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Chapter 13

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Matthias Nemier

Chapter 13: The Somatic Motor System  Motor system= all our muscles and the neurons that control them  Motor control= 2 parts= 1) Spinal cords command and control of coordinated muscle contraction and 2) the brains command and control of the motor programs in the spinal cord The Somatic motor System  Muscle types= smooth + striated  Smooth muscle= lines the digestive tract, arteries + related things and is innervated by nerve fiber from the ANS role in peristalsis= movement of material in small intestine + control of bp/flow  Striated muscle= cardiac (heart muscle + contracts rhythmically even without innervation innervation from ANS= slow or accelerates heart rate) and skeletal muscle (bulk of muscle mass= moves bones around joints, move eyes within head, control respiration, control facial expression and to produce speech.  Muscles have 100’s of muscle fibers= cells of skeletal muscle and they are innervated by one axon branch from CNS muscles + nervous system that conrols them= somatic motor system= under voluntary control + generates behavior  Elbow joint= ormed when the humerus= upper arm bone is bound by fibrous ligaments to the radius and the ulna= bones of the lower arm  Joint= forms like a hinge and movement in the direction that closes the knife= flexion and one that opens knife= extension  Flexion caused by brachialis  bieps branchi and coracobrachialis= both called flexors and are synergists  Sysnergistic muscles for extension of the elbow joints- triceps brachi and anconeous= extensors  Extensors and flexors= antagonists  Axial muscles= trunk movement, shoulder, elbows, pelvis and nee= proximal/girdle muscles, move hand/feet/digits= distal muscles  Axial musculature= for posture, proximal= for locomotion and distal= for object manipulation The Lower Motor Neuron  Somatic musculature is innervated by the somatic motor neurons in the ventral horn of the spinal cord= lower motor neurons higher order upper motor neurons supply input to the spinal cord  Only the lower motor neurons directly command muscle contraction= final common pathway for the control of behavior  Axons of lower motor neurons bundle together to form ventral roots= joins with a dorsal root= form spinal nerve that exit @ notches between vertebrae of spinal cord  Spinal nerves mixed spinal nerves= contain sensory and motor fibers=CTLS (8,12,5,5)  Motor neurons that innervate distal an proximal musculature are found in the cervical and lumbar=sacral segments of the spinal cord, whereas those innervating axial muscalture found @ all levels  Cells innervating axial muscles are medial to those innervating the distal muscles and the cells innervating flexors are dorsal to those innervating extensors  Alpha motor neurons: directly trigger the generation of force by muslces one alpha motor neuron and all the muscle fibers it innervates= motor unit= elementary aprt of motor control o Muscle contraction occurs via combined actions of these motor units collction of the a. motor neurons that innervate a single muscle= motor neuron pool o CNS controls muscle contraction by varying the firing rate of motor neurons a.motor neuron communicates with a muscle fiber by releasing the NT ach at the NMJ= synapse between nerve and skeletal muscl EPSP in muscle/endplate pot= post synaptic pot= twitch= rapid seq of contraction and relaxation in the muscle fiber  sustained contraction needs a continuous barrage of AP o Firing of motor units= how CNS grades muscle contraction o CNS also grades muscle contraction by recruiting additional synergistic motor units= extra tension by the recruitment depends on how many muscle fibers are in that unit innervation ratio is coherent with muscles involved o Most muscles have a range of motor unit sizes, and these motor units are recruited in the order of smallest first, largest last o A single AP in an alpha moyot neuron causes the muscle fiber to twitch and the summation of twitches causes a sustained contraction as the numbr and freq of incoming AP increase o Size principle: the idea that the orderly recruitment of motor neurons is due to variations in alpha motor neuron size o Alpha motor neurons excite skeletal muscles o lower motor neurons are controlled by synaptic inputs in the ventral horn there are only 3 major sources of input to an alpha motor neuron o 1) Dorsal root ganglion with axons that innervate a specialized sensory apparatus embedded within the muscle= muscle spindle o 2) derived from upper motor neurons in motor cortex and brain stem= important for initiation and ocntorl of voluntary movement o 3) largest input= from interneurons of SC= excitatory or inhibitory and is part of the circuitry that generates spinal motor programs  Types of Motor units: o Red and dark muscle fibers= large number of mitochondria nad enzymes needed for oxidative metablolism= slow to contract but can sustain contraction for long time without fatigue= in antigravity muscles of leg ad flight muscles of birds o Pale/ white muscle fibers= less mito and rely on aneroibic metab= rapid and powerful contraction and fatigue quickly= in escape reflex muscle i.e. jumping frog muscle o Fast motor units= white fibers and slow motor units= red ribers in them o Fast ones= bigger and larger diameter and faster conduction, high freq bursts of AP (30-60 impulses per se vs 10-20 of slow) o Neuromuscular match making: precise matching of motor neurons to muscle fibers= interesting  Experiment done in which normal innervation of a fast muscle removed and replaced with a nerve that normally innervated a slow muscle= resulted in muscle getting slow properties= type of contraction and also switch in underlying biochem= phenotypic change because the proteins expressed by the muscle were changed by new innervation  Muscle fibers are also changed by varying the absolute amount of activity long term consequence of increased actitivty= hypertrophy= too much growth of the muscle  Prolonged inactivity= atrophy= degeneration of muscle fibers Excitation-Contracting Coupling  Muscle contraction inititated by the release of ACH form the axon terminals of alpha motor neurons  ACH produces large EPSP due to activation of nictonic ACH receptor membrane has v gated NA channels= EPSP enough to evoke AP in muscle fiber  Via excitation-contraction coupling, AP/excitation= Ca release from muscle fiber which leads to contraction of the fiber relaxation occurs when Ca levels are lowered by reuptake into the organelle  Muscle fiber structure= formed early in fetal dev by fusion of muscle precursor cells/myoblasts = derived from mesoderm  Individual muscle cells= multinucleated muscle fibers are enclosed by an excitable cell membrane called the sarcolemma  Within muscle fiber= cylindrical structures called myofibrils that contract in response to AP, sweeping down the sarcolemma  Myofibrils are surrounded by the sarcoplasmic reticulum, an extensive intracellular sac that stores Ca  AP sweeping along the sarcolemma gain access to the sarcoplasmic reticulum inside the fiber via T tubules= inside out axons lumen of each T tubule is continuous with the EC fluid  Where t tubules come in close apposition to the SR, there is spacizlied protein coupling in 2 membranes voltage sensitive cluster of 4 Ca channels called a tetrad in the T tubule membrane is linked to a Ca release channel in the SR  Myofibrils are divided into segments by disks called Z lines a segment made up of 2 Z lines and the myofibril in between= sarcomere anchored to each side of the Z line= thin filaments rom adjacent Z lines face each other but don’t come in contact  Between 2 sets of thin filaments are thick filaments  Muscle contraction occurs when thin filaments slide along the thick filaments= bringing adjacent z LINES TOWARD one another= sarcomere becomes shorter in length=sliding filament model  Sliding of the filaments with respect to one another occurs because of the ineraction betweent he major thick filament protein myosin and major thin filament protein actin  Exposed heads of myosin bind actin molecules and then undergo a conformational change that cusauses them to pivot= cuasing the thick filament to move with respect ot the thin filament + @ the expense of ATP, the myosin heads then disengage and uncock so that the process can repeat itself= repeating cycle allows myosin head to walk along the actin filament  When the muscle is at rest, myosin cannot interact with actin because the myosin attachment sites on the actin molecule are covered by the protein troponin Ca starts muscle contraction by binding to troponin thereby exposing sites where myosin binds to actin= contraction continues as long as Ca and ATP available and relaxation occurs when the Ca is sequestered by the SR  Ca reuptake depends on SR and action of Ca pump= needs ATP  Death causes stiffening of muscles= rigor mortis= starving muscle cells of ATP prevents detachment of myosin head= leaves attachment site open for binding= permanent attachment formed between thick and thin filaments  Excitation 1) AP occurs in alpha motor neuron axon 2) Ach released by axon terminal of AMN at NMJ 3) Nictonic R channel in sarcolemma open and postsynaptic sarcolemma depols= EPSP 4) V gated Na channel open= AP made in muscle fiber= sweeps down sarco
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