Logan Steele
Skeletal muscles are made up of two types of muscle fibers: extrafusal and intrafusal. Extrafusal muscle fibers attach to tendons and then to the skeleton, producing force and movement. They are embedded in a connective tissue layer called the endomysium and arranged in bundles surrounded by another connective tissue layer called the perimysium. These bundles of muscle fibers are called fascicles. This paragraph will explore the structure and function of extrafusal muscle fibers and their role in muscle contraction.
| Characteristics | Values |
|---|---|
| Definition | Two types of muscle fibers: extrafusal and intrafusal |
| Location | Outside of the muscle spindle |
| Shape | Polygonal in cross-section |
| Nucleus Location | Next to the sarcolemma; 3–5% may have “internalized” nuclei |
| Encasement | Individual muscle fibers are surrounded by the endomysium |
| Attachment | Attach to tendons and then to the skeleton |
| Function | Produce force and movement |
| Generation | Can be generated in vitro from pluripotent stem cells through directed differentiation |
| Types | Slow and fast extrafusal muscle fibers |
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What You'll Learn
- Extrafusal fibres are attached to tendons and the skeleton, producing force and movement
- They are distinct from intrafusal fibres, which are found within muscle spindles
- Muscle activation increases muscle stiffness, as does death, leading to rigor mortis
- Muscle fibres are composed of myofibrils, which contain sarcomeres, the functional contractile unit of the muscle
- Neuromuscular spindles are recognised as rounded structures containing intrafusal fibres
Extrafusal fibres are attached to tendons and the skeleton, producing force and movement
Skeletal muscles are composed of specialised cells, which fuse during development to form two types of fibres: extrafusal and intrafusal. Extrafusal fibres attach to tendons and the skeleton, producing force and movement. Intrafusal fibres, on the other hand, contain muscle spindles and attach to muscles, serving a sensory function.
Extrafusal muscle fibres are responsible for muscle contraction. They are embedded in a connective tissue layer called the endomysium, which is a thin, delicate network of strands that also contains the microcirculation network. Individual muscle fibres are surrounded by the endomysium, which provides structural support and facilitates the exchange of nutrients and waste products between the blood vessels and the muscle cells.
The endomysium surrounds each extrafusal muscle fibre, and these fibres are then arranged in bundles, surrounded by another connective tissue layer called the perimysium. These bundles of muscle fibres are called fascicles. The perimysium is composed of concentric strands of connective tissue that enclose and provide support to the fascicles.
Extrafusal muscle fibres play a crucial role in producing force and movement in the body. They work in conjunction with muscle proteins, primarily myosin and actin, which form strands within the muscle fibres. Myosin molecules store kinetic energy by metabolising adenosine triphosphate (ATP). When a muscle is activated, this chemical energy is converted into mechanical force and work, resulting in movement.
The process of muscle activation involves a cascade of electrical and biochemical events. It begins with the release of acetylcholine by motor neuron synapses at the neuromuscular junction. This release of acetylcholine triggers a series of reactions that ultimately lead to the conversion of chemical energy into mechanical force by the muscle fibres.
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They are distinct from intrafusal fibres, which are found within muscle spindles
Extrafusal muscle fibres are distinct from intrafusal fibres. While extrafusal fibres are responsible for force and movement, intrafusal fibres are sensory in nature. Intrafusal fibres are found within muscle spindles and are innervated by both sensory and motor nerve fibres.
Muscle spindles are fusiform structures 0.5–3.0 mm in length found longitudinally at the edge of muscle fasciculi. They are enclosed in a thick fibrous capsule and contain multiple small intrafusal muscle fibres, nerve fibres, specialised nerve endings, and blood vessels. The muscle spindles are in parallel with extrafusal fibres, so that any tension applied to the long axis of the muscle will stretch both types of fibre.
Intrafusal fibres are a specialised cell population in skeletal muscle. They have mechano-sensory capabilities, forming part of the monosynaptic stretch-reflex arc, a key component responsible for proprioceptive function. They detect the amount and rate of change in length of a muscle.
The stretch reflex is important in clinical neurology because it is responsible for muscle tone and tendon reflexes. Intrafusal muscle fibres have a separate motor innervation that arises from gamma motoneurons (also located in the anterior horn). These neurons co-activate with alpha motoneurons to ensure that the intrafusal fibres remain under tension when muscles contract, so that stretch-sensitivity is maintained during movements.
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Muscle activation increases muscle stiffness, as does death, leading to rigor mortis
Skeletal muscles consist of two types of muscle fibres: extrafusal and intrafusal. Extrafusal muscle fibres, which attach to tendons and then to the skeleton, produce force and movement. They are embedded in a connective tissue layer called the endomysium and arranged in bundles surrounded by another connective tissue layer called the perimysium.
Muscle activation increases muscle stiffness. However, muscle stiffness also occurs when a person or animal dies, leading to a condition called rigor mortis. This phenomenon is caused by the skeletal muscles partially contracting and becoming locked in place. The onset of rigor mortis may range from 10 minutes to several hours, depending on factors including temperature. Under normal conditions, rigor mortis sets in within four hours.
Rigor mortis is caused by the depletion of the cell's energy molecule, adenosine triphosphate (ATP), which is crucial for muscle function. ATP provides the energy required for muscle contraction and relaxation. Without ATP, the myosin heads remain attached to actin, causing the muscles to stay contracted and become rigid.
In addition, after death, calcium flows freely into the cells. This influx of calcium continues to activate the contraction process even though ATP is no longer available to release the myosin heads from actin.
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Muscle fibres are composed of myofibrils, which contain sarcomeres, the functional contractile unit of the muscle
Skeletal muscles are composed of specialised cells, which fuse during development to form two types of fibres: extrafusal and intrafusal. Extrafusal fibres, which attach to tendons and then to the skeleton, produce force and movement. Each extrafusal muscle fibre is embedded in a connective tissue layer called the endomysium.
Muscle fibres are composed of myofibrils, which are long fibres composed of myofilaments that facilitate muscle contraction. Myofibrils are only approximately 1.2 μm in diameter, and hundreds to thousands can be found inside one muscle fibre.
Myofibrils contain numerous sarcomeres, the functional contractile unit of the muscle. Sarcomeres are the smallest functional unit of a skeletal muscle fibre and are a highly organised arrangement of contractile, regulatory, and structural proteins. The sarcomere is the region of a myofibril contained between two cytoskeletal structures called Z-discs (also called Z-lines or Z-bands).
The sarcomere comprises two main protein filaments: thin actin and thick myosin filaments. These are the active structures responsible for muscular contraction. The widely accepted theory describing muscular contraction is the sliding filament theory, which proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere.
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Neuromuscular spindles are recognised as rounded structures containing intrafusal fibres
Skeletal muscles consist of two types of muscle fibres: extrafusal and intrafusal. Extrafusal fibres, which attach to tendons and then to the skeleton, produce force and movement. Neuromuscular spindles are recognised as rounded structures containing intrafusal fibres. These intrafusal fibres are found within muscle spindles, which are structures that are 1–4 mm in length and surrounded by a fusiform capsule of connective tissue. The spindle has a thick surrounding fibrous capsule and contains multiple small variably sized intrafusal muscle fibres, nerve fibres, specialised nerve endings, and blood vessels.
The muscle spindles signal the length of extrafusal muscle fibres at rest, or during relaxation and contraction, and the velocity at which it occurs. Muscle spindles are richly innervated by mechanosensitive nerve endings that terminate in the non-contractile central portion of the fibres. These nerve endings are highly sensitive to changes in muscle fibre length, especially the rate of change in length. The muscle spindles contain three types of intrafusal fibres: nuclear bag1, nuclear bag2, and nuclear chain fibres. Different parts of intrafusal fibres are innervated by different neurons: the central (equatorial) part is in intimate contact with afferent proprioceptive sensory neurons, termed primary "group Ia afferents" and (if present) secondary or "group II afferents".
In addition to the sensory neurons, intrafusal muscle fibres are innervated by efferent gamma motoneurons in both polar regions, where they form a cholinergic synapse. The polar regions of intrafusal fibres contain most of the contractile elements. The gamma motoneurons induce contractions of sarcomeres in the polar region to exert tension on the central region of intrafusal fibres. This prevents the slackening of intrafusal fibres during muscle shortenings and allows for continuous adjustment. The muscle spindles sense muscle tension to coordinate body movement.
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Frequently asked questions
Extrafusal muscle fibers are one of the two types of muscle fibers, the other being intrafusal fibers. Extrafusal fibers attach to tendons and then to the skeleton, producing force and movement.
In cross-section, extrafusal muscle fibers in adults are polygonally shaped, with little space between them, and are rather uniform in size.
Extrafusal muscle fibers work concurrently with intrafusal fibers to coordinate muscle contraction and relaxation.
