TOPIC 6 The Sensorimotor System How You Do What You Do

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 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).

Major Areas of Sensorimotor Association Cortex 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

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 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

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

Dorsolateral Prefrontal Association Cortex 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

Secondary Motor 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

Primary 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

Motor homunculus

The 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

Cerebellum and Basal Ganglia Interact with different levels of the sensorimotor hierarchy Coordinate and modulate May permit maintenance of visually guided responses despite cortical damage

Cerebellum 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

Basal Ganglia 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

4 Descending Motor Pathways 2 dorsolateral Corticospinal Corticorubrospinal 2 ventromedial Cortico-brainstem-spinal tract Both corticospinal tracts are direct

Dorsolateral Tracts 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

Ventromedial 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

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 Proximal muscles Posture and whole body movement

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

Muscles 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

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

Reflexes 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.

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 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

The Development of Central Sensorimotor Programs 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