The cerebral hemispheres are the highest level of the Central Nervous System. They are like two mushroom caps bulging of the left and right front end of the brain stem, as shown in Figure 1 at the right. They control the activity of the lower levels, directing the overall plan and direction of behavior.
Figure 2 at the left represents the left cerebral hemisphere in humans. It shows four of the five lobes (the limbic lobe is on the inner surface) and the location of the primary sensory and motor areas.
Figure 3 at the right shows the variation among species in size of the cerebral hemispheres Compared to the rest of the brain, the hemispheres have undergone by far the greatest changes in the evolution. In fishes and amphibia (frogs, salamanders, etc.), they are little bumps on the brain stem. In mammals the hemispheres have expanded enormously to cover the front end of the brain stem. They are the largest in primates and cetaceans (whales, dolphins). The human cerebral hemispheres are about 3 times larger than the chimpanzee's. In humans, the hemispheres make up about 2/3 of the whole brain, hiding the front half the brain stem.
Figure 4 at the left is a cross section through the cerebral hemispheres, showing their internal structure. The outer surface of the hemispheres, colored pink in Figure 4, is called the cerebral cortex (Greek for rind), which is filled with cell bodies and dendrites, and synaptic connections from axons. The cortex contains about half of all neurons in the human brain and serves as the highest level of brain function. It is essential for the highest levels of mental and behavioral functions. Underneath the cortex are the axons connecting each cortical area with other parts of the brain. The underlying axon area is a creamy white and is calledwhite matter Because it is full of cell bodies, etc., the cortex looks darker and is called grey matter.
The cortex is divided into three different kinds of areas, as shown in Figure 5 below:
- Primary areas, one for each major sense and one for motor control. They are most directly connected to the input from the senses and output to the motor system.
- Sensory and motor association areas, several for each major sense and for motor control. These are involved in perceptual and motor organization.
- Higher order association areas, one on the back half of the brain and one in the front. These are essential for mental functions, like language that do no depend on one particular sense.
The inside of the cerebral hemispheres contain several large, interconnected clusters of cell bodies called the basal ganglia. These link sensory parts of the cerebral cortex to its motor parts, and connect the motor parts to brain stem and spinal cord.
Figure 4 above also shows the location of several internal strutures of the cerebral hemispheres
- White Matter made of the axons communicating to and from cerebral cortex.
- The Basal Ganglia, which are large clusters of grey matter made of cell bodies dendrites and axon terminals. They are links between motor cortex and the rest of the cerebral cortex and also with the brain stem motor systems. They are involved in initiating and directing responding.
