Presentation on theme: "TOPIC 6: The Sensorimotor System"— Presentation transcript:
1 TOPIC 6: The Sensorimotor System How You Do What You Do
2 Motor Systems Functions Guided by sensory systems Movement Posture & balanceCommunicationGuided by sensory systemsInternal representation of world & selfDetect changes in environmentexternal & internal ~
3 3 Classes of Movement 1. Voluntary Complex actions reading, writing, playing pianoPurposeful, goal-orientedLearnedimprove with practice ~
4 Continue… 3 Classes of Movement 2. ReflexesInvoluntary, rapid, stereotypedeye-blink, coughing, knee jerkGraded control by eliciting stimulus3. Rhythmic motor patternsCombines voluntary & reflexive actschewing, walking, runningInitiation & termination voluntaryonce initiated, repetitive & reflexive ~
5 Control of movement by the brain What are the principles of sensorimotor Function?
6 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.
7 3 Principles of Sensorimotor Function Hierarchical organizationAssociation cortex at the highest level, muscles at the lowestParallel structure – signals flow between levels over multiple pathsLike a company: President (association cortex) issues general commands and lower level (motor neurons and muscles) take care of detailsAdvantage: Higher levels are left free to focsu on complex fucntions.
9 Continue… Motor output guided by sensory input. Sensorimotor system monitors the external world (sensory input) and the consequences of its own actionsIt acts accordinglyLearning (experience) changes the nature and locus of sensorimotor controlWith 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.
10 2 Major Areas of Sensorimotor Association Cortex Each composed of several different areas with different functionsSome disagreement exists about how to divide the areas up:Posterior parietal association cortexDorsolateral prefrontal association cortex
11 A. Posterior Parietal Association Cortex Before we respond to sensory input, we integrates information aboutBody part location (Where are they?)External objectsReceives visual, auditory, and somatosensory informationMost outputs go to secondary motor cortex.
12 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
13 B. Dorsolateral Prefrontal Association Cortex Input comes from posterior parietal cortexProjects 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.
15 Secondary Motor Cortex (SMC) Input mainly from association cortexOutput mainly to primary motor cortexAt least 7 different areas of SMC in each hemisphere2 supplementary motor areas, 2 premotor areas (i.e.dorsal and ventral); 3 cingulate motor areasAll are interconnected, All send axons to the motor circuits of the brainstem.
16 Continue… SMC Produces complex movements when stimulated Activated before and during voluntary movementsAre active when either side of the body is involved in ta movement.Premotor cortex: Respond → visual and touch stimuli
17 Primary Motor Cortex (PMC) Precentral gyrus of the frontal lobeMajor point of convergence of cortical sensorimotor signalsMajor point of departure of signals from cortexSomatotopic – more cortex is devoted to body parts that make more movements (eg. face vs elbow)
19 The Motor HomunculusEg: 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)
20 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.
21 Cerebellum 10% of brain mass, > 50% of its neurons Input from 1° (PMC) and 2° (SMC) motor cortexInput from brain stem motor nucleiFeedback from motor responsesInvolved in fine-tuning and motor learningMay also do the same for cognitive responses.
22 Basal Ganglia A collection of nuclei Part of neural loops that receive information from various part of cortex and send output back via the thalamusModulate motor output and cognitive functionsEg: Sequencing of movements, expanded role in non-motor tasks (siognitive)
23 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)?
24 4 Descending Motor Pathways: 2 dorsolateral areas of spinal cordCorticospinal tractCorticorubrospinal tract2 ventromedial (each side of Spinal cord)Cortico-brainstem-spinal tract tractBoth corticospinal tracts are direct
25 Dorsolateral TractsDCorticospinalT = Axons from PMC descend through Medullary pyramids → then cross, continue to contralateral dorsolateral white matter of spinal cordBetz cells (in PMC) – synapse on motor neurons projecting to leg musclesWrist, hands, fingers, toesDCorticorubrospinalT = Axons from PMC – synapse at red nucleus and cross before the medullaSome control muscles of the faceDistal muscles of arms and legs
27 Ventromedial Tracts Corticospinal Cortico-brainstem-spinal Descends ipsilaterally (following the same side)Axons branch and stimulate interneuron circuits bilaterally in multiple spinal segments.Cortico-brainstem-spinalInteracts with various brain stem structures and descends bilaterally carrying information from both hemispheresSynapse on interneurons of multiple spinal segments controlling proximal trunk and limb muscles
29 Dorsolateral Vs Ventromedial Motor Pathways One direct tract, one that synapses in the brain stemTerminate in one contralateral spinal segmentDistal musclesLimb movementsVentromedialOne direct tract, one that synapses in the brain stemMore diffuseBilateral innervation/ stimulationProximal musclesPosture and whole body movement
30 REFLECTIVE BEHAVIOUR: Motor Units and Muscles Motor units – a motor neuron + muscle fibers, all fibers contract when motor neuron firesNumber of fibers per unit varies – fine control requires fewer fibers/neuronMuscle – fibers bound together by a tendon
31 MusclesAcetylcholine (NT) released by motor neurons at the neuromuscular junction causes contraction.Motor pool – all motor neurons innervating the fibers of a single muscleTypes of muscles fibers:Fast muscle fibers – fatigue quicklySlow muscle fibers – capable of sustained contraction due to vascularizationMuscles are a mix of slow and fast
32 Muscles Movement occurs at joints Flexors – bend or flex a joint Extensors – straighten or extend limbContraction & relaxation of opposing muscles
33 Movement & MusclesSynergistic muscles – any 2 muscles whose contraction produces the same movementAgonists: prime moversAntagonistic muscles – any 2 muscles that act in oppositionAntagonistscounterbalance agonistsdecelerate movement ~
34 Functions of muscles:Movement control more than contraction & relaxationAccurately time control of many musclesMake postural adjustment during movementAdjust for mechanical properties of joints & musclesinertia, changing positions ~
35 Receptor Organs of Tendons and Muscles Golgi tendon organsEmbedded in tendonsTendons connect muscle to boneDetect muscle tensionMuscle spindlesEmbedded in muscle tissueDetect changes in muscle length
38 Types of Reflexes:Stretch reflex – monosynaptic, serves to maintain limb stabilityWithdrawal reflex – multisynaptic, evoke by painful stimulus, before info reaches the brainReciprocal innervation – antagonistic muscles interact so that movements are smooth – flexors are excited while extensors are inhibited, etc.
40 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.
41 Motor equivalenceA 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).
42 The Development of Central Sensorimotor Programs Programs/controls: many species-specific behaviors established without practice for central sensorimotor programsFentress (1973) – mice without forelimbs still make coordinated grooming motionsPractice can also generate and modify programs:Through response chunkingShifting control to lower levels
43 The Development of Central Sensorimotor Programs Response chunkingPractice combines the central programs controlling individual responseShifting control to lower levelsFrees up higher levels to do more complex tasksPermits greater speed
44 Hierarchical Control of Movement 3 levels of controlSpinal cord (SC)BrainstemCortexDivision of responsibilityHigher levels: general commandsSpinal cord: complex & specificEach receives sensory inputRelevant to levels function ~
45 Hierarchical Control: Spinal Cord Automatic & stereotyped responsesreflexesrhythmic motor patternsCan function without brainSpinal interneuronssame circuits as voluntary movementPathways converge on a motor neuronsfinal common path ~
46 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?