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Central Nervous System (CNS)

The central nervous system(CNS) consists of the brain in the skull and the spinal cord, which threads through holes down the middle of the vertebrae (bones that make up the vertebral column ["back bone"]). It is the fuchsia colored area Figure 1. The red lines illustrate the peripheral nervous system, which carries neural signals from sensory receptors (detectors for sound, touch, etc.) to the CNS and from the CNS to the muscles and glands.

The central nervous system (CNS) is a very complex, highly organized structure. A couple of general principles will help you understand how it is put together.

  • The front half (roughly) of the CNS organizes and controls movement; the back half receives and processes information from the senses.
  • The CNS is organized as a hierarchy (organized in graded layers, higher layers controlling lower layers as in the military [general > colonel > major > lieutenant]). Each level from the cerebral hemispheres at the top controls the levels below it. Unlike most hierarchies, the higher the level the more parts to it, at least in the brains of mammals, especially humans. The hierarchy has three main levels, cerebral hemispheres > brain stem > spinal cord, as shown in Figure 2.

    • The spinal cord is the lowest level of the hierarchy. The spinal cord
      • receives sensory neurons from the body.
      • sends motor neurons to the muscles and some of the glands in the body.
      • does the simplest, most basic coordination of different body parts.
      • The brain stem is the next level. It is an expansion of the front (top in humans) end of the spinal cord. It does for much of the head what the spinal cord does for the body. In addition, the brain stem:
      • receives sensory neurons from the special senses (eyes, ears, balance) and thegut.
      • sends motor neurons to most muscles and some glands of the head.
      • coordinates more complex reflexes and actions that involve many parts of the body,
      • activates the rest of the brain and spinal cord.
      • The cerebral hemispheres are the highest level of the hierarchy. They are bulges on the left and right front end of the brain stem. In simple vertebrates like frogs and fishes, they are small bumps on the brain stem; in mammals, especially primates, they are much larger. They completely cover most of the brain stem (See Figure 1 in exercise E09_29e). The cerebral hemispheres perform the highest levels of mental activity. They:
      • extract the important information from the sensory input from brain stem and spinal cord (perception)
      • compare that information to past experience (memory) and interpret it
      • choose and plan course of action and tell the lower level to execute it
      • coordinate the actions of different parts of the brain stem and spinal cord.

Figure 3 summarizes this basic organization.

The cerebral cortex is the outer layer of the cerebral hemispheres. It contains roughly 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 inside the cerebral hemispheres, axons connect to and from the cortex. In addition the hemispheres contain several large, interconnected clusters of cell bodies, called the basal ganglia. They link most of the cerebral cortex to its motor parts, and connect the motor parts of the cortex to the brain stem and spinal cord.

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