Kenyon: Motor Neurons

Question 1

Cell bodies in the brainstem or spinal cord
The axons end in neuromuscular junctions with skeletal muscle fibers.
Excitatory

Answer 1

Lower motor neurons

Question 2

Cell bodies in the brainstem or cerebral cortex
The axons end in synapses on other upper motor neurons, local circuit neurons, lower motor neurons, or some combination.
Excitatory or inhibitory or both!

Answer 2

Upper motor neurons

Question 3

Describe the major pathway from upper motor neurons to skeletal muscles.

Answer 3

upper motor neurons in the motor cortex (for planning, initiating, directing voluntary movements) or in the brainstem (basic movements and posture) –> local circuit neurons (lower motor neurons) –> motor neuron pool (lower motor neurons)–> skeletal muscles

Question 4

Describe the minor pathway from upper motor neurons to skeletal muscles.

Answer 4

upper motor neurons in the motor cortex or brainstem centers –> motor neuron pools (lower motor neurons) –> skeletal muscles

Question 5

What is the final common pathway in the organization of motor control?

Answer 5

motor neuron pools –> skeletal muscles

Question 6

Where is there sensory input in the organization of motor control?

Answer 6

to local circuit neurons
to motor neuron pools
to the cerebellum

Question 7

Many spots along the motor control pathway may be excitatory or inhibitory. Which spot is only excitatory?

Answer 7

motor neuron pool –> skeletal muscle

Question 8

Lower motor neurons can synapse in what two places?

Answer 8

spinal cord

brainstem

Question 9

What do lower motor neurons that synapse in the ventral horn of the spinal cord do?

Answer 9

control the skeletal muscles of the body

Question 10

What do lower motor neurons that synapse in the somatic and branchial motor nuclei do?

Answer 10

control the muscles of the head

Question 11

What are the somatic motor nuclei that lower motor neurons may synapse in?

Answer 11

oculomotor
trochlear
abducens
hypoglossal

Question 12

What are the branchiomotor nuclei that lower motor neurons can synapse in?

Answer 12

trigeminal
facial
ambiguus
spinal accessory

Question 13

What are upper motor neurons in the primary motor cortex and premotor cortex for?

Answer 13

planning, initiating, and directing series of movements in limbs and eyes

Question 14

What are upper motor neurons in the cingulate gyrus for?

Answer 14

expressions of emotions especially facial muscles

Question 15

Where are upper motor neurons found in the brainstem?

Answer 15

1. vestibular nuclei
2. reticular formation
3. superior colliculus

Question 16

What do upper motor neurons in the brainstem do?

Answer 16

regulate muscle tone

orient eyes, head, and body with respect to vestibular, somatic, auditory, and visual sensory information

Question 17

Compares actual movement (proprioceptive input) with intended movement and commands corrections. Modulates ongoing movements

Answer 17

cerebellum

Question 18

The basal ganglia is made up of the forebrain nuclei. What are they?

Answer 18

caudate 
putamen
globus pallidus
substantia nigra
subthalamic nucleus

Question 19

What are the basal ganglia (forebrain nuclei) responsible for?

Answer 19

initiation of movements
suppressing unwanted movements
starting wanted movements

Question 20

Receive input from upper motor neurons and sensory neurons.

Excite or inhibit lower motor neurons

Answer 20

local circuit neurons

Question 21

In the spinal cord, the lower motor neurons for the axial (postural) muscles are located (blank), while the lower motor neurons for distal muscles (limbs) are located (blank)

Answer 21

medially; axially

Question 22

In the spinal cord, where are local circuit neurons controlling POSTURE located? Are these projections long or short? What is unique about them?

Answer 22

medially; long; they contract OR relax muscles over a large part of the body as needed in posture

Question 23

In the spinal cord, where are local circuit neurons controlling LIMBS located? Are their projections long or short?

Answer 23

laterally (near the motor neurons); projections are short **local circuit neurons controlling limbs control a more limited set of muscles

Question 24

Each lower motor neuron innervates from 3 to 3000 muscle fibers. The motor neuron and its fibers are called a (blank). The lower motor neurons for a given muscle are clustered in (blank) in one or more spinal cord segments.

Answer 24

motor unit; ventral horns

Question 25

The muscle fibers innervated by a single motor neuron are spread throughout the muscle. Compare the lower motor neurons of the gastroc and the eye.

Answer 25

the gastroc has 1 motor neuron for about 2000 muscle fibers, while the eye muscles have 1 motor neuron for every 3 muscle fibers (more fine movement)

Question 26

What are the different types of motor units?

Answer 26

slow fast fatigable fast fatigue-resistant

Question 27

red muscle, resist fatigue | Tend to be continuously active (e.g. posture).

Answer 27

slow motor units

Question 28

paler | large forces for short bursts

Answer 28

fast fatigable motor units

Question 29

easier to depolarize to threshold (needs fewer EPSPs), slow and steady firing rate, good for slow postural muscles

Answer 29

small motor unit

Question 30

harder to depolarize to threshold (needs more EPSPs), fast firing rate, good for fast fatigable muscles

Answer 30

large motor unit

Question 31

As the synaptic activity driving muscle contraction increases, which units are activated first?

Answer 31

slow units, then fast fatigue resistant then fast fatigable (stand, walk/run, jump/gallop)

Question 32

help control muscle length (deep tendon reflex, a.k.a. stretch reflex, a.k.a. myotatic reflex)

Answer 32

muscle spindles

Question 33

help control muscle tension

Answer 33

golgi tendon organs

Question 34

moves you away from an unpleasant event in the periphery

Answer 34

flexion reflex

Question 35

Activate contraction of 8 – 10 specialized intrafusal muscle fibers (muscle spindles) in proprioception

Answer 35

gamma motor neurons

Question 36

Describe the role of 1a afferent fibers in keeping your beer from spilling when it's heavy.

Answer 36

stretching of muscle causes 1a afferents to synapse on lower motor neurons which drive the activity in the homonymous muscle and inhibit the activity in the antagonist muscle

Question 37

stretching of the muscle spindle causes 1a afferents to activate the (blank) muscle, but inhibit the (blank) muscle

Answer 37

homonymous; antagonist

Question 38

Why must you activate gamma motor neurons as well as lower motor neurons in the muscle spindle reflex?

Answer 38

activation of lower motor neurons alone would cause the muscle to shorten and the 1a afferents would shut down *the cerebellum won't know what is happening

Question 39

Why is co-activation of lower and gamma motor neurons necessary?

Answer 39

An effective control center needs to know the activity in both the gamma efferents and the Ia afferent to know what is going on in the muscle

Question 40

Sensitivity of muscle spindles is under constant adjustment. When you're sitting or stretching is gamma activity high or low? When you're standing on a moving bus or skiing, is gamma activity high or low?

Answer 40

low; high

Question 41

Gamma motor neurons are wrapped in (blank) afferent neurons that sense muscle length

Answer 41

1a primary

Question 42

Encapsulated afferent nerve endings between the muscle fibers and the tendon

Answer 42

golgi tendon organs

Question 43

Golgi tendon organs utilize (blank) fibers

Answer 43

Ib

Question 44

What are golgi tendon organs activated by?

Answer 44

tension

Question 45

Passive stretch increases activity in afferents from both muscle spindles and golgi tendon organs. Spindles respond to (blank) change, while golgi organs respond only slightly to passive increase in (blank)

Answer 45

length; muscle tension

Question 46

What does contraction do to activity in spindle afferents? What does it do to activity in Golgi afferents?

Answer 46

decreases; increases

Question 47

Golgi afferents synapse on local circuit neurons that inhibit a response in the (blank) muscle and activate the (blank) muscle

Answer 47

homonymous; antagonis

Question 48

If tension gets to big, what do golgi afferents cause the muscle to do?

Answer 48

relax to prevent damage

Question 49

How do primary afferent neurons for pain and temp link to lower motor neurons?

Answer 49

directly and via interneurons

Question 50

What happens when you step on something unpleasant?

Answer 50

a cutaneous afferent fiber from the a nociceptor in the foot will synapse in the dorsal horn on a lower motor neuron causing you to flex the ipsilateral leg and extend the contralateral leg

Question 51

Can the spinal cord organize and generate sequences of movement without input from the cortex?

Answer 51

yes | **pattern generators in the spinal cord

Question 52

What happens to a cat after sectioning the dorsal roots of the spinal cord?

Answer 52

it can still walk on a treadmill!

Question 53

What do deficits due to damage or loss of lower motor neurons result in?

Answer 53

specific loss of neuro-muscular transmission

Question 54

List some symptoms of lower motor syndrome

Answer 54

``` paralysis paresis areflexia loss of tone atrophy fibrillations fasiculations ```