Lateral line system, electrical sensing and mechanoreception


Lateral line system is found in some fishes specialized organs for equilibrium and gravity detection, audition and magnetoreception have evolved from lateral line system of fishes.

The lateral line system for electrical sensing is in the head and body areas of most fishes, some amphibians and platypus. It consists of sensory pores in the epidermis of the skin that connect to canals leading into electroreceptors called ampullary organs. These organs can sense electrical currents in the surrounding water. Most living organisms generate weak electrical fields. The ability to detect these fields helps a fish to find motes, capture prey or avoid predators. This is valuable sense in deep, musky water, where vision is of little use. In fact some fishes actually generate electrical fields and then use their electroreceptors (electro communication) to detect how surrounding objects distort the field. This allows these fishes to navigate in musky waters.

Lateral line receptors lie in the lateral line canal on each side of trunk and tail. Each lateral line canal is continued into cephalic canal on the head. These canals lie in the dermis and have group of epidermal receptors or neuromasts embedded in their walls. The neuromasts are innervated by branches of seventh nerve and the lateral line branch of the vagus. Small tubes emerge from these canals and open on the surface through minute pores.

Each lateral line canal lies within the skin running along the side of entire body. In the head region two lateral canals are joined by a transverse commissural occipital canal above the head, then each lateral line canal runs forward as a post orbital canal which divides into two branches, a supra orbital canal above the orbit and infra orbital below the orbit, both sum up to the snout. The canals are lined with epithelium having many mucous gland cells which secrete mucous. Canals open at intervals on the surface by vertical tubes. The canals are filled with fluid and mucous. In the canals are neuromasts made up of groups of sensory receptor cell and supporting cell. The lateral line neuromasts are current receptors (rheoreceptors) detecting any vibration of water.

Pit organs:
They are found on dorsal and lateral surfaces of the head. They consist of ectodermal pits beneath which lie groups of receptor cells innervated by nerve fibres of seventh cranial nerve. Pit organs are scattered individual neuromasts found in all fishes. They are rheoreceptors.

Ampullae of Lorenzimi:
They are found in clusters on dorsal and ventral surfaces of the head embedded below the skin but opening externally on the surface of the skin. Ampullae of Lorenzimi are connected with the branches of facial nerves. They are arranged in groups formerly they were considered as neuromast organs and afterwards are called theroreceptor organs. The change in temperature of water is carried to the brain through ampullary receptors.

Mechanoreception:
Mechanoreceptors are sensitive to mechanical stimuli such as pressure, sound waves and gravity. Human skin contains various types of mechanoreceptors such as touch receptors and pressure receptors. A pressure receptor called paciman corpuscle is shaped like an onion and consists of series of concentric layers of connective tissue wrapped around the end (dendrite) of a sensory neuron. In contrast pain receptors are only unmylinated (naked) ends of fibres of sensory neurons. Some pain receptors are especially sensitive to mechanical stimuli and others are most sensitive to temperature or chemicals. Hair cells are often Mechanoreceptors in various specialized vertebrate sense organs.

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