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Skeletal muscles are made up of two types of muscle fibres: extrafusal and intrafusal. Extrafusal muscle fibres are slender muscle fibres that attach to tendons and then to the skeleton, producing force and movement. They are embedded in a connective tissue layer called the endomysium. Each extrafusal muscle fibre is surrounded by the endomysium, which is a thin, delicate network of strands between the fibres, also containing the microcirculation network. This paragraph will explore the structure and function of extrafusal muscle fibres and their role in muscle contraction.
| Characteristics | Values |
|---|---|
| Definition | Standard skeletal muscle fibres that generate tension by contracting, allowing for skeletal movement |
| Location | Outside of the spindle |
| Innervation | By alpha motor neurons |
| Function | Produce contraction and supply the muscle with power |
| Muscle Spindles | Located in parallel with extrafusal fibres |
| Muscle Composition | Chains of long cylindrical structures called myofibrils that extend the length of the muscle fibre |
| Muscle Connections | Neuromuscular junction where the neuron's signal is transduced to the muscle fibre by the neurotransmitter acetylcholine |
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What You'll Learn
- Extrafusal muscle fibres are standard skeletal muscle fibres
- They are innervated by alpha motor neurons
- They generate tension by contracting, allowing for skeletal movement
- They are attached to bone by fibrous tissue extensions (tendons)
- Each alpha motor neuron and the extrafusal muscle fibres innervated by it make up a motor unit
Extrafusal muscle fibres are standard skeletal muscle fibres
Extrafusal muscle fibres are not to be confused with intrafusal muscle fibres, which are found within muscle spindles (stretch receptors found within muscles) and are innervated by sensory nerve endings in central noncontractile parts and by gamma motor neurons in contractile ends. Intrafusal muscle fibres serve as sensory proprioceptors. The muscle spindles signal the length of extrafusal muscle fibres at rest, during relaxation, and during contraction, as well as the velocity at which it occurs.
The muscle fibres that form most of the muscle bulk and produce the contraction are called extrafusal fibres. They are long, cylindrical, multinucleated cells. The cytoplasm of a muscle fibre, the sarcoplasm, is enclosed by a plasma membrane called the sarcolemma. The sarcoplasm of the cell contains chains of long cylindrical structures called myofibrils that extend the length of the muscle fibre. Numerous nuclei occupy the space between adjacent myofibrils and the sarcolemma. Extrafusal muscle fibres receive motor innervation.
The muscle spindle is located in parallel with the extrafusal fibres, and it stretches along with the muscle. The muscle spindle signals muscle length and velocity to the CNS through two types of specialised sensory fibres that innervate the intrafusal fibres. These sensory fibres have stretch receptors that open and close as a function of the length of the intrafusal fibre.
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They are innervated by alpha motor neurons
Extrafusal muscle fibers are standard skeletal muscle fibers that are innervated by alpha motor neurons. They are responsible for generating tension through contraction, enabling skeletal movement. Each alpha motor neuron, along with the extrafusal muscle fibers it innervates, forms a motor unit. This neuromuscular junction facilitates the transduction of the neuron's signal, or action potential, to the muscle fiber via the neurotransmitter acetylcholine.
Alpha motor neurons, also known as lower motor neurons, stimulate skeletal muscle contractions that facilitate movement. These neurons release acetylcholine at the neuromuscular junction, leading to the propagation of an action potential along the muscle fiber. If the ends of the muscle remain fixed during contraction, the result is an increased force on the supports, known as isometric contraction. Conversely, if the muscle shortens without resistance, the outcome is a constant force or isotonic contraction.
The role of alpha motor neurons in innervating extrafusal muscle fibers is crucial for muscle function. These neurons send their axons from the spinal gray matter to terminate on extrafusal muscle fibers, causing their contraction. During active muscle contraction, both alpha and gamma motor neurons fire simultaneously. While alpha motor neurons target extrafusal fibers, gamma motor neurons innervate intrafusal fibers, which are located within the muscle spindle.
The coordination between alpha and gamma motor neurons, known as alpha-gamma coactivation, ensures that the muscle spindle maintains sensitivity to stretch over a wide range of muscle lengths. When a muscle is stretched, the muscle spindle and its intrafusal fibers also elongate due to being surrounded by extrafusal fibers. This stretch triggers the firing of action potentials in the 1a sensory axon, which wraps around the intrafusal fibers within the muscle spindle. As a result, the alpha motor neuron innervating the extrafusal muscle fibers increases its firing rate, leading to muscle contraction.
It is important to distinguish extrafusal muscle fibers from intrafusal muscle fibers. While extrafusal muscle fibers are innervated by alpha motor neurons and generate muscle contractions, intrafusal muscle fibers are innervated by sensory nerve endings and gamma motor neurons. Intrafusal fibers have a distinct function, serving as sensory proprioceptors and contributing to our sense of position and movement.
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They generate tension by contracting, allowing for skeletal movement
Extrafusal muscle fibers are the standard skeletal muscle fibers that enable skeletal movement through contraction-induced tension generation. These fibers, innervated by alpha motor neurons, constitute the majority of skeletal striated muscle tissue and are attached to bones via tendons, which are fibrous tissue extensions. Each alpha motor neuron, along with the extrafusal muscle fibers it innervates, forms a motor unit. The neuromuscular junction between the alpha motor neuron and the extrafusal muscle fiber facilitates the conversion of the neuron's signal, the action potential, into a muscle fiber response via the neurotransmitter acetylcholine.
Extrafusal muscle fibers are distinct from intrafusal muscle fibers, which are found within muscle spindles and act as stretch receptors. Intrafusal muscle fibers are innervated by sensory nerve endings in their central non-contractile regions and by gamma motor neurons in their contractile ends, functioning as sensory proprioceptors. While intrafusal fibers possess unique combinations of properties, extrafusal muscle fibers are the primary force-generating fibers during muscle contraction.
The contractile properties of extrafusal muscle fibers can be further understood by examining the behavior of their subtypes, namely bag1, bag2, and chain fibers. Bag1 fibers exhibit slow-twitch myosin properties, similar to the bag2 fibers in their extracapsular polar regions. On the other hand, the contractile portion of bag2 fibers, known as the juxta-equatorial region, expresses tonic myosin. This expression of tonic myosin in presumptive bag2 fibers is an early indicator of differentiation between intrafusal and extrafusal fibers.
Chain fibers, on the other hand, exhibit the fastest contraction rates among the subtypes. They demonstrate appropriate levels of alkaline-stable ATPase activity and are highly oxidative. The introduction of immunohistochemical staining techniques has further emphasized the unique characteristics of intrafusal fibers, including the expression of specific isoforms of MHC. Chain fibers, as expected, express a fast-twitch isoform.
The muscle spindle, composed of dynamic nuclear bag fibers, static nuclear bag fibers, and nuclear chain fibers, plays a crucial role in maintaining sensitivity to muscle length changes. The coordinated process of alpha-gamma coactivation involves the CNS instructing both alpha and gamma motor neurons to contract, ensuring the muscle spindle remains taut and sensitive during muscle contraction. This mechanism highlights the importance of extrafusal muscle fibers in generating tension through contraction, ultimately facilitating skeletal movement.
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They are attached to bone by fibrous tissue extensions (tendons)
Extrafusal muscle fibres are standard skeletal muscle fibres that are responsible for skeletal movement. They are attached to bones by fibrous tissue extensions, known as tendons.
The muscle fibres that form most of the muscle bulk and produce the contraction are called extrafusal fibres. They are highly contracting fibres that supply the muscle with its power. These fibres are innervated by alpha motor neurons and generate tension by contracting, thereby enabling skeletal movement. Each alpha motor neuron and the extrafusal muscle fibres innervated by it make up a motor unit.
The connection between the alpha motor neuron and the extrafusal muscle fibre is a neuromuscular junction. At this junction, the neuron's signal, the action potential, is transduced to the muscle fibre by the neurotransmitter acetylcholine.
Extrafusal muscle fibres are long, cylindrical multinucleated cells. The cytoplasm of a muscle fibre, the sarcoplasm, is enclosed by a plasma membrane called the sarcolemma. Each extrafusal muscle fibre is embedded in a delicate connective tissue layer called the endomysium.
The muscle spindle, a mechanoreceptor or stretch receptor, is located in parallel with the extrafusal fibres. This means that it stretches alongside the muscle. The muscle spindle signals muscle length and velocity to the CNS through specialised sensory fibres that innervate the intrafusal fibres.
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Each alpha motor neuron and the extrafusal muscle fibres innervated by it make up a motor unit
Extrafusal muscle fibres are the standard skeletal muscle fibres that generate tension and facilitate skeletal movement through contraction. They make up the bulk of skeletal striated muscle tissue and are attached to bones by fibrous tissue extensions called tendons.
Extrafusal muscle fibres differ from intrafusal muscle fibres, which are found within muscle spindles (stretch receptors within a muscle) and are innervated by sensory nerve endings in their central noncontractile parts and by gamma motor neurons in their contractile ends. Intrafusal muscle fibres serve as sensory proprioceptors and do not contribute significantly to muscle contraction.
Extrafusal muscle fibres, on the other hand, are innervated by alpha motor neurons, which supply the muscle with its power. Each alpha motor neuron, along with the extrafusal muscle fibres it innervates, forms a motor unit. This connection between the alpha motor neuron and the extrafusal muscle fibre is a neuromuscular junction, where the neuron's signal, or action potential, is transmitted to the muscle fibre through the neurotransmitter acetylcholine.
The muscle spindle, which contains intrafusal fibres, is located in parallel with the extrafusal fibres. When a muscle contracts, the muscle spindle stretches alongside it, signalling muscle length and velocity to the central nervous system (CNS) through specialised sensory fibres that innervate the intrafusal fibres. This process is known as alpha-gamma coactivation, where the CNS instructs both alpha and gamma motor neurons to contract accordingly.
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Frequently asked questions
Extrafusal muscle fibers are the standard skeletal muscle fibers that generate tension by contracting, allowing for skeletal movement.
They are long, cylindrical, multinucleated cells. The cytoplasm of a muscle fiber, the sarcoplasm, is enclosed by a plasma membrane called the sarcolemma.
Each extrafusal muscle fiber is embedded in a delicate connective tissue layer called the endomysium. They are innervated by alpha motor neurons and are the highly contracting fibers that supply the muscle with its power.
Intrafusal muscle fibers are found within muscle spindles (stretch receptors found within muscle) and are innervated by sensory nerve endings in central noncontractile parts and by gamma motor neurons in contractile ends. They serve as a sensory proprioceptor. Extrafusal muscle fibers, on the other hand, are the skeletal muscle fibers outside of the spindle and are responsible for muscle contraction and skeletal movement.
