Beta motor neurons (β motor neurons), also called beta motoneurons, are a kind of
lower motor neuron
Lower motor neurons (LMNs) are motor neurons located in either the anterior grey column, anterior nerve roots (spinal lower motor neurons) or the cranial nerve nuclei of the brainstem and cranial nerves with motor function (cranial nerve low ...
, along with
alpha motor neurons and
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. Beta motor neurons innervate
intrafusal fibers
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 ...
of
muscle spindles
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 ...
with collaterals to
extrafusal fibers
Extrafusal muscle fibers are the standard skeletal muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. They make up the large mass of skeletal striated muscle t ...
- a type of slow twitch fiber. Also,
axons of alpha, beta, and gamma motor neurons become
myelinated
Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be l ...
. Moreover, these efferent neurons originate from the
anterior grey column
The anterior grey column (also called the anterior cornu, anterior horn of spinal cord, motor horn or ventral horn) is the front column of grey matter in the spinal cord. It is one of the three grey columns. The anterior grey column contains motor ...
of 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 sp ...
and travel to
skeletal muscles
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 muscle ...
. However, the larger diameter alpha motor fibers require higher conduction velocity than beta and gamma.
Types
There are two kinds of beta motor neuron (as gamma motor neuron) that include:
*Static beta motor neurons. These motor neurons innervate
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 of muscle spindles, with collaterals to extrafusal muscle fibers.
*Dynamic beta motor neurons. The dynamic type innervates
nuclear bag fibers
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 ...
of muscle spindles, with collaterals to extrafusal muscle fibers.
Gamma motor neurons innervate only intrafusal fibers of muscle spindles, but extrafusal fibers (i.e. slow and fast fibers) are innervated by alpha motoneurons.
See also
*
Nerve fiber
*
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
*Williams & Warwick. Gray's Anatomy. Thirty-seventh edition. Churchill Livingstone.
*https://web.archive.org/web/20100725065352/https://musom.marshall.edu/anatomy/grosshom/Musclesensory.html
{{Nervous tissue
Somatic motor system
Efferent neurons