KIN 461 Week 3 Notes
KIN 461 Week 3 Notes KIN 461-401
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This 2 page Class Notes was uploaded by Tanski Notetaker on Sunday February 14, 2016. The Class Notes belongs to KIN 461-401 at University of Wisconsin - Milwaukee taught by Dr. Peterson in Spring 2016. Since its upload, it has received 27 views. For similar materials see Principles of Motor Learning in Kinesiology at University of Wisconsin - Milwaukee.
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Date Created: 02/14/16
Week 3 1 Introduction to Movement (continued) Sensory Receptors Muscle Spindles (cont.) o Response to stretch Myelinated sensory axons enter muscle spindle capsule in its central Sensory axons terminate on central portion and spiral around intrafusal fibers When intrafusal fibers stretch (called loading the spindle), sensory endings increase their firing rate due to elongation of afferent endings Unloading: stretch is released Intrafusal fibers slacken Decrease firing rate of afferent endings Golgi Tendon Organs o Sensitive to changes in tension o Located at junction between muscle fibers and tendon Myelinated Have slightly smaller diameter than group Ia fibers o Slender, encapsulated structures o Collagen bundles within capsule divide into fine fascicles that form braided structure o Stretching of tendon organ straightens the collagen bundles This compresses and elongates nerve endings, causing them to fire Because nerve endings intertwine with collagen fiber bundles, firing rate of tendon organs is very sensitive to changes in tension of the muscle Central Nervous systems controls sensitivity of the muscle spindles through the Gamma motor neurons o This ensures that information on changes in muscle length is transmitted during contraction o Intrafusal fibers innervated by small gamma motor neurons o Extrafusal fibers innervated by larger alpha motor neurons o Alphagamma coactivation: electrical stimulus of motor cortex and other higher centers leads to simultaneous activation of alpha and gamma motor neurons o Gamma motor neurons regulate activity of muscle spindle afferents Gamma neurons innervate ends of intrafusal fibers where contractile elements are located Activation of gamma neuron causes contraction and shortening of fiber ends This increases firing rate of sensory endings Makes afferent ending more sensitive to stretch in muscle spindles Side Notes: Week 3 2 Afferent carry sensory information from muscles to spinal cord Efferent carry motor information from spinal cord to muscles Advantage of decreasing 1a firing rate after an increase in velocity of joint movement o It makes movement steadier Exam Question: What would happen if an object was lighter than expected? Motor Units and EMG Force exerted by muscle is controlled by a population of alpha motor neurons whose cell bodies cluster in the ventral horn of the spinal cord Alpha motor neurons have extensive dendritic trees o Receive ~100,000 synaptic connection from descending pathways, spinal interneurons and afferent fibers 95% of the surface area of a motor neuron is located in the dendrites o The thousands of inputs that a motor neuron receives occur primary on dendrites Average muscle unit innervation number is 340 o However, they are not evenly distributed Contractile properties o Twitch: the basic contractile property of a motor unit o Tetanus Muscle fiber types o Three types of muscle fibers based on myosin ATPase activity Type I (slow twitch) Type IIa (fast twitch) Type IIx (fast twitch) o Beneficial for soleus, a lower limb muscle, to have 80% type I fibers We use it all of the time. Do not want it to fatigue quickly o Beneficial for orbicularis oculi (a muscle in the eye) to have 15% type I fibers? We need it to act quickly, for only a short period of time. Motor units and muscle force o Activation of the motor unit pool is proportional to the synaptic input it receives Synaptic input: chemical released that will either increase excitation or decrease excitation (increase inhibition) o The change in muscle force that occurs during a voluntary contraction is accomplished by the concurrent recruitment of motor units and variation of discharge rate
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