What Is the Medial Forebrain Bundle?

The medial forebrain bundle is a collection of long projections of nerve cells called axons that plays an important role in the reward system. A collection of structures in the brain, the reward system is involved in producing pleasurable effects in order to regulate human behavior. For this reason, the medial forebrain bundle is sometimes referred to as the reward circuit. Medical researchers believe that this area is one of the primary circuits that affects human behavior.

Forming a huge part of the medial forebrain bundle is the midbrain tegmentum. This is part of the midbrain, or the mesencephalon — the part of the central nervous system (CNS) involved in functions such as hearing, vision and alertness — that extends horizontally from its substantia nigra to its cerebral aqueduct. The substantia nigra in particular contributes to functions such as addiction, learning and eye movement. The medial forebrain bundle also contains components of the limbic system, a collection of brain structures such as the amygdala, the limbix cortex and the hippocampus, which accommodate functions such as emotion, long-term memory, and olfaction, or the sense of smell.

The axons that form the medial forebrain bundle come from three areas: the basal olfactory region, the periamygdaloid cortex and the medial olfactory area. All areas are concerned with olfaction. The basal olfactory region refers to the basal cells, units from the innermost layer of the epidermis, or the outer layer of skin. The periamygdaloid cortex, also known as the periamygdaloid region, is part of a section of the brain called the rhinencephalon. The medial olfactory region, better known as the septal area or the septal nuclei, comprises structures located at the corpus callosum, a collection of neural fibers underneath the brain's cortex.

Passing to the lateral hypothalamus, which regulates certain functions of the autonomic nervous system (ANS), the medial forebrain bundle forms part of the mesolimbic pathway. One of the dopaminergic pathways of the brain, the mesolimbic pathway transfers the neurotransmitter dopamine between different areas of the brain. Dopamine is a chemical involved in functions such as movement and emotion, and its reduction is linked to the development of the degenerative CNS disorder called Parkinson's disease. A particular area of the mesolimbic pathway that the medial forebrain bundle runs to is the ventral tegmental area, a neural bundle that can be found near the midbrain's floor.