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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 fibres. This information can be processed by the brain as proprioception. The muscle spindle has both sensory and motor components. The sensory component is conveyed by primary type Ia sensory fibres, which spiral around muscle fibres within the spindle, and secondary type II sensory fibres. The motor component is provided by motor neurons: up to a dozen gamma motor neurons, also known as fusimotor neurons, and to a lesser extent by one or two beta motor neurons.
| Characteristics | Values |
|---|---|
| Nature | Stretch receptors within the body of a skeletal muscle |
| Function | Detect changes in the length of the muscle |
| Activation | When a muscle is stretched |
| Type of fibre | Intrafusal muscle fibres |
| Type of sensory fibres | Primary type Ia and secondary type II |
| Type of motor neurons | Gamma and beta motor neurons |
| Type of afferents | Group Ia and group II afferents |
| Velocity | Dynamic and static |
| Location | Parallel to the extrafusal muscle fibres |
| Response | Contraction of the muscle |
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What You'll Learn
Muscle spindles are stretch receptors
The muscle spindle has both sensory and motor components. The sensory component is conveyed by primary type Ia sensory fibres, which spiral around muscle fibres within the spindle, and secondary type II sensory fibres. The primary type Ia fibres respond to both changes in muscle length and velocity and transmit this activity to the spinal cord in the form of changes in the rate of action potentials. The secondary type II fibres respond to muscle length changes and transmit this signal to the spinal cord, albeit with a smaller velocity-sensitive component.
The motor component of the spindle is provided by motor neurons: up to a dozen gamma motor neurons, also known as fusimotor neurons, and, to a lesser extent, one or two beta motor neurons. These activate the muscle fibres within the spindle. Gamma motor neurons supply only muscle fibres within the spindle, whereas beta motor neurons supply muscle fibres both within and outside of the spindle. Activation of the neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibres.
When a muscle spindle's associated muscle is rapidly stretched, the spindle can cause two things to happen. Firstly, it senses how much and how fast a muscle is lengthened or shortened. Secondly, it may signal to its muscle to contract to prevent it from going too far, too quickly in the stretch. This stimulation of a reflexive muscle contraction is known as the stretch or myotatic reflex.
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They detect changes in muscle length and velocity
Muscle spindles are stretch receptors that detect changes in muscle length and velocity. They are located within the body of a skeletal muscle and consist of multiple intrafusal muscle fibres. These intrafusal fibres are contractile proteins like actin and myosin, but unlike extrafusal fibres, they do not extend through the entire length of the muscle. Instead, they are present only at each end of the intrafusal muscle fibre, with the central region containing the muscle fibre's nuclei. The arrangement of these nuclei determines whether the intrafusal muscle fibres are considered nuclear bag or nuclear chain fibres.
When a muscle is stretched, the muscle spindle's primary type Ia sensory fibres respond to changes in both muscle length and velocity. They do this by transmitting signals in the form of changes in the rate of action potentials to the spinal cord. The secondary type II sensory fibres also respond to muscle length changes, although with a smaller velocity-sensitive component, and they transmit this signal to the spinal cord as well. The primary endings are thus considered to transmit information regarding velocity and muscle length, whereas the secondary endings sense muscle length.
The muscle spindles' response to changes in length plays a crucial role in regulating muscle contraction. For example, they can activate motor neurons via the stretch reflex to resist muscle stretch. This is known as the myotatic reflex, where the muscle spindle signals its muscle to contract to prevent it from being stretched too far, too quickly. Additionally, the stretch reflex can inhibit the opposing muscle, known as reciprocal inhibition, to prevent it from contracting and contributing to further stretching.
The sensitivity of the muscle spindles can be adjusted by the gamma motor neurons. These neurons activate the intrafusal muscle fibres, changing the resting firing rate and stretch-sensitivity of the afferents. This modification ensures that the muscle spindles' transduction capabilities are maintained within the appropriate range.
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They convey information to the central nervous system
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 fibres. This information can be processed by the brain as proprioception. The muscle spindle has both sensory and motor components.
The sensory information is conveyed by primary type Ia sensory fibres, which spiral around muscle fibres within the spindle, and secondary type II sensory fibres. The primary type Ia sensory fibres respond to both changes in muscle length and velocity and transmit this activity to the spinal cord in the form of changes in the rate of action potentials. The secondary type II sensory fibres respond to muscle length changes and transmit this signal to the spinal cord. The Ia afferent signals are transmitted to many alpha motor neurons of the receptor-bearing muscle. The alpha motor neurons are then transmitted via their efferent axons to the extrafusal fibres of the muscle, which generate force and resist the stretch.
The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons (also known as fusimotor neurons) and, to a lesser extent, by one or two beta motor neurons. Gamma motor neurons activate the intrafusal muscle fibres, changing the resting firing rate and stretch-sensitivity of the afferents. The function of gamma motor neurons is to modify the sensitivity of the muscle spindle sensory afferents to stretch. The activation of the neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibres.
The stretch-induced receptor potential is then translated into action potential signals that are conveyed to the central nervous system. This sensory transduction may be facilitated in an axon-autonomous manner. The discovery of synaptic-like vesicles containing glutamate suggested that the afferent endings can release glutamate in the spindle, increasing the firing rate of the autonomous afferent and improving the static but not the dynamic sensitivity.
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Gamma motor neurons activate intrafusal muscle fibres
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They are composed of intrafusal muscle fibres and are innervated by both sensory neurons and motor neurons. The motor neurons that innervate the intrafusal muscle fibres are called gamma motor neurons, or fusimotor neurons.
Gamma motor neurons are a type of lower motor neuron that takes part in the process of muscle contraction. They receive input from the reticular formation of the pons in the brainstem and their axons have a diameter of 5 μm. Unlike alpha motor neurons, which are responsible for muscle contraction, gamma motor neurons do not directly adjust the lengthening or shortening of muscles. Instead, they activate the intrafusal muscle fibres within the muscle spindle, changing the resting firing rate and stretch-sensitivity of the afferents.
The function of gamma motor neurons is to modify the sensitivity of the muscle spindle sensory afferents to stretch. When a gamma motor neuron is activated, it releases acetylcholine, causing the end portions of the intrafusal muscle fibres to contract and the non-contractile central portions to elongate. This opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium ions and an increase in the probability of action potential firing. This, in turn, increases the stretch-sensitivity of the muscle spindle afferents.
Gamma motor neurons play a crucial role in keeping the muscle spindles taut, allowing the continued firing of alpha neurons and subsequent muscle contraction. They also help maintain the sensitivity of the muscle spindles during dynamic muscle activity, preventing muscular damage. This is achieved by regulating the stretch of the intrafusal fibres so that they retain proper tension and sensitivity during muscle contraction or relaxation.
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Muscle spindles regulate muscle contraction
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in muscle length. They convey length information to the central nervous system via afferent nerve fibres. This information can be processed by the brain as proprioception. The muscle spindle has both sensory and motor components. The sensory component is conveyed by primary type Ia sensory fibres, which spiral around the muscle fibres within the spindle, and secondary type II sensory fibres. The motor component is provided by motor neurons: up to a dozen gamma motor neurons, also known as fusimotor neurons, and to a lesser extent, one or two beta motor neurons.
The gamma motor neurons activate the muscle fibres within the spindle, changing the resting firing rate and stretch-sensitivity of the afferents. When a muscle spindle's associated muscle is stretched, the spindle can cause two things to happen. Firstly, it senses how much and how fast a muscle is lengthened or shortened. Secondly, it may signal its muscle to contract to prevent it from being stretched too far, too quickly. This stimulation of a reflexive muscle contraction is known as the stretch or myotatic reflex. It can inhibit the opposing muscle, i.e., the antagonist to the muscle being stretched, to prevent it from contracting so that it can't contribute to any further stretching (reciprocal inhibition).
The function of the gamma motor neurons is not to supplement the force of muscle contraction provided by the extrafusal fibres but to modify the sensitivity of the muscle spindle sensory afferents to stretch. Upon release of acetylcholine by the active gamma motor neuron, the end portions of the intrafusal muscle fibres contract, thus elongating the non-contractile central portions. This opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium ions. This raises the resting potential of the endings, thereby increasing the probability of action potential firing and the stretch-sensitivity of the muscle spindle afferents.
Muscles that perform precise movements have many spindles per unit of mass to help ensure exact control of their contractile activity. A simple example of muscle spindle activity is the knee-jerk reflex, a sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon, which lies just below the kneecap. Tapping on the tendon of the knee extensor muscle group below the patella stretches the muscle spindle fibres. This causes activation of extrafusal muscle fibres in the same muscle. A knee jerk occurs as these fibres actively shorten, in turn shortening the intrafusal fibres and causing their discharge to cease.
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Frequently asked questions
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle.
When a muscle is stretched, the primary type Ia sensory fibres of the muscle spindle respond to both changes in muscle length and velocity. This activates the muscle spindle, which then transmits this activity to the spinal cord.
Muscle spindles convey length information to the central nervous system via afferent nerve fibres. This information is then processed by the brain as proprioception.
A muscle spindle consists of multiple intrafusal muscle fibres. These fibres are oriented parallel to the regular, power-producing extrafusal muscle fibres. The central region of the intrafusal muscle fibre does not contract, even though the ends do.
Problems with muscle spindles may manifest as abnormal muscle tone, such as spasticity. This is a velocity-dependent increase in resistance to passive stretch, resulting in exaggerated tendon reflexes or hypereflexia. Other issues include muscular dystrophy and basal ganglia disorder.
