1 Motor units and force production Overall organization of neural structures that control movement CNS lesions and function Copyright ©2002 Thomson Learning, Inc. Lesions to the CNS Figure A5 Internal structure of the spinal cord (Part 1)2 Damages both lower (innervate muscle fibers) and upper motor neurons (originate in brainstem and motor cortex: Overview ① How are neurons organized in the spinal cord? ② How do different types of motor units relate to force production and fatigue? ③ Training and motor unit types Figure 16.2 Spatial distribution of lower motor neurons in the ventral horn of the spinal cord Motor neuron pool Figure 16.3 Somatotopic organization of lower motor neurons3 Figure 16.4 Location of local circuit neurons that supply the medial region of the ventral horn Figure A5 Internal structure of the spinal cord (Part 2) Figure A1 (A) Anatomical terminology of the brain and brainstem (Part 1) Figure A1 (B) Major planes of section of the brain (Part 2)4 Overview ① How are neurons organized in the spinal cord? ② How do different types of motor units relate to force production and fatigue? ③ Training and motor unit types Figure 16.5 The motor unit Muscle # motor neurons # muscle fibers Innervation ratio First Dorsal Interosseus 119 40,500 340 Medial Gastrocnemius 579 1,120,000 1934 Rectus Lateralis 4150 22,000 5 Figure 16.6 The force and fatigability of the three different types of motor units (Part 1) Fast: large, pale TYPE IIB Fast: TYPE IIA Slow: small, red TYPE I5 Figure 16.6 The force and fatigability of the three different types of motor units (Part 2) RepeatedstimulationMuscle Fiber Distribution Muscle Percentage of Type I fibers Biceps Brachii 40% Vastus Lateralis 46% Gastrocnemius 49% Rectus Femoris 52% Tibialis Anterior 73% Soleus 80% First Dorsal Interosseus 57% Obicularis Oculi 15% Regulation of muscle force Regulation of muscular force: ① Motor unit recruitment – size principle ② Frequency of APʼs – rate coding ③ Synchronization Figure 16.7 Motor neuron recruitment in a cat muscle under different behavioral conditions Size principle6 Figure 16.8 The effect of stimulation rate on muscle tension Overview ① How are neurons organized in the spinal cord? ② How do different types of motor units relate to force production and fatigue? ③ Training and motor unit types Organism level plasticity • Properties of slow/fast muscle fibers change when innervation is surgically altered (animal research) • Resistance/endurance training: Low exertion -> Type S >; high exertion -> type FF/FR > • Exercise training: changes within fast-twitch (IIb to IIa) • Fibers do not exist in only three forms Box 16A(2) Motor Unit Plasticity (Part 1) SFF FR7 Overview ① How are neurons organized in the spinal cord? ② How do different types of motor units relate to force production and fatigue? ③ Training and motor unit
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