Nervous

The human central nervous system is composed of the brain and spinal cord, from which extend 31 pairs of spinal nerves that direct the various actions of the body.

The cerebrum, located at the very front of the human brain, is so large compared with the rest of the brain that it appears to envelop it. In the brains of humans and other primates the cerebrum is split into two halves, or hemispheres, which are connected only by a nerve tract called the corpus callosum. Each hemisphere of the brain is divided further by two deep grooves into four lobes, designated the frontal, parietal, temporal, and occipital lobes.

The cerebral cortex

Much of the neural activity of the cerebrum occurs within a thin, gray layer only a few millimeters tick on its outer surface, overlying a solid white region that consists of my elinated nerve fiber. This layer, the cerebral cortex mentioned earlier, is densely packed with neuron cell bodies. The human cerebral cortex contains more than 10 billion nerve cells, roughly 10% of all the neurons in the brain. The surface of the cerebral cortex is highly convoluted, particularly in human brains, a property that increases its surface area threefold.
There are four major regions of activity on the cerebral cortex, each of them referred to as a specialized cortex: the motor, sensory, auditory, and visual cortexes.
The motor cortex straddles the rearmost portion of the frontal lobe. Each point on its surface is associated with the movement of a different part of the body. Right behind the motor cortex, on the leading edge of the parietal lobe, lies the sensory cortex. Each point on the surface of the sensory cortex represents sensory receptors from a different part of the body, such as the pressure sensors of the fingertips or the taste receptors of the tongue. The auditory cortex lies within the temporal lobe; different surface regions of this cortex correspond to different sound frequencies. The visual cortex lies on the occipital lobe, with different sites corresponding to different positions on the retina.

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