The vertebrate nervous system

The central nervous system (CNS) is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish.
The CNS contains the majority of the nervous system and consists of the brain and the spinal cord. It is contained within the dorsal cavity, with the brain in the cranial cavity, and the spinal cord in the spinal cavity. In vertebrates, the brain is protected by the skull, the spinal cord is protected by the vertebrae, and both are enclosed in the meninges. The brain receives sensory input from the spinal cord as well as from its own nerves (e.g., olfactory and optic nerves). It devotes most of its volume (and computational power) to processing its various sensory inputs and initiating appropriate — and coordinated — motor outputs.
The spinal cord functions primarily in the transmission of neural signals between the brain and the rest of the body. However, it also contains neural circuits that can independently control numerous reflexes and central pattern generators. The spinal cord has three major functions:

1. To serve as a conduit for motor information, which travels down the spinal cord.
2. To serve as a conduit for sensory information, which travels up the spinal cord.
3. To serve as a center for coordinating certain reflexes.

                                  THE REFLEX ARC

A reflex arc is a neural pathway that controls an action reflex. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs.
A reflex is a rapid automatic response to a stimulus. When you accidentally touch a hot object and automatically jerk your hand away, this is a reflex action. It happens without you having to think about it.
Animals automatically blink when an object approaches the eye and cats twist their bodies in the air when falling so they land on their paws. The classic example of reflex reaction is the knee-jerk reaction when the patellar tendon is tapped just below the knee (Figure 1). Swallowing, sneezing, and the constriction of the pupil of the eye in bright light are also all reflex actions.

                                THE SPINAL CORD

The vertebrate spinal cord is divided into different segments running down the dorsal side of the body
At every segment, right and left pairs of spinal nerves (mixed; sensory and motor) form. Each segment of the spinal cord is associated with a pair of ganglia, called dorsal root ganglia, which are situated just outside of the spinal cord. These ganglia contain cell bodies of sensory neurons. Axons of these sensory neurons travel into the spinal cord via the dorsal roots.
Ventral roots consist of axons from motor neurons, which bring information to the periphery from cell bodies within the CNS. Dorsal roots and ventral roots come together and exit the intervertebral foramina as they become spinal nerves.
During early development of the vertebrate embryo, a longitudinal groove on the neural plate gradually deepens as ridges on either side of the groove (the neural folds) become elevated and ultimately meet, transforming the groove into a closed tube. This tube develops into the central nervous system. The hollow center of the tube becomes the central canal of the spinal cord. This canal and the hollow ventricles within the developing brain at the anterior (superior) end of the tube become filled with cerebrospinal fluid.

                        WHITE AND GRAY MATTER

The vertebrate nervous system can also be divided into areas called gray matter and white matter. Gray matter contains a high proportion of cell bodies of neurons. White matter is composed mainly of myelinated axons, and takes its color from the myelin sheathing that surrounds and insulates the axons.
White matter includes all of the peripheral nerves and much of the interior of the brain and spinal cord. Gray matter is found in clusters of neurons in the brain and spinal cord, as well as in cortical layers that line their surfaces.
There is an anatomical convention that a cluster of neurons in the brain or spinal cord is called a nucleus, whereas a cluster of neurons in the periphery is called a ganglion (Figure 3) . The white matter provides tracts or pathways for rapid transmission of neural impulses. Therefore, the white matter provides the common link between the CNS and the peripheral nervous system (PNS).