PSYC271 Lecture 19: Spinal Motor Control PDF

PSYC271 Lecture 19 Spinal motor control Sensorimotor Spinal Circuits Much of our motor output is controlled by simple circuits between receptors, the spinal cord, and muscles Why make a simple job complicated? Often no need to get the brain involved Simple = fast and reliable Mus...

PSYC271 Lecture 19 Spinal motor control Sensorimotor Spinal Circuits Much of our motor output is controlled by simple circuits between receptors, the spinal cord, and muscles Why make a simple job complicated? Often no need to get the brain involved Simple = fast and reliable Muscles Motor Unit: Single motor neuron and all of the individual skeletal muscle fibers it innervates Motor pool: All of the motor neurons that innervate a muscle (2 motor units) Neuromuscular junctions Where neuron meets muscle fibre Acetylcholine used at neuromuscular jct. Acetylcholinesterase causes channel to close in < 1 ms Motor end-plate: receptive area on muscle fibre at neuromuscular jct. Muscles Attached to bone by tendon Fast muscle fibres: forceful, poorly vascularized, fatigue quickly Slow muscle fibres: weaker, well-vascularized, high endurance Flexors & Extensors Synergistic muscles produce the same movement Antagonistic muscles produce opposite movements Isometric contraction increases muscle tension without movement Receptors in Tendons & Muscles Golgi Tendon Organs: Embedded in tendons Respond to muscle tension Help prevent over-contraction Muscle Spindles: Embedded in muscle Respond to change in muscle length Proprioception Extrafusal & Intrafusal Muscle The Stretch Reflex Adjusts muscle tension in response to external forces Intrafusal motor neuron adjust length of intrafusal fibers Monosynaptic: fast! Muscle-Spindle Feedback System  Withdrawal Reflex Sensory neurons carry pain signals Synapse on interneurons on flexors Withdrawal in just 1.6 ms Starts before information travels to the brain Reciprocal Innervation When a muscle contracts, antagonistic muscles relax Mediated by inhibitory interneurons Some degree of co-contraction occurs for smooth movement  Recurrent Collateral Inhibition Function: Distribute workload across motor pool Process: Motor neuron fires Axon collateral excites inhibitory interneuron (Renshaw cell) in ventral horn. Inhibits motor neuron that activated it It is recurrent Enforces rest after motor firing Walking: A Complex Sensorimotor Reflex Draws upon: visual info somatosensory info vestibular info Integrated movements involving the trunk, legs, feet and upper arms Program must be plastic to adjust to changes in terrain or external forces Central Sensorimotor Program Theory Lower levels of sensorimotor system contain programmed activities Complex movements are produced by activating the correct combo Execution does not require higher level oversight “Here is the goal. I don’t care about the details.” Cats with transected spinal cords will walk on a treadmill if suspended in a sling Characteristics of Central Sensorimotor Programs Motor equivalence: The same task can be completed using different muscles or the same muscles in different ways. e.g. Your signature is similar using different muscles or body parts Central sensorimotor programs are capable of motor equivalence Control is high in the system Characteristics of Central Sensorimotor Programs Sensory info controlling central sensorimotor programs not necessarily conscious Position of fingers approaching central disks for pickup consistent with actual sizes not their consciously perceived sizes Characteristics of Central Sensorimotor Programs Can develop without practice (e.g., species-typical behavior) Fixed Action Patterns: Behavioural sequences an animal follows to completion when triggered by a defined stimulus. Chick pecking the red area induces regurgitation. Characteristics of Central Sensorimotor Programs Characteristics of Central Sensorimotor Programs Practice can create or refine central sensorimotor programs. Transferring control to lower levels of CNS frees up higher areas. e.g., pianists can concentrate on how to interpret a musical piece. Increases speed because lower levels can operate simultaneously. e.g., when learning to type, start of the next keypress begins before the current press ends. Characteristics of Central Sensorimotor Programs Response-chunking hypothesis: Practice combines programs into programs of programs (chunks) of responses. Chunks can be combined into larger chunks. e.g., when learning to type, Finding and pressing the key: chunked into a keypress. Keypresses: chunked into words. Words: chunked into sentences. Characteristics of Central Sensorimotor Programs After a motor skill is acquired, it can be disrupted by consciously thinking about it. Bringing in the higher areas just causes problems because lower areas can already do the job Beilock et al. (2008)  Functional Neuroimaging of Sensorimotor Learning Dorsolateral Prefrontal Cortex: Mostly involved when motor sequences are conscious Active during new motor sequences Premotor Cortex: Important when behaviour guided by sensory stimuli Active during new motor sequences Supplementary Motor Cortex: Automatic behaviours Well-practiced motor sequences Primary Somatosensory and Motor Cortices: Active during new and well-practiced motor sequences Posterior Parietal Cortex and Cerebellum: Active primarily during new motor sequences  Neuroplasticity and Sensorimotor Learning Sensorimotor learning occurs via changes at both cortical and subcortical levels. Strengthening of inputs from the thalamus during learning Large increase in oligodendrocytes in subcortical white matter after learning  Bridging spinal injuries Spinal injuries can lead to almost complete paralysis Bridging severed spinal tracts the focus of many technologies