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