Movement- Spinal CordSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17SummaryMovement- Spinal Cord•How does the release of neurotransmitter from an alpha motor neuron result in contraction?•What are the inputs to alpha motor neurons?•How does the circuitry of the spinal cord result in reflexive movements? Why do we need spinal reflexes?Muscle FibersSynaptic Transmission to ContractionSynaptic Transmission to ContractionATP binds to myosin during a muscle contraction. What does the ATP do?a) Cause myosin to bind to actinb) Prevent the myosin head from movingc) Cause myosin to unbind actind) Cause myosin to bind troponin1. APs in motor neuron causes Ach release2. Ach binds to receptor and depolarizes the sarcolemma, generating APs3. APs invades the T-tubules (tunnels that are continuous with the sarcolemma)4. V-gated channels in T-tubules open and open Ca2+ channels in sarcoplasmic reticulum (Ca2+ storage organelle)5. Ca2+ enters cytosol and causes myofibrils to contract:a) Ca2+ binds to troponin and tropomyosin moves to expose myosin binding site on actinb) Myosin (thick filament) binds to actin (thin filament) and the head pivotsc) Myosin disengages from actind) If Ca2+ is still present, return to (b)6. Ach is degraded by acetylcholinesterase, receptors close, sarcolemma returns to rest, and Ca2+ is actively pumped back into the sarcoplamic reticulumWhy does rigor mortis (stiffening of the muscles) occur upon death?a) Ca2+ can no longer be released to allow myosin to bind to actinb) ATP is no longer present to release myosin from actinc) Action potentials can no longer be generated by the sarcolemmaReflexes- Myotatic reflex:Reverse myotatic reflex:Flexor/Crossed extensor reflex:Myotatic Reflex“offline”“online”Gamma motor neurons keep muscle spindles “online”What would happen if the upper motor neurons only innervated the gamma motor neurons?a) The muscle would not contract and the muscle spindle would be inactiveb) The muscle spindle would be activated, but there would not be muscle contractionc) The muscle spindle would be activated and there would be muscle contraction.The knee-jerk reflex- myotatic reflex with reciprocal inhibitionReverse Myotatic ReflexFlexor/Crossed Extensor ReflexSummary•Synaptic transmission at the neuromuscular junction results in the release of Ca2+ from the sarcoplamic reticulum, which allows myosin to bind to actin and shorten the length of the sarcomeres in myofibrils•Single action potentials in motor neurons result in twitches; a smooth contraction requires many action potentials close in time•Motor neurons receive inputs from upper MNs as well as muscle spindle Ia afferents and spinal interneurons•Spinal reflexes rely solely on the circuitry in the spinal cord and each serves to protect our bodies from