3.2 The Structure of the Brain

Since much of this thesis makes references to brain anatomy and function, this section deals with the terminology used in the discipline of neurology. There are four sections covering brain anatomy, brain cellular structure and brain function. Further reference can be obtained from the many good books on brain biochemistry and neuroanatomy [13,14,15].

3.2.1 Principal Axes and Planes of the Central Nervous System

In order to describe the position of structures relative to each other in the brain, neurology has a number of terms of direction, many of which are derived from the Latin or Greek. There are two axes which describe the organisation of the central nervous system (CNS). These are most easily understood in animals with the spinal cord running horizontally rather than vertically. In this case the rostral-caudal axis runs from nose to tail, and the dorsal-ventral axis runs perpendicularly to this as shown in Figure 3.4a. Now using this system in the human spinal cord, 'top to bottom' is 'caudal to rostral', and 'back to front' is 'dorsal to ventral' (Figure 3.4b). In the human brain however these axes turn through 90 degrees, so the front of the brain is rostral, top is dorsal, and base is ventral.


Figure 3.4 The axes of the central nervous system. (a) In animals, where the spinal cord runs horizontally. (b) In humans, where the spinal cord runs vertically.

In addition to these labels, there is another perpendicular set of axes which is the same for spinal cord and brain, that is anterior=front, posterior=back, superior=top, inferior=bottom.

The midline runs down the centre of the brain, separating left from right. If two structures are on the same side of the midline, they are said to be ipsilateral, whereas they are contralateral if they are on opposite sides. When comparing two structures, the one closest to the midline is medial, as opposed to the other which is lateral.

When viewing sections through the brain, three mutually perpendicular planes are commonly considered, as shown in Figure 3.5. These are axial (or transverse) coronal, and sagittal.

Figure 3.5 The three planes of section in the brain

3.2.2 Cellular Structure of the Brain

The neuron is the basic functional unit of the nervous system. The brain consists of several hundred billion neurons, communicating by billions of interconnections. All neurons consist of four distinct parts: cell body, dendrites, axons and axon terminals (Figure 3.6).

Figure 3.6 Diagram of a single motor neurone

The cell body (or soma) contains the nucleus of the cell, as well as the essential cellular organelles, such as the energy generating mitochondria. The cell body has many branches, called dendrites, which receive signals from other cells, and are often covered in dendritic spines. Extending from the cell body in one direction is an axon. The length of axons can be several centimetres or longer. Axons carry information from one neuron to another, and are terminated at the synaptic knob, which is attached to the dendrites or cell body of another neuron. Signals are transferred across the synapse by means of a chemical neurotransmitter.

Signals travel along the axon by generating and propagating an action potential. This is produced by letting the delicate balance of sodium, potassium and chloride ions across the cell membrane be altered, thus generating an electrical signal that flows along the axon. If the axon is coated in a fatty sheath, called myelin, the signal travels at higher speeds of anything up to 120 ms-1. When the signal reaches the synapse, the synaptic knob emits a neurotransmitter which acts either to encourage the next neuron to 'fire' (excitatory) or discourage the neuron to fire (inhibitory).

The CNS contains a number of different types of neurons, which are tailored to the job they perform. Signals from sensory receptors over the body feed along the spinal cord to the brain, and signals are sent from the brain to make muscles contract. Many medical conditions are caused by the failure of the CNS to function correctly, for example in Parkinson's disease there is a deficiency of the neurotransmitter dopamine.

For every neuron in the CNS there are also ten glial cells. These cells provide support for neurons, for example the microglia which perform a scavenger role, and oligodendrocytes which form the myelin sheath around the axons.

3.2.3 Brain Anatomy

The central nervous system consists of the spinal cord and the brain. The brain is then further divided into the forebrain, midbrain, and hindbrain (Figure 3.7).

Figure 3.7 Midline section through the brain showing the cerebrum, cerebellum, midbrain and spinal cord

The largest region is the forebrain, which contains the cerebral hemispheres, the corpus callosum, thalamus, hypothalamus, and hippocampus. the hindbrain consists of the cerebellum, pons, and medulla. The structure of each of these is described below.

The cerebrum is divided into two hemispheres, the left and the right, separated by the longitudinal fissure. Anatomically these are identical in form, each being split into four lobes; the frontal lobe, the parietal lobe on the top, the temporal lobe on the side, and the occipital lobe at the back (Figure 3.8). The frontal and parietal lobes are separated by the central sulcus, and the temporal lobe separated by the lateral fissure. The corpus callosum joins left and right hemispheres.

Figure 3.8 The four lobes of the cerebrum; frontal, temporal, parietal and occipital
The outer surfaces of the hemispheres contain neurons with unmyelinated axons, whereas the more central regions contain myelinaed axons. The presence of the myelin sheath gives these regions of the brain a white appearance, and is termed white matter, as opposed to the grey matter of the outer surface. The grey matter is folded forming many fissures and sulci (grooves) and gyri (elevations). The whole brain is surrounded with a watery fluid that acts as a cushion from physical shocks, called cerebrospinal fluid (CSF). There are also four cavities within the brain which contain CSF, two lateral ventricles, one in each hemisphere, and two lower in the brain. The lateral ventricles show up very clearly on MR images since CSF has a long T2 (Figure 3.9).

Figure 3.9 T2* Weighted image of a transaxial slice through the lateral ventricles

Under the surface of the cerebral hemispheres are bundles of fibres, the basal ganglia, connecting together many regions of the cerebral cortex. The thalamus, hypothalamus and hippocampus are located at the centre of the forebrain, just above the midbrain (Figure 3.10). At the rear of the brain is a more tightly folded structure called the cerebellum, which is connected to the pons, the medulla, and finally the spinal cord.

Figure 3.10 Coronal section through the brain showing the thalamus and hypothalamus

3.2.4 Functional Organisation of the Brain

The functional organisation of much of the brain is poorly understood. However many of the regions involved in sensory and motor function have been identified.

The primary visual cortex is located in the occipital lobe, which deals with the reception and interpretation of vision. The right visual field is mapped on to the left cerebral hemisphere, and the left visual field on to the right hemisphere (Figure 3.11). The signals from the retina travel along the optic tracts, which cross over at the optic chiasm.

Figure 3.11 Mapping of the visual field.
Just as visual stimuli are interpreted by an area on the opposite side of the brain to the eyes, auditory stimuli are interpreted on the opposite side of the brain to the ears. The primary auditory cortex is located in the temporal lobe, with the right ear mapping on to the left hemisphere, and vice-versa. Similarly signals from the many touch receptors over the body end up in the somatosensory cortex, which is located in the parietal lobe, just behind the central sulcus. The sensations of taste and smell are mediated by the gustatory and olfactory systems. The olfactory bulb is located on the inferior surface of the frontal lobe, whereas the gustatory cortex is in the temporal lobe. The cortical regions associated with the five primary sensory areas are highlighted in Figure 3.12.

Figure 3.12 Approximate location of the five primary sensory areas and the primary motor cortex

The organisation of the somatosensory cortex shows similarity to a map of the surface of the body[16]. This is illustrated in what is known as the sensory homunculus (Figure 3.13). A much greater part of the somatosensory cortex is associated with the hand and face, compared to regions not so important in tactile tasks such as the leg. Similarly there is a motor homunclus which illustrates the layout of the motor cortex. The hand is given much more cortical surface in the motor cortex than in the somatosensory cortex, representing the highly sophisticated tasks that the hands perform.

Figure 3.13 (a) A cross section through the brain, illustrating the sensory and motor sequences. (b) The motor homunculus, drawn such that the relative size of the organs represents the area of the corresponding cortex (from Penfield and Rasmussen (1952) `The Cerebral Cortex of Man')

All the regions mentioned so far are primary cortices because they are most closely involved with the brain's input and output. Much of the rest of the brain is given over to integrating these stimuli, and interpreting how to respond. The regions responsible for these more abstract tasks are termed secondary (Figure 3.14). For example, the secondary motor regions, which are more anterior in the frontal lobes to the primary motor cortex, are responsible for planning and initiating motion, and the secondary visual area, close to the primary visual cortex is involved in interpreting colours and movement in the visual information. The tertiary areas, or association cortices, are responsible for the higher brain functions such as interpretation and memory.

Figure 3.14 Approximate locations of the primary, secondary and tertiary sensory and motor cortices

Some specific regions of interest are those responsible for speech, which are located in the left hemisphere of most people. Broca's area is in the lower part of the frontal lobe (Figure 3.15), and is involved in the formation of sentences, and Wernicke's area, located in the temporal lobe is involved in the comprehension of speech.

Figure 3.15 Approximate locations of Broca's area, involved in the formation of sentences and Wernike's area, involved in the comprehension of speech
The cerebellum has a number of poorly understood functions, but is involved in the regulation of movement. Patients with damage to the cerebellum can still move, but the movements become more erratic and less controlled. The cerebellum is also involved in the 'automatic response' that is experienced when a new skill has been learnt. For example, when learning to play a piece on the piano, at first the cerebral cortex is required to control the fingers, but upon learning, the cerebellum takes over. The region at the centre of the brain, the thalamus acts as an intermediary in transferring information to the cerebral hemispheres. The hippocampus plays an important role in long term memory storage, and the hypothalamus mediates emotions, also being involved in the control of hormonal release. The lower structures in the brain, the reticular formation, medulla and pons, regulate alertness and participate in blood pressure and respiratory regulatory mechanisms.

This is just a brief sketch of brain function as it is understood at present. New literature is appearing at a rapid pace, confirming and evolving the models. The motor system is covered in more detail in Chapter 7.

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