Intro | Olfactory Nerve Axons | Olfactory Bulb | Olfactory Cilia | Olfactory Nerve
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Each Olfactory Nerve carries the information away from the olfactory bulbs to cortical areas for additional processing.
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The major output of the olfactory bulbs is typically referred to as the olfactory tract (referred to as the olfactory nerve in this figure) and alternatively as cranial nerve I. Information leaving the bulb travels to a region of old cortex called piriform cortex within the frontal lobe of the same hemisphere, located just in front of the optic chiasm. The primary projection of piriform cortex is to the dorsal medial nucleus of the thalamus, which in turn projects to orbitofrontal cortex. Orbitofrontal cortex is, therefore, primary olfactory cortex. Similar to the other sensory systems, there are two parallel routes for olfactory input to cortex. The second pathway runs from pyriform cortex to the lateral hypothalamus (link to hypothalamus in Fig. 7). This pathway is probably important to the response we make to food (to eat or not to eat!) and for the olfactory control of sexual behavior in mammals.
All mammals (except whales and dolphins) have an additional organ that receives olfactory input, the vomernasal organ. This structure plays an important role in the physiology and behavior of sexuality, and is sensitive to a class of `odors' called pheromones. A pheromone is a chemical that is released by one animal that affects the physiology or behavior of another animal. Examples of pheromonal effects are: 1) The Whitten effect, whereby the menstrual cycles of females living together become synchronized. This effect occurs in response to a pheromone present in male urine. 2) A similar phenomenon (discovered by McClintock) was recently described for human females in response to a pheromone released from the skin of the other females in a group. 3) The Vandenbergh effect, whereby the onset of puberty is quickened by the presence of a pheromone found in male urine. 4) The Bruce effect, whereby a pregnancy is terminated in response to the presence of a pheromone, in the urine of a male other than the one that impregnated the female. 5) Pheromones present in the vaginal secretions of females can stimulate sexual behavior in males. This molecule in hamsters is called aphrodisin. The confirmation of the presence of a vomernasal organ in humans arrived within the past decade (1994). Additional research will be needed to clarify its role in humans.
John Hughlings Jackson (1835-1911) is often called the `father' of British neurology as credit for the many important contributions he made during the early days of brain-behavior studies. Jackson's most significant contributions were to the study of epilepsy and aphasia (link to language disorders - Fig. 10). In 1899, he reported the observation that temporal lobe seizures are often preceded by olfactory auras (sensations). Later, it was noted that electrical stimulation of medial (toward the midline of the brain) temporal lobe (including the amygdala) (link to amygdala - Fig 8) will often elicit olfactory sensations. Furthermore, surgical removal of the temporal lobe typically results in a range of olfactory impairments, including reduced sensitivity to odors, deficits in odor memory, and deficits in odor identification (anosmia). Evidence of olfactory deficits following damage to the orbitalfrontal cortex (located in the region just above the olfactory bulbs) suggests that this cortical region is also involved in olfactory perception. Although the systematic study of olfaction in humans is relatively sparse, the clinical evidence does suggest an important role for this sense modality in social and sexual behavior.