Caudate Nucleus

The caudate nucleus is a key component of the basal ganglia, a collection of nuclei in the brain involved in motor control, learning, reward, and many other functions. The caudate nucleus, along with the putamen, constitutes the striatum, the main input station of the basal ganglia^[1].

The caudate nucleus is shaped like a C, its head bulging into the anterior horn of the lateral ventricle and its tail curving along the lateral wall of the lateral ventricle towards the temporal lobe. The caudate is divided anatomically into the head, body, and tail^[2].

The caudate nucleus receives glutamatergic inputs primarily from all areas of the cerebral cortex, and to a lesser extent from the thalamus. It also receives dopaminergic input from the substantia nigra pars compacta in the midbrain^[5].

The output from the caudate nucleus (and the rest of the striatum) goes to the globus pallidus and substantia nigra pars reticulata, via inhibitory GABAergic medium spiny neurons^[6].

Functionally, the caudate nucleus plays a role in several critical functions:

• Motor Control: The basal ganglia, including the caudate nucleus, play a role in the selection of movements at both the voluntary and subconscious levels. It assists in selecting appropriate motor responses among many potential ones, based on the contextual information received from the cerebral cortex^[7]. The caudate nucleus is also involved in procedural learning, which is learning how to do things through repeated practice^[8]. Both the 'direct pathway' (the 'GO' pathway) and the 'indirect pathway' (the 'NO-GO' pathway) involve the caudate nucleus^[9].

• Learning and Memory: The caudate nucleus plays a crucial role in goal-directed actions and habit formation^[10]. It helps process spatial memory and works in conjunction with other parts of the brain to facilitate associative learning^[11]. The caudate nucleus, together with other parts of the striatum, plays an important role in reinforcement learning^[12].

• Reward Prediction: One of the primary roles of the caudate nucleus in reward prediction involves coding prediction errors. The neurotransmitter dopamine, which is largely involved in reward and pleasure systems, signals these prediction errors. Dopamine neurons in the ventral tegmental area and substantia nigra send a burst of activity to the caudate nucleus when an unexpected reward occurs, signaling a positive prediction error. Conversely, if an expected reward does not occur, these neurons decrease their activity, signalling a negative prediction error^[13].

• Oculomotor control: The caudate nucleus also helps manage saccadic eye movements, which are rapid, jerky movements of the eyes from one fixation point to another.^[14] This is crucial for tasks such as reading or visually scanning the environment. In addition to helping direct where and when to move the eyes, the caudate nucleus and other parts of the basal ganglia may contribute to the decision-making processes behind these movements. For instance, when deciding to make a saccade towards a particular visual stimulus, the brain must weigh the potential reward or importance of the new stimulus against the cost of moving the eyes away from the current focus.It plays a role in the decision-making processes behind these movements, such as when to move the eyes away from the current focus^[15].

Disruption to the caudate nucleus and its connections can contribute to various neurological and psychiatric disorders, including Parkinson's disease, Huntington's disease, obsessive-compulsive disorder, dysarthria, and schizophrenia^[16].