Muscle spindle
Muscle spindles, seen here in longitudinal (top) and cross sections (bottom), are encapsulated stretch receptors in skeletal muscles that regulate muscle contraction and detect changes in muscle length. Muscle spindles are composed of intrafusal fibers which are modified skeletal muscle fibers of two types, nuclear bag and nuclear chain fibers. Spiraling sensory nerve endings surround the nuclear bag fibers. Intrafusal fibers also receive efferent innervation. A fluid-filled space separates the internal capsule from an outer external capsule. 800x
Nuclear bag fibers
Muscle spindles, seen here in longitudinal (top) and cross sections (bottom), are encapsulated stretch receptors in skeletal muscles that regulate muscle contraction and detect changes in muscle length. Muscle spindles are composed of intrafusal fibers which are modified skeletal muscle fibers of two types, nuclear bag and nuclear chain fibers. Spiraling sensory nerve endings surround the nuclear bag fibers. Intrafusal fibers also receive efferent innervation. A fluid-filled space separates the internal capsule from an outer external capsule. 800x
Nuclear chain fibers
Muscle spindles, seen here in longitudinal (top) and cross sections (bottom), are encapsulated stretch receptors in skeletal muscles that regulate muscle contraction and detect changes in muscle length. Muscle spindles are composed of intrafusal fibers which are modified skeletal muscle fibers of two types, nuclear bag and nuclear chain fibers. Spiraling sensory nerve endings surround the nuclear bag fibers. Intrafusal fibers also receive efferent innervation. A fluid-filled space separates the internal capsule from an outer external capsule. 800x
Sensory nerve endings
Muscle spindles, seen here in longitudinal (top) and cross sections (bottom), are encapsulated stretch receptors in skeletal muscles that regulate muscle contraction and detect changes in muscle length. Muscle spindles are composed of intrafusal fibers which are modified skeletal muscle fibers of two types, nuclear bag and nuclear chain fibers. Spiraling sensory nerve endings surround the nuclear bag fibers. Intrafusal fibers also receive efferent innervation. A fluid-filled space separates the internal capsule from an outer external capsule. 800x
Inner capsule
Muscle spindles, seen here in longitudinal (top) and cross sections (bottom), are encapsulated stretch receptors in skeletal muscles that regulate muscle contraction and detect changes in muscle length. Muscle spindles are composed of intrafusal fibers which are modified skeletal muscle fibers of two types, nuclear bag and nuclear chain fibers. Spiraling sensory nerve endings surround the nuclear bag fibers. Intrafusal fibers also receive efferent innervation. A fluid-filled space separates the internal capsule from an outer external capsule. 800x
Outer capsule
Muscle spindles, seen here in longitudinal (top) and cross sections (bottom), are encapsulated stretch receptors in skeletal muscles that regulate muscle contraction and detect changes in muscle length. Muscle spindles are composed of intrafusal fibers which are modified skeletal muscle fibers of two types, nuclear bag and nuclear chain fibers. Spiraling sensory nerve endings surround the nuclear bag fibers. Intrafusal fibers also receive efferent innervation. A fluid-filled space separates the internal capsule from an outer external capsule. 800x
Extrafusal muscle fibers >
The bulk of a skeletal muscle is composed of muscle fibers called extrafusal to differentiate them from the intrafusal fibers that form muscle spindles. As seen here, they surround the muscle spindles.
Image source >
Image from the Virtual Histology Laboratory website.
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