Anatomy And Physiology

Anatomy Physiology



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Every action or non-action in the body is controlled by the brain. Neurons are the cells that communicate between the brain and the other parts of the body. Neurons, like other cells, have a cell body, which is called the soma. Inside of the soma there is a center of the cell called a nucleus. The body of the neuron is made by a plasma membrane, which is made of a lipid bi-layer, meaning a double layer of lipid, or fat, cells. The neuron has branches that extend from the sides in all directions. Each of these branches is called a dendrite. The dendrites reach out to other neurons and receive signals from them. The signals from the other neurons come through a part of the neuron called the axon. The axon extends from the neuron at a point called the axon hillock, where the soma and axon connect. The axon also has a lipid bi-layer covering called the myelin sheath. In addition to the dendrites, the soma can receive signals from other neurons as well. The axon acts as the transmitter for the neuron, sending signals away. The end of the axon is called the terminal button. The terminal button converts an electric signal into a chemical signal, releasing neurotransmitters (chemicals) across a gap called the synapse and into the receiving cell.

The process through which a neuron receives and sends signals is electro-chemical. When a signal comes in it is received as an electrical charge, this is known as the action potential. Each neuron has a resting potential, that is, a charge that is present when there aren’t any signals coming in or going out. The resting potential is -70mV. When an action potential comes into the cell it causes a chain reaction by changing the resting potential of the cell. There are different types of potentials that could come in, one is excitatory and another is inhibitory. These are known as post-synaptic potentials (PSP). The plasma membrane across the dendrites, soma and axon has sodium (Na) and potassium (K) ions surrounding them, both inside and outside the cell structure.  These two ions are what make the resting potential negative. Both ions are positively charged, at the resting potential of the cell there are more positively charged ions outside the cell than inside, which makes the outside more positive. The action potential changes the electrostatic pressure by opening a channel for the ions to move from inside to out and outside in.

Along the membrane there are channels for sodium and potassium. These channels open and close based on electrostatic charge. When an action potential raises the resting potential from -70mV to about -55mV the sodium channel will open up. Due to the response from the change in charge, these channels are called voltage gated. Because it is true that opposites attract when dealing with positives and negatives, when this channel opens, the sodium ions rush into the cell because they are positive and the inside charge is negative, this is what electrostatic pressure is. Another force at play is called concentration gradient.  Concentration gradient is when there is a concentration – go figure – of one type of ion in an area. These ions like to spread out in order to create homeostasis. When the channel opens up electrostatic pressure and concentration gradient both work on the ions to move them from outside to inside. As this is occurring with the Na ions, the K ions are having a similar experience, only they move from inside to outside. After the Na channel opens a K channel opens.  They begin opening at the same time actually, but the K channel is slower.

As the ions begin switching places, the charge inside the cell changes from negative to positive in a process called depolarization. If the charge is compared to a globe, with a north pole, equator and south pole, the resting charge would be in the southern hemisphere, nearer the south pole. As the charge increase, it leaves the south pole, depolarizes, and crosses the equator into a positive charge. It won’t stay positive forever though, a sodium-potassium pump – another channel, with a different function – puts things back to the way they were. The pump takes three sodium ions from inside the cell and puts them outside, as well as takes two potassium ions from outside and puts them inside. Through this process the pump returns the cell to its resting potential. This process is called repolarization as the charge is returning the South Pole orientation. Due to the process, the pump isn’t able to set the ion distribution perfectly to get right back to the resting potential, so it goes even more negative on its repolarization trip. When the charge goes below -70mV it is called hyper polarization. This whole process of ion flow and charges going up and down sets off a chain reaction across the cell membrane and down the axon to the terminal button.

When the charge hits the terminal button, depending on the type of PSP it is, excitatory or inhibitory, it releases different neurotransmitters, the chemicals that will jump the synaptic gap and transfer the signal.  The neurotransmitters come in packets called vesicles. The vesicles are sent, merge with the membrane of the terminal button and release the chemicals into the synapse, which is just a space between sending and receiving cells. The receiving cell absorbs the neurotransmitters based on what type of chemical it is; each has its own reception area. The chemical then triggers an electrical response and the process continues on.

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