In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another neuron or to the target effector cell. Santiago Ramón y Cajal proposed that neurons are not continuous throughout the body, yet still communicate with each other, an idea known as the neuron doctrine; the word "synapse" – from the Greek synapsis, meaning "conjunction", in turn from συνάπτεὶν – was introduced in 1897 by the English neurophysiologist Charles Sherrington in Michael Foster's Textbook of Physiology. Sherrington struggled to find a good term that emphasized a union between two separate elements, the actual term "synapse" was suggested by the English classical scholar Arthur Woollgar Verrall, a friend of Foster; some authors generalize the concept of the synapse to include the communication from a neuron to any other cell type, such as to a motor cell, although such non-neuronal contacts may be referred to as junctions. Synapses are essential to neuronal function: neurons are cells that are specialized to pass signals to individual target cells, synapses are the means by which they do so.
At a synapse, the plasma membrane of the signal-passing neuron comes into close apposition with the membrane of the target cell. Both the presynaptic and postsynaptic sites contain extensive arrays of a molecular machinery that link the two membranes together and carry out the signaling process. In many synapses, the presynaptic part is located on an axon and the postsynaptic part is located on a dendrite or soma. Astrocytes exchange information with the synaptic neurons, responding to synaptic activity and, in turn, regulating neurotransmission. Synapses are stabilized in position by synaptic adhesion molecules projecting from both the pre- and post-synaptic neuron and sticking together where they overlap. There are two fundamentally different types of synapses: In a chemical synapse, electrical activity in the presynaptic neuron is converted into the release of a chemical called a neurotransmitter that binds to receptors located in the plasma membrane of the postsynaptic cell; the neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron.
Chemical synapses can be classified according to the neurotransmitter released: glutamatergic, GABAergic and adrenergic. Because of the complexity of receptor signal transduction, chemical synapses can have complex effects on the postsynaptic cell. In an electrical synapse, the presynaptic and postsynaptic cell membranes are connected by special channels called gap junctions or synaptic cleft that are capable of passing an electric current, causing voltage changes in the presynaptic cell to induce voltage changes in the postsynaptic cell; the main advantage of an electrical synapse is the rapid transfer of signals from one cell to the next. Synaptic communication is distinct from an ephaptic coupling, in which communication between neurons occurs via indirect electric fields. An autapse is a chemical or electrical synapse that forms when the axon of one neuron synapses onto dendrites of the same neuron. Synapses can be classified by the type of cellular structures serving as the pre- and post-synaptic components.
The vast majority of synapses in the mammalian nervous system are classical axo-dendritic synapses, however, a variety of other arrangements exist. These include but are not limited to axo-axonic, dendro-dendritic, axo-secretory, somato-dendritic, dendro-somatic, somato-somatic synapses; the axon can synapse onto a dendrite, onto a cell body, or onto another axon or axon terminal, as well as into the bloodstream or diffusely into the adjacent nervous tissue. It is accepted that the synapse plays a role in the formation of memory; as neurotransmitters activate receptors across the synaptic cleft, the connection between the two neurons is strengthened when both neurons are active at the same time, as a result of the receptor's signaling mechanisms. The strength of two connected neural pathways is thought to result in the storage of information, resulting in memory; this process of synaptic strengthening is known as long-term potentiation. By altering the release of neurotransmitters, the plasticity of synapses can be controlled in the presynaptic cell.
The postsynaptic cell can be regulated by altering the number of its receptors. Changes in postsynaptic signaling are most associated with a N-methyl-d-aspartic acid receptor -dependent long-term potentiation and long-term depression due to the influx of calcium into the post-synaptic cell, which are the most analyzed forms of plasticity at excitatory synapses. For technical reasons, synaptic structure and function have been studied at unusually large model synapses, for example: Squid giant synapse Neuromuscular junction, a cholinergic synapse in vertebrates, glutamatergic in insects Ciliary calyx in the ciliary ganglion of chicks Calyx of Held in the brainstem Ribbon synapse in the retina Schaffer collateral synapse in the hippocampus The function of neurons depends upon cell polarity; the distinctive structure of nerve cells allows action potentials to travel directionally, for these signals to be received and carried on by post-synaptic neurons or received by effector cells. Nerve cells have long been used as models f
A primate is a eutherian mammal constituting the taxonomic order Primates. Primates arose 85–55 million years ago from small terrestrial mammals, which adapted to living in the trees of tropical forests: many primate characteristics represent adaptations to life in this challenging environment, including large brains, visual acuity, color vision, altered shoulder girdle, dexterous hands. Primates range in size from Madame Berthe's mouse lemur, which weighs 30 g, to the eastern gorilla, weighing over 200 kg. There are 190 -- 448 species of living primates, depending on. New primate species continue to be discovered: over 25 species were described in the first decade of the 2000s, eleven since 2010. Primates are divided into two distinct suborders; the first is the strepsirrhines - lemurs and lorisids. The second is haplorhines - the "dry-nosed" primates - tarsier and ape clades, the last of these including humans. Simians are monkeys and apes, cladistically including: the catarrhines consisting of the Old World monkeys and apes.
Forty million years ago, simians from Africa migrated to South America by drifting on debris, gave rise to the New World monkeys. Twenty five million years ago the remaining Old World simians split into Old World monkeys. Common names for the simians are the baboons, macaques and great apes. Primates have large brains compared to other mammals, as well as an increased reliance on visual acuity at the expense of the sense of smell, the dominant sensory system in most mammals; these features are more developed in monkeys and apes, noticeably less so in lorises and lemurs. Some primates are trichromats, with three independent channels for conveying color information. Except for apes, primates have tails. Most primates have opposable thumbs. Many species are sexually dimorphic. Primates have slower rates of development than other sized mammals, reach maturity and have longer lifespans. Depending on the species, adults may live in solitude, in mated pairs, or in groups of up to hundreds of members; some primates, including gorillas and baboons, are terrestrial rather than arboreal, but all species have adaptations for climbing trees.
Arboreal locomotion techniques used include leaping from tree to tree and swinging between branches of trees. Primates are among the most social of animals, forming pairs or family groups, uni-male harems, multi-male/multi-female groups. Non-human primates have at four types of social systems, many defined by the amount of movement by adolescent females between groups. Most primate species remain at least arboreal: the exceptions are some great apes and humans, who left the trees for the ground and now inhabit every continent. Close interactions between humans and non-human primates can create opportunities for the transmission of zoonotic diseases virus diseases, including herpes, ebola and hepatitis. Thousands of non-human primates are used in research around the world because of their psychological and physiological similarity to humans. About 60% of primate species are threatened with extinction. Common threats include deforestation, forest fragmentation, monkey drives, primate hunting for use in medicines, as pets, for food.
Large-scale tropical forest clearing for agriculture most threatens primates. The English name "primates" is derived from Old French or French primat, from a noun use of Latin primat-, from primus; the name was given by Carl Linnaeus. The relationships among the different groups of primates were not understood until recently, so the used terms are somewhat confused. For example, "ape" has been used either as an alternative for "monkey" or for any tailless human-like primate. Sir Wilfrid Le Gros Clark was one of the primatologists who developed the idea of trends in primate evolution and the methodology of arranging the living members of an order into an "ascending series" leading to humans. Used names for groups of primates such as "prosimians", "monkeys", "lesser apes", "great apes" reflect this methodology. According to our current understanding of the evolutionary history of the primates, several of these groups are paraphyletic: a paraphyletic group is one which does not include all the descendants of the group's common ancestor.
In contrast with Clark's methodology, modern classifications identify only those groupings that are monophyletic. The cladogram below shows one possible classification sequence of the living primates: groups that use common names are shown on the right. All groups with scientific names are monophyletic, the sequence of scientific classification reflects the evolution
Brodmann area 24
Brodmann area 24 is part of the anterior cingulate in the human brain. In the human this area is known as ventral anterior cingulate area 24, it refers to a subdivision of the cytoarchitecturally defined cingulate cortex region of cerebral cortex, it occupies most of the anterior cingulate gyrus in an arc around the genu of the corpus callosum. Its outer border corresponds to the cingulate sulcus. Cytoarchitecturally it is bounded internally by the pregenual area 33, externally by the dorsal anterior cingulate area 32, caudally by the ventral posterior cingulate area 23 and the dorsal posterior cingulate area 31. Francis Crick, one of the co-discoverers of the structure of DNA, listed area 24 as the seat of free will because of its centrality in abulia and amotivational syndromes. In the guenon this area is referred to as area 24 of Brodmann-1905, it includes portions of the frontal lobe. The cortex is thin. Note that Brodmann divided this area into two areas, area 24 of Brodmann-1909 and area 25 of Brodmann-1909.
The area has been subdivided further, Vogt et al. make three division for the area in the rhesus monkey: 24a: "adjacent to the callosal sulcus" 24b: "has more defined layers II, III, Va" 24c: "the lower bank of the anterior cingulate sulcus" Brodmann area List of regions in the human brain Anterior cingulate cortex List of Brodmann areas
The orbitofrontal cortex is a prefrontal cortex region in the frontal lobes of the brain, involved in the cognitive process of decision-making. In non-human primates it consists of the association cortex areas Brodmann area 11, 12 and 13; the OFC is considered anatomically synonymous with the ventromedial prefrontal cortex. Therefore, the region is distinguished due to the distinct neural connections and the distinct functions it performs, it is defined as the part of the prefrontal cortex that receives projections from the medial dorsal nucleus of the thalamus, is thought to represent emotion and reward in decision making. It gets its name from its position above the orbits in which the eyes are located. Considerable individual variability has been found in the OFC of humans. A related area is found in rodents; the OFC is divided into multiple broad regions distinguished by cytoarchitecture, including brodmann area 47/12, brodmann area 11, brodmann area 14, brodmann area 13, brodmann area 10.
Four gyri are split by a complex of sulci that most resembles a "H" or a "K" pattern. Extending along the rostro-caudal axis, two sulci, the lateral and orbital sulci, are connected by the transverse orbital suclus, which extends along a medial-lateral axis. Most medially, the medial orbital gyrus is separated from the gyrus rectus by the olfactory sulcus. Anteriorly, both the gyrus rectus and the medial part of the medial orbital gyrus consist of area 11, posteriorly, area 14; the posterior orbital gyrus consists of area 13, is bordered medially and laterally by the anterior limbs of the medial and lateral orbital sulci. Area 11 makes up a large part of the OFC involving both the lateral parts of the medial orbital gyrus as well as the anterior orbital gyrus; the lateral orbital gyrus consists of area 47/12. Most of the OFC is granular, although the caudal parts of area 14 are agranular; these caudal regions, which sometimes includes parts of the insular cortex, responds to unprocessed sensory cues.
The connectivity of the OFC varies somewhat along a rostral-caudal axis. The caudal OFC is more interconnected with sensory regions, notably receiving direct input from the pyriform cortex; the caudal OFC is the most interconnected with the amygdala. Rostrally, the OFC receives fewer direct sensory projections, is less connected with the amygdala, but it is interconnected with the lateral prefrontal cortex and parahippocampus; the connectivity of the OFC has been conceptualized as being composed of two networks. The medial and orbital networks are sometimes referred to as the "visceromotor network" and the "sensory network", respectively; the OFC receives projections from multiple sensory modalities. The primary olfactory cortex, gustatory cortex, secondary somatosensory cortex and inferior temporal gyrus all project to the OFC. Evidence for auditory inputs is weak, although some neurons respond to auditory stimuli, indicating an indirect projection may exist; the OFC receives input from the medial dorsal nucleus, insular cortex, entorhinal cortex, perirhinal cortex and amygdala.
The orbitofrontal cortex is reciprocally connected with the perirhinal and entorhinal cortices, the amygdala, the hypothalamus, parts of the medial temporal lobe. In addition to these outputs, the OFC projects to the striatum, including the nucleus accumbens, caudate nucleus, ventral putamen, as well as regions of the midbrain including the periaqueductal grey, ventral tegmental area. OFC inputs to the amygdala synapse on multiple targets, including two robust pathways to the basolateral amygdala and intercalated cells of the amygdala, as well as a weaker direct projection to the central nucleus of the amygdala. Multiple functions have been ascribed to the OFC including mediating context specific responding, encoding contingencies in a flexible manner, encoding value, encoding inferred value, inhibiting responses, learning changes in contingency, emotional appraisal, altering behavior through somatic markers, driving social behavior, representing state spaces. While most of these theories explain certain aspects of electrophysiological observations and lesion related changes in behavior, they fail to explain, or are contradicted by other findings.
One proposal that explains the variety of OFC functions is that the OFC encodes state spaces, or the discrete configuration of internal and external characteristics associated with a situation and its contingencies For example the proposal that the OFC encodes economic value may be a reflection of the OFC encoding task state value. The representation of task states could explain the proposal that the OFC acts as a flexible map of contingencies, as a switch in task state would enable the encoding of new contingencies in one state, with the preservation of old contingencies in a separate state, enabling switching contingencies when the old task state becomes relevant again; the representation of task states is supported by electrophysiological evidence demonstrating that the OFC responds to a diverse array of task features, is capable of remapping during contingency shifts. The representation of task states may influence behavior through multiple potential mechanisms. For example, the OFC is necessary for ventral tegmental area neurons to produce a dopaminergic reward prediction error, the OFC may encode expectations
The visual cortex of the brain is that part of the cerebral cortex which processes visual information. It is located in the occipital lobe. Visual nerves run straight from the eye to the primary visual cortex to the Visual Association cortex. Visual information coming from the eye goes through the lateral geniculate nucleus in the thalamus and reaches the visual cortex; the part of the visual cortex that receives the sensory inputs from the thalamus is the primary visual cortex known as visual area 1, the striate cortex. The extrastriate areas consist of visual areas 2, 3, 4, 5. Both hemispheres of the brain contain a visual cortex; the primary visual cortex is located around the calcarine fissure in the occipital lobe. Each hemisphere's V1 receives information directly from its ipsilateral lateral geniculate nucleus that receives signals from the contralateral visual hemifield. Neurons in the visual cortex fire action potentials when visual stimuli appear within their receptive field. By definition, the receptive field is the region within the entire visual field that elicits an action potential.
But, for any given neuron, it may respond best to a subset of stimuli within its receptive field. This property is called neuronal tuning. In the earlier visual areas, neurons have simpler tuning. For example, a neuron in V1 may fire to any vertical stimulus in its receptive field. In the higher visual areas, neurons have complex tuning. For example, in the inferior temporal cortex, a neuron may fire only when a certain face appears in its receptive field; the visual cortex receives its blood supply from the calcarine branch of the posterior cerebral artery. V1 transmits information to two primary pathways, called the dorsal stream; the ventral stream begins with V1, goes through visual area V2 through visual area V4, to the inferior temporal cortex. The ventral stream, sometimes called the "What Pathway", is associated with form recognition and object representation, it is associated with storage of long-term memory. The dorsal stream begins with V1, goes through Visual area V2 to the dorsomedial area and Visual area MT and to the posterior parietal cortex.
The dorsal stream, sometimes called the "Where Pathway" or "How Pathway", is associated with motion, representation of object locations, control of the eyes and arms when visual information is used to guide saccades or reaching. The what vs. where account of the ventral/dorsal pathways was first described by Ungerleider and Mishkin. More Goodale and Milner extended these ideas and suggested that the ventral stream is critical for visual perception whereas the dorsal stream mediates the visual control of skilled actions, it has been shown that visual illusions such as the Ebbinghaus illusion distort judgements of a perceptual nature, but when the subject responds with an action, such as grasping, no distortion occurs. Work such as the one from Scharnowski and Gegenfurtner suggests that both the action and perception systems are fooled by such illusions. Other studies, provide strong support for the idea that skilled actions such as grasping are not affected by pictorial illusions and suggest that the action/perception dissociation is a useful way to characterize the functional division of labor between the dorsal and ventral visual pathways in the cerebral cortex.
The primary visual cortex is the most studied visual area in the brain. In mammals, it is located in the posterior pole of the occipital lobe and is the simplest, earliest cortical visual area, it is specialized for processing information about static and moving objects and is excellent in pattern recognition. The functionally defined primary visual cortex is equivalent to the anatomically defined striate cortex; the name "striate cortex" is derived from the line of Gennari, a distinctive stripe visible to the naked eye that represents myelinated axons from the lateral geniculate body terminating in layer 4 of the gray matter. The primary visual cortex is divided into six functionally distinct layers, labeled 1 to 6. Layer 4, which receives most visual input from the lateral geniculate nucleus, is further divided into 4 layers, labelled 4A, 4B, 4Cα, 4Cβ. Sublamina 4Cα receives magnocellular input from the LGN, while layer 4Cβ receives input from parvocellular pathways; the average number of neurons in the adult human primary visual cortex in each hemisphere has been estimated at around 140 million.
The tuning properties of V1 neurons differ over time. Early in time individual V1 neurons have strong tuning to a small set of stimuli; that is, the neuronal responses can discriminate small changes in visual orientations, spatial frequencies and colors. Furthermore, individual V1 neurons in humans and animals with binocular vision have ocular dominance, namely tuning to one of the two eyes. In V1, primary sensory cortex in general, neurons with similar tuning properties tend to cluster together as cortical columns. David Hubel and Torsten Wiesel proposed the classic ice-cube organization model of cortical columns for two tuning properties: ocular dominance and orientation. However, this model cannot accommodate the color, spatial frequency and many other features to which neurons are tuned; the exact organization of all these cortical columns within V1 remains a hot topic of current research. The mathematical modeling of this function has been compared t
Myelin is a lipid-rich substance formed in the central nervous system by glial cells called oligodendrocytes, in the peripheral nervous system by Schwann cells. Myelin insulates nerve cell axons to increase the speed at which information travels from one nerve cell body to another or, for example, from a nerve cell body to a muscle; the myelinated axon can be likened to an electrical wire with insulating material around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, each myelin sheath insulates the axon over a single section and, in general, each axon comprises multiple long myelinated sections separated from each other by short gaps called Nodes of Ranvier; each myelin sheath is formed by the concentric wrapping of an oligodendrocyte or Schwann cell process around the axon. More myelin speeds the transmission of electrical impulses called action potentials along myelinated axons by insulating the axon and reducing axonal membrane capacitance.
This results in saltatory conduction whereby the action potential "jumps" from one node of Ranvier, over a long myelinated stretch of the axon called the internode, before "recharging" at the next node of Ranvier, so on, until it reaches the axon terminal. Nodes of Ranvier are the short unmyelinated regions of the axon between adjacent long myelinated internodes. Once it reaches the axon terminal, this electrical signal provokes the release of a chemical message or neurotransmitter that binds to receptors on the adjacent post-synaptic cell at specialised regions called synapses; this "insulating" role for myelin is essential for normal motor function, sensory function and cognition, as demonstrated by the consequences of disorders that affect it, such as the genetically determined leukodystrophies. Due to its high prevalence, multiple sclerosis, which affects the central nervous system, is the best known disorder of myelin; the process of generating myelin is called myelination or myelinogenesis.
In the CNS, cells called oligodendrocyte precursor cells differentiate into mature oligodendrocytes, which form myelin. In humans, myelination begins early in the 3rd trimester, although only little myelin is present in either the CNS or the PNS at the time of birth. During infancy, myelination progresses with increasing numbers of axons acquiring myelin sheaths; this corresponds with the development of cognitive and motor skills, including language comprehension, speech acquisition and walking. Myelination continues through adolescence and early adulthood and although complete at this time, myelin sheaths can be added in grey matter regions such as the cerebral cortex, throughout life. Myelin is considered a defining characteristic of the jawed vertebrates, but axons are ensheathed by glial cells in invertebrates, although these glial-wraps are quite different from vertebrate compact myelin, formed, as indicated above, by concentric wrapping of the myelinating cell process multiple times around the axon.
Myelin was first described in 1854 by Rudolf Virchow, although it was over a century following the development of electron microscopy, that its glial cell origin and its ultrastructure became apparent. In vertebrates, not all axons are myelinated. For example, in the PNS, a large proportion of axons are unmyelinated. Instead, they are ensheathed by non-myelinating Schwann cells known as Remak SCs and arranged in Remak bundles. In the CNS, non-myelinated, intermingle with myelinated ones and are entwined, at least by the processes of another type of glial cell called the astrocyte. CNS myelin differs in composition and configuration from PNS myelin, but both perform the same "insulating" function. Being rich in lipid, myelin appears white. Both CNS white matter tracts and PNS nerves each comprise thousands to millions of axons aligned in parallel. Blood vessels provide the route for oxygen and energy substrates such as glucose to reach these fibre tracts, which contain other cell types including astrocytes and microglia in the CNS and macrophages in the PNS.
In terms of total mass, myelin comprises 40% water. Protein content includes myelin basic protein, abundant in the CNS where it plays a critical, non-redundant role in formation of compact myelin. In the PNS, myelin protein zero has a similar role to that of PLP in the CNS in that it is involved in holding together the multiple concentric layers of glial cell membrane that constitute the myelin sheath; the primary lipid of myelin is a glycolipid called galactocerebroside. The intertwining hydrocarbon chains of sphingomyelin strengthen the
An axon, or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, that conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons and glands. In certain sensory neurons, such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body, from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders which can affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, group C nerve fibers. Groups A and B are myelinated, group C are unmyelinated; these groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, Type IV. An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron.
Axons are distinguished from dendrites by several features, including shape and function. Some types of neurons have no transmit signals from their dendrites. In some species, axons can emanate from dendrites and these are known as axon-carrying dendrites. No neuron has more than one axon. Axons are covered by a membrane known as an axolemma. Most axons branch, in some cases profusely; the end branches of an axon are called telodendria. The swollen end of a telodendron is known as the axon terminal which joins the dendron or cell body of another neuron forming a synaptic connection. Axons make contact with other cells—usually other neurons but sometimes muscle or gland cells—at junctions called synapses. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse. At a synapse, the membrane of the axon adjoins the membrane of the target cell, special molecular structures serve to transmit electrical or electrochemical signals across the gap.
Some synaptic junctions appear along the length of an axon as it extends—these are called en passant synapses and can be in the hundreds or the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches. A single axon, with all its branches taken together, can innervate multiple parts of the brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system, a fascicle in the peripheral nervous system. In placental mammals the largest white matter tract in the brain is the corpus callosum, formed of some 20 million axons in the human brain. Axons are the primary transmission lines of the nervous system, as bundles they form nerves; some axons can extend up to more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot; the diameter of axons is variable. Most individual axons are microscopic in diameter.
The largest mammalian axons can reach a diameter of up to 20 µm. The squid giant axon, specialized to conduct signals rapidly, is close to 1 millimetre in diameter, the size of a small pencil lead; the numbers of axonal telodendria can differ from one nerve fiber to the next. Axons in the central nervous system show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain. There are two types of axons in the nervous system: unmyelinated axons. Myelin is a layer of a fatty insulating substance, formed by two types of glial cells Schwann cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of a myelinated axon. In the central nervous system oligodendrocytes form the insulating myelin.
Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an rapid mode of electrical impulse propagation called saltatory conduction; the myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in the brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum. Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures; the largest of these is the corpus callosum that connects the two cerebral hemispheres, this has around 20 million axons. The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, the axonal region as the other.
The axonal region or compart