The Brain
last authored: Sept 2009, David LaPierre
last reviewed:
Introduction
The brain is the most complicated structure in the universe, with each cubic millimeter of cerebral cortex containing approximately one billion synapses (Alonso-Nanclares et al, 2008). Some neurons have more than 100,000 synaptic connections.
The most important biological function of the brain is to generate behaviors that ensure survival. Brains control behavior either by activating muscles, or by causing secretion of chemicals such as hormones.
Components of the Brain
The brain is contained within the skull, and is composed of the cerebrum, cerebellum, and brainstem. With the skull and dura removed, the brain's gyri (folds) and sulci (grooves) are visible. Bigger grooves called fissures are used as landmarks and divide the brain into lobes.
The largest and most visible component of the brain is the cerebrum. It is composed of the cerebral hemispheres, or cortex, as well as the diencephalon, composed of the thalamus and hypothalamus. The cerebellum is a large mass located below the posterior cereberum. It is largely involved in skilled, coordinated activity.
Major components of the brain include:
- cortex: processing and outputs for motor and sensory functions; higher order brain activities
- cerebellum: skilled coordination of activity
- brain stem: essential functions for life (heart rate, breathing, etc); connects rest of brain with spinal cord
- limbic system: memory, emotional regulation
- thalamus: provides connections for motor and sensory pathways
- basal ganglia: coordination and initiation of movement
- cerebrospinal fluid
- hypothalamus
The brain contains several widespread divergent networks of neurons, which communicate with one another by means of long fibers called axons. These carry trains of signal pulses called action potentials to distant parts of the brain or body and target them to specific recipient cells.
Arterial System
The brain weighs only 2% of body weight, but receives 15% of cardiac output and accounts for up to 20% of its oxygen demand and up to 50% of its resting glucose use. The brain is highly aerobic, with oxygen as the limiting substance. Cerebral blood flow is normally about 50 ml/minute for every 100 gm brain tissue, with considerable regional variations.
Within 10 seconds, loss of consciousness occurs, and within 20 seconds, significant changes in electrical activity begins. Cell death occurs within 3-4 minutes.
- overview
- carotid artery system
- vertebral artery system
Overview
The brain's arterial supply is extensive; all the capillaries in the brain laid end to end are estimated to stretch from Halifax to Boston.
Each of the major arteries tends to supply a major area of the brain. Disruption of a given vessel - for example, by an ischemic stroke - will therefore have a predictable impact on brain vitality and function. However, please remember that as in other parts of the body, significant anatomic variation can be present.
Vessel |
Area Supplied |
Effect of disruption |
carotid system |
||
anterior carotid |
medial frontal and parietal lobes |
higher cognitive processes; lower limb motor |
middle cerebral |
frontal, parietal, and temporal lobes |
motor and sensory control; not lower limb
|
anterior choroidal |
limbic system, thalamus |
internal carotid artery
The internal carotids begin from the common carotid arteries and ascend to the base of the skull. They enter through the temporal bone through the carotid canal, which also contains the carotid plexus of sympathetic nerves. From there, the carotid runs through the cavernous sinus before branching.
The anterior cerebral areteries course forward, with branches to supply the medial frontal and parietal cortex. The anterior communicating artery connects these two major vessels.
The middle cerebral arteries travels above the temporal lobe and laterally to supply surface of parts of the frontal, parietal, and temporal lobes. They are associated with motor and sensory control for most of the body, save the lower limb.
The anterior choroidal arteries branch from MCA to supply the choroid plexus, as well as parts of temporal lobe (limbic system), portions of midbrain, internal capsule, and thalamus.
The posterior communicating arteries the connects the internal carotids with the posterior cerebral arteries, which originate in from the vertebral system.
vertebral artery
the two vertebral arteries join to form a single midline vessel called the baslilar artery.
The posterior cerebral artery travels laterally and splits, traveling over medial surface of occipital lobe and inferior surface of the temporal lobe. It suppliess the region around the calcarine fissure, involved with vision.
The superior cerebellar artery leaves basilar artery just before bifurcation into posterior cerebral arteries. It supplies the superior cerebellum, inferior midbrain, and superior pons. CN III travels between the posterior cerebral and superior cerebellar arteries.
The posterior choroidal supplies the choriod plexus of third ventricle.
The labyrinthe artery leaves basilar artery near the convergence of vertebral arteries.
The anterior inferior cerebellar artery supplies the anterior region under the cerebellar cortex and posterior pons.
The posterior inferior cerebellar artery leaves vertebral artery at level of medulla. It supplies the lateral cerebellum, dorsolateral medulla.
Anterior and posterior spinal arteries leave vertebral arteries and travel down through foramen magnum to supply the spinal cord.
Venous Return
The arachoid space is connected with the superior sagittal sinus, at the top of the skull, through arachnoid granulations and villi, which extend through the dura. CSF flows through the pressure-sensitive one-way valves of the granulations to enter the venous system.
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