The brain can be divided into two regions. The brainstem is the part of the brain extending downward toward the spinal cord. The cerebral hemispheres contain the wrinkled outer layer of the brain, called the cerebral cortex, and a few deeper structures including the basal ganglia, the hippocampus, and the amygdaloid nuclei.
A cerebral hemisphere is mainly made up of furrowed gray matter with an inner layer of white matter. The gray matter of the brain contains the neuron cell bodies. The white matter, on the other hand, is white because it contains the myelinated axons that extend from a neuron cell body. The crests of the gray matter wrinkles are called gyri, and the grooves are called sulci or fissures. The central sulcus is one of the most prominent sulci, which separates the precentral and postcentral gyri. The precentral gyrus is involved with motor functions, while the postcentral gyrus controls sensory input.
Each cerebral hemisphere can be divided into four lobes, each with distinct functions. The frontal lobe, which covers the area behind the forehead and the top of the head, is involved with planning and movement. The parietal lobe, located just behind the frontal lobe, manages sensory information. The occipital lobe at the base of the skull is involved with vision, and the temporal lobe, which lies on the side of the brain behind the ears, manages hearing.
The cerebral hemispheres control the contralateral sides of the body, meaning the right cerebral hemisphere controls the left side of the body and vice versa. Sensory information from the right hand, for example, crosses over to the left cerebral hemisphere, where it is processed. To move the right hand, the signal must come from the left side of the brain and cross over again.
A large bundle of nerves called the corpus callosum links the left and right cerebral hemispheres. The two hemispheres are not always exactly symmetrical, and they may be somewhat specialized. For example, Wernicke’s area and Broca’s area, the two parts of the brain involved in speech, are both located on the left cerebral hemisphere.
The deeper parts of the cerebral hemispheres are also critical to brain functioning. The basal ganglia, a collection of nuclei closely connected to the cortex, regulate motor movement. The hippocampus is involved with memory storage and retrieval. The amygdaloid nuclei controls emotional responses, especially to emotions like anger and fear.