3. Projection fibres. These long-range fibres connect very distant brain areas, such as the cortex and the thalamus (thalamic radiation), the cortex and the brainstem and the spinal cord. Projection fibres are categorized into two types: (1) motor tracts, also called descending tracts, which transmit information from the brain to the rest of the body, and (2) sensory tracts, also called ascending tracts, which send the signals picked up by sensory receptors located throughout the body, to the brain. The projection fibres pass through a wide band called the internal capsule (Figure 3), which is located under the corpus callosum. It is important to understand that neurons are unidirectional, that is to say, they can only be involved in one type of tract (up or down).

The gray matter is composed of a greater diversity of neurons parts, but also of other cell types. It includes cell bodies of neurons and their dendrites, unmyelinated axons, nerve endings and glial cells. Glial cells, also called neuroglia, are a collection of cell types that have various functions (mainly to support and protect the neurons) but do not transmit nerve impulses. The glial cells include protoplasmic astrocytes which have a star shape with short extensions, and which would play, among other things, a role in modulating synaptic transmission. They also include microglia and Schwann cells.

The gray colour of the gray matter is due to two factors, the low presence of myelin and the presence of Nissl bodies, structures located mainly in the cell body of neurons, and which have a gray colour.

In the brain and cerebellum, the gray matter is located mainly on the surface and forms the cerebral cortex and the cerebellar cortex respectively (Figure 1B). Gray matter is also found deep in the centre of the brain and form a collection of nuclei. These deep regions of gray matter are called basal ganglia. (Figure 3). The basal ganglia are composed of six nuclei, three in each of the cerebral hemispheres. In each hemisphere, two of the three nuclei are located next to the thalamus, namely the globus pallidus and the putamen. These two nuclei are often grouped together as the lenticular nucleus. The third nucleus is the caudate nucleus. In the shape of a long comma, it “surrounds” the lenticular nucleus (Figure 3). The basal ganglia receive nerve impulses from the cortex and transmit information to the entire cortex through circuits called cortical-subcortical loops. The basal ganglia have many functions. They contribute to the preparation and initiation of voluntary movements (e.g., talking or waving hello), the control of involuntary actions such as swinging the arms when walking or laughing, motor learning, and to the regulation of posture (muscle tone, posture, rigidity). Another important function of the basal ganglia is the regulation of reward and disgust. Finally, but not the least, the basal ganglia are also involved with different cognitive functions, such as attention, selection and inhibition of actions, and consciousness.