An understanding of the central nervous system — the CNS — and how it functions requires knowing its component parts and their specialized operations, and the contribution of each of the parts to the function of the whole. The ﬁrst section of this atlas introduces the student to the CNS from an anatomical and functional viewpoint. The subsequent section(Section B) will use these components to build the various systems, such as the sensory and motor systems. The blood supply and the detailed anatomical organization are found in Section C. Emotional behavior is discussed in Section D. cessing of information. These neurons are called interneurons, and more complex information processing, such as occurs in the human brain, is correlated with thedramatic increase in the number of interneurons in our brains. Communication between neurons occurs almost exclusively at specialized junctions called synapses, using biological molecules called neurotransmitters. These modify ion movements across the neuronal membranes of the synapse and alter neurotransmission — they can be excitatory or inhibitory in their action, or modulate synaptic excitability.The post-synaptic neuron will modify its ﬁring pattern depending on the summative effect of all the synapses acting upon it at any moment in time. The action of neurotransmitters depends also on the speciﬁc receptor type; there is an ever increasing number of receptor subtypes allowing for even more complexity of information processing within the CNS. Drugs are being designed to act on thosereceptors for therapeutic purposes. Much of the substance of the brain consists of axons, also called ﬁbers, which connect one part of the brain with other areas. These ﬁbers function so that the various parts of the brain communicate with each other, some going a short distance linking neurons locally and others traveling a long distance connecting different areas of the brain and spinal cord. Many ofthe axons are myelinated, an “insulation,” which serves to increase the speed of axonal conduction; the thicker the myelin sheath, the faster the conduction. Axons originating from one area (cortex or nucleus) and destined for another area usually group together and form a tract, also called a pathway (or fasciculus). The other major cells of the CNS are glia; there are more glia than neurons.There are two types of glial cells: • • Astrocytes, which are involved in supportive structural and metabolic events Oligodendrocytes, which are responsible for the formation and maintenance of the myelin that ensheaths the axons
The major cell of the CNS is the neuron. Human brains have billions of neurons. A neuron has a cell body (also called soma, or perikaryon);dendrites, which extend a short distance from the soma; and an axon, which connects one neuron with others. Neuronal membranes are specialized for electro-chemical events, which allow these cells to receive and transmit messages to other neurons. The dendrites and cell bodies of the neurons receive information, and the axons transmit the ﬁring pattern of the cell to the next neuron. Generally, eachneuron receives synaptic input from hundreds or perhaps thousands of neurons, and its axon distributes this information via collaterals (branches) to hundreds of neurons. Within the CNS, neurons that share a common function are usually grouped together; such groupings are called nuclei (singular nucleus, which is somewhat confusing as it does not refer to the part of a cell). In other parts of thebrain, the neurons are grouped at the surface, forming a cortex. In a cortical organization, neurons are arranged in layers and the neurons in each layer are functionally alike and different from those in other layers. Older cortical areas have three layers (e.g., the cerebellum); more recently evolved cortices have six layers (the cerebral cortex) and sometimes sublayers. Some neurons in the...