Lobes of the Brain The three main components of the brain; the cerebrum, the cerebellum, and the brain stem have distinct functions. The cerebrum is the largest and most developmentally advanced part of the human brain. It is responsible for several higher functions, including higher intellectual function, speech, emotion, integration of sensory stimuli of at types, initiation of the final common pathways for movement, and fine control of movement.

The cerebellum, the second largest area, is responsible for maintaining balance and further control of movement and coordination.

The brain stem is the final pathway between cerebral structures and the spinal cord. It is responsible for a variety of automatic functions, such as control of respiration, heart rate, and blood pressure, wake-fullness, arousal and attention.

The cerebrum is divided into a right and a left hemisphere and is composed of pairs of frontal, parietal, temporal, and occipital lobes.

The left hemisphere controls the majority of functions on the right side of the body, while the right hemisphere controls most of functions on the left side of the body The crossing of nerve fibres takes place in the brain stem. Thus, injury to the left cerebral hemisphere produces sensory and motor deficits on the right side, and vice versa.

One hemisphere has a slightly more developed, or dominant, area in which written and spoken language is organised Over 95% of right handed people and even the majority of left handed people have dominance for speech and language in the left hemisphere Thus, a left hemisphere stroke will be more likely to produce aphasia and other language deficits.

Circle of Willis At the base of the brain, the carotid and vertebrobasilar arteries form a circle of communicating arteries known as the circle of Willis.

From this circle other arteries -- the anterior cerebral artery (ACA), the middle cerebral artery (MCA), the posterior cerebral artery (PCA) - arise and travel to all parts of the brain.

Because the carotid and vertebrobasilar arteries form a circle, if one of the main arteries is occluded, the distal smaller arteries that it supplies can receive blood from the other arteries (collateral circulation).

Anterior Cerebral Artery The anterior cerebral artery extends upward and forward from the internal carotid artery. It supplies the frontal lobes, the parts of the brain that control logical thought, personality, and voluntary movement, especially the legs. Stroke in the anterior cerebral artery results in opposite leg weakness. If both anterior cerebral territories are affected, profound mental symptoms may result (akinetic mutism).

Posterior Cerebral Artery The posterior cerebral arteries stem in most individuals from the basilar artery but sometimes originate from the ipsilateral internal carotid artery. The posterior arteries supply the temporal and occipital lobes of the left cerebral hemisphere and the right hemisphere. When infarction occurs in the territory of the posterior cerebral artery, it is usually secondary to embolism from lower segments of the vertebral basilar system or heart.

Clinical symptoms associated with occlusion of the posterior cerebral artery, depend on the location of the occlusion and may include thalamic syndrome, thalamic perforate syndrome, Weber's syndrome, contralateral hemplegia, hemianopsia and a variety of other symptoms, including including colour blindness, failure to see to-and-fro movements, verbal dyslexia, and hallucinations. The most common finding is occipital lobe infarction leading to an opposite visual field defect.

Lenticulostriate Arteries Small, deep penetrating arteries known as the lenticulostriate arteries branch from the middle cerebral artery Occlusions of these vessels or penetrating branches of the circle of Willis or vertebral or basilar arteries are referred to as lacunar strokes. About 20% of all stokes are lacunar and have a high incidence in patients with chronic hypertension and in the elderly CT scanning shows signs of infarction in only approximately half of the most of the common form of lacunar stroke (pure motor stroke), but MRI has increased the yield: the probability that CT or MRI wit be positive is generally a function of the severity of the deficit .The cells distal to the occlusion die, but since these areas are very small Often only minor deficits are seen. When the infarction is critically located, however, more severe manifestations may develop, including paralysis and sensory loss.

Within a few months of the infarction, the necrotic brains cells are resorbed by macrophage activity, leaving a very small cavity a lake or lacune in French.

Cerebellum and Brainstem A stroke involving the cerebellum may result in a lack of coordination, clumsiness, shaking, or other muscular difficulties. These are important to diagnose early, since swelling may cause brainstem compression or hydrocephalus.

Strokes in the brainstem are usually due to basilar occlusion, although in many cases the clinical syndrome may fit the criteria for a lacunar stroke. Brainstem strokes can be serious or even fatal. People who survive may be left with severe impairments or remain in a vegetative state.