Lecture 11: Movement I - Lower and Upper Motor Neurons Flashcards
1
Q
What is the motor cortex responsible for?
A
- Planning, initiating, and directing voluntary movements
2
Q
What are the brainstem centers responsible for?
A
- Rhythmic, stereotyped movements and postural control
3
Q
What is the cerebellum responsible for?
A
- Coordination of ongoing movement
- Manages prediction error in movement
- Detects and attenuates difference b/n intended movement and performed movement
- Coordination
4
Q
What is the basal ganglia responsible for?
A
- 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
5
Q
What are local circuit neurons responsible for?
A
- Sensorimotor integration and central pattern generation
- Sensory inputs synapse here
6
Q
What are motor neuron pools?
A
- Lower motor neurons
- Effectors of movement
- Synapse onto skeletal muscles
7
Q
What is motor control?
A
- Control of our muscles and how they are directed to perform actions and movements
VOLUNTARY = skeletal muscle
INVOLUNTARY = reflexes, visceral
8
Q
What are the 4 main motor subsystems?
A
- Lower motor neurons
- Upper motor neurons
- Cerebellum
- Basal ganglia
9
Q
What are lower motor neurons?
A
- 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)
10
Q
What are upper motor neurons?
A
- 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
11
Q
Where is the dorsal horn?
A
- Toward back
- Sensory
12
Q
Where is the ventral horn?
A
- Toward stomach
- Motor
13
Q
Where are cell bodies that innervate distal muscles located in the spinal cord?
A
- More lateral
14
Q
What are alpha motor neurons?
A
- Large motor neurons
- Innervate extrafusial striated muscle fibers that generate forces needed for posture and movement (contraction)
15
Q
What are gamma motor neurons?
A
- Interspersed among alpha motor neurons
- Smaller
- Innervate intrafusial muscle spindles
16
Q
What does extrafusial mean?
A
- Outside of spindles
17
Q
What is a motor unit?
A
- 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
18
Q
What are slow (S) motor units?
A
- Small units important for sustained muscle contration
- Ex. standing
19
Q
What are fast-fatiguable (FF) motor units?
A
- Large units that require large force for brief periods
- Fatigue quickly
- Ex. running
20
Q
What are fast fatigue-resistant (FR) motor units?
A
- Intermediate units that generate twice the force of S units, but are less fast than FF units
- Ex. walking
21
Q
What is Henneman’s size principle?
A
- 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
22
Q
How does frequency of stimulation alter the amount of force produced by the muscle?
A
- 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)
23
Q
Under normal conditions, what does asynchronous firing of different LMNs provide?
A
- 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
24
Q
What is the muscle stretch reflex?
A
- 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
25
Q
Describe synergistic muscles.
A
- 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)
26
Q
Describe antagonistic muscles.
A
- Ia sensory afferents synapse onto inhibitory interneurons (called reciprocal-Ia-inhibitory interneurons) innervating alpha motor neurons that innervate antagonistic muscle (works in opposition)
27
Q
What kind of lower motor neurons does the medial ventral horn contain?
A
- 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
28
Q
What kind of lower motor neurons does the lateral ventral horn contain?
A
- 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
29
Q
How do corticobulbar axons travel?
A
- 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)
30
Q
How do corticospinal fibers travel?
A
- 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
31
Q
Where is the primary motor cortex located?
A
- Located in precentral gyrus
- Anterior to central sulcus
32
Q
Where is the premotor cortex located?
A
- Immediately anterior to primary motor cortex in frontal lobe
33
Q
Where do all motor cortices receive regulatory input from?
A
- Basal ganglia (initiating motor sequence)
- Cerebellum (coordination of ongoing movement and corrections)
- Via ventrolateral thalamus
34
Q
What is the topographical organization of cortical UMNs like?
A
- 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)
35
Q
What happens as a result of prolonged microstimulation of primary motor cortex?
A
- 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
36
Q
What is the premotor cortex involved in? Where does it receive input from?
A
- 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
37
Q
What are the differences between the premotor and primary motor cortex?
A
- 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
38
Q
Where are mirror neurons located?
A
- Ventral-anterior section of lateral premotor cortex
39
Q
When do mirror neurons fire?
A
- 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
