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Chapter 13 Spinal Control of Movement

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University of Toronto Scarborough
Matthias Niemeier

Chapter 13Spinal Control of Movement The motor system consists of all our muscles and neurons that control them complex patterns of behaviour can be generated without the participation of the brain MUSCLE CONTROL Spinal cords command and control of coordinated muscle contraction Brains command and control of motor programs in spinal cord THE SOMATIC MOTOR SYSTEM CNSaxon branchmuscle fiberskeletal musclesomatic motor system voluntary control Cardiac muscles STRIATED eg heart muscle MUSCLES Skeletal muscleseg move bones around joints move eyes within head respiration etc SMOOTH Digestive tract arteries structures innervated by nerve fibers in the autonomic nervous system important for peristalsis control of blood pressure and blood flow Major muscle that causes flexionBRACHIALIS whose tendons insert into the humerus at one end and into the ulna at the otherFlexing of an elbow joint CONTRACTION of synergistic flexor musclesRELAXATION of the antagonistic extensor musclesMusclesAXIALtrunk posturePROXIMALshoulder elbow pelvis knee locomotionDISTALhands feet fingers manipulation of small objectsTHE LOWER MOTOR NEURON Upper motor neurons of the brain supply input to the spinal cord Lower motor neurons are the only ones that directly command muscle contractions they are somatic motor neurons in the ventral horn of SC BranchialisTriceps brachii FlexionFlexorsExtensionExtensorsBiceps brachii Coracobrachialis Anconeus Flexors work TOGETHER therefore SYNERGISTS Flexors and extensors pull on the joint in the opposite direction therefore they are ANTAGONISTS to one another Axons of motor neurons bundle togetherform ventral rootsjoins with dorsal rootform spinal nerveexits cord through notches between vertebrae Skeletal muscles are not spread out evenly throughout the body and neither are the lower motor neurons within the SCThe greater number of motor neurons the larger the size of the ventral horn for accommodation eg cervical and lumbar where distal and proximal muscles are located Alpha Motor Neurons Lower motor neurons are classified into two types 1 ALHPA MOTOR and 2 GAMMA MOTORAlpha motor neurons directly trigger the generation of force by muscles Motor unitone alpha motor neuronall muscle fibers it innervateMotor neuron poolmany alpha motor neuronssingle muscle that it innervates Graded Control of Muscle Contraction by Alpha Motor NeuronsAlpha motor neurons release ACh at the neuromuscular junctionthe specialized synapse between a nerve and skeletal musclepresynaptic APEPSP in muscle fiber endplate potentialpostsynaptic APtwitchrapid sequence of contractionrelaxation CNS controls muscle contraction via the following 1 Variable firing rate of motor neuronssummation of twitches cause sustained contraction as the number and frequency of incoming AP increase 2 Recruiting additional synergistic motor unitseg antigravity muscles of legsLARGE motor unitsmany muscle fibers per alpha motor neuronlarge alpha motor neurons muscles controlling rotation of eyesSMALL motor unitsfew muscle fibers peralpha motor neuron muscles with largeof small motor units are FINELY controlled
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