WeвЂ™ve covered how the CNS and PNS exchange sensory and motor information, and how the PNS collects information from the sensory neurons in its afferent pathways and relays motor commands from the CNS out to the muscles and organs of the body. LetвЂ™s pause and take a look at how our mapвЂ™s doing. Fun fact: interneurons exist in the CNS and in the enteric nervous system (discussed in Part II).Д®fferent neurons carry an impulse from the integration center out to the appropriate muscles. This can happen either directly, as in the case of monosynaptic reflexes, or via interneurons. In the brainstem or posterior horn of the spinal cord, synaptic contact between sensory and motor neurons (integration) occurs. Somatic sensory receptors in the skin, muscles, and/or tendons send information to the spinal cord or brainstem via afferent neurons. The only involuntary movements associated with the somatic motor division are somatic reflexes, such as when you touch something hot and you pull your hand away without thinking about it. If you want to know more about the interactions between neurons and skeletal muscles, check out this post. ItвЂ™s the part of the nervous system we have the most conscious control over, with neural messages traveling from the primary motor cortex in the brain down through the spinal cord to the efferent neurons of the PNS. The key difference between these is that the somatic motor division sends voluntary commands to skeletal muscles. The motor division of the PNS also has visceral and somatic branches. Motor neurons send signals in the opposite direction from sensory ones, transmitting commands from the brain to control the contraction of smooth, skeletal, and cardiac muscles. Sensory neurons keep close tabs on the heart, lungs, stomach, and bladder so that the CNS can manage their regulation. Visceral sensory input comes from (surprise!) the viscera, or internal organs.

These organs transmit information we associate with the five senses. Somatic sensory input comes from the receptors of the eyes, ears, nose, tongue, and skin. There are two main flavors of sensory input received by the PNS: somatic and visceral. They are (in some cases literally) the eyes and ears of the nervous system, collecting information and keeping the CNS informed so it can formulate reactions. Sensory neurons relay information about stimuli such as temperature, pressure, light, pain, and certain chemicals back to the brain. The PNS has two different kinds of pathways: sensory (afferent) and motor (efferent), and weвЂ™re going to add those to the map next.

The officials there issue orders when problems arise.ДЄfferent and Efferent Pathways of the PNS These starships and stations interact with alien lifeforms, distant planets, and weird space anomalies, then send reports back to Starfleet HQ on Earth. You can see this distinction in the image below.Д«ecause IвЂ™m into both metaphors and Star Trek, letвЂ™s think of the CNS as Starfleet Command and the PNS as all the exploratory vessels and research stations scattered throughout the galaxy. ThatвЂ™s it-all the other pieces of the nervous system belong to the PNS. The CNS is composed of the brain and the spinal cord. I placed the CNS a little higher than the PNS because even though the PNS isnвЂ™t part of the CNS, the CNS is technically вЂњin charge.вЂќ The biggest and most basic distinction we make when talking about the nervous system is the one between the Central Nervous System (CNS) and the Peripheral Nervous System (PNS), so thatвЂ™s where weвЂ™re going to start our map. The map weвЂ™re creating is going to help us learn about all of these different pieces of the nervous system, what they do, and how they communicate with each other. It turns out that when we say вЂњtheвЂќ nervous system, weвЂ™re really referring to one entity that has a bunch of different divisions that work together to achieve a common goal (keeping you alive).

Well, if youвЂ™re feeling lost among the branches, never fear! The easiest way to get un-lost is to make a map, and thatвЂ™s what weвЂ™re going to do today. Autonomic, parasympathetic, somatic, peripheral.how many nervous systems can one person have?! Navigating the Nervous System Part I: The CNS, PNS, and Somatic Motor ControlДЄll the terminology that gets thrown around when youвЂ™re studying the nervous system can be a little overwhelming.

The somatic nervous system includes the