Putamen

The putamen is a round structure located at the base of the forebrain (part of the diencephalon) and is one of the principal components of the basal ganglia, along with the caudate nucleus, globus pallidus, substantia nigra, and subthalamic nucleus^[1].

The putamen lies within each hemisphere of the brain and forms the outermost part of the basal ganglia.

The putamen contains medium spiny neurons (MSNs), which are the primary type of cells in this area. These neurons have both D1 and D2 receptors for the neurotransmitter dopamine ^[4].

These two types of receptors are differentially distributed among the direct and indirect pathways of the basal ganglia.

Direct pathway: The direct pathway involves MSNs expressing D1 receptors. The activation of these receptors leads to an increase in cyclic adenosine monophosphate (cAMP) inside the cell. This increase, in turn, enhances the excitability of these neurons, facilitating the initiation and execution of voluntary movements^[5].

Indirect pathway: The indirect pathway, in contrast, includes MSNs expressing D2 receptors. The activation of D2 receptors has the opposite effect: it decreases the levels of cAMP, reducing the excitability of these neurons, and consequently, suppressing undesired or competing movements^[5].

The putamen is also enriched with various other neurotransmitters, including gamma-aminobutyric acid (GABA), acetylcholine, and glutamate^[6]. These neurotransmitters contribute to the various functions and pathways that the putamen is involved in.

Functionally, the putamen plays a role in a variety of complex processes, such as movement regulation, reinforcement of stimulus-response (habit) learning, and the creation and modulation of motor and action plans^[6].

Habit and reward-based learning

Dopaminergic neurons from the substantia nigra pars compacta project into the putamen, carrying information about prediction errors (differences between expected and actual outcomes), which is critical for reinforcement learning. Studies have shown that the putamen is also involved in the processing of various other cognitive tasks, including implicit learning, stimulus-response learning, and procedural memory. For instance, during procedural or "habit" learning, the putamen is believed to contribute to the gradual shift from goal-directed actions to habitual behaviour in response to consistent stimulus-reward associations^[8].

Motor function

Via the cortical-basal ganglia-thalamocortical loop, the putamen receives inputs from motor and somatosensory cortices and sends outputs to the motor cortex via the globus pallidus and the thalamus, influencing motor planning and execution^[9].

Speech and Language

Research has shown involvement of the putamen in various aspects of speech and language, including motor aspects of speech production and the learning of language sequences^[10].

Association with other structures

The putamen, along with the caudate nucleus, forms the dorsal striatum, which is primarily involved in motor and cognitive functions. Together with the nucleus accumbens, they form the ventral striatum, which is involved in reward and motivation. Together with the globus pallidus internus (GPi) and the globus pallidus externus (GPe), the putamen forms the lentiform nucleus^[11].

Damage or dysfunction of the putamen has been associated with a variety of neurological conditions, such as Parkinson's disease, Huntington's disease, and various types of dystonia. It's also been implicated in psychiatric disorders, such as obsessive-compulsive disorder, addiction, and schizophrenia, suggesting its broad influence on human brain function and behaviour.