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Spinal Reflexes



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Lecture 23 Spinal Reflexes Krakauer referenced figures are in fourth edition of Principles of Neural Science Introduction 1 The place of spinal reflex circuits in the motor hierarchy 2 For the accurate control of movement a continuous flow of sensory information is important both visually and from receptors in the moving part itself Any type of receptor in the moving body part that can give movement or position information is called a proprioceptor These include the muscle spindle the golgi tendon organ GTO cutaneous receptors and joint receptors The GTO Box 36 3 lies in series with the muscle and tendon and is sensitive to changes in tension The muscle spindle Box 36 1A lies in parallel with the muscle and tendon and is sensitive to changes in length There are up to 500 muscle spindles in a given muscle depending on the muscle size and each can range from 0 5 to 10 mm in length Thus they make up 10 20 of the length of the muscle Muscle spindles are made up of specialized cells called intrafusal muscle fibers Notice in Figure 36 3 that the spindles have both sensory and motor innervation This allows the CNS to alter the nature of incoming sensory information Please take note of the 3 types of intrafusal fiber the two types of sensory afferent and the two types of motor neurons called gamma to distinguish them from alpha motor neurons innervating the extrafusal muscle fibers Ramp and hold stretches of the soleus muscle in the decerebrate cat Type II afferents respond to constant changes in length whereas Type Ia fibers are sensitive to rate of change of length Importantly they only show linear behavior for very rapid small changes in length This means that overall muscle spindles are nonlinear receptors because they only show linear behavior for small changes in length We will come back to this non linear behavior later Response of Ia to gamma activation Fig 36 3C Why have gamma activation Because it enables muscle spindles to continue sensing changes in muscle



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