However, the brain is still susceptible to damage, disease, and infection. The brain is protected by the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier. The whole circuitry is driven by the process of neurotransmission.

Neurons connect to form neural pathways, neural circuits, and elaborate network systems. Brain activity is made possible by the interconnections of neurons and their release of neurotransmitters in response to nerve impulses. There are more than 86 billion neurons in the brain, and a more or less equal number of other cells. The cells of the brain include neurons and supportive glial cells. Underneath the cerebral cortex are several important structures, including the thalamus, the epithalamus, the pineal gland, the hypothalamus, the pituitary gland, and the subthalamus the limbic structures, including the amygdala and the hippocampus the claustrum, the various nuclei of the basal ganglia the basal forebrain structures, and the three circumventricular organs. Within the cerebrum is the ventricular system, consisting of four interconnected ventricles in which cerebrospinal fluid is produced and circulated. The cerebellum is connected to the brainstem by three pairs of nerve tracts called cerebellar peduncles. The brainstem consists of the midbrain, the pons, and the medulla oblongata. The cerebrum is connected by the brainstem to the spinal cord. The hemispheres are connected by commissural nerve tracts, the largest being the corpus callosum. Although the left and right hemispheres are broadly similar in shape and function, some functions are associated with one side, such as language in the left and visual-spatial ability in the right. Within each lobe, cortical areas are associated with specific functions, such as the sensory, motor and association regions. The frontal lobe is associated with executive functions including self-control, planning, reasoning, and abstract thought, while the occipital lobe is dedicated to vision. Each hemisphere is conventionally divided into four lobes – the frontal, temporal, parietal, and occipital lobes. The neocortex is made up of six neuronal layers, while the allocortex has three or four. The cortex has an outer layer, the neocortex, and an inner allocortex. Each hemisphere has an inner core composed of white matter, and an outer surface – the cerebral cortex – composed of grey matter. The cerebrum, the largest part of the human brain, consists of two cerebral hemispheres. The brain is contained in, and protected by, the skull bones of the head. It controls most of the activities of the body, processing, integrating, and coordinating the information it receives from the sense organs, and making decisions as to the instructions sent to the rest of the body. The brain consists of the cerebrum, the brainstem and the cerebellum. The human brain is the central organ of the human nervous system, and with the spinal cord makes up the central nervous system.

Skull sketch