Editing Central nervous system (section)

===Spinal cord===
{{main article|Spinal cord}}
[[File:Sobo 1909 615.png|thumb|right|Diagram of the columns and of the course of the fibers in the spinal cord. Sensory synapses occur in the dorsal spinal cord (above in this image), and motor nerves leave through the ventral (as well as lateral) horns of the spinal cord as seen below in the image.]]
[[Image:1508 Autonomic Control of Pupil Size.jpg|thumb|right|Different ways in which the CNS can be activated without engaging the cortex, and making us aware of the actions. The above example shows the process in which the pupil dilates during dim light, activating neurons in the spinal cord. The second example shows the constriction of the pupil as a result of the activation of the Eddinger-Westphal nucleus (a cerebral ganglion).]]

From and to the spinal cord are projections of the peripheral nervous system in the form of [[spinal nerve|spinal]] [[nerve]]s (sometimes segmental nerves<ref name="Moore's" />). The nerves connect the spinal cord to skin, joints, muscles etc. and allow for the transmission of [[Efferent nerve fiber|efferent]] motor as well as [[Afferent nerve fiber|afferent sensory signals]] and stimuli.<ref name="Kandel" /> This allows for voluntary and involuntary motions of muscles, as well as the perception of senses.
All in all 31 spinal nerves project from the brain stem,<ref name="Kandel" /> some forming plexa as they branch out, such as the [[brachial plexus|brachial plexa]], [[sacral plexus|sacral plexa]] etc.<ref name="Moore's" /> Each spinal nerve will carry both sensory and motor signals, but the nerves synapse at different regions of the spinal cord, either from the periphery to sensory relay neurons that relay the information to the CNS or from the CNS to motor neurons, which relay the information out.<ref name="Kandel" />

The spinal cord relays information up to the brain through spinal tracts through the final common pathway<ref name="Kandel" /> to the [[thalamus]] and ultimately to the cortex.
<gallery>
File:1615 Locations Spinal Fiber Tracts.jpg|Schematic image showing the locations of a few tracts of the spinal cord.
File:1507 Short and Long Reflexes.jpg|Reflexes may also occur without engaging more than one neuron of the CNS as in the below example of a short reflex.
</gallery>

====Cranial nerves====
Apart from the spinal cord, there are also peripheral nerves of the PNS that synapse through intermediaries or [[Ganglion|ganglia]] directly on the CNS. These 12 nerves exist in the head and neck region and are called [[cranial nerves]]. Cranial nerves bring information to the CNS to and from the face, as well as to certain muscles (such as the [[trapezius muscle]], which is innervated by [[accessory nerve]]s<ref name="Moore's" /> as well as certain [[cervical spinal nerve]]s).<ref name="Moore's" />

Two pairs of cranial nerves; the [[olfactory nerve]]s and the [[optic nerve]]s<ref name="Purves 2000"/> are often considered structures of the CNS. This is because they do not synapse first on peripheral ganglia, but directly on CNS neurons. The olfactory epithelium is significant in that it consists of CNS tissue expressed in direct contact to the environment, allowing for administration of certain pharmaceuticals and drugs.
<ref name="Gizurarson S. 2012 566–582"/>

{{multiple image
| align       = left
| direction   = horizontal
| total_width = 640
| image1      = Periferal nerve myelination.jpg
| alt1        = Image showing the way Schwann cells myelinate periferal nerves.
| width1      = 822
| height1     = 232
| caption1    =
| image2      = Neuron with oligodendrocyte and myelin sheath.svg
| alt2        = A neuron of the CNS, myelinated by an oligodendrocyte
| width2      = 410
| height2     = 485
| caption2    =
| footer      = A peripheral nerve myelinated by [[Schwann cell]]s (left) and a CNS neuron myelinated by an [[oligodendrocyte]] (right)
}}{{clear left}}