UMN and LMN

Question 1

Lower Motor Neurons vs Upper Motor Neurons

Answer 1

LMNs directly synapse on the muscle and have cell bodies in the ventral horn of the spinal cord. Two types: gamma (for sensory) and alpha motor neurons (axons go to skeletal muscles).

UMN synapse onto LMN or local circuitry neurons and have cell bodies in the motor cortex and brainstem.

Question 2

What are the 4 motor systems?

Answer 2

Basal Ganglia - gating proper initiation of movement

Cerebellum - sensory motor coordination of ongoing movement

UMN -
motor cortex - voluntary movements
brainstem - basic movements and postural control

Question 3

What is a motor unit? Whats one fact about a motor unit?

Answer 3

The alpha motor neuron and the muscle fibers it contacts define the motor unit

fibers of the same motor unit are distributed for even force throughout the muscle

DNTN: most extrafusal skeletal muscle fibers in mature mammals are innervated by only a single alpha motor neuron

Question 4

What are the 3 types of motor units?

Answer 4

Muscle fibers within a motor unit are all the same “type” 
	Slow – small motor neurons, low frequency sustained firing rate
	Fast fatigable (FF) – large motor neurons, brief high frequency firing rate
	Fast fatigue-resistant (FR) – intermediate

Question 5

With increased motor unit size, alpha motor neurons exhibit:

Answer 5

Increased
cell body size
dendritic complexity
short term EPSP potentiation with repeated activation
axonal diameter (i.e faster conducction)
number of axonal branches (i. e more muscle fibers innervated)

Decreased
   Input resistance
   excitability
   Ia EPSP amplitude
   PSP decay constant
   Duration of after-hyperpolarization

Question 6

What is the size principle and how does the muscle generate force?

Answer 6

The size principle: S motor units are recruited first (smallest), then FR, then FF (largest motor units)

Both rate of firing and number of active motor units increases to generate force

Question 7

Describe the stretch reflex circuit

Answer 7

Regulation of muscle length

α-MN innervate extrafusal muscle fibers

γ-MN innervate intrafusal muscle fibers

Types of intrafusal muscle fibers 
	Nuclear bag 
		-static (type II sensory neuron)
		-dynamic (type Ia sensory neuron)
	Nuclear chain (type II sensory neuron)

Question 8

Describe the Golgi Tendon Organ Reflex Circuit

Answer 8

Regulation of muscle force

Thought to a) protect the muscle by causing it to relax when exceptionally large forces are generated and b) maintain a steady level of force, counteracting effects that diminish muscle force such as fatigue.

GTO innervated by Ib sensory neuron

Question 9

What are Central Pattern Generators (CPGs)?

Answer 9

CPGs are neural networks that produce rhythmic patterned outputs with little to no sensory feedback or external motor commands

DNTM: Can generate fairly complex, stereotypical movements such as walking without voluntary motor control, using only local spinal cord CPGs.

Question 10

Describe the corticobulbar tract

Answer 10

Corticobulbar tract
Motor cortex (related to head/face) -> internal capsule -> descend with corticospinal fibers -> different motor nucleis

Termination - Cortex to BRAINSTEM

Function
Arises from areas related to head and face
Controls turning of head/neck

Question 11

Describe the motor cortex

Answer 11

The Corticospinal and Corticobulbar Tracts

Starts: 
posterior frontal lobe
known as primary motor cortex
Inputs:
-Basal ganglia, cerebellum
-Sensory regions of parietal lobe

Special cells:
Betz cells in layer 5 (largest cells in CNS)

Function:
Mediate planning and initiation of complex temporal sequences of voluntary movement

Question 12

Describe the lateral corticospinal tract

Answer 12

Cortex -> internal capsule -> cerebral peduncles -> pons -> medulla (CROSS OVER!!!) -> descend down spinal cord

Termination - Target LMN in lateral portion of anterior horn (LMN)

Function
LMNs responsible for innervation of distal muscles
Allow for precise skilled movements and synergisticc movement of limb

Question 13

Functional Organization of the 
Primary Motor Cortex

Answer 13

1930s – Sherrington and Penfield
Motor cortex contains spatial map of the contralateral body

1960s– more refined method (smaller magnitude of electrical current)
Small currents initiate excitation of several muscles (and suppression of others

1970s: spike-triggered averaging
Correlate muscle activity with discharge of single upper motor neurons

Motor map is less precise than sensory map – This allows for dynamic and flexible way of encoding higher order parameters that coordinate activation of multiple muscle groups

Question 14

The premotor cortex

Answer 14

Receives input from parietal lobe and prefrontal area
Projections: Over 30% of axons in corticospinal tract arise from neurons in premotor cortex

Difference between premotor and primary motor cortex?
Strength of their connections to lower motor neuron
Study-
Monkey trained to reach different directions depending on visual cue– premotor area fires before monkey receives signal to activity
Encode intention to perform movement

Question 15

The premotor cortex contains mirror neurons. describe

Answer 15

1. Premotor areas fires Mirror Motor Neurons
2. Responds less when same actions pantomimed without presence of behavioral goal (ie object to be grasped)
3. Can fire to goal-directed behavior even if final stage of action is hidden from view (because they understand the goal!

Therefore, these neurons encode intentions and involved in imitation learning

Question 16

What are the Motor Control Centers in the Brainstem?

Answer 16

Vestibular complex:
-Maintenance of balance

Reticular formation:
-Regulation of posture

Red nucleus:

• Flexor movement in upper limbs
• Source of learning signals for cerebellum

Superior Colliculus:

• Orientation of visual gaze
• Responsible for orienting head and neck during eye movements

Question 17

What is the difference between medial and lateral vesibular nuclei?

Answer 17

Medial Vestibular Nuclei

• Terminates bilaterally in medial ventral horn
• Regulates head position by reflex activation of neck muscles in response to semicircular canals

Lateral Vestibular Nuclei

• Terminates along medial lower motor neuronal pools, govern proximal limbs
• Activators limb extensors to stabilize balance and upright posture

Question 18

Describe the reticular formation in the brainstem

Answer 18

Complicated network of circuits– therefore, many functions

Motor: temporal and spatial control of limb and trunk movement

The reticular formation with the Vestibular Nuclei - maintains posture, responds to disturbance of body position and stability

Question 19

Vestibular nuclei vs Reticular Formation

Answer 19

Vestibular nuclei: direct projections from nuclei to spinal cord
Rapid compensatory feedback response to postural instability

Reticular formation
Controlled by cerebral cortex, hypothalamus, or brainstem
Initiates feedforward adjustments to stabilize during ongoing movements

Question 20

Describe the Anticipatory maintenance of body posture by Reticular Formation

Answer 20

Hear tone
Subject pulls handle
Contract biceps
Contraction of calf PRECEDES bicep contraction

Anticipatory = feedfoward mechanism

Question 21

What are the signs and symptoms of upper motor neuron damage?

Answer 21

weakness
spasticity
   increased tone
   hyperactive deep reflex
   clonus
babinski's sign
loss of fine voluntary movements

Question 22

Function of Cortical control of motor function

Answer 22

Influence LMNs in spinal cord
Influence motor nuclei in cranial nerves in the brainstem
Gating for spinal reflexes
Responsible for influences on afferent (sensory) systems