Why is Jacobson’s Organ Important in Animal Senses?
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.
[Image will be uploaded soon]
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.
FAQs on Jacobson’s Organ: Definition, Structure, and Functions
1. What is Jacobson's organ?
Jacobson's organ, also known as the vomeronasal organ (VNO), is a specialised, auxiliary olfactory sense organ found in many animals. It is typically located in the soft tissue of the nasal septum or the roof of the mouth. Its primary purpose is to detect specific non-volatile chemical signals, such as pheromones, which are not usually detected by the main olfactory system.
2. What is the main function of Jacobson's organ?
The primary function of Jacobson's organ is to facilitate chemical communication. It specialises in detecting pheromones, which are chemical signals used to convey information between individuals of the same species. This plays a crucial role in triggering social behaviours related to mating, territorial marking, and identifying prey or predators.
3. Which animals are well-known examples of possessing a Jacobson's organ?
Jacobson's organ is highly developed in many species. Notable examples include:
- Snakes and Lizards: They use their forked tongues to deliver scent molecules directly to the organ.
- Mammals: Many mammals, such as cats, dogs, horses, elephants, and rodents, have a functional VNO.
It is largely absent or non-functional in most birds, adult primates, and aquatic mammals.
4. How does Jacobson's organ function in snakes?
In snakes, the Jacobson's organ is essential for their highly developed sense of smell. A snake constantly flicks its forked tongue to pick up chemical particles from the air and ground. When the tongue is retracted, its forked tips are inserted into the two openings of the Jacobson's organ in the roof of the mouth. The organ's sensory cells analyse these chemicals, providing the snake with critical information about the location of prey, potential mates, or threats.
5. What is the Flehmen response and what is its importance?
The Flehmen response is a distinct behaviour seen in many mammals, such as horses, cats, and goats. It involves an animal curling back its upper lip, wrinkling its nose, and inhaling with its mouth open. This action helps to draw air and chemical compounds (especially pheromones) through ducts in the mouth that lead directly to the Jacobson's organ. This allows for a more thorough analysis of a particular scent, which is important for assessing reproductive status or investigating an interesting smell in their environment.
6. Do humans have a functional Jacobson's organ?
While human embryos have a structure called the vomeronasal pit, in most adults it is considered a vestigial organ. This means it lacks the necessary sensory neurons and connections to the brain to function as a chemical sensor. Although its potential role is still debated in scientific circles, there is no conclusive evidence that a functional Jacobson's organ plays a significant part in human behaviour or olfaction.
7. How does the Jacobson's organ differ from the main olfactory system?
The Jacobson's organ and the main olfactory system are two distinct sensory pathways. The main olfactory system detects volatile, airborne chemicals through the nose for general smelling. In contrast, the Jacobson's organ detects larger, non-volatile molecules like pheromones delivered directly to it. Furthermore, signals from the Jacobson's organ are processed in the accessory olfactory bulb in the brain, a separate region from the main olfactory bulb, which handles different types of behavioural responses, particularly social and reproductive ones.
