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Neurons are the cells of the nervous system, which conducts nerve impulses. The human brain and spinal cord are composed of approximately 100 billion neurons. The neuron is responsible for literally every action the human body makes. Without it, intelligent life would not be possible. It utilizes a remarkable design, completing unbelievable tasks in fractions of a second while using fairly simple organelle. An electrical charge is created in the neuron's cell body that turns into a chemical signal, jumps a gap, and is converted into an electric signal again once it reaches the receiving neuron. The neuron is fairly easy to study, it can be dyed to contrast its backdrop, and be viewed for any issues.
There are several different types of neurons throughout the human body. They offer variations in functions, sizes, and mechanisms; however they all have the same basic structure. The vast majority of neurons include a soma, axon, and dendrites. These few structures are vital to a neuron’s function.
The soma is basically the neuron’s cell body. It contains the nucleus, nucleolus, mitochondria, smooth and rough endoplasmic reticulum, ribosomes, and a Golgi apparatus. This is the brain of the neuron. The nucleus is in charge of the synthesis of the chemicals used to transmit messages across the axon. These neurotransmitters are the essential product of the soma. The nucleus also contains proteins, which are responsible for the coloration of the soma. The soma is most frequently found to be within .005mm and .1mm big.
The axon is a long, tubular fiber that transmits the action potentials from a soma to the dendrites or to an effector organ. The axon is responsible for conducting the action potential, an electrical current created in the soma, down to the dendrites. The axon can be extremely short or can even extend up to several feet. For example, the sciatic nerve stretches from the spinal cord all the way down to the end of each toe. Typically, the thinner the axon, the faster the action potential can travel. The axon in a myelinated neuron is covered in myelin sheathes. The axons can be classified by their two different sheaths. It can be either a Schwann cell, or an oligodendrocyte. The difference between the two is fairly simple. A Schwann cell can only cover one axon. This means is this axon is severed regeneration is impossible. This is what leads to various different conditions regarding the nervous system. The oligodendrocytes, on the other hand, can myelinate up to 50 axons, thus if one is severed there is a possibility for it to regenerate. This happens because when the axon is severed, the oligodendroctye “branches out” in search of another oligodendrocyte to attach to.
The axon’s myelin sheath is composed of fatty tissues. It is there to protect the axon from shorting itself out on accident. Because the neurons bundle themselves together to create a nerve the axons are in very close proximity. This could cause a potential issue for an individual neuron, because if one axon fired an action potential down its axon and it got intercepted by another axon it could cause serious problems.
Interestingly enough, the action potential does not stay inside of the myelin sheath and run along the axon. It skips along the tops of the myelin sheath and lands inside a small indentation that exposes a small amount of the axon. This indentation is known as the Node of Ranvier. They are found in myelinated axons. They offer a floor for saltatory conduction. Saltatory conduction happens when the action potential skips across the tops of the Nodes of Ranvier and meets with the axon after each node. This is an extremely efficient method of conduction because the action potential gets a boost every time it skips a node. It also requires much less sodium than the unmyelinated axons do because it does not employ the use of a sodium-potassium pump.
The dendrites are the receivers of the neuron. They are short, fibrous projections surrounding the soma. The dendrite is tipped with many junctions that make connections with other neurons easy. The dendrite branches out in order to increase its surface area. The purpose that they serve is simple: receive the signals and impulses sent by other neurons. They received these signals as a chemical message. This message is called a neurotransmitter. One neurotransmitter commonly found in the human body is Acetylcholinesterase. It is mainly found in the brain and muscles. It is one of the many neurotransmitters responsible for motor functions. It is found in high abundance in the spinal cord.
The mechanisms of the neuron are located inside of the soma. The soma contains the nucleus, nucleolus, mitochondria, smooth and rough endoplasmic reticulum, ribosomes, and a Golgi apparatus. The nucleus and nucleolus are the neuron’s brain. The mitochondria is responsible for producing energy for the neuron. It absorbs glucose and turns it into ATP. The neuron then turns the ATP into ADP, a phosphate, and energy. It utilizes this process for all of its active transport needs. The rough endoplasmic reticulum is responsible for protein synthesis. The smooth endoplasmic reticulum is not involved in protein synthesis, because they are free of ribosomes. They do function to detoxify the neuron and contribute to membrane synthesis. The ribosome aids the rough endoplasmic reticulum in the protein synthesis. The Golgi apparatus is responsible for packaging and transferring items outside of the soma inside of a vesicle.
The main function of a neuron is to transmit an electrochemical signal from one neuron to another or from a neuron to a muscle. This is achieved when the neuron fires an electrical pulse down the axon. This is known as an action potential. An action potential happens when an electrical signal is stored up and eventually released. The axon hillock is responsible for containing the impulse until the charge reaches a predetermined level. This is called a threshold stimulus. A threshold stimulus is created when the axon builds up enough charge to fire one single action potential. If there is not enough charge created the axon hillock will restrict the potential from firing. This is known as a subthreshold stimulus. Once the stimulus is released it travels down the axon and reaches the end of the neuron. At this point the electrical signal propagates itself into a chemical signal in the presynaptic terminal. This chemical signal is the neurotransmitter. At this point the neurotransmitter is transmitted across a small gap called a synapse. It is received at the other end, in the new neuron’s dendrites, across a thin membrane known as the synaptic cleft. The neurotransmitter is then read by the nucleus of the neuron and is propagated into an electrical signal again.
The action potential is conducted along the axon via active transport. This starts with the sodium-potassium pumps. Once the electrical signal reaches a pump it changes the net charge at that specific point on the axon from negative to positive. This causes potassium to be actively pumped into the neuron and sodium to be actively pumped out of the neuron at the same time. This briefly changes the net charge to positive at that specific point on the axon. This process repeats itself the whole way down the axon until it reaches the end of synaptic cleft and goes across a synapse.
- ↑ Nervous System Anatomy Encyclopædia Britannica Online, accessed May 17 2009.
- ↑ Neuron- Structure and FunctionNo Stated Author, Net Industries, accessed May 17, 2009.
- ↑ External Components of a Neuron Athabasca University, Jan. 24, 2006.
- ↑ Axon Definition MedicineNet, accessed May 17, 2009.
- ↑ Acetylcholinesterase (enzyme)Encyclopædia Britannica Online, accessed 17 May. 2009
- ↑ Mitochondria by John Kyrk, Johnkyrk.com, Sept. 21, 2008.
- ↑ Rough and Smooth Endoplasmic Reticulum Brooklyn College, February 20, 2002.
- ↑ The Golgi Apparatus by Kimball, Rcn.com, June 30, 2008.
- ↑ Human Physiology by Gary Ritchison, BIO 301, Sept 25, 2008