Gamma motor neuron in the context of Soma (biology)

A motor neuron (or motoneuron), also known as efferent neuron is a neuron that allows for both voluntary and involuntary movements of the body through muscles and glands. Its 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 control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

A single motor neuron may innervate many muscle fibres and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch. Innervation takes place at a neuromuscular junction and twitches can become superimposed as a result of summation or a tetanic contraction. Individual twitches can become indistinguishable, and tension rises smoothly eventually reaching a plateau.

Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar 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. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

While their cell bodies are found in the central nervous system (CNS), α motor neurons are also considered part of the somatic nervous system—a branch of the peripheral nervous system (PNS)—because their axons extend into the periphery to innervate skeletal muscles.

Beta motor neurons (β motor neurons), also called beta motoneurons, are a few kind of lower motor neuron, along with alpha motor neurons and gamma motor neurons. Beta motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers - a type of slow twitch fiber. Also, axons of alpha, beta, and gamma motor neurons become myelinated. Moreover, these efferent neurons originate from the anterior grey column of the spinal cord and travel to skeletal muscles. However, the larger diameter alpha motor fibers require higher conduction velocity than beta and gamma.

A type Ia sensory fiber (also group Ia afferent fiber, or primary afferent fiber) is one of two types of afferent (sensory) nerve fibers innervating muscle spindles - a type of stretch receptor encountered in nearly all striated muscles. Type Ia sensory fibers are fast-conducting, large-diameter, thickly myelinated muscle fibers conveying rapidly adaptating (dynamic) proprioceptive information regarding velocity (rate of change) of muscle stretch from the nuclear bag fibers. The other type of afferent fibres innervating muscle spindles are the slower-conducting type II sensory fibers which convey slowly adaptating (static) information regarding sustained state of muscle stretch.

Type Ia fibers innervate both the nuclear bag fibers and the nuclear chain fibers. Type Ia fibers coil around the non-contractive central region of each intrafusal muscle fiber of a muscle spindle to form so-called annulospinal primary endings or primary sensory endings (type II sensory fibers meanwhile innervate the periphery of the sensory middle portion of nuclear chain intrafusal fibers on one or both sides of the central area innervated by type Ia fibers). Type Ia fibers convey phasic responses to small changes in stretch. Dynamic gamma motor neurons adjust the tautness of the central intrafusal region to maintain the dynamic responsiveness of type Ia fibers.