lecture 13

Question 1

What is the upper motor neuron?

Answer 1

• really any motor neuron that controls the excitability of the lower motor neuron

Question 2

What is an important consideration in regards to nervous activity at any one time?

Answer 2

• usually inhibitory
• if you want to excite something, often done by removing the inhibitory signal - disinhibition
• reflex response becomes abnormally strong when normal inhibition from the brain is removed

Question 3

Where are descending motor control axons distributed?

Answer 3

• lateral white matter = axons from motor cortex, sometimes make direct synaptic connections to motor neurons, quite uncommon and really only in humans/primates. Tend to excite distal muscles more.
• medial/ventral white matter = axons from brainstem. Tend to make connections to interneurons that control proximal muscles more.
• usually synapse onto interneurons, which are usually inhibitory

Question 4

Do upper motor neurons cross the midline?

Answer 4

• general pattern that the ones that descend laterally to excite neurons related to distal limb muscles tend to cross the midline further up (i.e. right cerebral cortex for left hand)
• medial pools with upper motor neurons from brainstem tend to be ipsilateral (i.e. same side) but tend to spread across both sides at the location.

This makes sense because you want the midline muscles to be cohesive while you want distal muscles to be independent.

Question 5

Where do medial pool upper motor neurons come from?

Answer 5

• the brain stem (medulla, pons and midbrain; just above the spinal cord)
• ## vestibular nuclei (in the pons)- mostly into same some but some, probably via interneurons, would excite motorneurons on the other side, ipsilateral

Question 6

What tells us about our posture?

Answer 6

• balance organs in inner ear - vestibular organs, tell us about our relationship with gravity and acceleration
• pontine and medullary reticular formation: reticulospinal tract, big cells that are spread out with big, branching dendrites, goes from medulla, pons and through to mid brain though motor control tends to be pons/medulla, these neurons are associated with activating spinal cord for particular patterns of movement, ipsilateral, interneurons spread signal to medial part of grey matter
• colliculospinal tract: superior colliculus in mid brain, crosses in the brain stem and comes down contralaterally, sensory info integrated from vision, touch and hearing, synthesis of 3D space in the colliculus e.g. look in the direction of a noise heard. e.g. Visual looming would indicate falling down

Question 7

What is anticipatory maintenance of body posture?

Answer 7

• experiment where someone pulls on a level after the sounding of a tone
• biceps contract ~200ms after tone
• calf muscles contract ~100ms after tone
• calf muscles obviously don’t need to contract to pull a lever
• this is a postural control mechanism because if you pulled on lever without pulling on calves you would smack head into wall
• there is a threat to stability so the CNS stabilises posture in anticipation to the event occuring
• why rely on correcting things when they go wrong when you can anticipate outcomes and prevent them

Question 8

What are the feedforward and feedback mechanisms of postural control?

Answer 8

• central command: feedforward for anticipated postural instability –> limb movement and postural adjustment
• limb movement leads to unanticipated postural instability that feeds back to allow for postural adjustment

Question 9

Where does input to brainstem come from?

Answer 9

• sensory input
• voluntary motor centres in motor cortex - creating postural commands that are part of voluntary movement that we don’t think about

Question 10

Where are the primary motor cortex and premotor areas?

Answer 10

• primary motor cortex: in front of central sulcus, precentral gyrus, contralateral
• premotor cortex/motor association areas : in front of primary motor cortex, area that had more complicated sequences of movements

Question 11

What is the cytoarchitectonic appearance of the primary motor cortex?

Answer 11

• histologically defined by the presence of Betz cells in layer 5 (and sometimes 6)
• cells that project all the way through the brain, down the spinal cord etc
• also non-betz pyramidal cells

Question 12

What are the corticospinal and corticobulbar tracts?

Answer 12

• majority (90%) of axons coming from cortex decussate at medulla
• they don’t cross over in a big bundle - cross over in groups at different locations
• travel in lateral tract
• about 10% of corticospinal fibres travel in the ventral tract
• some fibres terminate in the brainstem – going to motor neurons in the brainstem (lower motor neurons that innervate the head

Question 13

Where are different parts of the body controlled in the primary motor cortex?

Answer 13

• face = lateral
• hands
• arm
• hip
• knee
• ankle
• toes = medial

simplistic
it is more like broad regions such as face/upper extremity etc

Question 14

do we get purposeful movements being coded for in the primary motor cortex?

Answer 14

• yes e.g. experiment with monkey
• stimulating one bit always brought the hand to the face regardless of start point
• similarly, stimulating another bit brought hand to chest
• important movements

Question 15

What do motor maps represent?

Answer 15

• hand to mouth
• defensive
• central space/manipulation
• reach
• outward arm movements
• climbing/leading
• i.e. we get specifc clusters of functions
• cerebrocortical neurons represent meaningful movements, not muscles not joints etc etc

Question 16

Do we have a tuning curve for a neurons involvement in a particular movement?

Answer 16

• yes
• e.g. monkey pushing lever in specific directions
• neuron active for a range of angles (~180º) but not the other
• very broad
• neurons aren’t very specific
• somehow ensembles of neurons with broad tuning curves get specific, accurate movements
• it is when neurons act together that you get well defined movements, acting on their own, upper motor neurons are not very specific/don’t really do anything

Question 17

What is the frontal eye field?

Answer 17

an area of the the motor associated region that specifically controls eye movement which is important to deciding further actions

Question 18

What are mirror motor neurons?

Answer 18

• a neuron activated when the monkey was watching someone doing a task
• this is how we learn things
• learn by recognising the things we see in the environment

Question 19

What is the Babinksi sign?

Answer 19

• if you trace an object up a person’s foot you will get plantar flexion
• if you trace it up a baby’s foot they have extensor plantar response
• changes as the baby learns to walk
• if you see it in an adult it is called the babinski sign –> indicative of a lack of upper motor inhibition of spinal cord

Question 20

What are signs and symptoms of upper and lower motor neuron lesions?

Answer 20

Lower motor neuron syndrome:

• When the lower motor neuron doesn’t work, muscle doesn’t work
• weakness or paralysis
• decreased superficial reflexes
• hypoactive deep reflexes
• decreased tone
• fasciculations and fibrillations
• severe muscle atrophy

Upper motor neuron syndrome

• weakness
• spasticity:
• • increased tone
• • hyperactive deep reflexes
• • clonus
• Babinski’s sign
• loss of fine voluntary movements