Lecture 6 - Motor Systems and Organization of Action

Question 1

What are the 3 types of movement?

Answer 1

Reflexive –> peripheral stimuli produces involuntary coordinated pattern of muscle contraction/relaxation (stretch reflex)
Rhythmic –> can be spontaneous or due to peripheral stimuli; repetitive (chewing, scratching)
Voluntary–> goal directed movement, improves with practice

Question 2

How is the spinal cord nerves organized?

Answer 2

31 nerves: cervical, thoracic, lumbar, sacral, coccygeal
Cervical: controls upper limb muscles
Thoracic: controls trunk and chest musculature
Lumbar: controls lower back and leg muscles
Sacral: controls bowel and bladder muscles

Question 3

What are the 4 types of spinal cord neurons

Answer 3

Local –> axons confined to adjacent spinal segments
Motor –> axons innervate muscle fibres; organized in a distal (lateral) to proximal (medial) fashion
Propriospinal –> axons connect distal spinal segments
Projection –> axons reach higher brain centres

Question 4

Describe motor units

Answer 4

One motor unit = motor neuron + all fibres it innervates
one motor neuron integrates multiple fibres, each fibre innervated by one neuron
units can be fast or slow twitch
size of motor units relates to amount of control needed (finer selection of motor fibres to contract if smaller units)
Number of motor units recruited related to force generated

Question 5

Describe how a muscle is innervated

Answer 5

Axons loses myelin sheath, branches into multiple extensions called synaptic boutons. It connects to a portion of the muscle membrane called the endplate. Boutons are positioned over junctional folds (deep depression in the post-synaptic muscle fiber) that have Ach receptors. Depolarization of axon causes Ca2+ to enter boutons, causes release of Ach. Ach crosses synaptic cleft, binds to the Ach receptors and depolarizes end plate to create potential. Ach receptors equally permeable to Na and K, but potential activates the voltage-gated Na+ receptors, which creates an action potential.

Question 6

How is muscle force increased

Answer 6

recruit more motor units
increase frequency of action potentials, results in complete tetany (twitch–> summation–> incomplete tetany–> complete tetany)

Question 7

What are spinal reflexes?

Answer 7

neuronal networks in spinal cord integrating responses to produce coordinated pattern of muscle contraction through divergent and convergent connections

Question 8

Explain the withdrawal and crossed-extension reflex

Answer 8

Nocioceptor (in foot) senses painful stimulus, sensory neuron relays signal to spinal cord. Neuron diverges, one pathway goes up to higher brain centres, another activates flexion reflex (excites flexors in leg, inhibits extensors). Crossed-extension reflex also occurs (extensors excited) to shift weight onto that side to maintain balance

Question 9

How does the spinal region control reflexes?

Answer 9

There is a resting membrane potential, and a threshold needed to produce action potential. Spinal cord changes resting membrane potential. Can have tonic excitatory input to raise resting potential, means that stimulus input will be more likely to generate potential. Can also be modulated to decrease reflex sensitivity.

Question 10

How are postural reflexes organized?

Answer 10

SLR, MLR, LLR –> short, medium, long latency response
integrates response at different regions
SLR=spinal cord, nonfunctional
MLR = cerebellum, brain stem
LLR = cortex (functional) modulates response, knows more about body state

Question 11

Describe task dependent reflexes

Answer 11

depends on task. Table vs. holding cup, amount of extensor activity changes

Question 12

Describe 7 neonatal reflexes and why we don’t see them with age, and the study showing off the smart spinal cord

Answer 12

1) Babinski= touch bottom of foot, toes fan out, big toe raised. test for spinal cord injury
2) STARtle reflex = star response due to sudden visual/auditory stimulus, arms/legs outwards, upwards, inwards, fists/feet open and clench
3) Tonic neck reflex = head turned to side –> fencer’s pose, arm and leg to side that head faces is extended, other side flexes
4) Grasp reflex = pressure on palm/fingers, results in grasping action
5) Swimming reflex = belly touches water/ground = rhythmic alternating contraction/extension of arms/legs
6) walking reflex = feet in contact with surface = lift and plant feet
7) Righting reflex = when sit baby up, attempt to keep head upright

Stop seeing because cortical regions develop, start to have descending pathway modulation, also start to have more control over movement so no need for reflex

Frog experiment –> apply stimulus to one area, spinal cord cut, spinal cord can still control muscle to wipe that area –> shows spinal integration and control of movement

Question 13

What is a CPG, how is it controlled

Answer 13

neuron network in the spinal cord that generates rhythmic pattern of reciprocal flexion and extension in absence of sensory input

Initiated by MLR (mesoencephalic locomotor region). walking initiated. see hip proprioception, hip flexor is stretched, causes burst of flexion (swing phase). when foot lands, GTO senses load, sends signal through 1b afferents to maintain extensor activity through positive feedback loop, creates flexor stretch in opposite leg.

Proven through cat –> flexion/extension alternate, sudden stretch of hip flexor causes abortion of extension, flexion triggered early

Question 14

How do neurons project to the spinal cord?

Answer 14

medial or lateral pathways
medial=control basic posture, axial/proximal muscles
reticulospinal, vestibulospinal, tectospinal
lateral = goal directed, voluntary movements, limb muscles
rubrospinal, corticospinal

Question 15

Describe 3 pathways

Answer 15

reticulospinal –> maintains posture and muscle tone
vestibulospinal –> maintains balance, orientation of head/body
rubrospinal –> excites neurons innervating proximal upper limb flexors

Question 16

What is the function of the cerebellum?

Answer 16

Receives feedback, comparison of motor plan with actual response
Involved in learning new motor skills
Produce smooth motion (timing)
Coordinates groups of muscles with cerebral cortex
Maintains equilibrium
Fine-tunes locomotor pattern by regulating timing/intensity of descending signals

Question 17

List name and function of 4 basal loops

Answer 17

Occulomotor loop –> controls eye movements
Motor loop –> movement, links to primary motor cortex, premotor cortesx
Prefrontal loop –> decision making, linked to DLPC
Limbic loop –> related to emotions

Question 18

What does the thalamus do?

Answer 18

connects regions

Question 19

What is the function of the PPC?

Answer 19

Sensorimotor transformations –> integrate sensory input from different coordinate systems and convert to motor coordinates, reference frame suitable for motor centre
Planning, on-line control of movement
Spatial maps/working memory

Question 20

What are the different PPC regions and their function?

Answer 20

Parietal arm field –> retinotopic locations of spaces that can actually be reached
Parietal eye field –> retinotopic location of objects, responds to auditory and visual input
parietal face field –> represents ultra near space of mouth/face through visual and tactile input
parietal grasp field –> shape information required to grasp objects

Question 21

When are neurons in the PPC active?

Answer 21

During foreperiod –> planning movement

Memory delay task –> PPC heavily active between cue and reach to plan movement

Question 22

How does the cortex project to the motor neurons?

Answer 22

corticobulbar and corticospinal fibres
corticobulbar –> controls motor nuclei in brainstem for facial muscles
corticospinal –> controls motor nuclei in spinal cord for voluntary control of trunk/distal limb muscles–> acts on spinal pathway or motoneurons
most fibers cross at pyrimidal decussation for contralateral control

Question 23

What is the function of the SMA?

Answer 23

supplementary motor area selects movement trajectories based on internal cues
involved with memory
important for bimanual movements
role in learning sequences

Question 24

What is the function of the premotor cortex

Answer 24

selects movement trajectories based on external cues
works with basal ganglia
role in decision making
Spatial choices experiment –> must plan for all trajectories, so heavily active before go signal, after go signal = burst to plan/select that trajectory. In one-target task, only has to plan for one trajectory, motor cortex neuron doesn’t fire until after go signal (no double hill)

Question 25

What is the function of the motor cortex?

Answer 25

Provides somatotopic organization
executes and adapts movements
neurons in the same column activate synergistic muscles, therefore neurons in different columns can activate same muscle

Question 26

How do individual motor cortex neurons affect force/direction of movement?

Answer 26

ex. neuron attached to flexor muscles.
When flexor load applied, neuron fires to activate flexors and oppose movement
when extensor load applied, neuron is silent b/c extension due to relaxation of antagonist

Neurons fire different amount based on direction of movement, therefore movement is determined by resolution of vectors of the firing rate of groups of neurons

Question 27

How does the cortex contribute to locomotion?

Answer 27

Motor cortex increased activity during step (enhanced muscle activity)
PPC increase before step (motor planning, gait modification)

Question 28

What does the DLPC do?

Answer 28

Dorsolateral prefrontal cortex involved in spatial working memory
role in decision making (how/when to move)
sensorimotor transformation

Question 29

Describe APAs

Answer 29

Anticipatory Postural Adjustments
postural change before postural disturbance to offset movement
maintains equilibrium and stabilizes position (minimizes disturbance)
Anticipates shift in centre of mass
Have a load–>unload tendency (kip-up) so COP has shift one way in order to shift the other way and thus make it easier for movement to occur (want to lift right leg, so shift onto right, then shift centre of mass to left to raise right leg. hip and shoulder shift to suport side, ankle shifts to moving side)

APAs are context dependent. Voluntary movement generates APA, but having support negates APA. Similarly, if there is an external stimulus, action in stabilizers more as a reflexive movement, not a true APA. And if stability constraints help with movement (moving floor), APA is redundant and can create balance deficit