Handout 6 Sol

Handout 6 Sol - TA: Anna Gruzman E-mail: agruzman@ucsd.edu...

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TA: Anna Gruzman Section 6 E-mail: agruzman@ucsd.edu 2/14/10 1. Describe the pathway for a monosynaptic and polysynaptic reflex. What would you classify autonomic reflexes as? Monosynaptic reflex: single synapse between the two neurons in the pathway - stimulus => receptor => sensory neuron => dorsal root => efferent neuron => target cell effector => response - An example of this is the knee-jerk reflex Polysynpatic reflex: three or more neurons in the pathway ( at least two synapses) and can have extensive branching to CNS - stimulus => receptor => sensory neuron => interneuron => effecrent neuron => target cell effector => response - ALL autonomic reflexes are polysynaptic with at least one synapse in the CNS and another in the autonomic ganglion. 2. Describe the function of alpha motor neurons, extrafusal muscle, fiber and muscle spindles. Explain the mechanism of contraction as well as the stretch reflex. What happens to spindle activity when a resting muscle contracts and shortens? Some vocabulary: - proprioceptors ( sensory receptors) o located in skeletal muscles, joint capsules, ligaments o monitor positions of limbs in space o - sensory neurons- carry input signal from proprioceptors to CNS - somatic motor neurons – carry output signals ( ex: alpha motor neurons) - alpha motor neurons: somatic motor neurons that innervate skeletal muscle contractile fibers ( cause extrafusal fibers to initiate contraction) - extrafusal muscle fibers: contractile skeletal muscles that are effectors o action potentials in alpha motor neurons cause extrafusal fibers to contract - Muscle spindles: stretch receptors that send information to the spinal cord and brain o Intrafusal fibers- small group of muscle fibers that have ends that are contractile Contractile ends innervated by gamma motor neurons Non contractile central region stimulated by stretch Stretch => intrafusal fibers => spinal cord => alpha motor neurons => innervate muscle o Gamma motor neurons- from CNS innervate intrafusal fibers o Muscle tone- certain level of tension maintained even at rest At rest central region of each muscle spindle stretched enough to activate sensory fibers Sensory neurons from spindles send constant action potentials to CNS
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At rest: sensory neuron (responds to stretch) => spinal cord integrates function => alpha motor neuron => extrafusal fibers ( tonic input) => maintain resting level of tension Stretch Reflex- reflex pathway in which muscle stretch initiates a contraction response o RESPONSE TO STRETCH: stretch => increased afferent signals to spinal cord => send sensory stretch info to the CNS => inc. AP freq => stimulates gamma motor neurons => innervate intrafusal fibers and alpha motor neurons => innervate extrafusal fibers=> fibers contract reversing stretch Contraction: alpha-gamma coactivation o Alpha motor neuron fires and gamma motor neuron fires => muscle contracts => stretch on centers of intrafusal fibers does not change 3. The following is an incomplete graph comparing muscle length to afferent sensory
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Handout 6 Sol - TA: Anna Gruzman E-mail: agruzman@ucsd.edu...

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