Nervous+System

= __Content Summary:__ =

**// Unit Overview: //**
Neurons are of extreme importance in the human body. Because of neurons, humans are able to feel stimuli and respond accordingly, maintain body heat, maintain posture, break down food, run, etc. Neurons are the cells that make every process in the human body a possiblity.

There are two different 'branches' of the nervous system: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made of the brain and spinal cord, and the PNS consists of the nerves that extend outward from the brain and spinal cord. The PNS contains only sensory and motor neurons (which sense and react to stimuli), and the CNS contains association neurons which are used as the 'integrating' part of a reflex cycle. These cells assess the information brought by the sensory neurons and tell the motor neurons how to respond (what to do). Without these different branches of the nervous system, the body would be completely useless because it takes both parts to function correctly and maintain life.



//__1. Neurons and Supportive Cells__//
Neurons are the functional cells of the nervous system that conduct electrical signals throughout the body. Neurons are made up of the cell body, dendrites, and the axon. The cell body is where the nucleus and organelles are housed. The dendrites are the part of the neuron that receive stimulation from another neuron or the environment. And the axon is where the signal is transmitted away from the cell body until it gets to the axon terminal where neurotransmitters (chemical signals) are released. These chemical can then either stimulate or suppress the next neuron, muscle, or gland cell. Neurons can be classified into three different functional types (sensory, motor, or association) and three different structural groups (bipolar, pseudounipolar, or multipolar). Sensory (afferent) neurons are found in PNS and are the neurons that sense a change in their environment and then relay that information to the CNS (brain and spinal cord). Motor (efferent) neurons are also found in the PNS, but relay the information from the CNS to a specific effector tissue (muscle or gland cell). The somatic motor neurons are under voluntary control and stimulate muscle contraction while the autonomic motor neurons are involuntary and stimulate smooth muscle, cardiac muscle, and glands. Association neurons are found in the CNS and are extremely important for thinking processes, memory, and reflexes. These neurons receive sensory information, analyze and integrate signals, and then stimulate motor neurons to conduct a certain response.

Bipolar neurons are only found in the eyes and ears and is called bipolar because there are two processes that originate from the cell body. Pseudounipolar neurons consist of most of the sensory neurons in the body and only one process originates from the cell body, but splits into two signals that go away from the cell body. Multipolar neurons are the structural group that contains motor and association neurons. These neurons have many processes that go away from the cell body; there are many dendrites and only one axon.

Found around the neurons, there are many different supportive cells that assist in the transmission of electrical impulses, protect the neurons, etc. The Schwann cells (PNS) and the oligodendrocytes (CNS) are the cells that form a myelin sheath around the axons which increases the speed of conduction of impulses down the axon. Satellite cells (PNS) form protective capsules around the neurons and are called "supportive cells". Astrocytes are found in the CNS and are very important in the blood-brain barrier. These cells control the permeability of capillaries in the CNS by forming tight junctions between the cells in the capillaries which forms a barrier. They can also form carrier proteins or ion channels to allow specific molecules to get into the brain, but it is extremely selective which protects the brain from harmful molecules. Microglia are the cells in the CNS that clean up foreign or dead material and keep the CNS clean and functioning properly. Ependymal cells forms the epithelial lining of the cavities throughout the brain and spinal cord (CNS).



//__2. Action Potential__//
Action potential is when the membrane potential of the cell rapidly depolarizes and then repolarizes; the resting potential is disrupted to allow a signal to be transmitted. During a resting potential, there is an over-all negative charge inside the cell (consisting mostly of negative ions, with a few positive potassium ions as well) and an over-all positive charge on the outside of the cell (primarily made of positive sodium ions). Resting potential is usually -70mV, but during depolarization, it becomes more positive until an action potential is reached.

During depolarization, the difference between the charges of the inside of the cell and the outside of the cell decreases. The over-all charge difference becomes more positive (-65mV, -60mV, -55mV, etc.) until it reaches a specific threshold which then quickly opens voltage-gated channels and slowly begins to open potassium channels (for the next step). When the voltage-gated channels open, sodium ions quickly rush into the cell which causes the inside of the cell to change from negatively charged to positively charged. The influx of sodium ions drives depolarization and causes an action potential.

The next step in conducting a signal down the axon is called repolarization. In this specific stage, the voltage-gated (sodium ion) channels are deactivated which ceases the flow of sodium into the cell. At the same time, potassium ion channels are completely opened to allow all of the potassium to flow out of the cell. This causes the inside of the cell to become negative and the outside to become positive again, but the sodium and potassium ions are on the opposite sides of the cell membrane that they belong on (during resting potential). Because of this, a sodium/potassium (Na+/K+) pump must be utilized to move the ions to their proper places. This pump requires a lot of energy, but is essential in maintaining and restoring membrane potential. The Na+/K+ pump carries three sodium ions out of the cell at the same time as it carries two potassium ions into the cell until the resting potential is restored. The out-flow of potassium ions is what drives repolarization and then the Na+/K+ pump restores balance.

During conduction of a signal, though, there is also a refractory period in which the neuron either can't respond to a stimulus at all (absolute refractory period) or a greater stimulus is needed to cause another action potential (relative refractory period). In an absolute refractory period, the neuron is currently carrying out an action potential and can't respond to a second stimulus. In a relative refractory period, though, the sodium ion channels can be activated again, but potassium continues to flow out of the cell which means that the cell would need a greater stimulus in order to carry out another action potential. Action potentials are the reactions carried out by the neurons in our bodies that allow us to move, see, hear, react to our environment, etc. They are very important to life as we know it.



//__3. Synapses__//
A synapse is the space between a neuron and another neuron, muscle, or gland cell in which a lot of important functions occur. Neurotransmitters released from an axon terminal on the presynaptic neuron bind to receptors on the cell following it which stimulates changes in that cell. These changes are usually a change in membrane potential which may cause an action potential to occur in the axon hillock (where the cell body and the axon meet).

<span style="font-family: Georgia,serif; font-size: 110%;">In the presynaptic neurons, the axon terminals contain vesicles which contain neurotransmitters. When an action potential reaches the axon terminal of the presynaptic neuron, it stimulates voltage-gated calcium (2+) channels to open and calcium ions rush into the axon terminal. The calcium ions promote exocytosis of the synaptic vesicles which causes the neuron to secrete neurotransmitters.

<span style="font-family: Georgia,serif; font-size: 110%;">These neurotransmitters then diffuse across the synapse and bind to the receptors on the postsynaptic cell membrane. The neurotransmitters can cause either an action potential throughout that cell or theycan inhibit the particular cell.



<span style="font-family: Georgia,serif; font-size: 110%;">The potentials in the synapses can add on to each other (summate) and can increase the chance of reaching the threshold and causing an action potential. Spatial summation is when there are many neurotransmitters from many different neurons acting on one postsyaptic cell, and temporal summation is when there is an increased amount of neurotransmitter being secreted by a particular neuron that acts on a single postsynaptic cell. Unlike within the neuron itself, there is no "all-or-nothing" reaction; the synaptic potentials are capable of adding together and causing a greater chance of reaching the threshold.

= __Application:__ = = = <span style="font-family: Georgia,serif; font-size: 110%;">Having extensive knowledge of the nervous system will be very convenient in the nursing field. There are many disorders throughout the world that affect the different parts of the nervous system and affect the body differently. Alzheimer's disease, multiple sclerosis, and muscular dystrophy are a few examples of nervous system disorders which work very differently on the body. Alzheimer's disease is a disease that affects memory; multiple sclerosis affects conduction of impulses; and muscular dystrophy affects the contractility of muscle fibers. The most interesting disease of the nervous system, to me, though, is Alzheimer's disease.

<span style="font-family: Georgia,serif; font-size: 110%;">Alzheimer's disease is a type of dementia found mostly in people over the age of 65 that is characterized by memory loss, confusion, aggression, etc. The individual becomes a completely different person by the final stages of their disease. The cause is yet to be found, but scientists are working really hard to figure out why this disease affects some people and how exactly it works on the brain.

<span style="font-family: Georgia,serif; font-size: 110%;">In Alzheimer's disease, neurons in the brain begin to break down. One part (so far unknown) of the cell stops functioning properly which causes a domino effect to occur. With that particular part of the cell not working correctly, other parts of the cell have to work even harder. These cells eventually get too exhausted, though, and also begin to dysfunction. As this process continues in the brain, the cells can no longer do what they are meant to do which leads to problems like not remembering things, speaking, moving, etc. depending on what part of the brain was affected by the disease. //(From: http://www.health-reports.com/alzheimers.html)//

<span style="font-family: Georgia,serif; font-size: 110%;">In my experience in nursing homes as a CNA, it is really difficult to stand by as families watch their loved ones endure this disease. It's painful to see what type of a person they've become when you know what kind of person they used to be. In my mind, a cure for Alzheimer's disease needs to be a top priority in the near future so that no one else has to watch their loved ones slowly slip away from them.

=__Essential Questions:__=

===<span style="font-family: 'Trebuchet MS',Helvetica,sans-serif;">**//1. Describe how the dendrite or cell body of the postsynaptic neuron is stimulated (excited) to send an impulse from the axon hillock to the rest of the neuron. (Include these key words in your description: neurotransmitter, EPSP, axon hillock, voltage-gated channels (Na+ and K+), action potential).//** ===

<span style="font-family: Georgia,serif; font-size: 110%;">In order to send an impulse from the axon hillock to the rest of the neuron, the dendrites or cell body must first get a signal from a presynaptic axon. The presynaptic axon secretes neurotransmitters that diffuse through the synaptic cleft and then attach to receptors on the postsynaptic cell. When the neurotransmitter binds to the receptors, ligand-gated ion channels open and ions enter the postsynaptic cells membrane. This creates a synaptic potential that can either be excitatory (EPSPs) or inhibitory (IPSPs). If the synaptic potential is inhibitory, chlorine (-) ions enter the cell membrane, but if the synaptic potential is excitatory, sodium (+) ions enter the membrane and cause depolarization. The conduction is then passed on to the axon hillock where depolarization continues until an action potential is created.

===//<span style="font-family: 'Trebuchet MS',Helvetica,sans-serif;">**2. Describe the sequence of events that occur to get an action potential (neuron impulse) to stimulate the release of neurotransmitters from the presynaptic axon. What happens when the neuron is inhibited? (Include these key words in your description: resting membrane potential [-70mv], depolarization, threshold [-55mv], action potential, hyperpolarization [<-70mv]__,__** voltage-gated channel (Na+, K+, Ca2+), repolarization, sodium/potassium pump, Ca2+, vesicles, neurotransmitters, exocytosis.)//===

<span style="font-family: Georgia,serif; font-size: 110%;">In order to get an action potential to stimulate the release of neurotransmitters, the resting potential (-70mV) must first be depolarized. When an impulse reaches the axon hillock, voltage-gated sodium ion (+) channels quickly open and potassium ion (+) channels slowly begin to open. Sodium ions rush into the cell which causes depolarization of the cell membrane. The membrane potential must then reach its threshold (usually -55mV) in order to open more voltage-gated channels further along the axon and continue sending the impulse. Once the threshold is reached, action potential will depolarize as much as possible. In the next step, repolarization, the sodium ion (+) channels are deactivated and potassium ion (+) channels are completely opened. The sodium stays in the cell while the potassium rushes out of the cell. Once the membrane potential drops under the threshold point (hyperpolarization), the potassium ion channel are closed again. Now, though, the sodium and potassium ions are on the wrong side of the cell - where they don't belong. In order to fix this, sodium/potassium (Na+/K+) pumps are activated and return the ions to the correct side of the membrane.

<span style="font-family: Georgia,serif; font-size: 110%;">As the action potential reaches the axon terminals, voltage-gated calcium ion (2+) channels open and calcium rushes in. The calcium then signals the cell to begin exocytosis of the synaptic vesicles which contain neurotransmitters. These neurotransmitters diffuse across the synaptic cleft and the process begins all over again.