In this term paper we will discuss about: How does the nervous system work? What are the parts of the nervous system and how do they work with the brain itself?
The body has two main messenger systems that allow the brain to send instructions to the various organs, glands and muscles throughout the body. While hormones flow through the bloodstream to reach the individual cells to transmit the necessary instructions for a variety of tasks, there is another method available to the brain for transmit commands. That secondary option is the nervous system.
The most important part of the nervous system is the brain itself. This is the powerhouse of the whole nervous system and the control center of the whole body. The nervous system itself has both voluntary and involuntary actions. One of the primary parts is the central nervous system (CNS).
The CNS is made of the combination of the brain plus spinal cord. A peripheral nervous system (PNS) is primarily made up of nerves that course throughout the body. These nerves are enclosed in bundles of long fibers, which are known as axons.
These axons essentially provide a connection from the CNS to our whole bodies and all our organs. While messages must be transmitted from the brain to the body, the body needs a way to respond. Therefore, the nerves transmitting the brain’s messages are called motor or efferent nerves. The nerves that return messages to the brain are called sensory nerves. Most of the bundles of nerves actually perform both functions, thus the term mixed nerves.
Now the PNS is also divided into three parts, the somatic, autonomic and the enteric. Somatic are in charge of voluntary movement. The automatic deals with the involuntary aspects of the nervous system. It can also be separated again into the sympathetic and the parasympathetic.
Sympathetic nervous system is particularly in high gear during emergencies to mobilize the body to get energized. The parasympathetic nervous system is active during the relaxed state.
The combination of these two systems allows for the individual’s flight and fight responses, in addition to many other functions that happen throughout the day. Consider this part of the nervous system as the background application that is always running behind the scenes, but with the ability to notify the main system when necessary.
Finally, the enteric nervous system is in charge of the gastrointestinal system. This also functions involuntarily as it controls digestion and the movement of food through that system. But to truly understand how this system functions throughout our bodies, it is important to learn about the nervous system from the cellular level up.
Let’s start with the types of cells often found throughout the nervous system.
Cells:
The nervous system contains two types of cells, the glial and neurons. The neuron’s fundamental property is how they communicate with other cells using synapses. These are membrane to membrane junctions that allow for rapid transmission of signals through electrical or chemical means on the molecular level. Neurons may also include an axon, which allows for thousands of potential synaptic contacts.
Before you start to think that neurons are cut from the same cloth, you need to understand that they exist in a large variety with different functions. Some of these neurons are sensory neurons that take physical stimuli into neural signals. Another type of neuron is the motor neuron, which transmits neural signals into action, either by the muscles or the glands themselves.
The glial cells provide support and nutrition, form myelin, work to maintain homeostasis, and participate in signal transmission within the nervous system. For example, within a human brain, it is estimated that the total number of neurons is roughly equal to the number of glial cells.
When it comes to describing the job of the glial cells, the best way to explain it is that they are the support staff for the neurons. Not only do they work to provide nutrition and keep the neurons in place, they also assist in fighting pathogens that might attack the neurons, as well as cleaning out the dead neurons.
However, the most important function of the glial cell is the creation of a fatty substance called myelin. This substance wraps around axons to provide electrical insulation to allow for rapid transmission of action signals. When myelin is damaged, it is not necessarily able to regrow, so the damage can be permanent.
There are diseases that actually strip myelin away from neurons, trapping an individual in their own head. Treatments continue to be researched to find ways to fix this problem for those with specific genetic disordered.
Now let’s move up to the chain to have a better understanding of the central nervous system (CNS) and peripheral nervous system, known as the PNS.
Central Nervous System and Peripheral Nervous System:
The CNS includes the spinal cord as well as the brain. The spinal canal is the home of the spinal cord, while the brain is housed with the cranial cavity. The protection for the CNS, the body has the meninges, which is a three-layered system of membranes.
One of these layers is the outer layer known as the Dura mater. The skull also protected by the skull, made of a hard and strong bone material. The spinal cord also has protection in the form of vertebrae, another type of strong bone. This is the part of the nervous system that most of us are familiar with.
The PNS describes all the other aspects of the nervous system that are not included in the CNS. Axon bundles are considered within the PNS, even though there are some axon bundles that can be found within various parts of the CNS, including the brain and spinal cord.
The PNS can be divided down even further, into two parts known as the somatic and visceral. The somatic includes nerves associated with the joints, skin and muscles. Dorsal root ganglia are considered the cell bodies of somatic sensory neurons. The visceral is often referred to as the autonomic nervous system. It contains neurons that deal with the internal organs, glands and blood vessels.
Now the visceral can be divided again, into two more parts. When mapping this out, it can sometimes appear similar to a long family tree. The two parts of the visceral are the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system is in charge of the body’s fight or flight response. However, when it does not need to fire up our defense mechanisms, the sympathetic nervous system is active in maintaining the body’s homeostasis.
The parasympathetic system compliments the sympathetic by stimulating the feed and breed or rest and digest responses within the body. Thus, digestion and other internal processes happen automatically without our conscious thought to keep it going. Breathing is another activity where the body depends on the automatic systems to keep it going, especially as we sleep or our conscious becomes involved in other activities.
But while a complex nervous system, most often found in creatures with vertebrae, there are other creatures that demonstrate variations of the nervous system. Scientists often refer to them as examples of simpler neural connections or alternatives to a complete nervous system. We will refer to them as neural precursors.
Neural Precursors:
When you look at a sponge, you do not see the typical indications of neural pathways. In fact, the sponge has been classified as having no nervous system at all. The typical synaptic junctions are missing, thus there are no neurons within these creatures. However, messages must be sent throughout a sponge’s body. So how is it done?
Sponges make groups of proteins that cluster together. When completed, the structure closely resembles a postsynaptic density, which is similar to the receiving end of a synapse. Yet the sponge’s use of this particular protein cluster remains unclear. What is known is that sponges currently communicate using calcium waves and other types of impulses. This allows for simple actions, such as the complete contraction of the body.
However, let’s face it. Sponges aren’t really going many places. Their job is filter ocean water, thus feeding themselves and cleaning the ocean at the same time. But within the ocean are more complex creatures that also have examples of neural precursors.
One such example are jellyfish. These creatures have a diffused nerve net, instead of a central nervous system. The nerve net is typically spread throughout the body in an even fashion. Made of sensory neurons that are sensitive to visual, tactile and chemical signals; motor neurons that activate body wall contractions; and intermediate neurons that provide the communication between the other two.
While this is a fairly unstructured nervous system, it clearly demonstrates that synapses occur in other creatures. However, the complexity of a human nervous system is unique due to the variety of messages that are constantly in motion through it. Messages on such a significant number of topics, from eating to comprehending information from the various senses and then acting on it.
So how does the nervous system manage all of this incoming and outgoing information? The answer is in a term called Bilateria.
Bilateria:
For a vast majority of animals with a nervous system, there is a division between their left and right sides. Simply put, the right and left side are almost mirror images of each other. Each side had its own cord or ganglion for each side, with the largest section of the ganglion being up front and often referred to as the brain. Our nervous system follows a similar pattern.
The spinal cord has segmented ganglia throughout the body that give each section of the body access to motor and sensory nerves. These segments feed into the main trunk and send messages to the brain. Depending on the creature, the main nerve cord will be either on the bottom or top of the body.
There are a variety of species, each with their own variations of this type of nervous system. The more complex the nervous system, typically the better developed sensory organs. For many arthropods, which is a group defined by insects and crustaceans, this means they have compound eyes and antenna. With these advanced sensory organs, they are able to process a variety of information from the world around them. We see this with flies all the time, as they quickly move at any sign of danger due to the abilities found in their compound eyes.
Nervous systems can be more or less complex, depending on the creature and the complexity of its organ structure. For instance, a jellyfish’s neural pathways are much less complex than the one found in a higher functioning mammal, including humans. But when we come down to it, the nervous system has some pretty basic functions.
The Functions of the Nervous System:
When we analyze it, this system is a major source of communication within the body. At its most basic level, it is a system that involves sending signals from one cell to another, creating chains that allow for messages to be transmitted from one area of the body to another.
What makes this stand out from the hormone type of messaging is its point to point signaling process. Think of it this way. When the government wants to get a message out to a large group of the population, they use a broadcast system. This spreads the message through multiple forums, but there is no guarantee that everyone will hear the message.
However, when someone receives a phone call, the message is clearly directed at them or targeted. The result is that there is confirmation the message was received. Our nervous system works much the same way. While hormones provide more of a broadcast type of delivery, the nervous system provides much more direct messages to specific targets throughout the body.
Not only is it targeted, but it moves much faster than a hormone message. Scientists have found that the fastest nerve signal will move through the body at speeds exceeding 100 meters in a single second. This is faster than just about any other type of movement. As a result, the body is able to quickly make changes and adjustments throughout our entire system in real time with minimal delay.
Yet it is so much more than a speedy message system. This is meant to control the body, both by acquiring information from the surrounding environment, but also then processing the information to find the appropriate response.
Then messages are sent to various parts of the body to facilitate the response action. What is amazing about this process is that it happens so quickly. Most of us do not even think twice about how much work goes into the brain’s translation of the input from our eyes, ears and touch.
Signals are sent via the axons in either a chemical or electrical signal. While electrical synapses make direct and specific connections, chemical synapses are the more common type and are also more diverse in their abilities.
On the molecular level, these cells have specific receptors and transmitters that facilitate all of the signals that flash throughout our bodies every second. But while all this is fascinating, there is one aspect of the nervous system we have not truly explored. Yet it deserves a special discussion because of how critical it is. Yes, we are talking about the control center of the nervous system and our bodies as a whole, the brain!