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.
Interestingly, Lashley found that, if the lesions had damaged motor co-ordination
so badly that the rats could no longer run the maze, they would still endeavour to
get to the end.
