Neurons and Glia: Structure and Function

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       The structure of a neuron is perfectly designed for its many functions. First a neuron collects information at the dendrites (in the form of excitatory or inhibitory neurotransmitters). Then it summates that information (adds up the positives and negatives to decide to fire or not to fire). If the decision is to fire it does so by deporalizing down its axon, at which point neurotransmitters are released to the next cell and the process occurs all over again. This all happens in a few milliseconds.


       There are two types of cells in the nervous system--neurons and glia. The term glia is derived from a Greek word meaning glue, and the glia cells were originally thought to be the cells that held the other neurons together. We know that glia serve many support functions for the neurons such as building myelin sheaths, removing waste material when neurons die, guiding migration of neurons as they grow, and exchanging chemicals with adjacent neurons. I like to think of glia as the Glee Club of the nervous system because of all the support functions they provide. Glial cells are ten times more numerous than neurons, but they are also much smaller.
       Neurons are the cells that send and receive information and they do so by conducting electrochemical impulses. There are three major components of a neuron, the soma (cell body), the axon and the dendrites. These three components allow the neuron to receive, integrate and transmit information efficiently.
       The soma is filled with cytoplasm which is the jellylike fluid within the cell membrane. The soma contains the nucleus as well as other structures. Inside the nucleus are chromosomes which are made of DNA. The DNA controls almost all the aspects of the cell’s metabolism by coding for the production of proteins. Each cell of an organism has the same genetic information, but each cell is different depending on which part of the genetic information is used. Different cells use different parts of their genetic material to make different proteins. The nucleus is surrounded by a nuclear membrane which controls the transport of cytoplasmic material into and out of the nucleus.
       Also found in the cytoplasm of the soma are mitochondia (sites of metabolic activity that provide energy for the cell) and the endoplasmic reticulum (passageways through the cell that move proteins and other substances). Ribosomes are the site of protein synthesis in the cell. The Golgi apparatus found in some cells is similar to endoplasmic reticulum and serves to concentrate proteins by removing water, then it wraps the proteins which then leaves the Golgi apparatus as vacuoles. Vacuoles migrate to the cell membrane.
       Dendrites, derived from the Greek word for tree, branch out from the soma to collect incoming information. The information is then channeled towards the soma to produce the action potential. As the dendrites radiate out from the cell they dramatically increase the surface area for gathering incoming information. Some types of neurons (examples are Purkinje cells in the cerebellum, pyramidal cells and spiny cells) are covered with spines. These dendritic spines further expand the area of the dendrites and contain the receptors where neurotransmitters are received from other cells. Dendrites can receive information from up to 1,000 adjoining neurons.
       Although a neuron can have many dendrites it has only one axon which extends away from the cell body to transmit messages to other neurons. The axon is like a fluid filled tube and this is where the action potential occurs. Axons may be microscopically tiny if they are in the brain, or several feet long if they extend down the length of the spinal cord. Many axons are encased by a myelin sheath, a fatty white substance that provides insulation that speeds up the transmission of electrical signals along the axon. The axon may branch to form a number of collaterals that reach out to other neurons. At the end of each axon terminal is a small swelling called a synaptic knob which almost--but never quite--touches the next neuron. The synaptic vesicle is a small container inside the axon terminal that holds the neurotransmitter. At the end of action potential, in a process called exocytosis, the vesicles merge with the membrane and spill the neurotransmitter into the synaptic cleft.

       The synapse, a gap approximately 20 mm wide, separates neurons. Neurotransmitters diffuse across the synaptic cleft and hook up with receptors on the postsynaptic cell.

What determines whether a stimulus will produce a nerve impulse in a neuron?

First lets review some key concepts. When a neuron is polarized a state of opposition exists and it is ready to fire. We call this resting potential. As a neuron’s membrane depolarizes it allows more positive ions in and is close to firing (action potential). Depolarization opens the sodium gates (voltage activated open) and action potential occurs. The neuron then hyperpolarizes (increases the opposition) and becomes harder to fire. This is known as the refractory period. Hyperpolarization can occur when the interior of the neuron becomes more negative (-70 mv to -90 mv), usually due to inhibitory neurotransmitters. Hyperpolarization also occurs when the interior goes from a negative state, passes zero and becomes slightly more positive (+20 mv). This occurs after action potential and is considered the refractory period .

       Not every stimulus that is detected by a neuron causes that neuron to fire or reach action potential. A stimulus (or combination of stimuli) must be strong enough to cause a neuron to reach threshold. Threshold can be defined as the minimal voltage necessary to cause the neuron to depolarize to the point of action potential.

     Sub-threshold stimuli are referred to as Excitatory Post Synaptic Potentials (EPSPs) when they depolarize the membrane (allowing positive ions in) and are called Inhibitory Post Synaptic Potentials (IPSPs) when they hyperpolarize the membrane (keeping positive ions out or holding positive K+ ions in). The EPSPs and IPSPs summate (add up) to determine whether or not the neuron fires. Once a stimulus (or group of stimuli) reaches threshold, the size and the strength of the resulting action potential are the same. This is known as the all-or-none principle. In other words a neuron either fires all the way or it doesn’t fire at all. Action potentials are the same strength not matter what the strength nor the number of the stimuli that caused them.

       When a neuron becomes stimulated, the area of the membrane at the point of stimulation becomes more permeable to Na+ (it is depolarized). This depolarization voltage activates the Na+ gates at the nodes of Ranvier to open. Na+ rushes inside the cell. At the same time the K+ gates, which were partially open allowing K+ to pass freely all along, open wider and K+ rushes out. This rapid exchange of ions reverses the electrical gradient because the influx of positive ions cancels out the negative interior causing the valence to reach zero. Then the influx of Na+ takes the interior to a slightly more positive state (+20 mv). This is the hyperpolarized state. (Remember, hyperpolarization means making it harder to fire, either by increasing the negative interior or going past zero to a slightly positive interior.) This hyperpolarization (known as the refractory period) is only momentary as the rapid outflux of K+ returns the interior to a negative state. The sodium/potassium pump is also working to re-establish the resting potential. In other words the outflux of K+ combined with the action of the sodium/potassium pump restores the electrical and concentration gradients. Within milliseconds the polarity is re-established and the neuron is once again ready to fire.