ballistic movements
high speed, all or none movements that cannot be modified once initiated
- not influenced by sensory feedback
ex. swatting a fly
general model of sensorimotor system
association cortex, motor pathways, skeletal muscle
- hierarchical organization
- motor output guided by sensory input
- learning can change nature and locus of motor control (less high-level areas need to be involved as you get better and better at executing a task)
sensorimotor association cortex
2 major areas to initiate mmvt; posterior parietal and dorsolateral prefrontal cortices
posterior parietal cortex
- integrates info on body position and objects in space from vis, aud and somato systems; directs attention and is responsible for intention to perform an action/knowledge of having performed an action; output to motor cortex, dorsolateral prefrontal cortex, secondary motor cortex and frontal eye field (to control conscious eye mvmt)
- specialized for guiding mvmt of eyes, head, arms, hands
- cells can be activated by simply imagining actions
damage to posterior parietal association cortex
sensorimotor deficits
- perception and memory of spatial relationships
- reaching/grasping accuracy
- control of eye mvmt
- attention
apraxia
disorder of voluntary mvmt
- can’t make specific mvmt when requested, esp if out of context (ex. mime hammering a nail)
- not due to motor deficit (paralysis/weakness) or deficit in motivation/comprehension
- symptoms manifest bilaterally, but are usually caused by unilateral damage in the left posterior parietal cortex
contralateral neglect
inability to respond to stimuli on opposite side of damage (which is usually in the right posterior parietal lobe, but produces no other sensorimotor deficits)
-as if the left side of the world doesn’t exist; motor and sensory systems otherwise fully intact, but tend not ro respond to left sides of objects, regardless of location in visual field
dorsolateral prefrontal cortex
- receives input from posterior parietal lobe
- projects to secondary and primary motor cortices, front eye field and posterior parietal lobe
- evaluates external stimuli and initiates reactions accordingly
- neuronal activity depends on object characteristics, location of objects, motor responses (neurons here are the first to fire in response to anticipation of motor responses (which is why decisions to initiate mvmt may occur here) and continue to fire until response complete)
secondary motor cortex
receives input from both association cortices (posterior parietal and dorsolateral prefrontal)
-output to primary motor cortex
composed of 2 major areas: supplementary motor area (extends into longitudinal fissure) and premotor cortex (lateral surface of frontal lobe, anterior to primary motor)
mirror neurons
fire when performing a goal directed hand movement, as well as when observing another individual perform that same goal directed hand movement
- encode goals and understanding of actions
- have yet to be confirmed in humans
primary motor cortex
in precentral gyrus of frontal lobe
- major convergence point for sensorimotor signals (major point of departure too)
- direction of motor output isn’t encoded by specific neurons, but rather the location of target of motion is
- in addition to motor homunculus, there is an action map of sorts
- system is inherently plastic according to feedback and to the reproduce patterns of muscle contraction from diff start/end points
effects of primary motor cortex (PMC) lesions
mild effects
- large lesions may disrupt mvmt of body parts independently of others
- asterognosia
- may reduce speed, accuracy and force of mvmt
- does not eliminate voluntary mvmt, which is process in a parallel path that jumps from M2 to subcortical structures (bypassing M1)
