1. Ch 8 Sensorimotor System

2. 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

3. 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

4. Association Cortex

5. 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

7. 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

8. 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

12. 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

13. 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.

14. Mirror Neurons
Video

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

16. 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

18. 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

19. 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

20. 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

21. 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

22. 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)

23. 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

24. 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

25. 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