TOPIC 6: The Sensorimotor System How You Do What You Do

Motor Systems Functions Guided by sensory systems Movement Posture & balance Communication Guided by sensory systems Internal representation of world & self Detect changes in environment external & internal ~

3 Classes of Movement 1. Voluntary Complex actions reading, writing, playing piano Purposeful, goal-oriented Learned improve with practice ~

Continue… 3 Classes of Movement 2. Reflexes Involuntary, rapid, stereotyped eye-blink, coughing, knee jerk Graded control by eliciting stimulus 3. Rhythmic motor patterns Combines voluntary & reflexive acts chewing, walking, running Initiation & termination voluntary once 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 cortex Dorsolateral prefrontal association cortex

A. Posterior Parietal Association Cortex Before we respond to sensory input, we integrates information about Body 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 hemisphere 2 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 cord Corticospinal tract Corticorubrospinal tract 2 ventromedial (each side of Spinal cord) Cortico-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 Cortico-brainstem-spinal 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 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 joint Extensors – straighten or extend limb Contraction & relaxation of opposing muscles

Movement & Muscles Synergistic muscles – any 2 muscles whose contraction produces the same movement Agonists: prime movers Antagonistic muscles – any 2 muscles that act in opposition Antagonists counterbalance agonists decelerate movement ~

Functions of muscles: Movement control more than contraction & relaxation Accurately time control of many muscles Make postural adjustment during movement Adjust for mechanical properties of joints & muscles inertia, changing positions ~

Receptor Organs of Tendons and 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

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 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

Hierarchical Control of Movement 3 levels of control Spinal cord (SC) Brainstem Cortex Division of responsibility Higher levels: general commands Spinal cord: complex & specific Each receives sensory input Relevant to levels function ~

Hierarchical Control: Spinal Cord Automatic & stereotyped responses reflexes rhythmic motor patterns Can function without brain Spinal interneurons same circuits as voluntary movement Pathways converge on a 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?