Akhavan M.D.  

Human's Creation and Evolution

Creation and evolution of human being is one of the most fascinating subjects to discuss. Up to 30-40 years ago most scientists believed in genetic mutation, Darvinian evolution and Newtonian pure materialism.With invention of electron microscope and functional brain scanning, many cell biologists and neuroscientists are now thinking of epigenetics, Lamark's views of evolution and quantum para physical fenomena. We start first with a summery of intra uterine development of human nervous system:

I-Intra-uterine Life
II-Neural Development

1- Fertilization: An ovum from a female and a sperm cell from a male unite to form a diploid cell name "zygote" which contain DNA derived from both the mother and the father. This provides all the genetic information necessary to form a new individual.
2- Morula: Morula is an embryo at an early stage of development, consisting of 16 cells (called blastomeres) in a solid ball.The morula is produced by the rapid mitotic division of the zygote. After reaching the 16-cell stage, the cells of the morula differentiate. The inner blastomeres will become the inner cell mass and the blastomeres on the surface will later flatten to form the trophoblast. Morula travels to the uterus around 3-4 days after fertilization and is implanted into the womb's endometrium.


3- Blastula: 7 days after fertilization(one week embryo), morula which developes a fluid-filled cyst in the center (blastocoel) is now called blastocyst. It has a spherical layer of 128 cells in the attachment pole of the embryo (embryoblast) and flattened one layer trophblasts in the periphery.


4- Week two: Gastrulation is a phase during which the morphology of the embryo is dramatically restructured by cell migration. The purpose of gastrulation is to position the three embryonic germ layers, the endoderm, ectoderm and mesoderm. These layers later develop into certain bodily systems.


It is the ectoderm that develops into the skin, nails, the epithelium of the nose, mouth and anal canal; the lens of the eye, the retina and the "nervous system".
How all the organs form and become functional(organogenesis) is programed in the zigot's genes, although the maternal effect during in utero development is getting more and more attention lately(epigenetics). Genes act like large libraries with many books(DNA) which contain all the information of evolution from initial mono cellular life entity to the human being with hundreds of trillions cells!

2- Neural Development
Development of nervous system is one of the earliest one to begin and the last one to be completed after birth. It generates the most complex structure within the embryo, fetus and newborn.
Week 3 : is about trilaminar embryo development and the beginning of each layer differentiation. At the trilaminar embryonic disc stage, the central portion of ectoderm will form the neural plate which folds to becomes neural tube. It eventually make the brain and spinal cord (CNS). A special group of cells at the edge of the neural plate (neural crest) have an important role to form "PNS" ie peripheral nervous system (both neurons and glia) consisting of sensory ganglia (dorsal root ganglia), sympathetic and parasympathetic ganglia and neural plexuses within specific tissues/organs), head development and many tissues throughout the body.


The Brain:
At the front end of neural tube, the ventricles and cord swell to form three vesicles that are the precursors of the forebrain (the cerebral cortex and basal ganglia), midbrain (the thalamus and hypothalamus), and hindbrain (the cerebellum, pons and medulla oblongata). Each of these areas contains proliferative zones at which neurons and glia cells are generated; the resulting cells then migrate, sometimes for long distances, to their final positions.

The triune brain: is a model proposed by Paul D. MacLean to explain the function of traces of evolution existing in the structure of the human brain. In this model, the brain is broken down into three separate brains that have their own special intelligence, subjectivity, sense of time and space, and memory. The triune brain consists of the R-complex, the limbic system, and the neocortex.
1- The R-complex, also known as the "Reptilian brain", includes the brain stem and cerebellum. The term "Reptilian brain" comes from the fact that a reptile's brain is dominated by the brain stem and cerebellum which controls instinctual survival behaviors and thinking. This brain controls the muscles, balance and autonomic functions (e.g. breathing and heartbeat). It is, thus, primarily reactive to direct stimuli.
2-The "limbic system"- comprises the amygdala, the hypothalamus, and the hippocampus. It is the source of emotions and instincts (e.g.. feeding, fighting, fleeing, and sexual behaviour—also known as "the 4 F's"). When this part of the brain is stimulated, such as by mild electric current, emotions are produced. MacLean observed that everything in the limbic system is either "agreeable or disagreeable." Survival is based upon the avoidance of pain (disagreeable) and the recurrence of pleasure (agreeable).
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3- The Neocortex:
The neocortex, also known as the cerebral cortex, is found in the brain of higher mammals, and is responsible for higher-order thinking skills, reason, speech, and sapience. It consists of the grey matter, or neuronal cell bodies and unmyelinated Axon fibers.
Brain is the governing center of the nervous system in all vertebrate, and most invertebrate animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. The cerebral cortex of the human brain contains many billions neurons linked with up to 10,000 synaptic connections each (neural network).



Neuron:

A neuron (pronounced /ˈnjʊərɒn/ N(Y)OOR-on, also known as a neurone or nerve cell) is an electrically excitable cell that processes and transmits information by electrochemical signaling, via connections with other cells called synapses. Neurons are the core components of the nervous system, which includes the brain, spinal cord, and peripheral ganglia. A number of specialized types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli affecting cells of the sensory organs that then send signals to the spinal cord and brain. Motor neurons receive signals from the brain and spinal cord and cause muscle contractions and affect glands. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord.
These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body and target them to specific recipient cells. A typical neuron possesses a cell body (often called soma), dendrites, and an axon. Dendrites are filaments of protoplasm that extrude from the cell body, often extending for hundreds of microns and branching multiple times, giving rise to a complex "dendritic tree". An axon is a special protoplasmic filament that arises from the cell body at a site called the axon hillock and travels through the body, often for a great distance. The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon, although the axon may branch hundreds of times before it terminates. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another. There are, however, many exceptions to these rules: neurons that lack dendrites, neurons that have no axon, synapses that connect an axon to another axon or a dendrite to another dendrite, etc.All neurons are electrically excitable, maintaining voltage gradients across their membranes by means of metabolically driven ion pumps, which combine with ion channels embedded in the membrane to generate intracellular-versus-extracellular concentration differences of ions such as sodium, potassium, chloride, and calcium. Changes in the cross-membrane voltage can alter the function of voltage-dependent ion channels. If the voltage changes by a large enough amount, an all-or-none electrochemical pulse called an action potential is generated, which travels rapidly along the cell's axon, and activates synaptic connections with other cells when it arrives. Neurons do not undergo cell division, and usually cannot be replaced after being lost. In most cases they are generated by special types of stem cells, although astrocytes (a type of glial cell) have been observed to turn into neurons as they are sometimes pluripotent.

Overwiew:
A neuron is a special type of cell that is found in the bodies of most animals (all members of the group Eumetazoa, to be precise—this excludes only sponges and a few other very simple animals). The features that define a neuron are electrical excitability and the presence of synapses, which are complex membrane junctions used to transmit signals to other cells. The body's neurons, plus the glial cells that give them structural and metabolic support, together constitute the nervous system. In vertebrates, the majority of neurons belong to the central nervous system, but some reside in peripheral ganglia, and many sensory neurons are situated in sensory organs such as the retina and cochlea. Although neurons are very diverse and there are exceptions to nearly every rule, it is convenient to begin with a schematic description of the structure and function of a "typical" neuron. A typical neuron is divided into three parts: the soma or cell body, dendrites, and axon. The soma is usually compact; the axon and dendrites are filaments that extrude from it. Dendrites typically branch profusely, getting thinner with each branching, and extending their farthest branches a few hundred microns from the soma. The axon leaves the soma at a swelling called the axon hillock, and can extend for great distances, giving rise to hundreds of branches. Unlike dendrites, an axon usually maintains the same diameter as it extends. The soma may give rise to numerous dendrites, but never to more than one axon. Synaptic signals from other neurons are received by the soma and dendrites; signals to other neurons are transmitted by the axon. A typical synapse, then, is a contact between the axon of one neuron and a dendrite or soma of another. Synaptic signals may be excitatory or inhibitory. If the net excitation received by a neuron over a short period of time is large enough, the neuron generates a brief pulse called an action potential, which originates at the soma and propagates rapidly along the axon, activating synapses onto other neurons as it goes. Many neurons fit the foregoing schema in every respect, but there are also exceptions to most parts of it. There are no neurons that lack a soma, but there are neurons that lack dendrites, and others that lack an axon. Furthermore, in addition to the typical axodendritic and axosomatic synapses, there are axoaxonic (axon-to-axon) and dendrodendritic (dendrite-to-dendrite) synapses. The key to neural function is the synaptic signalling process, which is partly electrical and partly chemical. The electrical aspect depends on properties of the neuron's membrane. Like all animal cells, every neuron is surrounded by a plasma membrane, a bilayer of lipid molecules with many types of protein structures embedded in it. A lipid bilayer is a powerful electrical insulator, but in neurons, many of the protein structures embedded in the membrane are electrically active. These include ion channels that permit electrically charged ions to flow across the membrane, and ion pumps that actively transport ions from one side of the membrane to the other. Most ion channels are permeable only to specific types of ions. Some ion channels are voltage gated, meaning that they can be switched between open and closed states by altering the voltage difference across the membrane. Others are chemically gated, meaning that they can be switched between open and closed states by interactions with chemicals that diffuse through the extracellular fluid. The interactions between ion channels and ion pumps produce a voltage difference across the membrane, typically a bit less than 1/10 of a volt at baseline. This voltage has two functions: first, it provides a power source for an assortment of voltage-dependent protein machinery that is embedded in the membrane; second, it provides a basis for electrical signal transmission between different parts of the membrane. Neurons communicate by chemical and electrical synapses in a process known as synaptic transmission. The fundamental process that triggers synaptic transmission is the action potential, a propagating electrical signal that is generated by exploiting the electrically excitable membrane of the neuron. This is also known as a wave of depolarization.
Histology:Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous system, there is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter.[1] * The soma is the central part of the neuron. It contains the nucleus of the cell, and therefore is where most protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter.[2] * The dendrites of a neuron are cellular extensions with many branches, and metaphorically this overall shape and structure is referred to as a dendritic tree. This is where the majority of input to the neuron occurs. * The axon is a finer, cable-like projection which can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve signals away from the soma (and also carries some types of information back to it). Many neurons have only one axon, but this axon may—and usually will—undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called the axon hillock. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. This makes it the most easily-excited part of the neuron and the spike initiation zone for the axon: in neurological terms it has the most negative action potential threshold. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons. * The axon terminal contains synapses, specialized structures where neurotransmitter chemicals are released in order to communicate with target neurons. Although the canonical view of the neuron attributes dedicated functions to its various anatomical components, dendrites and axons often act in ways contrary to their so-called main function. Axons and dendrites in the central nervous system are typically only about one micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a human motoneuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the dorsal columns, over 1.5 meters in adults. Giraffes have single axons several meters in length running along the entire length of their necks. Much of what is known about axonal function comes from studying the squid giant axon, an ideal experimental preparation because of its relatively immense size (0.5–1 millimeters thick, several centimeters long). Fully differentiated neurons are permanently amitotic;[3] however, recent research shows that additional neurons throughout the brain can originate from neural stem cells found throughout the brain but in particularly high concentrations in the subventricular zone and subgranular zone through the process of neurogenesis.

Direction:
* Afferent neurons convey information from tissues and organs into the central nervous system and are sometimes also called sensory neurons. * Efferent neurons transmit signals from the central nervous system to the effector cells and are sometimes called motor neurons. * Interneurons connect neurons within specific regions of the central nervous system. Afferent and efferent can also refer generally to neurons which, respectively, bring information to or send information from the brain region.
The neuron doctrine: is the now fundamental idea that neurons are the basic structural and functional units of the nervous system. The theory was put forward by Santiago Ramón y Cajal in the late 19th century. It held that neurons are discrete cells (not connected in a meshwork), acting as metabolically distinct units. Later discoveries yielded a few refinements to the simplest form of the doctrine. For example, glial cells, which are not considered neurons, play an essential role in information processing.[14] Also, electrical synapses are more common than previously thought,[15] meaning that there are direct, cytoplasmic connections between neurons. An electrical synapse is a mechanical and electrically conductive link between two abutting neuron cells that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. At gap junctions, such cells approach within about 3.5 nm of each other (Kandel et al. 2000), a much shorter distance than the 20 to 40 nm distance that separates cells at chemical synapse (Hormuzdi et al. 2004). In organisms, electrical synapse-based systems co-exist with chemical synapses. Compared to chemical synapses, electrical synapses conduct nerve impulses faster, but unlike chemical synapses they do not have gain (the signal in the post synaptic neuron is always smaller than that of the originating neuron). Electrical synapses are often found in neural systems that require the fastest possible response, such as defensive reflexes. An important characteristic of electrical synapses is that most of the time, they are bidirectional, i.e. they allow impulse transmission in either direction.[1] However, some gap junctions do allow for communication in only one direction.
Dendrites (from Greek δένδρον déndron, “tree”) are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project. Electrical stimulation is transmitted onto dendrites by upstream neurons via synapses which are located at various points throughout the dendritic arbor. Dendrites play a critical role in integrating these synaptic inputs and in determining the extent to which action potentials are produced by the neuron. Recent research has also found that dendrites can support action potentials and release neurotransmitters. This property was originally believed to be specific to axons. The long outgrowths on dendritic cells are also called dendrites. These dendrites do not process electrical signals. Despite the critical role that dendrites play in the computational tendencies of neurons, very little is known about the process by which dendrites orient themselves in vivo and are compelled to create the intricate branching pattern unique to each specific neuronal class. It is likely that a complex array of extracellular and intracellular cues modulate dendrite development.
synapse : In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell. The word "synapse" comes from "synaptein", which Sir Charles Scott Sherrington and colleagues coined from the Greek "syn-" ("together") and "haptein" ("to clasp"). Synapses are essential to neuronal function: neurons are cells that are specialized to pass signals to individual target cells, and synapses are the means by which they do so. At a synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target (postsynaptic) cell. Both the presynaptic and postsynaptic sites contain extensive arrays of 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, but some presynaptic sites are located on a dendrite or soma. There are two fundamentally different types of synapse: * In a chemical synapse, the presynaptic neuron releases a chemical called a neurotransmitter that binds to receptors located in the postsynaptic cell, usually embedded in the plasma membrane. Binding of the neurostransmitter to a receptor can affect the postsynaptic cell in a wide variety of ways. * In an electrical synapse, the presynaptic and postsynaptic cell membranes are connected by channels that are capable of passing electrical current, causing voltage changes in the presynaptic cell to induce voltage changes in the postsynaptic cell.

Synapses:
are functional connections between neurons, or between neurons and other types of cells.[2][3] A typical neuron gives rise to several thousand synapses, although there are some types that make far fewer.[4] Most synapses connect axons to dendrites,[5][6] but there are also other types of connections, including axon-to-cell-body[7][8], axon-to-axon,[7][8] and dendrite-to-dendrite.[6] Synapses are generally too small to be recognizable using a light microscope except as points where the membranes of two cells appear to touch, but their cellular elements can be visualized clearly using an electron microscope. Chemical synapses pass information directionally from a presynaptic cell to a postsynaptic cell and are therefore asymmetric in structure and function. The presynaptic terminal, or synaptic bouton, is a specialized area within the axon of the presynaptic cell that contains neurotransmitters enclosed in small membrane-bound spheres called synaptic vesicles. Synaptic vesicles are docked at the presynaptic plasma membrane at regions called active zones (AZ).Immediately opposite is a region of the postsynaptic cell containing neurotransmitter receptors; for synapses between two neurons the postsynaptic region may be found on the dendrites or cell body. Immediately behind the postsynaptic membrane is an elaborate complex of interlinked proteins called the postsynaptic density (PSD). Proteins in the PSD are involved in anchoring and trafficking neurotransmitter receptors and modulating the activity of these receptors. The receptors and PSDs are often found in specialized protrusions from the main dendritic shaft called dendritic spines. Between the pre- and postsynaptic cells is a gap about 20 nm wide called the synaptic cleft. The small volume of the cleft allows neurotransmitter concentration to be raised and lowered rapidly. The membranes of the two adjacent cells are held together by cell adhesion proteins. Signaling in chemical synapses [edit] Overview Here is a summary of the sequence of events that take place in synaptic transmission from a presynaptic neuron to a postsynaptic cell. Each step is explained in more detail below. Note that with the exception of the final step, the entire process may run only a few tenths of a millisecond, in the fastest synapses. 1. The process begins with a wave of electrochemical excitation called an action potential traveling along the membrane of the presynaptic cell, until it reaches the synapse. 2. The electrical depolarization of the membrane at the synapse causes channels to open that are permeable to calcium ions. 3. Calcium ions flow through the presynaptic membrane, rapidly increasing the calcium concentration in the interior. 4. The high calcium concentration activates a set of calcium-sensitive proteins attached to vesicles that contain a neurotransmitter chemical. 5. These proteins change shape, causing the membranes of some "docked" vesicles to fuse with the membrane of the presynaptic cell, thereby opening the vesicles and dumping their neurotransmitter contents into the synaptic cleft, the narrow space between the membranes of the pre- and post-synaptic cells. 6. The neurotransmitter diffuses within the cleft. Some of it escapes, but some of it binds to chemical receptor molecules located on the membrane of the postsynaptic cell. 7. The binding of neurotransmitter causes the receptor molecule to be activated in some way. Several types of activation are possible, as described in more detail below. In any case, this is the key step by which the synaptic process affects the behavior of the postsynaptic cell. 8. Due to thermal shaking, neurotransmitter molecules eventually break loose from the receptors and drift away. 9. The neurotransmitter is either reabsorbed by the presynaptic cell, and then repackaged for future release, or else it is broken down metabolically.

Neurotransmitter release
The release of a neurotransmitter is triggered by the arrival of a nerve impulse (or action potential) and occurs through an unusually rapid process of cellular secretion, also known as exocytosis: Within the presynaptic nerve terminal, vesicles containing neurotransmitter sit "docked" and ready at the synaptic membrane. The arriving action potential produces an influx of calcium ions through voltage-dependent, calcium-selective ion channels at the down stroke of the action potential (tail current).[10] Calcium ions then trigger a biochemical cascade which results in vesicles fusing with the presynaptic membrane and releasing their contents to the synaptic cleft within 180µsec of calcium entry.[10] Vesicle fusion is driven by the action of a set of proteins in the presynaptic terminal known as SNAREs. As calcium ions enter into the presynaptic neuron, they bind with the proteins found within the membranes of the synaptic vesicles that allow the vesicles to "dock." Triggered by the binding of the calcium ions, the synaptic vesicle proteins begin to move apart, resulting in the creation of a fusion pore. The presence of the pore allows for the release of neurotransmitter into the synapse.[11] The membrane added by this fusion is later retrieved by endocytosis and recycled for the formation of fresh neurotransmitter-filled vesicles. [edit] Receptor binding Receptors on the opposite side of the synaptic gap bind neurotransmitter molecules and respond by opening nearby ion channels in the postsynaptic cell membrane, causing ions to rush in or out and changing the local transmembrane potential of the cell. The resulting change in voltage is called a postsynaptic potential. In general, the result is excitatory, in the case of depolarizing currents, or inhibitory in the case of hyperpolarizing currents. Whether a synapse is excitatory or inhibitory depends on what type(s) of ion channel conduct the postsynaptic current display(s), which in turn is a function of the type of receptors and neurotransmitter employed at the synapse. [edit] Termination After a neurotransmitter molecule binds to a receptor molecule, it does not stay bound forever: sooner or later it is shaken loose by random temperature-related jiggling. Once the neurotransmitter breaks loose, it can either drift away, or bind again to another receptor molecule. The pool of neurotransmitter molecules undergoing this binding-loosening cycle steadily diminishes, however. Neurotransmitter molecules are typically removed in one of two ways, depending on the type of synapse: either they are taken up by the presynaptic cell (and then processed for re-release during a later action potential), or else they are broken down by special enzymes. The time course of these "clearing" processes varies greatly for different types of synapses, ranging from a few tenths of a millisecond for the fastest, to several seconds for the slowest. [edit] Modulation of synaptic transmission Synaptic transmission can be modulated by e.g. desensitization, homosynaptic plasticity and heterosynaptic plasticity: [edit] Desensitization Desensitization of the postsynaptic receptors is a decrease in response to the same neurotransmitter stimulus. It means that the strength of a synapse may in effect diminish as a train of action potentials arrive in rapid succession--a phenomenon that gives rise to the so-called frequency dependence of synapses. The nervous system exploits this property for computational purposes, and can tune its synapses through such means as phosphorylation of the proteins involved. [edit] Homosynaptic plasticity Homosynaptic plasticity is a change in the synaptic strength that results from the history of activity at a particular synapse. This can result from changes in presynaptic calcium as well as feedback onto presynaptic receptors, i.e. a form of autocrine signaling. Homosynaptic plasticity can affect the number and replenishment rate of vesicles or it can affect the relationship between calcium and vesicle release. Homosynaptic plasticity can also be post-synaptic in nature. It can result in either an increase or decrease in synaptic strength. One example is neurons of the sympathetic nervous system (SNS), which release noradrenaline, which, besides affecting postsynaptic receptors, also affects presynaptic α2-adrenergic receptors, inhibiting further release of noradrenaline. [12] This effect is utilized with clonidine to perform inhibitory effects on the SNS. [edit] Heterosynaptic plasticity Heterotropic plasticity is a change in synaptic strength that results from the activity of other neurons. Again, the plasticity can alter the number of vesicles or their replenishment rate or the relationship between calcium and vesicle release. Additionally, it could directly affect calcium influx. Heterosynaptic plasticity can also be post-synaptic in nature, affecting receptor sensitivity. One example is again neurons of the sympathetic nervous system, which release noradrenaline, which, in addition, generate inhibitory effect on presynaptic terminals of neurons of the parasympathetic nervous system.
Relationship to electrical synapses: An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. At gap junctions, cells approach within about 3.5 nm of each other, rather than the 20 to 40 nm distance that separates cells at chemical synapses.[17][18] As opposed to chemical synapses, the postsynaptic potential in electrical synapses is not caused by the opening of ion channels by chemical transmitters, but by direct electrical coupling between both neurons. Electrical synapses are therefore faster[9] and more reliable than chemical synapses. Electrical synapses are found throughout the nervous system, yet are less common than chemical synapses.

Neuroscience:
is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. Nevertheless, it is currently an interdisciplinary science that involves other disciplines such as psychology, computer science, statistics, physics, philosophy, and medicine. As a result, the scope of neuroscience has broadened to include different approaches used to study the molecular, developmental, structural, functional, evolutionary, computational, and medical aspects of the nervous system. The techniques used by neuroscientists have also expanded enormously, from biophysical and molecular studies of individual nerve cells to imaging of perceptual and motor tasks in the brain. Recent theoretical advances in neuroscience have also been aided by the use of computational modeling of neural networks. The term neurobiology is usually used interchangeably with neuroscience, although the former refers specifically to the biology of the nervous system, whereas the latter refers to the entire science of the nervous system.