The olfactory tract passes sensory information from the nose to the rest of the brain, allowing it to interpret odors. Sense of smell is one of the oldest and most rudimentary of the senses, and the olfactory system has a number of unique features that set it apart from other sensory pathways. For example, neurons in the olfactory epithelium are continually replaced with new ones, which is unusual for nerve cells, and the olfactory tract can directly signal the area of the brain known as the olfactory cortex.
These nerve pathways start at the olfactory bulb, which responds to signals sent by nerve cells in the olfactory epithelium. Specialized nerve cells are sensitized to respond to specific odors, activating neurons which can send a signal along the olfactory tract. These signals pass both to the olfactory cortex and into the limbic system for distribution to other parts of the brain. It can combine this information with other sensory input for perception and understanding of the surrounding environment.
In the skull, the olfactory tracts can be seen extending from the paired bulbs located just below the brain. They pass through the ethmoid bone, which has a structure called the cribiform plate to accommodate them. A series of small holes called foramina allow the nerves through while still allowing the bone to protect the skull from impact, infection, and other potential threats. Once in the brain, the sensory neurons route in several directions to carry signals from the nose.
Head injuries sometimes disrupt the olfactory tract. If the cribiform plate is damaged, it may sever or crush the nerves, making it difficult for them to carry signals to the brain. Swelling in the brain related to injuries can put pressure on the olfactory tract and limit its information transmission abilities. Likewise, tumors can grow large enough to displace or damage the nerves; changes to sense of smell can sometimes be a warning sign of a brain tumor.
Disorders related to sense of small can occur for a variety of reasons, including genetics, injuries, and disease. Loss of smell is typically not regarded as life threatening, but can decrease quality of life because people may experience an impaired sense of taste. In addition, for people with anosmia, a total loss of the ability to smell, there can be a risk of unwittingly eating spoiled food or not being aware of a chemical leak because the nose can’t detect the problem.