Have you ever touched a hot object and without any hesitation removed your hand quickly from the object to avoid burning yourself? The nervous system in your body is doing its job. How does this happen? Well, the nervous system consists of the brain, spinal cord, peripheral nerves and autonomic nerves, which all work synergistically to coordinate movement, thought and sensations that the human being experiences. During this blog I will go in depth with defining the structure and function of the nervous system, and how neurons communicate with one another among the tissues of the body. I will finish with some of the issues that can develop when nerves become damaged or diseased.
The nervous system is the control center of the body. It is the electrical system of the body that communicates with every cell, tissue of the body. All movement is directly controlled by the NS. Heart rate, breathing, blood pressure and body temperature are all regulated by the NS. The master controller of the NS is the brain. The brain coordinates the nerves to react to all situations that the body experiences. The brain coordinates with the spinal cord. The spinal cord will then send the appropriate signal to the area that the brain is wanting to elicit. Nerves are electrochemical in nature. They are divided into four categories.
* Cranial Nerves: Signal action from the (eyes, ears, nose, mouth) to the brain.
* Central Nerves: Connects the brain to the spinal cord
* Peripheral Nerves: Connects the brain to the limbs (arms legs).
* Autonomic Nerves: Connects the brain and spinal cord to the organs of the body
The Central Nervous System consists of the brain, spinal cord, cranial and central nerves. Peripheral nerves are the ones that respond quickly to external stimuli that we touch, i.e; touching a hot object.
The Spinal Cord and Neurons
The Spinal Cord is like a telephone cable that is protected by vertabras, located in the back of the body. The spinal column contains many nerve cell bodies known as grey matter, and axons known as white matter. Both grey matter and white matter run from the brain and throughout the body. The peripheral nerves run in and out of the openings of the vertebras. Inside each vertebra the nerves are divided into dorsal roots and ventral nerves. Dorsal roots are sensory axons and cell bodies. Ventral nerves are motor nerve cell processes. Autonomic nerve cell bodies, on the other hand, are long chain like nerves that run alongside parallel to the spinal cord and inside the vertebra. Their axons exit the spinal nerve sheaths.
A nerve cell is known as a neuron. The brain and spinal cord have billions of nerve cells. The neuron is the master cell in that it gathers and transmits all electrochemical signals in the body. Nerve cells differ from other cells in that they can transmit electricalchemical signals up to several feet. They also communicate to each other. For example, the autonomic nerves in the stomach (afferent and efferent nerves) communicate with each other to send information to the brain to eat and to stop eating.
The Three Basic Parts of A Neuron
Cell Body, Axon, Dendrite
The Cell Body is the main part of the neuron. It contains a nucleus, endoplasmic reticulum and ribosomes and mitochondria. The nucleus is the command center of the cell that tells the organelles what to do. It also contains the DNA. The endoplasmic reticulum and ribosomes help manufacture proteins. And the mitochondria produces Adenosine Triphosphate (the high energy compound of the body). Cell bodies form clusters in the body known as ganglia. Ganglia are located in various parts of the brain and spinal cord. When the cell body dies so does the neuron. This is why spinal cord injuries are so serious because the nerves are damaged to the point of non repair.
Axons: Act as electrochemical transmitters that send messages to other parts of the cell. In some cases the signal is a few feet in length. Think of them as being like an electrical cable that connects to the house to provide electricity. The transformer would act as the Cell Body. Like the electrical cable in incased with a protective sheath, the axon is covered by a myelin sheath. The myelin sheath is made up of essential fat. The sheath helps speed up the transmission along the axon. Most of all myelinated neurons are found in the peripheral nerves. The non-myelinated neurons are found in the brain and spinal cord.
NOTE: When the myelin sheath deteriorates this causes nerve disease. This is known as Multiple Sclerosis. Essential fat is crucial in the diet to help keep the neurons protected.
Dentrites: These are the nerve endings. They are small, branchlike projections of the cell that allow neurons to communicate with other cells. Dentrites can be found on one or both ends of a cell.
The Size of Neurons
Neurons vary in shape and size depending where and what they are doing in the body. The small sensory neurons of the finger have long axons, that are the length of the arm. Where as, neurons in the brain only extend a couple of millimeters. Motor neurons, the ones that are associated to activating muscles, contain a cell body on one end, a long axon in the middle and dendrites on the other end.
Neurons Vary In Function
Sensory nerves transmit information from the peripheral aspect of the body to the central nervous system.
Motor neurons transmit information from the central nervous system to the muscles, skin, glands.
The body is comprised of billions of receptors that sense the environment and what the body is experiencing every second of life. How the receptors perceive the information is sent to the sensory neurons via electrochemical messages. It is at the sensory nerves where the information is encoded. The information is dispersed to the interneuron network connecting to the neurons of the brain and spinal cord.
In the peripheral and autonomic nerve network, the axons are bunched into groups. How they are bunched depends on where they are coming from and going to. These bundles of axons are protected by membranes known as fasciculi. The nerve is supplied with blood vessels that bring in oxygen and help remove waste products. The peripheral nerves travel near major arteries deep within the extremities.
Neural Pathways and Action Potentials: How cells are stimulated.
Depending on the complexity of a particular task, nerve impulses respond to all types of stimuli. There are different types of neural pathways that respond to stimulation. The simplest type of neural pathway is a monosynaptic reflex pathway. This type of pathway is when the nerve fires off a sequence that elicits a response without having to connect to the brain. An example of this type of monosynaptic reflex pathway would be hitting your knee below the patella, or hitting the funny bone in the elbow. The sensory neuron transfers the information to a motor unit of the closest muscle. The muscle then contracts in response. The body has many reflex pathways like this. However as the task becomes more complicated the higher levels of the nervous system are required to elicit a response to the stimuli.
How does the nerve cell transmit electrochemical energy? The answer: It transmit energy by an action potential.
An action potential is a series of chemical reactions that produce a charge in the electrical components of the nerve. How this is done is by depolarization and repolarization cycles. The body is comprised of mostly water. The cell membrane is made up of fats known as phospholipids. The phospholipids are like batteries in that they have a charged electrical head that sits near a water supply and two polar tails that repel water. The cell membrane has a negative and positive polarization factor. What this means is that inside the cell has a negatively charged polarization, where as the outside of the cell has a positive electrical charge. The membrane is designed to keep the electrically charged ions from entering into the cell. The positively charged ions (sodium, potassium, calcium, chloride ions) can only get into the cell by a series of events that allow the proteins to lock into a certain sequence along the receptors of the cell membrane walls. This sequence is much like a lock and key configuration. During this selective permeability the electrically charged particles can enter the cell influences the appropriate organelles to perform their task. The cell then becomes polarized. The polarized cell then gets depolarized. The electrical energy that has entered the cell is then pushed out by a series events that allow the sodium, potassium, pumps to repolarize the cell. The type of cellular response depends on the type of ion that is presented at the gates of the receptors.
Nerve Growth and Regeneration
Nerve cells are the slowest growing of all cells in the body. It can takes months for a nerve to grow one to two centimeters. When nerve cells grow they secrete NGF (nerve growth factor). NGF attracts to already established healthy nerve cells and begin to establish connections. Unfortunately, the nerves in the central nervous system do not act in this manner. When nerves are damaged or injured in the spinal column or brain, the recovery rate and re-growth is poor. In severe damage the re-growth is impossible. It appears the the peripheral and autonomic nerves have a greater re-growth rate. The reason for this is still unknown.
The action potential is what is responsible for the polarization of sodium and potassium ions. The resting cell has a negative charge of about 70-80Mv. When there is enough sodium ion pressure outside of the cell, the positive charge forces the cell to open. Sodium pumps open to allow the sodium to enter the cell. This change in energy depolarizes the cell allowing more sodium to enter forcing an action potential. The cell now becomes positive inside and negative outside. The sodium channels then shut off discontinue sodium uptake. The increased cellular positivity alerts the potassium channels to open. Potassium ions leave the cell. This results in the inside of the cell to become negative and the outside to become positive again, the resting state. This is repolarization. The potassium pumps turn off once the resting values are re-established.
During the action potential once the ions initiate their threshold they cannot be reversed. This is known as the “all or none” response. This mostly occurs in excitable tissue like muscle. Once the cell fires off there is a refractory period. Meaning that that cell is no longer available until it is repaired and repolarized. During weight training the cells are “blown out” during the exercise. As long as the exercise persists the recruitment of other muscles are fired up to keep up with the constant tension. The more intense the exercise the more tissue damage that occurs. Meaning you will need more rest to repair the tissue.
How Nerves Communicate With Each Other
Please Refer to my blog on Synaptic Transmission posted 8/13/10
When nerves get damaged the following can be the cause.
Toxic chemicals can interfere with the sodium and potassium pumps. Ultimately, altering the action potential.
Physical tissue damage (broken bones, muscle tears, etc). can destroy nerves retarding conductivity to that area. Most injuries can be repaired and the nerves can regenerate among the peripheral nerves depending on the severity of the injury. The more serious injuries could result in total loss of the nerves.
The most common nerve damage occurs in the lower back. Many people suffer from the pinching of the sciatic nerve. This is usually the case when the vertebras of the lower lumbar break down causing the bone and cartilage to press against the nerve. The conduction is impaired and the result is usually pain and numbness. The more severe the damage the more the pain and numbness there is.
The nervous system is a remarkeable part of the human body. Without nerves we wouldn’t exist. The nerves are delicate electrical transmitters. Keeping your body healthy and eating healthy will help protect the nerves and keep them functioning for many years. Exercise is a great way to condition the neuromuscular system and to create stronger action potentials among muscle cells. The more muscle cells that you can develop the greater the neural response becomes. The more nerves you can activate the faster you can respond to varying stimuli. If the muscle tissue becomes dormant you begin to lose the ability to react to a stimulus at a fast rate which could lead to injury. One of the goals for weight training should be in developing the sychronization of the muscle cell and the central nervous system. The greater this connection the stronger the body becomes. The stronger the body, the more you can endure and do in regards of physical activity. Also, keeping the nervous system stimulated by exercise can help nourish and oxygenate the brain. The more oxygen and nutrition you can pump through the brain the better the nerves are stimulated possibly reducing the onset some neurological diseases.
I hope that this has provided you a good introduction of how nerves work in the body.
Daryl Conant, M.Ed.
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