TOPIC 6:The Sensorimotor System

How You Do What You Do

Motor Systems Functions

MovementPosture & balanceCommunication

Guided by sensory systemsInternal representation of world & selfDetect changes in environment

external & internal ~

3 Classes of Movement

1. VoluntaryComplex actions

reading, writing, playing pianoPurposeful, goal-orientedLearned

improve with practice ~

Continue… 3 Classes of Movement2. Reflexes

Involuntary, rapid, stereotyped

eye-blink, coughing, knee jerkGraded control by eliciting stimulus

3. Rhythmic motor patternsCombines voluntary & reflexive acts

chewing, walking, runningInitiation & termination voluntaryonce initiated, repetitive & reflexive ~

Control of movement by the brain What are the principles of

sensorimotor Function?

3 Principles of Sensorimotor Control

The sensorimotor system is hierachically organized.

Motor output is guided by sensory input.

Learning can change the nature and locus of sensorimotor control.

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

Like a company: President (association cortex) issues general commands and lower level (motor neurons and muscles) take care of details

Advantage: Higher levels are left free to focsu on complex fucntions.

Continue… Motor output guided by sensory input.

Sensorimotor system monitors the external world (sensory input) and the consequences of its own actions

It acts accordingly Learning (experience) changes the nature

and locus of sensorimotor control With regards to new tasks, after much practice at the

lower levels – they mastered the task. These well-learned tasks need little involvement from

the higher level.

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 cortexDorsolateral prefrontal association

cortex

A. Posterior Parietal Association Cortex Before we respond to sensory input, we

integrates information aboutBody part location (Where are they?)External objects

Receives visual, auditory, and somatosensory information

Most outputs go to secondary 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 hemiphere.

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

B. Dorsolateral Prefrontal Association Cortex Input comes from posterior parietal cortex Projects output to secondary motor cortex,

primary motor cortex, and frontal eye field. Evaluates external stimuli (i.e.

characteristic, location, response fr object) Initiates voluntary reactions – supported

by neuronal responses.

Secondary Motor Cortex (SMC)

Input mainly from association cortex Output mainly to primary motor cortex At least 7 different areas of SMC in each

hemisphere2 supplementary motor areas, 2 premotor areas

(i.e.dorsal and ventral); 3 cingulate motor areas All are interconnected, All send axons

to the motor circuits of the brainstem.

Continue… SMC

Produces complex movements when stimulated

Activated before and during voluntary movements

Are active when either side of the body is involved in ta movement.

Premotor cortex: Respond → visual and touch stimuli

Primary Motor Cortex (PMC)

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 is devoted to

body parts that make more movements (eg. face vs elbow)

Motor homunculus

The Motor Homunculus

Eg: Control of hands involves a network of widely distributed neurons on PMC → each area of PMC received feedback from muscles and joints it influences.

Stereognosis – ability of recognisation of object from touches – requires input from sensory (skin) and motor systems (feedback).

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

Subcortical sensorimotor structures: Cerebellum and Basal Ganglia

Interact with different levels of the sensorimotor hierarchy but neither participates directly in signal transmission to the spinal cord.

Coordinate and modulate activities at various level of the sensorimotor system.

Cerebellum

10% of brain mass, > 50% of its neurons Input from 1° (PMC) and 2° (SMC) 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 information

from various part of cortex and send output back via the thalamus

Modulate motor output and cognitive functions

Eg: Sequencing of movements, expanded role in non-motor tasks (siognitive)

SCL:

How do principle of sensorimotor system relate to our daily life?

What will happen when cerebellum is damaged?

How is Primary Motor Cortex organised and what is/are its main function(s)?

4 Descending Motor Pathways:

2 dorsolateral areas of spinal cordCorticospinal tractCorticorubrospinal tract

2 ventromedial (each side of Spinal cord)Corticospinal tractCortico-brainstem-spinal tract tract

Both corticospinal tracts are direct

Dorsolateral Tracts

DCorticospinalT = Axons from PMC descend through Medullary pyramids → then cross, continue to contralateral dorsolateral white matter of spinal cord Betz cells (in PMC) – synapse on motor neurons

projecting to leg muscles Wrist, hands, fingers, toes

DCorticorubrospinalT = Axons from PMC – 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 (following the same side) Axons branch and stimulate 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/

stimulation Proximal muscles Posture and whole

body movement

REFLECTIVE BEHAVIOUR: 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 requires fewer fibers/neuron

Muscle – fibers bound together by a tendon

Muscles

Acetylcholine (NT) released by motor neurons at the neuromuscular junction causes contraction.

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

Types of muscles fibers: Fast muscle fibers – fatigue quickly Slow muscle fibers – capable of sustained contraction

due to vascularization Muscles are a mix of slow and fast

Muscles

Movement occurs at joints Flexors – bend or flex a jointExtensors – straighten or extend limbContraction & relaxation of opposing muscles

Movement & Muscles Synergistic muscles – any 2 muscles

whose contraction produces the same movementAgonists: prime movers

Antagonistic muscles – any 2 muscles that act in oppositionAntagonists

counterbalance agonists decelerate movement ~

Functions of muscles: Movement control more than contraction

& relaxationAccurately time control of many musclesMake postural adjustment during

movementAdjust for mechanical properties of joints &

muscles inertia, changing positions ~

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 reflex

Types of Reflexes:

Stretch reflex – monosynaptic, serves to maintain limb stability

Withdrawal reflex – multisynaptic, evoke by painful stimulus, before info reaches the brain

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 is 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 (SMC)

Eg: You can sign your name with left or right hand. Signature is very similar and the SMC for preferred right hand is activated (even when left hand signs).

The Development of Central Sensorimotor Programs Programs/controls: many species-specific

behaviors established without practice for central sensorimotor programs

Fentress (1973) – mice without forelimbs still make coordinated grooming motions

Practice can also generate and modify programs:Through response 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

Hierarchical Control of Movement 3 levels of control

Spinal cord (SC) Brainstem Cortex

Division of responsibilityHigher levels: general commandsSpinal cord: complex & specific

Each receives sensory inputRelevant to levels function ~

Hierarchical Control: Spinal Cord Automatic & stereotyped responses

reflexesrhythmic motor patterns

Can function without brain Spinal interneurons

same circuits as voluntary movement Pathways converge on motor neurons

final common path ~

SCL:

How does our brain control muscles? What are the neural pathways?

What are the concepts related to Central Sensorimotor program and its important functions?