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Biological Sciences
Peter A.Bowler

e109 10/26/12 -Muscle spindles: stretch receptors (sensor) in skeletal muscles “normal muscle cells”—extrafusal muscle fibers (regular muscle fibers) innervated by alpha motor neurons “muscle spindle cells”—intrafusal fibers innervated by motor neurons Muscle spindles monitor muscle length and prevent overstretching -red things: extrafusal muscle fibers 1. extrafusal muscle fibers at resting length 2. sensory neuron is tonically active 3. spinal chord (CNS) integrates function 4. 4. Alpha motor neurons to extrafusal fibers receive tonic input from muscle spindles 5. Extrafusal fibers maintain a certain level of tension at rest (goes to the same muscle) Muscle stretch Muscle stretch can trigger a stretch reflex. When muscles stretch and lengthen, muscle spindle sensory afferent neurons fire more. The reflex response is mucle contraction to prevent damage from over stretching 1. Muscle stretch 2. Increased afferent signals to spinal cord 3. Spinal cord 4. Increased efferent output through alpha motor neurons 5. muscle contracts More frequent action potentials=more of a stretch of the muscle Alpha (extrafusal) and gamma (intrafusal) motor neurons fire together (normally happens) As the muscle shortens, so does the muscle spindles this keeps the muscle spindle “ready” or “recalibrated” to respond to a stretch Alpha-gamma coactivation maintains spindle function when muscle contracts and shortens! -this keeps the muscle spindle “ready” or “recalibrated” to respond to a stretch Gamma motor neurons cause intrafusal fibers (of spindle organ) to contrac - without gamma motor neurons, muscle contraction causes the spindle firing rate to decrease - this is NOT what usually happens—normally there is no decrease in action potential frequency in spindle sensory neuron Stretching: Shortening: Golgi tendon organ Measure tension directly -protects muscle from being damaged (inhibits muscle contraction if large pull on tendon) Go through 13-5 and 13-6 More complex reflex: Inhibition of antagonistic musclesAND stimulation of other muscles Figure 13-7 1. pain receptor feels person’s pain when stepping ona tack 2. sends action potential to the spinal chord 3. the spinal chord (CNS) has different collateral (branches). One collateral leads to the ascending pathway to the brain. The other collaterals head to the muscle. 4. Intraneuron passes the message to the motor neuron a. negative sign shows that it is an inhibitory intraneuron b. the other muscle cells prevent the person from falling offer (the inhibitors of the flexors of the other leg are inhibited) Tetanus in a muscle vs. tetanus the disease -tetanus shots protect you from “lockjaw” or the disease tetanus -C.t. bacteria produce tetanospasmin
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