The
nucleus accumbens (NAcc), also known as the
accumbens nucleus or as the
nucleus accumbens septi (Latin for
nucleusIn neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex. In anatomical sections, a nucleus shows up...
leaning against the septumIn anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.- In human anatomy :* Interatrial septum, the wall of tissue that separates the left and right atria of the heart...
), is a collection of
neuronA neuron is an excitable cell in the nervous system that processes and transmits information by electrochemical signaling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves...
s within the
striatumThe striatum also known as striate body or striate nucleus is a subcortical part of the telencephalon/cerebrum. It is the major input station of the basal ganglia system...
. It is thought to play an important role in
rewardA reward may refer to:*Bounty , reward, often money, offered as an incentive*Reward website, website that offers rewards for performing tasks-Science:*Reward system, collection of brain structures which induce pleasurable effects...
,
laughterLaughter is an audible expression or appearance of happiness, or an inward feeling of joy . It may ensue from jokes, tickling, and other stimuli. Inhaling nitrous oxide can also induce laughter; other drugs, such as cannabis, can also induce episodes of strong laughter...
,
pleasurePleasure describes the broad class of mental states that humans and other animals experience as positive, enjoyable, or otherwise to be sought out. It thus includes more specific mental states such as happiness, entertainment, enjoyment, ecstasy, and euphoria...
,
addictionThe term "addiction" is used in many contexts to describe an obsession, compulsion, or excessive psychological dependence, such as: drug addiction The term "addiction" is used in many contexts to describe an obsession, compulsion, or excessive psychological dependence, such as: drug addiction The...
,
fearFear is an emotional response to a threat. It is a basic survival mechanism occurring in response to a specific stimulus, such as pain or the threat of danger. Some psychologists such as John B. Watson, Robert Plutchik, and Paul Ekman have suggested that fear is one of a small set of basic or...
, and the
placebo effectPlacebo effect may refer to:* Placebo effect, the tendency of any medication or treatment, even an inert or ineffective one, to exhibit results simply because the recipient believes that it will work...
.
Each half of the brain has one nucleus accumbens. It is located where the head of the
caudateThe caudate nucleus is a nucleus located within the basal ganglia of the brains of many animal species. The caudate nucleus is an important part of the brain's learning and memory system.-Anatomy:...
and the anterior portion of the
putamenThe putamen is a round structure located at the base of the forebrain . The putamen and caudate nucleus together form the dorsal striatum. It is also one of the structures that comprises the basal ganglia. Through various pathways, the putamen is connected to the substantia nigra and globus pallidus...
meet just lateral to the
septum pellucidumThe septum pellucidum is a thin, triangular, vertical membrane separating the lateral ventricles of the brain. It separates the anterior horn of the left and right lateral ventricles. It runs as a sheet from the corpus callosum down to the fornix. When the hemispheres are cut apart, the septum...
.
The nucleus accumbens and the olfactory tubercle collectively form the ventral striatum, which is part of the
basal gangliaThe basal ganglia are a group of nuclei in the brains of vertebrates, situated at the base of the forebrain and strongly connected with the cerebral cortex, thalamus and other areas. The basal ganglia are associated with a variety of functions, including motor control and learning...
.
The nucleus accumbens can be divided into two structures -- the
nucleus accumbens core and the
nucleus accumbens shell. These structures have different morphology and function.
Cell types
The principal neuronal cell type found in the nucleus accumbens is the
medium spiny neuronThe medium spiny neurons are a special type of inhibitory cells representing approximately 90% of the neurons within the corpus striatum of the basal ganglia...
. The
neurotransmitterNeurotransmitters are endogenous chemicals which relay, amplify, and modulate signals between a neuron and another cell. Neurotransmitters are packaged into synaptic vesicles that cluster beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they...
produced by these neurons is
gamma-aminobutyric acidγ-Aminobutyric acid is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays an important role in regulating neuronal excitability throughout the nervous system. In humans, GABA is also directly responsible for the regulation of muscle tone...
(GABA), one of the main inhibitory neurotransmitters of the central nervous system. These neurons are also the main projection or output neurons of the nucleus accumbens.
While 95% of the neurons in the nucleus accumbens are medium spiny GABA-ergic projection neurons, other neuronal types are also found such as large aspiny
cholinergicA receptor is cholinergic if it uses acetylcholine as its neurotransmitter.Cholinergic means related to the neurotransmitter acetylcholine, and is typically used in a neurological perspective. The parasympathetic nervous system is entirely cholinergic...
interneuronAn interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways...
s.
Output and input
The output neurons of the nucleus accumbens send axon projections to the ventral analog of the
globus pallidusThe globus pallidus is a sub-cortical structure of the brain. It is a major element of the basal ganglia system. In this system, it is a major constituent of the basal ganglia core, which consists of the striatum and its direct targets: globus pallidus and substantia nigra...
, known as the ventral pallidum (VP). The VP, in turn, projects to the
medial dorsal nucleusThe medial dorsal nucleus is a large nucleus in the thalamus.It receives inputs from the Pre-Frontal Cortex and the Limbic System and in turn relays them to the Pre-Frontal Association Cortex...
of the dorsal
thalamusThe thalamus is a midline paired symmetrical structure within the brain of vertebrate animals, including humans. It is between the cerebral cortex and the midbrain, both in terms of its location and its neurological connections...
, which projects to the
prefrontal cortexThe prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...
as well as the
striatumThe striatum also known as striate body or striate nucleus is a subcortical part of the telencephalon/cerebrum. It is the major input station of the basal ganglia system...
. Other efferents from the nucleus accumbens include connections with the
substantia nigraThe substantia nigra is a brain structure located in the mesencephalon that plays an important role in reward, addiction, and movement. Substantia nigra is Latin for "black substance", as parts of the substantia nigra appear darker than neighboring areas due to high levels of melanin in...
and the pontine reticular formation. Major inputs to the nucleus accumbens include prefrontal association cortices, basolateral
amygdalaThe ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans...
, and dopaminergic neurons located in the ventral tegmental area (VTA), which connect via the
mesolimbic pathwayThe mesolimbic pathway is one of the dopaminergic pathways in the brain. The pathway begins in the ventral tegmental area of the midbrain and connects to the limbic system via the nucleus accumbens, the amygdala, and the hippocampus as well as to the medial prefrontal cortex...
. Thus the nucleus accumbens is often described as one part of a cortico-striato-thalamo-cortical loop.
Dopaminergic input from the VTA is thought to modulate the activity of neurons within the nucleus accumbens. These terminals are also the site of action of highly-addictive
drugsRecreational drug use is the use of psychoactive drugs for recreational purposes rather than for work, approved medical or spiritual purposes, although the distinction is not always clear ....
such as
cocaineCocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system and an appetite suppressant...
and
amphetamineAmphetamine is a psychostimulant drug that is known to produce increased wakefulness and focus in association with decreased fatigue and appetite. Amphetamine is related to drugs such as methamphetamine and dextroamphetamine, which are a group of potent drugs that act by increasing levels of...
, which cause a manifold increase in
dopamineDopamine is a neurotransmitter occurring in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this phenethylamine functions as a neurotransmitter, activating the five types of dopamine receptors — D
1, D
2, D
3, D
4, and...
levels in the nucleus accumbens. In addition to cocaine and amphetamine, almost every recreational drug has been shown to increase dopamine levels in the nucleus accumbens.
Research
In the 1950s,
James OldsJames Olds was an American psychologist who co-discovered the pleasure center of the brain with Peter Milner while he was a postdoctoral fellow at McGill University in 1954...
and Peter Milner implanted electrodes into the septal area of the rat and found that the rat chose to press a lever which stimulated it. It continued to prefer this even over stopping to eat or drink. This suggests that the area is the 'pleasure center' of the brain. The septal nuclei are not directly connected to the nucleus accumbens, however.
Although the nucleus accumbens has traditionally been studied for its role in addiction, it plays an equal role in processing many rewards such as food and sex. A recent study found that it is involved in the regulation of emotions induced by
musicMusic is an art form whose medium is sound. Common elements of music are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...
, perhaps consequent to its role in mediating dopamine release. The Nucleus Accumbens plays a role in rhythmic timing and is considered to be of central importance to the limbic-motor interface (Mogensen).
In April 2007, two research teams reported on having inserted electrodes into the nucleus accumbens in order to use
deep brain stimulationDeep brain stimulation is a surgical treatment involving the implantation of a medical device called a brain pacemaker, which sends electrical impulses to specific parts of the brain...
to treat severe
depressionMajor depressive disorder is a mental disorder characterized by an all-encompassing low mood accompanied by low self-esteem, and loss of interest or pleasure in normally enjoyable activities...
.
Additionally, in July 2007, researcher Jon-Kar Zubieta published findings that the nucleus accumbens is central to the machinery of the
placebo effectPlacebo effect may refer to:* Placebo effect, the tendency of any medication or treatment, even an inert or ineffective one, to exhibit results simply because the recipient believes that it will work...
. His group has confirmed that specific neural circuits and neurotransmitter systems respond to the expectation of benefit during placebo administration and that these expectations induce measurable physiological changes.
External links