The locus caeruleus is a part of the formatio reticularis in the bridge (pons) and consists of four nuclei. Its connections to the forebrain (prosencephalon), diencephalon, brainstem (truncus cerebri), cerebellum, and spinal cord are involved in specific arousal processes. Neurodegenerative diseases such as Alzheimer’s dementia, Down syndrome, and Parkinson’s syndrome can damage the locus caeruleus, which also plays a role in various mental disorders.
What is the locus caeruleus?
The locus caeruleus is a part of the central nervous system. It is located in the bridge (pons), which in turn belongs to the hindbrain (metencephalon) and thus to the rhombencephalon. Functionally, the locus caeruleus can be assigned to the ascending reticular activating system (ARAS). The name of the locus caeruleus comes from Latin and translates roughly to “sky-blue place.” The name derives from the coloration in this brain region found by early anatomists during their investigations, which is due to pigments. Other spellings for the locus caeruleus are locus coeruleus and locus ceruleus.
Anatomy and structure
The locus caeruleus is located at the border of the mesencephalon near the fourth ventricle of the brain. It is part of the bridge (pons) that connects the medulla oblongata (medulla oblongata) to the midbrain (mesencephalon). Within the pons, the locus caeruleus represents part of the formatio reticularis. This is a network of different nuclei and neural pathways that run throughout the brainstem (midbrain, bridge, and medulla oblongata). Four structures join to form the locus caeruleus, at the center of which is the central nucleus; its tissue is clearly demarcated from surrounding areas. The anterior part of the locus caeruleus contains the anterior nucleus, while the posterior part contains the dorsal subnucleus. The fourth part of the locus caeruleus is the nucleus subcaeruleus, although some definitions consider it to be an independent area. Numerous nerve fibers connect the locus caeruleus with structures in the forebrain (prosencephalon), diencephalon (diencephalon), brainstem (truncus cerebri), cerebellum (cerebellum), and spinal cord. These neuronal pathways play a critical role in the function of the locus caeruleus.
Function and tasks
Originally, researchers assumed that the locus caeruleus had a nonspecific role in controlling arousals. In fact, however, the functions of the locus caeruleus are both more extensive and more specific than initially thought. Norepinephrine occurs as the major neurotransmitter of the locus caeruleus, where it can bind to various adrenoreceptors, triggering an electrical signal in the downstream neuron. The connections between the prosencephalon and the locus caeruleus include nerve fibers that connect the structure in the pons to the neocortex. The neocortex belongs to the cerebral cortex (cortex cerebri) and, from an evolutionary perspective, embodies its youngest area. Activation in the locus caeruleus is accompanied by an increase in activity in the neocortex and is reflected in subjective experience as increased alertness according to current knowledge. This function of the locus caeruleus also contributes to general arousal. Other fibers lead to the pars basalis telencephali and are also involved in wakefulness and arousal there, among other functions. Furthermore, the locus caeruleus is connected to the limbic system, which is responsible for emotional processes. Crucial structures include the hippocampus, which is significant for memory function, and the amygdala, whose activity is correlated with anxiety. Neural pathways between the locus caeruleus and brainstem connect to motor and premotor functions, sensory processing, parasympathetic activity, and wakefulness. In the diencephalon, nerve fibers from the locus caeruleus terminate at the thalamus and hypothalamus; the cerebellum, whose functions include movement control and coordination, is also connected to the locus caeruleus. Some fibers originating in the locus caeruleus pass directly to the spinal cord.
Diseases
Several nervous diseases can affect the locus caeruleus. Neurodegenerative diseases are diseases characterized by the loss of nerve cells.One example is Alzheimer’s dementia, which is characterized by the progressive loss of neurons. The degeneration leads to various mental and neurological symptoms, including memory impairment, agnosia, speech and language disorders, and the inability to perform (even simple) practical tasks. Particularly in the third and final stages, affected individuals suffer from apathy and are usually bedridden. The exact cause of Alzheimer’s dementia is still unknown. The three leading hypotheses assume a disorder related to plaques, neurofibrils, or certain glial cells that trigger, accompany, or follow neuronal cell loss. Down syndrome may also be associated with an impairment of the locus caeruleus. The congenital disorder is based on a genetic defect: affected individuals have a third chromosome 21, which is why Down syndrome is also known as trisomy 21. In the context of Parkinson’s syndrome, the locus caeruleus can also be affected. The clinical picture manifests itself in the form of four cardinal symptoms: muscle tremor (tremor), muscle stiffness (rigor), slowing of movements (bradykinesis), and postural instability (postural instability). For a diagnosis to be made, at least bradykinesis and one other core symptom must be present. The symptoms are due to atrophy of the substantia nigra, which is part of the extrapyramidal motor system. In addition, the locus caeruleus seems to be associated with various mental disorders. For example, abnormalities were shown in the context of depression, anxiety disorders, panic disorder, and stress. In addition, the locus caeruleus contributes to the development of physical substance dependence; researchers were able to show a corresponding correlation for opiates and alcohol.
