ch eight: sensorimotor system Flashcards
motor cortical network
P- provides
P - plans
O -organizes
P- produces
1) posterior cortex provides sensory inofrmation to the frontal cortex
2) prefrontal cortex plans movements
3) premotor cortex organizes movement sequences
4) motor cortex produces specific movements
see diagram 8a
posterior parietal cortex
Plays a critical role in transforming sensory input into motor output
- directs attention
- monitors limb position
- locates objects
- sensory integration
- movement initation
posterior parietal damage; optic ataxia
Damage results in deficits in visuomotor control
Optic ataxia - difficulty reaching towards and grasping objects
posterior parietal damage; limb apraxia
- what is ideomotor?
- what is ideational?
- what is pantomine?
- what is imitation?
Limb apraxia - an inability to perform purposeful movement in the absence of a primary motor deficit
- Ideomotor - incorrect execution of the action
- Ideational - lose knowledge of the goal of the action
lesions of the left PPC or left premotor cortex
1) Pantomime – asking the subject to generate a movement without an object present, i.e., “Show me how you would use a hammer”
2) Imitation – asking the subject to imitate your movement
posterior parietal damage; ideomotor apraxia
Ideomotor apraxia: These patients have deficits in their ability to plan or complete motor actions that rely on semantic memory. They are able to explain how to perform an action, but unable to “imagine” or act out a movement such as “pretend to brush your teeth” or “pucker as though you bit into a sour lemon.”
posterior parietal damage; ideational/conceptual apraxia
Ideational/conceptual apraxia: Patients have an inability to conceptualize a task and impaired ability to complete multistep actions.
- Consists of an inability to select and carry out an appropriate motor program. For example, the patient may complete actions in incorrect orders, such as buttering bread before putting it in the toaster, or putting on shoes before putting on socks.
- There is also a loss of ability to voluntarily perform a learned task when given the necessary objects or tools. For instance, if given a screwdriver, the patient may try to write with it as if it were a pen, or try to comb one’s hair with a toothbrush.
contralateral neglect
The inability to respond or attend to stimuli in contralesional space
Contralateral neglect is sometimes manifested in terms of gravitational coordinates, sometimes in terms of object-based coordinates.
dorsolateral prefrontal cortex
Receives inputs from PPC
Sends projections to M1 and secondary motor cortex and frontal eye fields
Neurons in DLPFC fire prior to movement onset
Object characteristics
Object location
Planning
dorsolateral prefrontal cortex
Receives inputs from PPC
Sends projections to M1 and secondary motor cortex and frontal eye fields
Neurons in DLPFC fire prior to movement onset
- Object characteristics
- Object location
- Planning
observing others
~ Rizzolatti et all. (1998)
Rizzolatti et al. (1998)
Accidently discovered a population of neurons in monkeys that fired when the monkey reach and grasped a banana
BUT ALSO
A subset of those neurons fired when the monkey saw the experimenter reach and grasp the banana!
mirror neurons
Rizzolatti et al. (2006)
Neurons in the premotor cortex that respond to grasping objects, as well as observing someone else grasp objects
“grasping” the intentions of others
- mirror neurons code the “__” of actions
Iacoboni et al. (2005)
Brain activity was greatest in the intention conditions
Mirror neurons code the “why” of actions and respond differently to different intentions
primary motor cortex
Somatotopic – more cortex devoted to body parts that make complex movements
- Motor homunculus
Until recently, each neuron was thought to encode the direction of movement
notes:
Earlier studies by Penfield used brief trains of stimulation
Elicited individual muscle movements
Hands are special in that they receive feedback from proprioceptive receptors and skin receptors
coding of movements in M1
Neurons in M1 code for specific movement directions
Direction coding is maintained irrespective of starting position
new insights into the role of M1
- M1 represents 3 types of organization. What are they?
Recent research in monkeys used longer trains of stimulation in M1
Graziano suggests that M1 represents 3 types of organization:
1) The body part to be moved
2) The endpoint of the movement (posture)
3) The goal of the movement
notes:
For example, stimulation of one site always caused the hand to close in a grip, the arm to bring the hand to the mouth, and the mouth to open.
Stimulation of another site always caused the grip to open, the palm to face away from the body, and the arm to extend, as if the monkey were reaching to grasp an object.
Other sites evoked other complex movements. The behavioral repertoire of the animal seemed to be rendered onto the cortical sheet.
Sites that moved a body part overlapped with other body parts
damage to m1
- small lesions vs large lesions?
- what is stereognosia?
Small lesions often produce minimal effects
Large lesions may disrupt a patient’s ability to move one body part independently of others
Large lesions may also produce stereognosia
- deficit in stereognosia is the inability to identify an object by touch
why understanding the motor system is important
- neural-prostetics
If we understand how the motor system controls our limbs we can develop neural-prosthetics for fine-tuned actions
human neural prostheses
Using brain activity to perform actions
cerebellum + basal ganglia
Interact with different levels of the sensorimotor hierarchy
Coordinate and modulate
May permit maintenance of visually guided responses despite cortical damage
basal ganglia
- basal ganglia is composed of 3 nuclei. what are they?
Comprised of 3 nuclei: caudate, putamen, globus pallidus
- Receives input from most areas of cortex
- Plays a critical role in modulating the force of movements
- Also plays an important role in motor learning
see diagram
cerebellum
1) Plays a critical role in maintaining balance and posture
- Connections with the vestibular nuclei and spinal cord
2) Helps coordinate and accurately time motor output
Limb movements, eye movements
3) Important for motor learning based on feedback signals from the sensory system
cerebellum and movement accuracy
Prisms induce a misalignment between the eye and the hand
Participants learn to adjust movements in direction opposite the prism shift to compensate
After prisms are removed there is an after-effect in the opposite direction
Patients with cerebellar lesions do not adjust movements as easily
Demonstrate a reduced or absent after effect
cerebellum and prediction
When trying to touch a moving object, you will miss the object if you steer your hand toward its current location, which will be out of date by the time your hand arrives there.
4 descending motor pathways
Two dorsolateral
- Corticospinal
- Corticorubrospinal
Two ventromedial
- Corticospinal
- Cortico-brainstem-spinal tract
Both corticospinal tracts are direct
dorsolateral tracts
Most synapse on interneurons of spinal gray matter
dorsolateral tracts; corticospinal
Corticospinal – descend through the medullary pyramids, then decussate
- Betz cells – synapse on motor neurons projecting to leg muscles
- Control of wrist, hands, fingers, toes
dorsolateral tracts; corticorubrospinal
Corticorubrospinal – synapse at red nucleus and cross before the medulla
- Some control face muscles
- Distal muscles of arms and legs
ventromedial tracts; corticospinal
Corticospinal
- Descends ipsilaterally
- Axons branch and innervate interneuron circuits bilaterally in multiple spinal segments
ventromedial tracts; cortico-brainstem-spinal
Cortico-brainstem-spinal
- Interacts with various brainstem structures and descends bilaterally carrying information from both hemispheres
- Synapse on interneurons of multiple spinal segments controlling proximal trunk and limb muscles
dorsolateral motor vs ventromedial motor pathway
1) dorsolateral
- One direct tract, one that synapses in the brain stem
- Terminate in one contralateral spinal segment
- Distal muscles
- Limb movements
2) ventromedial
- One direct tract, one that synapses in the brain stem
- More diffuse
- Bilateral innervation
- Proximal muscles
- Posture and whole body movement
sensorimotor spinal circuits
motor circuits of the spinal cord show considerable complexity
- independent of signals from the brain
Striking the tendon stretches the muscle spindle in the attached muscle, sending a sensory signal via the dorsal root ganglion to the spinal cord.
After crossing an interneuron, the signal returns to the muscle to trigger a counterbalancing contraction
sensorimotor system function
Walking – a complex reflex in some animals
muscles
1) Motor units – an alpha motor neuron plus muscle fibers; all fibers contract when motor neuron fires
2) Number of fibers per unit varies – fine control, fewer fibers/neuron
3) Muscle – muscle fibers bound together by a tendon
receptors for proprioception
1) Muscle spindles
- Embedded in muscle tissue
- Detect changes in muscle length
2) Golgi tendon organs
- Embedded in tendons
- Detect muscle tension
recurrent collateral inhibition
Motor neurons branch to inhibitory interneurons before innervating muscles
- RENSHAW CELLS
Renshaw cells give muscle fiber a rest after every contraction
Contraction is taken over by other motor neurons in the motor pool
central motor programs
Some motor programs may be innate (e.g., pincer grip)
Children will spontaneously make pincer movements by 3 months of age
By 12 months children start grasping objects
brain during motor learning
Functional brain-imaging studies in humans have generally supported changes in brain function with practice
