Sensorimotor System: Motor Output and Sensory Input

Sensorimotor System Overview

• The sensorimotor system functions as a parallel, functionally segregated, hierarchical system
• Information mainly flows up through the hierarchy in sensory systems
• Information mainly flows down in the sensorimotor system

Motor Output and Sensory Input

• Sensory feedback plays a key role in directing the continuation of responses produced
• Ballistic movements are brief, all-or-none, high-speed movements like swatting a fly, and are not normally influenced by sensory feedback
• Many motor output adjustments responding to sensory feedback are unconsciously controlled by lower levels

Learning and Sensorimotor Control

• Sensorimotor learning causes changes
• Initially, each individual response is performed under conscious control
• After practice, responses become organized into continuous sequences adjusted by sensory feedback without conscious regulation
• Organization of individual responses into continuous motor programs and transfer of control to lower CNS levels characterize most sensorimotor learning

General Model of Sensorimotor System

• Principles include hierarchical structure, functional segregation of levels, parallel connections between levels and numerous feedback pathways
• Focuses on neural structures that control voluntary behavior
• Traces major motor signals as they descend the sensorimotor hierarchy to skeletal muscles

Sensorimotor Association Cortex

• Association cortex is the top of the sensorimotor hierarchy
• Posterior parietal association cortex
• Dorsolateral prefrontal association cortex compose sensorimotor association areas
• There is no consensus on how to best divide either of them for analysis or even how comparable the areas are in humans, monkeys, and rats

Posterior Parietal Association Cortex

• The nervous system requires the original positions of the body parts needed to moved
• Also requires positions of any external objects the body interacts with before movement
• Posterior parietal association cortex integrates these kinds of information
• Directs behavior by providing spatial information
• Directs attention

Posterior Parietal Cortex Damage

• Apraxia, a disorder of voluntary movement not due to simple motor deficit/comprehension/motivation deficits, can result from left posterior parietal cortex or its connections unilateral damage
• Patients have difficulty making specific movements when requested, but can perform the same movements under natural conditions
• Contralateral neglect can arise from posterior parietal cortex damage. Neglect is the inability to respond to stimuli on the opposite side of the body occurs
• Damage can produce deficits including spatial relationships, accurate reaching/grasping, eye movement control, and attention skills

Hierarchical Organization

• Operations are directed by commands cascading down through the levels of the hierarchy
• Commands from the association cortex specify general goals rather than specific plans of action
• Advantage is higher levels are free to perform more complex functions

Functional Segregation

• The sensorimotor and company hierarchies are characterized by functional segregation
• Each level of the sensorimotor and company hierarchies tend to be composed of different units (neural structures or departments), each of which performs a different function

Dorsolateral Prefrontal Association Cortex

• Plays a key role in sensorimotor function
• Receives projections from posterior parietal cortex
• Sends projections to secondary motor cortex, primary motor cortex, and frontal eye field areas
• Activity of some neurons depends on object characteristics
• Activity of others depends on object locations
• Activity of still others depends on a combination of both.

Secondary Motor Cortex

• Receives input from association cortex (posterior parietal and dorsolateral prefrontal)
• Sends output to primary motor cortex
• Supplementary motor area and premotor cortex were the original two known areas
• Electrical stimulation can elicit complex movements, often involving both body sides
• Neurons become more active just before voluntary movement begins and continue throughout the movement
• Areas are thought to program specific movement patterns after receiving general instructions from the dorsolateral prefrontal cortex

Mirror Neurons

• Fire when an individual performs a particular goal-directed movement or when they observe the same goal-directed movement performed by another
• Provide a possible mechanism for social cognition via mapping the actions of others onto one's own action repertoire to facilitate social understanding
• Respond to the understanding of an actions purpose, not necessarily to some superficial characteristics of the action itself

Primary Motor Cortex

• Located in the precentral gyrus of the frontal lobe
• Major convergence point of cortical sensorimotor signals
• Major departure point of sensorimotor signals from cerebral cortex

Conventional View of Primary Motor Cortex Function

• The conventional view of primary motor cortex
• Primary motor cortex mapped by electrical stimulation of conscious human patients during neurosurgery shows each cortical site activates a particular contralateral muscle for a simple movement
• Organized somatotopically based on a body map
• Commonly referred to as the motor homunculus
• Each site receives sensory feedback from receptors in muscles and joints it influences

Current View of Primary Motor Cortex Function

• Recent studies using longer bursts of current show complex natural-looking response sequences instead of contractions of individual muscles

• Exhibits looser somatotopic organization

• Sites that move a body part overlap with sites that move other body parts.

• Studies with freely moving monkeys show firing of primary motor cortex neurons is often related to the particular end point of a movement, not to the direction of the movement

  • If stimulation of a site causes bending of a straight arm at the elbow to a 90-degree angle, stimulation of the same site causes a tightly bent arm to straighten to the same 90-degree angle
  • The same motor cortex stimulation can produces opposite movements depending on the starting position
  • Each particular site has the ability to the get body part (e.g., an arm) to a target location regardless of the starting position

• Primary motor cortex seems to contain an action map (in addition to a topographic map)

Cerebellum and Basal Ganglia

• Both structures are important and highly interconnected sensorimotor pathways
• Interact with different levels of the sensorimotor hierarchy
• Coordinate and modulate its activities
• Neither is a major part of the pathway where signals descend through via the sensorimotor hierarchy

Cerebellum

• Receives input from primary and secondary motor cortex, brainstem motor nuclei, and feedback from motor responses via the somatosensory and vestibular systems
• Thought to compare these sources of input and correct ongoing movements deviating from their intended course.
• Major role in motor learning, specifically in learning of movement sequences where timing is critical
• Monitors and adapts ongoing movement patterns
• Involved in learning from errors and prediction of errors

Basal Ganglia

• Perform a modulatory function
• Contribute few fibers to descending motor pathways, instead forming neural loops via reciprocal connections with cortical areas and the cerebellum
• Involved in several cognitive functions and many motivational aspects
• Partake in learning (ie: habit learning)
• Responsible for movement vigor :control of he speed and amplitude of movement based on motivational factors
• Active suppression of potentially inappropriate/unhealthy motor activity is performed for movement execution

Descending Motor Pathways

• Neural signals are conducted from the primary motor cortex to the motor neurons of the spinal cord over four different pathways

Dorsolateral Motor pathways

• Descend in the dorsolateral region of the spinal cord
• Terminate in the contralateral half of one spinal cord segment, sometimes directly on a motor neuron
• Activate motor neurons that project to distal muscles (e.g., finger muscles)

Ventromedial Motor Pathways

• Descend in the ventromedial region of the spinal cord

• Innervate interneurons on both sides of the spinal gray matter and in several segments

• Activate motor neurons that project to proximal muscles of the trunk and limbs (e.g., shoulder muscles)

• All tracts originate in the cerebral cortex, all presumed to mediate voluntary movement

• Ventromedial tracts involved posture control and whole body movements; can exert limb movement control during these activities

• Dorsolateral tracts control limb movement

Sensorimotor Spinal Circuits: Muscles

• Motor units are the smallest units of motor activity
• Each contains the single motor neuron and the skeletal muscle fibers that it innervates
• Differ in the number of muscle fibers they contain

Fast muscle fibers

• Contract/relax quickly
• Fatigues quickly because they are poorly vascularized

Slow muscle fibers

• Although slower/weaker, are capable of more sustained contraction because they are more richly vascularized
• Each muscle applies force only in one direction (elastic, rather than inflexible and cable-like)

Isometric contraction

• Activation of a muscle can increase the tension that it exerts on two bones without shortening/pulling them together

Dynamic contraction

• Activation can shorten and pull them together
• Tension in a muscle is increased by increasing motor pool neurons that are firing, by increasing already firing rates, or by a combination of these changes

Receptor Organs of Tendons and Muscles

• Monitors skeletal muscles

Golgi tendon organs are embedded in the tendons

• Connect each skeletal muscle to bone
• Respond to increases in muscle tension

Muscle spindles are embedded in the muscle tissue itself

• Respond to changes in muscle length
• Golgi tendon organs provide the central nervous system with muscle tension information
• Their protection prevents extreme contraction damage, causing muscle relaxation via exciting inhibitory interneurons in the spinal cord

Stretch Reflex

• Elicited by a sudden external stretching force on a muscle (stretch reflex)

• The sudden stretch initiates a chain of events - Stretch receptors initiate volley of action potentials carried from the stretch receptor into spinal cord by spindle afferent neurons via the dorsal root - Action potentials excite the motor neurons in the ventral horn of the spinal cord - Motor neurons responds with action potentials back the previously stretched the muscle - Compensatory muscle contraction and leg extension

Withdrawal Reflex

• The withdrawal reflex is not monosynaptic and it only relates to the action of withdrawing from a painful stimulus

• Involves one interneuron through a neural signal to cross two synapses

  • 1.6 milliseconds
  • First responses are recorded in flexor motor neurons of the arm

• Signals trigger motor neuron responses to arm flexors after the initial volley

• Signals have travelled over multisynaptic pathways

Reciprocal Innervation

• Antagonistic muscles are innervated permits a smooth, unimpeded motor response - Single sensory input produces a coordinated motor output: activities of agonists and antagonists are automatically coordinated by the internal circuitry of the spinal cord

Recurrent Collateral Inhibition

• Collateral inhibition is produced by the local feedback circuits - Inhibition is called recurrent collateral inhibition

  • Small inhibitory interneurons that mediate recurrent collateral inhibition are called Renshaw cells - A motor neuron fires, it momentarily inhibits itself, then shifts the responsibility to the muscle's motor pool to contract a a particular muscle

Central Sensorimotor Programs and Learning

• System of sensorimotor programs by which the spinal cord directs its various parts on what to do - Comprises a hierarchy of them - All the levels can move as prompted by each program
• Able to operate on the sense feedback without directly controlled by a level above it - Individuals barely notice these movements

Characteristics of Central Programs

Capable of Motor Equivalence

• Not always accomplish a particular task in exactly the same way. - The same basic movement can be carried out in different ways involving different muscles
• Is a demonstration of the plasticity in the sensorimotor system
• General programs are stored higher, with a situationally oriented adaptation
• Those for signing one's name in specific store in motor cortex regions, but those regions also activate if one were signing their name with their feet (instead)

Sensory Information is Not Always Conscious

• The neural mechanisms of conscious visual perception differs from the perception in the behavior of dorsal streams
• Perceptually separated individuals can act with (dorsal)/ without (ventral) conscious awareness

Programs Can Develop Without Practice

• Can be established for many species typical with or without explicit practice
• Deprived individuals who have innate traits can show this proficiency without practice of sensory feedback

Practice Can Create Programs

• Practice modifies or generates central programs and there are 2 emphasizes in sensoritimotor learning - Chunking - Levels shifting