Olfactory bulb

Bulbus olfactorius

Also known as: olfactory glomeruli, mitral cells, tufted cells, granule cells, olfactory tract

The oval, layered forebrain structure resting on the cribriform plate, where olfactory nerve axons synapse in glomeruli onto mitral and tufted cells, whose axons form the olfactory tract.

Overview

Glomeruli per bulb>≈5 500 (adult human)

Layers6

Each olfactory bulb lies on the cribriform plate beneath the orbital surface of the frontal lobe, in the olfactory sulcus lateral to the gyrus rectus, and continues posteriorly as the olfactory tract. It is an outgrowth of the telencephalon with a laminar organisation (superficial → deep): olfactory nerve layer; glomerular layer; external plexiform layer; mitral cell layer; internal plexiform layer; and granule cell layer.

Glomeruli are spherical neuropil units (≈50–150 µm) where thousands of ORN axons expressing the same receptor converge and synapse (glutamatergic) onto the apical dendrites of a few dozen mitral and tufted cells — the bulb's projection neurons. Humans have more than ≈5 500 glomeruli per bulb (≈16 glomeruli per receptor type), which declines with age. This massive convergence increases sensitivity and signal-to-noise ratio.

Inhibitory interneurons sharpen the odour map: periglomerular cells (GABA/dopamine) act between glomeruli, and granule cells (GABAergic, axonless) form reciprocal dendrodendritic synapses with mitral cell lateral dendrites, providing lateral inhibition and synchronous oscillations. The bulb receives extensive centrifugal input from olfactory cortex and from cholinergic, noradrenergic and serotonergic neuromodulatory systems, which adjust processing according to behavioural state. New interneurons are added throughout life in rodents (via the rostral migratory stream); in adult humans this is minimal.

Location

On the cribriform plate of the ethmoid bone, inferior to the frontal lobe (olfactory sulcus), anterior cranial fossa.

Parts & subdivisions

Glomerular layer

Glomeruli receiving convergent ORN axons; periglomerular interneurons.

Mitral cells

Large projection neurons with one apical dendrite to a single glomerulus; axons form the lateral olfactory tract.

Tufted cells

Smaller projection neurons in the external plexiform layer; project mainly to the anterior olfactory nucleus and olfactory tubercle.

Granule cells

Axonless GABAergic interneurons; dendrodendritic lateral inhibition of mitral cells.

Olfactory tract

Carries mitral/tufted axons; contains the anterior olfactory nucleus; divides into olfactory striae at the olfactory trigone.

Anatomy

Arterial supply

Branches of the anterior cerebral artery (orbitofrontal/olfactory branches).

Relations

Above: orbital surface of the frontal lobe (gyrus rectus medially). Below: cribriform plate and the nasal cavity roof. Olfactory groove meningiomas arise beneath it.

Development

Evagination of the rostral telencephalon induced by arriving olfactory axons; bulb agenesis/hypoplasia in Kallmann syndrome (absent on MRI).

Clinical significance

Kallmann syndrome

X-linked (KAL1/ANOS1) or autosomal genetic failure of GnRH neurons and olfactory axons to migrate from the olfactory placode: anosmia + hypogonadotropic hypogonadism (delayed puberty, low LH/FSH); hypoplastic or absent bulbs on MRI.

Foster Kennedy syndrome

An olfactory groove (or frontal) meningioma compresses the ipsilateral olfactory bulb and optic nerve: ipsilateral anosmia, ipsilateral optic atrophy and contralateral papilloedema (from raised ICP).