Efferent Motor Pathways - Lower Motor And Local Circuit Neurons (10A)

Question 0

What does the input of local circuit neurons ultimately allow for lower motor neuron coordination of?

Answer 0

Provide coordination between muscle groups that is essential for coordinated movements, such as walking, chewing, etc.
* these activities are “preprogrammed” events that may be initiated independent of higher brain input (local circuit/lower motor neuron interactions literally responsible for a chicken running around w/ it’s head cut off)

Question 1

What do local circuit neurons do?

Answer 1

Are the major source of synaptic input for lower motor neurons - serve to integrate info coming in from sensory systems and also descending projections from higher brain centers

Question 2

What is the function of lower motor neurons?

Answer 2

Final common pathway for movement - all commands for movement, whether reflexive or involuntary are ultimately conveyed to the muscles by lower motor neurons

Question 3

Where are cell bodies of upper motor neurons found? (2 places)

Answer 3

In the brainstem and cerebral cortex

Question 4

What is the main different feature of an upper motor neuron compared to lower motor neurons?

Answer 4

Upper motor neurons are entirely contained w/in the CNS(no part leaves it), while the axons of lower motor neurons project into the periphery

Question 6

What do upper motor neurons synapse with?

Answer 6

Usually with local circuit neurons (less frequently may directly synapse with lower motor neurons)

Question 7

Upper motor pathways arising from where are responsible for initiation of voluntary movement and complex skilled movements like fine motor control?

Answer 7

Cerebral cortex

• projections from primary motor cortex, promotor cortex! and supplementary motor cortex are essential for planning, initiating, and directing the sequences of voluntary movement

Question 8

Upper motor neurons that regulate muscle tone, and orient the eyes head and body with respect to incoming sensory info originate from where?

Answer 8

Brainstem

• these are critical for basic navigational movement and control of posture

Question 9

How do the cerebellum and the basal ganglia alter the output of motor function?

Answer 9

They modulate the output of the upper motor neurons to the lower motor neurons (don’t influence lower motor neurons directly)

Question 10

Where are the cell bodies of the 1st order neurons in the Corticospinal (pyramidal) Tracts located? What type do motor neurons are these?

Answer 10

In the precentral gyrus ; these are upper motor neurons

Question 11

What structure do fibers in the corticospinal (pyramidal) tract form after leaving the primary motor cortex (precentral gyrus)?

Answer 11

Corona radiata; they then converge to pass through the posterior limb of the internal capsule

Question 12

What structure do fibers in the corticospinal (pyramidal) tract pass through in the anterior midbrain?

Answer 12

Crus cerebri (basis pedunculi)

Question 13

What structure do fibers of the corticospinal tract form in the anterior medulla?

Answer 13

Pyramids

Question 14

Where do fibers of the corticospinal tract decussate? What are they called afterwards?

Answer 14

At the junction of the medulla and spinal cord - now known as the lateral corticospinal tract

*85-90% of the fibers decussate; the rest remain on the ipsilateral side

Question 15

What are the few fibers of the corticospinal tract that don’t decussate at the pyramids called as they progress caudally?

Answer 15

Ventral (anterior) corticospinal tract

Question 16

Where do the ventral (anterior) corticospinal tract neurons decussate? Via what structure do they cross over?

Answer 16

At the level they synapse on the lower motor neurons in the ventral horn of the spinal cord
* fibers that decussate cross over via the anterior white commisure

Question 17

Which corticospinal tract innervates limb (distal) musculature? Do they do this ipsilaterally or bilaterally?

Answer 17

Lateral corticospinal tract upper motor neurons; synapse only ipsilaterally to innervate limb musculature

Question 18

Which corticospinal tract neurons innervate axial (proximal) musculature lower motor neurons? Do they do so ipsilaterally or bilaterally?

Answer 18

Anterior (ventral) corticospinal tract upper motor neurons synapse bilaterally on lower motor neurons to axial musculature

Question 19

What lamellae in spinal cord grey matter are local circuit interneurons found?

Answer 19

VII-VIII

*in between the intermediate zone(VII) and ventral horn(VIII)

Question 20

What lamella are the lower motor neurons found in?

Answer 20

IX (ventral horn)

Question 21

Where in the ventral horn spinal cord grey matter are neurons innervating distal structures located? Axial structures? (Somatotropin distribution)

Answer 21

Distal structures - lateral portions

Axial structures - medial portions

Question 22

What is a motor unit? What kind of motoneuron is involved?

Answer 22

An alpha-motoneuron and the several muscle fibers in a single muscle it innervates
*activation of a single motor unit represents the smallest unit of force that can be generated by a single muscle fiber

Question 23

What does the size of the alpha-motoneuron cell body have to do with its function?

Answer 23

The greater the size of the motor neuron the greater number of fibers it synapses with.
* alpha-motoneurons are the same as lower motor neurons

Question 24

How is more precise control achieved for fine motor movements, like those in the fingers or eyes?

Answer 24

By having a single motor unit innervating fewer fibers.

But having more fibers per motor unit gives more power

Question 25

Which of the three types of muscle fiber types Is dark meat composed of? Do they have a high or low ratio of muscle fibers per neuron?

Answer 25

Slow, fatigue resistant fibers - are red due to increased blood supply and ample myoglobin; tire slowly due to aerobic metabolism
* have a low number of muscle fibers per neuron(thus smaller motoneuron cell bodies)
Aka Type I fibers

Question 26

What type of muscle fiber types are characteristic of white meat? Why are they pale? What is the ratio of muscle fiber types per neuron in these fibers?

Answer 26

Fast, fatiguable fibers - are paler due to less blood supply and less myoglobin; have a high ratio of fibers to neurons (so large motoneuron cell bodies, and more powerful, less control)
Aka Type II fibers

Question 27

What neuron subtype send afferents from muscle spindle cells to the spinal cord to synapse on alpha-motoneurons as part of the muscle stretch (monosynaptic) reflex? Why do they also synapse on inhibitory interneurons?

Answer 27

1A (sensory from muscle spindle)

Activation of interneurons inhibit the lower motoneurons of the antagonistic muscle set

Question 28

What is the name for the connection made when a sensory afferent nerve (1A) provides excitatory information to an alpha-motoneuron?

Answer 28

Monosynaptic connection

Question 29

What kind of amino acids are common excitatory NTs for motoneurons?

Answer 29

Acidic a.a.

Question 30

In addition to input from sensory neurons in the dorsal root ganglion in the same spinal cord segment, what other two sources of input do alpha-motoneurons have? Do these synapse directly on the motoneurons?

Answer 30

1. DRG neurons from spinal segments above and below
2. Input from higher motor centers (upper motor neurons)
   * do not synapse directly, input is received via interneurons; both excitatory and inhibitory interneurons exist

Question 31

What two afferent neuron subtypes do muscle spindles give rise to?

Answer 31

Types IA and II

Question 32

What type of motoneurons innervate muscle spindles?

Answer 32

Gamma-motoneurons (are the efferent fibers to muscle spindle intramural fibers)

Question 33

What type of sensory organs are located at semitendinous junctions where muscle fibers terminate to form a tendon? What subtype of afferent nerves do these organs give rise to?

Answer 33

Golgi tendon organs; give rise to type IIb afferent nerves

Question 34

What are annulospiral endings?

Answer 34

Specialized sensory nerve endings that encircle intrafusal nerve endings within muscle spindles - they are responsible for the APs fired when the muscle spindle is stretched (such as in response to reflex testing)

Question 35

What role do gamma-motoneurons have in maintaining function of the muscle spindles?

Answer 35

They fire in concert with alpha-motoneurons to keep muscle spindle intrafusal fibers contracting at the same time as the extrafusal fibers (ones making up the normal muscle) - this keeps the mechanical stretch receptors (annulospiral endings) from becoming relaxed and non-functional when the rest of the muscle contracts

Question 36

What is the main purpose of the muscle spindles?

Answer 36

Monitor and regulate muscle tone throughout the full range of muscle extension and flexion to make sure it is a smooth process and relatively constant muscle tone is maintained throughout the movement

Question 37

What information do type II afferent nerves from the muscle spindle encode?

Answer 37

Encode muscle fiver length information - increasing length increases the frequency of AP firing; decreasing the length decreases the frequency of AP firing
*remember, type IA and II are the afferents from muscle spindle fibers

Question 38

What information about muscle fibers is encoded by type IA afferent nerves from muscle spindles?

Answer 38

Both length and velocity of change in length

• frequency of APs increases dramatically during a rapid increase in muscle fiber length; frequency decreases dramatically when length rapidly shortens
• when length is constant, the basal frequency of AP firing reflects the length of the fiber (this info Los conveyed by the type II afferents

Question 39

What does the combined action of muscle fiber afferents (II and IA) and their monosynaptic connections allow for functionally?

Answer 39

Allows the motor unit to monitor its own length and thus control movement at the level of the spinal cord without input from higher CNS structures; this allows you to hold objects w/o looking or to touch your nose with closed eyes, for example

Question 40

What is the main purpose of Golgi tendon organs? What type of afferents do they use again?

Answer 40

They monitor muscle force use the info to protect against excessive and potentially harmful forces = are basically an emergency shut-off switch in the event of muscles contracting too hard
- use IB afferent nerves

Question 41

What kind of neurons do type IB afferents from GTO’s synapse on?(2 targets) Are GTOs connected in series or parallel with their respective muscle/tendon?

Answer 41

1. Inhibitory interneurons = inhibit the alpha-motoneurons that innervate the same muscle as the Golgi tendon organ
2. Excitatory interneurons = contraction of the antagonist muscle groups to those of the GTO
   - connected in series with muscles/tendons (in contrast to intrafusal muscle spindle fibers, which are connected in parallel w/ their muscles)

Question 42

T or F? Muscle spindle and GTO reflex mechanisms do not receive input from higher cortical and brainstem centers.

Answer 42

False; higher cortical and brainstem centers have substantial effects on these reflexes (via input to local circuit interneurons)

Question 43

What is the flexor reflex? What two components does it have?

Answer 43

Reflex withdrawal due to uncomfortable stimulus on skin (like touching hot stove)
Has components of agonist muscle activation (via excitatory interneurons) and antagonist muscle inhibition

Question 44

Gamma-motoneuron activity reflects ______ gain or bias on muscle tone. What does higher gain mean?

Answer 44

Positive; higher the gain, the greater the muscle tone and force of contraction

Question 45

What two inputs alter gamma-motoneuron gain/bias?

Answer 45

1. Local reflex circuitry

2. Upper motor neuron input

Question 46

What two things alter Golgi tendon organ activity?

Answer 46

1. Local sensory circuitry

2. Upper motor neuron input

Question 47

Do upper or lower motor neuron lesion cause weakness?

Answer 47

Yes, both do

Question 48

Does an upper or a lower motor neuron cause atrophy?

Answer 48

Only lower motor neuron lesions cause atrophy

Question 49

In what kind of motor lesion (upper or lower) are fasciculations seen?

Answer 49

Lower motor lesions only

Question 50

What happens to reflexes in upper motor neuron lesions? Lower?

Answer 50

Upper motor neuron lesion - increased reflexes

Lower motor neuron lesion - decreased reflexes

Question 51

What happens to muscle tone in upper motor neuron lesions? Lower motor neuron lesions?

Answer 51

Upper - increased muscle tone

Lower - decreased muscle tone

Question 52

What is the basic function of the cerebellum?

Answer 52

Detects difference (error) between intended movement and the movement actually performed, both real-time and long term 
*long term is motor learning process - improvement over time