The putamen or outer lenticular nucleus is a structure in the brain that belongs to the corpus striatum or nucleus lentiformis. Its function is to process neural signals relevant to the control of motor processes. Damage to the putamen may accordingly be accompanied by disturbances in voluntary movements.
What is the putamen?
The putamen is a nuclear area of the brain that contains numerous nerve cell bodies and is part of the corpus striatum. Together with the caudate nucleus, it thus participates in the control of voluntary movements. Functionally, the putamen belongs to the basal ganglia: the motor, limbic and cognitive core areas of the brain. They are not part of the pyramidal system, which is also responsible for movement processes and whose pathways ascend or descend through the spinal cord. However, pyramidal nerve tracts run in the brain immediately adjacent to the putamen through the capsula interna; it also includes numerous other nerve fibers and forms the connection between the cortex and deeper areas such as the cerebral peduncles (crura cerebri). The putamen belongs not only to the corpus striatum but also to the nucleus lentiformis or lenticular nucleus, the other half of which forms the pallidum. This division is independent of the caudate nucleus – which forms the other part of the striatum but is not part of the lentiform nucleus.
Anatomy and structure
In the cerebrum, the putamen lies symmetrically in both halves (hemispheres). It is located adjacent to the capsula interna, a cup-shaped collection of many nerve fibers that pass through the brain and belong to different functional pathways. Outwardly, the putamen is adjacent to the pallidum, with which it forms together the nucleus lentiformis. The neurons within the putamen essentially belong to two distinct types: cholinergic interneurons and inhibitory projection neurons. In biology, interneurons are neurons that are the connecting link between two other neurons. Cholinergic interneurons make use of the neurotransmitter acetylcholine in signal transmission. Projection neurons are also known as principal neurons and have longer axons that allow them to connect brain structures that are not directly adjacent to each other. Because these projection neurons exert an inhibitory effect in the putamen, biology also refers to them as inhibitory projection neurons.
Function and tasks
As a core area, the putamen computes information from various neurons that are interconnected and that the human body ultimately needs to control movement. As usual, the computation follows the principle of spatial and temporal summation: within a nerve fiber, neuronal information travels as an electrical signal known as an action potential. The electrical insulation of the nerve fiber by a myelin layer allows the action potential to propagate more quickly. Brain areas with many nerve fibers and few cell bodies form the white matter of the brain, while gray matter is characterized by many cell bodies and few (myelinated) nerve fibers. When a nerve fiber abuts a cell body, a synapse there forms the junction between the nerve fiber of the preceding cell and the body (soma) of the second neuron. The action potential ends at a thickening of the nerve fiber called the terminal button. Within it are small bubbles (vesicles) filled with molecular messengers, which, in response to the electrical stimulus, pass from the vesicles into the space between the terminal knob and the nerve cell body. This interstitial space or synaptic cleft connects the two nerve cells. At the opposite end, the membrane of the downstream (postsynaptic) neuron contains receptors to which neurotransmitters can dock. Their stimulation leads to the opening of ion channels in the membrane and causes a change in the electrical charge of the cell. Excitatory neurotransmitters trigger an excitatory or excitatory postsynaptic potential (EPSP), while inhibitory synapses result in an inhibitory postsynaptic potential (IPSP). The cell accounts for EPSP and IPSP summatorily, also taking into account the strength of the respective signal. This signal strength depends first on the number of electrical action potentials in the presynaptic nerve fibers and then on the amount of biochemical neurotransmitters.Only when the sum of all EPSP and IPSP exceeds the critical threshold of charge change in the cell body, a new action potential is generated at the axon hillock of the postsynaptic neuron.
Diseases
Because of its involvement in motor control, disorders of the putamen may be reflected in the form of motor complaints. In many cases, the putamen is not affected in isolation; rather, the basal ganglia as a whole are often impaired in function under such circumstances. One example is Parkinson’s disease: this neurodegenerative disorder is based on the atrophy of the dopaminergic substantia nigra, resulting in a dopamine deficiency. Dopamine serves as a neurotransmitter; its deficiency causes synapses to fail to correctly transmit neuronal signals between nerve cells. Therefore, for Parkinson’s disease, motor symptoms include muscle rigidity (rigor), muscle tremor (tremor), slowed movements (bradykinesis) or inability to move (akinesis), and postural (postural) instability. Treatment may include the use of L-dopa, which is a precursor of dopamine and is thought to at least partially compensate for the neurotransmitter deficiency in the brain. In the context of Alzheimer’s dementia, the putamen can also suffer damage along with other areas of the brain. The most prominent symptom of the disease is memory loss, with short-term memory typically affected first and more severely than long-term memory. The causes of Alzheimer’s disease are still unknown; one of the leading theories involves deposits (plaques) that impair signal transmission and/or supply to neurons, eventually leading to their atrophy.
