The striatum is the largest nucleus, i.e. cluster of neurons in the basal nuclei that plays a vital role in the reward and motivation system of the brain. In this way, this subcortical structure also participates in the processes of learning, mood, and addiction.
What Is Basal Nuclei of the Brain?
The basal nuclei (or: basal ganglia) refers to a collection of structures in the cortex of the brain. These brain structures are one of the most fundamental processing units of the brain, responsible for a broad spectrum of functions, including the control of voluntary motor movements, procedural and habit learning, movements of the eyes, cognition, as well as emotion.
The basal nuclei of the brain consist of:
- striatum
- globus pallidus
- ventral pallidum
- substantia nigra pars reticulata
- subthalamic nucleus
- amygdaloid nuclear complex
Globus Pallidus
This structure of the brain is one of the main output nuclei of the basal ganglia. The neurons comprising the globus pallidus form a pathway that can influence neurons in the motor cortex. In this way, they inform the parts of the brain responsible for initiating movements.
Ventral Pallidum
Being one of the major outputs for the transmission of limbic signals to the nuclei of the thalamus, the ventral pallidum is also one of the main reward mechanisms in the brain, as well as a center for the regulation and control of motor functions.
Substantia Nigra Pars Reticulata
The substantia nigra pars reticulata is the largest structure of the basal ganglia. Composed of the pars compacta (SNpc) and the pars reticulata (SNpr), this brain structure plays an important role in the reward and movement systems of the brain.
Subthalamic Nucleus
This cluster of neurons labeled as a subthalamic nucleus produces the neurotransmitter glutamate and transmits the sensory information to the globus pallidus.
Amygdaloid Nuclear Complex
The amygdaloid nuclear complex is a center in the brain for the control and regulation of the signals and information received from our senses and our internal organs. It comprises a group of more than 10 nuclei that are located in the mid temporal lobe.
Location and Structure of the Striatum
The striatum is the largest brain structure located within the basal ganglia, i.e. deep in the hemispheres of the brain. It comprises three nuclei: caudate, putamen, and ventral striatum, all three of which comprise the structure labeled as the corpus striatum.
This brain structure has its own micro-circuit of neurons that both receive and send fibers among each other from all parts of the striatum. It consists of two parts:
- Dorsal striatum
- Ventral striatum
Dorsal Striatum
On the basis of its functions and connections, the dorsal striatum is divided into the following clusters of neurons:
- Caudate nuclei – These striatum nuclei are a large, C-shaped gray matter cluster of neurons that directly contributes to decision-making, action selection, and initiation.
The caudate nucleus plays a vital role in how the brain learns, especially when it comes to storing and processing memories. By using information from past experiences, it also regulates and controls future actions and decisions. This is important for the development and use of language.
In addition, having both motor and behavioral functions, the caudate nucleus supports the movements of the arms, legs, and the entire body.
- Caudate putamen – This large, round nucleus located at the base of the forebrain is one of the main input areas of the basal ganglia. This structure of the striatum supports the planning of movements and prepares the parts of the body for movement.
The caudate-putamen nuclei are interconnected with the cerebral cortex, the thalamus, and the brainstem.
Ventral Striatum
The ventral striatum plays a big role in decision making and reward-related behavior. It receives fibers from the amygdala and hypothalamus.
This striatum nucleus is composed of the following clusters of neurons in the cerebrum:
- Nucleus accumbens
- Olfactory tubercle
Nucleus Accumbens
This cluster of neurons labeled as nucleus accumbens is involved in the cognitive processing of motor function related to the analysis and processing of rewards and reinforcing stimuli. This striatum structure is composed of two sub-structures:
- Nucleus accumbens shell – This is the outer area of the nucleus accumbens, involved in the cognitive processing of the reward cycles in the brain.
- Nucleus accumbens core – This is the inner area of the nucleus accumbens, involved in the motor processing of the voluntary movements of the body.
The nucleus accumbens connects with the mesolimbic pathway from the ventral tegmental area. It is involved in the reward processes of the brain and voluntary motor movement. Hence, this structure is activated by pleasurable stimuli, motivational salience, and positive reinforcement.
Located in the preoptic area of the forebrain, the nucleus accumbens cluster is also responsible for the regulation of slow-wave sleep.
Olfactory Tubercle
The olfactory tubercle refers to a layer of gray matter located in the frontal lobe of the brain that transmits the sensory information from the olfactory receptors (i.e. receptors of smell) to the thalamus, ventral pallidum, and nucleus accumbens in the central nervous system of the brain.
This makes it a multi-sensory processing center of the brain within the ventral striatum and the cerebral cortex. It is especially active in the locomotor processes, in the cognitive and behavioral processes of attention, as well as in sensory responsiveness.
The olfactory tubercle receives direct outputs from the olfactory bulb, an essential structure of the olfactory system that relays the smell stimuli from the nose to the dedicated areas of the brain. This is the area of the brain where they are further processed and translated into odours.
Function of the Striatum
The corpus striatum plays the most significant role in the reward and reinforcement circuit of the brain, as well as in the process of sleep. The nucleus accumbens and the olfactory tubercle of the ventral striatum are especially active during these processes.
On the other hand, the dorsal striatum has a special function in the cognitive control of the motor functions by regulating and controlling voluntary movements. Upon receiving the sensory stimuli from various parts of the body, the dorsal striatum processes the sensory information in order to translate them into a movement.
What Is the Reward System of the Brain?
Almost all structures of the brain are activated by the rewards (i.e. reward stimuli) in social situations. Together, these brain areas and the dedicated neural pathways comprise the reward system of the brain.
When the brain is exposed to the reward stimuli (such as sugar, addictive drugs, or anything else that the brain structures translate as pleasurable or a reward), it releases the neurotransmitter dopamine that relays the information about the pleasurable stimuli to all these brain structures.
Here, the mesolimbic dopamine pathway participates the most, by connecting the ventral tegmental area of the brain with the nucleus accumbens in the dorsal striatum – an area of the brain dedicated to the processing of the reward and motivation stimuli.
What Happens When the Striatum Is Dysfunctional?
Damage or a lesion to the striatum can cause various problems and loss of abilities of the affected individual, such as difficulty in controlling speech, maintaining body posture, starting, stopping, or sustaining movements, etc.
The list of disorders and other severe medical conditions that can occur due to a dysfunctional striatum includes the following:
Parkinson’s disease
A neurological disorder that affects the part of the brain that controls the movement of our body. Due to a loss of dopaminergic innervation to the dorsal striatum, it results in tremors, rigid muscles, difficulty making quick and smooth movements, as well as difficulty in standing and walking.
Addiction
The striatum is the key brain structure involved in all addictions since it’s the most vital participant in the reward and motivation processes in the brain. The addiction is a severe psychiatric disorder characterized by a compulsion to abuse certain substances or behaviors in order to control an insatiable desire for the signaled reward.
Depression
A mood disorder that affects the way an individual feels, perceives events and surroundings, and affects their manner of thinking and behaving. This can lead to behavior with negative consequences.
Attention-deficit disorder (ADD)
A neurological disorder characterized by a wide range of behavior problems, such as difficulty in following instructions, focusing on performing a task, inability to complete a task, being disorganized and forgetful, difficulty in staying on topic while talking, etc.
Obsessive-compulsive disorder (OCD)
the basal ganglia are part of the brain commonly associated with OCD – a mental disorder in which the individuals conduct unwanted behaviors in order to get rid of their obsessive ideas, thoughts, feelings, or sensations.
Huntington’s disease
A fatal genetic disorder that worsens an individual’s physical and mental abilities. It’s characterized by a loss of memory, a feeling of hopelessness, lack of interest in things and events, and problems with memory and movement. By gradually breaking down the nerve cells of the brain, it leads to the loss of language, thinking, and reasoning abilities of the affected individual.
Dyskinesias
A medical condition characterized by difficulty in initiating and performing new movements. Any movement the affected individual tries to perform is slow and with the absence of swinging while the patient is trying to move the arms.
Chorea
A neurological disorder triggered by a stroke or tumor in the striatum area of the brain. This disorder is characterized by sudden involuntary movements of the shoulders, hips, and the face of the affected individual.
Fun Facts
Did you know?
- The striatum is also known under the names corpus striatum, neostriatum, as well as the striate nucleus.
- The caudate nucleus of the basal ganglia functions as a feedback processor. Namely, it uses our memories from previous experiences in order to influence our future actions and reactions.
- If the caudate nucleus is not sending enough stimulation and information to the centers of the brain associated with motivation, we can lose our motivation to be productive.
- Social contacts have long-lasting effects on human behavior due to the functions performed in the striatum, as a part of the reward system in the brain.
- The putamen regulates our automated learning, which we use in activities such as riding a bike, driving a car, as well as all other repetitive actions which do not require upper-level thinking.
- There are two nuclei of the caudate nucleus – one on each side of the brain.
- The Latin name of the olfactory tubercle is tuberculum olfactorium.
- The ventral striatum reaches adulthood faster than the other structures of the brain, which influences the manner of thinking and reasoning in teenagers.
- Pleasure is only a part of the reward, but not all rewards bring pleasure. These stimuli are called extrinsic rewards.
- The stimuli that are naturally pleasurable are labeled as intrinsic rewards.