TOPIC 6 The Sensorimotor System

1. TOPIC 6The Sensorimotor System How You Do What You Do

2. 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. 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 sensorimotorcontrol. • E.g. from Conscious behavior to automatic. From conscious control (cortical level) to "Automatic Pilot" (lower levels).

5. Major Areas of SensorimotorAssociation 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

6. Posterior Parietal Association Cortex • This cortex receives input from • The somatosensorysystem, •  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).

7. Posterior Parietal Association Cortex • This cortex receives input from • The somatosensorysystem, •  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).

8. Posterior Parietal Association Cortex • Integrates information about • Body part location • External objects • Receives visual, auditory, and somatosensory information • Outputs to motor cortex

9. 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

10. 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

12. 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

13. 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

14. 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

15. Motor homunculus

16. 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

17. 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

18. 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

19. 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

20. 4 Descending Motor Pathways • 2 dorsolateral • Corticospinal • Corticorubrospinal • 2 ventromedial • Corticospinal • Cortico-brainstem-spinal tract • Both corticospinal tracts are direct

21. 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

23. 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

25. Dorsolateral Vs Ventromedial Motor Pathways • Ventromedial • one direct tract, one that synapses in the brain stem • More diffuse • Bilateral innervation • Proximal muscles • Posture and whole body movement • Dorsolateral • one direct tract, one that synapses in the brain stem • Terminate in one contralateral spinal segment • Distal muscles • Limb movements

26. 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

27. 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

28. 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

29. 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

31. Knee-jerk reflex

32. 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.

34. 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

35. 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

36. 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

37. 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