Lecture 11: Movement I - Lower and Upper Motor Neurons

Question 1

What is the motor cortex responsible for?

Answer 1

• Planning, initiating, and directing voluntary movements

Question 2

What are the brainstem centers responsible for?

Answer 2

• Rhythmic, stereotyped movements and postural control

Question 3

What is the cerebellum responsible for?

Answer 3

• Coordination of ongoing movement
• Manages prediction error in movement
• Detects and attenuates difference b/n intended movement and performed movement
• Coordination

Question 4

What is the basal ganglia responsible for?

Answer 4

• Initiation of intended movement and suppression of unwanted movement
• Impaired in Parkinson’s
• Prepare motor circuit for movement initiation and prevent unwanted movements
• Strong connectivity with motor cortex

Question 5

What are local circuit neurons responsible for?

Answer 5

• Sensorimotor integration and central pattern generation

- Sensory inputs synapse here

Question 6

What are motor neuron pools?

Answer 6

• Lower motor neurons
• Effectors of movement
• Synapse onto skeletal muscles

Question 7

What is motor control?

Answer 7

• Control of our muscles and how they are directed to perform actions and movements
  VOLUNTARY = skeletal muscle
  INVOLUNTARY = reflexes, visceral

Question 8

What are the 4 main motor subsystems?

Answer 8

• Lower motor neurons
• Upper motor neurons
• Cerebellum
• Basal ganglia

Question 9

What are lower motor neurons?

Answer 9

• Alpha motor neurons
• Cell bodies located in ventral horn of spinal gray matter, and in motor nuclei of cranial nerves in the brainstem
• Final common pathway for transmitting information from a variety of upper motor neuron sources to skeletal muscles
• Form distinct clusters (motor neuron pool) in the ipsilateral ventral horn
• Rod-shaped clusters down one or more spinal cord segments (arranged according to which muscle)

Question 10

What are upper motor neurons?

Answer 10

• Modulate activity of lower motor neurons by influencing local circuitry (interneurons)
• Cell bodies are in brainstem centers (vestibular nuclei, superior colliculus, reticular formation) and in the cortex
• Cortical UMN = planning and initiation of voluntary complex motor sequences, facial expressions, and speech
• Brainstem UMN = muscle tone and orientation of the head, eyes, body with respect to vestibular, somatic, auditory, and visual sensory information (react to stimuli and considering broader context)
• Descent from higher brain centers to influence output of LMNs via modulation of activity of local circuit neurons in brainstem/spinal cord
• Maintains somatotopic organization of spinal cord

Question 11

Where is the dorsal horn?

Answer 11

• Toward back

- Sensory

Question 12

Where is the ventral horn?

Answer 12

• Toward stomach

- Motor

Question 13

Where are cell bodies that innervate distal muscles located in the spinal cord?

Answer 13

• More lateral

Question 14

What are alpha motor neurons?

Answer 14

• Large motor neurons

- Innervate extrafusial striated muscle fibers that generate forces needed for posture and movement (contraction)

Question 15

What are gamma motor neurons?

Answer 15

• Interspersed among alpha motor neurons
• Smaller
• Innervate intrafusial muscle spindles

Question 16

What does extrafusial mean?

Answer 16

• Outside of spindles

Question 17

What is a motor unit?

Answer 17

• An alpha motor neuron and all the muscle fibres that it innervates
• Alpha motor neuron is excited/inhibited by many inputs
• An AP generated by motor neuron brings all contacted muscle fibers to contraction
• Determines whether alpha MN will reach threshold and generate force
• Important to understand what changes excitability of alpha MN because leads to force

Question 18

What are slow (S) motor units?

Answer 18

• Small units important for sustained muscle contration

- Ex. standing

Question 19

What are fast-fatiguable (FF) motor units?

Answer 19

• Large units that require large force for brief periods
• Fatigue quickly
• Ex. running

Question 20

What are fast fatigue-resistant (FR) motor units?

Answer 20

• Intermediate units that generate twice the force of S units, but are less fast than FF units
• Ex. walking

Question 21

What is Henneman’s size principle?

Answer 21

• Sensory input adds EPSPs and IPSPs onto MN to reach threshold
• Recruited in order (smallest to largest; S->FR->FF)
• When synaptic input to motor pool increases, progressively larger motor units that generate larger forces are recruited

Question 22

How does frequency of stimulation alter the amount of force produced by the muscle?

Answer 22

• At low frequency, each AP in motor neuron results in single twitch of related muscle fibers
• At higher f, twitches sum to produce a force greater than that produced by single twitches
• At even higher f, force is greater, but individual twitches are still apparent (unfused tetanus)
• At highest f, individual twitches are no longer apparent (fused tetanus)

Question 23

Under normal conditions, what does asynchronous firing of different LMNs provide?

Answer 23

• Steady level of input to muscle
• Causes contraction of relatively constant # of motor units
• Averages out changes in tension due to contractions/relaxations of individual motor units

Question 24

What is the muscle stretch reflex?

Answer 24

• Sensory response to muscle stretch which provides direct excitatory feedback to motor neurons innervating the muscle that has been stretched
• Originates in muscle spindles
• Stretching spindle increases activity in group Ia afferents resulting in reciprocal innervation
• Reflex arc maintains appropriate muscle length

Question 25

Describe synergistic muscles.

Answer 25

• Ia sensory afferents synapse onto alpha motor neurons that innervate the same muscle (homonymous)
• Also synapse onto alpha motor neurons that innervate synergistic muscles (Work in synergy to affect movement)

Question 26

Describe antagonistic muscles.

Answer 26

• Ia sensory afferents synapse onto inhibitory interneurons (called reciprocal-Ia-inhibitory interneurons) innervating alpha motor neurons that innervate antagonistic muscle (works in opposition)

Question 27

What kind of lower motor neurons does the medial ventral horn contain?

Answer 27

• Neurons that govern posture, balance, locomotion and orienting movements of head and neck during shifts of visual glaze
• Receive descending input from pathways that originate mainly in brainstem, course through anterior-medial white matter of spinal cord, terminate bilaterally

Question 28

What kind of lower motor neurons does the lateral ventral horn contain?

Answer 28

• Mediate expression of skilled voluntary movements of distal extremities
• Receive major descending projection from contralateral motor cortex via main/lateral division of corticospinal tract, which runs in lateral white matter of spinal cord

Question 29

How do corticobulbar axons travel?

Answer 29

• Originate in motor cortex
• Travel through internal capsule and come together in ventral midbrain in cerebral peduncle
• Continue through pons to reach medulla
• Bilateral innervation of brainstem nuclei (stop at spinal cord and bifurcate)

Question 30

How do corticospinal fibers travel?

Answer 30

• Originate in motor cortex
• Travel through internal capsule and come together in ventral midbrain in cerebral peduncle
• Continue through pons to reach medulla
• 90% cross caudal medulla to form lateral corticospinal tract
• Remaining 10% form ventral corticospinal tract
• Distal extremities are controlled by cortical area that is contralateral

Question 31

Where is the primary motor cortex located?

Answer 31

• Located in precentral gyrus

- Anterior to central sulcus

Question 32

Where is the premotor cortex located?

Answer 32

• Immediately anterior to primary motor cortex in frontal lobe

Question 33

Where do all motor cortices receive regulatory input from?

Answer 33

• Basal ganglia (initiating motor sequence)
• Cerebellum (coordination of ongoing movement and corrections)
• Via ventrolateral thalamus

Question 34

What is the topographical organization of cortical UMNs like?

Answer 34

• Rough
• Much more coarse than representation of contralateral body in primary somatosensory cortex
• Identifies brain regions of UMNs that control different groups of muscles, rather than individual muscles (movement specific, broader)

Question 35

What happens as a result of prolonged microstimulation of primary motor cortex?

Answer 35

• Elicits coordinated movements (not individual muscle contraction)
• Could elicit stereotyped movements in limbs of monkeys with electrodes and stimulation of single UMNs (similar pattern at different points of stimulation)
• Indicates that UMNs were connected by local circuits and these local circuits were operating in coordinated movements

Question 36

What is the premotor cortex involved in? Where does it receive input from?

Answer 36

• Preparation
• Extensive multisensory input from parietal cortex
• Receives complex prefrontal cortical inputs related to motivation/intention (executive function and decisions)
• Extensive reciprocal connectivity with primary motor cortex (work together)
• Axons that project through corticobulbar and corticospinal pathways to directly influence local circuits

Question 37

What are the differences between the premotor and primary motor cortex?

Answer 37

• 30% of axons in corticospinal tract arise from premotor cortical neurons
• More UMNs in primary motor cortex than premotor make monosynaptic connections to alpha motor neurons
• Action goals encoded by primary motor cortex tend to be localized to personal space
• Action goals encoded by premotor cortex localized toward extra-personal space (incorporates context in which movement will occur)
• Premotor cortical coding for intention to move, not movement command itself

Question 38

Where are mirror neurons located?

Answer 38

• Ventral-anterior section of lateral premotor cortex

Question 39

When do mirror neurons fire?

Answer 39

• During passive observation of human hand placing morsel of food on tray, as well as during execution of similar action to retrieve food
• Same neuron does not respond when food is placed with pliers, but it does fire during monkey’s reaching/retrieval when monkey is allowed to observe its reach
• Encodes observed actions of others