Olfactory cortex pyramidal neuron
Olfactory Cortex pyramidal cell is a neuron found in vertebrates.
Pyramidal cells are the principal neurons in the olfactory cortex, i.e., they are considered to be the primary integrative and information storage units (1, 5). Principally, olfactory cortex pyramidal cell activity encodes the odor quality (3). Although there are several olfactory cortical areas, the most studied and largest area is the pirifom cortex (PC), which is why the PC is often called the primary olfactory cortex in mammals (2, 6).
Neuronal Type: Principal
Typically, olfactory cortex pyramidal neurons have a cytoplasm containing a low to moderate concentration of organelles, a relatively smooth-surfaced nucleus and a regular lattice of microtubules in dendrites (1). While there is a variation in the morphological details of pyramidal cells in the olfactory cortex, their basic structural plan is as follows: a cell body and initial segments found in layers II and III of the PC, a large apical dendritic tree directed toward the surface (through layer I), a less substantial basal dendritic tree, a profusion of small spines on apical and basal dendrites concentrated in layer III and a deep-directed myelinated axon (1, 2).
- Neurotransmitter: Glutamate
Pyramidal cells have excitatory interactions with other nearby pyramidal cells largely via basal dendrites and with distant pyramidal cells largely via apical dendrites (1). Each pyramidal cell makes a small number of synaptic contacts on a large number (>1000) of other cells in PC at disparate locations (2).
Electron microscopy of pyramidal cells has also revealed probable inhibitory input: feedback inhibition is mediated via interneurons that are excited by local axon collaterals of pyramidal cells (1).
Spike trains of individual pyramidal cells represent low stimulus frequency (below 30 Hz) but do not represent input in the gamma frequency range. Gamma frequency is thought to be a direct outcome of synaptic currents generated in the apical dendrites of pyramidal cells. Individual pyramidal cell spike trains represent input associated with the theta rhythm (3-12 Hz), respiration (0.8-2.0 Hz), exploratory sniffing (4-11 Hz) and the mean spike rates of olfactory bulb mitral cells (3-20 Hz) (6). From a functional perspective, it is thought that individual piriform cortex pyramidal cells only code for low-frequency stimuli because: 1) neural processes involved in gamma oscillations do not contain stimulus-related information, 2) information contained in the gamma frequency events is represented in a network-level code rather than in spike trains of individual neurons 3) dendritic processes transform synaptic inputs in such a way that high-frequency information is represented in the spike train despite the bandwidth limitations imposed by the spike initiation zone (6).
Unlike other sensory systems, the olfactory system lacks a highly specific feature detector cells. Olfactory pyramidal neurons do not appear to be tuned to “primary” odors or to simple spatial or temporal patterns of receptive surface activation. Rather they tend to respond to a large number of different odors according to a seemingly indecipherable scheme (1). This characteristic has led many different workers to postulate that odors are coded in the form of complex, spatially distributed patterns (1). Pyramidal cell activity does not only encode the odor quality but it is also related to contextual information about past experience and future action (3).
(1) Haberly, L.B., “‘Neuronal Circuitry in Olfactory Cortex: Anatomy and Functional Implications’”, Chemical Senses, 10 (1985) 219-238
(2) Dawn M.G Johnson, Kurt R Illig, Mary Behan, and Lewis B. Harley, “New Features of Connectivity in Piriform Cortex Visualized by Intracellular Injection of Pyramidal Cells Suggest that “Primary” Olfactory Cortex Functions Like “Association” Cortex in Other Sensory Systems”, The Journal of Neuroscience, 18 (2000) :6984-6974
(3) L.E. Zinyuk, F. Datiche, M. Cattarelli, “Cell Activity in the Anterior piriform cortex during an olfactory learning in the rat”, Behavioral Brain Research, 124 (2001) 29-32
(6) Alexander D. Protopapas, James M. Bower, “Spike Coding in Pyramidal Cells of the Piriform Cortex of Rat”, Journal of Neurophysiology, 86 (2001) 1504-1510