Under the menninges is the brain. The outside of the brain is known as the cerebral cortex, and this is where most of the nerve cells of the brain reside. The cerebrum (frontal, parietal, temporal and occipital lobes) constitutes the largest mass of the brain where most perception, memory, and motor functions are performed.
At the base of the cerebrum are a number of phylogenetically "older" portions of the brain that are involved in the primitive functions of memory, emotion, and autonomic nervous activity. These structures include the hippocampus, amygdala, hypothalamus, and other elements of the so-called "limbic" system. Deep gray matter nuclei of the cerebrum includes the thalamus and basal ganglia, which have a less well defined function than the cerebral cortex, but receive inputs from all elements of the nervous system and provide broad neural control and modulation as well coordinative functions. The cerebellum, which occupies the posterior cranial fossa, controls coordination and other modulative functions. It receives input from the cerebrum and other higher centers as well as to and from the brainstem. The brainstem is comprised of the midbrain, pons, and medulla oblongata; it controls consciousness, respiration, various autonomic functions, cranial nerve function including eye movements, equilibrium, and coordination. The brainstem also houses the main motor and sensory pathways going to the body. The medulla connects to the cervical spinal cord, and thence to the rest of the cord and peripheral nervous system.
The brain can be injured either directly, through the application of physical forces focally, or globally. In focal injuries physical forces may be transmitted into the brain by an object such as a bullet or blade that punctures the brain. The skull may be sufficiently deformed so that the brain is bruised directly as in a blow, or by major deformation of the malleable skull, as in a young infant, so that the brain may separate from itself locally causing a "contusional" tear. If the whole brain is in motion and is subjected to accelerative (or decelerative) forces, the brain may take on complex motions with respect to itself. In these instances it causes torsional-rotational or shearing forces which may produce deep injury to brain structures. Other processes can focally or diffusely injure the brain. The brain requires moment-to-moment uninterrupted supplies of oxygen, glucose, and blood. Interruption of any of these supplies can result in global damage to the brain which may be irreversible dependent upon the severity and length of time of the interruption.
All forms of brain injury produce swelling/edema of the brain which can lead to its own secondary damage to the brain locally or as a whole.
There are four main equilibria that operate in the brain, that are interlinked and the disruption of which can have profound effects on the brain. These are: cerebral circulation and vascular autoregulation; production and absorption of cerebrospinal fluid; the blood-brain barrier systems; and the pressure/volume equilibrium. Brain trauma and brain hypoxia/ischemia that invariably accompany brain trauma at any age have profound effects on all of these systems and govern the clinical symptomology, clinical presentation, treatment options, and prognosis in any case of head/brain injury.
When one or more of these equilibria are disturbed, the brain may swell, increasing in volume, and may herniate.