Intrafusal muscle fibers are
skeletal muscle fibers that serve as specialized
sensory organs (
proprioceptors).
They detect the amount and rate of change in length of a
muscle.
[Casagrand, Janet (2008) ''Action and Movement: Spinal Control of Motor Units and Spinal Reflexes.'' University of Colorado, Boulder.] They constitute the
muscle spindle
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...
,
and are innervated by both sensory (afferent) and motor (efferent) fibers.
Intrafusal muscle fibers are not to be confused with
extrafusal muscle fibers, which contract, generating skeletal movement and are innervated by
alpha motor neurons.
Structure
Types
There are two types of intrafusal muscle fibers:
nuclear bag fibers and
nuclear chain fibers.
They bear two types of sensory ending, known as annulospiral and flower-spray endings. Both ends of these fibers contract, but the central region only stretches and does not contract.
Intrafusal muscle fibers are walled off from the rest of the muscle by an outer
connective tissue sheath consisting of flattened
fibroblasts
A fibroblast is a type of biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework ( stroma) for animal tissues, and plays a critical role in wound healing. Fibroblasts are the most common cells ...
and
collagen.
This sheath has a spindle or "fusiform" shape, hence the name "intrafusal".
Innervation
They are innervated by
gamma motor neuron
A gamma motor neuron (γ motor neuron), also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of ( Aγ) fibers going to the muscle. Like ...
s and
beta motor neuron
Beta motor neurons (β motor neurons), also called beta motoneurons, are a kind of lower motor neuron, along with alpha motor neurons and gamma motor neurons. Beta motor neurons innervate intrafusal fibers of muscle spindles with collaterals to e ...
s.
Gamma efferents from small
multipolar neuron
A multipolar neuron is a type of neuron that possesses a single axon and many dendrites (and dendritic branches), allowing for the integration of a great deal of information from other neurons. These processes are projections from the neuron cell ...
s from anterior gray column innervate it. These form a part of
neuromuscular spindles.
Function
Intrafusal muscle fibers detect the amount and rate of change in muscle length.
It is by the sensory information from gamma motor neurons and beta motor neurons that an individual is able to judge the position of their muscles.
See also
*
Alpha motor neuron
*
Beta motor neuron
Beta motor neurons (β motor neurons), also called beta motoneurons, are a kind of lower motor neuron, along with alpha motor neurons and gamma motor neurons. Beta motor neurons innervate intrafusal fibers of muscle spindles with collaterals to e ...
*
Extrafusal muscle fiber
*
Gamma motor neuron
A gamma motor neuron (γ motor neuron), also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of ( Aγ) fibers going to the muscle. Like ...
*
Type Ia sensory fiber
A type Ia sensory fiber, or a primary afferent fiber is a type of afferent nerve fiber. It is the sensory fiber of a stretch receptor called the muscle spindle found in muscles, which constantly monitors the rate at which a muscle stretch cha ...
*
Type II sensory fiber
Type II sensory fiber (group Aβ) is a type of sensory fiber, the second of the two main groups of touch receptors. The responses of different type Aβ fibers to these stimuli can be subdivided based on their adaptation properties, traditional ...
References
External links
Histology at ucsd.edu*
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Muscular system