The brain is the center of intelligence, thought, and instinct in most higher creatures, including humans. In human beings, the basic brain structures are the cerebrum, the cerebellum, and the brainstem. Each of these is composed of smaller organs and structures that maintain different mental and physical functions. All brain structures are composed of nerve cells, or neurons, that interact with great efficiency, processing sensory input and undertaking appropriate responses in fractions of a second.
The brain is an extension of the central nervous system, a network of neurons existing throughout the human body. These neurons relay sensory data and muscle commands to and from the brain via the spinal cord, located within the protective spine, or backbone. At the top of the spinal cord, the brainstem forms the base of the brain itself. Like many brain structures, the brainstem regulates specific bodily functions, in this case involuntary actions such as breathing, blood circulation, and sleep. The brainstem includes other brain structures, including the pons and the medulla oblongata.
Just behind the brainstem is the cerebellum, which coordinates movement and orientation by processing information from the ears and other sensory organs. Just above the cerebellum and the brainstem are the basal ganglia, also known as the limbic system. These brain structures, including the caudate nucleus, putamen, and globus pallidus, also regulate motor functions. The hippocampus and the amygdala, involved in basic emotional response and memory, are also part of the limbic system.
The central structure of the brain is the cerebrum; its outermost layer is the cerebral cortex, also called the neocortex. Like the other brain structures, the cerebral cortex is composed of neurons. In preserved brains, neurons are gray, so the cerebrum is often referred to as gray matter. In the cerebrum, millions of neurons communicate with each other through connections called synapses, maintaining higher brain functions such as speech, memory, and complex problem-solving.
The cerebral cortex is divided into right and left hemispheres; each controls different mental and physical processes. Beneath this outer layer, the so-called white matter is composed of axons, the communicating bodies of the cerebrum’s neurons. This white matter contains brain structures such as the thalamus and hypothalamus, which deal with simpler functions such as pain, appetite, and hormonal response. Within the white and gray areas of the cerebral cortex, the various brain structures process information, make decisions, and initiate actions. The brain can coordinate millions of such interactions in any given second.