Jacobson's Organ

Jacobson’s organ is also known as the vomeronasal organ. They are present in pairs. It is an olfactory sense organ. This means that this organ is related to smell. It is located near the soft tissue of the nasal septum. It is present in the nasal cavity, above the roof of the mouth. The name vomeronasal organ is derived from the fact that this organ is present near the unpaired vomer bone. This organ is present in all snakes and lizards. It is also present in mammals such as dogs and cats. This organ is present in human beings also, but it is vestigial and non-functional in nature. 

About Vomeronasal Organ

The vomeronasal organ has the bodies of sensory neurons. They have receptors that detect non-volatile organic compounds. These non-volatile compounds are liquid in nature. These receptors help in conveying these signals to the body from the environment. These compounds can range from sex pheromones, prey, predators and many more. When this organ is triggered, they induce a behavioural response. When certain chemicals are bound to the G protein-coupled receptors, then these organs are activated. They are expressed by three receptors that are V1R, V2R and FPR. Cranial nerve zero is the name given to the axons of these neurons. They induce responses in the olfactory bulb and then target the amygdala and further activates the anterior hypothalamus. These all structures together constitute the accessory olfactory system. There is a flehmen response that is triggered by these receptors in some of the mammals. This organ was discovered in 1732 by Ruysch and later on by Jacobson in 1813. 

Structure

As we read above, Jacobson’s organ is present at the base of the nasal cavity. This organ is split into two parts and these parts are divided by the nasal septum. Both sides of these organs possess an elongated C-shaped structure that is called the lumen. This is present inside a bony or cartilaginous capsule. This capsule opens into the base of the nasal cavity.

The vomeronasal receptor neurons and these neurons possess axons that travel from the vomeronasal organ to the accessory olfactory bulb. This bulb is also known as a vomeronasal bulb. In the medial concave surface of the crescent lumen, these receptors are present. Non-sensory ciliated cells cover the lateral and the convex surface of the lumen. Basal cells are also found at this point. Vomeronasal glands are present at the dorsal and ventral side of the lumen. These glands help in filling the vomeronasal lumen with fluids. Blood vessels are also present adjacent to the lumen. These vomeronasal glands form the nasal placode at the time of embryo development. 

Receptors of Vomeronasal Organ

The nasal septum separates the two pairs of the vomeronasal gland. On the concave side, we have pseudostratified epithelium. This epithelium has three types of cells that are supporting cells, receptor cells and basal cells. The basal cells are present at the membrane that is near to the non-sensory epithelium. The supporting cells are present superficially on the membrane. These receptors are coupled by G-proteins and they help in detecting the pheromones. There are three types of G-protein receptors present that are V1R, V2R and FPR. These are present in the main olfactory epithelium. The activation of these V1R and V2R receptors is done with the help of distinct ligands. Lipophilic odorants activate the G1 proteins. Non-volatile receptors help in activating the G0 receptors. 

• The V1 receptors are linked to the G-proteins. These G proteins help in signalling them in more than one direction. The V1R receptors are located in the apical compartment. This V1R is significantly expressed in the rodent species. 

• The V2 receptors are also linked to the G-proteins. The NH2 terminals are long and extracellular in nature. These receptors are expressed in the basal part of the neurons in the VNO. 

Functions of the Vomeronasal Organ

In mammals, this organ has sensory neurons. These sensory neurons help in detecting the non-volatile chemical cues. This requires direct physical contact with the source of the odour. They also help in detecting pheromones. The vomeronasal organ functions by sending neural signals to the olfactory bulb and then the same signals to the amygdala and the hypothalamus. As the hypothalamus is stimulated and the hypothalamus help in controlling the body temperature so these signals bring changes in the body temperature of the animal. This hypothalamus is responsible for regulating the reproductive hormones. These reproductive hormones are required for the purpose of reproduction and breeding. The main olfactory system also helps in detecting some hormones. 

Animals that Possess Vomeronasal Organ

It is found in all snakes and lizards. Also, mammals possess these organs. In human beings, this organ has become vestigial. 

• Nose tapping behaviour in the salamanders is performed with the help of this organ.

• The snakes are able to sense their prey with the help of this organ. They stick their tongue out to capture the scents from the prey and they touch these organs so that the smells can be retracted.

• Lemurs and lorises have well developed vomeronasal organs.

• In elephants, it is present at the roof of their mouth. It helps in capturing the chemosensory stimuli. 

• Turtles also use this organ to smell underwater. 

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Flehmen Response

There is a phenomenon called Flehmen response that is associated with some mammals such as cats and horses. These animals show certain facial movements that help the inhaled compounds to Jacobson's organ. These distinctive movements are known as flehmen’s response. They lift up their head to find the fragrance or smell, then start wrinkling their nose while lifting up their lips and this helps them cease to breathe momentarily. These animals that show this response have incisive papilla and ducts. Their vomeronasal organs are found behind their teeth. These animals with the help of the vomeronasal organ are able to differentiate between similar smelling products.