The brain - a multi-level organisational structure
A century ago, it was the common belief that the brain was one homogeneous mass. This has since been shown to be false. The brain is actually made up of many different areas and regions dedicated to certain tasks, or groups of processes. Broca, for instance, showed that a certain region of the brain was associated with the production of speech. Autopsies performed on stroke sufferers who were unable to speak, yet could understand language, found that they all suffered damage in the same area of the brain, now called Broca’s area. Wernicke, using the same method, identified another area of the brain (Wernicke's area)that was key to the comprehension of language. Since all the different specialised regions of the brain work together, sub-groups of areas contributing to a certain more complex task, then that task forming another sub-group, the brain can be thought as being organised on many different levels.
There are three basic divisions of the whole nervous system in which all the specialised areas can be placed. These are; the central nervous system (CNS), the peripheral nervous system (PNS), and the autonomic nervous system (ANS). The CNS could be seen as having the highest functioning of the three, as it contains the brain and the spinal chord. The PNS serves the CNS by conveying information into and out of it. The ANS is responsible for the regulation of the endocrine and exocrine systems, motivational and emotional states, and the basic physiology of the body (such as blood pressure and heart rate). The ANS is itself regulated by the CNS, specifically, by the hypothalamus in the brain.
The CNS is further constructed of several levels or sub-regions. Firstly, there is the spinal chord. Its main function is to serve as a conduit to pass affectory information from the brain to locations in the body, such a muscle movements, and to conduct other information, such as somatosensory information, back to the brain. However, even though it is effectively an extension of the brain, the spinal chord does have a certain amount of autonomy. For instance, reflex actions are pretty much entirely contained within the spinal chord. Their basic function, in the case of someone moving their hand away from contact with a sharp object is as follows. Pain receptors send signals to the spinal chord to be relayed on to the brain, but when the signals reach the spinal chord, they trigger neurons connected to certain muscles in the arm. These neurons cause the muscles to contract, thus moving the hand away from the source of pain. This is only a crude explanation – the real effect is much more complex. The autonomy of the spinal chord exists, but is easily overcome by higher brain functions. More cognitive processes can overcome these reflex actions – for instance, if we carry a hot cup of coffee that is burning our hand, the reflex action will be directed to letting go of the cup, but a more cognitive influence might value having a clean carpet and inhibit the reflex action.
Another region of the CNS, rostral (further "forward") to the spinal chord, is the medulla oblongata (or just the medulla). This controls and regulates vital functions such as respiration and the cardio-vascular system. Rostral to the medulla is another region of the CNS known as the pons. Among the major functions of the pons is the relay of information to the cerebellum (another major area of the CNS), and also it helps the timing sequence and pattern of muscle movements. The midbrain, rostral to the pons, is yet another CNS region, with functions including eye movements and motor control of skeletal muscles.
Rostral to the pons is a region of the CNS that has itself two further sub-regions. The Diencephalon is a structure that contains the thalamus and the hypothalamus. The hypothalamus, as mentioned above, helps regulate the ANS – regulating hormonal secretions by the pituitary gland for example. It controls four basic animal functions which physiological psychologists have, rather amusingly, termed “the four F’s” – fighting, feeding, fleeing, and mating. The thalamus is concerned with sensory input. Even the thalamus is made of constituent nuclei – each one projecting the input from a different sensory modality to different areas of the cerebral cortex.
The final sub-region of the CNS are the cerebral hemispheres. There are effectively two levels to this area – the cerebral cortex and the basal ganglia. These areas, known collectively as the cerebrum, are involved in perceptual, cognitive, and higher motor functions. The cerebral cortex – like other regions – also has sub-regions. It has two hemispheres, each hemisphere consisting of four lobes; the frontal lobe (the most rostral part of the brain - at the front), the parietal lobe (in a dorsal medial location - at the top), the temporal lobe (in a lateral location - at the sides), and the occipital lobe (the most caudal area - at the back).
All these main areas of the cerebral cortex have specialised regions within them. There are large areas dedicated to the complex tasks involved in movement. The primary motor cortex is located on the precentral gyrus (in the area just between the frontal and parietal lobes). There are also many areas dedicated to sensory information processing – the primary visual cortex (located at the back of the occipital lobe), the primary auditory cortex (located in the temporal lobes), and the somatic sensory cortex (located on the postcentral gyrus). All these primary sensory and motor areas each have associated secondary and tertiary areas – further levels or regions. These allow greater analysis and integration of the relevant information. The primary, secondary, and tertiary sensory areas for each modality then have further association areas which integrate all this diverse information to help provide purposeful action.
In fact, the sensory and motor areas of the brain can also be considered as two collaborative levels, since they interact with each other constantly. Catching a ball, for instance, requires sensory and motor functions to work together very closely. There is constant somatic sensory feedback from the muscles, which feed to the motor areas so that the muscle movements can be reassessed and altered, while also taking into account new visual information.
There are two further major regions of the brain that have not been considered yet. These are the left and right hemispheres. The brain is split into two approximately symmetrical hemispheres, connected by a large group of nerve fibres called the corpus callosum. The hemispheres of the brain control and receive information cross-laterally – i.e. from the opposite side of the body. The hemispheres, although roughly symmetrical in appearance, are not necessarily symmetrical in function. Although each side of the brain will control things like cross-lateral motor function, symmetrical areas may have their function in the same thing, but control different aspects of it. For instance the left side of the brain has an area which controls the actual production of words in speech (Broca's area), but the corresponding area of the right hemisphere controls the more emotive side of speech, such as intonation. Some interesting effects of the differences between the two hemispheres arise when the corpus callosum is severed. The severing of the corpus callosum is a treatment for very severe epilepsy. After the severing, the two hemispheres of the brain are unable to communicate with each other. Patients report that, for instance, their left hand has “a mind of its own”. This is because the left hemisphere cannot “find out” what the right is doing. The right hemisphere controls the movements of the left hand, while the left hemisphere controls speech production, hence the left hemisphere cannot articulate what the right hemisphere is doing. Patients engrossed in a book they are reading will sometimes find that their left hand will suddenly and spontaneously put the book down. This is due to the right hemisphere being unable to read and because of this, it gets bored with the book.
Beyond the areas, regions, and levels described so far, there is another level. Every part of the brain is made up of neurons. The whole brain and nervous system is based upon these cells – it is the ultimate, final level to the brain’s organisation. The flowing of Sodium, Potassium, and Chlorine ions in and out of these cells is the fundamental basis of all the functions of the brain. There is also chemical level made up of the many different kinds of neurotransmitters regulating function too, such as dopamnine and GABA.
The brain is organised in many levels, starting with neurons and building up though more complex structures. The levels of organisation are very much a hierarchical structure of complexity.