copyright 2008 Cheryl K. Hosken, BSN, MS Psych.
All of your behavior from the simple to the complex: every emotion you have ever had, from mild to extreme: all your thoughts, from the trivial to the profound - all of these can be reduced to molecules of chemicals racing in and out of microscopically tiny cells that comprise your nervous system. Regardless of the complexity of a stimulus or behavior, there is a remarkable series of biochemical and physiological reactions that take place in your body.
The neuron is the basic cell of the nervous system, a microscopically small cell that transmits information in the form of impulses from one part of the body to another. Neurons were discovered at the beginning of the 20th century. There are an estimated 100 billion cells in the human brain.
There are three types of basic neurons in your nervous system: sensory neurons, motor neurons, and inter-neurons. Each performs a different function. Most of them have a few structures in common.
Meditate Word By Word On These Verses:
Luke 5:12-16.
The first structure they have in common is the cell body, which is the largest mass of the neuron. It contains the nucleus of the cell in which the genetic information is located. This information keeps the cell functioning. Protruding from the cell are several tentacle-like structures called dendrites and a particularly long structure called the axon. The dendrites reach out to receive impulses from nearby neurons. These impulses are sent to the cell body down the axon to other neurons, muscles, or glands. Some axons are quite long - as much as two or three feet in the spinal cord.
Question:
1. What are the parts of a neuron?
(Select the best answer.)
The cell body, the ganglia and the axioms.
The cell body, the dendrites and the axioms.
The cell body, the dendrites and the axons.
On some neurons there is a covering of a white substance that is composed of fat and protein called myelin. About one half of the adult's nervous system has myleinated axons. Myelin covers the axon in segments rather than in a continuous coating. Between each segment of myelin is an unmyelinated segment called a node. It is the absence or presence of myelin that allows us to distinguish between the gray matter (dendrites, cells, and unmyleinated axons) and the white matter (myelinated axons) in the nervous system tissue. We tend to find myelin on axons that carry impulses for relatively long distances. Neurons that carry impulses up and down the spinal cord have myelinated axons, whereas those that carry impulses back and forth across the spinal cord do not.
Myelin does several things. It acts as an insulator, separating the activity of one neuron from those nearby. It protects the long, delicate axon. it speeds the impulses along the length of the axon. Myelinated axons carry nerve impulses ten times faster than unmyelinated ones - up to 120 meters per second. Myelin is found only in vertebrate animals.
Question:
2. What are the functions of myelin?
Axon terminals are located at the ends of axons. They are a series of branching bare end points. At the axon terminal, the impulse is sent on to other neurons.
Virtually no neurons are generated after birth. We are born with twice as many as we will ever need. Those that are not strengthened by experience eventually die off. Bryan Kolb gives us an analogy of how neurons die off. He states that the normal brain is "constructed" in a manner rather like that in which a statue is chipped away from a block of granite. Rather than building up the finished project in one small step at a time, more material is than one needs is available. What is needed is used, and the rest dies away. For example, a young infant (six to ten months) has neural circuitry that allows him or her to distinguish between most human speech sounds. Experience with his native language strengthens those neurons that are important to the native language. Those that are not used atrophy and die off. The result is that an infant that could discriminate between speech sounds from his language and other languages can no longer do so at 10-12 months of age.
While it is true that some neurons die off, the interconnections between the neurons become more numerous and complex as the brain develops. Here is an observation: To have 100 billion neurons at birth, our brain cells must be generated at a rate of about 250,000 per minute while that brain is being formed. There are implications for prenatal care here. For example, a mother who drinks alcohol while pregnant concentrates the alcohol in the infant's gray matter of the brain. The unborn baby cannot eliminate alcohol as fast as the mother can, therefore the damage done by alcohol is greater. The baby's fine motor skills are affected negatively by the mother's alcohol use. Fine motor skills are those used for playing a musical instrument, working with small machinery, writing, and other such skills. The functions of the lost neurons can be taken over by those that remain.
From one cell to another: the Synapse
In the nervous system, there are billions of points where neurons interface with one another. The location where an impulse is passed from one neuron to another is called a synapse. At the synapses, the neurons do not physically touch one another. Instead, there is a microscopic gap between the axon terminal of one neuron and another neuron. This gap is called the synaptic cleft.
Synaptic Transmission
At the end of the axon are many branches called axon terminals. Throughout any neuron, but concentrated in the axon terminals are incredibly small containers called vesicles. Vesicles hold chemicals called neurotransmitters. Neurotransmitters are chemical molecules that excite or inhibit the transmission of an impulse in the next neuron after crossing the synapse. When an impulse reaches the axon terminal, the vesicles near the neural membrane burst open and release the neurotransmitter they have been holding. The released neurotransmitter floods out into the synaptic cleft - the tiny space between the two neurons. Once in the synaptic cleft, some neurotransmitter molecules move into the membrane where they fit into receptor sites, which are special places on a neuron where neurotransmitters can be received. Think of a neurotransmitter as being a key that fits a particular lock on a receptor site. Once the key fits into the lock, the neurotransmitter enters the membrane of the next neuron.
Question:
3. What is a synapse?
(One or more of the following answers may be correct.)
A vesicle containing a neurotransmitter.
One of billions of points where neurons interface with one another.
A microscopic gap between the axon terminal of one neuron and another neuron.
The membrane where neurotransmitters fit into receptor sites.
Then what happens? A number of things occur. The most reasonable scenario for synaptic activity is that where the neurotransmitters float across the synaptic cleft, enter the receptor sites of the next neuron in a chain of nerve cells, excite that neuron to release its action potential, and fire a new impulse down its axon terminals. These new neurotransmitter chemicals are released from the vesicles, which cross the synaptic cleft and stimulate the next neuron in sequence. These neurotransmitters are referred to as excitatory. As it happens, there are also many neurons throughout our nervous systems that hold neurotransmitters that have the opposite effect. They prevent the next nerve from firing. These are called inhibitory neurotransmitters.
In this way, the process we have just described causes muscle cells to contract or a gland to secrete a hormone into the body.
Neurotransmitters
To understand how neurotransmitters work, we must examine the two components that make a synapse work: the neurotransmitter and the receptor site. Not long ago, it was believed that neurons produced and released one of two neurotransmitters. They either excited further impulses or inhibited impulses. Now we know that this view is too simple. We know now that there are at least 60 neurotransmitters, and there are more to be discovered. There was a view that each neurotransmitter had one receptor site. However, we know now that there are over 1,000 receptors for each neurotransmitter. However, thus far, only 60 neurotransmitters have been identified. To make matters more complicated, a single neurotransmitter may have more than one receptor site. So there are subtypes of receptor sites for various brain chemicals.
The biochemistry of the brain is wonderfully complex. The discovery of neurotransmitters and receptor sites opened new areas of research for the brain. More neurotransmitters, receptor sites, and special sites indicate a potential for clearer and more complex communication within the nervous system. For example, the hypothalamus receives separate signals for two different emotions of fear and surprise. If there were only one neurotransmitter and receptor site, the different emotions could not be recognized. More neurotransmitters and receptor sites allow the independent signals to be sorted, separated, and more clearly recognized.
Question:
4. How is the neural impulse transmitted from neuron to neuron?
There are probably over 100 different neurotransmitters. Here are the functions of four neurotransmitters:
Acetylcholine: It is found throughout the whole nervous system and can act as an excitatory or inhibitory neurotransmitter. It was the first to be discovered. It works in synapses between neurons and muscles tissue cells. One form of food poisoning, botulism, blocks the release of acetylcholine at the neuron/muscles cell synapses that causes paralysis and death. Other poisons have just the opposite effect. The venom of one spider has this opposite effect causing excessive amounts of acetylcholine to be released that results in muscle spasms that are severe enough to be deadly. Nicotine is a chemical that in small amounts tends to increase the functioning of acetylcholine, but large doses of nicotine can overcome the action of acetylcholine and cause death. The effect of smoking or chewing tobacco is not great to cause such changes. If nicotine is taken in large amounts, it stimulates the brain center that controls vomiting before too much has been absorbed into the body. Acetylcholine affects normal memory function and therefore the disease of Alzheimer's is studied in conjunction with acetylcholine levels in the brain.
Norepinephrine: This transmitter is involved in mood regulation. It is involved in physiological reactions that regulate levels of emotional arousal, such as increased heart rate, perspiration and blood pressure. When there is an increased amount of norepinephrine in the spinal cord or brain, feelings of agitation, increased physical arousal, or anxiety may result. The addictive drug cocaine increases the release of norepinephrine leading to a state of agitation and "high" mood. Too little norepinephrine in the brain is associated with depressed feelings.
Dopamine: This is also a common neurotransmitter involved in mood regulation. It is involved in a wide range of reactions. Either too much or too little with in the nervous system seems to produce a number of effects, depending primarily on which system of nerve fibers in the brain is involved. Dopamine has been associated with the thought and mood disturbances of some psychological disorders. It is also associated with impaired movement. When there is not enough dopamine, there is difficulty in voluntary movement; too much and we note involuntary tremors. Those persons who suffer from Parkinson's syndrome are prescribed various doses of dopamine to control symptoms.
Endorphins: (there are several of them) are natural pain suppressors. The pain threshold - the ability to tolerate different levels of pain - is a function of endorphin production. With excess levels of endorphins, we feel little pain: a deficit results in increased experience of pain. When we are under extreme stress, endorphin levels rise. Many long distance runners often report an euphoric high after they run great distances, as though the endorphins have kicked in to protect them against the pain of physical exhaustion.
Question:
5. Which of the 4 neurotransmitters discussed in this lecture affect mood regulation? What do they do?
These are but a small number of neurotransmitters that work in our brains and bodies. They are the agents that either excite or inhibit the transmission of neural impulses throughout the nervous system. That excitation or inhibition can have a considerable effect on our thoughts, feelings, and behavior.
Question:
6. How many neurotransmitters are there and how do they work?
This has been a simplified description of what we know. Neural impulse transmission is seldom a matter of one neuron stimulating another neuron in a chain reaction. Any neuron can have hundreds or thousands of axon terminals and synapses, and has the potential for exciting or inhibiting (or being excited or inhibited by) many other neurons.
Question:
7. How many axon terminal sites are on a neuron for the transmission of an impulse?
(Only one of the following answers is correct.)
Four.
Sixty.
Hundreds or thousands.