For specific values as wells as alterations in disease, see Kandel et al. For humans, however, less-invasive forms of brain-computer interfaces are more conducive to clinical application. Please help improve this article by adding citations to reliable sources. Each oligodendroglial cell has several such "shovels", forming myelin around several axons. The Integrative Action of the Nervous System. This is an article that will show you how to examine histology slides.
Central Nervous System
Then, as the brain begins to process sensory information, some of these synapses strengthen and others weaken. Eventually, some unused synapses are eliminated completely, a process known as synaptic pruning, which leaves behind efficient networks of neural connections.
Other forms of neuroplasticity operate by much the same mechanism but under different circumstances and sometimes only to a limited extent. These circumstances include changes in the body, such as the loss of a limb or sense organ , that subsequently alter the balance of sensory activity received by the brain.
In addition, neuroplasticity is employed by the brain during the reinforcement of sensory information through experience, such as in learning and memory , and following actual physical damage to the brain e. Today it is apparent that the same brain mechanisms—adjustments in the strength or the number of synapses between neurons—operate in all these situations. Sometimes this happens naturally, which can result in positive or negative reorganization, but other times behavioral techniques or brain-machine interfaces can be used to harness the power of neuroplasticity for therapeutic purposes.
In some cases, such as stroke recovery, natural adult neurogenesis can also play a role. 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. 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. These are among the smallest and most numerous neurons in the body. The Purkinje cell layer contains large cell bodies of Purkinje cells , the sole output cells for the cortex. The outer molecular layer consists principally of the dendrites of Purkinje cells and the axons of granule cells.
The odd name "molecular layer" derives from the fine texture of this layer, due to its composition largely of dendrites and fine axon terminals.
Nuclei in this layer belong mostly to glial cells. The pattern of connections among various axons and dendrites in the cerebellum is extremely elegant and regular, and has been described in extensive detail. Any thorough neuro text e. Both the paravertebral ganglia of the sympathetic nervous system and the scattered ganglia of the parasympathetic nervous system consist of small clusters of nerve cell bodies.
Parasympathetic ganglia may turn up in sections of various visceral organs, where they can be recognized by the classic appearance of nerve cell bodies. Like other "pieces" of the nervous system, peripheral nerves are a part of a functioning, highly organized whole.
Each "piece" must be understood in relation to the rest of the system. Examples of peripheral nerves are often fairly easy to find in sections of the skin. Larger nerves also often run in parallel with blood vessels. Peripheral nerves consist of axons bundled together within an epineurium connective tissue sheath.
Peripheral nerves are only meaningful in relation to their connections. All of the axons which travel along peripheral nerves begin and end somewhere else.
Motor axons originate with cell bodies in the spinal cord's ventral horn or in the brainstem's motor nuclei or in peripheral sympathetic or parasympathetic ganglia. Motor axons terminate at muscles including smooth muscle along blood vessels or glands. Somatosensory axons begin with a peripheral receptor e.
These sensory axons then travel toward their cell bodies in a dorsal root ganglion or trigeminal ganglion , and finally terminate at synapses within the spinal cord or brain stem.
Note that somatosensory axons are an exception to the rule that axons always conduct impulses away from the cell body. All the cellular nuclei which are obviously visible within a peripheral nerve belong not to nerve cells but to Schwann cells or to fibroblasts. All three are eosinophilic, and all contain scattered, elongated nuclei. Several features may be used to distinguish nerves from smooth muscle or other fibrous tissue.
Note that the texture of peripheral nerves can differ from site to site, depending on axon size and especially on the proportion of myelinated to unmyelinated axons. Nerves in the tongue, with many large myelinated axons, are much more obvious than are autonomic nerves in Auerbach's plexus of the gut, where most axons are smaller and unmyelinated.
In peripheral nerve cross sections stained for myelin, the myelin is generally visible as a dark or black frame around each pale myelinated axon. The typical round shape is often distorted by tissue preparation. In longitudinal sections containing large myelinated axons, nodes of Ranvier can be easily seen where the myelin appears to be "pinched".
Seldom can a single axon be followed throughout an entire internode i. Nevertheless, the length of each internode can be estimated by measuring the total length of all axons visible in a field of view and dividing by the number of nodes that appear. Less-than-ideal fixation also often distorts the relationship, so the axon may not be centered within the halo.
Many details of peripheral nerves cannot be well-appreciated by light microscopy. For electron micrographs of 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.
For sensory receptors in skin, see skin innervation. For sensory receptors associated with muscle, see muscle innervation.
The organization of the central nervous system is based upon interconnections across varying distances among billions of individual nerve cells. The basic principle of neural organization is quite straightforward. Nervous tissue consists of nerve cells communicating with other nerve cells. This simple yet fundamental concept can easily become lost in the forest of details presented in standard textbooks.
Here, then, is a brief guide to nervous tissue, including the classification and nomenclature of nerve cells. Each nerve cell has a cell body in one place and an axon which travels some distance to synapse with the cell bodies and dendrites of other neurons. The microscopic appearances of gray matter and white matter may be conveniently contrasted in a section of spinal cord.
Various stains have various effects on gray matter. Note that a popular neuroanatomical stain Weigert's , used to highlight different brain regions, colors myelin black. Thus, paradoxically, in many pictures of the brain, white matter appears black while gray matter appears pale. Where cell bodies and dendrites are common, the gross color of fixed dead brain tissue is gray. Hence we have the term gray matte r. Note that gray matter is not just a place where cell bodies and dendrites happen to be.
Gray matter is the cell bodies and dendrites. Note that gray matter necessarily contains both the beginnings and endings of axons, even though the greater portion of many axons' length is contained within the fiber tracts of white matter. Gray matter is gray not because it lacks myelin, but because it contains lots of other stuff besides myelinated axons.
Axons from many different neurons often gather together in large numbers at some distance from their cell bodies. In such regions, the relatively large amount of myelin confers a white color, hence, white matter. Myelin is largely fat, which is white in both living and fixed condition. Note that although white matter consists of myelinated axons and unmyelinated axons as well , myelinated axons are not excluded from gray matter.
Myelinated axons must begin and end somewhere, and that place is with cell bodies and dendrites of gray matter.
Gray matter just has a lot of other stuff in it besides myelinated axons. Also note that in many neuroanatomical images, white matter has been stained black. Sensory neurons convey sensory information into the central nervous system.
Primary sensory neurons receive their information directly through sense receptors rather than dendrites. Second, third and higher order sensory neurons relay information to sequentially higher levels in the brain.
Motor neurons or motoneurons convey information out from the central nervous system to muscles or glands. Lower motor neurons , located in the ventral horn of the spinal cord or in motor nuclei of the brainstem, send their motor axons out peripheral nerves. Upper motor neurons , pyramidal cells located in the motor cortex , relay information to the lower motor neurons. All other neurons are interneurons. They interconnect neurons with other neurons.
Nearly all the nerve cells in the central nervous system are interneurons. Their axons arise in one region of the CNS where the cell body resides and end somewhere else sometimes several other places. Second, third and higher order sensory neurons can be considered as ascending interneurons; upper motor neurons can be considered as descending interneurons.
Information from primary sensory neurons does not reach the highest levels the cerebral cortex directly. Rather it is relayed at least twice once in the spinal cord or brain stem , again in the thalamus.
At each relay, incoming afferent , presynaptic axons terminate by synapsing onto the dendrites of the next neurons in the series. The outgoing axons of these neurons then relay the information to the next level. At each relay site, some information processing and distribution can occur, so the information can be altered as it travels upward. Similarly, muscle commands are relayed downward from motor cortex and other motor centers to the " final common pathway ", the lower motor neurons of cranial nerve nuclei and the anterior horn of the spinal cord.
Because each relay occurs at synapses onto dendrites and cell bodies of the next neurons in the pathway, each relay is associated with gray matter.
Conversely, every gray matter region nucleus or cortex is associated with relaying information from one set of axons the afferent axons that enter the region in question to another the efferent axons that leave the region.
Sometimes it is sufficient just to know the beginning and ending points of an entire pathway. Other times knowing how far the neurons of each relay extend will be necessary to determine the site or effects of a lesion. All gray matter regions of the brain, both cortex and nuclei, are associated with afferent "input" and efferent "output" axons.