The olfactory bulb, as an essential structure of the olfactory system, is a rounded tissue in the brain. It consists of several different types of cells that support the translation of the sensory information received from the nose as a smell.
What Is Olfaction?
In the most simple terms, olfaction is our ability to smell, and it’s supported by specialized sensory receptor cells.
It’s a kind of a special sensory perception of the brain that is activated when odorants bind to the olfactory receptor neurons in order to relay the sensory stimuli received through the nasal cavity.
Anatomy and Structure of the Olfactory Bulb
This forebrain structure is located in the lower side of the cerebral hemisphere, near the frontal lobe of the cerebral cortex. Right above the nasal cavity, it connects with the other structures of the brain through the olfactory tract.
The olfactory bulb consists of two parts: the main olfactory bulb and the accessory olfactory bulb. The nerve tissue comprising these structures is labeled as glomeruli.
The glomeruli connect the axon’s fibers of the receptor cells with the outer dendritic fibers of the interneurons. All of the glomeruli are located near the surface of the olfactory bulb.
What Are the Cell Structures of the Olfactory Bulb?
The olfactory bulb comprises of different cell structures that are organized in layers:
- Glomerular cell structure
- External plexiform cell structure
- Mitral cell layer
- Internal plexiform cell structure
- Granule cell layer
Astrocytes, mitral cells, periglomerular cells, interneurons, tufted cells, and granular cells, are the main carriers of chemical processes within these layers of the olfactory bulb.
Astrocytes
Astrocytes are star-shaped glial cells found in the connective tissue of the central nervous system. This type of glial cells are, in fact, synapses that support the flow of chemical and electrical impulses between the cells and are arranged according to their chemical and electrical communication with the other cells.
This type of cell in the brain coordinates the actions of the neuron cells by guiding them to their final destination, thus playing an important role in neurotransmission. By removing the pathogens from the brain, they also protect the central nervous system from inflammation and infection.
These non-neuronal cells play a vital role in determining the functional interactions among specific neuronal substructures involved in the control of the metabolic processes in the human body.
Mitral and Tufted Cells
The mitral cells are located in the olfactory bulb. Upon receiving the sensory data from the axons of the olfactory receptor neurons, they form synapses in the glomeruli. Hence, their main function is to relay the sensory information to the other areas of the cerebral cortex.
Together with the tufted cells, they are the main neurons of the olfactory bulbs, but they process different aspects of the information received through the sense of smell.
The tufted cells are located in the olfactory glomeruli and function as projection neurons, i.e. they relay a sharpened olfactory signal deeper into the brain in order to be processed.
Periglomerular and Granular Cells
The periglomerular and the granular cells are both GABAergic interneurons that form inhibitory synapses with the mitral and tufted cells in the olfactory bulb. These two types of cells are the key to the detection and translation of odor.
Interneurons
The interneurons (or association neurons) are found only in the central nervous system, i.e. in the brain and the spinal cord. They enable the communication between the CNS and the sensory and motor neurons.
These associative neurons receive impulses from the sensory neurons, as well as interpret the information they receive from the other neurons so that they can transmit the electrical impulses to the motor neurons. In this way, the motor neurons are signaled the appropriate response to the sensory stimuli.
What Is the Function of the Olfactory Bulb?
The main function of the olfactory bulb is to relay the smell stimuli from the nose to the dedicated areas of the brain, where they are processed and translated.
However, the olfactory bulbs also support the discrimination among odors (distinguishing pleasant from unpleasant smells, as well as distinctive smells, like the scent of roses, lemon, fish), they filter the background odors in order to support the transmission of a few selected odors, and they also aid the other brain areas in modulating the detection or filtering of the odors.
What Is an Olfactory System?
The olfactory system is the sensory system that enables us to smell. It’s comprised of:
- Peripheral olfactory system
- Transduction
- Central olfactory system
Peripheral Olfactory System
The peripheral olfactory system is made of the following elements:
- Nostrils – The nose channels. Their upperparts are lined with a mucous membrane that supports the perception of smell, while the lower parts serve for respiration.
- Ethmoid bone – A bone in the skull that separates the nasal cavity from the brain.
- Nasal cavity – An air-filled space behind the nose, in the middle of the face.
- Olfactory epithelium – epithelial tissue in the olfactory system responsible for the detection of smell.
Transduction
The transduction of smell is a series of processes that occur in the nasal cavity. These processes begin when the molecules that bear scent bind to the cells in the nose. Then they send the electrical signals to the brain, where they are translated as a distinct smell.
Central Olfactory System
The central olfactory system is the area where the main olfactory bulb relays the information to both the mitral and the tufted cells. These cells can also determine the differences between similar odors.
The central olfactory system transmits the data received from the sensory stimuli to various parts of the amygdala, thalamus, hypothalamus, hippocampus, brain stem, retina, auditory cortex, as well as to the olfactory system.
How Is Smell Transmitted to the Brain?
When the molecules of the olfactory bulb carry an impulse received from the sensory organ of smell, they connect with the cilia on the surface of the olfactory receptor cells in the nasal cavity. From here, the electric nerve impulses travel along the nerve fibers to a network of small blood vessels (labeled as glomeruli) in the olfactory bulb.
The axons of the smell receptor cells branch into the olfactory bulb, where the information on smell is processed. This is how the olfactory bulb transmits what we perceive as a ‘scent’ from the nose to the brain.
What Neuron Pathways Do Scents Use?
The olfactory receptors are located in the mucous membrane of the upper nasal cavity. Each olfactory receptor has projections: a peripheral dendrite and an olfactory nerve fiber.
The olfactory nerve fiber branches through the cranial cavity in order to reach the olfactory bulb in the forebrain. When the chemical substances become dissolved in the thin layer of fluid that covers the mucous membrane, these chemical molecules that carry the sensory information of the smell come in touch with the tiny olfactory hairs. This is where the smell is formed from the received information.
All smell receptor cells have a different structure and sensitivity. Hence, they are able to detect different odor-spreading substances.
Which Other Brain Structures Support the Processing of Information Received From the Olfactory Bulb?
The olfactory bulb receives the sensory information of smell from the amygdala, hippocampus, neocortex, and substantia nigra.
It also sends the olfactory information back to the amygdala, the neocortex, substantia nigra, and the hippocampus, in order to be further processed. Hence, it plays a role in the emotions, memory, and learning triggered by a memorized scent.
Amygdala
The amygdala is the brain area that is responsible for deriving emotional responses by processing and translating sensory stimuli. It is located in the temporal lobe, positioned right in front of the hippocampus.
The neurons of the medial nucleus of the amygdala receive, process, and transmit the sensory information from the olfactory pathways to the hypothalamic nuclei. The amygdala in the olfactory cortex processes pheromone, allomone, and kairomone.
Hippocampus
This part of the brain relays the information to the other regions of the cerebral cortex which are responsible for giving meaning to the received sensory signals.
By conducting complex neurological processes and connecting them with other related memories, this brain structure interacts mostly with the amygdala and the hypothalamus.
Neocortex
The neocortex is the outermost, cortical part of the human brain. This structure of the brain functions as a bridge to the other parts of our brain by building junctions among the neural cells from different areas of the cerebral cortex, including the olfactory bulb.
Each of the lobes of the neocortex is a center responsible for the processing of specific stimuli. It’s a highly organized structure responsible for processing, regulating, and controling cognitive and sensory information, as well as consciousness, emotions, and all sensory stimulations among which are the olfactory stimuli by the odorants, too.
Substantia Nigra
The substantia nigra is a midbrain structure composed of two parts: substantia nigra pars reticulata and substantia nigra pars compacta.
The substantia nigra pars reticulata is a brain area whose neuronal cells project their axons to the thalamus, midbrain, and the brain stem. Its main function is to direct the information received from the sensory stimuli back to the striatum, or to the thalamus and the brain stem.
Fun Facts
Did you know?
- All of the knowledge we accumulate during our life creates wrinkles in our neocortex. Thus, the more wrinkled our brain is, the smarter we are!
- Sheep have larger olfactory bulbs than humans. This gives them a stronger sense of smell as one of the means for their survival in nature.
- We are able to detect more that one trillion odors with our nose.
- Some of the best and most pleasing smells in the world include cookies taken fresh out the oven, freshly baked bread, fresh morning coffee, fresh sea air, lemons, the smell of a freshly bathed baby, lavender, melting chocolate, the smell of the rain, the smell of a new book, freshly mown grass, freshly washed hair, etc.
- On the other side of the spectrum, there are the worst smells in the world. The list of most displeasing smells includes body odor (smelly feet, sweat), garbage bins, sewage, public toilets, vomit, the smell of burning plastic, fish, garlic breath, etc.
- We have around 50 olfactory bulbs, but over 10 million olfactory bulb neurons that help us detect any smell in our surrounding.
- Behind every thought, action, scent, or dream there is a complex chemical process developed and conducted in our brain.
- There are around 1000 billion neurons in the human brain.
- We can actually smell fear and disgust through the sweat, after which we can experience the same emotions.
- Dogs have almost 44 times more scent cells than humans. This explains why they are such good ‘sniffers’.