Commonly known as the sense of smell, olfaction is the sensory process that detects and responds to airborne chemical stimuli. The human sense of smell is very sensitive to certain odourants with detection levels as low as a few parts per trillion parts of air (green pepper odour for example). Thresholds for odour detection span approximately 1012 orders of magnitude. The physicochemical events that determine human olfactory sensitivity apply equally well to animal sensitivities. However the absolute sensitivity of most mammals is orders of magnitude greater than in humans. This is primarily related to the num-ber of receptors in the olfactory mucosa, being in the order of 100 million in rabbit contrasting with 10 million in human.
The structure of the nose varies from a simple sac in salamanders, frogs and tortoises to a more complex structure in mammals, almost completely occupied by the TURBINATE BONES, which bear the OLFACTORY EPITHELIUM. Within this epithelium the receptor neurons are arranged in a mosaic between supporting cells, and overlie a single layer of basal cells. The receptor neurons have apical cilia entering the mucous layer which is secreted from the Bowman’s glands, and their unmyelinated axons synapse in complex GLOMERULI in the OLFACTORY BULB. Cilia have long been established as a feature of the olfactory neuron and emphasis has been placed on their role in the perception of odours. They increase the surface area of the receptor-cell surface, and are believed to contain the seven-transmembrane receptors for odorant ligands (see LIGAND).
Most mammals, the exceptions being higher primates, possess a functionally well-developed dual olfactory system. The main olfactory system has its neural receptors located in the nasal cavity. These give rise to axons which ascend into the cranial cavity through the CRIBRIFORM PLATE to synapse on MITRAL CELLS in the olfactory bulbs. Behaviour that is initiated by olfactory cues but can be modified by complex variables, such as past experience, requires integrative NEURAL NETWORKS, and is probably represented neuroanatomically by this main olfactory pathway which relays in PYRIFORM CORTEX, THALAMUS and ORBITO FRONTAL CORTEX. This allows for a degree of plasticity in the behavioural response and, in contrast to insects, mammals rarely show fixed actions patterns of behaviour in response to odour cues. A second set of chemoreceptor neurons located in the VOMERONASAL ORGAN send axons via the VOMERONASAL NERVE to the ACCESSORY OLFACTORY BULB. In contrast to that of the main bulb, the projection of the accessory bulb is directly into the limbic brain with connections to the AMYGDALA and HYPOTHALAMUS. The accessory olfactory pathway has, therefore, a fairly direct connection with those neural structures involved in pri mary motivated behaviours such as FEEDING and SEXUAL BEHAVIOUR and neuroendocrine function, but unlike the main olfactory bulb appears not to have access to the thalamus and in turn neocortical regions that integrate sensory information.
Although it is generally accepted that most mammals use all sensory systems to assess their environment, some clearly rely on olfactory information more than others, especially in the context of REPRODUCTION. Thus, not only do mice identify the sex of an individual by its odour, but their physiological reproductive state is determined largely by chemical cues. Mice, like most nocturnal mammals, are categorized as MACROSMATIC. PRIMATES, on the other hand, including man, have all of their senses well developed and with the evolutionary enlargement of the NEOCORTEX have the capacity to assimilate and integrate information rapidly from a number of sensory channels simultaneously. More pertinently, primates possess the ability to attend to whichever sensory channel is most relevant at the time and behaviour is not dominated by any one sense.
The olfactory neurons of VERTEBRATES, including mammals, are continuously replaced, the turnover occurring even in adult life. The BASAL CELLS, located close to the BASAL LAMINA, serve as STEM CELLS for mature neurons. Active division of these cells is stimulated following damage to the olfactory mucosa. Complete experimental section of the olfactory sensory axons induces degeneration of the olfactory neurons which is followed by an outburst of mitotic activity in the basal cells which subsequently differentiate into mature neurons. Their axons establish anatomical and functional synaptic connections with the apical dendrites of the mitral cells, their first relay in the olfactory bulb. The continual regeneration of olfactory receptor neurons poses some special problems for olfactory LEARNING and MEMORY. To complicate matters further, cells from the SUBVENTRICULAR ZONE of the cortex migrate rostrally to reach the olfactory bulb, where they differentiate into intrinsic neurons called GRANULE CELLS. These granule cells form dendro-dendritic synapses with mitral cells which are crucial to the role of the olfactory bulb in olfactory learning. This migration occurs not only in the postnatal period, but is also observed in the adult. These regenerative events raise the question as to how this instability in the organization of the neural network can sustain olfactory memories for any length of time. Perhaps this accounts for why olfactory memories are difficult to recall in the absence of the cue, and even the process of recognition may require continual updating.
