The Brain

Good day everyone, Today we are going to discuss fully on The brain ;

Learning outcomes

After studying this section, you should be able to:

• describe the blood supply to the brain
• name the lobes and principal sulci of the brain
• outline the functions of the cerebrum
• identify the main sensory and motor areas of the cerebrum
• outline the position and functions of the thalamus and hypothalamus
• describe the position and functions of the midbrain, pons, medulla oblongata and reticular activating system
• describe the structure and functions of the cerebellum.

Definition of The Brain

The brain is a large organ weighing around 1.4 kg that lies within the cranial cavity. Its parts are:

• cerebrum
• thalamus
• hypothalamus
• midbrain
• pons
• medulla oblongata
• cerebellum

Brain supply and venous drainage

The circulus arteriosus and its contributing arteries play a vital role in maintaining a constant supply of oxygen and glucose to the brain when the head is moved and also if a contributing artery is narrowed. The brain receives about 15% of the cardiac output, approximately 750 mL of blood per minute. Autoregulation keeps blood flow to the brain constant by adjusting the diameter of the arterioles across a wide range of arterial blood pressure (about 65-140 mmHg) with changes occurring only outside these limits.

Venous blood from the brain drains into the dural venous sinuses and then downwards into the internal jugular veins.

Cerebrum

This is the largest part of the brain and it occupies the anterior and middle cranial fossae. It is divided by a deep cleft, the longitudinal cerebral fissure, into right and left cerebral hemispheres, each containing one of the lateral ventricles. Deep within the brain, the hemispheres are connected by a mass of white matter (nerve fibres) called the corpus callosum. The falx cerebri is formed by the dura mater. It separates the two cerebral hemispheres and penetrates to the depth of the corpus callosum. The superficial part of the cerebrum is composed of nerve cell bodies (grey matter), forming the cerebral cortex, and the deeper layers consist of nerve fibres (axons, white matter).

The cerebral cortex shows many infoldings or furrows of varying depth. The exposed areas of the folds are the gyri (convolutions) and these are separated by sulci (fissures). These convolutions greatly increase the surface area of the cerebrum.

For descriptive purposes each hemisphere of the cerebrum is divided into lobes which take the names of the bones of the cranium under which they lie:

• frontal
• parietal
• temporal
• occipital.

The boundaries of the lobes are marked by deep sulci. These are the central, lateral and parieto-occipital sulci.

Cerebral tracts and basal ganglia

The surface of the cerebral cortex is composed of grey matter (nerve cell bodies). Within the cerebrum the lobes are connected by masses of nerve fibres, or tracts, which make up the white matter of the brain. The afferent and efferent fibres linking the different parts of the brain and spinal cord are as follows.

• Association (arcuate) tracts are most numerous and connect different parts of a cerebral hemisphere by extending from one gyrus to another, some of which are adjacent and some distant.
• Commissural tracts connect corresponding areas of the two cerebral hemispheres; the largest and most important commissure is the corpus callosum.
• Projection tracts connect the cerebral cortex with grey matter of lower parts of the brain and with the spinal cord, e.g. the internal capsule.

The internal sple tract that lies deep within the brain between the basal ganglia and the thalamus. Many nerve impulses passing to and from the cerebral cortex are carried by fibres that form the internal capsule Motor fibres within the internal capsule form the pyramidal tracts (corticospinal tracts) that cross over (decussate) at the medulla oblongata and are the main pathway to skeletal muscles. Those motor fibres that do not pass through the internal capsule form the extrapyramidal tracts and have connections with many parts of the brain including the basal ganglia, thalamus and cerebellum.

Basal ganglia

The basal ganglia are groups of cell bodies that lie deep within the brain and form part of the extrapyramidal tracts. They act as relay stations with connections to many parts of the brain including motor areas of the cerebral cortex and thalamus. Their functions include initiation and fine control of complex movement and learned coordinated activities, such as posture and walking. If control is inadequate or absent, movements are jerky, clumsy and uncoordinated.

Functions of the cerebral cortex

There are three main types of activity associated with the cerebral cortex:

• higher order functions, i.e. the mental activities involved in memory, sense of responsibility, thinking, reasoning, moral decision making and learning
• sensory perception, including the perception of pain, temperature, touch, sight, hearing, taste and smell
• initiation and control of skeletal muscle contraction and therefore voluntary movement.

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Functional areas of the cerebral cortex

The main functional areas of the cerebral cortex have been identified but it is unlikely that any area is associated exclusively with only one function. Except where spe cially mentioned, the different areas are active in both hemispheres, however, there is some variation between individuals. There are different types of functional area:

• motor, which direct skeletal (voluntary) muscle movements
• association, which are concerned with integration and processing of complex mental functions such as intelligence, memory, reasoning, judgement and emotions.
• sensory, which receive and decode sensory impulses enabling sensory perception

In general, areas of the cortex lying anterior to the central sulcus are associated with motor functions, and those lying posterior to it are associated with sensory functions.

Motor areas of the cerebral cortex

The primary motor area. This lies in the frontal lobe immediately anterior to the central sulcus. The cell bodies are pyramid shaped (Betz’s cells) and they control skeletal muscle activity. Two neurones involved in the pathway to skeletal muscle. The first, the upper motor neurone, descends from the motor cortex through the internal capsule to the medulla oblongata. Here it crosses to the opposite side and descends in the spinal cord. At the appropriate level in the spinal cord it synapses with a second neurone (the lower motor neurone), which leaves the spinal cord and travels to the target muscle. It terminates at the motor end plate of a muscle fibre. This means that the motor area of the right hemisphere of the cerebrum controls voluntary muscle movement on the left side of the body and vice versa. Damage to either of these neurones may result in paralysis.

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In the motor area of the cerebrum the body is represented upside down, i.e. the uppermost cells control the feet and those in the lowest part control the head, neck, face and fingers. The sizes of the areas of cortex representing different parts of the body are proportional to the complexity of movement of the body part, not to its size.

Motor speech (Broca’s) area. This is situated in the frontal lobe just above the lateral sulcus and controls the muscle movements needed for speech. It is dominant in the left hemisphere in right-handed people and vice versa.