Motor Systems Flashcards
What is the Moravec’s paradox ?
“Moravec’s paradox is the discovery by artificial
intelligence and robotics researchers that, contrary to
traditional assumptions, high-level reasoning requires very
little computation, but low-level sensorimotor skills require
enormous computational resources.” (Wikipedia)
1997 Deep Blue versus Garri Kasparow (AI beat chess master)
what is electromygraphy?
measurement of muscle activity, measuring action potentials (by electrodes), that lower motoneurons elicit on musclefibers
How many muscles does a human body have?
Which are under voluntary control?
Which are not?
- Humans: > 600 muscles
- Under voluntary control:
- Skeletal muscles
- Not under voluntary control:
- Cardiac muscles
- Smooth muscles (e.g. peristaltic)
how are upper and lower motor neurons connected and what is that good for?
upper motor neuron origins in neocotex, travels down spinal cord and there synapses with lower motor neuron, which then innervates the muscle.
in case of danger, rapid reflexive movement is possible without loop over neocortex but initiated in the spinal cord
what s a motor unit?
(Divergence, Neurotransmitter)
every muscle fiber is innervated by only one alpha motor neuron, but one motor neuron innervates several muscle fibers.
all muscle fibers innervated by the same motor neuron, including this motor neuron, is a motor unit.
the higher the degree of divergence (many fibers activated by 1 neuron), the lesser the fine control
neurotransmitter: ACTH
Motor Systems
Alpha Motor Neurons
- Function?
- Origin?
- Exit?
- Terminate?
- Input?
- After AP?
- Force of muscles depends on?
Function
- Innervate muscle fibers
- Translate nerve signals into mechanical actions
Origin
- Spinal Chord
Exit
- Ventral Root
Terminates
- Muscle Fibers
Input
- Muscle Spindles
- sensory receptors embedded in the muscles, provide information about how much the muscle is stretched
- axons form afferent nerve, enters SC via dorsal root, synapses directly on corrsponding efferent AMN
- Spinal Inter Neurons (see card!)
After AP
- Release of neurotransmitteracetylcholine (which makes muscle fibers contract)
Force of Muscles Depends on?
- Number of muscle fibers
- Number / frequency of APs
Motor Systems
What are Gamma motor neurons?
- part of the proprioceptive system
- important for sensing and regulating the length of muscle fibers
Motor Systems
Spinal Inter Neurons
- Where?
- Innervate?
- Innervated by?
- Integration results in?
Where?
- Spinal Chord
Innervate?
- Alpha Motor Neurons
Innervated by?
- Afferent Sensory Nerves (skin, muscles, joints)
- Descending Motor Fibers (upper motor neurons, originate in (sub-)cortical structures)
Integration results in?
- sensory feedback + motor commands → voluntary movement
- agonist gets excitatory signals and antagonist gets inhibitory signals → else: stretch reflex
how does the patellar reflex work?
muscle spindle below the patellar is stretched, sensory signal goes to dorsal spinal cord, synapses with a motor neuron in the ventral spinal cord (without interneuron). this motorneuron innervates the quadriceps femoris muscle and elicits the reflex.
Motor Systems
Extrapyramidal Tracts
- What are they not part of?
- Do they decussate?
What are they not part of?
- the pyramidal tracts (axons that travel directly from the cortex to the spinal segments)
- a primary source of indirect control over spinal activity modulating posture, muscle tone, and movement speed
- they receive input from subcortical and cortical structures
Do they decussate?
- Yes (Exeption: Cerebellum)
Motor Systems
Subcortical Motor Structures - Cerebellum
- Input
- Output
- Organisation
- What are the three regions of the cerebellum?
(Fun Fact: Containins more neurons than the rest of the central nervous system combined. Most of these neurons are contained in the layers of the cerebellar cortex)
Input
- Inputs to the cerebellum primarily project to the cerebellar cortex
Output
- The output from the cerebellum originates in the deep cerebellar nuclei, projecting to brainstem nuclei and the cerebral cortex via the thalamus
Organisation
- Ipsilateral (input from and output to the cortex both cross over to the contralateral side; The right side of the cerebellum is associated with movements on the right side of the body, and the left side is associated with movements on the left side of the body)
What are the three regions of the cerebellum?
- Vestibulocerebellum
- smallest and oldest region
- works with the brainstem vestibular nuclei to control balance and coordinate eye movements (VOR -> vestibulo-ocular reflex) with body movements
- Spinocerebellum
- medial region
- receives sensory information from the visual and auditory systems as well as proprioceptive information from the spinocerebellar tract.
- The output from the spinocerebellum innervates the spinal cord and nuclei of the extrapyramidal system.
- Lesions of the spinocerebellum can result in an unsteady gait and disturbances of balance.
- Cells in this region are especially sensitive to the effects of alcohol.
- Chronic alcohol abuse can cause persistent problems with balance.
- Even with acute alcohol use, cerebellar symptoms can be observed: Tests used by police on suspected drunk drivers are essentially assessing cerebellar function.
- Neocerebellum
- newest region
- heavily innervated by descending fibers originating from many regions within the parietal and frontal lobes
- Output from the neocerebellum projects back to the cortex via the thalamus and the thalamic projections terminate in the primary motor, lateral premotor, and prefrontal cortices
- Lesions to the neocerebellum produce ataxia, problems with sensory coordination of the distal limb movements, thus disrupting fine coordination.
Motor Systems
Subcortical Motor Structures - Basal Ganglia
- What nuclei?
- Function?
- Input?
- Output?
- What are the remaining components doing?
What nuclei?
- Striatum (Caudate Nucleus, Putamen)
- Globus Pallidus
- Subthalamic Nucleus
- Substantia Nigra
Function
- plays a critical role in motor control, especially in the selection and initiation of actions
Input
- mainly to the two nuclei forming the striatum
Output
- almost exclusively by way of the internal segment of the globus pallidus and part of the substantia nigra
What are the remaining components doing?
- The remaining components (the rest of the substantia nigra, the subthalamic nucleus, and the external segment of the globus pallidus) modulate activity within the basal ganglia.
Motor Systems
Corticospinal Tract (CST)
- why is it referred to as the pyramidal tract?
- Origin?
- Terminates on?
- Axons cross where?
Why is it referred to as the pyramidal tract?
- because the mass of axons resemble a pyramid as they pass through the medulla oblongata
Origin?
- Most corticospinal fibers originate in the primary motor cortex
- some originate in premotor cortex, supplemental motor area, and even somatosensory cortex
Terminates on?
- Spinal interneurons
- Directly on alpha motor neurons. (These are the longest neurons in the brain—some axons extend for more than 1 meter.)
Axons cross where?
- 80 % of the axons cross (decussate) at the junction of the medulla and the spinal cord
- 10 % cross when they exit the spinal chord
Motor Systems
Cortical Regions Involved in MotorControl - Primary Motor Cortex
- Input?
- Output?
- Anatomical Subdivisions?
- Map?
- Lessions?
Input?
- receives input from almost all cortical areas implicated in motor control. These areas include the parietal, premotor, supplementary motor, and frontal cortices as well as subcortical structures such as the basal ganglia and cerebellum
Output?
- the output of the primary motor cortex constitutes the largest signal in the corticospinal tract
Anatomical Subdivisions?
- rostral region
- homologous among many species
- terminate on spinal interneurons
- caudal region
- present only in humans and some of our primate cousins
- terminate directly on alpha motor neurons (these motor neurons project to muscles of the upper limb. Functionally, this relatively recent adaptation is thought to provide more direct control of effectors essential for volitional movement. It allows greater dexterity as well as the ability to produce novel patterns of motor output)
Map?
- Different regions represent different body parts
- the somatotopic organization in M1 is not nearly as distinct as that seen in the somatosensory cortexb(more like a mosaic pattern)
- the representation of the effectors does not correspond to their actual size but reflects the importance of that effector for movement and the level of control required for manipulating it (fingers span a large portion of the human motor cortex)
Lessions?
- usually result in hemiplegia (loss of voluntary movements on the contralateral side)
- not a matter of will or awareness
- usually affects the most distal effectors
- most frequently results from a hemorrhage in the middle cerebral artery
- recovery minimal
- reflexes take over due to loss of cortical influence
Motor Systems
Cortical Regions Involved in MotorControl - Secondary Motor Areas
- Location?
- Functions?
- Anatomical Subdivisions?
- Map?
- Lessions?
Location?
- anterior to the premotor cortex
Functions?
- Secondary motor areas are involved with the planning and control of movement
Anatomical Subdivisions?
- premotor cortex
- has strong reciprocal connections with the parietal lobe, providing the anatomical substrate for external sensory-guided actions, such as grabbing a cup of coffeeor catching a ball
- The lateral area is important for linking action with visual objects in the environment
- ventral premotor cortex (PMv)
- dorsal premotor cortex (PMd)
- supplementary motor area (SMA)
- SMA, in contrast, has stronger connections with medial frontal cortex, areas are associated with internally guided personal preferences and goals. For example, SMA might help decide which object to choose (e.g., coffee or soda), or with the planning of a sequence of learned actions (e.g., playing the piano
Map?
- Multiple somatotopic maps are found within the secondary motor areas
- although, as with M1, the maps are not clearly delineated and may not contain a full body representation
Lessions?
- Apraxia
- problems in performing purposeful and coordinated movements
- The patients can produce simple gestures, like opening and closing their fist or moving each finger individually. Nonetheless, they cannot link these gestures into meaningful actions, such as sequencing an arm and wrist gesture to salute
- Ideomotor apraxia
- the patient appears to have a rough sense of the desired action but has problems executing it properly. If asked to pantomime how to comb his hair, the patient might knock his fist against his head repeatedly.
- Ideational apraxia
- the patient’s knowledge about the intent of an action is disrupted. He may no longer comprehend the appropriate use for a tool. For example, one patient used a comb to brush his teeth, demonstrating by the action that he could make the proper gesture, but used the wrong object to do it.
Motor Systems
Cortical Regions Involved in MotorControl - Association Motor Areas
- Somatosensory cortex provides a representation of the body and how it is situated in space.
- Broca’s area and the insular cortex (medial to Broca’s area) are involved in the production of speech movements.
- Frontal eye fields, a region (as the name implies) that contributes to the control of eye movements.
- Anterior cingulate cortex is also implicated in the selection and control of actions, evaluating the effort or costs required to produce a movement.
Central Pattern Generators
- What is it?
- Why is it considered hierarchical?
- Why did this mechanism evolve?
What is it?
- A mechanism for the hierarchical control of movement
Why is it considered hierarchical?
- Brain structures would not have to specify patterns of muscle activity. Rather, they would simply activate the appropriate pattern generators in the spinal cord, which in turn would trigger muscle commands. The system is truly hierarchical, because the highest levels are concerned only with issuing commands to achieve an action, whereas lower-level mechanisms translate the commands into a specific neuromuscular pattern to produce the desired movement.
Why did they evolve?
- Central pattern generators most likely evolved to trigger actions essential for survival, such as locomotion.
contralaterality.
where do efferent nerve fibers on the way from cerebral cortex through the spinal cord (cortico spinal tract) change sides?
80 % of CST cross at the medullary pyramid (junction of medulla oblongata and spinal cord, and also the reason why it’s called pyramidal tracts)
Motor Systems
Neural Coding of Movement
- What are population vectors?
- Coding for location or for direction?
- Predictor of what?
- 2D or 3D?
What are population vectors?
- Activity is distributed across many cells, each with their unique preferred direction.
- Each neuron can be considered to be contributing a “vote” to the overall activity level. The strength of the vote will correspond to how closely the movement matches the cell’s preferred direction: If the match is close, the cell will fire strongly; if the match is poor, the cell will fire weakly or even be inhibited.
- The activity of each neuron can be described as a vector, oriented to the cell’s preferred direction with a strength equal to its firing rate. The population vector is the sum of all the individual vectors
Coding for location or direction?
- activity of the cells in the primary motor cortex correlates much better with movement direction than with target location
- the population vector provides an excellent predictor of movement direction
- The population vector is not limited to simple 2-D movements; it also has proven effective at representing movements in 3-D space
Motor Systems
Alternative Perspectives on Neural Representation of Movement
- The population vector and the planing of movement
- Shift of tuning
- Dynamic properties of sensory- and motor cortex
The population vector and the planing of movement
- the population vector shifts in the direction of the upcoming movement well before the movement is produced, suggesting that at least some of the cells are involved in planning the movement and not simply recruited once the movement is being executed
- by looking at the population vector, which was recorded more than 300 ms before the movement, the direction of the forthcoming movement can be precisely predicted.
Shift of tuning
- the tuning may be inconsistent: The tuning exhibited by a cell before movement begins may shift during the actual movement.
- many cells that exhibit an increase of activity during the delay phase show a brief drop in activity just before movement begins, or a different firing pattern in preparation and execution of a movement.
- This result is at odds with the assumption that the planning phase is just a weaker, or subthreshold version of the cell’s activity during the movement phase.
Dynamic properties of sensory- and motor cortex
- Rather than viewing neurons as static representational devices (e.g., with a fixed directional tuning), we should focus on the dynamic properties of neurons, recognizing that movement arises as the set of neurons move from one state to another. By this view, we might see that neurons wear many hats, coding different features depending on time and context
- People produce movements in anticipation of their sensory consequences and we use sensory information to adjust our actions. Thus, the motor cortex isn’t just “motor,” and the sensory cortex isn’t just “sensory.” For example, in rats, the neurons that control whisker movements are predominantly in somatosensory cortex.
- In monkeys, sensory inputs rapidly reshape motor activity. In fact, some evidence suggests that the directional tuning of some motor cortex neurons is more about “sensory” tuning. Consider the same shoulder movement induced by two different sensory events. One is caused by a nudge to the elbow and the other following a nudge to the shoulder. As early as 50 ms, well before the sensory signals in sensory cortex would have been processed and sent to the motor system, M1 neurons show differential responses to the two types of nudges. It appears that the sensory information was processed within M1 directly, allowing for fast, nearly real-time feedback
M1: direction or muscle coding?
32% of neurons in M1 predominately code for muscles , 50 % for direction
(comparison of neuronal activity for movements in the same direction, but with different hand position/ different muscles. if coded for direction all movements show similar activation, if coded for muscles both movements would have to correspond with different neurons.)
Does input to pyramidal tracts / efferences only stem from M1?
No!
M1: only codes for simple aspects of movement (like direction) and the more complicated it gets, the more other regions are involved.
PMC & SMA: movement sequences
S1: somatosensory feedback
posterior parietal cortex: sensory motor integration
area 8
PFC
what is open loop control in movement?
ballistic movement without sensory feedback (fixed trajectory), no adjustments
- patient with sensory neuropathy can draw figures in the dark - no visual feedback either : sequence of events can be generated in an open loop
- eye movements (saccades are preplanned), tennis serves
describe the direct and indirect pathway ini the substantia nigra.
(maybe rather relevant for neuroanatomy and physiology)
The direct pathway
- Involves fast, direct, inhibitory connections from the striatum to the GPi and SNr.
The indirect pathway
- Takes a slower, roundabout route to the GPi and SNr.
- Striatal axons inhibit the external segment of the globus pallidus (GPe), which in turn inhibits the subthalamic nucleus and GPi.
- The output from the basal ganglia via the GPi and SNr is also inhibitory.
again, what was going on in the Libet experiment?
EEG from M1 und SMA.
subjects pressed a key whenever they wanted,
reported at which time they were aware of wanting to move.
‘readiness potential’ observed 350 ms before report of intention and 550 my before actual response
discussion of free will
later experiment included the choice which hand participants used for their response, observation of lateralized readiness potential showed that awareness of intention is related rather to specific movement than generalized intention
Motor Systems
Action Goals and Movement Plans
- Affordance Competition Hypothesis
the processes of action selection (what to do) and specification (how to do it) occur simultaneously within an interactive neural network, and they evolve continuously.
Even when performing one action, we are preparing for the next.
The brain uses the constant stream of sensory information arriving from the environment through sensorimotor feedback loops to continuously specify and update potential actions and how to carry them out.
That’s the affordance part.
This sensory nformation is constrained by our internal drive states, longer-range goals, expected rewards, and anticipated costs, and we use all this information to assess the utility of the different actions.
This is the competition part. At some point, one option wins out over the other competitors.
An action is selected and executed.
This selection process involves many parts of the motor pathway, where interactions within frontoparietal circuits have a prominent role (see Figure 8.16). This schema implies that decision-making processes are embedded in the neural systems associated with motor control, not carried out by some sort of detached central control center. Is there any evidence supporting this? Let’s start with the notion that an action has multiple goals, and each goal is linked with the plan to accomplish it.
Cisek (2005) developed his model based on evidence obtained in single-cell recordings from the premotor cortex of monkeys. In each trial of his study, the animal was presented with two targets, either of which it could reach with its right arm. After a delay period, a cue indicated the target location for the current trial. During this delay period, neural signatures for both movements could be observed in the activity of premotor neurons, even though the animal had yet to receive a cue for the required action. These signatures can be viewed as potential action plans. With the onset of the cue, the decision scales were tipped. Activity associated with movement to that target became stronger, and activity associated with the other movement became suppressed. Thus, following the cue, the initial dual representation consolidated into a single movement
