62 The Brain

The Brain

The brain is the neurological center of an organism.

The human brain is the center of the human nervous system. It has the same general structure as the brains of other mammals, but is larger than expected on the basis of body size when compared to other primates. Estimates for the number of neurons ( nerve cells) in the human brain range from 80 to 120 billion. Most of the expansion comes from the cerebral cortex, especially the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, the human brain is susceptible to many types of damage and disease. This includes degenerative disorders such as Parkinson’s disease, multiple sclerosis, and Alzheimer’s disease. A number of psychiatric conditions, such as schizophrenia and depression, are thought to be associated with brain dysfunction, although the nature of such brain anomalies is not well understood.

Cerebral Cortex

The cerebral hemispheres form the largest part of the human brain and are situated above most other brain structures. They are covered with a cortical layer and have a convoluted topography. The cerebral cortex is essentially a sheet of neural tissue folded in a way that allows a large surface area to fit within the confines of the skull. Anatomists call each cortical fold a sulcus and the smooth area between folds a gyrus. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other hand, is more convoluted than the cortex of a human.

Cortex Divisions

The left and right hemispheres of the cerebral cortex are nearly symmetrical. The hemispheres are connected by the corpus callosum, the largest white-matter structure in the brain. Anatomists conventionally divide each hemisphere into four lobes: the frontal (control of specialized motor control, learning, planning, and speech), parietal (control of somatic sensory functions), occipital (control of vision), and temporal lobes (control of hearing and some speech). The division into lobes does not actually arise from the structure of the cortex itself. Instead, each section is named after the skull bone that covers it. The borders between lobes are placed beneath the sutures that link the skull bones together. The only exception is the border between the frontal and parietal lobes, which is shifted backward from the corresponding suture to the central sulcus. This deep fold marks the line where the primary somatosensory cortex (main sensory receptive area for the sense of touch) and primary motor cortex (one of the principal areas of the brain involved in motor function) come together. Functionally, the cortex is commonly described as comprising three parts: sensory, motor, and association areas.

Brain Stem and Cerebellum

The cerebrum is attached to a stalk-like structure called the brain stem, which consists of the midbrain, pons, and medulla. At the rear of the brain beneath the cerebrum and behind the brainstem is the cerebellum. This structure has a horizontally-furrowed surface with an appearance that is distinct from all other brain areas. These same structures are present in other mammals, although the cerebellum is not so large relative to the rest of the brain in non-human mammals.

Hippocampus

Found deep in the temporal lobe, the seahorse-shaped hippocampus is responsible for memory. The amygdala is a small, almond-shaped structure deep inside the anteroinferior region of the temporal lobe. It plays an important role in the mediation and control of activities and feelings such as love, friendship, affection, and mood expression. The amygdala is the center for danger identification, a fundamental part of self-preservation.

Thalamus

The thalamus is perched on top of the brainstem near the center of the brain, with nerve fibers projecting out to the cerebral cortex in all directions. Its functions includes relaying sensory and motor signals to the cerebral cortex and regulating consciousness, sleep, and alertness. It likely acts as a relay between a variety of subcortical areas and the cerebral cortex.

Hypothalamus

The hypothalamus is a small part of the brain located just below the thalamus on both sides of the third ventricle. The hypothalamus also plays a role in emotion. Specifically, the lateral parts seem to be involved with pleasure and rage, while the medial part is linked to aversion, displeasure, and a tendency toward uncontrollably loud laughing. When the physical symptoms of emotion appear, the threat they pose returns to the limbic centers via the hypothalamus, then to the prefrontal nuclei, increasing anxiety.