Olfactory Cortex & Basal Forebrain
The olfactory cortex & basal forebrain
- Here, we divide this difficult anatomical region into olfactory cortex [specifically the medial, intermediate, and primary (aka lateral) cortices (innervated by the medial, intermediate, and lateral striae, respectively)] or the basal forebrain.
- Unfortunately, this division is imperfectly defined throughout the literature but this is a great way to make sense of this anatomy.
- As a simplification, we'll create two coronal illustrations: one anterior and one posterior.
- The anterior-lying structures generally fall within the olfactory cortex whereas the posterior-lying ones generally fall within the basal forebrain – although this is an oversimplification, it helps us group the regions and draw meaning from their relationships.
As a key landmark, in the anterior plane, we find the optic chiasm, whereas in the posterior plane, we find the optic tract.
The olfactory cortex
- Basal Ganglia: Putamen, Caudate (head), Nucleus accumbens (their ventral connection).
- Internal capsule, Frontal horn of the lateral ventricle, Corpus callosum, Medial hypothalamus, Medial temporal lobe (limbic lobe), and Insula.
From medial to lateral, we show the:
The medial olfactory cortex (aka medial olfactory area) also comprises:
- The paraterminal gyri.
Next, we show the:
- Periamygdaloid cortex in between the piriform cortex, laterally, and the entorhinal cortex, medially.
The anterior olfactory nucleus is also variably listed as part of the primary olfactory cortex.
- The amygdala separates it into:
- The corticomedial division, which categorizes within the olfactory cortex (later we'll see it categorize within the basal forebrain).
- The basolateral division, which isn't a part of either the olfactory cortex or basal forebrain.
The basal forebrain
- Basal Ganglia: Putamen, Caudate (head)
- Internal capsule, Frontal horn of the lateral ventricle, Corpus callosum, Hypothalamus, Medial temporal lobe (limbic lobe), and Insula.
In between the subcallosal gyrus and the nucleus accumbens (in the anterior diagram), lies the diagonal band of Broca, which categorizes within the basal forebrain.
- Superiorly, in midline: the septal nuclei (later we'll see their relationship to the bed nucleus of the stria terminalis).
- The substantia innominata lies lateral to the hypothalamus and beneath the basal ganglia.
- The anterior commissure lies above the substantia innominata and below the basal ganglia.
- The basal nucleus of Meynert within the substantia innominata.
Note that the basal nucleus of Meynert is variably considered to be either synonymous with the substantia innominata or to reside within this "unnamed substance" as it is drawn, here. The ventral pallidum lies just below the anterior commissure.
- The amygdala extends to the surface of the temporal lobe.
- The corticomedial division (here, it categorizes within the basal forebrain).
- The basolateral division.
- The extended amygdala most notably includes the bed nucleus of the stria terminalis and the nucleus accumbens, and refers to those areas of the basal forebrain with prominent connections to the corticomedial amygdala.
As a clinical corollary, cholinesterase inhibitors, which prevent the breakdown of acetylcholine, promote cholinergic health in the basal forebrain and are used to slow the progression of Alzheimer's disease (unfortunately, with limited effect).
- Structures with the most notable cholinergic properties are the:
- Medial septal nuclei
- Diagonal band of Broca
- Basal nucleus of Meynert
Additional considerations
Olfactory Cortex
1. The olfactory tubercle is variably considered part of the primary olfactory cortex, as well as intermediate olfactory cortex.
2. It's the anterior entorhinal cortex that categorizes within the primary olfactory cortex.
3. The distinction between the primary and secondary olfactory cortices is highly variable; as an example, certain texts include the entorhinal cortex as secondary olfactory cortex rather than as primary cortex.
4. Although the piriform cortex in rats, the animal model for olfaction, is extensive and functionally important, its size and significance in humans is not yet known.
Basal Forebrain
1. The ventral pallidum and nucleus accumbens are alternatively (or additionally) categorized as part of the basal ganglia.
2. The corticomedial amygdala is commonly organized along with both the olfactory cortex and basal forebrain.