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Unformatted text preview: Neuroscience Study Guide 3 Movement and Cerebellum Neuromuscular Junction o In muscle contraction, spinal neurons send their axons out of the ventral roots to the periphery. Near the muscle these axons split into several collaterals, each of which innervates a muscle fiber within a muscle. The axon terminals innervate muscle fibers at the Neuromuscular Junction . Almost every AP that reaches the NMJ releases enough ACh to produce an action potential in the postsynaptic muscle fiber. Motor Units o Motor units consist of a single motor axon and all of the muscle fibers it stimulates. The innervation ratio is the ratio of motor axons to muscle fibers. Muscles involved in fine movements (such as movements of the eye) have high innervation ratios, whereas muscles that act on large body masses (such as leg muscles) have low innervation ratios. Propioceptors: Gogli tendon organs and muscle spindles o The integrative mechanisms of the brain and spinal cord require continual information about the state of the muscles, position of limbs, and the instructions being issued by the motor centers. Collecting this information is known as proprioception. There are two main types of receptors involved in proprioception: muscle spindles and Gogli tendon organs Muscle Spindles: Muscle spindles are complex structures consisting of both afferent and efferent elements. Small fibers within the spindle are called intrafusal fibers , whereas normal fibers are called exrtrafusal fibers. Two types of afferent fibers are involved in proprioception: primary and secondary afferent fibers. Primary fibers wrap around the middle of the spindle (the nuclear bag) and are innervated when an intrafusal fiber is initially stretched. Secondary fibers have endings on the tips of the spindle and become innervated when an intrafusal fiber has been stretched for a prolonged period of time. Thus primary fibers are called dynamic and secondary fibers are called static indicators of muscle length. The efferent elements of the spindle are composed of Gamma motorneurons , which inform fibers of planned ongoing movements, causing spindles to contract. Gogli Tendon Organs: Gogli tendon organs respond to muscle contraction, rather than muscle stretch. Their main purpose is to detect overloads that could possibly cause muscular damage. Thus, when they are stimulated they inhibit the motorneurons supplying overbearing muscles, and by releasing this tensiono they are able to prevent muscle damage. Describe Population Coding in M1 o Many neurons in the M1 change their firing rates according to the direction of a certain movement, and for any one neuron, discharge rates are highest in one particular direction. Although different cells prefer different directions, each cell carries only partial information about the direction of the movement. The ultimate movement is encoded by the average discharge of the entire population of M1 neurons involved in the movement....
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- Spring '08