Amygdala in the context of "Olfactory tract"

Facial expression is the motion and positioning of the muscles beneath the skin of the face. These movements convey the emotional state of an individual to observers and are a form of nonverbal communication. They are a primary means of conveying social information between humans, but they also occur in most other mammals and some other animal species.

Humans can adopt a facial expression voluntarily or involuntarily, and the neural mechanisms responsible for controlling the expression differ in each case. Voluntary facial expressions are often socially conditioned and follow a cortical route in the brain. Conversely, involuntary facial expressions are believed to be innate and follow a subcortical route in the brain. Facial recognition can be an emotional experience for the brain and the amygdala is highly involved in the recognition process.

The olfactory bulb (Latin: bulbus olfactorius) is a neural structure in the forebrain of vertebrates that is involved in olfaction, or the sense of smell. It transmits olfactory information to the other brain regions including the amygdala, orbitofrontal cortex (OFC) and hippocampus where it contributes to emotion, memory and learning.

The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. The accessory olfactory bulb resides on the dorsal-posterior region of the main olfactory bulb and forms a parallel pathway.

The anterior commissure (also known as the precommissure) is a white matter tract (a bundle of axons) connecting the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix. In all but five species of mammal the great majority of fibers connecting the two hemispheres travel through the corpus callosum, which in humans and all non-monotremes is more than 10 times larger than the anterior commissure. Other routes of communication pass through the hippocampal commissure or, indirectly, via subcortical connections. Nevertheless, the anterior commissure is a significant pathway that can be clearly distinguished in the brains of all mammals.

The anterior commissure plays a key role in pain sensation, more specifically sharp, acute pain. It also contains decussating fibers from the olfactory tracts, vital for the sense of smell and chemoreception. The anterior commissure works with the posterior commissure to link the two cerebral hemispheres of the brain and also interconnects the amygdalae and temporal lobes, contributing to the role of memory, emotion, speech and hearing. It also is involved in olfaction, instinct, and sexual behavior.

Evolutionary explanations for the existence of discrete emotions such as fear and joy are one of many theoretical approaches to understanding the ontological nature of emotions. Historically, evolutionary theoretical approaches to emotions, including basic emotion theory, have postulated that certain so-called basic emotions (usually fear, joy, anger, disgust, and sadness) have evolved over human phylogeny to serve specific functions (for example, fear alerts a human mind of imminent danger). So-called basic emotions are often linked causally to subcortical structures of the brain, including the amygdala (pronounced uh-MIG-duh-luh). In other words, subcortical structures have historically been considered the causes of emotions, while neocortical (neo- meaning new, recent and cortical meaning relating to cortex) structures, especially the prefrontal cortex, are almost invariably understood as the cause of reason. Those ideas about the brain are old; they're traceable at least to Aristotle and were later incorporated into Paul MacLean's mistaken model of brain organization, the "triune brain." These ideas have led to the widespread, erroneous belief that animal brains, including human brains, evolve in a linear fashion, such that, along the course of evolution, new layers of brain tissue are stacked upon older layers of brain tissue, much like the formation of sedimentary rocks. Brain evolution is a lot more complicated than that.

Evolution and natural selection has been applied to the study of human communication, mainly by Charles Darwin in his 1872 work, The Expression of the Emotions in Man and Animals. Darwin researched the expression of emotions in an effort to support his materialist theory of unguided evolution. He proposed that much like other traits found in animals, emotions apparently also evolved and were adapted over time. His work looked at not only facial expressions in animals and specifically humans, but attempted to point out parallels between behaviors in humans and other animals.

The basolateral amygdala, or basolateral complex, or basolateral nuclear complex consists of the lateral, basal and accessory-basal nuclei of the amygdala. The lateral nuclei receives the majority of sensory information, which arrives directly from the temporal lobe structures, including the hippocampus and primary auditory cortex. The basolateral amygdala also receives dense neuromodulatory inputs from ventral tegmental area (VTA), locus coeruleus (LC), and basal forebrain, whose integrity are important for associative learning. The information is then processed by the basolateral complex and is sent as output to the central nucleus of the amygdala. This is how most emotional arousal is formed in mammals.

The central nucleus of the amygdala (CeA or aCeN) is a nucleus within the amygdala. It serves as the major output nucleus of the amygdala and participates in receiving and processing pain information.

CeA connects with brainstem areas that control the expression of innate behaviors and associated physiological responses.

The Intercalated cells of the amygdala (ITC or ICCs) are GABAergic neurons situated between the basolateral and central nuclei of the amygdala that play a significant role in inhibitory control over the amygdala. They regulate amygdala-dependent emotional processing like fear memory and social behavior. Their function has been best studied with selective ITC ablation which impairs fear extinction, fear generalization, and social behavior. Studies have begun to recognize that ITC clusters may be implicated in reward, addiction, and withdrawal circuits given their heavy expression of dopamine and opioid receptors.

In rodents, ITCs are organized into distinct clusters that wrap the basolateral amygdala (BLA). Each cluster is unique in connectivity, intrinsic properties, and function. These clusters are named by their location relative to the BLA with medial ITC clusters towards the central amygdala.

Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli. Axons of the mitral cells transfer information to a number of areas in the brain, including the piriform cortex, entorhinal cortex, and amygdala. Mitral cells receive excitatory input from olfactory sensory neurons and external tufted cells on their primary dendrites, whereas inhibitory input arises either from granule cells onto their lateral dendrites and soma or from periglomerular cells onto their dendritic tuft. Mitral cells together with tufted cells form an obligatory relay for all olfactory information entering from the olfactory nerve. Mitral cell output is not a passive reflection of their input from the olfactory nerve. In mice, each mitral cell sends a single primary dendrite into a glomerulus receiving input from a population of olfactory sensory neurons expressing identical olfactory receptor proteins, yet the odor responsiveness of the 20-40 mitral cells connected to a single glomerulus (called sister mitral cells) is not identical to the tuning curve of the input cells, and also differs between sister mitral cells. Odorant response properties of individual neurons in an olfactory glomerular module. The exact type of processing that mitral cells perform with their inputs is still a matter of controversy. One prominent hypothesis is that mitral cells encode the strength of an olfactory input into their firing phases relative to the sniff cycle. A second hypothesis is that the olfactory bulb network acts as a dynamical system that decorrelates to differentiate between representations of highly similar odorants over time. Support for the second hypothesis comes primarily from research in zebrafish (where mitral and tufted cells cannot be distinguished).