The Organization of the Human Nervous System

    I think of it this way. Cells. Multiple cells make up tissues. Multiple tissues make up organs and organ systems consist of multiple organs. Similarly, the nervous system is organized in the same way. Neurons and axons make up nerves, nerves make up neural circuits, and neural circuits create neural systems. Neural circuits which share similar tasks, come together to make neural systems.

  

    Such neural systems include sensory systems (observe and process information about the environments like the visual system, auditory system etc.) and the motor systems (reactions to certain input information. They include reflexes and counter balancing some moves).  Associational systems are composed of large numbers of cells and circuits which are located between both motor and sensory systems. Associational systems also carry out some of the most intricate processes and brain functions.

    As you know, the Central Nervous System (CNS) is composed of the brain (all 4 regions), and the spinal cord. And then, there is also the peripheral nervous system (PNS) which is all the cranial nerves which branch off the brain, and the many nerves which branch off the spinal cord. The PNS also includes sensory neurons which link sensory receptors (parts of sensory neurons which receive sensory information) to circuits in the CNS (missed information about neural circuits? Click here to view my previous post about them).

    The movement (motor) portion of the PNS is divided into 2 branches which both rely on one another in various ways. There is the somatic motor division which consists of nerves and axons which connect the brain and spinal cord to skeletal muscles. And, an autonomic nervous system which controls smooth muscles (which are muscles that move with conscious control ex: Biceps) and involuntary muscles. Both are crucial for survival, as they provide you, your muscle controls and gland controls to live. Without the somatic motor system, the autonomic system cannot move arms and legs, and without your autonomic system, you will need to keep remembering to execute many of your involuntary muscle movements!

    The neurons which lie within the PNS are located in ganglia which are lumps of nerve cells and supporting cells. Bundles of peripheral axons come together to make nerves, just like how thread is made of many thin fibers bundled and twisted. Those fibers are like axons. In nerves, all-around the body, lie glia. The only glia in the periphery though, are Schwann cells which lie layers of Myelin to help speed up neural messages.

In the PNS it is ganglia but in the CNS it is known as nuclei. Local accumulations of cells in the CNS are known as nuclei (not to get confused with the other meaning of nuclei – control center in most cells. Nuclei are found throughout the CNS, including the Spinal cord and division of the main area of the brain. In addition, there are cortices (cortex-singular) which are flat sheets of nerve cells. 

Tracts in the CNS consist of axons, just like those in the PNS. And those tracts that cross the midline division of the brain are called commissures. The area that contains axons, tracts, and commissures is known as white matter (named so because of its color due to its heavy content of myelin). On the other hand, gray matter is the large clumps of cell bodies and neuropil (click here for more info about neuropil). It is located in the brain and spinal cord.