Types of cortical neuroplasticity
In some cases, such as stroke recovery, natural adult neurogenesis can also play a role. Axon length may exceed a meter for many sensory and motor axons , and commonly extends for several centimeters. The main types of cells in the human body are listed below:. However, transcription factors that determine the type of nociceptor remain unclear. Monocytes circulate in the bloodstream between one and three days before entering the tissues of the body where they become macrophages. Tachycardia may be treated with beta blockers.
Red Blood Cells
The enteric nervous system is a meshwork of nerve fibers that innervate the viscera gastrointestinal tract, pancreas, gall bladder. The following table shows how the nervous system can be divided. The bottom row of the table contains the names of specific areas within the brain.
From a top view, notice how the brain is divided into two halves, called hemispheres. Each hemisphere communicates with the other through the corpus callosum , a bundle of nerve fibers. Another smaller fiber bundle that connects the two hemispheres is called the anterior commissure. The word "cortex" comes from the Latin word for "bark" of a tree. This is because the cortex is a sheet of tissue that makes up the outer layer of the brain.
The thickness of the cerebral cortex varies from 2 to 6 mm. The right and left sides of the cerebral cortex are connected by a thick band of nerve fibers called the "corpus callosum. A bump or bulge on the cortex is called a gyrus the plural of the word gyrus is "gyri" and a groove is called a sulcus the plural of the word sulcus is "sulci". Lower mammals, such as rats and mice, have very few gyri and sulci. The word "cerebellum" is derived from the Latin word for "little brain.
The brain stem refers to the area of the brain between the thalamus and spinal cord. Structures of the brain stem include the pons, medulla oblongta, tectum, reticular formation and tegmentum.
The brain stem is important for maintaining basic life functions such as breathing, heart rate and blood pressure. The hypothalamus is composed of several different areas and is located at the base of the brain. One function of the hypothalamus is the control of body temperature.
The hypothalamus detects changes in body temperature and sends commands to adjust the temperature. For example, the hypothalamus can detect fever and respond by sending a command to expand capillaries in the skin. The expansion of the capillaries cools the blood and results in a drop in body temperature. The hypothalamus also controls the pituitary.
The thalamus receives sensory information from other areas of the nervous system and sends this information to the cerebral cortex. The thalamus is also important for processing information related to movement. The limbic system or the limbic areas is a group of structures that includes the amygdala, the hippocampus, mammillary bodies and cingulate gyrus. These areas are important for controlling the emotional response to a given situation. The hippocampus is also important for memory.
The basal ganglia are a group of structures, including the globus pallidus, caudate nucleus, subthalamic nucleus, putamen and substantia nigra, that are important in coordinating movement. The midbrain includes structures such as the superior and inferior colliculi and red nucleus. There are several other areas also in the midbrain. Secondary symptoms may include high level cognitive dysfunction and subtle language problems.
PD is both chronic and progressive. Myasthenia gravis is a neuromuscular disease leading to fluctuating muscle weakness and fatigability during simple activities.
Weakness is typically caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppressants , cholinesterase inhibitors and, in selected cases, thymectomy. Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves.
When myelin degrades, conduction of signals along the nerve can be impaired or lost, and the nerve eventually withers. This leads to certain neurodegenerative disorders like multiple sclerosis and chronic inflammatory demyelinating polyneuropathy. Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acute axonal degeneration, which is rapid separation of the proximal and distal ends within 30 minutes of injury.
Degeneration follows with swelling of the axolemma , and eventually leads to bead like formation. Granular disintegration of the axonal cytoskeleton and inner organelles occurs after axolemma degradation.
Early changes include accumulation of mitochondria in the paranodal regions at the site of injury. Endoplasmic reticulum degrades and mitochondria swell up and eventually disintegrate.
The disintegration is dependent on ubiquitin and calpain proteases caused by influx of calcium ion , suggesting that axonal degeneration is an active process. Thus the axon undergoes complete fragmentation. The signaling pathways leading to axolemma degeneration are currently unknown. Neurons are born through the process of neurogenesis, in which neural stem cells divide to produce differentiated neurons. Once fully differentiated neurons are formed, they are no longer capable of undergoing mitosis.
Neurogenesis primarily occurs in the embryo of most organisms. It has been demonstrated that neurogenesis can sometimes occur in the adult vertebrate brain, a finding that led to controversy in The body contains a variety of stem cell types that have the capacity to differentiate into neurons. A report in Nature suggested that researchers had found a way to transform human skin cells into working nerve cells using a process called transdifferentiation in which "cells are forced to adopt new identities".
From Wikipedia, the free encyclopedia. This article is about cells in the nervous system. For other uses, see Neuron disambiguation. For other uses, see Glial cell. Synapse and Chemical synapse. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. May Learn how and when to remove this template message.
The New York Times. Journal of Clinical Neurophysiology. A journey through the brain. State Commission in Lunacy. Anatomy and Physiology' Ed. Frontiers in Cellular Neuroscience. Ion conductances related to shaping the repetitive firing in rat retinal ganglion cells. Action Potentials in Nerve Cells. Archived from the original on August 27, A highly efficient coding scheme for neural networks.
Parallel processing in neural systems , Elsevier, pp. Ramon y Cajal's first paper on the Golgi stain was on the bird cerebellum, and it appeared in the Revista in He acknowledged that he found the nerve fibers to be very intricate, but stated that he could find no evidence for either axons or dendrites undergoing anastomosis and forming nets.
He called each nervous element "an absolutely autonomous canton". In his paper, Waldeyer , The word "neuron" was born this way. Today, Wilhelm von Waldeyer-Hartz is remembered as the founder of the neurone theory, coining the term "neurone" to describe the cellular function unit of the nervous system and enunciating and clarifying that concept in Annual Review of Neuroscience. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences.
The History of Its Discovery. Retrieved March 19, EPSP attenuation and spike trigger zones". The Journal of Neuroscience. The Journal of Physiology. A review of years of cell counting". The Journal of Comparative Neurology. National Institute on Aging. Archived from the original on 15 January Retrieved 28 December The New England Journal of Medicine.
Archived from the original on 4 January Retrieved 18 July The International Neuromodulation Society. Evidence and Remaining Questions". By transforming cells from human skin into working nerve cells, researchers may have come up with a model for nervous-system diseases and perhaps even regenerative therapies based on cell transplants.
The achievement, reported online today in Nature , is the latest in a fast-moving field called transdifferentiation, in which cells are forced to adopt new identities.
In the past year, researchers have converted connective tissue cells found in skin into heart cells, blood cells, and liver cells. Find more about Neuron at Wikipedia's sister projects. Astrocyte Radial glial cell Ependymal cells Tanycyte Microglia.
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Parts Soma Axon hillock. 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.
Afferent axons enter the region from somewhere else i. Efferent axons arise from cell bodies within the region and leave the region to go somewhere else. Thus every long-distance axon is both efferent with respect to its source, the location of its cell body and afferent with respect to its destination.
The terms "afferent" and "efferent" are relational terms. Neither can be used precisely without specifying a region of reference e. These terms may often correlate with " afferent " and " efferent ", at least when the reference is high, like cortex. In fact, "afferent" and "efferent" are sometimes used as synonyms of "ascending" and "descending", respectively.
But they also have a relational meaning, defined above. But both sensory and motor information can be passed up, down, and sidewise, so these words should not be carelessly interchanged.
Gray matter typically contains both many short-axon neurons and a smaller number of long-axon neurons. The largest and most conspicuous cell bodies in a particular region of gray matter are sometimes referred to as the principal cells of that region. These cells generally have very long axons which leave the local region to go elsewhere, usually traveling within some white matter fiber tract.
The axons of these projection neurons may extend for appreciable distances, from a few centimeters to well over a meter. Long-axon neurons are responsible for communicating with other brain regions. Every parcel of gray matter has a class of long-axon neurons; otherwise information would come in but never go out.