Ch 8 Sensorimotor System

Introduction The sensorimotor system is organized hierarchically, has functional segregation & parallel processing (like the somatosensory system) Sensory feedback plays a large role in motor movements A movement will need conscious control at first, but as it is repeated & learned, it can be performed without conscious involvement This chapter will focus primarily on voluntary movements

Sensorimotor Association Cortex 2 major areas Posterior parietal association cortex Assesses & integrates info on current body position & the location of the object with which you are going to interact Provides spatial info & directs attention Input from visual, auditory & somatosensory systems Dorsolateral prefrontal cortex Evaluates external stimuli & initiates voluntary reactions to them

Association Cortex

Damage to posterior parietal cortex can cause: Apraxia A disorder of voluntary movement that is not attributable to a motor deficit (paralysis, weakness) People with this condition fail to perform the action when asked, but can successfully do it naturally Most pronounced when action is without context Contralateral neglect Inability to respond to stimuli on the body side contralateral to a brain lesion, without sensory or motor deficit Will act as if one side of their world doesn’t exist Often don’t recognize that there is a problem

Secondary Motor Cortex 2 main areas Supplementary motor area Premotor cortex There is some evidence for up to 8 areas Overall, 2ndary cortex is involved in programming specific patterns of movement after taking instructions from the association cortex

Primary Motor Cortex Located in precentral gyrus of frontal lobe Major departure point of motor signals from the cortex Like the somatosensory system, it is organized somatotopically & has its own homunculus However, recent research suggests it may not be so clear-cut Stimulating a specific area for longer can elicit a full motor behavior, not just one muscle contraction Damage doesn’t result in as much detriment as you’d think

Mirror Neurons Neurons that fire when a goal-directed hand movement is performed OR when you observe the same movement being performed by someone else Provides a possible mechanism for social cognition The knowledge of the perceptions, ideas & intentions of others Could facilitate social understanding, cooperation & imitation

Mirror Neurons Can fire even when the action is not being done, but there is a clue that it is being done Ex: sound of a cracking peanut So far they have only been studied in primates, but indirect evidence suggests humans have them too.

Mirror Neurons Video

Cerebellum & Basal Ganglia Interact with the sensorimotor pathway to coordinate & modulate its activities Help connect sensory & motor areas

Cerebellum The cerebellum contains over half of the brain’s neurons!! Damage to the cerebellum is devastating to motor function Primarily responsible for fine tuning/editing motor movements & is involved in motor learning http://www.youtube.com/watch?v=BTH2KvFLcHo

Basal Ganglia Composed of a complex system of interconnected nuclei Neural loops; receive various inputs from the cortex and transmit it back to the cortex via the thalamus

Descending Motor Pathways Neural signals descend from the primary motor cortex to the motor neurons of the spinal cord through 4 pathways 2 in the dorsolateral region Control movement in limbs 2 in the ventromedial region Involved in control of posture & whole-body movements

Sensorimotor Spinal Circuits A motor unit is the smallest unit of motor activity A single motor neuron & the muscle fibers it innervates When the neuron fires, its muscles fibers contract The motor units with the fewest fibers allow for the most fine detail motor control Ex: In the fingers & face

Muscles A skeletal muscle is made of 100,000s of muscle fibers bound together & attached to a bone by a tendon ACh is released by motor neurons at neuromuscular junctions & activates the motor end-plate on each muscle fiber, causing it to contract

Muscles 2 basic types of muscle fibers Fast Slow Contract & relax quickly Great force, but fatigue quickly Poorly vascularized Involved in quick movements (ex: jumping) Slow Slower & weaker But capable of sustained contraction Richly vascularized Involved in gradual movements (ex: walking)

Muscles Muscles only generate force in one direction (contraction) All the motor neurons innervating a muscle are called its motor pool Joints that can move in more than one direction must be controlled by a pair of opposite muscles: flexors (bend) & extensors (straighten) Ex: bicep/tricep Muscles are elastic

Walking Requires the complex involvement of numerous muscles & neural structures Must integrate visual info; somatosensory info from the feet, knees, arms, etc.; balance info from vestibular system; etc. AND send out the appropriate motor signals; to the feet, toes, legs, trunk, etc. Must be plastic & able to quickly respond to changes (incline, weight, intercepting objects, ground texture, etc.) Walking can be controlled by circuits in the spinal cord (without the brain) Asimo 1 Asimo 2

Central Sensorimotor Programs Theory that the sensorimotor system comprises a hierarchy of central sensorimotor programs States that all but the highest levels of the sensorimotor system have certain patterns of activity programmed into them & complex movements are produced by activating the right combination of these programs Each level can operate by itself based on sensory feedback Don’t constantly require involvement from higher levels Explains why so many of your actions can be done seemingly automatically, without even thinking about them