Motor system Flashcards
Movement systems and sensory information
- We talk about movement systems and sensory perception seperatly but they are not seperate - they are constantly interacting with each other.
- You use sensory information to initiate movement.
- As you are making the movement, you are getting continuous sensory feedback. You are getting somatic sensory feedback and all that feedback is helping to guide the movements in action while they are happening.
- Most movements you don’t need to think about it (except for complex movements).
- Your visual system is actually taking in all of this informaton and constructing a coherent world out of it. Movement system is incredibly complicated too. Our capacity for movement is really quite amazing.
How does a movement start
- Movement starts with an internal representation (the goal of the intended movement) and translates it into the appropriate motor commands to achieve the goal.
- Movement control in a sense is the opposite in how sensory systems work. Your sensory system starts with little bits of information scattered over the entire sensory space and then assemble it together into more complex things (hierarchy going up).
- Movement system are the opposite. When you make a movement, you do not start by thinking. All you do is think this is what I want to do and then it happens.
- The point is that the person is able to right the sentence with his hand, feets, mouth…
- Movement starts with an abstract goal, it starts with what you want to do, how you want to move. And somehow that through a series of steps get translated into the details of specifically which muscles need to contract and the timing of those muscle contractions. How do we go from an abstract goal down to the specific comands to specific muscles and the timing of those movement comands.
Organization of motor systems
The motor system has both hierarchical and parallel organization.
* Image summarizes the fundamental hierarchy of the core movement system.
* Has 3 levels of hierarchical levels: cerebral cortex (motor region of cerebral cortex), brainstem (complex stereotyped movements - a way station on way to spinal cord), spinal cord (final output for movements of your limbs and movements of your body, it has built in circuitry that sort of pre programs certain kinds of stereotyped movements).
* There’s also a hierarchy within the motor regions of the cortex itself. There are regions in the cortex that are more involved in abstract goals.
* This core pathway is often reffered to as the pyramidal tract
How is the core pathway modulated?
- The core pathway is modulated/regulated by two other brain regions/systems:
1. the cerebellum: involved in motor coordination, and also motor timing and motor learning (the more you practice something, the better you get at it).
2. the basal ganglia: set of nuclei underneath the cerebral cortex which are also involved in motor learning and selection and amplification of movements in the appropriate ways. - These two systems are working in parallel with this core system to enable you to engage in complex voluntary movements.
Central Sulcus
- Central sulcus forms the boundary for the frontal lobe.
- The frontal lobe is involved in action.
- It is functionally organized along a rostralcaudal gradient (it has a hierarchical organization): most anterior part (the front) is invovled in the abstract aspect of action and the more middle parts are involved in translating our abstract goals into more specific movement plans (abstract goals into more specific movement plans = directly involved in movements). The most caudal/posterior is involved in the actual execution of the movements.
- The abstract aspects of action (e.g., “I’m going to make a sandwich.”) are rostral, whereas the specific action required to achieve the abstract goal (e.g., the movements involved in slicing bread) are caudal.
BA of the motor system
- Notice how much of the frontal lobe is not actually directly invovwled in movement. It is involved in action, planning and setting goals but not actually involved in directly controlling movement.
- Motor cortex comprises area 4 (primary motor cortex, M1) and area 6 (lateral premotor cortex, supplemental motor area (SMA) and preSMA)
Primary motor cortex (BA 4) = region that is the most directly involved in controlling movements.
lateral premotor cortex (BA 6) = involved more in the planning of movements and what we call sensory motor integration.
supplementary motor area and pre-supplementary motor area: SMA is mostly on the medial surface. Involved in 3 different aspects of controlling movement.
What do the different regions of motor cortex do?
- Primary motor cortex (most involved in controlling movement), premotor cortex and supplemental motor area mediate different aspects of movement.
- Lesions of primary motor cortex result in contralateral paralysis and increased muscle tone (spasticity). Movements requiring dexterity are especially affected.
- Premotor cortex is involved in using sensory information to guide movement.
- Lateral premotor cortex is especially important for translating vision into movement.
- SMA - involved in complex movement sequences (ie: playing the piano) and internally generated movements.
- In contrast, lesions to premotor cortex or SMA affect the organization and control of movements. (e.g., alien limb syndrome for SMA lesions; loss of ability to use sensory information to reach for objects for premotor lesions).
Primary motor cortex
Primary motor cortex is most directly connected to movement.
Where is the motor map found?
- Early work by Wilder Penfield and others showed that primary motor cortex contained a map of the body musculature that paralleled the somatotopic map in primary somatic sensory cortex (lines up perfectly with map of primary somatic sensory cortex).
- There are extensive connections between these two maps. It makes sense that they are close to each other so that they can talk very efficiently.
- The map implies that primary somatic sensory cortex is sort of a final output pathway.
Neurons in primary motor cortex
- Neurons in primary motor cortex fire before and during voluntary movements of contralateral muscles.
- Just like the sensory system, everything crosses over. Meaning the primary motor cortex on the left side of your brain is controling the right side of your body and vice versa.
1) These neurons fire only for that specific movement, so they’re specific to that movement. So, neurons in the primary motor cortex only fire when you are moving and the individual neurons only fire in response to specific movements.
2) The neuron begings to fire APs just before the movement begins and continues to fire during the movement and then stoprs firing when the movement is over.
Premotor and supplemental motor areas
The premotor and supplemental motor areas organize the motor programs for complex voluntary movements.
Lateral premotor cortex
- Reciprocal connections between the parietal lobe and premotor cortex mediate sensory-motor transformations, the computations that enable sensory information to guide interactions with objects in the environment.
- Parietal lobe is a higher cortical area where visual and somatic sensory information are being combined together, and then these neurons project and feed forward to the lateral premotor cortex. These neurons are conveying information about vision and somatic sensation to premotor cortex. So, the parietal regions combined with the lateral premotor regions work together in this phenomenon called sensory motor transformation.
- Sensory motor transformation: transforming a sensory input into the appropriate movement commands that enable you to interact with objects in the environment.
Parietal Premotor Network
A network comprising the parietal lobe, dorsal premotor cortex and primary motor cortex is involved in directing arm movements toward objects.
* Research where monkeys see a cue and then need to reach forward and grab something.
* This research has identified two subnetworks in the parietal premotor network: one of the networks is a more dorsal network, it goes from the parietal lobe to a dorsal region of the premotor cortex and then from the premotor cortex it foes back to the arm region of the primary motor cortex (that is the final output region). This region is invovled in the animal using its arm to reach out. Arrows are two way because there is two way communication between these two regions
Within the parietal lobe, there are neurons that are integrating vision and somatic sensation.
- When we record APs from this region, we discover that there are neurons in this region of the parietal lobe that will respond to both visual input and to somatic sensory input. But the visual input and somatic sensory input are aligned with each other.
- Many neurons in the parietal lobe respond to both tactile and visual stimuli with receptive fields that are spatially in register. It is a visual somatic sensory neuron (it is integrating the two together).
- These neurons are thought to be involved construction of a peripersonal spatial map used to guide goal-directed movements.
- This is what you need to do if you are going to use vision and somatic sensation to enable you to guide movements.
Premotor neurons
- monkey fixates a central spot, cue appears telling him where to reach, can only move once the yellow light turns green. So, there is a delay where the monkey knows what it is supposed to do.
- The recording from the premotor neuorns shows that neurons start to fire as soon as the monkey sees where his arm is supposed to go. Fires a burst of AP when the monkey sees the cue. Then another burst of AP when it reaches out to touch the spot.
- Premotor neurons fire both during movements and during an imposed delay prior to the movement, suggesting they are involved in the planning and preparation to move.
But maybe this neuron in the monkey’s brain is just a visual neuron?
- If the monkey is given the same cue, but preceded by an instruction not to move, the neurons do not fire during the delay. So, when the moneky knows it does not have to move or plan a movement, the neuron does not fire AP.
- This result shows that they are not sensory neurons. They are connecting the sensory input to the appropriate action.
Neurons in premotor cortex vs neurons in primary motor cortex
- The premotor neuron on the left is active in preparation for and during the execution of an arm movement toward a target, regardless of which arm is used.
- In contrast, the neuron in primary motor cortex is active only during the execution phase and only for the contralateral arm.
- Some premotor neurons will actually fire for the same movement by either arm.
The more ventral pathway
A network comprising the parietal lobe, ventral premotor cortex and primary motor cortex is involved in shaping the hand to grasp objects.
Pathway from a slightly different region in the parietal lobe to a more ventral region of the premotor cortex and then to the hand region of the primary motor cortex.
* When you are reaching for something, you start to shape your hand in a way that is appropriate to interact with the object.
* Parietal premotor network is calculating what the object looks like and the various ways your hand can interact with the object and choses the appropriate way to interact with the object.
Different neurons like different shapes
- Many ventral premotor neurons respond to preferred shapes.
- This neuron prefers a ring and responds strongly both when the stimulus is presented and when the monkey reaches for the object. It responds much less strongly to a sphere.
- The response of a monkey vPMC neuron when different shaped objects become visible (red) and when the monkey reaches for the objects (green).
Mirror neurons
- Mirror neurons were first discovered in premotor cortex.
- Mirror neurons respond when the monkey reaches for an object and when he watches the experimenter reach for the object. They do not respond to the object alone or to non-goal directed movements of the experimenter’s arm.
- Mirror neurons are a subset of neurons in the premotor cortex that fire when someone else is executing the movement - not only when it itself does the movement. This demonstrates that the monkey understands other’s goals and intentions:
- Then moneky is watching the experimenter do that movement and this activates the same motor areas in the monkey as if it was doing the action.
- These neurons in the monkey will not fire it it is not a purposeful goal-directed movement.
Role of SMA
- The SMA has been proposed to be involved in internally generated (i.e “free-willed”) movements, especially learned complex movement sequences and linking together these complex movement sequences. Seems to be involved in activating the movement
- In support of this hypothesis, lesions to SMA and pre-SMA can cause paradoxical effects on volitional movement, including alien limb syndrome (limbs acting on their own - they do purposeful things but the person has no conscious control over it), or, conversely, loss of spontaneous movement (cannot do anything for a long period of time - won’t talk or move).
- SMA is not actually connected to any kind of movement.
How does a neuron identify where a sound is coming from?
SMA and imagined movements
- SMA is active during complex learned movement sequences even when they are just imagined.
- experiment: subjects are asked to do very simple hand motion with their fingers.
- When you imagine yourself making that complex movement sequence, the SMA is the area activated.
SMA neurons respond to…
- SMA neurons respond to selective components of learned movements sequences.
- This monkey SMA neuron is active prior to the turning motion in a sequence of movements, but only when the turn is followed by a pull and not a push movement.