Olfactory system

You use your nose to smell.

Tiny bits of smell float in the air. They go into your nose when you breathe. They also move up from your throat when you chew. Inside your nose, a wet layer catches these bits. Tiny hairs feel the smells. These hairs send a message to your brain. Your brain then tells you what the smell is. This helps you enjoy tasty food. It also helps you stay safe from bad smells.

How Do We Smell?

Your sense of smell is very special. It helps you find tasty food. It also helps you stay safe. Most mammals and reptiles have two parts for smelling. One part finds things in the air. The other part senses things in liquids.

Smell starts in your nose. When you breathe, odor molecules enter your nostrils. You can also smell through your throat when you chew. Inside your nose, a wet layer called mucus catches these molecules. Tiny hairs called cilia have receptors on them. These receptors catch the molecules. This starts a process called transduction. This is the way a smell becomes a signal for the brain.

The signal travels through nerves to your brain. It passes through a bone called the ethmoid bone. The signal reaches the olfactory bulb. This part of the brain helps you tell smells apart. It even helps you notice if a smell is strong or weak. Other parts of your brain help you remember smells. They also help you feel happy or sad about them.

Sometimes, people lose their sense of smell. This is called anosmia. It can happen because of age or a cold. It can also happen from a head injury. Doctors can test your smell using things like coffee or mint.

Your sense of smell is a vital way to understand the world. It is part of the chemosensory system. This system includes both smell and taste. Both senses give your brain information about chemicals. This happens through a process called transduction. This is the way a chemical signal becomes an electrical one.

Most mammals and reptiles have two parts for this. The main system finds things in the air. An accessory system senses things in liquids. This helps living things navigate their surroundings.

Smell begins when odor molecules enter your nasal cavity. You can breathe them in through your nostrils. You can also smell through your throat when you chew. This is called retro-nasal olfaction. Inside the nose, a wet layer called mucus dissolves the molecules. Tiny hairs called cilia sit on the olfactory epithelium. These hairs have receptors that bind to the molecules. This causes an electrical response in the olfactory neurons.

This signal travels through nerve fibers to the brain.

The signal must pass through the ethmoid bone. This bone has a part called the cribriform plate. The nerves pass through this plate to reach the olfactory bulbs. In the bulbs, cells called mitral and tufted cells help. They help the brain decide how strong a smell is. This is known as a timing code. These cells also help you tell similar smells apart.

The brain then sends this data to the olfactory cortex. This area includes the amygdala and the hippocampus.

Different parts of the brain handle different tasks. The amygdala helps process signals from other species. The hippocampus helps you form new memories of smells. The olfactory tubercle helps check if a smell is real. It also helps your brain connect smells to rewards. The orbitofrontal cortex helps you expect a reward or punishment.

This system is very complex. It links many areas of the brain together.

Sometimes, the sense of smell does not work right. Losing your sense of smell is called anosmia. This can be total or just on one side. People may also have hyposmia, which is partial loss. Other people might have phantosmia, which are fake smells. Age is a very common cause of smell changes. In the US, 12.4% of people have smell problems. This number rises as people get older. Other causes include head injuries or viral infections like COVID-19.

The olfactory system is the sensory system responsible for the sense of smell, also known as olfaction. It is a vital chemosensory system that allows organisms to detect the chemical composition of their environment. Along with the gustatory system, or sense of taste, it provides the brain with essential data through a process called transduction. Transduction is the method by which chemical signals are converted into electrical impulses that the brain can understand. Most reptiles and mammals possess two distinct parts: a main olfactory system for airborne substances and an accessory olfactory system for fluid-phase stimuli.

The process begins in the peripheral olfactory system, which includes the nostrils, nasal cavity, and the olfactory epithelium. This epithelium is a layer of thin tissue covered in mucus that lines the nasal cavity. Odor molecules enter this system in two ways. First, they enter through the nostrils during inhalation, a process called olfaction. Second, they reach the nasal cavity through the throat during chewing or swallowing. This second pathway is known as retro-nasal olfaction. Once inside, mucus dissolves the odor molecules so they can interact with the sensory cells.

Inside the olfactory epithelium, specialized cells perform the work of detection. Olfactory sensory neurons have tiny hairs called cilia that contain olfactory receptors. These receptors bind to the dissolved odor molecules, triggering an electrical response. This response travels through the sensory neuron to the olfactory nerve fibers at the back of the nasal cavity. These fibers then carry the information toward the central olfactory system. To reach the brain, the nerve fibers must pass through the cribriform plate, which is a part of the ethmoid bone.

Once the signals pass the cribriform plate, they arrive at the olfactory bulbs in the brain's limbic system. The main olfactory bulb uses specific cells to process these incoming pulses. Mitral cells have low firing rates and are easily inhibited by neighboring cells. In contrast, tufted cells have high firing rates and are more difficult to inhibit. Together, these cells use a "timing code" to help the brain determine odor concentration. This mechanism also allows the brain to distinguish between highly similar odors.

The information then moves to the olfactory cortex, located within the uncus. This area includes several important structures like the amygdala, the hippocampus, and the piriform cortex. The olfactory tubercle is a particularly busy hub, possessing 27 inputs and 20 outputs. It helps ensure that signals are actual odors rather than mere irritation of the nasal tissue. It also integrates auditory information and helps transmit positive signals to reward sensors. The amygdala processes signals like pheromones, which are chemical signals between members of the same species.

Other parts of the brain connect smell to memory and emotion. The hippocampus receives olfactory data through the amygdala to form and reinforce new memories. The parahippocampal gyrus provides a topographical map for olfaction, helping to contextualize scenes. The orbitofrontal cortex (OFC) plays a major role in decision-making by representing the expectation of reward or punishment. Interestingly, humans often struggle to identify individual components in a complex mixture of smells. This may be because the olfactory cortex sends feedback to the bulb to suppress background odors through olfactory adaptation.

Problems with the olfactory system are medically significant and can greatly impact quality of life. Total loss of smell is called anosmia, while partial loss is known as hyposmia or microsmia. Some people experience dysosmia, which is distorted smell, or phantosmia, which involves smelling things that are not there. In the United States, a 2012–2014 survey found that 12.4% of people had olfactory dysfunction. This prevalence increases significantly with age, rising to 39.4% for those aged 80 and older. Age is the strongest cause of decline, often due to the closure of the cribriform plate or cumulative damage to receptors.

Other causes of olfactory issues include viral infections, head trauma, and neurodegenerative diseases like Parkinson's or Alzheimer's. Common viruses such as influenza, herpes, and COVID-19 can cause permanent loss of smell. Toxic chemicals, including pesticides and heavy metals like cadmium or nickel, can also damage the epithelium. Traumatic brain injury can disrupt the system depending on the severity of the impact. Because smell helps detect hazards, dysfunction can be dangerous; for example, 20% of people aged 70 and older cannot identify smoke. Understanding these connections helps doctors diagnose whether damage is permanent or temporary.