What type of neuron innervates the muscle spindle

This nerve fiber is a type Ia fiber averaging 17 micrometers in diameter, and it transmits sensory signals to the spinal cord at a. Details of nerve connections from the nuclear bag and nuclear chain muscle spindle fibers.

Modified from Stein RB: Peripheral control of movement. Physiol Rev , Secondary Ending. Usually one but sometimes two smaller sensory nerve fibers—type II fibers with an average diameter of 8 micrometers—innervate the receptor region on one or both sides of the primary ending, as shown in Figures and This sensory ending is called the secondary ending; sometimes it encircles the intrafusal fibers in the same way that the type Ia fiber does, but often it spreads like branches on a bush.

There are also two types of muscle spindle intrafusal fibers: 1 nuclear bag muscle fibers one to three in each spindle , in which several muscle fiber nuclei are congregated in expanded "bags" in the central portion of the receptor area, as shown by the top fiber in Figure , and 2 nuclear chain fibers three to nine , which are about half as large in diameter and half as long as the nuclear bag fibers and have nuclei aligned in a chain throughout the receptor area, as shown by the bottom fiber in the figure.

The primary sensory nerve ending the micrometer sensory fiber is excited by both the nuclear bag intrafusal fibers and the nuclear chain fibers. Conversely, the secondary ending the 8-micrometer sensory fiber is usually excited only by nuclear chain fibers.

These relations are shown in Figure When the receptor portion of the muscle spindle is stretched slowly, the number of impulses transmitted from both the primary and the secondary endings increases almost directly in proportion to the degree of stretching, and the endings continue to transmit these impulses for several minutes.

This effect is called the static response of the spindle receptor, meaning simply that both the primary and secondary endings continue to transmit their signals for at least several minutes if the muscle spindle itself remains stretched. When the length of the spindle receptor increases suddenly, the primary ending but not the secondary ending is stimulated especially powerfully.

This excess stimulus of the primary ending is called the dynamic response, which means that the primary ending responds extremely actively to a rapid rate of change in spindle length. Even when the length of a spindle receptor increases only a fraction of a micrometer, if this increase occurs in a fraction of a second, the primary receptor transmits tremendous numbers of excess impulses to the large micrometer sensory nerve fiber, but only while the length is actually increasing.

As soon as the length stops increasing, this extra rate of impulse discharge returns to the level of the much smaller static response that is still present in the signal.

Conversely, when the spindle receptor shortens, exactly opposite sensory signals occur. Thus, the primary ending sends extremely strong, either positive or negative, signals to the spinal cord to apprise it of any change in length of the spindle receptor. The gamma motor nerves to the muscle spindle can be divided into two types: gamma-dynamic gamma-d and gamma-static gamma-s. Type Ia sensory fiber. Edit source History Talk 0.

Ia afferents from the muscle spindle terminate on the proximal dendrites of motor neurones. Categories Sensory system Add category. Cancel Save. Fan Feed 1 Types of gestures 2 Human sex differences 3 Impregnation fetish. Universal Conquest Wiki. This page uses Creative Commons Licensed content from Wikipedia view authors. Tap on each box on the right to highlight the inputs blue and outputs red of each region.

The spinal cord is the first level of the motor hierarchy. It is the site where motor neurons are located. These circuits execute the low-level commands that generate the proper forces on individual muscles and muscle groups to enable adaptive movements. The spinal cord also contains complex circuitry for such rhythmic behaviors as walking. Because this low level of the hierarchy takes care of these basic functions, higher levels such as the motor cortex can process information related to the planning of movements, the construction of adaptive sequences of movements, and the coordination of whole-body movements, without having to encode the precise details of each muscle contraction.

Alpha motor neurons also called lower motor neurons innervate skeletal muscle and cause the muscle contractions that generate movement. Motor neurons release the neurotransmitter acetylcholine at a synapse called the neuromuscular junction. When the acetylcholine binds to acetylcholine receptors on the muscle fiber, an action potential is propagated along the muscle fiber in both directions see Chapter 4 of Section I for review. The action potential triggers the contraction of the muscle.

If the ends of the muscle are fixed, keeping the muscle at the same length, then the contraction results on an increased force on the supports i sometric contraction.

If the muscle shortens against no resistance, the contraction results in constant force isotonic contraction. The motor neurons that control limb and body movements are located in the anterior horn of the spinal cord, and the motor neurons that control head and facial movements are located in the motor nuclei of the brainstem. Even though the motor system is composed of many different types of neurons scattered throughout the CNS, the motor neuron is the only way in which the motor system can communicate with the muscles.

Thus, all movements ultimately depend on the activity of lower motor neurons. Motor neurons are not merely the conduits of motor commands generated from higher levels of the hierarchy. They are themselves components of complex circuits that perform sophisticated information processing.

As shown in Figure 1. Two terms are used to describe the anatomical relationship between motor neurons and muscles: the motor neuron pool and the motor unit. If a muscle is required for fine control or for delicate movements e. That is, each motor neuron will innervate a small number of muscle fibers , enabling many nuances of movement of the entire muscle. If a muscle is required only for coarse movements e. A motor neuron controls the amount of force that is exerted by muscle fibers.

There are two principles that govern the relationship between motor neuron activity and muscle force: the rate code and the size principle. The upper trace on the oscilloscope shows the action potentials generated by the alpha motor neuron. The lower trace shows the force generated by the isometrically contracting muscle. PLAY 1: Single spikes by the motor neuron produce small twitches of the muscle. PLAY 2: Multiple spikes in succession summate to produce larger contractions.

PLAY 3: Very high rates of spikes produce maximal contraction called tetanus. Because motor units are recruited in an orderly fashion, weak inputs onto motor neurons will cause only a few motor units to be active, resulting in a small force exerted by the muscle Play 1. With stronger inputs, more motor neurons will be recruited, resulting in more force applied to the muscle Play 2 and Play 3.

Moreover, different types of muscle fibers are innervated by small and larger motor neurons. Small motor neurons innervate slow-twitch fibers ; intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibers ; and large motor neurons innervate fast-twitch, fatigable muscle fibers.

The slow-twitch fibers generate less force than the fast-twitch fibers, but they are able to maintain these levels of force for long periods. These fibers are used for maintaining posture and making other low-force movements. Fast-twitch, fatigue-resistant fibers are recruited when the input onto motor neurons is large enough to recruit intermediate-sized motor neurons.

These fibers generate more force than slow-twitch fibers, but they are not able to maintain the force as long as the slow-twitch fibers. Finally, fast-twitch, fatigable fibers are recruited when the largest motor neurons are activated. These fibers produce large amounts of force, but they fatigue very quickly. They are used when the organism must generate a burst of large amounts of force, such as in an escape mechanism.

Most muscles contain both fast- and slow-twitch fibers, but in different proportions. Thus, the white meat of a chicken, used to control the wings, is composed primarily of fast-twitch fibers, whereas the dark meat, used to maintain balance and posture, is composed primarily of slow-twitch fibers. Upper trace of oscilloscope represents the action potentials of a descending pathway axon. With low rates of activity of the descending pathway, only small alpha motor neurons are activated, producing small amounts of muscle force lower trace of oscilloscope.

With increasing rates of descending pathway activity, intermediate-size alpha motor neurons are activated in addition to the small neurons. Because more motor units are activated, the muscle produces more force. Finally, with the highest rates of descending activity, the largest alpha motor neurons are recruited, producing maximal muscle force.

The motor system requires sensory input in order to function properly.