Lecture 10

Lecture 10 - Lecture 10 Reflexes, Spinal Cord, SCI Outline...

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Lecture 10 – Reflexes, Spinal Cord, SCI Outline 1. Sensory organs of the muscle a. Muscle spindle b. Golgi tendon organ 2. Reflexes a. Myotatic stretch reflex b. Inverse myotatic stretch reflex c. Cutaneous reflex 3. Disorders of the SC a. Herniated disc b. Spinal tumor c. Syringomelia Sensory Organ of the Muscle - Muscle spindle: muscle receptor which provides information on length and rate of change in the muscle Muscle Spindle - Intrafusal muscle fibers o Specialized muscle fibers inside the spindle o Nuclear bag: clump of nuclei o Nuclear chain: cell bodies all in a row - Extrafusal fibers o Connected to Intrafusal fibers o All connected, run parallel Muscle Spindle - Afferents: Sensory receptors 1
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o Ia (large diameter): wrap around central (middle) region of each fiber ; register quick and tonic stretch of the spindle o II (smaller diameter): wrap or end around nuclear chain fibers; only register tonic stretch Stimulation of muscle spindle - [Middle, central] Sensory portion of the intrafusal muscle fiber is sensitive to tension - Lengthening of the Extrafusal muscle fiber causes lengthening of the intrafusal fibers; initiates the action potential of the afferent fibers - When a muscle is passively stretched (static), both Ia and II axons fire - Velocity of change in muscle length is signaled only by the Ia afferents - When muscle length is shortened, the spindle is slack (firing ceases) Muscle Spindle - So, what if the muscle is shortened? Does the spindle go slack? Yes. - Efferents: o Gamma: innervate intrafusal fibers Only the contractile portions ( 2 polar regions) => mid (central) portion to lengthening => fire - (a) Muscle lengthened; afferents fire - (b) Muscle shortened: Ia fibers silent; spindle is “off the air,” no info. is sent providing info about the muscle length
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- (c) Gamma motor neuron activates and contracts polar ends The Gamma Motor Neurons - Innervate contractile elements at the poles of the muscle spindles, which causes the central region of the muscle spindle to stretch - Activation of the gamma motor neuron prevents the muscle spindle from slackening during shortening of the Extrafusal fibers – this maintains the tension of the intrafusal fibers of the muscle spindle - Because of this, the sensory fibers fire continuously in response to muscle shortening (as well as lengthening) - Information from the muscle spindles is transmitted to the spinal cord where it is- o Used for reflex actions o Sent to the somatic sensory cortex Alpha Motor Neurons - LMNs that innervate extrafusal fibers - Can be directly stimulated by Ia afferents (sensory) - Ia endings: o Have direct excitatory connections with alpha motor neurons supplying the same muscle o Connect to the DCML tract to send info to the brain What’s firing? -
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This note was uploaded on 09/02/2010 for the course OT 441 taught by Professor Stein during the Summer '10 term at USC.

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Lecture 10 - Lecture 10 Reflexes, Spinal Cord, SCI Outline...

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