Pinel basics ch06

Copyright © 2007 by Allyn and Bacon

Chapter 6The Sensorimotor System

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3 Principles of Sensorimotor Function Hierarchical organization

Association cortex at the highest level, muscles at the lowest

Parallel structure – signals flow between levels over multiple paths

Motor output guided by sensory input Learning (experience) changes the nature

and locus of sensorimotor controlConscious to automatic, for example



2 Major Areas of Sensorimotor Association Cortex Each composed of several different

areas with different functions Some disagreement exists about how

to divide the areas up Posterior parietal association cortex Dorsolateral prefrontal association

cortex


Posterior Parietal Association Cortex

Integrates information aboutBody part locationExternal objects

Receives visual, auditory, and somatosensory information

Outputs to motor cortex


What affect does damage to the posterior parietal area have? Apraxia – disorder of voluntary

movement – problem only evident when instructed to perform an action – usually a consequence of damage to the area on the left

Contralateral neglect – unable to respond to stimuli contralateral to the side of the lesion - usually seen with large lesions on the right


Dorsolateral Prefrontal Association Cortex Input from posterior parietal cortex Output to secondary motor cortex,

primary motor cortex, and frontal eye field

Evaluates external stimuli and initiates voluntary reactions – supported by neuronal responses



Secondary Motor Cortex

Input mainly from association cortex Output mainly to primary motor cortex At least 7 different areas

2 supplementary motor areas SMA and preSMA

2 premotor areas dorsal and ventral

3 cingulate motor areas


Secondary Motor Cortex

Subject of ongoing research May be involved in programming

movements in response to input from dorsolateral prefrontal cortex

Many premotor neurons are bimodal – responding to 2 different types of stimuli


Primary Motor Cortex

Precentral gyrus of the frontal lobe Major point of convergence of cortical

sensorimotor signals Major point of departure of signals from

cortex Somatotopic – more cortex devoted to

body parts which make many movements


Motor homunculus


The Motor Homunculus

Control of hands involves a network of widely distributed neurons

Stereognosis – recognizing by touch – requires interplay of sensory and motor systems

Some neurons are direction specific – firing maximally when movement is made in one direction


Cerebellum and Basal Ganglia

Interact with different levels of the sensorimotor hierarchy

Coordinate and modulate May permit maintenance of visually

guided responses despite cortical damage


Cerebellum

10% of brain mass, > 50% of its neurons Input from 1° and 2° motor cortex Input from brain stem motor nuclei Feedback from motor responses Involved in fine-tuning and motor learning May also do the same for cognitive

responses


Basal Ganglia

A collection of nuclei Part of neural loops that receive

cortical input and send output back via the thalamus

Modulate motor output and cognitive functions


4 Descending Motor Pathways

2 dorsolateralCorticospinal Corticorubrospinal

2 ventromedialCorticospinalCortico-brainstem-spinal tract

Both corticospinal tracts are direct


Dorsolateral Tracts

Most synapse on interneurons of spinal gray matter Corticospinal - descend through the medullary pyramids,

then cross Betz cells – synapse on motor neurons projecting to

leg muscles Wrist, hands, fingers, toes

Corticorubrospinal – synapse at red nucleus and cross before the medulla Some control muscles of the face Distal muscles of arms and legs



Ventromedial Tracts

Corticospinal Descends ipsilaterally Axons branch and innervate interneuron circuits

bilaterally in multiple spinal segments Cortico-brainstem-spinal

Interacts with various brain stem structures and descends bilaterally carrying information from both hemispheres

Synapse on interneurons of multiple spinal segments controlling proximal trunk and limb muscles



Dorsolateral Vs Ventromedial Motor Pathways Dorsolateral one direct tract, one

that synapses in the brain stem

Terminate in one contralateral spinal segment

Distal muscles Limb movements

Ventromedial one direct tract, one

that synapses in the brain stem

More diffuse Bilateral innervation Proximal muscles Posture and whole

body movement


Motor Units and Muscles

Motor units – a motor neuron + muscle fibers, all fibers contract when motor neuron fires

Number of fibers per unit varies – fine control, fewer fibers/neuron

Muscle – muscle fibers bound together by a tendon


Muscles

Acetylcholine released by motor neurons at the neuromuscular junction causes contraction

Motor pool – all motor neurons innervating the fibers of a single muscle

Fast muscle fibers – fatigue quickly Slow muscle fibers – capable of sustained

contraction due to vascularization Muscles are a mix of slow and fast


Muscles

Flexors – bend or flex a joint Extensors – straighten or extend Synergistic muscles – any 2

muscles whose contraction produces the same movement

Antagonistic muscles – any 2 muscles that act in opposition


Receptor Organs of Tendons and Muscles Golgi tendon organs

Embedded in tendonsTendons connect muscle to boneDetect muscle tension

Muscle spindlesEmbedded in muscle tissueDetect changes in muscle length



Knee-jerk (patellartendon) reflex


Reflexes

Stretch reflex – monosynaptic, serves to maintain limb stability

Withdrawal reflex – multisynaptic Reciprocal innervation – antagonistic

muscles interact so that movements are smooth – flexors are excited while extensors are inhibited, etc.



Central Sensorimotor Programs

Perhaps all but the highest levels of the sensorimotor system have patterns of activity programmed into them and complex movements are produced by activating these programs

Cerebellum and basal ganglia then serve to coordinate the various programs


Motor equivalence

A given movement can be accomplished various ways, using different muscles

Central sensorimotor programs must be stored at a level higher than the muscle (as different muscles can do the same task)

Sensorimotor programs may be stored in 2° motor cortex


The Development of Central Sensorimotor Programs Programs for many species-specific

behaviors established without practice Fentress (1973) – mice without forelimbs

still make coordinated grooming motions Practice can also generate and modify

programsResponse chunkingShifting control to lower levels


The Development of Central Sensorimotor Programs Response chunking

Practice combines the central programs controlling individual response

Shifting control to lower levelsFrees up higher levels to do more

complex tasksPermits greater speed

Technology

Pinel basics ch06