The fasciculus retroflexus (FR) also known as the habenulointerpeduncular tract is a bundle of fibers located at the base of the

midbrain The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

vertebrates Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

. Connected to the habenula (Hbn) and the

interpeduncular nucleus The interpeduncular nucleus (IPN) is an unpaired, ovoid group of neurons at the base of the midbrain tegmentum. In the midbrain it lies below the interpeduncular fossa. As the name suggests, the interpeduncular nucleus lies ''in between'' the cere ...

(IPN), the fasciculus retroflexus is involved in a variety of bodily phenomena, some being sleep retention. and drug addiction. It acts as a channel through which messages are sent between the stria medullaris and the mid- and

hindbrain The hindbrain, rhombencephalon (shaped like a rhombus) is a developmental categorization of portions of the central nervous system in vertebrates. It includes the medulla, pons, and cerebellum. Together they support vital bodily processes. Met ...

. The fasciculus retroflexus, along with the stria medullaris, the habenula, and the medial forebrain bundle forms a unit for the transfer of neurological impulses. In this unit, the fasciculus retroflexus mediates the transfer of information for processes such as pain, pleasure, and motor control

Anatomy

The fasciculus retroflexus is the main efferent track of the habenula. The FR is an extremely condensed bundle of fibers which consists of two concentric regions. The first of these is the inner fibers of the FR, beginning at the medial habenula and span to

, sometimes referred to as the Hbn-FR-IPN complex. The outer portion runs from the lateral habenula to the RMTg.

Function

The FR functions as the connecting segment between the Hbn and IPN in the Hbn-FR-IPN complex. The function of these three portions are extremely interconnected in their neurological purpose. Signals sent from the habenula to the interpeduncular nucleus must first pass through the core of the fasciculus retroflexus.

Associations

REM sleep

The placement of the FR makes it a vital pathway for contribution to proper functioning of sleep-wake cycles (Circadian rhythm). The FR has significant influence on

REM sleep Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals (including humans) and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the s ...

, specifically. When FR function is hindered, REM sleep was reduced by more than 50% in a study done on rats.

Drug addiction

The habenula is largely identified as the reward center of the brain, and so a prime target for drug addiction. By association, the fasciculus retroflexus plays a key role in transmitting the messages which cause dependence on certain

controlled substance A controlled substance is generally a drug or chemical whose manufacture, possession and use is regulated by a government, such as illicitly used drugs or prescription medications that are designated by law. Some treaties, notably the Sing ...

s. Many trials have been done specifically linking these connections to

nicotine Nicotine is a natural product, naturally produced alkaloid in the nightshade family of plants (most predominantly in tobacco and ''Duboisia hopwoodii'') and is widely used recreational drug use, recreationally as a stimulant and anxiolytic. As ...

addiction. The basis of

drug addiction Addiction is a neuropsychological disorder characterized by a persistent and intense urge to use a drug or engage in a behavior that produces natural reward, despite substantial harm and other negative consequences. Repetitive drug use can ...

is tether to the fluctuating levels of

dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. It is an amine synthesized ...

found in our brains. A key

inhibitor Inhibitor or inhibition may refer to: Biology * Enzyme inhibitor, a substance that binds to an enzyme and decreases the enzyme's activity * Reuptake inhibitor, a substance that increases neurotransmission by blocking the reuptake of a neurotransmi ...

, which functions to keep these dopamine levels in check, is

GABA GABA (gamma-aminobutyric acid, γ-aminobutyric acid) is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervous system. GA ...

. The Hbn-FR-IPN makes up the main pathway through which both GABA and dopamine flow. Once drugs are introduced, this area of the brain, the reward and pleasure center is heavily involved. The inhibitory

GABAergic In molecular biology and physiology, something is GABAergic or GABAnergic if it pertains to or affects the neurotransmitter gamma-aminobutyric acid (GABA). For example, a synapse is GABAergic if it uses GABA as its neurotransmitter, and a GABAergic ...

cells are located in the nuclei of the RMTg, which is the output of the fasciculus retroflexus's inner portion. The modulation, activation, and inhibition of dopamine is mediated by the habenula and by association the inner and outer fasciculus retroflexus for the duration of drug use; this idea exemplifying its participation in the formation of drug addiction.

Development

Early brain development

Early brain development of the fasciculus retroflexus, like most tissues of the central nervous system, persists when embryotic non-specified neural tissues begin to grow and differentiate. Both the fasciculus retroflexus and the habenula have been observed to be dependent of the presence Wnt1, as shown in a rat study.

Clinical significance

When FR are lesioned artificially, a reduction in signal transduction is observed. This leads to an understanding that, at least for specific

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

s, the FR is vital to the signaling pathways related to the habenula; further solidifying the proposed structure and function of the FR, proving that its core channel is necessary for response output. The study of these

lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by injury or diseases. The term ''Lesion'' is derived from the Latin meaning "injury". Lesions may occur in both plants and animals. Types There is no de ...

s leads to an understanding of how defects in a mammal's Hbn-FR-IPN complex, or more specifically their fasciculus retroflexus, could potentially lead to decreased response time and

signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events. Proteins responsible for detecting stimuli are generally termed receptor (biology), rece ...

; insinuating that defects hold consequences in the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

. Likewise, since the FR was found to be such a vital part of habenula function, loss of functionality in the FR would lead to co-malfunction in the habenula, which has been found to cause imbalances leading to

anxiety Anxiety is an emotion characterised by an unpleasant state of inner wikt:turmoil, turmoil and includes feelings of dread over Anticipation, anticipated events. Anxiety is different from fear in that fear is defined as the emotional response ...

, depression, and even

schizophrenia Schizophrenia () is a mental disorder characterized variously by hallucinations (typically, Auditory hallucination#Schizophrenia, hearing voices), delusions, thought disorder, disorganized thinking and behavior, and Reduced affect display, f ...

References

{{reflist Animal anatomy