Chapter 8

• Association cortex —> company president
• Muscles —--> workers
• 1. The sensorimotor system is hierarchically organized.
  2. Motor output is guided by sensory input.
  3. Learning can change the nature and the locus of sensorimotor control.
• Functional segregation - each level of the sensorimotor and company hierarchies tends to be composed of different units (neural structures or departments), each of which performs a different function.
• Sensory Feedback - plays an important role in directing the continuation of the responses that produced it.
• Ballistic Movements - only responses that are not normally influenced by sensory feedback.
• Posterior Parietal Association Cortex

  • the portion of parietal neocortex posterior to the primary somatosensory cortex
  • plays an important role in integrating these two kinds of information, in directing behavior by providing spatial information, and in directing attention.
  • classified as association cortex because it receives input from more than one sensory system.
• Posterior Parietal Association Cortex receives information from the three sensory systems:

  1. Visual System
  2. Auditory System
  3. Somatosensory system
• Frontal Eye Field - a small area of prefrontal cortex that controls both eye movements and shifts in attention.
• Apraxia is a disorder of voluntary movement that is not attributable to a simple motor deficit or to any deficit in comprehension or motivation.
• Contralateral Neglect is a disturbance of a patient’s ability to respond to stimuli on the side of the body opposite (contralateral) to the side of a brain lesion in the absence of simple sensory or motor deficits.
• Dorsolateral Prefrontal Association Cortex

  • The other large area of association cortex that has important sensorimotor functions
  • receives projections from the posterior parietal cortex
  • it sends projections to areas of secondary motor cortex, to primary motor cortex, and to the frontal eye field.
• secondary motor cortex

  • Areas that receive much of their input from association cortex and send much of their output to primary motor cortex.
• supplementary motor area

  • wraps over the top of the frontal lobe and extends down its medial surface into the longitudinal fissure.
• premotor cortex

  • runs in a strip from the supplementary motor area to the lateral fissure.
• Mirror neurons

  • neurons that fire when an individual performs a particular goal-directed movement or when they observe the same goal-directed movement performed by another.
• Primary Motor Cortex

  • located in the precentral gyrus of the frontal lobe
  • major point of convergence of cortical sensorimotor signals, and it is the major, but not the only, point of departure of sensorimotor signals from the cerebral cortex.
• motor homunculus is the somatotopic layout of the human primary motor cortex.
• stereognosis is the process of identifying objects by touch.
• Large lesions to the primary motor cortex may disrupt a patient’s ability to move one body part (e.g., one finger) independently of others.
• The cerebellum and the basal ganglia are both important and highly interconnected sensorimotor structures.
• Cerebellum - constitutes only 10 percent of the mass of the brain. It contains more than half of the brain’s neurons.
• basal ganglia are a complex heterogeneous collection of interconnected nuclei. It does not contain as many neurons as the cerebellum, but in one sense they are more complex. 
• movement vigor is the control of the speed and amplitude of movement based on motivational factors.
• Two pathways descend in the dorsolateral region of the spinal cord—collectively known as the dorsolateral motor pathways, and two descend in the ventromedial region of the spinal cord—collectively known as the ventromedial motor pathways.
• The ventromedial tracts are much more diffuse.
• Acetylcholine activates the motor end-plate on each muscle fiber and causes the fiber to contract.
• Contraction is the only method that muscles have for generating force, thus, any muscle can generate force in only one direction.
• All of the motor neurons that innervate the fibers of a single muscle are called its motor pool.
• • Fast muscle fibers - are those that contract and relax quickly. Although they are capable of generating great force, they fatigue quickly because they are poorly vascularized (have few blood vessels, which gives them a pale color)
• • Slow muscle fibers - although slower and weaker, capable of more sustained contraction because they are more richly vascularized (and hence much redder)
• • Flexors act to bend or flex a joint
• • Extensors act to straighten or extend it.
• • without shortening and pulling them together; this is termed isometric contraction.
• • it can shorten and pull them together; this is termed dynamic contraction.
• Golgi tendon organs - respond to increases in muscle tension
• • Muscle spindles - respond to changes in muscle length, but they do not respond to changes in muscle tension.
• Patellar tendon reflex (patella = “knee”)

  • is a stretch reflex that is elicited by a sudden external stretching force on a muscle.
• Spindle afferent neurons 

  • carries action potentials from the stretch receptors into the spinal cord.
• Reciprocal innervation

  • an important principle of spinal cord circuitry. It refers to the fact that antagonistic muscles are innervated in a way that permits a smooth, unimpeded motor response, in short, when one is contracted, the other relaxes.