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Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in muscle length and velocity. They convey length information to the central nervous system (CNS) via afferent nerve fibres. The muscle spindle has both sensory and motor components. The sensory component is comprised of type Ia and type II sensory fibres, which are responsible for detecting muscle length and velocity. The motor component is provided by motor neurons, which activate the muscle fibres within the spindle. The muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far.
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What You'll Learn
Intrafusal muscle fibres
Intrafusal fibres are classified as either nuclear bag fibres or nuclear chain fibres. Nuclear bag fibres are thicker and contain many nuclei that are centrally located. These fibres extend beyond the spindle capsule and attach to the connective tissue of the extrafusal fibres. Nuclear chain fibres, on the other hand, are shorter and thinner and have fewer nuclei in the central area of the fibre. The central region of the intrafusal fibres does not contain contractile elements, representing the sensory receptor area of the spindle. Both types of intrafusal fibres contain contractile elements at their distal poles.
The intrafusal fibres of the spindle are innervated by two types of sensory nerves: annulospiral and flower-spray neurons. The branches of the large, myelinated annulospiral neurons wrap around the centre of both types of intrafusal fibres. The sensory innervation of the muscle spindle arises from both group Ia and group II afferent fibres, which differ in their axonal conduction velocity. The primary type Ia sensory fibres spiral around all intrafusal muscle fibres, ending near the middle of each fibre. These fibres respond to both changes in muscle length and velocity. The secondary type II sensory fibres, on the other hand, end adjacent to the central regions of the static bag and chain fibres. These fibres respond to muscle length changes but are less sensitive to velocity changes.
The efferent nerve fibres of gamma motor neurons also terminate in muscle spindles, regulating the sensitivity of the sensory afferents located in the non-contractile central (equatorial) region. The contractile proteins in intrafusal fibres, such as actin and myosin, are only present at each end of the fibre, causing the central region to remain non-contractile.
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Primary and secondary sensory fibres
Muscle spindles are stretch receptors located within the body of muscles that provide feedback to the central nervous system about changes in muscle length and the rate of change in length. They are essential for the control of movement and posture, as well as for maintaining muscle tone.
Now, let's focus on the primary and secondary sensory fibres within the context of muscle spindles:
The muscle spindle is a stretch receptor organ embedded in the muscle belly and
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Contractile elements
Muscle spindles contain intrafusal muscle fibres, which are up to 8mm long in humans. Each muscle spindle contains on average 8–20 intrafusal fibres. These fibres are much thinner than extrafusal muscle fibres.
Intrafusal fibres assume two forms: nuclear bag fibres and nuclear chain fibres. Nuclear bag fibres are thicker and contain many nuclei that are centrally located. These fibres extend beyond the spindle capsule and attach to the connective tissue of the extrafusal fibres. Nuclear chain fibres are shorter and thinner and have fewer nuclei in the central area of the fibre. The arrangement of the nuclei determines whether the intrafusal muscle fibres are considered nuclear bag fibres or nuclear chain fibres.
Both types of intrafusal fibres contain contractile elements at their distal poles. The central region of the fibres does not contain contractile elements; this represents the sensory receptor area of the spindle. The contractile elements in the polar regions of intrafusal fibres contain sarcomeres.
The contractile proteins actin and myosin are present at each end of a intrafusal muscle fibre. The central region of a intrafusal muscle fibre does not contain contractile proteins, and therefore does not contract, even though the ends do.
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Connective tissue sac
A muscle spindle is an encapsulated proprioceptor that detects changes in length. It consists of a connective tissue sac, 5mm long and 0.2mm in diameter, containing 2 to 12 specially adapted muscle fibres, known as intrafusal fibres. These intrafusal fibres are narrower than extrafusal fibres and come in two forms: nuclear bag fibres and nuclear chain fibres.
Nuclear bag fibres are thicker and contain many nuclei that are centrally located. These fibres extend beyond the spindle capsule and attach to the connective tissue of the extrafusal fibres. Nuclear chain fibres, on the other hand, are shorter and thinner and have fewer nuclei in the central area of the fibre. The central region of these fibres does not contain contractile elements, representing the sensory receptor area of the spindle.
The intrafusal fibres of the spindle are innervated by sensory nerves called annulospiral and flower-spray neurons. The branches of the large, myelinated annulospiral neurons wrap around the centre of both types of intrafusal fibres. The sensory innervation of the muscle spindle arises from both group Ia and group II afferent fibres, which form irregular coils with branches and varicose swellings.
The muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far. This is achieved through the detection of changes in muscle length and velocity, which are conveyed to the central nervous system (CNS) via afferent nerve fibres. The CNS then computes the position and movement of our extremities in space, contributing to motor control, posture maintenance, and a stable gait.
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Gamma motor neurons
Muscle spindles are innervated by both sensory neurons and motor neurons, including gamma motor neurons. Gamma motor neurons are relatively small in size, but they make up approximately 30% of the motor neurons leaving the spinal cord. They are one of three types of lower motor neurons involved in muscle contraction, the others being alpha motor neurons and beta motor neurons.
The muscle spindle is a group of smaller and specialised muscle fibres within a muscle. Typical spindles contain two bag fibres and about five chain fibres. The intrafusal fibres are innervated by motor nerves (the gamma motor neurons) and two different types of sensory fibres: the type Ia stretch receptors and the type II afferent sensory receptors. The type Ia sensory fibres then synapse with alpha motor neurons, completing the gamma-loop.
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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.
Each muscle spindle contains multiple intrafusal muscle fibres. These fibres have contractile proteins like actin and myosin.
Intrafusal muscle fibres are up to 8mm long in humans. They are much thinner than extrafusal muscle fibres.
The two types of intrafusal fibres are nuclear bag fibres and nuclear chain fibres. Nuclear bag fibres are thicker and contain many nuclei that are centrally located. Nuclear chain fibres are shorter and thinner and have fewer nuclei in the central area of the fibre.
Muscle spindles inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. This information is used to compute the position and movement of our extremities in space, which is important for motor control, maintaining posture and a stable gait.
