Organization of the Motor System

Question 1

What are the six parts of the motor system?

Answer 1

• Motor cortex
• Basal ganglia
• Cerebellum
• Brainstem
• Spinal cord
• Motor neurons

Question 2

Explain the function of the motor system during a hand movement

Answer 2

• Visual system (what part of object should be grasped)
• Information relayed to corticomotor regions (frontal lobe motor regions - plan and initiate movement)
• Instructions sent to spinal cord (muscles of arm and hand receive message from motor neurons)
• Information from fingers travels to spinal cord
• Sent to sensory regions of the cortex that interpret touch (basal ganglia judges grasp forces; cerebellum corrects movement errors)
• Sensory cortex informs motor cortex that object is now being held

Question 3

What are the four regions in the neocortex used for skilled movement?

Answer 3

• Posterior cortex (specify movement goals and send information to PFC)
• PFC (generates plans for movement)
• Premotor cortex (recognizes movement of other and selects similar or different actions, organizes movement sequences; “lexicon” of movements)
• Primary motor cortex (executes movements)

Question 4

In what areas of the brain does blood flow increase when a subject uses a finger to press a lever?

Answer 4

• Primary somatosensory cortex

- Primary motor cortex

Question 5

In what areas of the brain does blood flow increase when a subject performs a sequence of movements?

Answer 5

-Premotor cortex

Question 6

In what areas of the brain does blood flow increase when a subject use a finger to find a route through a maze?

Answer 6

-Prefrontal, temporal, and parietal cortex

Question 7

What area of the cortex is the precentral gyrus known as? What is the motor homunculus?

Answer 7

• The primary motor cortex
• Contains motor homunculus (different areas can be stimulated to cause movement in different areas of body)
• Disproportionate in relative sizes of body areas; areas that are larger have more motor control

Question 8

What is the motor cortex specialized for?

Answer 8

• Control of movements, rather than control of individual muscles
• Movements include: defensive postures of the face, movement of hand to mouth, manipulation and shaping of the hand, outward reach with the hand, climbing and leaping

Question 9

What is the evidence for a human movement lexicon?

Answer 9

• The pincer grasp (used to reach for small objects, babies begin to spontaneously make the movement)
• When lesions occur to the thumb region of the cortex, weakness appears in all fingers

Question 10

What are the functions of motor neurons?

Answer 10

• Calculate distance and direction of movement
• Each neuron is maximally responsive to a particular direction
• Activity is decreased in proportion to the displacement from the preferred direction

Question 11

What are mirror neurons? What function do the serve?

Answer 11

• Neurons that fire when we see others make a movement
• Encode a complete action that can be used for imitating and for understanding others’ actions
• Can “fill in the blanks” when part of movement is absent. Don’t fire when target object is missing
• Can have a broad range of responses
• In humans mirror neurons are typically located in the left hemishpere, they play a role in self-action, perception of action, self-awareness, and awareness of the intention and actions of others. Also important for gestures and verbal movement

Question 12

Basal ganglia damage causes what kind of disorders?

Answer 12

• Disorders of too much force (Huntington’s chorea and Tourette’s syndrome)
• Disorders of too little force (Parkinson’s disease) Involuntary movements vs. difficulty making movements

Question 13

What are the two pathways of the basal ganglia? How do they work?

Answer 13

• Excitatory (high excitation (excitatory > inhibitory) in the globus pallidus internal (GPi) which causes a decrease in the force of movements)
• Inhibitory (high inhibition (inhibitory > excitatory) of GPi which causes amplification of movement)

Question 14

What is the function of the cerebellum?

Answer 14

• Acquiring and maintaining motor skills
• Timing of movements
• Maintain movement accuracy

Question 15

How is the cerebellum involved in timing of movements? What are the two tests of timing?

Answer 15

• Cerebellum acts like a pacemaker and ensures that both movements and perceptions are appropriately timed
• Motor test of timing - tap a finger in rhythm with a metronome, metronome is turned off, attempt to keep tapping with same beat
• Perceptual test of timing - presented with two pairs of tones, silence between tone in second pair varies in length (longer or shorter than first)
• Patients with damage to cerebellum perform poorly on both tasks

Question 16

How is the maintenance of movement accuracy tested in a person?

Answer 16

• Throw darts at a target. Glasses displace target to the left
• Patients who are normal adjust their throws until accuracy is restored. Patients with cerebellum damage do not correct for error
• When the glasses are removed, the normal patients first throw darts too far right, and adjust until they regain their former accuracy. Cerebellum damage patients show no aftereffects and good aim because they never adjusted for glasses to begin with

Question 17

What are the two versions of movement? How does the feedback circuit work for this?

Answer 17

• Intended movement and actual movement
• Cortex sends instructions to spinal cord to throw a dart
• Copy sent to cerebellum through inferior olivary nucleus
• Actual movement sent through spinal cord to cerebellum
• Cerebellum calculates error and tells cortex how it should correct movement

Question 18

What is the organization of the cerebellum?

Answer 18

• Flocculus (a small lobe) projects from the ventral surface and takes part in balance
• Medial parts control face and midline of the body
• Lateral parts are associated with movements of the limbs, hands, feet, and digits

Question 19

Describe spinal cord motor neurons (their location, how they are arranged, how the muscle pairs in limbs are arranged)

Answer 19

• Located in ventrolateral spinal cord and just out to form the ventral horns
• Arranged as a homunculus
• Limbs are arranged in muscle pairs (extensor moves limb away from the body; flexor moves limb toward the body)