The human brain is hugely interconnected but three major components can be identified: the cerebrum, the cerebellum and the brain stem.
The brainstem which includes the medulla, the pons and the midbrain, controls breathing, digestion, heart rate and other autonomic processes, as well as connecting the brain with the spinal cord and the rest of the body.
The cerebellum plays an important role in balance, motor control, but is also involved in some cognitive functions such as attention, language, emotional functions (such as regulating fear and pleasure responses) and in the processing of procedural memories.
The cerebrum (or forebrain), which makes up 75% of the brain by volume and 85% by weight, is divided by a large groove, known as the longitudinal fissure, into two distinct hemispheres. The left and right hemispheres (“left” and “right” refer to the owner’s point of view, not an outside viewer’s) are linked by a large bundle of nerve fibres called the corpus callosum, and also by other smaller connections called commissures.
Most of the important elements of the cerebrum are split into symmetrical pairs in the left and right hemispheres. Thus, we often speak of the temporal lobes, hippocampi, etc (in the plural), although this website generally follows the convention of speaking of the temporal lobe, hippocampus, etc (in the singular), which should, therefore, be taken to mean both sides, within both hemispheres. The two hemispheres look similar, but are slightly different in structure and perform different functions. The right hemisphere generally controls the left side of the body, and vice versa, although popular notions that logic, creativity, etc, are restricted to the left or right hemispheres are largely simplistic and unfounded.
The cerebrum is covered by a sheet of neural tissue known as the cerebral cortex (or neocortex), which envelops other brain organs such as the thalamus (which evolved to help relay information from the brain stem and spinal cord to the cerebral cortex) and the hypothalamus and pituitary gland (which control visceral functions, body temperature and behavioural responses such as feeding, drinking, sexual response, aggression and pleasure). The cerebral cortex itself is only 2 – 4 mm thick and contains six distinct but interconnected layers. It is intricately grooved and folded into the familiar convoluted pattern of folds, or gyri, allowing a large surface area (typically almost 0.12m2) to fit within the confines of the skull. Consequently, more than two-thirds of the cerebral cortex is buried in the grooves or sulci.
About 90% of all the brain’s neurons are located in the cerebral cortex, mainly in the “grey matter”, which makes up the surface regions of the cerebral cortex, while the inner “white matter” consists mainly of myelinated axons, over 170,000 km of them. As many as five times that number of glial cells exist to support the active nerve cells.
The cerebral cortex plays a key role in memory, attention, perceptual awareness, thought, language and consciousness. It is divided into four main regions or lobes, which cover both hemispheres: the frontal lobe (involved in conscious thought and higher mental functions such as decision-making, particularly in that part of the frontal lobe known as the prefrontal cortex, and plays an important part in processing short-term memories and retaining longer term memories which are not task-based); the parietal lobe (involved in integrating sensory information from the various senses, and in the manipulation of objects in determining spatial sense and navigation); the temporal lobe (involved with the senses of smell and sound, the processing of semantics in both speech and vision, including the processing of complex stimuli like faces and scenes, and plays a key role in the formation of long-term memory); and the occipital lobe (mainly involved with the sense of sight).
The medial temporal lobe (the inner part of the temporal lobe, near the divide between the left and right hemispheres) in particular is thought to be involved in declarative and episodic memory. Deep inside the medial temporal lobe is the region of the brain known as the limbic system, which includes the hippocampus, the amygdala, the cingulate gyrus, the thalamus, the hypothalamus, the epithalamus, the mammillary body and other organs, many of which are of particular relevance to the processing of memory.
The hippocampus, for example, is essential for memory function, particularly the transference from short- to long-term memory and control of spatial memory and behaviour. The hippocampus is one of the few areas of the brain capable actually growing new neurons, although this ability is impaired by stress-related glucocorticoids. The amygdala also performs a primary role in the processing and memory of emotional reactions and social and sexual behaviour, as well as regulating the sense of smell.
Another sub-cortical system (inside the cerebral cortex) which is essential to memory function is the basal ganglia system, particularly the striatum (or neostriatum) which is important in the formation and retrieval of procedural memory