Exam 3: Upper Motor System

Question 1

Anatomy—location of primary motor cortex and premotor cortex, layers of M1, location and function of Betz cells and non-Betz pyramidal neurons:

Answer 1

The pyramidal cells of cortical layer 5 are the upper motor neurons of the primary motor cortex.
It was originally thought that the majority of corticospinal fibers originate from the Betz cells in layer 5 of M1
Betz cells contribute less than 5% of the corticospinal fibers
Most corticospinal fibers originate from non-Betz pyramidal neurons of layer 5 in the premotor cortex and M1

Question 2

Corticobulbar tract:

Answer 2

bilateral innervation of brainstem nuclei. Carries motor information to cranial nerve motor nuclei, Controls muscles of the face, head, and neck
Innervates cranial motor nuclei bilaterally (with a few exceptions)

Question 3

Lateral corticospinal tract:

Answer 3

decussate in medulla. 90% of the fibers in the corticospinal tract, Decussate in the medullary pyramids = pyramidal decussation
Terminates primarily in lateral ventral horn, Controls distal movements

Question 4

Ventral corticospinal tract:

Answer 4

10% of the fibers in the corticospinal tract, Do not decussate, Terminate bilaterally, Controls movement of trunk and proximal limb muscles

Question 5

Somatotopic organization in M1

Answer 5

Somatotopic organization in M1 similar to the somatosensory cortex: roughly true, but somatotopic map in M1 is much more coarse–overlapping, intermingled, and fractured

Question 6

M1 organized in a way that activation of particular cortical neurons activates a particular muscle?

Answer 6

Even small microstimulations elicit responses in several muscles
Particular movements can be elicited by stimulation of different areas

Question 7

What does activity in M1 reflect? (movements vs. individual muscles)

Answer 7

Organized movements rather than individual muscles are represented in the map
Neurons in nearby regions linked by local cortical circuits
Microstimulation of M1 can cause activation of ⍺-motoneurons with very short latencies (down to7 ms)
When preparing a voluntary movement, activation of M1 usually starts on the order of 100 ms before motoneurons are activated.
Studies suggest that populations of neurons in M1 carry information about various movement parameters (movement direction, required force, speed, etc.)

Question 8

What is spike-triggered averaging and why would you use it?

Answer 8

correlate the timing of a single cortical neuron’s activity with onset times of contractions in muscle

Question 9

What is a muscle field?

Answer 9

the peripheral group of muscles which are activated by a given upper motor neuron

Question 10

Describe the evidence showing that M1 contains a map of behaviorally relevant
postures. What is the argument against this?

Answer 10

stimulation of a single site specified a final posture rather than the direction of movement
If it were true that M1 neurons only care about final postures or final spatial locations, in recording studies one should find neurons that are maximally active for movements to a particular spatial location, regardless of the starting position. This has not been observed.

Question 11

Describe the likely model of mapping between motor cortex and muscles:

Answer 11

Probably there is no one-to-one mapping between neurons in motor cortex and particular muscles.
Likely there is a many-to-many mapping and this mapping can still be modified by sensory feedback at both the cortical and the spinal cord level.

Question 12

What type of coding is necessary for representing movement direction in M1? Why?

Answer 12

Upper motor neurons in M1 are broadly tuned for direction
Firing rate of individual upper motor neurons prior to each direction of movement
Population vector (red arrows) represent movement direction encoded by the simultaneous activity of the entire population of recorded units

Question 13

What are the functions of the premotor cortex and supplementary motor areas?

Answer 13

Microstimulation in premotor cortex can elicit more complex movements than stimulation in M1, but larger currents are required
Premotor lesions–movements can still be executed, but obstacles can no longer be avoided Mental rehearsal of movements activates premotor areas, but not the primary motor cortex.
Premotor areas are more involved in motor planning.

Question 14

Explain the evidence showing that responses in M1 are limb-specific while responses in
The premotor cortex is not:

Answer 14

Activity of a premotor cortex neuron in a monkey during an instructed-delay reaching task.
Neuron is directionally tuned with a preference for rightward movements.
The directional tuning is identical whether the left or right arm is used.
Relatively inactive during movement execution

Question 15

What do neurons in SMA encode?

Answer 15

Some neurons in the SMA encode a specific sequence of motor acts

Question 16

Mirror neurons—what do they respond to? Where are they located?

Answer 16

Located in the ventrolateral subdivision of the PM
Respond in preparation for a particular movement, as well as when the same action is observed
Do not respond when actions are just pantomimed•Respond during observation of behavior even when final stage of action is hidden

Question 17

Anatomy of the basal ganglia:

Answer 17

4 nuclei:
Striatum (caudate, putamen, ventral striatum)
Globus pallidu
Substantia nigra
Subthalamic nucleus

Question 18

Symptoms of damage to the basal ganglia:

Answer 18

Deficits in the basal ganglia do not lead to paralysis, but to involuntary movements or to immobility without paralysis. 
Parkinson disease (diminished movement)
Huntington disease (excessive movement). 
Basal ganglia are also involved in cognitive/behavioral deficits.

Question 19

Direct and indirect basal ganglia pathways:

Answer 19

Direct pathway: release thalamic neurons from tonic inhibition > activate upper motor neurons
Indirect pathway: increase tonic inhibition > inhibit upper motor neurons

Question 20

Parkinson’s disease—causes, symptoms, treatment:

Answer 20

Dopamine deficiency resulting from degeneration of dopaminergic neurons in the substantia nigra
Relative overactivity of the indirect pathway
Consequence: output neurons of basal ganglia are more active than usual•Stronger inhibition of thalamus/cortex than usual
Consequence: hypo-or akinesia (problems with initiating or stopping movements)

Question 21

Parkinson Possible Treatments

Answer 21

Drug therapy: administration of L-DOPA (dopamine precursor). Surgical intervention. Subthalamic nucleus: remove excitatory input to Gpi. GPi: remove inhibitory input to the thalamus/cortex. Deep brain stimulation: GP, Subthalamic nucleus, thalamus

Question 22

Huntington’s disease—causes, symptoms, treatment:

Answer 22

Caused by a genetic defect
Leads to cell apoptosis in the striatum (primary, but not only target), affecting the indirect pathway
Consequence: indirect pathway does not provide sufficient inhibitory input to neurons in GPe
The output neurons of the basal ganglia provide less inhibition to the thalamus/ cortex. Consequence: hyperkinesia (unwanted movements)
The size of the striatum is dramatically reduced in people with advanced HD

Question 23

Function of the cerebellum, what happens if it’s damage?

Answer 23

Involved in the adjustment of movements while in progress and during repetition
Projects mainly to premotor and motor systems of the cortex and the brain stem
Cerebellar systems are ipsilateral to the controlled muscles (in contrast to the cortex).
Damage disrupts spatial accuracy and temporal coordination of movements and impairs motor learning and certain cognitive functions.

Question 24

Anatomy of the cerebellum

Answer 24

Spinocerebellum
Cerebrocerebellum
Vestbullocerebellum
Vermis
Nodulus
Flocculus
Deep cerebellar nuclei: Festigar 
Input mainly to the cerebellar cortex, Output mainly from the deep nuclei

Question 25

Defects observed in cerebellar diseases

Answer 25

Delayed movement
Range of movement errors
Patterned movement errors

Question 26

Feedback controller:

Answer 26

Measures the current temperature and calculates an error; if there is an error, the controller activates a heating or cooling device
Advantage: will almost always reach the target temperature (without having to learn anything)
Disadvantage: relatively slow

Question 27

Feedforward controller:

Answer 27

Receives a multitude of sensory information about events that might have an impact on the temperature; e.g., may have learned that opening a window in winter leads to a drop in temperature; if open window detected > predicts an error > takes counteraction (activate a heating device) before the temperature drops
Advantage: very fast (errors are anticipated rather than detected)
Disadvantage: needs a lot of experience (learning)

Question 28

Cerebellum feedback controller

Answer 28

Feedback: The cerebellum compares internal feedback signals that report the intended movement with external feedback signals that report the actual motion.
Feedforward: also able to generate corrective, anticipatory signals that operate on descending motor systems