neurobio 919 ch 13 of Bear's book: spinal control of movement part 2 (flexor extensor alpha motor
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what is troponin?
a protein that attaches to the myosin attaachment sites on the actin molecule, which prevents myosin from binding to it.
what occurs in the myofibrils when Ca binds to troponin?
the myosin binding sites on actin are exposed and the myosin heads bind actin. The myosin heads pivot. Myosin heads then disengage at the expense of ATP
what are intrafusal fibers
the modified skeletal muscle fibers within the muscle spindle's fibrous capsule
what are extrafusal fibers?
muscle fibers that lie outside the spindle and form the bulk of muscle
what is a gamma motor neuron?
a type of lower motor neuron that is found within muscle fibers. they innervate the intrafusal fibers within the muscle spindle.
what occurs in the muscle spindle when the extrafusal muscle fibers shorten?
the activation of alpha neurons causes the extrafusal muscle biers to shorten. If the muscle spindle begins to slack, it goes "off the air" and no longer reports the length of the muscle. Activation of gamma motor neurons causes the poles of the spindle to contract, keeping it "on-the-air"
by what structure do the intrafusal muscle fibers communicate with alpha motor neurons?
the Ia afferent axon
what is the function of the golgi tendon organs?
they encode muscle tension information
what is the gamma loop?
the gamma loop changes the set point of the myotatic feedback loop (when the muscle contracts, it changes the length of the intrafusal muscle fibers, which causes the gamma motor neuron to change the length of the intrafusal fibers-- this changing of the lenght of the fibers is known as gamma loop).
spindles are in ____ with muscle fibers; golgi tendon organs are in ____ with muscle fibers
spindles are in parallel with muscle fibers; golgi tendon organs are in series with muscle fibers
what is purpose of the reverse myotatic reflex?
the function of the reverse myotatic reflex is to regulate muscle tension within optimal ranges
describe the process involved in the reverse myotatic reflex
as muscle tension increases, the Ib axons of the golgi tendon organs enter the spinal cord, branch repeatedly, and synapse on interneurons in the ventral horn. Some of these interneurons form inhibitory connections with the alpha motor neurons innervating the same muscle. the inhibition of the alpha motor neuron slows muscle contraction; as muscle tension falls, the inhibition of the alpha motor neuron is reduced, and muscle contraction increases . This type of proprioceptive feedback is thought to be particularly important in the proper execuation of fine motor acts, such as the manipulation of fragile objects with the hands.
the neural circuitry for walking resides where?
the spinal cord
what are central pattern generators?
in general, neural circuits that give rise to rhythmic motor activity are called central pattern generators
explain what is meant by "intrinsic pacemaker activity"
some neurons respond to the activation of NMDA receptors with rhythmic depolarization-- which give these neurons an intrinsic pacemaker quality.
describe the molecular basis behind the rhythmic activity in an NMDA-receptor neuron
- In the resting state, the NMDA receptor channels and the calcium-activated potassium channels are closed. Glutamate causes the NMDA receptors to open, the cell membrane to depolarize, and Ca2+ to enter the cell. PThe rie in intracellular Ca concentration causes the Ca-activated potassium channels to open. Potassium ions leave the neuron, hyperpolarizing the membrane. the hyperpolarization allows Mg2+ to enter and clog the NMDA channel, arresting the flow of Ca. As Ca concentration falls, the potassium channels close, resetting the membrane for another oscillation
how is rhythmic activity actually mediated?
a combination of intrinsic pacemaker properties and synaptic interconnections produce rhythmic activity
Which is recruited first, a fast motor unit or a slow motor unit? Why?
Most muscles have a range of motor unit sizes. These motor units are recruited in order of size–the smallest being recruited first and the largest last. This explains why finer control is possible when muscles are under light loads than when they are under greater loads. Small motor units have small alpha motor neurons and large motor units have large alpha motor neurons. Small neurons are more easily excited by signals descending from the brain.
When and why does rigor mortis occur?
The stiffening of muscles after death is a condition known as rigor mortis. Muscle contraction occurs because of the interaction between myosin, the major thick filament protein, and actin, the major thin filament protein, during excitation contraction coupling. The heads of myosin filaments bind to actin filaments and undergo a conformational change. This causes the thick filament to move with respect to the thin filament, shortening the muscle fiber during muscle contraction. ATP is required to release the myosin heads from the actin filament. When no ATP is available because the tissue is dead, the attachment between the thick and thin filaments becomes permanent.
which muscles are involved in maintaining posture?
axial muscles. axial muscles are those that move the trunk. this is important for maintaining posture
what do we mean by the somatic motor system?
the skeletal muscles and the parts of the nervous system that control them
where are the motor neurons that innervate the skeletal muscles located?
the motor neurons that innervate the skeletal muscles are located in the ventral horn of the spinal cord. Their axons exit the spinal cord through the ventral roots, which join with the sensory fibers of the dorsal horn in the mixed spinal nerves
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