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Crustaceans, such as crabs, lobsters, and crayfish, possess muscles that enable them to move their appendages. The meat inside a crab's shell is essentially its muscle, which it uses to contract and extend its legs and claws. Crustacean muscles have unique properties, differing from those of vertebrates, and are innervated by phasic and/or tonic motoneurons. The understanding of crustacean muscles and their control is an active area of research, with scientists investigating the role of muscle receptors, tendon tension receptors, and the impact of neural trophic influences.
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What You'll Learn
- Crustacean muscles contain muscle fibres of variable properties
- Crustacean muscles are innervated by phasic and/or tonic motoneurons
- Crustaceans have muscle receptors and tendon tension receptors
- Crustacean muscles are controlled by mechanoreceptors in their walking legs
- Crustacean muscles differ physiologically from vertebrates
Crustacean muscles contain muscle fibres of variable properties
The muscle fibres in crustaceans are innervated by phasic and/or tonic motoneurons, which may possess synapses of diverse physiological properties. For example, phasic motor axons usually innervate short-sarcomere phasic muscle fibres, while tonic motor axons innervate long-sarcomere tonic muscle fibres. However, some muscles that receive a single (tonic) motor axon contain both phasic and tonic muscle fibres.
The physiology of striated muscle in Crustacea differs from that of vertebrates in several ways. For instance, the inhibitory process is peripherally located, and there are separate motor fibres for slow and fast contractions. The crustacean muscle is surrounded by a dense network of nerve axons, with each muscle fibre receiving a large number of endings from different types of nerve fibres.
Crabs, lobsters, and other Decapod Crustacea possess proprioceptors and mechanoreceptors in their walking legs and chelipeds. These include joint receptors, muscle receptors, and tendon tension receptors, which are functionally analogous to the corresponding sense organs in vertebrates.
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Crustacean muscles are innervated by phasic and/or tonic motoneurons
Crustacean muscles, which are essentially the meat inside their shells, are used to contract and extend their appendages. These muscles are innervated by phasic and/or tonic motoneurons, which display differential physiology and have morphologically distinct synaptic terminals.
Phasic motor neurons are specialized for the rapid initial release of transmitters and generate large EPSPs in the innervated muscle fibres. They release much more transmitter per impulse and have filiform terminals. Phasic neurons exhibit much higher quantal release per synapse and per muscle fibre, along with more rapid synaptic depression and less short-term facilitation.
Tonic motor neurons, on the other hand, release less transmitter initially and generate smaller EPSPs that exhibit marked facilitation as impulse frequency increases. They have larger terminals with prominent varicosities. Tonic neurons support moderate to high frequencies of nerve impulses intermittently or continuously during locomotion.
The diversity in the physiology and morphology of crustacean motor neurons suggests that other properties of the nerve terminal are responsible for the differences observed in their synaptic content. Some neural trophic effects have been found in crustaceans, indicating that neural trophic influences may play a role in establishing the definitive properties of their muscles.
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Crustaceans have muscle receptors and tendon tension receptors
Crustaceans, such as crabs and crayfish, have muscles that enable them to move their limbs rhythmically. The meat inside a crab's shell is its muscle, which it uses to contract and extend its appendages.
Crustacean muscles contain muscle fibres of variable properties and are innervated by phasic and/or tonic motoneurons. Tonic motor axons innervate long-sarcomere tonic muscle fibres, while phasic motor axons innervate short-sarcomere phasic muscle fibres. However, some muscles with a single (tonic) motor axon contain both phasic and tonic muscle fibres.
Electrophysiological studies have been carried out to analyse the segmental connections of proprioceptors in the various joints and their roles in the coordinated control of muscles. Staining procedures can be used to trace the location of tension receptors within the apodeme.
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Crustacean muscles are controlled by mechanoreceptors in their walking legs
Crustacean muscles are known to contain muscle fibres of variable properties and to be innervated by phasic and/or tonic motoneurons. The meat inside a crab's shell is its muscle, which it uses to contract and extend its appendages.
Crabs, crayfish, lobsters, and other decapod crustaceans are well-endowed with proprioceptors and other mechanoreceptors in their walking legs and chelipeds. These include joint receptors (chordotonal organs), muscle receptors, and tendon (or apodeme) tension receptors. The limb muscle receptors lack any peripheral inhibitory control upon the sensory neurones themselves.
The muscle receptors in the walking legs of crustaceans are non-impulsive stretch receptors. They are non-spiking muscle receptors that are controlled by a tonic sensorimotor synapse.
A single muscle moves a crustacean limb joint rhythmically by acting against a spring containing resilin. The protein resilin has the elastic properties to store energy during contraction and then release it to power the recoil.
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Crustacean muscles differ physiologically from vertebrates
Phasic motor axons typically innervate short-sarcomere phasic muscle fibres, while tonic motor axons innervate long-sarcomere tonic muscle fibres. Interestingly, some muscles receiving a single (tonic) motor axon contain a mix of both phasic and tonic muscle fibres. This diversity in muscle fibre types and innervation patterns is a unique feature of crustaceans.
Another difference lies in the peripheral location of the inhibitory process. Crustacean muscles have separate motor fibres for slow and fast contractions, which is not commonly observed in vertebrates. Additionally, each muscle fibre in crustaceans is surrounded by a dense network of nerve axons, with forty or more nerve endings on its surface. This distinct arrangement of nerve endings contributes to the unique physiological characteristics of crustacean muscles.
Furthermore, crustaceans, including crabs, lobsters, and other Decapod Crustacea, possess a variety of mechanoreceptors in their walking legs and chelipeds. These include joint receptors, muscle receptors, and tendon tension receptors, which are functionally analogous to corresponding sense organs in vertebrates. However, the limb muscle receptors in crustaceans lack peripheral inhibitory control upon the sensory neurons, unlike the 'accessory nerves' found in the abdominal muscle receptor organs of lobsters and crayfish.
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Frequently asked questions
Yes, crustaceans have muscles.
Crustaceans move their limbs by contracting a single muscle that works against a spring containing a protein called resilin.
Crabs are well-endowed with proprioceptors and mechanoreceptors in their walking legs and chelipeds. Crabs control their muscle receptors by reversing a walking leg reflex.
The striated muscle of Crustacea differs from vertebrates in the peripheral location of the inhibitory process and the presence of separate motor fibres for slow and fast contractions.
Some examples of crustaceans include crabs, crayfish, lobsters, and the parasitic copepod, Pennella elegans.
