Deep within the brain lies the striatum, a central hub for coordinating thought, emotion, and action. Within this structure are specialized clusters of neurons known as striosomes, which form a distinct and organized network. Scientists now understand that striosomes are involved in some of the brain’s most complex operations, including weighing the costs and benefits of our decisions, regulating mood, and shaping movements. Understanding their role provides a clearer picture of how the brain navigates the world and what happens when these systems are compromised by disease.
The Striatum’s Two Compartments
The striatum is not a uniform structure but is organized into two distinct, intermingled compartments: the striosomes and the much larger surrounding area known as the matrix. A useful way to visualize this arrangement is to think of striosomes as islands distributed throughout a sea of matrix tissue. This division represents two functionally separate, parallel-processing systems within the basal ganglia.
The primary differences between these compartments are their neurochemical makeup and connections to other brain regions. Striosomes have a high density of mu-opioid receptors, involved in pain relief, reward, and emotional responses. In contrast, the matrix is rich in the enzyme acetylcholinesterase, a component in the signaling pathway that uses acetylcholine to modulate motor control.
These chemical distinctions correspond with their unique wiring. Striosomes receive input from parts of the cerebral cortex associated with emotion and executive function, such as the prefrontal cortex. They then send this information to the dopamine-producing cells of the substantia nigra. The matrix primarily receives input from cortical areas that process sensory information and control voluntary movement. This separation allows information related to mood to be processed differently from information for motor execution.
Role in Decision-Making and Mood
Striosomes play a direct role in complex choices, particularly those involving a conflict between cost and benefit. A circuit connecting the prefrontal cortex to striosomes becomes highly active when an individual must weigh a large reward against a significant risk. This pathway integrates the emotional and cognitive aspects of a decision, allowing the brain to calculate whether a potential outcome is “worth it.”
This function is prominent in situations that provoke anxiety. When faced with a choice that has both a desirable and an aversive component, the striosomes help navigate the conflict. By processing inputs from emotion-related brain regions, this system guides behavior away from negative consequences or toward actions that, while difficult, promise a greater reward.
The influence of striosomes on decision-making under stress links them to mood regulation. Dysregulation in this system can alter how an individual perceives and reacts to negative situations, which has implications for mood disorders. Chronic stress, for example, can elevate the firing rates of striosomal neurons, a change associated with flawed cost-benefit calculations and avoidance behaviors.
This connection suggests the striosome pathway modulates mood states like anxiety and anhedonia, the inability to feel pleasure. When this circuit functions properly, it helps maintain a balance between seeking rewards and avoiding harm. When disrupted, it can bias the system toward avoidance and a negative perception of outcomes, contributing to the states seen in mood disorders.
Connection to Movement Control and Habit Formation
While the matrix compartment is more directly associated with executing movements, striosomes have a distinct modulatory function in controlling action. They do not send simple “go” or “no-go” signals. Instead, they influence the dopamine-producing neurons that adjust the strength of motor pathways originating in the matrix, allowing striosomes to shape motor output based on emotional and motivational context.
This system is involved in the selection and sequencing of actions. By integrating information about value and emotional state, the striosome pathway helps prioritize certain movements and suppress unwanted actions. This function is not about the mechanics of movement but about ensuring that chosen actions align with internal goals and potential consequences.
This modulatory role extends to habit formation, which are actions that become automatic through repetition. The striosome pathway is involved in the initial, goal-directed phase of learning a new behavior when cost-benefit analysis is most active. As a behavior becomes ingrained, its control may shift more toward matrix-driven circuits, but striosomes remain involved in updating these habits when circumstances change.
Implications in Neurological and Psychiatric Disorders
The specialized functions of striosomes mean their dysfunction is linked to the symptoms of several major brain disorders. In Huntington’s disease, a fatal genetic disorder, the degeneration of striosomal neurons is one of the earliest and most severe signs. This early damage is believed to cause the mood disorders, like depression and anxiety, that often appear years before motor symptoms.
The later decline in motor control in Huntington’s, including the uncontrolled movements known as chorea, is linked to the subsequent degeneration of the matrix. In Parkinson’s disease, the death of dopamine-producing neurons disproportionately affects striosome-related circuits. This disruption contributes to the difficulty in initiating voluntary movements, a classic symptom of the disease.
Imbalances in striosome activity are also implicated in psychiatric conditions. Given their role in cost-benefit decision-making, striosome dysfunction is an area of research for major depressive disorder and anxiety disorders. Overactivity in these circuits has been linked to the repetitive behaviors seen in obsessive-compulsive disorder (OCD) and Tourette’s syndrome, where action selection becomes stuck in a loop.