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Sensory Receptors
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Slide 7: Muscle spindle in cross section. This is a stretch receptor as in the knee jerk response. There are two types of inrafusal fibers found within a connective tissue capsule: nuclear bag and nuclear chain fibers. Both of them have afferent nerve fibers ending on them which are fired when the muscle is stretched. Basically, the action potential is carried along the afferent fiber to the spinal cord, and in the simplest situation it makes a synaptic connection with an efferent fiber in the motor horn. These efferent or motor fibers end on the extrafusal (normal striated muscle cells) fibers outside the connective tissue capsule causing them to contract. In real life, however, the situation is generally much more complicated than this with motor fibers going to some of the intrafusal fibers to fine-tune the system, and occasional association neurons being involved within the spinal cord and perhaps even the brain. For more information, read the section in the text. Locate the intrafusal and extrafusal fibers, connective tissue capsule, and myelinated nerve. H & E.