Intrafusal muscle fiber
Encyclopedia
Intrafusal muscle fibers (or intrafusal muscle fibres) are skeletal muscle fibers
that comprise the muscle spindle
and are innervated by gamma motor neurons. These fibers are a proprioceptor that detect the amount and rate of change of length in a muscle. These fibers are walled off from the rest of the muscle by a collagen
sheath. This sheath has a spindle or "fusiform" shape, hence the name "intrafusal."
There are two types of intrafusal muscle fibers: nuclear bag
and nuclear chain fiber
s. They bear two types of sensory ending, known as annulospiral and flower-spray endings.
Intrafusal muscle fibers are not to be confused with extrafusal muscle fiber
s, which are innervated by alpha motor neuron
s and contract, generating skeletal movement.
It is by the sensory information from these two intrafusal fiber types that one is able to judge the position of their muscle, and the rate at which it is changing.
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
that comprise the muscle spindle
Muscle spindle
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...
and are innervated by gamma motor neurons. These fibers are a proprioceptor that detect the amount and rate of change of length in a muscle. These fibers are walled off from the rest of the muscle by a collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
sheath. This sheath has a spindle or "fusiform" shape, hence the name "intrafusal."
There are two types of intrafusal muscle fibers: nuclear bag
Nuclear bag fiber
A nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle. Each has a large number of nuclei concentrated in bags and they cause excitation of both the primary and secondary nerve fibers....
and nuclear chain fiber
Nuclear chain fiber
A nuclear chain fiber is a specialized sensory organ contained within a muscle. Nuclear chain fibers are intrafusal fibers which, along with nuclear bag fibers, make up the muscle spindle responsible for the detection of changes in muscle length....
s. They bear two types of sensory ending, known as annulospiral and flower-spray endings.
Intrafusal muscle fibers are not to be confused with extrafusal muscle fiber
Extrafusal muscle fiber
Extrafusal muscle fiber is a term given to standard muscle fibers as to distinguish them from intrafusal muscle fibers. Extrafusal muscle fibers are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement...
s, which are innervated by alpha motor neuron
Alpha motor neuron
Alpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
s and contract, generating skeletal movement.
It is by the sensory information from these two intrafusal fiber types that one is able to judge the position of their muscle, and the rate at which it is changing.
See also
- Extrafusal muscle fiberExtrafusal muscle fiberExtrafusal muscle fiber is a term given to standard muscle fibers as to distinguish them from intrafusal muscle fibers. Extrafusal muscle fibers are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement...
- Type Ia sensory fiberType Ia sensory fiberType Ia Sensory Fiber also called Primary Afferent Fiber is a type of sensory fiber. It is a component of a muscle fiber's muscle spindle which keeps track of how fast a muscle stretch changes .-Function of muscle spindles:...
- Type II sensory fiberType II sensory fiberType II sensory fiber is a type of sensory fiber, the second of the two main groups of stretch receptors. They are non-adapting, meaning that they keep responding even when the muscle has stopped changing its length. They are the second most highly myelinated fibers in the body.Their firing rate...
- Alpha motor neuronAlpha motor neuronAlpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
- Gamma motor neuron
- Beta motor neuronBeta motor neuronA beta motor neuron is a kind of lower motor neuron, along with alpha motor neuron and gamma motor neuron. These motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers . Axons of beta motor neurons are myelinated...