Lower Motor Neurons, Brainstem Control

Front 1

Where do the upper motor neurons live?

Back 1

Motor cortex; basal ganglia; cerebellum; brainstem

Front 2

Where do the lower motor neurons live?

Back 2

Spiral cord and brainstem. Directly innervate skeletal muscles (visceral muscles is by autonomic ns)

Front 3

single motor neuron, its axon fibers and the muscle fibers that it innervates.

Back 3

motor unit

Front 4

group of motor neurons that innervate fibers within the same muscle

Back 4

motor neuron pool

Front 5

Compare small vs. large motor neurons

Back 5

Small have a low innervation ratio and fine delicate control (e.g., for extraocular muscles)

Large have a high innervation ratio and work for force generation (e.g., gastrocnemius, 2000:1)

Front 6

Where are the cell bodies of the lower motor neurons?

Back 6

Ventral horn of spinal cord gray matter and in motor nuclei of the cranial nerves in brainstem

Front 7

Where are the cell bodies of the upper motor neurons?

Back 7

either in cortex or in brainstem centers (like nuclei such as vestibular nucleus)

Front 8

How is the mapping/topography of motor neuron pools accomplished?

Back 8

Neurons innervating axial musculature (eg postural muscles of trunk) are located medially. More distal motor neurons innervate more distal muscles.

Front 9

intrafusal muscle fibers

Back 9

muscle spindles embedded within connective tissue capsules in the muscle. innervated by sensory axons that send info to the CNS about length and tension of muscle

Front 10

What are the α motor neurons?

Back 10

Type of lower motor neuron that innervates the extrafusal muscle fibers (the striated muscle fibers that actually generate the forces needed for posture and movement)

Front 11

What are the γ motor neurons?

Back 11

Innervate muscle spindles. gamma motor neurons are involved in reflexes and are involved in adjusting the tension on muscle spindles.

Adjust sensitivity of muscle spindles.

Front 12

T/F individual motor axons branch within muscles to synapse on many different fibers

Back 12

T. There are many more muscle fibers than motor neurons.

Front 13

Characteristics of S (slow) motor units

Back 13

-Contract slowly
-Generate small forces
-Fatigue resistant
-Impt for activates requiring sustained muscular contraction
-recruited the most quickly

Front 14

Characteristics of fast-fatigable (FF) motor units

Back 14

-Larger α motor neurons
-Innervate larger, pale muscle fibers that generate more force
-Fibers have sparse mitochondria
-Especially impt for brief exertions
-Takes a lot of neural stimulation to recruit them

Front 15

Characteristics of fast fatigue-resistant (FR) motor units

Back 15

-Intermediate size
-Not as fast as FF but faster than S
-Generate 2x force of S
-More resistant to fatigue than FF

Front 16

What is the size-recruitment principle?

Back 16

Small motor units fire first, have the lowest threshold, and generate the least force.

Large motor units have higher threshold but generate more force. They fire last.

Front 17

What is the order of recruitment for FR, FF, and S.

Back 17

1) S
2) FR
3) FF

Front 18

T/F Increasing stimulation within a motor neuron pool progressively activates larger and larger neurons that in turn generate more and more force.

Back 18

T

Front 19

What is the stretch reflex?

Back 19

A sensory response to muscle stretch.

Muscle spindle afferents fire when muscle is stretched. This synapses with α motor neurons in ventral horn of spinal cord that innervate the same muscle and forms inhibitory connections with α motor neurons of antagonistic muscles (aka reciprocal innervation)

Front 20

What is reciprocal innervation?

Back 20

A sensory afferent forms excitatory connections with one muscle group and inhibitory connections with an opposing/antagonist muscle group. Allows for rapid contraction of muscle and simutaneous relaxation of the antagonist muscle.

Front 21

What kind of afferents does the stretch/myotatic reflex use?

Back 21

Group Ia (very fast) afferents in the muscle spindle.

Front 22

What is unusual about the stretch/myotatic reflex?

Back 22

It's monosynaptic. The sensory neuron directly synapses on the motor neuron. (basis of the knee, ankle, etc. reflexes tested)

Front 23

What is the γ bias (aka γ gain)?

Back 23

The level of the γ motor neuron activity. It can be adjusted by upper motor neurons pathways as well as local reflex circuitry.

If the gain is high, a small amount of stretch applied to the intrafusal fibers will produce large increases in the number of α motor neurons recruited.

Front 24

What happens to the response of the Ia fiber as the muscle contracts

a) in presence of γ neurons
b) without γ neurons

Back 24

a) there is no decrease in Ia firing during muscle shortening

b) the response of the Ia fiber decreases as the muscle contracts

Front 25

T/F In general, the baseline activity of γ fibers is high if a movement is difficult and requires precise execution

Back 25

T

Front 26

What group of sensory axon is golgi tendon organ?

Back 26

Ib (slightly smaller than the Ia axons that innervate muscle spindles)

Front 27

WHat encapsulated afferent nerve ending is located at the junction of muscle and tendon?

Back 27

Golgi tendon organ

Front 28

T/F Golgi tendon organs are in parallel with muscle spindles

Back 28

F. They're in series.

Front 29

How are golgi tendon organs more sensitive to increases in muscle tension than stretch?

Back 29

When a muscle actively contracts, force acts directly on the tendon, leading to an increase in the tension of the collagen fibrils in the tendon organ and compression of the intertwined sensory receptors.

Front 30

How is the golgi tendon circuit a negative feedback system?

Back 30

it decreases the activation of a muscle when exceptionally large forces are generated, protecting the muscle.

tends to maintain a steady level of force, counteracting forces that decrease force (fatigue) and increase

Front 31

T/F Golgi tendon organ also responds to passive muscle stretch

Back 31

F

Front 32

What happens in the golgi (inverse myotatic) reflex?

Back 32

When the Golgi tendon organ is stimulated by muscle tension there is an increase in Ib afferent activity.

Ib fibers synapse on inhibitory neurons in the spinal cord that in turn inhibits the activity of motor neurons innervating the homonymous muscle.

Ib fibers activity also causes an excitation of motor neurons innervating antagonist muscles.

Front 33

very generally, what happens in the flexion reflex pathway?

Back 33

A noxious stimulus leads to activation of local spinal cord circuits that flex/withdraw the affected limb and extend the unaffected side to provide balance/support.

Front 34

What stimulates the flexion/withdrawal reflex?

Back 34

Touch, temperature, and pain

Front 35

What fiber types carry info in the flexion/withdrawal reflex pathway?

Back 35

Flexor Reflex Afferents (FRAs). Group II, III (Aδ), and IV (C)

Front 36

What happens to the ipsilateral side stimulated by the flexion/withdrawal reflex?

Back 36

Flexor muscles on ipsilateral side are <b>stimulated</b>.

Extensors on ipsilateral side are <b>inhibited.</b>

Front 37

What happens to the contralateral side stimulated by the flexion/withdrawal reflex?

Back 37

Extensor muscles on contralateral side are <b>stimulated</b> while flexors on contralateral side are <b>inhibited</b>.

Front 38

What are the following for the <b>stretch reflex</b>:

A) Stimulus (type of receptor)
B) Neuron afferent type
C) How many synapses?
D) Response

Back 38

A) Muscle spindle
B) Ia
C) Monosynaptic
D) Contraction

Front 39

What are the following for the <b>golgi reflex</b>:

A) Stimulus (type of receptor)
B) Neuron afferent type
C) How many synapses?
D) Response

Back 39

A) Golgi tendon organ
B) Ib
C) Disynaptic
D) Relaxation

Front 40

What are the following for the <b>flexor withdrawal reflex</b>:

A) Stimulus (type of receptor)
B) Neuron afferent type
C) How many synapses?
D) Response

Back 40

A) Touch, temp, pain
B) II,III (A&delta;), IV(C)
C) Polysynaptic
D) Ipsilateral flexion, Contralateral extension

Front 41

What are the four distinct but highly interactive motor subsystems?

Back 41

Local circuits in spinal cord and brainstem, descending upper motor neuron pathways, basal ganglia, and cerebellum

Front 42

Directly link the nervous system and muscles to form motor unit

Back 42

&alpha; motor neurons located in spinal cord and cranial nerve nuclei

Front 43

What are central pattern generators?

Back 43

Specialized local circuits that provide much of spatial coordination and timing of muscle activation

Front 44

What are symptoms of lower motor neuron syndrome?

Back 44

Paralysis (loss of movement) or paresis (weakness)
Loss of reflexes (areflexia)
Loss of muscle tone
Muscle atrophy (with time)
Fasciculations (spontaneous twitches)

Front 45

What kinds of neurons degenerate in ALS?

Back 45

lower motor neurons in spinal cord and upper motor neurons in motor cortex

Front 46

Symptoms of ALS?

Back 46

Progressive muscle weakness, atrophy, muscle twitching.

Sensory function remains intact; mental status remains intact

Front 47

Pyramidal pathways

Back 47

Corticospinal, Corticobulbar. Have fibers passing thru and crossing in medullary pyramids

Front 48

What are extrapyramidal structures responsible for?

Back 48

Originate in brainstem centers. Responsible for control of posture and spatial orientation.

Front 49

What are the extrapyramidal structures?

Back 49

Red nucleus; superior colliculus; pontine and medullary reticular formation; vestibular nuclei

Front 50

Rubrospinal tract: origin and function

Back 50

Red nucleus; facilitation of flexors and inhibition of extensors

Front 51

Difference between medial and lateral systems

Back 51

Medial systems generally control axis/postural adjustment. Lateral systems do more peripheral movement, limbs in 3d, etc.

Front 52

Pontine/medial reticulospinal tract:origin and function

Back 52

pons; projects medially.

Has excitatory effect on LMNs.
<b>Maximum excitation on extensors</b>.

Front 53

Which brainstem center has maximum excitatory effect on extensors?

Back 53

Pontine/medial reticulospinal tract

Front 54

Medullary/lateral reticulospinal tract:origin and function

Back 54

medulla, projects laterally.

Inhibitory effect on LMNs.
<b>Maximum inhibition on extensors.</b>

Front 55

Lateral vestibulospinal tract:origin and function

Back 55

Deiter's nucleus.

Excitation of extensors and inhibition of flexors.

Has a "medial" function even though "lateral" is in name.

Front 56

tectospinal/colliculospinal tract: origin and function

Back 56

Superior colliculus. Fibers project to cervical spinal cord where they control neck muscles involved in head movement.

Front 57

Effect of <b>motor cortex</b> on:

A) Extensors
B) Flexors
C) Pontine Reticular Formation
D) Medullary reticular formation

Back 57

A)+
B) ++
C) -
D) +

Front 58

Effect of red nucleus on:

A) Extensors
B) Flexors

Back 58

A) -
B) +

Front 59

Effect of pontine reticular formation on:

A) Extensors
B) Flexors

Back 59

A) ++
B) +

Front 60

Effect of lateral vesticular/Deiter's nucleus on:

A) Extensors
B) Flexors

Back 60

A) +
B) -

Front 61

Effect of Medullary reticular formation on:

A) Extensors
B) Flexors

Back 61

A) --
B) -

Front 62

What 3 things happen in spinal cord transection?

Back 62

A) Complete loss of voluntary movements below lesion (paraplegia)
B) Loss of conscious sensations
C) Initial loss of reflexes

Front 63

What is spinal shock?

Back 63

Loss of reflexes and flaccid limbs following spinal cord lesion. It's the result of removal of background facilitation by higher centers on mostly &alpha; and &gamma; motor neurons. Partial recovery may occur due to hypersensitivity development.

Front 64

What causes the initial loss of reflexes after spinal cord lesions?

Back 64

Removal of excitatory influence of UMNs onto LMNs in spinal cord.

Front 65

What causes the hyperreflexia occurring several weeks after spinal cord lesions?

Back 65

Neurons in spinal cord recover and become hyperactive and hypersensitive due to build up of neurotransmitter.

Front 66

What are the brainstem centers most impt to the maintenance of muscle tone in antigravity muscles?

Back 66

Pontine reticular formation; deiter's nucleus/lateral vestibulospinal tract

Front 67

What normally modulates/inhibits the excitatory effect on extensors of the pontine reticular formation?

Back 67

central/cortical projections

Front 68

What happens if there is a lesion above the level of the pontine reticular formation/mid-collicular cut?

Back 68

Inhibitory influence of cortical projections is removed, leading to increase in tone of antigravity/postural muscles (mainly extensors).

Front 69

What is decerebrate rigidity?

Back 69

An increase in tone from the pontine reticular formation following removal of inhibitory cortical projections.

An exaggerated activation in muslce tone of extensors (arms and legs extended, back arched, head dorsiflexed, toes curled).

Front 70

What happens if there is a lesion above the midbrain (red nucleus)?

Back 70

No decerebrate rigidity because red nucleus still provides inhibition via rubrospinal tract on extensor activity.

Front 71

What is the righting reflex? Stages of?

Back 71

Refers to the ability of an animal to reflexively right itself from an abnormal position.

1) Input from vestibular system allows the head to orient into a normal vertical position
2) Distortion from neck muscles provide info to allow the trunk to come into alignment with the head.

Front 72

What is decorticate rigidity?

Back 72

Turning head to one side activates ipsilateral extensors and contralateral flexors.

Front 73

Symptoms of upper motor neuron lesion

Back 73

Weakness, spasticity (increased tone, hyperactive deep reflexes, clonus), Babinski's sign, Loss of fine voluntary movements

Front 74

Symptoms of lower motor neuron lesion

Back 74

Weakness or paralysis, decreased superficial reflexes, hypoactive deep reflexes, decreased tone, fasciculations and fibrillations, severe muscle atrophy