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Portrait of Olfactory bulb

Anatomy · Eye, ear & senses

Olfactory bulb

The first relay of the sense of smell

Forebrain · underside of the frontal lobe

A pair of small, oval swellings on the underside of the frontal lobes, each resting on the perforated cribriform plate of the ethmoid bone above the roof of the nose. Bundles of olfactory nerve fibres (cranial nerve I) pass up through the plate and connect within the bulb, where signals are sorted in ball-shaped glomeruli. The mitral and tufted cells then send them along the olfactory tract to the olfactory cortex, without a relay through the thalamus first.

How to spot them

  • ▸A small grey oval on the underside of the frontal lobe, one on each side of the midline
  • ▸Sits on the cribriform plate of the ethmoid bone, above the roof of the nose
  • ▸Continues backwards as the olfactory tract, which divides into two stria
  • ▸Fibres of the olfactory nerve pierce the plate below it in about twenty bundles on each side

What to know

Sits in a groove on the ethmoid bone, pierced by tiny holes like a sieve, through which nerve fibres pass upward

The olfactory bulb rests on the cribriform plate of the ethmoid

Ball-shaped knots of neuropil, each collecting fibres from receptor cells that all carry the same odorant receptor

Olfactory glomeruli, the sorting stations of the olfactory bulb (Buck and Axel, Nobel 2004)

The mitral cells send their long axons along a tract straight to the cortex, bypassing the thalamus

The olfactory bulb — smell is the only sense whose primary pathway reaches the cortex without a thalamic relay

A blow to the back of the head can shear off the tiny fibres as they pass through the sieve-like bone, causing loss of smell

Post-traumatic anosmia from tearing of the olfactory fibres at the cribriform plate

Absent or small in a syndrome that combines failure of puberty and no sense of smell

Kallmann syndrome — hypoplastic olfactory bulbs with hypogonadotropic hypogonadism

A tumour here can cause loss of smell on one side, optic atrophy on that side and swelling of the optic disc on the other

Foster Kennedy syndrome from an olfactory-groove meningioma compressing the bulb

One of only a few sites in the adult mammalian brain where new nerve cells are added, at least in rodents

Adult neurogenesis in the olfactory bulb, from the subventricular zone

Key ideas

  • Each olfactory receptor neuron expresses one of about 400 functional human odorant-receptor genes, and all neurons with the same receptor converge on the same few glomeruli, making the bulb a map of smell.
  • Because receptor neurons are exposed in the nose and are replaced every month or two, the bulb is one of the few places where the brain is in direct contact with the outside world.

Quick-fire facts

  • •Olfactory nerve (cranial nerve I) fibres are unmyelinated axons of bipolar receptor neurons in the roof of the nasal cavity
  • •Mitral and tufted cells are the principal output cells; their axons form the lateral olfactory tract to the piriform cortex, amygdala and entorhinal cortex
  • •Olfactory receptors were identified in 1991 by Linda Buck and Richard Axel, who shared the 2004 Nobel Prize in Physiology or Medicine
  • •Humans have about 400 functional odorant-receptor genes; mice have about 1,000
  • •Each bulb is only about 1 cm long in a human, proportionally far smaller than a dog's
  • •Loss of smell (anosmia) was a common symptom of COVID-19; the virus infects support cells rather than the neurons themselves
  • •The olfactory tract widens into the olfactory trigone and divides into medial and lateral striae
  • •Smell is the only sense linked directly to the limbic system, which may help explain the link between odours and memory

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