervate how many
fibers?

Answer 18

Tens

Question 19

In a large motor unit, 1 α motor neuron may innervate how many
fibers?

Answer 19

Hundreds to thousands

Question 20

Large muscles that carry out strong but crude actions, such as those in the leg, have what kind of motor units?

Answer 20

Large

Question 21

Muscles that carry out fine motor movements, such as fingers or eyes, have what kind of motor units?

Answer 21

Small

Question 22

What kind of skeletal muscle fibers do gamma motor neurons (a type of LMNs) innervate?

Answer 22

intrafusal muscle fibers of skeletal muscle

• part of the neuromuscular spindle

Question 23

How are α and γ motor neurons arranged within lamina IX of ventral horn of the spinal cord?

Answer 23

somatotopically
• LMNs that innervate extremity muscles are located
more laterally
• LMNs that innervate trunk muscles are located
medially

Question 24

Lower motor neurons that innervate extremity muscles are located where in lamina IX of the ventral horn of the spinal cord?

Answer 24

More laterally

Question 25

Lower motor neurons that innervate trunk muscles are located where in lamina IX of the ventral horn of the spinal cord?

Answer 25

More medially

Question 26

Neuromuscular spindles (NM spindles) are what kind of receptors?

Answer 26

encapsulated mechanoreceptor

Question 27

Where are NM spindles found?

Answer 27

In skeletal muscle

Question 28

What is the function of NM spindles?

Answer 28

Monitor amount or velocity of stretch

Question 29

What is the distribution of NM spindles in skeletal muscles?

Answer 29

uneven; clustered near tendon ‐ more NM spindles in muscles involved in skilled movement

Question 30

What is the morphology of NM spindles?

Answer 30

fusiform surrounded by CT capsule attached to CT that encloses extrafusal muscle fibers (endomysium)

Question 31

Spinal cord reflexes require:

Answer 31

- afferent limb - efferent limb - specialized sensory receptors - contractile elements

Question 32

What is the afferent limb of spinal cord reflexes?

Answer 32

Sensory input provided by 1˚ sensory | neurons → fibers enter spinal cord via dorsal roots of spinal nerves

Question 33

What is the efferent limb of spinal cord reflexes?

Answer 33

Motor output provided by lower motor neurons → motor neuron cell bodies located in ventral horn (lamina IX)

Question 34

The CT capsule of NM spindles encloses what kind of muscle fibers?

Answer 34

Intrafusal muscle fibers

Question 35

What are the “main” muscle fibers of skeletal muscle and comprise the mass of the muscle?

Answer 35

Extrafusal muscle fibers

Question 36

How are extrafusal muscle fibers arranged in relation to neuromuscular spindles?

Answer 36

They are arranged in parallel to the | neuromuscular spindles

Question 37

What happens when extrafusal muscle fibers are stretched?

Answer 37

Increased tension in NM spindle and increased tension of intrafusal fibers

Question 38

What happens when extrafusal muscle fibers are contracted?

Answer 38

Decreased tension in NM spindle and decreased tension of intrafusal muscle fibers

Question 39

NM spindles receive both sensory and motor innervation. Sensory innervation is via?

Answer 39

* Group A α (type Ia) fibers -> form annulospiral endings | * Group A β (type II) fibers -> form flower spray endings

Question 40

How are fibers and endings of NM spindles activated?

Answer 40

when muscle is stretched -> tension in NM spindle increases -> NM spindles are activated -> endings stimulated -> proprioceptive data produced

Question 41

extrafusal muscle fibers stretched= ________ firing rate of sensory fibers

Answer 41

Increased

Question 42

extrafusal muscle fibers contracted= _______ firing rate of sensory fibers

Answer 42

Decreased

Question 43

NM spindles receive both sensory and motor innervation. Motor innervation is via?

Answer 43

γ motor neurons

Question 44

γ motor neurons control length/tension of ________ within NM spindles and determine sensitivity to stretch of ________.

Answer 44

- intrafusal fibers - extrafusal fibers I.e. increased firing rate of gamma motor neurons = contraction of intrafusal fibers; increased sensitivity to stretch of extrafusal fibers

Question 45

Stretch Reflex (aka deep tendon reflex; DTR) requires 2 neurons:

Answer 45

- ‘afferent limb’: sensory neuron (type Ia, II fibers) - - ‘efferent limb’: alpha motor neuron -> supplies extrafusal fibers

Question 46

What is a monosynaptic reflex?

Answer 46

two neurons, one synapse as seen in stretch reflex

Question 47

High velocity muscle stretch initiates _____.

Answer 47

Stretch Reflex (aka deep tendon reflex; DTR)

Question 48

Tapping the quadriceps/infrapatellar tendon will cause what chain of events?

Answer 48

- high velocity muscle stretch - spindle (sensory endings) activation - sensory info relayed to spinal cord via Type Ia and II fibers - some collateral processes join ascending pathways - some collateral processes synapse on alpha motor neurons - alpha motor neurons cause contraction of extrafusal fibers of homonymous muscle

Question 49

During muscle contraction caused by deep tendon reflex, why does the contraction not last indefinitely?

Answer 49

Because, during the contraction, the spindle tension decreases which decreases firing of sensory fibers, loss of activation of alpha motor neurons and muscle relaxation.

Question 50

What kind of receptors are Golgi tendon organs (GTOs; neurotendinous spindles)?

Answer 50

encapsulated proprioceptors that contain thin capsule of CT that surrounds some collagenous fibers of the tendon

Question 51

Where are Golgi tendon organs (GTOs; neurotendinous spindles) most numerous?

Answer 51

near attachments of tendons to muscle

Question 52

Golgi tendon organs (GTOs; neurotendinous spindles) are supplied by what group of fibers?

Answer 52

type Ib fiber type sensory | afferents

Question 53

What do Golgi tendon organs (GTOs; neurotendinous spindles) monitor in muscle-tendon units?

Answer 53

Tension

Question 54

What happens to GTO tension during muscle contraction? What does this do to the firing of the type Ib fibers?

Answer 54

GTO tension increases = Increased type Ib fiber firing

Question 55

UMNs are able to influence motor activity (muscle tone) by two mechanisms:

Answer 55

- directly synapsing on γ (gamma) motor neurons | - indirectly synapsing on α (alpha) motor neurons

Question 56

What does a poly synaptic reflex involve?

Answer 56

More than 2 neurons and more than 1 synapse

Question 57

What is a flexor reflex?

Answer 57

protective reflex involving withdrawal of a body part from a noxious stimulus

Question 58

Interneurons of the flexor reflex will synapse on two things:

Answer 58

- α motor neurons of flexor muscles to stimulate them | - α motor neurons of extensor muscles to inhibit them

Question 59

What is reciprocal innervation?

Answer 59

activation of flexor muscles, inhibition of antagonist muscles (extensors)

Question 60

In the flexor reflex, how is significant muscle activation achieved?

Answer 60

Motor neurons at multiple cord levels must be activated via collaterals traveling to superior and inferior segments

Question 61

What two pathways can the flexor reflex travel to different spinal cord levels in order to involve motor neurons at multiple levels for sufficient muscle activation in the flexor reflex?

Answer 61

- collaterals of Lissauer’s tract | - interneurons within fasciculus proprius (propriospinal tract)

Question 62

Where is fasciculus proprius located?

Answer 62

immediately adjacent to spinal cord gray

Question 63

The neurons that travel in fasciculus proprius are termed________.

Answer 63

Spinospinalis neurons

Question 64

What is crossed extension/extensor reflex?

Answer 64

some dorsal horn neurons (interneurons) have axons that decussate and synapse on neurons in the contralateral ventral horn for activation of contralateral ventral horn neurons to induce muscle contraction of contralateral extensor muscles

Question 65

When GTOs ‘fire’ due to increased tension and send sensory impulses through type Ib fibers along ascending conscious pathways and onto interneurons in the spinal cord gray to do what?

Answer 65

inhibit alpha motor neurons that | supply the homonymous muscle

Question 66

What is the gamma reflex loop?

Answer 66

“functional complex’ of structures that play an | important role in controlling muscle tone

Question 67

What are the components of the gamma reflex loop?

Answer 67

a. γ motor neurons b. intrafusal muscle fibers of the NM spindle c. type Ia, II fibers d. α motor neurons e. extrafusal muscle fibers

Question 68

Interruption of the gamma reflex loop results in:

Answer 68

- no paralysis - decreased muscle tone - loss of associated DTR - loss of stretch reflex - lack of strength and precision of muscle contraction

Question 69

descending motor control fibers from higher centers that influence LMNs terminate on gamma motor neurons resulting in:

Answer 69

increased activity of γ motor neurons → increased type Ia, II fiber firing → info travels to spinal cord → increased facilitation of α motor neurons → increased tone of extrafusal fibers → increased muscle tone