Nervous System´╗┐This chapter discusses the nervous system which is an organ system composed of specialized cells, called neurons, which coordinate actions and transmit signals from other parts of the body. *Content Summary:*Neurons and other supporting cells:Neurons are the basic structure of the nervous system. They are trained to respond to physical and chemical stimuli, release chemical regulators and conduct electrochemical impulses. A neuron is broken down into a dendrite, cell body and axon. Sensory neurons conduct impulses towards the CNS while motor neurons conduct away from the CNS. The somatic motor nerves work to innervate skeletal muscles while the autonomic nerves work the smooth and cardiac muscle as well as glands.

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Acetylcholine as a NeurotransmitterAcetylcholine (ACh) is used as an excitatory neurotransmitter by neurons of the CNS and somatic neurons at the neuromascular junction. The two subtypes of ACh receptors are called either nicotinic or muscarinic. Nicotinic receptors (found in regions of the brain, skeletal muscle fibers and ganglia) open when ACh bind to the site causing depolarization. Muscarinic ACh receptors (found in the plasma membrane of smooth muscle cells, cardiac muscle cells and cells of particular glands) open ion gated channels through G-Proteins when the ACh binds to the receptor site. I've included a simple and short video showing ACh in the nervous system.



*Monoamines as Neurotransmitters: Monoamine is a type of neurotransmitter that contains one amino group. Examples include serotonin, dopamine, epinephrine, histamine and norepinephrine. Specific transporter proteins called monoamine transporters exist that transport monoamines in or out of a cell. These are the dopamine transporter, serotonin transporter, and the norepinephrine transporter in the outer cell membrane and the vescular monoamine transporter in the membrane of intracellular vesicles. After release into the synaptic cleft, monoamine neurotransmitter action is ended by reuptake into the presynaptic terminal. There, they can be repackaged into synaptic vesicles or degraded by the enzyme monoamine oxidase (MAO), which is a target of monoamine oxidase inhibitors, a class of antidepressants. http://en.wikipedia.org/wiki/Monoamine_neurotransmitter´╗┐ Image of Monamine Families. Showed up fuzzier than I would have liked!


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Application: In the nursing field, we constantly monitor for physiological changes as well as psychological. In the event that a patient is suffering from a mentals disorder or a change in mental state, we give a plethora of drugs to treat the condition including anti-anxiety medications (zanax, ativan) or anti-depressants (prozac, cymbalta). It's important to understand what happens physiologically in the brain and how the medications work before giving them. For instance, some anti-depressants block the reuptake of serotonin (SSRI's). I recently had the learning opportunity to do a week of clinicals at the state mental hospital. What most people don't understand is that mental disorders can be just as debilitating as visible or physical disorders. I was able to see first hand how abnormal levels of chemicals in the brain, or diseases of the brain, can change a person's entire life and how an individual can really suffer from it. It was heartbreaking and very eye opening. This chapter, although extremely difficult, was important to me in understanding how these disorders take place and how the different medications can work to resolve them. *Critical Thinking Questions:1. Describe how the dendrite or cell body of the postsynaptic neuron is stimulated to send an impulse from the axon hillock to the rest of the neuron.
A neurotransmitter is released from the presynaptic axon. It diffuses across the synaptic cleft to the membrane of the postsynaptic cell. The neurotransmitter binds to specific receptor proteins on the membrane.This causes chemically gated channels in the dendrites and cell body to open. Na+ diffuses into the cell causing depolarization called excitatory postsynaptic potential (EPSP).EPSP spreads to the axon hillock and opens voltage-gated Na+ channels. Na then enters the cell causing it to become more positive. When a membrane potential of +30 mv, Na+ channels close and K+ channels open. The neuron fires action potentials. The action potentials are passed to the next region of the postsynaptic cell.

2a. Describe the sequence of events that occur to get an action potential to stimulate the release of neurotransmitters from the presynaptic axon.
Cells have a RMP of -70mv. When stimulated by an external stimulus/neurotransmitter, the neuron begins to change. Depolarization causes the neuron to become more negative. When the neuron has a threshold level at -55 mv, voltage-regulated Na+ channels open and more Na+ comes into the cell causing more depolarization. This causes more and more Na+ channels to open, thus allowing more Na+ to come in! The charge of the the ion eventually reaches around +30mv. The K+ voltage-gated channels open and K+ ions diffuse out of the cell quickly. The inside of the cell becomes more negative. The membrane potential actually becomes less than the RMP of -70mv, called hyperpolarization. The K+ channels close and the RMP is restored. Now there is too much K+ extracellularly and Na+ intracellularly. The sodium/potassium pump is now needed. After being fully restored, the neurons releases an impulse.When the action potentials reach the axon terminals, voltage-gated Ca2+ channels open and Ca2+ ions enter the cytoplasm and bind to a sensor protein.
(Our textbook was used heavily for these answers).
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