Type 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

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...

 which keeps track of how fast a muscle stretch changes (the velocity of the stretch).

Function of muscle spindles

In order to control movements, the nervous system

Nervous system

The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

 must receive continuous sensory information from muscles and joints. For this purpose the body has specialized sensory receptor

Sensory receptor

In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...

s called proprioceptors

Proprioception

Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...

. Muscle spindles are a type of proprioceptor, and they are located inside the muscle itself. They are sensitive to muscle length because they are in parallel with the contractile fibers.

Types of sensory fibers

This change in length of the spindle is transduced (transformed into electric membrane potential

Membrane potential

Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...

s) by two types of sensory afferents, whose cell bodies are located in dorsal root ganglia located next to the spinal cord

Spinal cord

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

.

The two kinds of sensory fibers are different in respect to the kind of potentials they generate:

Type

Primary/secondary

Response >- | Type Ia

primary

>- | Type Ib

N/A

>- | Type II

secondary

Respond only to changes in length

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

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....

s 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. These connections are also called "annulospiral endings ".

Efferent innervation

In addition, the spindle also has a motor efferent innervation carried by gamma motor neurons, which is used by the nervous system to modify the spindle's sensitivity.

Termination of afferents

Ia afferents from the muscle spindle terminate on the proximal dendrites of motor neurones.

See also

• Intrafusal muscle fiber
  Intrafusal muscle fiber
  Intrafusal muscle fibers 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...
  
  
• Type II sensory fiber
  Type II sensory fiber
  Type 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...
  
  
• Gamma motor neuron
• Beta motor neuron
  Beta motor neuron
  A 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...
  
  
• Proprioception
  Proprioception
  Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
  
  
• Motor system
  Motor system
  The motor system is the part of the central nervous system that is involved with movement. It consists of the pyramidal and extrapyramidal system....
  
  
• Muscle
  Muscle
  Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
  
  
• 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...
  
  
• Reflex action
  Reflex action
  A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
  
  
• Phasic stretch reflex
• Tonic stretch reflex

External links

• Lecture notes from John D.C. Lambert on neurophysiology.
• http://www.lib.mcg.edu/edu/eshuphysio/program/section8/8ch3/s8ch3_15.htm