Lecture 14

Question 1

What is neural control of striated muscle contraction orchestrated by?

Answer 1

Lower and upper motor neurons

LMN- spinal or brainstem motor neuron; directly innervates muscle (primary, α)
Lower motor neurons actually contact muscles themselves

UMN- gives rise to a descending projection that controls the activity of LMNs in the brainstem and spinal cord (initiation of voluntary movements)
Upper motor neurons control lower motor neurons (synapse on them)

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Question 2

What are the 2 major groups of descending spinal tracts?

Answer 2

Lateral pathway- voluntary movement of distal muscles under direct cortical control

Ventromedial pathways- control of posture and locomotion under brainstem control
Control posture, locomotion

Slide 4

Question 3

What is the somatotopic organization in the ventral horn?

Answer 3

Slide 5
Lateral motor system
Medial motor systems

Question 4

What is the pathways of descending motor control?

Answer 4

Slide 6-7
Lateral ventral horn has LMNs that mediate expression of skilled voluntary movements of distal extremities
They receive major descending projection from contralateral motor cortex via corticospinal tract
Medial ventral horn has LMNs that govern posture, and orientating movements of the head and neck during shifts of visual gaze
They receive descending input from pathways that originate mainly in the brainstem, course through the anterior medial white matter of the spinal cord and terminate bilaterally

Question 5

What is the corticospinal tract path?

Answer 5

Motor cortex
Internal capsule
Midbrain
Pons
Medullary pyramids
Pyramidal decussation 
Lateral column of the spinal cord
Terminate on LMNs in the lateral region of the ventral horns that control the movements of the distal extremities

Slide 8

Question 6

What are the ventromedial pathways?

Answer 6

4 ventromedial pathways use sensory info about balance, body position, and visual environment to reflexively maintain balance and body posture

Vestibulospinal tracts
Tectospinal tract
Pontine (medial) reticulospinal tract
Medullary (lateral) reticulospinal tract

Slides 9-10

Question 7

What are the vestibulospinal tracts?

2 types

Answer 7

Bilateral projection- vestibular nuclei to medial ventral horn, head position by reflex activation of neck muscles

Ipsilateral projection- upright balanced posture, proximal extensor (antigravity) muscles
For balance and holding yourself up

Slide 9

Question 8

What is the tectospinal tract?

Answer 8

Superior colliculus -> decussation -> contralateral termination- orienting response of head and eyes in response to primary visual stimuli

Slide 9

Question 9

What is the pontine reticulospinal tract?

Answer 9

Medial
Originated in the pontine reticular formation- enhances antigravity reflexes of the spinal cord by facilitating extensors of lower limbs

Slide 10

Question 10

What is the medullary reticulospinal tract?

Answer 10

Lateral
Medullary reticular formation- opposite effect, it liberates the antigravity muscles from reflex control

Slide 10

Question 11

What is the feed forward mechanism used to stabilize posture?

Answer 11

Motor centres in the reticular formation initiate fred forward adjustments that stabilize posture during ongoing movements
Anticipatory maintenance of body posture

Slide 11

Question 12

What is paresis?

What is paralysis?

Answer 12

Paresis- partial damage (weakness)
Paralysis- complete severing (loss of movement), areflexia (absence of spinal reflexes), atrophy (loss of size)

Damage to upper parts of motor system (motor cortex, brainstem, motor tracts) starts an immediate period of spinal shock where functions of the spinal cord appear shut down: reduced muscle tone, etc

Slide 12

Question 13

What is the homunculus?

Answer 13

Electrical stimulation of the motor cortex elicits contractions of muscles and the cortex contains a completes disproportionate representation or map of the body’s musculature

Musculature used in tasks requiring fine motor control occupies a greater amount of space in the motor map

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Question 14

What do motor maps represent?

Answer 14

Organized movements rather than individual muscles are represented

Neurons in nearby regions are linked by local circuits in the cortex and spinal cords to organize these movements

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Question 15

Where are purposeful movements mapped?

Answer 15

Purposeful movements are mapped in the primary motor cortex and their somatotopic organization is best understood in the context of ethologically relevant behaviours

Slide 18

Question 16

What is broad directional tuning?

Answer 16

M1 cells firing across a range of angles but most firing at a certain angle

The activity of each cell is represented as a direction vector pointing in the direction that was best for that cell; the length of the vector represented how active that cell had been during a particular movement

Slide 19

Question 17

What is population coding for movement direction?

Answer 17

Movement direction encoded by collective activity of a population of neurons
Both cells fire during movement in a range of directions
Cell 1 fires best upward movement
Cell 2 responds best when movement is from left to right

For any direction, direction vectors of individual cells are averaged to yield a population vector, reflecting strength of the response of both cells during this movement

Slide 20-21

Question 18

What are malleable motor maps?

Answer 18

Cortical reorganization may provide a basis for learning fine motor skills because cortical cells in M1 can switch allegiance from participation in one type of movement to another as skills are learned

Slide 22

Question 19

What are the cortical areas involved in planning and directing voluntary movements?

Answer 19

The highest level- prefrontal and association areas of cortex are concerned with the goal of the movement and the movement strategy that beat achieves the goal

Posterior parietal cortex mental body image generation by somatosensory, proprioceptive, visual inputs

Area 6 controls the planning of complex movements premotor area (PMA) supplementary motor area (SMA)

Motor cortex (M1) sequences of muscle contractions, arranged in space and time, required to smoothly and accurately achieve the strategic goal

Slide 23-24

Question 20

What is the neuronal activation before a movement?

Answer 20

PMA and SMA play an important role in the planning
These areas lie at junction where signals encoding what actions are desired are converted du to signals that specify how the actions will be carried out

Movement strategies are devised and held until executed in the areas

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Question 21

What is the mirror motor neuron?

Answer 21

Neurons that respond not just in prep for execution of particular movements (precision grip to retrieve food), but also when the same action is observed, being performed by another (monkey or human) play a role in encoding the actions of others

Mirror motor neuron fires during the passive observation of a human hand placing the morsel of food on the tray as well as during the execution of a similar action to retrieve the food

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