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The mysterious lateral line system: How did ancient fish evolve this amazing sense?
The lateral line system, also known as the lateral line organ, is a sensory system found in fish that detects motion, vibrations, and pressure gradients in the surrounding water. This sensory capability is achieved through modified epithelial cells, known as hair cells, that respond to displacements caused by movement and convert these signals into electrical signals through excitatory synapses. The lateral line system plays an important role in fish schooling behavior, feeding, and positioning.
The lateral line system is an ancient and fundamental sensory system that dates back to fish more than 400 million years ago.
Function
The lateral line system enables fish to detect motion, vibrations and pressure gradients around them in the water, which is critical for positioning, feeding and schooling behaviour. This system provides spatial awareness, allowing fish to navigate and hunt in environments with poor vision. Research shows that the lateral line system should be an effective passive sensing system capable of distinguishing potential obstacles based on their shape.
For example, blind predatory fish are still able to capture prey, but this ability is impaired when lateral line function is inhibited by cobalt ions. The lateral line system also plays a role in the schooling behavior of fish. Even fish that have lost their sight can still blend into a school of fish, whereas fish with their lateral lines cut off cannot. In addition, this system may further evolve to allow fish to forage in dark caves. In Mexican blind cave fish, the ganglia around the eyes are more sensitive than in fish that live on the surface.
The function of swarming may be to confuse the lateral line system of predatory fish, as the pressure gradients produced by many closely swimming fishes override simple patterns produced by a single prey fish.
Anatomy
The lateral lines usually appear as faint, hole-like lines on the sides of the fish's body. The functional units of the lateral line are nerve branches, which are mechanoreceptors that sense movement in water. The nerve branches in the lateral line system are mainly divided into two categories: canal nerve branches and superficial nerve branches. Superficial nerve branches are located on the surface of the body, while canal nerve branches are found in fluid-filled tubes under the skin.
Each nerve branch is composed of receptor hair cells whose tips are covered by soft glial-like vesicles. Hair cells typically have stable glutamatergic afferent connections and cholinergic efferent connections. These receptive hair cells are modified epithelial cells and usually have 40 to 50 microvilli. These microvilli serve as functional units of mechanoreceptors, and their lengths are distributed in a ladder-like manner.
Signal transduction
When hair cells are stimulated, their tufts move toward the highest tufts, causing cations to enter through mechanically gated channels, leading to depolarization or hyperpolarization reactions. Depolarization opens calcium channels in the basement membrane. Hair cells utilize a coding system to convey a sense of direction of stimuli. By transmitting mechanical motion through the water to the nerve branches, the glial bending causes the hair strands to move with the intensity of the stimulation.
Deflection toward long hairs induces hair cell depolarization, increasing the rate of neurotransmitter release at excitatory afferent junctions; whereas deflection toward short hairs induces hyperpolarization, thereby decreasing neurotransmitter release. release. These electrical signals are transmitted to the brain via lateral line afferent neurons.
Electrophysiology
Mechanoreceptive hair cells in lateral line structures are also integrated into more complex circuits through their afferent and efferent neural connections. Synapses involved in the transduction of mechanical information are excitatory afferent connections, which use glutamate. Different species differ in the number of their nerve branches and afferent connections to hair cells, thus providing different mechanoreceptive properties.
Evolution
Mechanosensory hair cells are functionally homologous to hair cells in the auditory and vestibular systems, showing a close connection between these systems. Due to the overlap in multiple functions and similarities in structure and development, the lateral line system and the fish inner ear are often collectively referred to as the eight-lateral system. The lateral line system is capable of detecting particle speed and acceleration below 100 Hz, while the auditory system detects pressure fluctuations above 100 Hz. The antiquity of the lateral line system suggests that it existed in the common ancestor of vertebrates.
With the evolution of time, the mysterious sensory system of the lateral line is still receiving close attention from biologists. This not only reveals the survival wisdom of fish, but also makes us more curious about the evolution of the sensory system. How do you think this amazing system affects the survival and reproduction strategies of fish?
