Where is neuron cell found

Each mammalian neuron consists of a cell body , dendrites , and an axon. When neurons receive or send messages, they transmit electrical impulses along their axons, which can range in length from a tiny fraction of an inch or centimeter to three feet about one meter or more. Many axons are covered with a layered myelin sheath, which accelerates the transmission of electrical signals along the axon.

This sheath is made by specialized cells called glia. In the brain, the glia that make the sheath are called oligodendrocytes, and in the peripheral nervous system, they are known as Schwann cells. The brain contains at least ten times more glia than neurons. Glia perform many jobs. Researchers have known for a while that glia transport nutrients to neurons, clean up brain debris, digest parts of dead neurons, and help hold neurons in place. Current research is uncovering important new roles for glia in brain function.

Synapse is the junction that allows signals to pass from a nerve cell to another cell or from one nerve cell to a muscle cell. The synaptic cleft is the gap between the membrane of the pre- and postsynaptic cell. In a chemical synapse the signal is carried by a diffusable neurotransmitter. The cleft between the presynaptic cell and the postsynaptic cells is 20 to 40 nm wide and may appear clear or striated. Recent studies have indicated that the cleft is not an empty space per se, but is filled with carbohydrate-containing material.

Synaptic vesicles are small spherical organelles in the cytoplasm of neurons that contain neurotransmitter and various proteins necessary for neurotransmitter secretion.

Vesicles containing inhibitory neurotransmitter are often flat or elliptical whereas vesicles that contain excitatory neurotransmitter are usually more spherical. Tap on the different glial cells to view the details of their structure and function. The most numerous cellular constituents of the central nervous system are the non-neuronal, neuroglial "nerve glue" cells that occupy the space between neurons.

This section will cover the general classifications of the neuroglial cells and describe some of the general properties that distinguish neuroglia from neurons. Neuroglia are classified based on size and shape fo their nucleus and distinguished from neurons, at the light microscopic level.

Alkaline basic dyes are used to show nuclear morphology. In addition, several metal stains are used show the shape of the cell and cytoplasmic architecture.

Characteristics of nuclei, including size, shape, staining intensity, and distribution of chromatin, are used to distinguish cell types in pathological material. Cell body characteristics, including size, shape, location, branching pattern, and density of processes, are also used. Neuroglia are divided into two major categories based on size, the macroglia and the microglia. The macroglia are of ectodermal origin and consist of astrocytes , oligodendrocytes and ependymal cells.

Microglia cells are probably of mesodermal origin. A comparison of the various neuroglial types is shown in Figure 8. Click on a glial cell to move to the related section. Protoplasmic astrocytes are found primarily in gray matter. With silver or glial specific stains, their cell bodies and processes are very irregular.

The processes may be large or very fine, sometimes forming sheets that run between axons and dendrites, and may even surround synapses. These fine sheet-like processes give the protoplasmic astrocyte cell body a "fuzzy" or murky appearance under the light microscope.

Bundles of fine fibrils may be seen within the cytoplasm. The nucleus of a protoplasmic astrocyte is ellipsoid or bean-shaped with characteristic flecks of chromatin. Specific types of intercellular junctions have been noted between the processes of protoplasmic astrocytes. These probably mediate ion exchange between cells. Fibrous astrocytes are found primarily in white matter, have a smoother cell body contour than do protoplasmic astrocytes as seen with glial-specific stains, and have processes that tend to emerge from the cell body radially.

These processes are narrower and branch to form end feet on blood vessels, ependyma, and pia. Consequently, the processes of fibrous astrocytes do not form sheets and do not tend to conform to the shape of the surrounding neurons or vascular elements. The major distinguishing feature of fibrous astrocytes, as the name suggests, is an abundance of glial fibrils arranged in parallel arrays in the cytoplasm and extending into the processes.

In Nissl stains, the fibrous astrocytes have a nucleus essentially the same as that of the protoplasmic type with a flecked appearance. Intercellular adherences have also been observed between fibrous astrocytes. No single astrocyte would project to all of these structures.

Both types of astrocytes function to support the neurons in their immediate vicinity. They provide a physical barrier between cells, maintain the ionic and pH equilibrium of the extracellular space around neurons, and continually modify the chemical environment of the neighboring cells. As shown in Figure 8.

During development, they form scaffolding along which nerve cells migrate to achieve their mature structure. During injury, the astrocytes proliferate and phagocytize dead cells. This often leads to the formation of glial scar. In addition to these general functions, astrocytes also act in more specialized ways to facilitate neuron function. They metabolize neurotransmitters by removing them from the synaptic cleft.

For example, the amino acid glutamate is taken up by astrocytes and inactivated by conversion to glutamine. Glutamine is then transported to the neuron to be re-synthesized into glutamate see Chapter More recent evidence indicates that the astrocytes can dramatically change size as part of their physiological regulation of the neuronal environment.

These functions will be discussed in later sections. Oligodendrocytes are also located in both gray and white matter. They are the predominant cell type in white matter where they are often located as rows of cells between groups of neuronal processes. These are termed interfascicular oligodendroglia and are involved in the formation and maintenance of the myelin surrounding the neuronal processes nearby.

In gray matter, oligodendroglia are usually located near neurons and, therefore, are known as perineuronal satellite cells. Cell bodies of oligodendroglia are often located near capillaries, but they lack the definite perivascular end feet characteristic of astrocytes.

The processes of oligodendrocytes are fewer and more delicate than astrocytes, and the cell body shape is polygonal to spherical. The oligodendrocyte nucleus is smaller than that of the astrocyte, is eccentrically located in the cell body, contains clumps of chromatin and can be stained by alkaline dyes.

The cytoplasm of oligodendrocytes tends to be darker than that of astrocytes with silver stains, and does not contain glial fibrils although they do contain microtubules. View an oligodendrocyte EM. The role of oligodendroglia in the central nervous system, particularly of the interfascicular oligodendrocytes , is the formation and maintenance of myelin.

Myelin is the sleeve of membranous material described by Dr. Byrne, that wraps the neuronal axon as shown in Figure 8. Myelin is composed of concentric layers of membranes compacted against one another with an internal i. Site search Search Menu. What is a neuron? Home The Brain Brain anatomy. What does a neuron look like? The tree-like structure of a neuron. Dendritic spines are small structures that receive inputs from the axons of other neurons.

Bottom-right image: a segment of dendrite from which spines branch off, like leaves off a tree branch. Concepts and definitions Axon — The long, thin structure in which action potentials are generated; the transmitting part of the neuron. Author: Dr Alan Woodruff. These molecules cross the synaptic cleft and bind to receptors in the postsynaptic ending of a dendrite.

Neurotransmitters can excite the postsynaptic neuron, causing it to generate an action potential of its own. Electrical synapses can only excite. They occur when two neurons are connected via a gap junction. This gap is much smaller than a synapse, and includes ion channels which facilitate the direct transmission of a positive electrical signal.

As a result, electrical synapses are much faster than chemical synapses. However, the signal diminishes from one neuron to the next, making them less effective at transmitting. Neurons vary in structure, function, and genetic makeup. Given the sheer number of neurons, there are thousands of different types, much like there are thousands of species of living organisms on Earth. In terms of function, scientists classify neurons into three broad types: sensory, motor, and interneurons.

Sensory neurons are triggered by physical and chemical inputs from your environment. Sound, touch, heat, and light are physical inputs. Smell and taste are chemical inputs. For example, stepping on hot sand activates sensory neurons in the soles of your feet. Those neurons send a message to your brain, which makes you aware of the heat. Motor neurons play a role in movement, including voluntary and involuntary movements.

These neurons allow the brain and spinal cord to communicate with muscles, organs, and glands all over the body. There are two types of motor neurons: lower and upper.

Lower motor neurons carry signals from the spinal cord to the smooth muscles and the skeletal muscles. Upper motor neurons carry signals between your brain and spinal cord. When you eat, for instance, lower motor neurons in your spinal cord send signals to the smooth muscles in your esophagus, stomach, and intestines.

These muscles contract, which allows food to move through your digestive tract.