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The Nervous System





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The NERVOUS SYSTEM, yes be amazed....
FIRST OFF...the CNS otherwise known as the central nervous system includes the brain and longitudinal nerve chord and the neurons that carry information in and out of the CNS are part of the perpetual nervous system otherwise known as the PNS
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  • neurons - nerve cells that transfer information throughout the body
    • It consists of: long - distance electrical signals and short distance chemical signals
      • The shape of the neurons uses the electrical pulses to receive, transmit, and regulate the flow of information throughout the body
      • They transfer many types of information:
        • transmit sensory information
        • control heart rate
        • coordinate hand and eye movement












      • This information is transferred through electrical impulses consisting with the movements of ions
      • The connections made by neurons specifies what is being transferred

    • Brain - consists of grouped neurons in highly complex animals
    • Ganglia - simpler clusters of the neurons grouped in the brain

  • 48.1
    • Information Processing
      • For example, when a squid sees its prey, the signal transfers from the brain to the neurons in its mantle case, using muscle contractions it propels itself forward
        • it uses sensory input, integration, and motor output
        • Sensory neurons - transmit stimuli from eyes and other and other sensors that detect external stimuli (like light, sound, and/or heat) or internal stimuli (such as blood pressure, muscle tension, or blood carbon dioxide level)
          • this information is sent in the brain or in the ganglia.
















        • Neurons in the brain or in the ganglia analyze and interpret the message comparing it to the animal's past experience
        • The major parts of the brains neurons are made up of interneurons, which make local connections
        • Motor neurons - transmit signals to muscle cells, causing them to contract
          • motor output relies on neurons that extend out of processing centers in bundles called nerves.
            • generate output by triggering muscle or gland activity










    • Neuron Structure and Function
      • Cell body - location of most of the neurons organelles, including the nucleus
      • A typical neuron has many dendrites, or highly branched extensions that receive signals from other neurons
      • A neuron has a single axon, an extension that transmits signals to other cells.
        • They are often much longer than dendrites, and some can reach from spinal cord of a giraffe to the muscle cells of its feet, which are over a meter long
        • The cone shaped region of an axon where it joins the cell body is called axon hillock
          • This is the region where the signals that travel down the axon are generated.
          • In the other end, the axon usually divides into several branches












        • Each branched end of an axon transmits information to another cell at the junction called a synapse.
          • The part of each axon branch that forms this specialized junction is a synaptic terminal.
          • chemical messengers called neurotransmitters pass information from the transmitting neuron to the receiving cell
          • the transmitting neuron is the presynaptic cell, and the muscle, neuron, or gland cell that receives the signal is called the postsynaptic cell.





        • depending on the number of synapses a neuron has with other cells, its shape can vary from simple to complex.
        • some interneurons take part in 100,000 synapses and neurons with simpler dendrites have much fewer synapses
        • neurons of vertebrates and most invertebrates require supporting cells called glial cells, or glia.
          • they may nourish neurons, insulate axons of neurons, or regulate the extracellular fluid surrounding neurons
          • glia outnumber the neurons in the mammalian brain 10- to 50-fol





  • 48.2
    • Membrane potential is voltage across the cell's plasma membrane .
      • In neurons, it can come from other neurons or specific stimuli, which can cause changes in this membrane potential that act as signals, transmitting and processing information
      • The membrane potential of a resting neuron - one that is not sending signals - is called resting potential.
        • it is in between -60 and -80 mV (milivolts)
          • the minus sign indicates that inside the neuron at rest is negative relative to the outside
















    • Formation of the Resting Potential
      • Potassium (K+) and sodium ions (Na-) play a crtical role in the formation of the resting potential
        • for each there is a concentration gradient across the plasma membrane of the neuron
        • the concentration of K+ ions is 140 millimolar (mM) inside the cell, but only 5 mM on the outside --> refer to the picture
        • the concentration gradients are maintained by sodium-potassium pumps in the plasma membrane
        • they uses the energy of ATP hydrolysis to actively transport Na- out of the cell and K+ into the cells --> refer to the picture
        • they represent a form of chemical energy
          • converting this chemical potential to an electrical potential involves ion channels, pores formed by clusters of specialized proteins that span the membrane
            • they allow the ions to diffuse back and forth across the membrane
            • as they diffuse through the channels, they carry with them units of electrical charge
            • net movement of +/- charge will generate a voltage, or a potential, across the membrane
















      • the ion channels that establish the membrane potential have selective permeability, meaning that they allow only certain ions to pass
        • a potassium channel only allows K+ ions to pass not Na- ions




      • In the mammalian neuron, the channels allow K+ ions to pass in either direction across the membrane
        • Because the concentration of K+ ions is greater inside the cells, the K+ ions move out and since other anions like Cl- can't come, the inside of the cell becomes negative
          • and this is the source of membrane potential
          • and so there is not a big negative build up inside the cell because then the K+ cells flow back into the cell creating electrical gradient
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      • Modeling of the Resting Potential
        • the net flow of K+ ions out of neurons proceeds until the chemical and electrical forces are in balance
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          • this membrane contains ion channels, which only allow K+ ions to diffuse across








      • To produce the concentration gradient of K+ ions, you can place 140 mM of KCl in the inner chamber and 5 mM KCl in the outer chamber
        • K+ ions diffuse down concentration gradients into the outer chamber, Cl- ions lack a means for crossing the membrane, there would be a great amount of negative charge in the inner chamber




      • When the neuron reaches equilibrium, there is no diffusion of K+ ions because the electrical gradient balances the chemical gradient
      • The magnitude of the membrane voltage at equilibrium for a particular ion is called that ion's equilibrium potential
      • For a membrane permeable to a single type of ion, you can use the Nernst equation
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        • which makes K = -90 mV; the minus sign indicates that K+ is at equilibrium when inside of the membrane is 90 mV more negative than the outside
        • the resting potential of a mammalian neuron is a little less negative
        • The difference reflects the steady movement of Na+ across the few open sodium channels in a resting neuron
          • the concentration gradient of Na+ has a direction opposite to K+, Na+ diffuses into the cell and makes the inside of the cell less negative
            • for example, if you model a selectively permeable membrane to Na+ ions, there is a higher concentration of Na+ ions in the outer chamber












          • neither K+ or Na+ ions is at an equilibrium in a resting neuron, each ion has a net fowl across the membrane
          • resting potential is steady, which means the K+ and Na+ currents are equal or opposite

48.3

    • Gated ion channels are ion channels that open or close in response to stimuli
      • it is the basis to all the electrical signaling in the nervous system
      • which alters the membrane permeability to some ions, which then alters the membrane potential
      • For example, if more passages of K+ ions open up then, the membrane permeability will increase for the K+ ions, which will increase the K+ diffusion out of the neuron and it makes the inside more negative
      • The increase in magnitude for membrane potential is called hyperpolarization
        • results from a stimulus that increases the inflow of negative ions and outflow of positive ions








      • The reduction in magnitude for the membrane potential is called depolarization
        • often involves sodium gate channels, the permeability of the membrane increases towards Na+ ions and causing depolarization




      • types of hyperpolarization and depolarization are called graded potentials
        • the magnitude of the change in membrane potentials depends on the strength of the stimulus
          • the larger the stimulus, the larger the change








        • graded potentials have a great effect on neurons
    • Production of Action Potentials
      • Voltage-gated channels are gated ion channels in neurons
        • they open and close when they change in membrane potential
        • if depolarization opens more sodium gated channels, the Na+ ions flow into the neuron which results in more sodium channels opening
          • and because the sodium channels are all voltage gated, more sodium channels open up rapidly













      • A massive change in membrane potential is called action potential. Like the one described above.
        • They carry nerve impulses or messages along the axons
        • they occur when depolarizations increases the membrane potential to a certain value called threshold




        • they represent all or nothing towards a stimuli - they need to have a certain potential to activate the stimuli or nothing happens
          • it reflects depolarization and more sodium channels open causing further depolarization




        • once action potential is initiated, it is independent of this strength to affect the stimuli
        • The positive feedback loop of depolarization and channel opening triggers action potential whenever the membrane potential reaches the threshold
    • Generation of Action Potentials: A Closer Look
      • action potentials are so brief that neurons can produce many hundreds per second
        • the frequency of the neuron producing the action potential depends on its input; or the strength of the signal
          • loud sounds produce more frequent action potentials
          • Screen_shot_2011-03-16_at_10.23.06_AM.png
            Screen_shot_2011-03-16_at_10.23.06_AM.png
          • The picture above shows a large change in membrane potential resulting from ion movement through the voltage-gated channels of sodium and potassium
          • depolarization opens both the potassium and sodium channels that respond independently
            • the sodium channel opens first forming the action potential and then as the action potential proceed, the sodium channel becomes inactivated
              • and they remain inactivated until the membrane reaches the resting potential and the channels clos




















            • potassium channels remain open through the action potentials
      • Stages
        • voltage gated channels effect the action potential
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        • The sodium channels remain inactivated during the falling phase and the early part of the undershoot
          • if a second depolarizing occurs during this period, and it would not be able to trigger the action potential
          • "downtime" following an action potential when a second action potential can not be initiated is called refractory period
            • sets a limit on the maximum frequency at which action potentials can be generated
            • the refractory period is due to the inactivation of sodium channels, not to change in the ion gradients across the plasma membrane
            • The flow of charged particles during an action potential involves far too few ions to change the concentration on either side of the plasma membrane












    • Conduction of Action Potentials
      • action potential functions as a long-distance signal by gegenration
        • travels from cell body to the synaptic terminals








      • At the site where the action potential is created --> pic
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        • the inward current that depolarizes the axon membrane ahead of the action potential cannot produce another action potential behind it
        • this makes the action potential move in only one direction




    • Conduction Speed
      • factors that affect the speed at which action potentials are conducted:
        • axon diameter - wider axons conduct action potentials more rapidly than narrow ones
        • the resulting depolarization can spread farther along the interior of a wide axon
        • the giant axons function in rapid behavioral responses, like muscle contractions
        • vertebrate axons have a narrow diameter - the adaptation that enables fast conduction in narrow axons is myelin sheath
          • it is a layer of electrical insulation that surrounds vertebrate axons
            • they are produced by two types of glia - oligodendrocytes in the CNS and Schwann cells in the PNS
























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            • in the myelinated axon, voltage-gated ion channels are concentrated in the nodes of Ranvier, small gaps between successive Schwann cells
            • action potentials can be generated only at these nodes
            • nerve impulse "jumps" from node to node, resulting in a faster mode of transmission known as saltatory conduction

48.4 Neurons communicate with other cells at synapses

At synapses, neurons exchange information. There are two types of synapses: Electrical synapses contain gap junctions that allows electrical current to pass between neurons. They are the cause for rapid and unvarying motions. And: Chemical synapses, which are the most common ones. They release a chemical neurotransmitter by the presynaptic neurons. At each terminal, the presynaptic neuron synthesizes the neurotransmitter and packages it in multiple membrane-bounded compartments called synaptic vesicles. The neurotransmitter then diffuses across the synaptic cleft, the narrow gap that separates the presynaptic neuron from the postsynaptic cell. Information transfer is more modified in chemical synapses than electrical ones because of its variety to factors that affect the neurotransmitter.

Generation of Postsynaptic Potentials
At many chemical synapses, ligand-gated ion channels exist that can bind to neurotransmitters are clustered in the membrane of the postsynaptic cell. When the neurotransmitter binds to the channel, ions cross the postsynaptic membrane, resulting in postsynaptic potential, a change in the membrane potential. When the channels open, the postsynaptic membrane depolarizes as the membrane potential approaches a value roughly midway between EK and ENa . These depolarizations bring the membrane potential towards threshold – therefore the depolarizations are called excitatory postsynaptic potentials (EPSP). On the other hand, hyperpolarizations result in inhibitory postsynaptic potentials (IPSP). Various mechanisms then rapidly clear neurotransmitters from the synaptic cleft, terminating their effect of postsynaptic cells.

Summation of Postsynaptic Potentials
Postsynaptic potentials are graded – their magnitude varies by a numerous number of factors, and they do not regenerate.

A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron. When two EPSP’s occur at a single synapse in rapid succession, they add together, an effect called temporal summation. When the same thing happens, but with different synapses, it is called spatial summation.

A single neuron has many synapses on its dendrites and cell body. Whether it generates an action potential depends on the temporal and spatial summation of EPSPs and IPSPs at the axon hillock.

Modulated Synaptic Transmission
There are synapses in which, instead of the neurotransmitter directly binding to an ion channel, the receptor for the neurotransmitter activated a signal transduction pathway, involving a second messenger. Compared with postsynaptic potentials with channels, this second messenger system is slower but lasts longer in the postsynaptic cell.

Neurotransmitters
There are several types of neurotransmitters: Acetylcholine is common in both invertebrates and vertebrates, and it is an excitatory transmitter. It binds to receptros on ligand-gated channels in the muscle cell, producing an EPSP. Biogenic amines are neurotransmitters derived from amino acids; some amines are serotonin, dopamine, epinephrine, and norepinephrine. Gamma-aminobutyric acid (GABA) and glutemate are the major neurotransmitters in vertebrates. Neuropeptides are relatively short chains of amino acids that serve as neurotransmitters that operate via signal transduction pathways. Some examples are substance P and endorphins. Gases can also act as local regulators.

Chapter 49
49.1 Nervous Systems consist of circuits of neurons and supporting cells

Most cnidarians have a series of interconnected nerve cells that form a diffuse nerve net, which controls the contraction and expansion of the gastrovascular cavity. However, in more complex animals, the axons of multiple nerve cells are often bundled together, forming nerves. They structure and organize information flow along the nervous system.
Animals with elongated, bilaterally symmetrical bodies have more specialized nervous systems, exhibiting cephalization, an evolutionary trend toward a clustering of sensory neurons and interneurons at the anterior end. An animal’s nervous system reflects their lifestyle.
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Organization of the Vertebrate Nervous System
The brain and spinal cord of the vertebrate CNS are tightly coordinated; the brain provides the power for the complex behaviour of vertebrates; while the spinal cord, which runs lengthwise, conveys information to and from the brain, generates basic patterns of locomotion, and produces reflexes. A reflex is the body’s automatic responses to certain stimuli, protecting the body by triggering a rapid, involuntary response to a particular stimulus.

During development, the hollow cavity of the embryonic nerve cord gives rise to the central canal of the spinal cord and the ventricles of the brain, filled with cerebrospinal fluid, which is formed by the filtration of arterial blood in the brain. The brain and the spinal cord contain gray matter, made up of neurons, dendrites, and unmyelinated axons, and white matter, made up of bundled axons with myelin sheaths which give the axons a whiteish appearance. Among the types of glia are astrocytes, which provide structural support for neurons and regulate the extracellular concentration of ions and neurotransmitters. During development, astrocytes make capillaries in the CNS form tight junctions, resulting in the blood-brain barrier , which restricts the passage of substances into the CNS, permitting tight control over the rain and spinal cord. In an embryo, radial glia form tracks along which newly formed neurons move from the neural tube.

The Peripheral Nervous System
The peripheral nervous system (PNS) transmits information to and from the central nervous system (CNS) and regulates much of an animal’s movement and internal environment.
The cranial nerves connect the brain with locations mostly in organs of the head and upper body. The spinal nerves run between the spinal cord and parts of the body below the head.
The PNS consists of the motor system and the autonomic system. The motor system consists of neurons that carry signals to skeletal muscles, mainly in response to external stimuli. The autonomic system regulates the internal environment by controlling smooth and cardiac muscles; it has three divisions: sympathetic, parasympathetic, and enteric. Activating the sympathetic division results in arousal and energy generation. Parasympathetic division generally causes opposite responses that promote calming and a return to self. The enteric division consists of networks of neurons in the digestive tract, pancreas, and gallbladder.
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49.2 The vertebrate brain is regionally specialized

The Brainstem
Pons and medulla in the brainstem serve as checkpoints for information traveling between the PNS and the higher brain. The reticular formation, a network of neurons within the brainstem, regulates sleep and arousal .

The Cerebellum
The cerebellum helps coordinate motor, perceptual, and cognitive functions; learning, and remembering motor skills.

The Diencephalon
The diencephalon contains several important parts of the brain: The thalamus is the main center through which sensory and motor information passes to the cerebrum. The hypothalamus regulates homeostasis and basic survival behaviours. The suprachiasmatic nucleus in the hypothalamus acts as the pace maker for circadian rhythm.

The Cerebrum
The cerebrum functions in movement, sensory processing, the sense of smell, language, communication, learning, and memory. It has two hemispheres. Each consists of cortical gray matter overlying white matter and basal nuclei, which are important in planning and leaning movements. In mammals, the convoluted cerebral cortex is also called the neocortex. A thick band of axons, the corpus callosum, provides communication between the right and left cerebral cortices.
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49.3

What controls how we move and think?
As noted above, it is the cerebral cortex. The basic breakdown of the cerebral cortex is that it has four lobes, the frontal, temporal, occipital, and parietal lobes. Note that these lobes are named from parts of the skull bones. As science has progressed, it has come to our knowledge that these lobes have a number of functional areas, which include primary sensory areas and association areas. The primary sensory areas receive and process a specific type of sensory information while the association areas integrate information from different parts of the brain.
DID YOU KNOW…that when mammals evolved, the cerebral cortex grew larger all thanks to the expansion of the association areas? This is exactly the reason why human’s cerebral cortex mostly deals with the association areas responsible for more complex behaviors whereas rats, which have a smaller cerebral cortex, deal more with primary sensory areas. external image 0lx_5U--xHrQpAAjVmLDsnTlc9tEt89Dm4eJZEh6fZviHljghgMa8akSPptEb5Wx72TFacyHihiY4SL8WD6T9d4KNds4YiHmv6Zxx-DpM8jZ0Ldxm7s

How is information processed in the cerebral cortex?

The cerebral cortex gets sensory input from two types of sources. Some of the input comes from dedicated sensory organs like the eyes and nose. Others come from somatosensory receptors that provide information about touch, pain, temperature, or the position of muscles and limbs like the hands or scalp. Most of the sensory information that goes into the cortex is directed via the thalamus to the primary sensory areas within the brain lobes. Depending on what type of input, the thalamus directs them to specific locations like how visual information goes to the occipital lobe or auditory input goes to the temporal lobe and somatosensory information travels to the parietal lobe. Interestingly enough, input on taste also goes to a part of the parietal lobe, a region specifically for somatosensory input. Olfactory information travels to the regions of the cortex first, which just happens to be the same in mammals and reptiles, then with the help of the thalamus, it gets sent to an interior part of the frontal lobe.
The information the primary sensory areas received is then passed along to the nearby association areas, which processes particular features in the sensory input. For example, in the occipital lobe, some neuron groups from the primary visual area are sensitive to how certain light rays are oriented. The visual association area combines information related to such features in a region that is dedicated to recognizing complex images, just like a face.
The integrated sensory information then passes to the frontal association area, which helps plan actions and movements. The cerebral cortex then is able to generate motor commands that cause particular behaviors—saying hello or wiggling a finger. These commands consist of action potentials that are produced near the rear of the frontal lobe by neurons in the motor cortex. These action potentials travel along axons to the brainstem and spinal chord where they excite motor neurons, which, just like the domino effect, excite skeletal muscle cells.
Neurons are distributed in an orderly fashion, in both the somatorsensory cortex and motor cortex, according to the part of the body that generates the sensory input of receives the motor command. (FIGURE 49.16) like how neurons that process sensory information from the legs and feet are located in the region of the somatosensory cortex that lies closets to eh midline. Neurons that control muscles in the legs and feet are located in the corresponding region of the motor cortex. As seen in the picture above, the cortical surface area that is dedicated to a specific body part is not proportional to the size of the part. However, the surface area correlates with the extent of neuronal control needed for muscles in a particular body part (the motor cortex0 or the number of sensory neurons that extent axons to that part (the somatosensory cortex) therefore the surface area of the motor cortex that is devoted to the face is much larger than that devoted to the trunk which shows just how much facial muscles are involved in communication.
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Language and Speech

In the 1800s physicians started to learn that damages to a particular region of the cortex by injuries, strokes, or tumors could really change a person’s behavior. Pierre Borca, a French physician, performed postmortem examinations of patients who were unable to speak but could still understand what people said. He discovered that patients had defects in a small region of the left frontal lobe, which is now known as Broca’s area, and is located in front of part of the primary motor cortex that controls muscles in the face. The German physician Karl Wernicke also found that damage to a posterior portion of the left temporal lobe, now called the Wernicke’s area, made a person unable to understand speech but did not hinder their ability to speak, when he conducted postmortem examinations. As time passed, studies of brain activity using fMRI and positron-emission tomography otherwise known as PET have confirmed that Broca’s area is active during speech generation while Wernicke’s area is active when speech is heard (below image, upper left image).
Broca’s area (the image below, lower left image) and Wernicke’s area, however, are part of a much larger network of the brain regions that involve language. The visual cortex is activated when a person reads a word that is pointed out without talking (check out the image below upper right image) whereas reading aloud a printed word activates BOTH the visual cortex and Broca’s area.
When the frontal and temporal areas become active, it means that the person starts to put meaning to words, like when a person generates verbs to go with nouns or groups related to words or concepts (image below, lower right image).
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What does lateralization of the cortical function mean?

Even if each cerebral hemisphere in humans have sensory and motor connection for the opposite side of the body, the two hemispheres do not have identical functions. For example, the left side of the cerebrum majors in language as noted in the location of both Broca’s area and Wernicke’s area. However, there are also subtler distinctions in the functions of both hemispheres. Just like how the left hemisphere is better at math and logical operations whereas the right hemisphere leans more to the recognition of faces and patterns or spatial relations and nonverbal thinking. We lateralized the functions of the brain when we established the differences in the functions of the hemisphere.
Some lateralization relates to he handedness, when a person prefers to use their hands for certain motor activities. DID YOU KNOW…that around 90% of the human population are more skilled with their right hand than with their left? Studies, with the help of fMRI, have revealed how languaged processing differs in relation to which hand is your dominant hand. When subjects thought of words without speaking them out loud, their brain activity was localized to the left hemisphere in 96% of right-handed subjects but only in 76% of left-handed people.
However, the two hemispheres normally work together by trading information back and forth through fibers of the corpus callosum. This is really important, because those whose corpus callosum had been surgically severed show a “split-brain” effect where they see a familiar word in their left eye but they cannot read the word. This is because the sensory information that normally travels from the left field of vision to the right hemisphere can no longer reach the language centers in the left hemisphere. This is because these patient’s two hemispheres are now working without the help from each other. Luckily, people only remove the cerebral hemisphere that connects both hemispheres for extreme forms of epilepsy, as a last resort.

Emotions

When a person feels and generates emotions, they involve various different parts of the brain. One such region, as seen in the image below, contains the limbic system, a group of structures surrounding the brainstem in mammals. It does not have a specific function because it controls a diverse set of functions like emotion, motivation, olfaction, behavior, and memory. The thalamus, hypothalamus, hippocampus, prefrontal cortex, olfactory bulb, and amygdala are all part of the limbic system, as seen from the figure below. There are also parts of the brain that deal with emotion, not just the limbic system. For example, emotions that are show themselves through behavior like laughing involve an interaction between parts of the limbic system and the sensory areas of the cerebrum. Emotional feelings, like aggression, feeding, and sexuality, are attached to basic survival related functions controlled by the brainstem by structures in the forebrain.
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Emotional experiences are stored as memories that can be recalled by similar circumstances. However, fear’s emotional memory is stored separately from the memory system that supports explicit recall of events. Emotional memory is focused in the amygdala, which is located in the temporal lobe. In order to study the human’s amygdala’s function, researchers usually show the subject a picture followed by an electric shock. After this occurs several times, the subjects experience AUTONOMIC AROUSAL—an increase in heart rate or sweating, ect—when they see the image again. However those with brain damage can recall the image because their explicit memory is intact, but they do not experience autonomic arousal.
The prefrontal cortex, a part of the frontal lobes that deals with emotional experience, is also important in temperament and decision making. A person can get a personality change when their frontal lobe is badly damaged by getting frontal lobotomy, which severs the connection between the prefrontal cortex and the limbic system, and tumors.

Consciousness

DID YOU KNOW...that the study of human consciousness used to not be considered science because it was rather broad and subjective? There is growing support that consciousness is an emergent property of the brain because it has lots of activities in many ares of cerebral cortex.

Changes in synaptic connections underlie memory and learning

Regulated gene expression and signal transduction establish the basic structure of nervous system during embryonic development. Two more processes complete the development of nervous system. First off, neurons compete for growth-supporting factors, which are not produced in large numbers by tissues that direct neuron growth, which causes half the neurons die; as a result the surviving neurons are located where they need to be in the nervous system. Then synapse elimination occurs, when the developing neurons form too many synapses so that the more than half are lost

Neural Plasticity

This term means when a nervous system changes according to its own activities by reshaping occurs during synapses which helps refine the system.Remodeling and refining our nervous system is usually necessary, like how it helps us develop the ability to sense our surrounding. They are also quite essential to the nervous system’s limited ability to recover from an injury or disease.external image Zd5pRorMZ0mpzhlb-IMob0kJKPPVL35KK68LJ35p0SegWl5B3_zeKde1S1Jum4fSplJhpzHOUljKWLzPpzMMyziXEGXwAR8MrilaPLYlllsAS5e89Jo


Memory and Learning

Apparently, humans are constantly comparing what just happened a few moment ago to what is happening now. Short term memory is when we remember information until we think we have no more use for it and information is accessed via temporary links formed in the hippocampus. Long term memory is when we wish to remember something that is needed for a long time. The link’s in the hippocampus are replaced by permanent connections within the cerebral cortex. DID YOU KNOW...People with damaged hippocampus are somewhat trapped in past, and they cannot form new lasting memories? Which is quite sad, obviously.
What are the advantages from the different organization of short and long term memories? Actually, it is thought that, because of the delay in forming connections in the cerebral cortex, long term memories can be integrated into existing knowledge and experiences. The information transferred from short to long term memory is enhanced by the association of new data with data that was already considered a long term memory. An example of this is when one finds it easy to learn a card game because they already know how to play other card games.
Motor skills like walking, writing ect, are learned by repetition and can be performed without remembering the exact details of the procedure. Learning skills and procedures like riding a bike, on the other hand, involve cellular mechanisms similar to those responsible for brain growth and development.

Long term potential

LTP is a lasting increase in the strength of synaptic transmission. It has presynaptic neurons that releases excitatory neurotransmitter glutamate. In order for LTP to occur, a brief high frequency series of action potentials must occur in the presynaptic neuron, and these actions must arrive at the synaptic terminal at the same time that the postsynaptic cell receives a depolarizing stimulus, as seen in the images below. Two glutamate receptors, NMDA and AMPA, both artifically activate particular receptors. This results in a stable increase in size of postsynaptic potentials at synapse and can last for days or weeks in dissected tissues.
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Nervous system disorder can be explained in molecular terms

Schizophrenia, depression, drug addiction, Alzheimer’s disease, Parkinson’s disease are all, obviosuly, nervous system disorders. Sadly, all these make more hospitalized patients than disease or cancer in the US alone. However, many disorders that influence mood or behaviour can be treated with medication. Nervous system disorders usually result from chemical or anatomical change in the brain. However, Alzheimer’s and many others lead to nervous system degeneration, which is hard to cure, if it can be cured at all. Nervous system disorders are influenced by genetic contributions, but they are mostly influenced by environmental factors.
Schizophrenia is a severe mental disturbance characterized by psychotic episodes in which the patients have distorted perception of reality. This includes patients suffering from hallucinations, like hearing voices only they can hear or delusions, thinking that people plotting to harm them.Scizophrenia does not necessarily mean multiple personalities, but rather a fragmentation of what are normally integrated brain functions. Evidence says that this disorder affects neuronal pathways that use dopamine as neurotransmitter.The drug amphetamine, “speed,” that stimulates dopamine release can produce the same set of symptoms as schizophrenia. Many drugs that alleviate symptoms of schizophrenia block dopamine receptors which can also change glutamate signaling. Thankfully medication can alleviate major symptomsexternal image fSn9bjrbJkfNkYKZT5HVWcWIeKcVtaJO4mLVp_RV7XZ-nWN71tHNsl-PWAG7mcUPmLFJrgRuFDqqAGEyLtedZI15DhuWCDO9W7MBiAyQ-JC4TvOZ_fQ

Depression

Depression is characterized by depressed moods and abnormalities when sleeping, eating, or energy level. There are a few different types of depression. A major depressive disorder has periods, that can last months, where once enjoyable activities no longer provide pleasure or invokes another’s interest. Statistics say that it affects 1 in 7 adults, and 2x as many in women as in men. Another type of depression is bipolar disorder or maniac depressive disorder which deals with mood swings from high to low. It affects 1% of world’s population, and may seem like a small number, but considering the amount of people in the world...it probably isn’t. The maniac phase is characterized by high self esteem and an increase in energy, flow of ideas and they can become chatter box and very risky risk taker. The milder forms can be associated with great creativity, some well known artists, musicians, and literary figures were pretty productive during their maniac phase. DID YOU KNOW...that some of these people included Van Gogh, Schumann, and even Hemingway? However, the depressive phase comes with a lowered ability to feel pleasure, a loss of motivation, sleep disturbances, and feelings of worthlessness. It is very severe to the point of suicide. Interestingly enough, some patients prefer to endure the depressive phase than take medication because they risk losing the enhanced creative output during their maniac phase if they take medication. Drugs like fluoxetine (Prozac) increases activity of biogenic amines in the brain, and depressive disorders can also be treated with anticonvulsant drugs or lithium.

Drug addiction and brain reward system

This disorder is comprised of compulsive consumption of drug and loss of control in limiting intake. Any number of drugs that have different effects on the CNS can be very addictive, like cocaine and amphetamine, which act as stimulants, and heroin, which is a pain relieving sedative. Obviously, they are very addictive, because they increase the activity of the brain’s reward system, and the neural circuitry that normally functions in pleasure, motivation, and learning. Those with no drug addiction, their reward system provides motivation for activities like eating in response to hunger, but for addicts, “wanting” is directed to further drug consumption. Experiments on rats show that they’ll rather self administer drugs even when they’re near death. Inputs to the reward system are received by neurons in regions near the base of brain called ventral tegmental area (VTA). Plainly put, drugs enhance activity of dopamine pathway as addiction develops, which causes long lasting changes are made in the reward circuitry. Check the image below. The results are a craving for drugs that are present independent of any pleasure associated with consumption. external image nj7kTtDeKm3CLweQ1bmiqioKMDIiirpQXrJA_b5btAgzaLed-LSB9-BnMcGrDmoorD9tpmQ4BkQHpkMPeZkgVlyAn3f_Z24nFK88MsME0M_RKy0vy9I

Alzheimer’s disease

As many know, Alzheimer’s disease is a mental deterioration or dementia, and is often characterized by confusion, memory loss, and a variety of other symptoms. This disease is age related, and can be seen rising from 10% at age 65 to 35% at age 85. This disease is rather progressive with patients gradually becoming less able to function and eventually needing outside help to do daily activities. Their personality changes, often for the worse and they lose the ability to recognize people. Basically this disease kills neurons in many areas of brain including the hippocampus and cerebral cortex. It involves a massive shrinkage of brain tissue. However, many symptoms recognized in Alzheimer’s disease are also shared with other forms of dementia so it’s hard to identify and diagnose until it is too late. During postmortem, two features were found: amyloid plaques and neurofibrillary tangles in the remaining brain tissue, as seen in the figure below.
The plaques are aggregates of β-amyloid, an insoluble peptide that is cleaved from a membrane protein found in neurons. Membrane enzymes, called secretases, catalyze this cleavage, causing the β-amyloids to accumulate in plaques outside the neurons. It is these plaques that appear to trigger the death of surrounding neurons. Tau protein are neurofibrillary tangles found in Alzheimer’s disease. Tau was originally supposed to help regulate movement of nutrients along microtubules however disease makes it bind to itself. Yes, there is currently no known cure for Alzheimer’s disease.
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Parkinson’s Disease

This is a motor disorder, which has the characteristics of having difficulty in moving, and the person moves very slowly with rigidity. The person inflicted with this disorder also experiences muscle tremors, poor balance, flexed posture, shuffling gait and, if that was not bad enough, their facial muscles become rigid. It is another progressive brain disease and protein aggregate like the Alzheimer’s disease. Also like the Alzheimer’s disease, it is common with advancing age. 1% get it at age 65 and 5% at age 85. The disorder results from the death of neurons in the midbrain that usually releases dopamine during synapses in the basal nuclei. Majority of the Parkinson’s disease have no clear cause, except for the rare form of the disease when it appears in young adults, then there is a clear genetic basis. There have been studies of mutations: disruption of genes required for certain mitochondrial defects. Also like the Alzheimer’s disease, there is no known cure for the Parkinson’s disease, but a person can always try brain surgery, deep brain stimulation, and drugs like L-dopa, a molecule that has ability to cross blood-brain barrier and be converted to dopamine in CNS. HOWEVER there IS a potential cure which includes implanting dopamine secreting neurons in the midbrain or basal nuclei.

Stem Cell Based Therapy

Currently there is a search for ways to replace brain tissue that no long work properly. Unlike PNS, mammalian CNS cannot fully repair itself when damaged or diseased. There was a news report in 1998 saying that adult human brains produce new neurons, and from there came the possibility of fixing diseased or damaged brain with new cells. As usual there were experiments. One experiment was able to happen when a group of terminally ill cancer patients donated their brain for research when they died. In order to monitor the tumor growth, patients were given bromodeoxyuridine (BrdU, an altered nucleotide that is incorporated into DNA during replication. Apparently, DNA with BrdU can be easily identified especially when the patient’s brains were dissected, there was evidence of newly divided neurons in the hippocampus of the brain. LOOK AT THE LAST PICTURE BELOW!!!!!
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Now the goal is to find a way to induce the body’s own neural progenitor cells to differentiate into specific types of neurons or glia when and where they are needed. Being as greedy as they are, humans have another goal: to restore function to damaged CNS by transplanting cultured neural progenitor cells. Sometimes...it’s ok to be greedy. xD

Recap of the Nervous System