Cerebral blood flow is influenced by sympathetic nerves. (which are present around arteries as they pass through the subarachnoid space). Adrenergic nerve fibres within the brain also end on blood vessels.
Blood flow through the brain does not markedly vary with alterations in blood pressure. Blood flow through a part of the brain increases when that part is “active”. Such areas can be visualised by using the technique of positron emission tomography (PET). Studies using the technique are throwing much light on functions of various areas. PET can be combined with magnetic resonance imaging to provide accurate localisation of the areas showing altered blood flow.
An intracranial haemorrhage may be of the following types.
1. Extradural haemorrhage (between dura mater and overlying bone) is usually a result offracture of the skull. In factures of the base of the skull blood may leak into the nose, the pharynx or the ear. An extradural haematoma may form. One important cause of this is bleeding from the middle meningeal artery in fractures of the squamous temporal bone.
2. Subdural haemorrhage (between dura mater and arachnoid mater) is usually caused byrupture of cerebral veins as they pass through this space to reach venous sinuses. This can happen in head injuries. Such bleeding can be extensive and can act as a space occupying lesion.
3. Subarachnoid haemorrhage (between arachnoid mater and pia mater) often occurs by ruptureof aneurysms on the base of the skull.
4. Haemorrhage into brain tissue (e.g. cerebral haemorrhage) is usually a result of rupture ofan artery supplying the region.