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 Stretch receptors are mechanoreceptors responsive to distention of various organs and muscles, and are neurologically linked to the Medulla oblongata, medulla in the brain stem via Afferent nerve fiber, afferent nerve fibers. Examples include stre ...

called 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 ...

found in

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

s, which constantly monitors the rate at which a muscle stretch changes. The information carried by type Ia fibers contributes to the sense of

proprioception Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...

Function of muscle spindles

For the body to keep moving properly and with finesse, the

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...

has to have a constant input of sensory data coming from areas such as the muscles and

joints A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

. In order to receive a continuous stream of sensory data, the body has developed special sensory receptors called

proprioceptors Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...

. Muscle spindles are a type of proprioceptor, and they are found inside the muscle itself. They lie parallel with the contractile fibers. This gives them the ability to monitor muscle length with precision.

Types of sensory fibers

This change in length of the spindle is transduced (transformed into electric membrane potentials) by two types of sensory afferents, whose cell bodies are located in

dorsal root ganglia A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the dorsal ...

located next to the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

. The two kinds of sensory fibers are different with respect to the kind of potentials they generate: The first of the two main groups of stretch receptors wrapping the intrafusal fibers are the Ia fiber, which are the largest and fastest fibers, and they fire when the muscle is stretching. They are characterized by their rapid adaptation, because as soon as the muscle stops changing length, the Ia stop firing and adapt to the new length. Ia fibers essentially supply proprioceptive information about the ''rate of change'' of its respective muscle: the derivative of the muscle's length (or position). Type Ia fibers connect to both

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 many nuclei concentrated in bags and they cause excitation of the primary sensory fibers. There are two kinds of bag fibers based up ...

s and

nuclear chain fiber A nuclear chain fiber is a specialized sensory organ contained within a muscle. Nuclear chain fibers are intrafusal fibers that, along with nuclear bag fibers, make up the muscle spindle responsible for the detection of changes in muscle len ...

s. These connections are also called "annulospiral endings", deriving from the Latin word ''annulus'' which means "a ring-shaped area or structure".

Efferent innervation

In addition, the spindle also has a motor efferent innervation carried by the

efferent nerve fiber Efferent nerve fibers refer to axonal projections that ''exit'' a particular region; as opposed to afferent projections that ''arrive'' at the region. These terms have a slightly different meaning in the context of the peripheral nervous syste ...

s of

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, which is used by the nervous system to modify the spindle's sensitivity.

Termination of afferents

Proprioceptive afferents send central axons through the

dorsal root of spinal nerve The dorsal root of spinal nerve (or posterior root of spinal nerve or sensory root) is one of two "roots" which emerge from the spinal cord. It emerges directly from the spinal cord, and travels to the dorsal root ganglion. Nerve fibres with the v ...

bifurcating into ascending and descending branches, which in turn send branches to multiple spinal segments. Some branches synapse at the dorsal horn and some at the ventral horn (where the

motor neurons A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly ...

are) which mediate different responses including

stretch reflex The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a muscle contraction in response to stretching within the muscle. The reflex functions to maintain the muscle at a constant length. The term deep tendon reflex is ...

. Ascending pathways to the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

have some similarities with the cutaneous afferents, but are different because the proprioceptive information also has to reach the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

which controls the timings of

muscle contractions Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...

for voluntary movements.

References

External links