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Peripheral Nervous System
The apex of the pyramid points toward the cortical surface. There are specific nociceptor transducers that are responsible for how and if the specific nerve ending responds to the thermal stimulus. This article describes the cell types of the blood, focusing on their histological features and functions. Patients with emotional disorders have been trained to self-regulate a region of the brain known as the amygdala located deep within the cerebral hemispheres and believed to influence motivational behaviour by self-inducing sadness and monitoring the activity of the amygdala on a real-time fMRI readout. Arthralgia joint Bone pain Myalgia muscle Muscle soreness: Therefore, the spinal cord is much shorter than the vertebral column.

divisions of the nervous system


From this, you may deduce that the bulk of nervous tissue consists of nerve cell processes rather than nerve cell bodies. The study of neuroanatomy consists largely of understanding the routes travelled by nerve cell axons.

Unfortunately, the organization of neural processes, most particularly the full length of axons and dendrites and the synaptic interactions between them, can seldom be visualized directly. In most other tissues of the body, what you can see in the microscope is directly informative.

Consider skin , where a routine section of epidermis reveals almost everything interesting about the size, shape and growth sequence of epidermal cells. Electron microscopy of similar specimens simply adds more finely resolved detail.

But making any sense at all of nervous tissue requires that you "see" with concepts acquired over decades of research with many special techniques.

When you examine microscope slides or micrographs of nervous tissue, patterns of functional connection cannot usually be seen. Nevertheless, what you can observe should be interpreted in terms of neuronal functions and connectivity, including unseen axons, dendrites and synapses as well as associated supporting cells. Thus your job for comprehending nervous tissue is not just to look-and-learn, but to think rather deeply, to fit many different views and facts together. Most of the listed vocabulary terms for neuronal and glial structures are well defined in standard textbooks.

You just have to make sense of it all. How to read this page. You might read this page straight down, from top to bottom.

But it is written with hyperlinks to facilitate browsing. You might more profitably check out each link, at least if it suggests a question in your mind, and use your browser's "back" arrow to return.

And return repeatedly to the outline at the top of this page to choose the topic that most closely engages your current curiosity. Nerve cells comprise the "enchanted loom" that is our brain. Every nerve cell has three distinctive portions -- a cell body , one axon , and several dendrites. Nerve cells come in extreme variety. In every region of the brain are several different nerve cell types, each distinguished by its own characteristic soma size, dendritic shape, source of synaptic input, destination of axonal output, and chemistry.

Occasional nerve cell types may have characters which depart from the the typical description presented below. Because of this immense variety of nerve cell types, there is no "one-size-fits-all" description. So textbook descriptions of nerve cells tend to present overwhelmingly abundant detail. Although details of nerve cell shape and connectivity are usually insignificant for clinical practice, they can be quite beautiful and are essential for understanding research on brain function.

It is also often necessary to learn some "irrelevant" detail in order to understand the particular examples used to demonstrate basic functional principles. Nerve cell bodies look more or less like other body cells, although they do have certain characteristic features. Extending out from each nerve cell body are long cytoplasmic processes, one axon and several dendrites. These processes usually cannot be distinguished in routine histological preparation.

A typical nerve cell body contains only a small fraction of the total cell volume; the rest is contained in the axon and dendrites. The spaces between nerve cell bodies with a feltwork of these axonal and dendritic processes, called neuropil which also includes glial cell processes. Myelin is a fatty covering which envelops many axons and permits action potentials to be propagated at a much greater velocity.

Myelin is formed by support cells Schwann cells in the peripheral nerve system, oligodendroglia in the CNS wrapping around the axons. Myelin is not part of, nor produced by, the nerve cell whose axon it envelops. In peripheral nerves, myelin consists of Schwann cell membrane wrapped around and around an axon, while most of the Schwann cell cytoplasm lies alongside the axon. See oligodendroglia for myelination of CNS axons. The image should be animated, if you watch patiently.

A Schwann cell is illustrated with brown cytoplasm. Observe that as the growing Schwann cell spirals inward around the axon, it wraps its membrane into layers of myelin.

The myelin of one Schwann cell wraps about one or two millimeters of an axon. To myelinate the entire length of the axon, many of these Schwann cell wrappings line up end-to-end along the axon. The points between segments of myelin are called nodes of Ranvier.

The stretch of axon between nodes is called an internode. The spacing of nodes is critical for propagation of action potentials. Along myelinated axons, action potentials are regenerated only at the nodes. Myelin provides insulation and, more importantly, decreased capacitance so that the ionic currents at one node can flow efficiently and quickly to the next node.

In contrast, action potentials propagating along un myelinated axons are regenerated at each point along the way, a much slower process. Clinical notes Because the currents generated at one node are generally sufficient to depolarize axonal membrane two or three nodes away, local anesthesics which block action potentials but do not prevent current flow must be distributed across several nodes several millimeters in order to produce effective anesthesia.

Because myelinated axons have voltage-dependent sodium channels only at nodes of Ranvier, demyelination such as that which occurs in multiple sclerosis effectively prevents the propagation of action potentials. Many details of myelin cannot be well-appreciated by light microscopy. For electron micrographs of myelin in peripheral nerves, see the online Electron Microscopic Atlas of Mammalian Tissues the text is in German, but most figure labels can be deciphered fairly easily.

Although axons reach into all parts of the body, the vast majority of nerve cell bodies occur in the central nervous system brain and spinal cord , in those regions described as gray matter. Relatively few nerve cell bodies occur peripherally, in the ganglia small clusters of nerve cells of sympathetic and parasympathetic nervous systems. Wherever they occur, nerve cell bodies have a distinctive appearance. Because these features of nerve cell bodies are related to the heavy metabolic demands imposed by extensive processes, they are exaggerated i.

Nerve cells with the most extremely long, large diameter axons -- such as pyramidal cells of motor cortex and motor neurons of spinal cord -- are often illustrated as "typical" neurons simply because they are big and hence especially easy to visualize. Cerebellar Purkinje cells comprise another "popular" type of nerve cell, also large but with a huge dendritic tree rather than an especially long axon. Special stains, like the silver-based Golgi stain , can reveal entire neurons or glial cells at least as much as fits within the thickness of a single section by impregnating them with opaque silver.

But this technique yields elegant results only by suppressing any staining of most neighboring cells, so neurons appear in splendid isolation when their essence is one of complex interaction. Similarly, electron microscopy can display elegant synapses, but the narrow view offers few clues about the cells to which the pre- and post-synaptic profiles belong. Sections of central nervous tissue routinely show neuron cell bodies surrounded by a finely-textured fibrous material often called neuropil which should not be confused with connective tissue.

This feltwork consists of axons and dendrites and glial processes , with all the comings and goings that these processes entail. Individual axons and dendrites can be distinguished only in fortuitous sections, and then only for a short length.

The so-called "molecular" layers of cerebral and cerebellar cortex consist of neuropil containing relatively few cell bodies most of the cell bodies lie in deeper layers. Note that a common artefact , resulting from tissue shrinkage, is for a clear " halo " to appear around cell bodies and blood vessels.

Although the presence of such halos can be misleading there is no such space in intact, living nervous tissue , this consistent artefact serves to highlight or emphasize the locations for these structures. Schwann cells -- Support cells in peripheral nerves. Many of the small, heterochromatic nuclei that can be seen within peripheral nerves belong to Schwann cells.

Some of the remaining nuclei belong to fibroblasts of the endoneurium , perineurium , and epineurium i. Perineurium also contains squamous perineural cells perineural epithelium which form a continuous layer that isolates the axons within from surrounding connective tissue.

Fibroblast nuclei tend to be smaller and more densely heterochromatic than Schwann cell nuclei, but in most ordinary preparations that include peripheral nerves, it is impractical to distinguish these nuclei.

Note that none of the nuclei visible in peripheral nerves belong to nerve cells, since peripheral nerves do NOT contain nerve cell bodies, only axons of nerve cells whose cell bodies lie elsewhere. Schwann cells can form tumors called schwannomas see WebPath: MRI , gross , dissection , microscopic low X , high X. The most numerous cells within the central nervous system are glial cells. The name "glia" means "glue" filling the interstices of nervous tissue , reflecting old but enduring ignorance of their function and the inadequacy of classical histology to offer much insight.

The small nuclei of glial cells may be readily observed in any section of central nervous tissue. Unfortunately, like neurons, these cells are difficult to visualize satisfactorily. Although glial cells vastly outnumber nerve cells approx.

Ignorance of glial function is beginning to dissipate. For a recent review, see:. Far more active than once thought, glial cells powerfully control synapse formation, function, and blood flow. They secrete many substances whose roles are not understood, and they are central players in CNS injury and disease.

Quite possibly the most important roles of glia have yet to be imagined. The two most common types of glia, oligodendroglia and astroglia , both have extensive cytoplasmic processes and are intimately involved in the function of nervous tissue. A third glial type, microglia , function similarly to macrophages. In most of our reference slides, both in the spinal smear and in sections of brain and spinal cord, only the nuclei of glial cells are clearly seen, with no indication of cytoplasmic shape.

The characteristic processes of glia can show up nicely in some of the Golgi-stained sections in your reference collection variously cerebellum or cerebral cortex. Even with electron microscopy, it is difficult to trace CNS myelin to the arms of the oligodendroglia from which it forms. Separately distinguishing among astroglia , oligodendroglia and microglia is a skill for specialists i.

Oligodendrocytes form myelin in the CNS and hence are responsible for normal propagation of action potentials. Myelin formation by oligodendroglia is slightly different than that by Schwann cells , each of which wraps myelin around a single axon. Each of the several glial cell processes extends to and then myelinates a segment of one axon. If the myelin of one oligodendrocyte process were unrolled, the process would be shaped rather like a wide-bladed shovel the thin shovel blade would represent the membrane that rolls around the axon to form myelin and the shovel handle would represent the process which extends back to the glial cell body.

Each oligodendroglial cell has several such "shovels", forming myelin around several axons. Recent evidence from mouse, based on gene transcription profiles, indicates that oligos form several populations; for example, "One population was responsive to motor learning, and another, with a different transcriptome, traveled along blood vessels" Science 10 Jun , Foot-processes of astrocytes line every surface where central nervous tissue contacts other body tissues, not only the obvious outer surface immediately underlying the pia mater where they form the glia limitans but also along every blood vessel and capillary which penetrates into the brain and spinal cord.

Other astrocyte foot processes approach nerve cells at any sites where the nerve cell membrane is not otherwise occupied by synapses or by oligodendroglia. There is growing awareness that astrocytes play several critical roles.

Recent evidence shows that activity of individual astrocytes can correspond closely with that of associated neurons, and can also modulate local blood flow Schummers, et al.

Additional functions and pathologies include all of the following from Ransom, et al. Recent research also implicates astroglia in the " glymphatic system " which allows recirculation of CSF and brain interstitial fluid along paravascular channels. A report in Science Local variation in blood flow through brain capillaries may be regulated by activity of pericytes , which in turn can respond to neural activity.

Controlled capillaries , Nature , 12 October doi: In contrast to vessels in most other parts of the body, most molecules can NOT pass freely between blood to interstitial space. The integrity of the blood-brain barrier is established by continuous capillary endothelium together with the absence of endothelial vesicular transcytosis. The only substances which cross this barrier are those which can diffuse through endothelial plasma membranes or those for which specific endothelial membrane channels exist.

The blood-brain barrier is a concept with considerable clinical significance, not only because it limits the delivery of drugs to the central nervous system but also because pathological disturbance of the barrier can seriously impact brain function. Read a more extensive description of the anatomy and physiology of the blood-brain barrier at the University of Arizona Health Science Center, Blood Brain Barrier.

Microglia are also implicated in the maturation, plasticity, and remodelling of synaptic circuits Science As described by Kembermann and Neumann Microglia: In addition, the inflammatory mediators released by microglia during an innate immune response strongly influence neurons and their ability to process information. Recent research indicates that microglia in mice are "an ontogenically distinct population in the mononuclear phagocyte system," originating during embryonic development Science , published online October 21, ; DOI: Central nervous tissue is highly vascular, so blood vessels should be a significant feature in any histological specimen of CNS.

Large vessels generally remain on the surface of the brain or spinal cord, so only smaller vessels penetrate into gray and white matter.

Such small vessels may not be immediately recognizable as such. As in other regions of the body, capillaries may be quite inconspicuous due to small size. Even venules and arterioles may be small enough that the layers in their walls are not clearly visible.

Blood cells may be washed out during preparation. Nevertheless, such vessels should be noticed, since they play a crucial role in brain function and pathology. Also see note on microvasculature , above. Blood vessels are generally the largest structural elements in neuropil and in white matter i. The thumbnails below link to several spinal cord specimens in which blood vessels may be observed.

Blood vessels appear similar in any region of the brain. Note that a clear "halo" commonly appears around blood vessels as well as neuronal and glial cell bodies.

This an artifact of histological preparation, resulting from tissue shrinkage when the central nervous tissue is fixed. The ventricular system of the brain is lined by a simple cuboidal epithelium called ependyma , a remnant of the embryonic neuroectoderm which once formed the neural tube.

At certain sites the posterior margin of the lateral ventricles, the midline of the 3rd ventricle, the roof of the 4th ventricle , this ependyma lies adjacent to overlying connective tissue. Here the ependyma is extensively wrinkled, with blood vessels which are caught up in the folds, to form choroid plexus. Choroid plexus is the source for cerebrospinal fluid CSF.

CSF is actively secreted by the ependymal cells of choroid plexus and like aqueous humor in the eye accumulates at a steady rate even if drainage points become occluded. In composition , CSF differs considerably from blood. Although osmolarity and sodium concentrations are similar in blood and CSF, CSF has somewhat more chloride; less potassium, calcium, magnesium and glucose; much less protein, and practically no white blood cells.

For specific values as wells as alterations in disease, see Kandel et al. CSF and brain interstitial fluid are exchanged through the so-called " glymphatic system " of paravascular channels. The layout of choroid plexus is perhaps most easily appreciated embryologically -- click on the thumbnail for an image of embryonic choroid plexus.

Cerebrospinal fluid accumulates not only from the action of choroid plexus but also from the interstitial spaces of the brain. It flows, under positive pressure developed by its active secretion, through the ventricular system, thence out through holes in the roof of the 4th ventricle into the subarachnoid space, finally draining through " arachnoid villi " into the venous sinuses of the cranial cavity.

The central nervous system is enveloped by specialized layers of connective tissue. This section offers a guide for microscope lab i. Using your reference slides, the best view of "whole" neurons is provided by the slide labelled "nerve cells, ox spinal cord". This is a slide of spinal smear, not a slice but a small amount of gray matter squished onto the slide. The largest nerve cells in this preparation represent spinal motor neurons , the cells whose very long axons extend out peripheral nerves to the muscles.

From the nerve cell body extend several dendrites ; these are broad at their base and contain Nissl but decrease in diameter and basophilia with increasing distance from the soma.

The full extent of the dendritic arborization is not visible, since the fine distal branches are hidden in the background texture of the slide. Each neuron also has a single axon , which can be readily identified only if it begins on the edge of the cell body as opposed to the top or bottom, as viewed in the slide.

The axon, unlike the dendrite, has a uniform diameter and does not contain basophilic Nissl bodies. It begins at the axon hillock , a specialized site on the cell body where the cytoplasm is clear like the axoplasm, it lacks Nissl bodies.

The axon, even more so than the dendrites, disappears into the distance and cannot be followed to its end. In this same preparation, smaller cells with similar features represent spinal interneurons. Scattered throughout this preparation are also very many cells whose nuclei are smaller than those of the neurons, oval with clumps of heterochromatin, and whose cytoplasm is inconspicuous.

These are the glial cells. Numerous capillaries , narrow tubular profiles wandering across the slide, may also be seen. The spinal cord consists of ascending and descending axonal pathways i. Use your preferred neuro text to rehearse the functions associated with the following regions in the spinal cord. Spinal motor neurons are lost in amyotrophic lateral sclerosis ALS -- for more, see: Some sections of spinal cord may include dorsal and ventral roots containing respectively sensory and motor axons.

The cerebral cortex forms the surface of gyri an sulci over each entire cerebral hemisphere. Its composition is complex after all, it is the seat of conscious perception and thought!

These include many local interneurons stellate cells and granule cells as well as the much larger and more conspicuous pyramidal cells , some of whose axons enter the underlying white matter and travel to other cortical areas or to other regions of the brain. The cerebral cortex is traditionally but rather arbitrarily described as having six layers.

Although these layer cannot be readily distinguished they are arbitrary, after all , they can be roughly approximated by looking for the following features. Layer I the "molecular layer" is the outermost layer. This layer contains relatively few nerve cell bodies. The odd name "molecular layer" derives from the fine texture of this layer, due to its composition largely of dendrites and fine axon terminals and glia, of course.

Layer II the "outer granular layer" , typically contains many very small cells granule cells. Layer III the "outer pyramidal layer" contains cell bodies of small pyramidal cells. Axons from these cells typically project to the upper layers of neighboring cortical regions. Layer IV the "inner granular layer" contains axonal ramifications of afferent fibers, such as sensory axons from the thalamus.

Axons from the lateral geniculate nucleus the visual relay of the thalamus are so numerous that the primary visual cortex which receives these axons Brodmann's area 17, at the occipetal pole of each hemisphere is sometimes called "striate cortex", because these axons conspicuously divide the cortex into layers that are visible to gross inspection.

Layer V the "inner pyramidal layer" contains cell bodies of large pyramidal cells. Axons from these cells typically project to more distant cortical regions, to other parts of the brain, or to lower centers such as spinal motor neurons. The larger size of these pyramidal cells compared the the smaller cells of layer III is associated with the greater length of their axons. Recall that cell bodies provide most of the basic cellular functions needed to maintain the axon, while the axonal surface membrane and axoplasmic volume may be many times greater than the surface and volume of the cell body.

Layer VI the "layer of pleiomorphic cells typically contains cells of assorted size and shape hence, "pleiomorphic". Variations in the detailed appearance "cytoarchitecture" of the several cortical layers, as described a century ago by K. Brodmann , formed the original basis for recognizing regional differentiation of the cortex " Brodmann's areas ". Now, of course, this cytoarchitectural differentiation is known to correspond with functional localization in the cortex. See WebPath for cortical changes associated with Alzheimer's disease.

The cortex of the cerebellum consists of three very well-defined layers. The most prominent nerve cells are Purkinje cells , whose cell bodies all lie in a discrete layer. The inner granular layer is packed with nuclei of vastly many cerebellar granule cells. As a result, neurogenesis has spurred an interest in stem cell research, which could lead to an enhancement of neurogenesis in adults who suffer from stroke, Alzheimer disease , Parkinson disease , or depression.

Developmental plasticity occurs most profoundly in the first few years of life as neurons grow very rapidly and send out multiple branches, ultimately forming too many connections. In fact, at birth, each neuron in the cerebral cortex the highly convoluted outer layer of the cerebrum has about 2, synapses. By the time an infant is two or three years old, the number of synapses is approximately 15, per neuron. This amount is about twice that of the average adult brain. The connections that are not reinforced by sensory stimulation eventually weaken, and the connections that are reinforced become stronger.

Eventually, efficient pathways of neural connections are carved out. During early childhood, which is known as a critical period of development, the nervous system must receive certain sensory inputs in order to develop properly. Once such a critical period ends, there is a precipitous drop in the number of connections that are maintained, and the ones that do remain are the ones that have been strengthened by the appropriate sensory experiences.

American neuroscientist Jordan Grafman has identified four other types of neuroplasticity, known as homologous area adaptation , compensatory masquerade, cross-modal reassignment, and map expansion.

Homologous area adaptation occurs during the early critical period of development. If a particular brain module becomes damaged in early life, its normal operations have the ability to shift to brain areas that do not include the affected module. The function is often shifted to a module in the matching, or homologous, area of the opposite brain hemisphere. The downside to this form of neuroplasticity is that it may come at costs to functions that are normally stored in the module but now have to make room for the new functions.

An example of this is when the right parietal lobe the parietal lobe forms the middle region of the cerebral hemispheres becomes damaged early in life and the left parietal lobe takes over visuospatial functions at the cost of impaired arithmetical functions, which the left parietal lobe usually carries out exclusively. Timing is also a factor in this process, since a child learns how to navigate physical space before he or she learns arithmetic.

The second type of neuroplasticity, compensatory masquerade, can simply be described as the brain figuring out an alternative strategy for carrying out a task when the initial strategy cannot be followed due to impairment. One example is when a person attempts to navigate from one location to another. Most people, to a greater or lesser extent, have an intuitive sense of direction and distance that they employ for navigation.

However, a person who suffers some form of brain trauma and impaired spatial sense will resort to another strategy for spatial navigation, such as memorizing landmarks.

The only change that occurs in the brain is a reorganization of preexisting neuronal networks. The third form of neuroplasticity, cross-modal reassignment, entails the introduction of new inputs into a brain area deprived of its main inputs. A classic example of this is the ability of an adult who has been blind since birth to have touch , or somatosensory, input redirected to the visual cortex in the occipital lobe region of the cerebrum located at the back of the head of the brain—specifically, in an area known as V1.

Sighted people, however, do not display any V1 activity when presented with similar touch-oriented experiments. Moreover, all the sensory cortices of the brain—visual, auditory, olfactory smell , gustatory taste , and somatosensory—have a similar six-layer processing structure. Because of this, the visual cortices of blind people can still carry out the cognitive functions of creating representations of the physical world but base these representations on input from another sense—namely, touch.

This is not, however, simply an instance of one area of the brain compensating for a lack of vision. It is a change in the actual functional assignment of a local brain region.

Map expansion, the fourth type of neuroplasticity, entails the flexibility of local brain regions that are dedicated to performing one type of function or storing a particular form of information. This phenomenon usually takes place during the learning and practicing of a skill such as playing a musical instrument. Specifically, the region grows as the individual gains implicit familiarity with the skill and then shrinks to baseline once the learning becomes explicit.

Implicit learning is the passive acquisition of knowledge through exposure to information, whereas explicit learning is the active acquisition of knowledge gained by consciously seeking out information. But as one continues to develop the skill over repeated practice, the region retains the initial enlargement. Map expansion neuroplasticity has also been observed in association with pain in the phenomenon of phantom limb syndrome. The relationship between cortical reorganization and phantom limb pain was discovered in the s in arm amputees.

Later studies indicated that in amputees who experience phantom limb pain, the mouth brain map shifts to take over the adjacent area of the arm and hand brain maps. In some patients, the cortical changes could be reversed with peripheral anesthesia. Some of the earliest applied research in neuroplasticity was carried out in the s, when scientists attempted to develop machines that interface with the brain in order to help blind people.

The machine consisted of a metal plate with vibrating stimulators. A camera was placed in front of the patient and connected to the vibrators. The camera acquired images of the room and translated them into patterns of vibration, which represented the physical space of the room and the objects within it.

After patients gained some familiarity with the device, their brains were able to construct mental representations of physical spaces and physical objects. Thus, instead of visible light stimulating their retinas and creating a mental representation of the world, vibrating stimulators triggered the skin of their backs to create a representation in their visual cortices.

A similar device exists today, only the camera fits inside a pair of glasses and the sensory surface fits on the tongue. Today neuroscientists are developing machines that bypass external sense organs and actually interface directly with the brain.

For example, researchers implanted a device that monitored neuronal activity in the brain of a female macaque monkey. Thus, the monkey became capable of moving a robot arm with its thoughts. This means that the motor cortex does not control the details of limb movement directly but instead controls the abstract parameters of movement, regardless of the connected apparatus that is actually moving. For humans, however, less-invasive forms of brain-computer interfaces are more conducive to clinical application.

For example, researchers have demonstrated that real-time visual feedback from functional magnetic resonance imaging fMRI can enable patients to retrain their brains and therefore improve brain functioning. Patients with emotional disorders have been trained to self-regulate a region of the brain known as the amygdala located deep within the cerebral hemispheres and believed to influence motivational behaviour by self-inducing sadness and monitoring the activity of the amygdala on a real-time fMRI readout.

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