Olfactory bulb

The olfactory bulb is an elongated neural structure located on the inferior surface of the anterior forebrain, resting above the cribriform plate of the ethmoid bone. It is the first central part of the olfactory pathway and receives and begins processing information about smell.

Afferent input and glomeruli

Olfactory receptor neurons in the olfactory epithelium detect odorants. Their axons form numerous small bundles that together make up the olfactory nerve (CN I). These fibres pass through the foramina of the cribriform plate and enter the olfactory bulb on the same side.

Within the bulb, the olfactory nerve fibres end in small spherical structures called olfactory glomeruli. Here, they synapse mainly with the dendrites of mitral cells, which are the principal projection neurons of the olfactory bulb. Tufted and periglomerular cells also participate in this local circuitry.

Each olfactory receptor neuron contains a particular type of odorant receptor. Neurons containing the same receptor type send their signals to particular glomeruli. A single odorant usually activates several receptor types, so each smell produces a characteristic pattern of activity across several glomeruli. Different smells produce different patterns, allowing the nervous system to distinguish between different odours.

Intrinsic processing within olfactory bulb

The olfactory bulb does not simply pass smell information onward; it also modifies and refines the incoming signals.

Signals entering the glomeruli activate mitral and tufted cells. Local neurons, particularly periglomerular and granule cells, then help regulate this activity. Granule cells interact with mitral cells and provide inhibitory signals, helping reduce competing or background activity. This makes the important pattern of olfactory activity clearer before it leaves the olfactory bulb.

Therefore, the basic sequence is:

Olfactory receptors → olfactory nerve → glomeruli → local processing → mitral and tufted cells → olfactory tract

Efferent pathway

Axons of mitral and tufted cells leave the olfactory bulb posteriorly and contribute to the olfactory tract. The tract passes posteriorly along the inferior surface of the forebrain and divides principally into the medial and lateral olfactory striae, which carry olfactory information toward central olfactory regions.

Clinical correlate

• Trauma involving the cribriform plate may tear the delicate olfactory nerve fibres as they pass through it, resulting in anosmia (loss of smell).

• If the meninges are also damaged, CSF rhinorrhoea may occur, providing a potential route for infection to enter the cranial cavity.