Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs (proprioceptors) that detect the amount and rate of change in length of a muscle. They constitute the muscle spindle and are innervated by two axons, one sensory and one motor. Gamma effrents from small multipolar cells from anterior gray coloumn innervate it. These form a part of neuromuscular spindles. Intrafusal muscle 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 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.
It is by the sensory information from these two intrafusal fiber types that an individual is able to judge the position of their muscle, and the rate at which it is changing.
|Intrafusal muscle fiber|
A muscle spindle, with γ motor and Ia sensory fibers
|Part of||Skeletal muscle|
|Anatomical terms of microanatomy|
Afferent nerve fibers refer to axonal projections that arrive at a particular region ; as opposed to efferent projections that exit the region. These terms have a slightly different meaning in the context of the peripheral nervous system (PNS) and central nervous system (CNS).
In the PNS, afferent and efferent projections are always from the perspective of the spinal cord (see figures). PNS afferents are the axons of sensory neurons carrying sensory information from all over the body, into the spine. PNS efferents are the axons of spinal cord motor neurons that carry motor-movement signals out of the spine to the muscles.In the CNS, afferent and efferent projections can be from the perspective of any given brain region. That is, each brain region has its own unique set of afferent and efferent projections. In the context of a given brain region, afferents are arriving fibers while efferents are exiting fibers.Extrafusal muscle fiber
Extrafusal muscle fibers are the skeletal standard muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. "The Public Library of Science says that in most mammals, skeletal muscle comprises about 55% of individual body mass and plays vital roles in locomotion, heat production during periods of cold stress, and overall metabolism." They make up the large mass of skeletal muscle tissue and are attached to bone by fibrous tissue extensions (tendons).
Each alpha motor neuron and the extrafusal muscle fibers innervated by it make up a motor unit. The connection between the alpha motor neuron and the extrafusal muscle fiber is a neuromuscular junction, where the neuron's signal, the action potential, is transduced to the muscle fiber by the neurotransmitter acetylcholine.
Extrafusal muscle fibers are not to be confused with intrafusal muscle fibers, which are innervated by sensory nerve endings in central noncontractile parts and by gamma motor neurons in contractile ends and thus serve as a sensory proprioceptor.
Extrafusal muscle fibers can be generated in vitro (in a dish) from pluripotent stem cells through directed differentiation. This allows study of their formation and physiology.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.
There are two kinds of bag fibers based upon contraction speed and motor innervation.
BAG2 fibers are the largest. They have no striations in middle region and swell to enclose nuclei, hence their name.
BAG1 fibers, smaller than BAG2.Both bag types extend beyond the spindle capsule.
These sense dynamic length of the muscle. They are sensitive to length & velocity.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 found in muscles called the muscle spindle, which constantly monitors how fast a muscle stretch changes. (In other words, it monitors the velocity of the stretch).