Keith E Rice - The Brain

therefore, be produced with the involvement of multiple areas of the brain.

This has been confirmed in some instances by PET scans.

More recently the idea of Connectionism (distributed function) has taken hold. This developed in the first place from the work of Donald Hebb (1949) who suggested that learning is consolidated in the brain by the activation of cell assemblies, groups of neurons that fire systematically to respond to a specific stimulus. This causes structural changes in the cells, possibly even the growth of new synapses. As this reoccurs, the cell assembly ‘learns’ and responds each time, forming the neural basis of memory.

Connectionism is a holistic perspective in which the brain functions as a whole. All areas are inter-connected and have multiple tasks to carry out. Information is distributed in networks made up of millions of neurons.

This concept allows for cortical specialisation but the interconnections mean no one area has overall control.

However, even this attempt at an all-encompassing view of brain activity falls down in face of the fact that some brain functions are strictly localised. Eg: no evidence has yet been produced to suggest that temperature control is the responsibility of any other part of the brain other than the hypothalamus.

and adults from brain trauma. Provided the lesion is not too severe or it occurs on one side only, the corresponding area on the other side takes over the function of the damaged area. This is especially true of speech.

The Principle of Multiple Control clearly applies to some parts of the brain which are involved in several different behaviours. Eg: the hypothalamus is implicated in both eating/ drinking and aggressive behaviour.

Certain behaviours may,

The brain is within the skull and the spinal cord is within the vertebrae. The brain and spinal cord are protected by 3 membranes called meninges; the space between the 2 inner meninges is filled with cerebrospinal fluid.

The brain contains around 80% of all neurons and about 10 times as many glial cells (or glia) which provide nutrition and waste removal.

As the brain develops, ‘pruning’ takes place periodically to reduce the number of dendrites forming connections between neurons, thereby reducing the amount of ‘grey matter’ (cell bodies and synapses) in the brain. However, the amount of ‘white matter’ (axons) increases due to the process of myelination. (See Neurons & Neurotransmitters.)

The brain is divided into 3 main areas.

The forebrain or cerebrum has two symmetrical halves - left and right cerebral hemispheres which are joined by fibres including the corpus callosum. Any information reaching one hemisphere first is rapidly and automatically transferred to the other hemisphere via the corpus callosum. Each half has 4 lobes - frontal, parietal, occipital and temporal. The grey outer 6 mm of the cerebrum is the cerebral cortex. This is the center of higher mental processes, where sensations are registered, voluntary actions initiated, decisions made, and plans formulated. It is thought that the frontal lobe in particular is responsible for thinking and decision-making. It is also thought that memories are stored by the cortex - but it is not known exactly where or how.

Some motor and somatosensory areas have been defined such as the visual cortex and the auditory cortex. Along the central sulcus is the primary motor cortex and its associated area. The other side of the central sulcus is the somatosensory cortex which integrates information from the body senses such as touch, pressure and pain.

Within the cerebrum are various subcortical structures, including the thalamus, the limbic system and the basil ganglia.

The thalamus (‘deep chamber’) is an egg-shaped mass of grey matter which serves as a relay station for information flowing in from the sense organs to the cortex. It also receives information from the cortex dealing with complex limb movements and forwards them to the cerebellum. Another part of the thalamus is involved in sleep and waking.

The basil ganglia (‘nerve knots’) is involved in aspects of memory and emotional expression as well as planning sequences of behaviour.

The limbic system is concerned with actions that satisfy basic needs and with emotion. A key part of it is the hypothalamus (‘under the thalamus’) which regulates endocrine activity via the pituitary gland and such life-maintaining processes as metabolism and temperature control. It produces the responses of anger, sexual arousal or fear via its control of the ANS. The amygdala is another part of the limbic system which plays a key role in motivation and emotional expression - eg: fear and anger - while the hippocampus is involved in motivation, learning and emotional memory.)

The second main area of the brain is the midbrain which contains part of the reticular formation (or reticular activating system) and part of the brainstem. The ascending reticular activating system (ARAS) carries mainly sensory information to the forebrain. The descending reticular activating system carries mainly motor information. The ARAS is critical to maintaining our general level of arousal and alertness. It also is involved with (amongst other things) selective attention.

The third part is the hindbrain which contains the cerebellum, the pons and the medulla oblongata.

The cerebellum (‘little brain’) co-ordinates the muscles so that movement and delicate hand co-ordination is smooth and precise. Once learned, complex movements (such as walking and talking) seem to be ‘programmed’ into the cerebellum so that we can do them without having to consciously think what we are doing.

The pons (‘bridge’) is a bulge of white matter which connects the 2 halves of the cerebellum and is vital in integrating the movements of the 2 sides of the body. Together with the midbrain, it activates the muscles of the eye – whether voluntarily or involuntarily (as in REM sleep).

The medulla oblongata (‘rather long marrow’) is really a thick extension of the spinal cord, containing vital reflex centres which control breathing, cardiac function, swallowing, vomiting, coughing, chewing, salivation and facial movements.

Localisation of Function in the Cerebral Cortex

Building on the work of John Hughlings Jackson’s (1865) observations of the effects of strokes on control of the human body and Gustav Fritsch & Eduard Hitzig’s (1870) experiments in electrically stimulating areas of dogs’ brains to produce movements, Wilder Penfield & Edwin Boldrey (1937) stimulated human brains (exposed during surgery) to produce movement in certain body parts. Over the next two decades, Penfield continued and extended such experiments, leading to what was effectively a typographical map of the motor and somatosensory cortexes (Penfield & Theodore Rasmussen, 1950).

Penfield’s work seemed to establish three clear principles with regard to the localisation of function in the brain:-

The left hemisphere of the brain controls the muscles of the right side of the body and the right hemisphere the left side - ie: contralateral
The body is represented upside down in the brain so that stimulation near the top of the head produces movement of the lower body while stimulation lower in the motor area led to movement of the upper body
Some areas of the body have large areas of the brain devoted to them while others had only small representations - this difference relating not to size of the body area but to the degree of control, coordination and sensitivity required.

However, the Law of Mass Action and the Law of Equipotentiality, as proposed by Karl Lashley (1929), work against the concept of localisation, as demonstrated by Penfield.

The Law of Mass Action was derived by Lashley experimenting with making lesions in the cortexes of rats which had been trained to run a maze: it wasn’t the location of the lesion which affected the rats’ memory but the size of the lesion. Lashley then put forward the notion that the cortex as a whole is equipotential for some processes such as learning or problem-solving.

The Law of Mass Action has been supported to some extent by Steve Rose (1976) who found that children up to 3 years old could recover much more strongly than older children

The nervous system is the network of all the neurons (nerve cells) in the body - there are between 10 and 12 billion.

Functionally, it subdivides into the central nervous system (CNS) and the peripheral nervous system (PNS). The brain and the spinal cord together comprise the CNS, the function of which is to analyse information arriving from the PNS and initiate appropriate responses to be sent via the PNS to the muscles and organs of the body.

The somatic nervous system carries messages to and from the muscles controlling the skeleton while the autonomic nervous system (ANS) carries messages to and from the body’s internal organs.