Often, neurons that look similar have strikingly different properties. For example, they utilize and respond to different neurotransmitter s. This section reviews the cellular components of nervous tissue. Students should be able to describe neurons and glia, their morphological components as seen with the light and electron microscope, and some of the fundamental functional roles these cell types play in the nervous system.
Figure 8. After reviewing the Model Neuron above, learn more about the function of each structure by tapping from the list below.
Click the identified structures on the model neuron to move to the related section. The region of the neuron containing the nucleus is known as the cell body , soma , or perikaryon Figure 8. The cell body is the metabolic center of the neuron. The interior of the soma consists of cytoplasm, a gel within a microtrabecular lattice formed by the microtubules and associated proteins that make up the cytoskeleton.
Energy producing metabolism and the synthesis of the macromolecules used by the cell to maintain its structure and execute its function are the principal activities of the neuronal soma.
As described in Chapter 6 , it also acts as a receptive area for synaptic inputs from other cells. Embedded within the neuronal cytoplasm are the organelles common to other cells, the nucleus , nucleolus , endoplasmic reticulum , Golgi apparatus , mitochondria , ribosomes , lysosomes , endosomes , and peroxisomes.
Many of these cell inclusions are responsible for the expression of genetic information controlling the synthesis of cellular proteins involved in energy production, growth, and replacement of materials lost by attrition. Place cursor over image to identify organelles.
The membrane of the neuron functions as a receptive surface over its entire extent; however, specific inputs termed afferents from other cells are received primarily on the surface of the cell body and on the surface of the specialized processes known as dendrites. The dendritic processes may branch extensively and are often covered with projections known as dendritic spines. Spines provide a tremendous increase in the surface area available for synaptic contacts.
The dendritic processes and spines of neurons are essentially expansions of cytoplasm containing most of the organelles found in the cell body. Dendrites contain numerous orderly arrays of microtubules and fewer neurofilaments see below.
The microtubule associated proteins MAPs in the dendrite have a higher molecular weight than those found in the axon. An example is MAP2. In addition, microtubules in dendrites have their positive ends toward the cell soma. Mitochondria are often arranged longitudinally. Rough endoplasmic reticulum and ribosomes are present in large but not small dendrites.
The shape and extent of the "dendritic tree" of an individual neuron is indicative of the quantity and variety of information received and processed by that neuron. The dendritic spines often contain microfilaments which is the cytoskeletal element responsible for changes in spine shape observed in some examples of synaptic plasticity.
Information is received by the dendrite through an array of receptors on dendrite surface that react to transmitters released from the axon terminals of other neurons. Dendrites may consist of a single twig-like extension from the soma or a multi-branched network capable of receiving inputs from thousands of other cells. For instance, an average spinal motor neuron with a moderate-sized dendritic tree, receives 10, contacts, with 2, of these on the soma and 8, on the dendrites.
The cone-shaped region of the cell body where the axon originates is termed the axon hillock. This area is free of ribosomes and most other cell organelles, with the exception of cytoskeletal elements and organelles that are being transported down the axon.
The neurofilaments in the axon hillock become clustered together as fascicles. The region between the axon hillock and the beginning of the myelin sheath is known as the initial segment. In many cases, this region is the anatomical location for the initiation of the action potential.
The area under the axolemma in this region has material that stains darkly when viewed by EM. This region is shown in Figure 8. At the distal-most end of the axon and its collaterales are small branches whose tips are button-shaped cytoplasmic enlargements called terminal boutons or nerve endings.
The other type of process in the idealized neuron is the axon. Each neuron has only one axon and it is usually straighter and smoother than the dendritic profiles. Axons also contain bundles of microtubules and neurofilaments and scattered mitochondria. The most MAPs in an axon have a lower molecular weight than those in the dendrite. A predominant MAP in axons is tau. Microfilaments within the axon are usually associated with an area adjacent to the plasmalemma and often are the most dense at the nodes of Ranvier.
Beyond the initial segments, the axoplasm lacks rough endoplasmic reticulum and free ribosomes. The branches of axons are known as axon collaterales.
The axon itself is often surrounded by a membranous material, called the myelin sheath, formed by glia cells. The myelin sheath acts to insulate the plasmalemma of the axon in a way that necessitates the more rapid spread of the depolarization of the plasmalemma and increases the speed of conduction of the nerve impulse see Chapter 3. The part of the plasma membrane of the nerve ending that is specialized to form functional contacts with other cells is the synapse. View an EM of a nerve ending with spherical vesicles.
When neurons interact with muscle fibers, the region of functional contact is called the neuromuscular junction or motor endplate Chapter 4. According to the classical definition of synapse, when a nerve ending synapses on a dendrite or soma of a second neuron it is termed either an axodendritic or an axosomatic synapse , respectively Chapter 7. However, almost all possible combinations of pre- and postsynaptic elements have been found in the central nervous system.
These different types of synapse are designated by combining the name of the structure of the presynaptic element with that of the postsynaptic structure. For example, when the transfer of information occurs from an axon to axon or from one terminal to another, the synapse involved is called an axoaxonic synapse.
Regions of functional contacts between neurons synapses have distinct morphological characteristics. Although a great deal of variation exists in the size and shape of boutons of individual neurons, synapses can be identified by the presence of the following:.
View an EM of a nerve ending with flat vesicles. Numerous variations of the "model" neuron described above exist. An important modification, which occurs especially in receptor neurons, involves the designation of a neuronal process as a dendrite or as an axon.
Classically, the axon has been identified as the myelinated or unmyelinated process that transmits signals away from the cell body. The classical view of the dendrite is that of an unmyelinated tube of cytoplasm which carries information toward the cell body. However, this distinction does not hold for ALL neurons. Some cells have a myelinated process that transmits signals toward the cell body.
Morphologically the "dendrite" and the "axon" may, therefore, be indistinguishable. Neither the position of the cell body nor the presence or absence of myelin is always a useful criterion for understanding the orientation of the neuron. The region of impulse initiation is more reliable guide to understanding the functional focal point of the cell. This region is analogous to the initial segment of the model neuron, discussed above.
Routinely the fiber or process, which contains the initial segment or trigger zone, is referred to as an axon. Note, as shown in Figure 8. A number of conventions have evolved to classify and name neurons.
Through this approach cells are classified as unipolar, bipolar and multipolar neurons as shown in Figure 8. Unipolar cells have only one cell process, and are primarily found in invertebrates. However, vertebrate sensory neurons are another form of this type of cell. Because these cells start out developmentally as bipolar neurons and then become unipolar as they mature, they are called pseudo-unipolar cells.
Bipolar cells are present in the retina and the olfactory bulb. Multipolar cells make up the remainder of neuronal types and are, consequently, the most numerous type. These have been further sub-categorized into Golgi type II cells that are small neurons, usually interneurons, and Golgi type I cells that are large multipolar neurons.
Cells are also named for their shape e. More recently, cells have been named for their function or the neurotransmitter they contain e. For instance, when you touch something hot, sensory neurons in your fingertips send a signal to interneurons in your spinal cord. Some interneurons pass the signal on to motor neurons in your hand, which allows you to move your hand away. Other interneurons send a signal to the pain center in your brain, and you experience pain.
For instance, until recently, researchers believed that neuron creation occurred in adults in a region of the brain called the hippocampus. The hippocampus is involved in memory and learning. But a recent study is calling beliefs about hippocampal neurogenesis into question. After analyzing hippocampus samples from 37 donors, researchers concluded that adults produce relatively few new hippocampal neurons.
Though the results have yet to be confirmed, they come as a significant setback. Nervous system cells are called neurons. They have three distinct parts, including a cell body, axon, and dendrites. These parts help them to send and receive chemical and electrical signals. While there are billions of neurons and thousands of varieties of neurons, they can be classified into three basic groups based on function: motor neurons, sensory neurons, and interneurons.
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Health Conditions Discover Plan Connect. What Are Neurons? Medically reviewed by Seunggu Han, M. Parts Function Types Research Takeaway Overview Neurons, also known as nerve cells, send and receive signals from your brain.
Parts of a neuron. Function of neurons. Types of neurons. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance.
Develop and improve products. List of Partners vendors. A neuron is a nerve cell that is the basic building block of the nervous system. Neurons are similar to other cells in the human body in a number of ways, but there is one key difference between neurons and other cells. Neurons are specialized to transmit information throughout the body. These highly specialized nerve cells are responsible for communicating information in both chemical and electrical forms. There are also several different types of neurons responsible for different tasks in the human body.
Sensory neurons carry information from the sensory receptor cells throughout the body to the brain. Motor neurons transmit information from the brain to the muscles of the body. Interneurons are responsible for communicating information between different neurons in the body. The cell bodies of both cell types contain organelles that support the life of the cell, including mitochondria, Golgi bodies, and cytoplasm.
Neurons stop reproducing shortly after birth. Generally, when neurons die they are not replaced, although neurogenesis , or the formation of new nerve cells, does occur in some parts of the brain. Neurons have a membrane featuring an axon and dendrites, specialized structures designed to transmit and receive information.
Neurons release chemicals known as neurotransmitters into synapses, or the connections between cells, to communicate with other neurons. There are three basic parts of a neuron : the dendrites, the cell body, and the axon. However, all neurons vary somewhat in size, shape, and characteristics depending on the function and role of the neuron.
Some neurons have few dendritic branches, while others are highly branched in order to receive a great deal of information.
Some neurons have short axons, while others can be quite long. The longest axon in the human body extends from the bottom of the spine to the big toe and averages a length of approximately three feet! How do neurons transmit and receive information?
In order for neurons to communicate, they need to transmit information both within the neuron and from one neuron to the next. This process utilizes both electrical signals as well as chemical messengers.
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