A mitral cell is a type of neuron that is part of the olfactory system .
These cells are a main output channel of the olfactory bulb and act by sending signals to various peripheral cortical structures. Let us know, in more detail, about this type of cell.
What is a mitral cell
Within the olfactory system, the flow of information from the periphery to the mitral cells is mediated by direct synaptic inputs from olfactory sensory neurons .
The extensions of the sensory neurons form a bundle of nerve fibers (the olfactory nerve) that transmits information about the volatile compound to the central nervous system, and more specifically to the olfactory bulb. In this structure the information is mainly received by two types of cells: mitral cells and plumed cells .
Mitral and plume cells are functionally similar and together they constitute the sensory neurons that with their axons connect the olfactory bulb to the central nervous system.
The olfactory bulb and glomeruli
The contact sites between the primary sensory neurons and the cells of the olfactory bulb give rise to a series of spherical structures called glomeruli. These have a relevant role, since they are the place where information from all the sensory cells converge and perceive the same volatile compound at the same time.
Mitral cells, which have received information from the axons of olfactory neurons, are involved in the synapse with the neuropyl (the region between various cell bodies or somas of neurons) of the olfactory glomeruli.
After the olfactory bulb, the axons of the mitral cells transmit information to other areas of the brain . These regions include the pyriform cortex, the area responsible for detecting odours and involved in the storage of memories; the amygdala, the main nucleus of control of emotions; and the entorhinal cortex, related to memory, smell and orientation.
Morphology of mitral cells
A mitral cell is distinguished by the position of its soma (the cell body) in an ordered row in the layer of mitral cells in the olfactory bulb. They usually show a single primary dendrite (prolongation of the neuron dedicated to the reception of the nerve impulse from other neurons) that projects into a single glomerulus.
In addition, this type of cell shows a few lateral dendrites that project to the outer plexiform layer (region that connects the photoreceptor cells and the bipolar cells).
The morphology of mitral cells has been an advantage in the first studies of synaptic processing carried out since it is possible to independently stimulate both the soma and the main dendrite by means of electrodes conveniently located in different layers of the olfactory bulb.
Once the information from the odour molecules has been captured, transformed and sent to the olfactory bulb, it is processed in the glomeruli and the mitral cells send that information to the relevant brain regions.
But what happens in the main core of olfactory information processing? These are the main functions carried out by the olfactory bulb:
1. Odour differentiation
The olfactory bulb is mainly responsible for distinguishing between different types of odours . This differentiation is made through different patterns of activation of neurons responsible for olfactory perception, which react according to the perceived odour and the shape and structure of the olfactory particles.
2. Emotional processing of information
The amygdala, the brain’s main center of emotion control, has connections with the olfactory bulb both directly and indirectly, through the primary olfactory cortex or piriform cortex, and allows certain emotions to be linked to certain olfactory stimuli .
In addition, our sense of smell, unlike other senses such as sight or hearing, does not need to take over in the thalamus; that is why its connection with the limbic system is more direct, generating more powerful and explicit connections, which makes it easier for us to evoke intense memories of past experiences through smells.
3. Taste perception
The senses of smell and taste are closely related and interconnected . Many times, we have the feeling that we are tasting something when we are simply sniffing.
In this sense, the olfactory bulb also plays an important role in the perception of taste by this very fact. An example of how these two senses relate to each other is the inability of people suffering from anosmia (loss of the sense of smell) to perceive flavours.
Synaptic connections with other cells
Mitral cells play a significant role in the circuit of connections in the olfactory bulb, since they receive information from at least four cell types: olfactory sensory neurons, external plume cells, periglomerular neurons and granular cells. The first two are excitatory, while the other two are inhibitory.
Through their primary dendrites, mitral cells receive excitatory synapses that come from olfactory sensory neurons and external plume cells. In addition, they also receive inhibitory signals from granular cells in their lateral dendrites or in the soma, and from periglomerular cells in the dendritic tuft.
It appears from research that plume cells receive strong innervation of the olfactory nerve and trigger their action potentials near the onset of inhalation and their frequency of firing is relatively insensitive to odor concentration; in contrast, mitral cells receive little innervation of the olfactory nerve and strong periglomerular (around the glomeruli) inhibition, which delays their firing relative to plume cells.
A hypothesis that is handled in animals is that mitral cells transform the strength of the olfactory signal into a synchronized code , where the odour concentration is encoded in the trigger frequency of the action potentials relative to the inhalation cycle.
- Bradford, H.F. (1988). Fundamentals of Neurochemistry. Labor.
- Dhawale et.al (2010) Non-redundant odor coding by sister mitral cells revealed by light addressable glomeruli in the mouse. Nature Neuroscience 13, pp. 1404 – 1412.