The human brain is a network of more than 100 billion individual cells interconnected in neural circuits that construct our perception of the extenal world, fix our attention, and control the machinery of our actions.

The specificity of the synaptic connections established during development underlie perception, action, emotion, and learning. Genes contribute to behavior, but behavior itself is not inherited. Genes encode proteins that are important for the development and regulation of the neural circuits that underlie behavior. The environment becomes of prime importance after birth, and environmental contingencies can influence behavior by altering gene expression.

The central nervous system is the part of the nervous system consisting of the brain and spinal cord; with the exception of the spinal cord, these are the components of the brain:

Operations responsible for our cognitive abilities occur primarly in the cerebral cortex. It comprises two hemispheres and is divided into four distinct lobes: frontal, parietal, occipital and temporal. The frontal lobel relates to memory, planing and movement, parietal in body image and extrapersonal space, occipital vision, and temporal herating, learning, momory and emotion. Each hemisphere is concerned with sensory and motor processes in the opposite side of the body. The hemispheres are neither completely symmetrical in structure nor equivalent in function.

Paul Broca first to identify specific areas of the brain concerned with language. In 1864 announced "We speak with the left hemisphere". Wernicke proposed the functional areas in the brain only map to the most basic mental functions, but the components of a single behavior are likely to be processed in several regions of the brain — distributed processing is now a central tenet of neuroscience. For instance, the angular gyrus processes the initial steps in spoken or written words by associating information for visual and auditory regions. Spoken or written words are transformed into a neural code shared by both speech and writting.

Lashley later proposes that cortex subdivisions are meaningless, since with minor brain lesions, the brain is still able to learn. However, around the 1930s, localization of function became overwhelming — Specialization is a key organizing principle, but it’s an emergent property.

For example, Hirsh found out that when adults learn a second language, each language localizes in distinct regions of the brain within Broca’s area; but when learnt eaerly in life, both are localized in the same region. However, Wernicke’s area retains similar structure even in the presence of multiple languages since evidence exists this area processes grammatical rules. Similarily, other areas exist in the visual system focused with where an object is located, what that object is, and so on.

Wernicke’s area is also related to comprehension of emotion in speech, Broka’s in expressing the emotional aspects of speech.

It is now believed that many cognitive abilities are the result from interactions across several regions. This mental process consists of parallel paths that ultimately converge to a set of cells; otherwise, the brain would function as independent units competing against each other which can happenin some circumstances. For instance, each hemisphere is self-aware but converge in the corpus callosum. Gazzaniga found out that when the corpus callosum is damaged, the affected individual would put on clothers with their left hand while putting them off with the right one.

Sensory receptors are connected to the brain, which selects important events from the sensors and organizes perception.

There are two clases of cells in the brain, neurons and glial cells. Neurons are the basic unit of the brain and there are about 100 billion of them. Neurons produce different actions by the way they are interconnected, which is a key organizational principle of the brain.

A typical neuron has four regions: The dendrites, cell body, axon and presynaptic terminals.

Ramon y Cajal also defined that signals flow in one diretion, from dendrites to axons, this principle is refered as dynamic polarization. Ramon y Cajal also defined that neurons make specific connections — neurons are not randomly connected to one another, known as the connectional specificity principle.

Glia cells can be divided into microglia and macroglia. Microglia are immune cells and macroglia form about 80% of the brain and subdivide into oligodendrocytes, swann and astrocytes cells. Half of macroglia is comprised of oligodendrocytes and half of astrocytes cells. Oligodendrocytes and swann cells insulate the axons while the astrocytes:

Action potentials usually lead to excitatory synapses; however, there are also neurons with inhibition mechanisms to prevent other neurons from firing their action potential using a combination of K+ and Na+. Action potentials are all-or-nothing, there is no decay in the axon and travel at 100 m/s. In 1920, Adrian finds out that action potentials are indistinguishable even between sensors and motor axons.

There is a class of neurons, beating neurons, which produce an action potential in the absents of stimulation at regular intervals. Other neurons, bursting neurons, fire in brief bursts of action potentials. These neurons respond differently to the same exhitatory input by initiating action potentials or regulating the rate of firing action potentials.

Action potentials release neurotransmitters in the neuron terminals, which bind to receptors in the postsynaptic neuron to generate a synaptic potentials. The synaptic potential can be exhitatory or inhibitory depending on the receptor and independent to the neurotransmitter being used.

Synapses can have short-term physiological changes lasting for hours to increase the synaptic effectiveness, long-term changes lasting days can happen by prunning existing synapses or growing new ones — known as the plasticity hypothesis, which forms the basis for learning.

The branch of computer science known as artificial intelligence, originally used traditional algoritmic processing to simulate the brain.^[3] This approach served well for some tasks, like playing chess; but performed poorly in other tasks, like face and speech recognition.

Theoretical neurologists have turned to create models they call neural networks, which process information using feed-forward and feedback connections. Neural networks capture well the architecture of most actual neural circuits and also the ability of the brain to function in the absence of specific sensory input such as during sleep. Neural networks also show that analyzing individual neurons may not be enough to understand an action potential. This makes the brain a remarkable information processing organ is not the complexity of the neurons but in the interconnections between its elements.

All behaviors are shaped by the interplay of genes and the environment, but they do not control behavior directly; instead, they specify the development of programs that assemble in the brain. In 1883, galton built a strong case for gene heritability by studing identical twins; however, heritability is behavioral traits is substantially less than 100%, demonstrating that the environment is an important factor — behavioral traits from twin studies range around 50%, but can be lower or higher for particular traits.

Genes are madee of DNA passed between generations and is made of two strands to ensure accurate copying of DNA during replication. Each strand is made of four nucleotides: adenine, guanine, thymine, and cytosine.

Most genes encode protein products. The brain expressese a greated number of genees than any other organ in the body and, within the brain, different neuron populations express different groups of genes. This permits a fixede numbeere of genees to generate a vastyly largeer number of neuronal cell types and connections in the brain.

Although genes express the initial development of the nervous system, the experience of an individual and activity in specific neural circuits can alter the expression of genes.

Sokolowski found out a forager gene in larvae and honeybees which encodes the kinase protein which regulates many neuronal functions, specially those that transform short-term neural signals into long-term changes in a neuron. High levels of this gene are expressed in foraging behavior, low levels in stationary behavior. Crowded envrionemnts favors foraging behavior, whereas sparse environments favor stationary behavior to exploit food sources more thoroughly.

Social behaviors are highly variable beetween species, yet they have a large innate component controlled genetically. In worms, the npr1 gene encodes a neuropeptide receptor, is involved in signaling between neurons. Neuropeptides have a role in coordinating behaviors across networks of neurons. Mammalian neuropeptides have been implicated in feeding behavior, sleep, pain, and other behavior and psycological processes. For instance, the neuropeptides oxytocin and vasopressin. Oxytocin has been found to regulate pair-bonding formation and paternal behavior, while vasopressin has been show to regulate how closely male rodents help raise offpring.

In humans, defects in speech articulation led to the identification of the foxp2 gene, which controls a human-specific pattern of development of the brain, larynx and mouth. This gene might be one of the adaptations that made human speech possible.

The neuregulin gene affects cell migration and synapse formation, together with the disc1 gene suggest a developmental defect that can underlie schizophrenia. Several gene mutations in autistic patients affect transmembrane signaling proteins called neurexins and neuroligins, which affect the strength of synapses. They suggest that subtle alterations in synaptic transmission play a role in autism. However, only rarely will genetic alteration at a single gene fully explains a disease or behavior in humans. Instead, the interplay of genetics, environment, chance and individual choice is what ultimately determines the behavioral differences between individuals. The challenge of genetics is to underestand the effeect of genes, while acknowledging that many factors influence human behavior.

You learned some of the neuroscience principles: neurons are the building blocks (neuron doctrine), they are arranged in functional groups (cellular connectionism), process behavior in multiple regions (distributed processing), produce different actions by the way the are connected (organizational principle), signals flow in one direction (dynamic polarization), neurons are not randomly connected (connectional specificity), and that connectivity changes are the basis for learning (plasticity hypothesis).