17Su11Lec3

17Su11Lec3 - CS17 Neurobiology of Cognition Spring 2011...

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CS17 Neurobiology of Cognition Spring 2011 Lecture 3: Control of Movement Muscles Three types Smooth (internal organs), Cardiac (heart, endogenous activity) and Striate (Skeletal, Facial) Striate : Band of parallel fibers, each fiber made of many cells but acts as unit; Attached by tendons to bones; - Come in Antagonistic Pairs : Flexor moves bone toward body, Extensor move same bone from body Neuro-Muscular Junction (primarily involving Striate Muscles); - Motor Neurons (“Alpha Motor Neurons”) exit Ventral Root of Spine >> Striate muscles - Neuron synapses directly onto muscle, releasing Acetylcholine (ACh), always sufficient to trigger response - Usually 1 axon branches to multiple muscle fibers; the fewer fibers per axon the more precise the control - Like in a neuron , Muscle fiber response is all/nothing depolarization => Na+ and then K+ gates open/close - Ca++ enters muscle cells => triggers energy-requiring process that causes muscle contraction (See below) - Afterward, Ca++ is actively pumped out, and a Na+/K+ Pump repolarizes fiber for next contraction Contractile Mechanism - Within each fiber is a string of Sarcomeres (the contractile units) each consisting of…. Myosin = thick protein filament with knobby bead-like clusters (“Cross Bridges”) along it, and Actin = thin protein filament, a coiled double-strand braid, anchored to muscle - Contraction = Myosin Cross Bridges hook into (grab) coiled Actin, bend to tighten coil, release, repeat - So note, the only active muscle response is contraction – all stretch is passive. Reflexes – Most involve Spinal Cord circuits (i.e. without brain participation) Stretch Reflex - Proprioceptors called Spindles in muscle detect passive stretch of muscle (e.g. while walking, lifting, being tapped on knee by a doctor “testing your reflexes”, etc.) - Axon of Spindle to Spinal Cord, excites Motor Neuron back to same muscle, contracts to counter stretch - Note: this is the only “mono-synaptic ” reflex Golgi Reflex - Proprioceptors called Golgi Tendon Organs in tendons detect intensity of muscle contraction - If contraction is too strong (threatens to tear muscle apart) sends signal to Interneurons in Spinal Cord that inhibit the Motor Neurons causing that contraction, lessening their rate of firing
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17Su11Lec3 - CS17 Neurobiology of Cognition Spring 2011...

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