The nucleus accumbens is situated within the basal forebrain. This small yet essential structure is a significant part of the ventral striatum and is placed between the caudate and putamen. The nucleus accumbens is partitioned into two physiological features- the central core and outer shell.
The nucleus accumbens acts as a neuronal intersection. Its complex network facilitates various cognitive functions- voluntary movement, operant conditioning, motivation, and slow-wave sleep. The two divisions of the nucleus accumbens- the shell and core- function in distinct yet cooperative ways.
The nucleus accumbens primary role is its elaborate neuronal and chemical engagement in maintaining the reward system. However, the nucleus accumbens performs numerous cognitive functions due to its intricate neuronal connections to other cranial features. More study is needed to fully determine the full significance and contribution of the nucleus accumbens.
The Nucleus Accumbens Anatomy
The nucleus accumbens is circular and flat on the top (dorsally). It's situated on the border of the anterior commissure and aligns parallel to the midline. Despite its relatively diminished stature, the nucleus accumbens enacts an array of functions. There are two distinct structural elements of the nucleus accumbens- the core and shell. These function in an individual yet supportive manner.
The outer shell is the nucleus accumbens substructure and resides on the exterior of the nucleus accumbens. The shell is also understood to extend as a feature of the rostral amygdala. The central core is the inner area of the nucleus accumbens. Scholars and neurologists believe the core comprises the ventral striatum and basal ganglia.
The core and shell of the nucleus accumbens are distinguished from each other via the following criteria: electrophysiological, histochemical, cellular make-up, and neuronal connections. In addition, these two components differ in their cortical afferent density. However, the shell and core maintain neuronal inputs from the globus pallidus, ventral pallidum, and amygdala.
The functional distinction between the shell and core is divided into two roles. The shell functions primarily with the limbic system and the core with the motor system. This difference is more noticeable in rats than in humans. Differences in the shell and core performance properties mainly arise due to connectivity and not neuronal make-up.
The Nucleus Accumbens- Outer Shell
The nucleus accumbens shell transmits neuronal projections mainly to the brain's limbic areas. These areas include the lateral hypothalamus, bed nucleus of the stria terminalis, central amygdala, and the ventromedial ventral pallidum.
The nucleus accumbens shell is tasked with the subjective reactions to pleasurable stimuli. Termed an opioid hedonistic hotpot, this desire hub reacts to pleasant stimuli and is situated medially within a small partition. Addictive drugs' dopamine release is significantly higher in the shell than in the core.
An interesting component of the shell is its role in instrumental conditioning. The shell has been observed to regulate Pavlovian-instrument transfer. This type of learning is when a classically conditioned stimulus results in reward-driven behavior.
This phenomenon is described as operant behavior and Pavlovian learning. Essentially, it determines behavior according to the level of reinforcement or aversion. The nucleus accumbens responds to stimuli and regulates the mind’s internal state. It does this according to the reward-based value system of the surrounding environment and cultural dynamics. This feature is a fascinating study.
The microcircuitry of the nucleus accumbens shell relays "liking" responses. The synaptic engagement of the axons within the nucleus accumbens shell tempers reward-seeking or aversion-driven behavior. The shell focuses on emotionally relevant information for encoding and assessing environmental stimuli. The shell is involved in risked based decision-making.
The Nucleus Accumbens- Central Core
The nucleus accumbens core is related to the motor system. Mainly, the core transmits neuronal projections to cranial areas associated with bodily movement. These regions, like the ventral pallidum and the subthalamic nucleus, are responsible for motor performance. More specifically, they’re tasked with motivation to action.
Due to this intricate connectivity, the core is involved in goal-oriented motor movements. The core reacts to environmental stimuli and facilitates necessary learning- regulated by reward and reinforcement. Its specialized function is to encode freshly learned action strategies- that will enable bodily motion directed to receive the desired reward.
The core of the nucleus accumbens is also tasked with slow-wave sleep modulation. Some neurons in the nucleus accumbens core present adenosine receptors (ADORA2A) to facilitate slow wave sleep. The activity stimulation level determines the onset of sleep in the core. Slow-wave sleep modulation is understood by motivating factors and how these impact the core.
As with the nucleus accumbens shell, the core is also responsible for generating Pavlovian-instrument transfer. Lesions to the nucleus accumbens core in rats demonstrate an inability of the rats to learn conditioned aversion responses. Therefore, highlighting the role of the core in conditioned learning of operant behavior.
The Nucleus Accumbens Efferent and Afferent Fibers
The efferent nerve fiber connections of the nucleus accumbens are output fibers. Output fibers travel from the nucleus accumbens to the basal ganglia and globus pallidus. The globus pallidus connection pathway directs output fibers to the medial dorsal nucleus of the thalamus- these project to the striatum and the prefrontal cortex.
Other efferent projections directed from the nucleus accumbens include those sent to the ventral tegmental area, the reticular formation of the pons, cingulum, lateral hypothalamus, and the substantia nigra.
The afferent fibers that connect to the nucleus accumbens are glutaminergic, histaminergic, and dopaminergic. These are the input fibers traveling to the nucleus accumbens. Histaminergic fibers connect the nucleus accumbens to the tuberomammillary nucleus. The tuberomammillary nucleus is the sole source of histamine in the brain.
The primary glutaminergic input fibers arrive from the basolateral aspect of the amygdala, the prefrontal cortex, the ventral hippocampus, and the nuclei of the thalamus. Major glutaminergic fibers also originate from the ventral tegmental area.
The ventral tegmental area relays dopaminergic neurons via the mesolimbic pathway. Due to this connection, the nucleus accumbens is recognized for its integral, functional role in the cortico-striato-thalamo-cortical loop.
The dopaminergic influence within the nucleus accumbens regulates neuronal activity. A clinically significant aspect of the dopaminergic neuronal terminals is their role in highly addictive drugs. Drugs like cocaine, nicotine, and amphetamine significantly increase dopamine levels in the nucleus accumbens. It's been observed that all recreational drugs contribute to higher dopamine quantities.
The Nucleus Accumbens' Neuronal Make-Up
The principal neurons which make up the nucleus accumbens are the medium spiny neurons. These specialized neurons of the nucleus accumbens produce neurotransmitters (chemical messengers) called gamma-aminobutyric (GABA). Environmental factors influence the development of these cells.
The medium spiny neurons present dopamine D1-type or D2-type receptors on the neuron's membrane. These are tasked with a variety of cognitive processes. The D1-type regulates reward-related cognitive processes. The D2-type neurons govern aversion-associated cognition.
The GABA-ergic neuron is an inhibitory neurotransmitter. It's one of the predominant inhibitory chemical messengers of the central nervous system. GABA cells generate calm, mediative moods and regulate neuron hyperactivity. Emotions that are otherwise induced by cell hyperactivity are anxiety, stress, and fear. (GABA cells also play an excitatory role)
The medium spiny neurons are also the principal cells of the nucleus accumbens- 95% of the cellular make-up comprises these GABAergic projections. In addition to the medium spiny neurons, there are cholinergic interneurons. These are characterized by their large size and absence of spines (aspiny).
The Nucleus Accumbens Neurochemistry
Various chemicals, hormones, and neuromodulators are transmitted throughout the nucleus accumbens. These neuromodulator molecules carry signals throughout the nucleus accumbens- which acts as a neuronal interface.
The most prevalent neurochemical is dopamine (mentioned above). Dopamine is discharged into the nucleus accumbens after exposure to rewarding environmental stimuli. External stimuli that impact dopamine levels include nicotine, cocaine, generic amphetamines, and other recreational drugs. To reiterate, the nucleus accumbens' role in drug addiction is significant.
The reward system is an immense network of intricate and sophisticated interlacing neurons tasked with associative learning, incentive salience, and positive valence emotions. The circuitry of the reward network is a highly relevant and integral feature of the nucleus accumbens.
Tyramine and phenethylamine are trace amines (dopaminergic neuromodulators). Essentially, these two compounds govern the discharge and re-uptake of dopamine within the nucleus accumbens. They're synthesized in neurons that exhibit a specific enzyme (AADC). They interact within the mesolimbic dopamine neuron's axon terminal.
Glucocorticoids are fascinating neurochemicals that play an interesting role in psychosis. Glucocorticoids belong to a category of corticosteroids, which are steroid hormones. Glucocorticoids are the only steroid hormones present in the outer shell of the nucleus accumbens. These, along with L-DOPA and steroids, are endogenous compounds that provoke psychosis.
Contemporary research and academia are harnessing their understanding of the role of hormonal administration and its impact on dopaminergic neurons. A more thorough understanding of glucocorticoid receptors could result in fresh insights and treatments for the psychotic mind.
Gamma-aminobutyric (GABA) are inhibitory neurotransmitters. (Mentioned above) The GABAA receptors overturn behavior otherwise affected by dopamine. In comparison, the GABAB receptors inhibit behavior governed by acetylcholine.
Glutamate enables the nucleus accumbens to relay information about spatial awareness in behavior. The prefrontal cortex and hippocampus transmit glutamate projections to the nucleus accumbens. These cortical regions are responsible for spatial memory.
Studies have illustrated that the effects of local blockade of glutamatergic NMDA receptors inhibit spatial learning. In addition, NMDA and AMPA glutamate receptors are essential in mediating instrumental behavior. Spatial memory consolidation requires synthesizing MNDA-type glutamate receptors for prolonged storage of data.
Serotonin (5-HT) is discharged into the nucleus accumbens and stimulates motivational behavior. However, the exact means by which serotonin regulates the activity of the nucleus accumbens needs further research.
Largely, serotonin (5-HT) synapses feature more extensively with greater amounts of synapses in the nucleus accumbens shell compared to the core. In the outer shell, the serotonin (5-HT) is more sizeable in length and thickness in the shell. In addition, the serotonin (5-TH) body vesicles are more robust and prominent than their shell counterparts.
The Nucleus Accumbens Location
The nucleus accumbens is in each cerebral hemisphere. It's situated in the basal forebrain, anterior to the hypothalamus' pre-optic region. Together with the olfactory tubercle, the nucleus accumbens structures the ventral striatum. The ventral and dorsal striatum form the striatum, a predominant feature of the basal ganglia.
The nucleus accumbens or nucleus accumbens septi (adjacent to the septum) is a set of neurons in the striatum. These nuclei (plural for nucleus) are a significant part of the septal area and the basal ganglia. The nucleus accumbens exists in the ventral part of the striatum and is the primary input nucleus of the basal ganglia.
Initially, there was debate amongst psychologists, neurologists, and scholars alike as to whether the nucleus accumbens’ operative capacity was integral to the septal system or the basal ganglia. After years of extensive research, it’s revealed that the nucleus accumbens is a specialized feature of the striatal complex. It’s closely aligned to the caudate-putamen.
The nucleus accumbens is approximate to the striatum's caudate and putamen. It continues dorsolaterally into the putamen and dorsomedially into the caudate nucleus. However, there isn't an apparent boundary between these regions.
The location of the nucleus accumbens in the ventral striatum and basal ganglia isn't anatomically apparent. The physiological characteristics of the nucleus accumbens aren't readily discernable from the ventral striatum and basal ganglia.
The ventral striatum is comparable to the nucleus accumbens concerning opiate receptor distribution, dopamine levels, neural connections, enzyme histochemistry, and acetylcholine quantities. Currently, scholars collectively agree that the nucleus accumbens is intrinsic to the ventral striatum- yet manifests as a specialized feature.
The Function of the Nucleus Accumbens
The nucleus accumbens serves the brain and body via a variety of connections. This collection of nuclei performs numerous roles: addiction, laughter, pleasure, satisfaction, behavior, reward, fear, and the placebo effect.
A main function of the nucleus accumbens is its assimilation of motivational states in directing voluntary movement. This is also true for the role of the nucleus accumbens in directing action motivated by social cues of reward and punishment.
Reward motivating behavior, integral to the nucleus accumbens, is exhibited in its midbrain dopamine projection. Its significant influence stimulates reward incentivized actions/behavior governed by appetite. However, the exact mechanism of appetite-induced-reward-incentivized behavior of the nucleus accumbens remains unknown.
In light of the nucleus accumbens' role in appetite-governed behavior, it functions not as a reward hub. Instead, it regulates action selection. It processes frontal and temporal cognitive and affective information. This neuronal network merging determines motivated behavior, either aversively or desire driven.
Action selection requires flexibility for adaptation and survival. In addition, it relies on the capacity for adaptation to adjust to internal states. The emphasis is on behavioral outcomes and their current motivational value- the nucleus accumbens functions as a reward predictor.
The Nucleus Accumbens as a Neuronal Interface
The nucleus accumbens is the key anatomical structure that serves as the limbic motor neuronal intersection. The nucleus accumbens receive direct and indirect neuronal inputs from the limbic forebrain structures via the ventral tegmental area. It then transmits signals through the globus pallidus to the motor system.
The ventral tegmental area projects directly to the nucleus accumbens. These two cranial organs are the principal areas that engage with addictive, psychoactive drugs. The ventral tegmental area comprises glutamate neurons, GABA, and dopamine. Projections to the nucleus accumbens reinforce drug-directed behavior.
The immense, complex connectivity of the nucleus accumbens suggests that it functions in spatial memory and object recognition. These are the more complex processes of learning, unlike procedural memory. This difference in complexity is because spatial memory and object recognition require flexible maintenance of sensory information.
Conclusion
The nucleus accumbens is a small yet vital anatomical cranial structure that facilitates numerous essential functions. Acting in the capacity of a neuronal interface, the intricate network of the nucleus accumbens with the motor and limbic system presents reward-seeking-voluntary behavior, motivational salience, and conditional learning. Divided into the outer shell and central core, the nucleus accumbens displays distinct yet complementary neuronal and chemical interactions. Medium-spiny neurons comprise 95% of the nucleus accumbens. Dopamine is a predominant chemical attributed to the nucleus accumbens. The role of addiction of the nucleus accumbens is significant.
