this was the second unit in the motor system (apologies for not getting to motor systems part 1 in time for the last test) and dealt with the motor pathways that start in the CNS, the different motor cortices and their roles, and some aspects of higher motor processing.
upper motor neurons originate in various places within the brain and project downwards through various descending tracts: the lateral corticospinal tract (recall that this was the major descending motor pathway from the spinal cord unit) represents 90% of the corticospinal tract which decussates in the medulla, follows the lateral funiculus, and is involved in fine movements. the remaining 10% does not cross over, follows down the anterior funiculus, and is involved in trunk muscles and posture. the vestibulospinal is a descending tract which receives information from the vestibular portion of the vestibulocochlear nerve and projects down to the spinal cord, ultimately to the flexors of the lower limb and extensors of upper limb. the reticulospinal is another descending tract which projects down to trunk muscles, extensors of lower limb, and flexors of the upper limb.
lower motor neuron disease happens when alpha motor neurons are damaged and thus dysfunctional motor innervation leads to muscle atrophy and flaccid paralysis. on the other hand, upper motor neuron disease occurs because a stroke in the higher motor centers disrupts the descending tracts' communication with cranial nerves and spinal motor neurons.
there are different motor cortices which all play different roles in processing and executing motor tasks. for example, the association cortex is involved in setting up tasks or sequences of motor programs, which it then sends to the primary motor cortex which executes the programs by contraction of fine muscles. the supplementary cortex is involved in mental rehearsal of the motor task, regardless of the external environment (also regardless of whether the action is actually performed). the premotor cortex is involved in "conditional motor tasks"-- deciding the appropriateness of movements and encoding intention behind them. broca's area is a part of the prefrontal cortex which controls the motor aspects of speech production.
different areas of the motor cortex are also involved in higher processing of motor information with incoming sensory input to both predict the future states of motion or position of a body part, as well as generally create the sensations of self-agency, ownership, and thus contributes to self-awareness. the motor cortex receives sensorimotor input from the posterior parietal cortex and visual cortex, which it then integrates with the motor program and feeds an "efference copy" into the somatosensory cortex for prediction of future limb states. when the efference copy matches the intention of a person, a feeling of self-agency is created; the sense that one is in control of one's actions- that the predicted body position or movement has occured. when incoming sensory data matches the intention, this creates the feeling of self-ownership.
another higher order function of the motor cortex occurs with mirror neurons, which are in the "core mirror area": inferior premotor cortex and inferior parietal cortex. these neurons were originally discovered in monkeys and are shown to fire when both performing certain movements as well as simply observing them. it is proposed that in humans they are fundamental in acquiring social behavior and thus the development of mirror neurons might have been an evolutionary landmark that allowed us to adapt and acquire behaviors that facilitated survival simply by observing and imitating.
questions
overview...
1. what do upper motor neurons control?
2. which areas of the brain have upper motor neurons that mediate balance, posture, and fine extremity movement?
3. what are the descending tracts in the spinal cord related to motor control?
4. what are the ascending tracts in the spinal cord related to motor control?
5. what does the vestibulospinal nuclei in the medulla receive and project?
6. what does the vestibulospinal tract activate in the upper and lower limbs?
7. what does the reticulospinal tract project down to?
descending tracts...
8. corticospinal tracts includes tracts to...
9. what are the three tracts that the motor cortex forms?
10. where does the somatosensory cortex send tracts down to?
11. how does the corticoreticular tract smooth general movements?
12. describe the lateral corticospinal tract.
13. describe the anterior corticospinal tract.
14. what other CNS structures do the corticospinal tracts pass through?
motor neuron disease...
15. what is lower motor neuron disease?
16. what is upper motor neuron disease?
17. why does upper motor neuron disease lead to a decrease in fine control of movement?
18. why does upper motor neuron disease lead to spasticity?
19. what does spasticity involve?
20. what is the babinski sign?
21. what is the babinski sign due to and who is it generally found in?
primary, association, supplementary, premotor cortices...
22. what role does the association motor cortex play in producing movements?
23. what role does the primary motor cortex play in producing movements?
24. where is the primary motor cortex located?
25. what is the role of the supplementary motor cortex in producing movements?
26. what is the role of the premotor cortex in producing movements?
27. what is broca's area?
higher functioning...
28. what does selecting appropriate movements require of the motor cortex?
29. where does the motor cortex receive information from during sensorimotor integration?
30. why are copies of the motor program fed back into the somatosensory cortex?
31. what does self-awareness or self-recognition depend upon?
32. what is the forward model?
33. what is self-agency and how is it manifested in the brain?
34. what is self-ownership and how is it generated in the brain?
35. what are mirror neurons?
36. what is the "core mirror area" in humans?
37. mirror neuron activity often represents what aspects of observed behavior?
answers
1. posture, balance, movements.
2. brainstem has pathways for balance and posture. motor cortex has fine extremity movement pathways.
3. lateral and anterior corticospinal tract, vestibulospinal, reticulospinal tracts.
4. spinocerebellar, gracile/cuneate fasciculi.
5. receives input from vestibular system (semicircular duct, utricle, saccule) about head position and movement, relays to spinal cord.
6. extensors in upper limb, flexors in lower limb.
7. trunk muscles, flexors of upper limb and extensors of lower limb.
8. brainstem and spinal cord.
9. corticospinal to the ventral spinal cord, corticonuclear to the cranial nerve nuclei, and corticoreticular to the medullary and pontine reticular formation.
10. to the brainstem to regulate sensitivity of sensory pathways.
11. by limiting inhibition in the extensors of the lower limb.
12. represents the 90% of corticospinal tract that crosses over in the medulla and goes down the lateral funiculus. fine control of movement.
13. represents the 10% of corticospinal tract that does not cross over; goes into the anterior funiculus and projects bilaterally. posture of neck and trunk.
14. the internal capsule, cerebral peduncles, and pyramids in medulla.
15. lesions in the alpha motor neurons blocking motor input to muscles, resulting in atrophy and flaccid paralysis.
16. a stroke in the motor cortex disrupting descending pathways to the cranial nerves and spinal motor neurons.
17. because of the disruption of the corticospinal tracts.
18. because of the disruption of the cortical projections to the reticular formation.
19. hypertonicity, clasp knife reflex, hyperreflexia, antigravity posture. (gravity causes the knife reflex in tonic water)
20. the fanning of toes due to upward stroking of the sole of the foot.
21. present in newborns, due to unmyelination of corticospinal tracts
22. develops strategies and programs/sequences of movements, which are then sent to the primary motor cortex.
23. activates small groups of muscles for fine movements.
24. in the precentral gyrus.
25. ensures correct motor sequences independent of external conditions; activated during mental rehearsal of movements.
26. involved in encoding intention of a movement and selection of movements based on environment or memory.
27. a part of the premotor cortex that controls motor aspects of speech production.
28. integration of the spatial aspects of environment with proprioceptive and somatosensory data.
29. the posterior parietal and visual cortex.
30. for integration with incoming sensory input to predict future sensation and limb position / movement.
31. sense of agency and ownership, possible through the integration of intention and sensory feedback in the "forward model".
32. the combination of the efference motor program copy, incoming sensory information, and internal model of the dynamics of the limb to predict the current state of the limb.
33. knowing that one's intentions lead to external actions; occurs when efference copy matches subject's intentions.
34. the sense that you are the one who is undergoing the experiences, generated when sensory feedback matches intentions.
35. the neurons that fire during an action as well as the observation of that action in others.
36. the inferior premotor cortex, and inferior parietal cortex.
37. the intention or goal of the action.
No comments:
Post a Comment