Problem 1 - Action Flashcards
The motor control hierarchy can be represented by 3 distinct levels from high to low respectively.
Name them.
- Association areas of the neocortex + basal ganglia of the forebrain
–> strategy/goal of the movement
- Motor cortex + cerebellum
–> tactics/sequences of motor movements
- Brian stem + spinal cord
–> execution/activation of motor + interneuron pools
Role of the Lateral pathways
Are involved in voluntary movement of the distal musculature + reflex modulation
–> are under direct cortical control
a) Corticospinal tract
b) Rubrospinal tract
Role of the Ventromedial pathways
Are involved in the control of posture and locomotion
–> are under brain stem control
Corticospinal tract
Lateral pathway
Consists of axons that exit the cortex + directly project to the spinal cord
- -> originates in the neocortex + is the longest and largest one
- -> 2/3 of its axons originate in the motor cortex (area4)
- -> the most important component of LP
- -> also called pyramidal tract
Extrapyramidal tract
Refer to the motor pathways within the brainstem that send direct projections down the spinal cord (involuntary)
–> vestibular, reticular formation nuclei, substantia nigra
BUT: Not part of the pyramidal tract
Rubrospinal tract
Lateral pathway
- Originates in the red nucleus of the midbrain
- Then decussate from the pons to join the axons of the corticospinal tract (=indirect)
–> smaller component of the lateral pathway, that gains larger role when CST is lesioned
What do lesions of the lateral pathways lead to ?
Inability to make fractioned movements of the arms + hands
–> no independent movements of shoulders, elbows, wrists + fingers
BUT: Corticorubrospinal pathway is able over time to partially compensate for any loss of the corticospinal tract input
Ventromedial pathways
Contain 4 descending tracts that originate in the brain stem + terminate in the spinal interneurons that contain proximal + axial muscles
–> uses sensory info about balance to maintain it
a) Vestibulospinal tract
b) Tectospinal tract
c) Pontine Reticulospinal tract
d) Medullary Reticulospinal tract
Vestibulospinal tracts
Originate in the vestibular nuclei of the medulla
–> relays sensory info to the ear
Name the 2 components of the vestiulospinal tracts and their respective functions.
- Projects bilaterally down the spinal cord to activate cervical spinal circuits
–> guiding head movements
- Projects ipsilaterally to the lumbar spinal cord
–> maintaining an upright + balanced posture
Tectospinal tract
Originates in the superior colliculus of the midbrain, that receives direct input from the retina
–> creating a map of the environment
Where does the superior colliculus receive its inputs from ?
- Retina
- Visual cortex
- Afferent axons receiving somatosensory + auditory info
–> thus constructs a map of the world
Reticulospinal tract
Originates from the reticular formation
–> has 2 descending tracts crucial for motor control
- Pontine reticulospinal tract
- Medullary reticulospinal tract
Pontine reticulospinal tract
Enhances the antigravity reflexes of the spinal cord
Medullary reticulospinal tract
Liberates the antigravity muscles from reflex control
–> has the opposite effect to pontine RT
Which regions of the frontal lobe are called the motor cortex ?
- Area 4 (Primary motor cortex/M1)
- -> lies anterior to the central sulcus, on the precentral gyrus - Area 6
- -> lies anterior to area 4
How did Penfield discover that there is somatotopic organization in the pre-central gyrus ?
By electrically stimulating these regions and discovering a twitch of the muscles in a particular region of the body on the CONTRALATERAL side
- What is area 6 specialized for ?
- It also includes 2 somatotopically organized motor maps.
Name them.
- Skilled voluntary movement
- -> desired actions are translated into how the actions will be carried out (planning)
2.
a) Premotor area (PMA)
b) Supplementary motor area (SMA)
BUT: both perform similar function but on different groups of muscles
Premotor area (PMA)
Refers to the lateral region of area 6
–> connects with reticulospinal neurons that innervate PROXIMAL motor units, visually guided movements
Supplementary motor area (SMA)
Refers to the medial region of area 6
–> connects directly with DISTAL motor units
Which cortical regions are involved in creating a mental image of the body in order to be able to execute movement ?
Inputs to the Posterior parietal cortex, especially
- Area 5
- -> receive inputs from primary somatosensory areas 1,2,3,7
Which regions represent the highest levels of the motor control hierarchy ?
Why ?
- Regions of the anterior frontal lobes + Posterior parietal cortex
- Here, decisions are made about what actions to take + their likely outcome
–> both send signals that converge on area 6
Describe the firing of the relevant neurons when planning + executing a moment.
(Ready, Set, Go)
- Ready
- -> frontal + posterior parietal lobes - Set
- -> increased firing of SMA + PMA neurons (area 6) before the movement - Go
- -> firing of the PMA + SMA during the movement, stop when it ends
Ventral lateral nucleus
VL
Refers to a nucleus of the dorsal thalamus
–> generates major subcortical input to area 6, by receiving signals from the basal ganglia
Name the brain regions and their consecutive places in the loop of where information travels to initiate movement.
Info travels from
- Cortex
- Basal ganglia + Thalamus
(Striatum, Globus P., VLo) - Cortex ( esp. SMA)
–> therefore, selecting + initiating voluntary movements
Striatum
Refers to the Caudate + Putamen
–> is the target of the cortical input to the basal ganglia
Function of the Globus pallidus
Source of the output to the thalamus
Motor strip
Refers to the full length of the precentral gyrus
Layer 5 of M1 is heavily interconnected with SMA.
Name its Input vs Output stations.
- Has pyramidal neurons that receive their inputs from 2 sources
a) Thalamus
- -> VLc (part of VL), that relays info from cerebellum
b) Other cortical areas
- -> area 6 + areas 1,2,3 (areas adjacent to area 4)
- Sends outputs to
a) Spinal cord (directly
b) Subcortical sites involved in sensorimotor processing
- -> esp. brain stem
What does the activity of each M1 cell suggest/mean when it comes to the execution of voluntary movement ?
Each cell represents a single vote for a particular direction of movement (=direction vector)
–> as suggested by “Centre out task”
What determines the direction of a particular movement ?
The average of the votes registered by each cell of the M1 population (=population vector)
–> the larger the population, the finer the movement
Effector
Refers to a part of the body that can move
ex.: arm, hand
Motor neurons
- Originate in the spinal cord
- Exit through the ventral root
- Terminate in the muscle fibers
–> an AP here releases a neurotransmitter, to make the muscle contract
Spinal interneurons
Lie within the spinal cord and innervate motor neurons
–> intergate sensory feedback with motor commands resulting in voluntary movement
Pyramidal tract
Refers to the motor neurons that travel directly from the cortex to the spinal cord
Basal ganglia
Is a collection of 5 nuclei
- Caudate nucleus
- Putamen
- Globus pallidus
- Subthalamic nucleus
- Substantia nigra
–> plays a critical role in the selection + initiation of actions
Damages to the posterior parietal cortex can lead to …
- Apraxia
- -> Difficulty performing skilled movement when asked out of context - Contralateral neglect
- -> Disturbance of the ability to respond to stimuli on the side opposite to the side of the brain damage
Posterior parietal cortex
Directs movement by providing spatial info (Mental body image)
–> includes:
a) Area 5
- -> input from primary somatosensory cortical areas
b) Area 7
- -> input from higher visual areas
Alpha motor neurons
Innervate muscle fibers + provide the physical basis fro translating nerve signals into mechanical action
Name the pathway of Alpha neurons.
- Spinal cord (Origin)
- Ventral root (Exit)
- Muscle fibres (Termination)
- ACh (Release)
- Contraction of the muscle
Muscle spindles
Inform neurons on how stretched the muscle is
–> receptor organ
Why do alpha motor neurons have to be inhibitory + excitatory at the same time ?
To overcome the stretch reflex + permit voluntary movement
Amyothrophic lateral Sclerosis (ALS)
Is a disease that results in muscle weakness and atrophy
What are the possible causes vs treatment options of ALS ?
1.
a) Genetic mutation affecting the enzyme superoxide dismutase
b) Excitotoxiticity
- -> elevated levels of glutamate or defective glutamate transporter resulting in cell death
- Using Riluzole which block glutamate release
–> only slows the progress by few months
Duchenne Muscular Dystrophy
Is a disease that involves progressive weakness + detonation of muscles in boys, that is passed on by mothers
What are possible causes vs treatment options of DMD ?
- Defective region on X chromosome encoding dystrophin
–> contributes tp cytoskeleton which lies just under the sarcolemma in muscles
- Gene therapy to replace the defective gene, by sending virus or transplanting stem cells
Myasthenia Gravis
Is a disease that involves weakness + fatiguability of voluntary muscles
–> can be fatal when respiration is compromised
What are possible causes vs treatment options of MG ?
- Autoimmune disease
- -> NS generates antibodies against own receptors, thus bind + interfere with normal actions - Drugs inviting the reuptake of antibodies
Non-primary motor areas
Refer to all the areas in the FL that can influence motor output at the level of M1 + spinal cord
Premotor areas
Have weak direct influence on spinal motor neurons
–> majority terminates in the intermediate zone of the spinal cord
How is motor movement processed ?
Name all the necessary steps.
- Info is processed by association + higher order systems
- Info is communicated to non-primary motor system
- PM areas coordinate the output level of M1 + spinal cord
Malleable motor map
The larger the population of neurons representing a type of movement, the finer there possible control
Gamma motor neurons
Involved in regulating the length of the muscle fibers
–> gets signals from muscle spindles
Extrafusal fiber vs Intrafusal fiber
Extrafusal
–> muscle fibers that are innervated by alpha motor neurons and generate tension by contracting
Intrafusal
–> muscle fibers that serve as specialized sensory organs that detect the amount and rate of change in length of a muscle (part of the muscle spindle)
