Index
To fulfill the function of conducting electrical signals, it is made up of neurons and glial cells .
Neurons
There are two broad classes of cells in the nervous system: neurons, which process information, and glia, which provide neurons with mechanical and metabolic support. Three general categories of neurons are commonly recognized.
Receptors are highly specialized neurons that act to encode sensory information. For example, photoreceptors in the eye transform variations in light intensity into electrical and chemical signals that can be read by other nerve cells.
It is the receptor cells that begin the process of sensation and perception. Interneurons form the second category of nerve cells. These cells receive signals and send signals to other nerve cells.
Interneurons serve to process information in many different ways and make up the majority of the human nervous system.
Effectors or motor neurons are the third class of neurons. These cells send signals to the muscles and glands of the body, therefore, they directly control the behavior of the body.
In all neurons, the cell membrane separates the interior of the cell from the surrounding fluids. This outer membrane is critical to the information processing functions of the neuron.
Today, the cell membrane is known to be a highly complex and specialized molecular machine that performs a wide variety of functions in cell function. Furthermore, the membrane has different properties in different specialized functional regions of the neuron.
A typical neuron can be divided into three distinct parts: its cell body, dendrites, and axon. The cell body, or soma, contains the nucleus of the cell and its associated intracellular structures.
Dendrites are specialized extensions of the cell body. They work to obtain information from other cells and carry that information to the body of the cell. Many neurons also have an axon, which carries information from the soma to other cells, but many small cells do not.
Axons end in terminal feet, or terminal buttons (buttons), which transmit information to the receiving cell. Dendrites and axons, both extensions of the cell body, are also known as processes.
The point of communication between one neuron and another is called a synapse. Synapses are generally directional in function, with activity at the end of the sending cell (presynaptic cell) that affects the behavior of the receiving cell (postsynaptic cell).
In most neurons, the postsynaptic membrane is usually in the cell body or in dendrites, but synapses also occur between axons.
Most neurons have several dendrites and an axon. Due to their multiple processes, these are called multipolar neurons. Simpler unipolar (one process) and bipolar (two processes) neurons are much less common in vertebrates than in invertebrate nervous systems.
A primary function of neurons is to process information and integrate the influences of the cells from which they receive information.
In the human brain, it is not unusual for a single neuron to receive information at 20,000 or 30,000 synapses. Therefore, the information-processing functions of neurons in the brain can be quite complex.
It is often helpful to distinguish between cell types based on their appearance, as shape can provide clues to function. Perhaps the most important distinction is between neurons with and neurons without long axons.
Long axon cells, called major neurons, transmit information over long distances from one region of the brain to another. Major neurons provide the communication pathways within the nervous system.
In contrast, local circuit neurons, lacking long axons, must exert all their effects in the local region of their cell bodies and dendrites. They are located in areas of the brain served by the major long axon neurons and act to affect activity in these pathways.
Local circuit neurons perform integrative and modulatory functions in local brain regions. The size and shape of neurons are often related. The main neurons, with their long axons, generally have large cell bodies.
In part, this is because the axon depends on the cell body for metabolic energy and for the proteins it needs to function and maintain itself. Furthermore, cells with large dendritic trees also tend to have large cell bodies.
In contrast, local circuit neurons, with their short dendrites and small axons (when present), generally have small, compact cell bodies.
Dendritas
Dendrites can be thought of as a continuation of the cell body membrane, and that this receptive sensory surface penetrates the surrounding nervous tissue. It is not surprising to find that the dendritic branching pattern differs widely between cells and reflects the functions that the cell performs.
In some cases, the functional properties of a neuron can be fully predicted from its dendritic spread pattern. Dendrites, with their thin, branched, and arborescent shapes, greatly increase the opportunity for synaptic connections in brain tissue.
Electron microscopy confirms the concept of dendrites as extensions of the cell body. The same types of intracellular substructures that characterize the cell body of a neuron are also present in dendrites.
Many types of neurons have dendrites with a special form of synaptic connection, dendritic spines. These are small (1-2 um), spiny protrusions of the dendrite that form the postsynaptic element of most synapses in the brain.
Dendritic spines appear to reach out to contact nearby axons. The pattern of the dendritic spines changes throughout the dendrite. Near the cell body, the spines are usually small, and the relatively simple enlargements protrude slightly from the side of the dendrite.
At greater distances, the spines get larger and more elaborate. The spines emerge from the dendrite and expand, sometimes dividing into a double column with multiple synapses.
At the very least, the spines increase the synaptic surface of the dendrite, allowing a maximum of synaptic content with a minimum of dendritic volume.
About 80 percent of all excitatory synapses (those that act to evoke activity in the postsynaptic cell) are found on dendritic spines; the rest involve other parts of the dendrite.
In contrast, less than a third of all inhibitory synapses involve spines, and when they do, they bind to an excitatory synapse in the same spinal column. The specific reasons for this agreement are a matter of growing interest.
It has also been suggested that dendritic spines are modifiable structures that can change with learning and other factors. Whatever their function, dendritic spines are an important anatomical feature of many classes of neurons in the human nervous system.
The cell body
The cell body integrates the synaptic input and determines the message that the axon must transmit to other cells, but that is not its only function. The cell body is also responsible for a variety of complex biochemical processes.
For example, the cell body contains the metabolic machinery necessary to transform glucose into high-energy compounds that meet the energy needs of other parts of the neuron.
Additionally, highly active proteins that serve as chemical messengers between cells are manufactured and packaged in the cell body. The cell body contains a series of smaller specialized substructures, called organelles, or small organs, that carry out many of the cell’s functions.
Mitochondria
Providing metabolic energy to the cell in a form that can be easily used is a major role of the mitochondria. These organelles have their own outer membrane that encloses a folded inner membrane.
The main source of energy for the nervous system is glucose from sugar, which is derived from carbohydrate foods.
Mitochondria contain the enzymes necessary to transform glucose into high-energy compounds, primarily adenosine triphosphate (ATP). The ATP molecules can then be transported to other regions of the cell where their energy is used.
Core
The manufacture of neuronal active compounds and other large protein molecules within the cell body is more complex. The protein synthesis process begins in the nucleus of the cell.
The nucleus of a neuron is separated from the intracellular fluid and other organelles of the cell, which contains the genetic information that guides cell function. The genetic template is stored as encoded strands of deoxyribonucleic acid (DNA).
Each DNA molecule contains the genetic codes for all cells in the body; only a selected part of this genetic blueprint is used by nerve cells.
The nucleus begins the process of building protein molecules by transcribing the relevant portion of the DNA code into a complementary ribonucleic acid (RNA) molecule. The nucleus then releases RNA molecules into the surrounding intracellular fluid, where the protein synthesis process actually takes place.
Nucleolo
The nucleolus is a separate structure within the nucleus, which is also involved in the process of protein synthesis. However, the nucleolus does not make proteins directly. Instead, it builds molecular complexes, called ribosomes, that are involved in protein synthesis.
Ribosomes are complexes of RNA and proteins that are expelled from the nucleolus and nucleus into the cell body, where they do their work.
Endoplasmic reticulum apparatus and Golgi apparatus
Two other organelles are mainly responsible for the cellular manufacture of proteins, the endoplasmic reticulum and the Golgi apparatus. Together, they form a miniature manufacturing and packaging plant. The endoplasmic reticulum is a system of tubes, vesicles, and sacs built from membranes similar to those that surround the neuron.
The rough endoplasmic reticulum is the initial segment of structure that begins to form protein molecules; It gains its rough appearance from the presence of a large number of ribosomes attached to its surface.
Rough endoplasmic reticulum ribosomes build large segments of protein molecules in the sequence of steps prescribed by RNA released by the cell nucleus.
These segments of the protein molecule move down through the endoplasmic reticulum in a manner similar to a product that is assembled on an industrial assembly line. When completed, the segments are released into the smooth endoplasmic reticulum, which lacks ribosomes, and is transported by it to the Golgi apparatus.
The Golgi apparatus, named after Camillo Golgi, is a complex of membranes that completes the assembly of the protein and encloses the resulting molecules in its own membrane for release into the cell.
It is important that proteins are packaged in this way because they have a strong effect on neuronal function. When encased in a constructed sphere of membrane, a vesicle, proteins can safely move to the part of the cell where they will eventually be used.
For example, the neurotransmitters released by a cell at a synapse are manufactured by the endoplasmic reticulum and the Golgi apparatus in the cell body, locked in a vesicle, and then transported along the axon to the synapse where they will eventually be used. .
Axon
The axon of a neuron arises from the cell body and extends into the synaptic contact region or regions. Axons are specialized processes that are characterized by having an excitable membrane, a membrane that is capable of generating or propagating an action potential.
An action potential is a distinctive length of the axon. Cells generally have only one axon, but they can emit collaterals or branches to carry the action potential to more than one region of the brain.
A Golgi stain of a single neuron located in the brain stem releases numerous collaterals and thus affects activity in many areas of the brain. However, this degree of branching is far from typical. Most cells with prominent axons have far fewer collaterals, if any.
Axón loma
The axon emerges from the cell body in a conical cone of membrane that forms the axon mound.
This structure is very different from the rest of the cell body when examined microscopically: it is completely devoid of the ribosomes and endoplasmic reticulum that characterize the rest of the cell body and the neighboring portions of dendrites.
Instead, there are numerous microtubules and microfilaments, which form the basis of a transport system for the axon, which aids in the movement of substances from the cell body to the end.
Terminal feet
As an axon approaches its synaptic targets, it often branches into several smaller processes, each of which ends at one end. Within each of the terminal feet are both mitochondria and synaptic vesicles.
Synaptic vesicles contain neurotransmitter substances, which are released into the space between the presynaptic membrane of the foot of the foot and the postsynaptic membrane of the receptor cell. The space between the presynaptic and postsynaptic membranes is called the synaptic cleft.
Cell membrane
The membrane that separates the neuron from other cells and from the extracellular fluid is extremely important in understanding neuronal function. All the information received by a neuron must enter through this membrane; all the messages that a neuron can send to other cells must also go through it.
Much has been learned about the cell membrane, particularly neuronal membranes, in the last two decades. The neuronal membrane is a complex molecular machine with several important adaptations that perform specific information-processing functions for the cell.
The neural membrane is a very old invention in evolution, which was so successful that it was not modified in the nervous systems of invertebrates and vertebrates. Its main structural components are phospholipids, or fatty acids.
These long, thin molecules have read that it is hydrophilic, or “water loving,” and a tail that it is hydrophobic, or “hating water.” When phospholipids are dissolved in a suitable agent (such as benzene) and a few drops are placed on a surface of water, a remarkable biochemical self-organizing effect occurs.
Each molecule orients itself with its hydrophilic head on the surface of the water and its hydrophobic tail extends from the water into the air. Since both intracellular and extracellular fluids are solutions of water and salts, one could imagine a cell membrane composed of two layers of phospholipids.
In this two-layer model, both the inner and outer surfaces of the membrane are composed of the hydrophilic heads of the phospholipid molecules; the inner portion of the membrane consists of the intercalated hydrophobic tails of fatty acids. There is ample evidence to support this view of the membrane.
For example, if a piece of membrane of a known area is split into its constituent phospholipid molecules and these molecules float on water, the resulting area of the recognized molecules is exactly twice that of the original piece of membrane.
The inner and outer layers of the biological membrane have become one on the surface of the water. The second main characteristic of the membrane is the protein molecules that are embedded in it. Proteins are complex organic molecules formed from chains of amino acids.
The protein molecules within the membrane are called integral proteins, which function as specialized biochemical machines within the membrane. Integral proteins provide a series of mechanisms that link the cell’s inner environment to its outer environment.
One function of these proteins is transport, selectively moving particular molecules like glucose across the membrane.
Whole proteins are particularly important at synapses, where a variety of specialized functions are performed. The functional aspects of membrane proteins are discussed in later chapters.
In addition to the integral membrane proteins, there are also important peripheral proteins. These large molecules adhere to the surface of the inner or outer membrane, where they serve a number of specialized functions.
Glial cells
The focus of attention in studying the biological basis of behavior is on neurons and their activities, but neurons are not the only cells in the central nervous system. They are compatible with the cells of the glia, which appear to perform a variety of cleaning functions in the brain.
The term gllia means “glue,” a reflection of the fact that glial cells actually hold the brain together, occupying the space between neurons. The glia are generally very small cells, but there are many of them.
Therefore, although a little more than half of the brain’s weight is contributed by glial cells, they outnumber neurons by a factor of 10 to 50.
There are two types of glial cells in the nervous system: the large macroglia and the smaller microglia. There are two classes of macroglia in the central nervous system: astrocytes and oligodendrocytes.
When examined at higher magnification, these small cells show a characteristic of a lack of organelles within their cell bodies. Apparently, astrocytes are not heavily involved in synthetic functions, such as building proteins.
Astrocytes were once thought to form an important part of the blood-brain barrier, which protects the brain from a variety of substances in the general circulation, but recent evidence suggests this is not true.
Astrocytes are now believed to provide structural support for neurons in brains and aid in the repair of neurons after damage to the brain. They also regulated the flow of larger ions and molecules in the region of the synapses, a fact of unknown significance.
A second type of macroglial cells are oligodendrocytes. These are small cells that lack the spider processes of astroglia. Oligendrocytes differ from astrocytes in that their cell bodies contain a large number of organelles.
They also contain many microtubules that are arranged in parallel arrays. Oligodendrocytes can fulfill a number of functional roles within the central nervous system, but only one is known for certain.
The oligendrocytes produce myelin, which surrounds the axons of many neurons. This insulating coating is called a myelin sheath.
Outside of the central nervous system, along the peripheral nerves that connect the brain and spinal cord to the muscles, glands, and sensory organs of the body, there is another type of support cell that is similar in many ways to oligendrocytes.
In the developing nervous system, the Shwann cell first surrounds an axon, then wraps itself around the neuron, forming a myelin sheath. As it moves, the cytoplasm is pushed forward, leaving only the Shwann cell membrane wrapped around the axon, which was once uncovered.
Myelination greatly increases the speed with which action potentials are carried along an axon. In contrast, microglia perform “cleaning” functions within the central nervous system. Among its functions is the elimination of dead cells within the brain.
It is estimated that approximately 100,000 of the 100 billion neurons in the brain die each day, a fact that explains the slight contraction of the brain as we age.
In summary
Neurons are the information-processing cells of the nervous system. They are classified as receptors, interneurons, or effectors, depending on their function.
The dendrites of a neuron provide an extended receptive surface for the cell, greatly increasing the number of synaptic inputs. Many dendrites have dendritic spines at their most distant synapses.
The cell body integrates information from dendrites and other synaptic inputs to determine the messages that will be transmitted to other cells through its axon.
The cell body also contains a number of specialized substructures: its nucleus, mitochondria, ribosomes, endoplasmic reticulum, and Golgi apparatus. These substructures fulfill metabolic functions or create complex molecules for use in other regions of the cell.
The axon carries messages in the form of action potentials from the cell body to its terminal feet, which synapse on other neurons or effector organs. Cells with long axons are called head neurons.
These cells establish the pattern of connectivity within the nervous system. Cells with short axons or no axons are called local circuit neurons; affect activity within your immediate neighborhood.
The cell membrane, which completely separates the cell from its external environment, is composed of a phospholipid bilayer into which large protein molecules can be inserted. Proteins serve as molecular machines that are responsible for all transactions between the neuron and its environment.
The glia is the other type of cell within the central nervous system. There are many glial cells, but very little is known about their functions.
They are presumed to serve primarily support functions for neurons. One type of glia, oligodendrocytes, produce the myelin sheaths that insulate the axons of many neurons in the central nervous system.
