All parts of the brain › Smell

What part of the brain controls smell?

No single part. Smell enters the brain at the olfactory bulb under the frontal lobe; the olfactory tract carries it to the olfactory cortex (the piriform cortex on the front of the parahippocampal gyrus and amygdala, and the entorhinal cortex beside it), and from there to the orbitofrontal cortex and the insula. Smell is the only sense whose signals reach the cortex before any stop in the thalamus. [1][2][3][4]

Not lit: the olfactory bulb and olfactory tract, in the olfactory sulcus under the frontal lobe. The olfactory bulb and tract are not a separate shape in this atlas (BodyParts3D), so no marker is placed at a guessed position. [2][5]

The piriform cortex is a region on the parahippocampal gyrus, not a separate shape. The 3D model has no shape for it, so the whole parahippocampal gyrus is lit; the region’s exact border is not drawn.

The 3D brain needs JavaScript. The list of parts still says what each one does for smell.

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All 4 parts are lit together. Drag to turn, pinch or scroll to zoom, tap a part on the brain or a name in the list to focus it; .

Smell: 4 parts lit

Parahippocampal gyrus, amygdala, orbitofrontal cortex and insula. Tap one to see where it is and what it does.

The parts and their roles

Its front end carries the primary olfactory cortex: the lateral olfactory stria bends toward its uncus, and the piriform cortex and the entorhinal cortex there receive the olfactory tract directly. The hippocampus gets almost all of its smell input by way of the amygdala instead. [2][3][6][7] Where is the parahippocampal gyrus?
Takes smell in directly from the olfactory bulb and olfactory cortex; its cortical part is counted as part of the piriform cortex, and its cortical and medial nuclei link smell to the hypothalamus. [8][3] Where is the amygdala?
Receives the primary olfactory cortex both directly and through the mediodorsal nucleus of the thalamus, and is thought to represent smell, taste and their reward value; neurons here respond to smells as well as tastes and sights of food. [9][4][10] Where is the orbitofrontal cortex?
Its front part, the anterior insula, is tied more to smell, taste and gut feelings than its rear part, and neurons in the primary taste cortex there often answer smells as well as tastes. [11][10] Where is the insula?

Named areas on the lit gyri

The piriform cortex, on the : The main primary olfactory cortex. In humans it has been described as the cortical amygdala, the uncus and the front of the parahippocampal gyrus, between the insula and the temporal lobe, in front of and beside the amygdala; work on human odour coding points to its rear part. [3]

How they work together

The route: odour molecules bind receptors on 10–20 million receptor neurons in the olfactory epithelium high in the nose; their axons pass through holes in the cribriform plate of the ethmoid bone into the olfactory bulb; and mitral cells in the bulb send the signal down the olfactory tract to the olfactory cortex and the limbic system. [1][12][5]
Humans have about 400 working odour-receptor genes (mice have about 1,000). Axons from receptor neurons of one type converge in the bulb on small knots of about 50 micrometres called glomeruli, so each glomerulus collects one receptor type. [12][4]
Smell is the one sense that does not stop in the thalamus before reaching the cortex. A second route does go through the thalamus: the piriform cortex projects to the mediodorsal nucleus of the thalamus, which projects to the orbitofrontal cortex. [1][4]
Because smell axons run straight into the limbic system (the tract reaches the amygdala and entorhinal cortex directly) and the hypothalamus, smells become linked with long-term memories and emotions, which is why a smell can bring back a place or a moment. [1][2]
Olfactory receptor neurons are replaced throughout life from stem cells in the nasal lining, unusually for neurons; the new axons grow along the ones already in place to find their targets in the bulb. [12][1]
Honest limit: the piriform and entorhinal cortex are regions with no shape of their own on this model, so the whole parahippocampal gyrus is lit, and the piriform cortex also takes in part of the amygdala; the olfactory bulb, tract and tubercle and the nose are not in the model at all; and how many odours a person can tell apart is an estimate: the old figure of about 10,000 came from impressionistic findings in the 1920s, while a 2014 study of mixtures built from 128 odour molecules estimated more than a trillion. [3][4][7]

Smell and taste work together: what we call flavour is taste plus smell plus the touch-like trigeminal sense, and the insula and orbitofrontal cortex take part in both. [13][9] What part of the brain controls taste?

Sources

Facts describe anatomy and normal function only. A part lit here takes part in smell; it does not do it alone, and it does other jobs too. The 3D model is the adult male BodyParts3D brain (credits and licence); on it the brainstem includes the midbrain, pons and medulla, so lighting the brainstem lights all three.