TOPIC 6The Sensorimotor
System
How You Do What You Do
1. The sensorimotor system is hierachically organized
2. Motor output is guided by sensory input
3. Learning can change the nature and locus of sensorimotor control
3 Principles of Sensorimotor Control
1. Hierarchical organization◦ Association cortex at the highest level, muscles at the lowest i.e
from general goals (cortical level) to specific details of action (lower levels).
◦ Parallel structure – signals flow between levels over multiple paths
◦ Information flow is down, while in the Sensory system informtion flows through the hierarchy.
2. Motor output guided by sensory input◦ Sensory feedback plays an important role in the control of
movement (exception: ballistic movements).
3. Learning (experience) changes the nature and locus of sensorimotor control.
◦ E.g. from Conscious behavior to automatic. From conscious control (cortical level) to "Automatic Pilot" (lower levels).
3 Principles of Sensorimotor Function
Sensory information is integrated in Association cortex.
Two Major areas of Sensorimotor Association Cortex are:-◦ Posterior parietal association
cortex◦ Dorsolateral prefrontal
association cortex Each composed of several
different areas with different functions
Major Areas of Sensorimotor Association Cortex
This cortex receives input from ◦ The somatosensory system, ◦ The visual system and ◦ The auditory system.
This information specifies the initial conditions for the programming of action: The original position of the body parts to be moved. The position of external objects Damage to the posterior parietal cortex causes Apraxia and Contralateral
neglect. Apraxia: difficulty in executing a movement when ordered to do so, but able to
do it when not thinking about it. Lesion is often on the left side. Contralateral Neglect: Patient does not respond to sensory stimulation from
the side opposite to the lesion of the parietal cortex (usually on the right side). The output of the Posterior Parietal Association Cortex goes to the Dorsolateral
Prefrontal Association Cortex, and to the Frontal Eye Field (Fig. 8.2).
Posterior Parietal Association Cortex
This cortex receives input from The somatosensory system, The visual system and The auditory system. This information specifies the initial conditions for the programming of
action: The original position of the body parts to be moved. The position of external objects Damage to the posterior parietal cortex causes Apraxia and Contralateral
neglect. Apraxia: difficulty in executing a movement when ordered to do so, but able
to do it when not thinking about it. Lesion is often on the left side. Contralateral Neglect: Patient does not respond to sensory stimulation from
the side opposite to the lesion of the parietal cortex (usually on the right side). The output of the Posterior Parietal Association Cortex goes to the
Dorsolateral Prefrontal Association Cortex, and to the Frontal Eye Field (Fig. 8.2).
Posterior Parietal Association Cortex
Integrates information about◦ Body part location◦ External objects
Receives visual, auditory, and somatosensory information
Outputs to motor cortex
Posterior Parietal Association 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
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
Dorsolateral Prefrontal Association 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
Secondary 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
Primary Motor Cortex
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
The Motor Homunculus
Interact with different levels of the sensorimotor hierarchy
Coordinate and modulate May permit maintenance of visually guided
responses despite cortical damage
Cerebellum and Basal Ganglia
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
Cerebellum
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
Basal Ganglia
2 dorsolateral◦Corticospinal ◦Corticorubrospinal
2 ventromedial◦Corticospinal◦Cortico-brainstem-spinal tract
Both corticospinal tracts are direct
4 Descending Motor Pathways
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
Dorsolateral 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
Ventromedial Tracts
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
Dorsolateral Vs Ventromedial Motor Pathways
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
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
Muscles
Golgi tendon organs◦ Embedded in tendons◦ Tendons connect muscle to bone◦ Detect muscle tension
Muscle spindles◦ Embedded in muscle tissue◦ Detect changes in muscle length
Receptor Organs of Tendons and Muscles
Knee-jerk reflex
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.
Reflexes
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
Central Sensorimotor Programs
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
Motor equivalence
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 programs◦Response chunking◦Shifting control to lower levels
The Development of Central Sensorimotor Programs
Response chunking◦ Practice combines the central programs
controlling individual response Shifting control to lower levels
◦ Frees up higher levels to do more complex tasks◦ Permits greater speed
The Development of Central Sensorimotor Programs