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_Basic physiology of a neuron and how it fires _
By: Anonymous
Describe the basic physiology of a neuron. Detail the manner by which neurons
fire action potentials and how neurons communicate with one another across
synapses. Outline the process of how an action petential occurs and hoe it
propagates down an axon. Explain how chemical transmission occurs at synapses
and how this allows neurons to activate of inhibit one another. [Picture from
"Answer to Neuron Structure"] Neurons are the basic units of the brain. Above
is a picture of a prototypical neuron with its parts labeled by number. The
objects labeled by the number one are Dendrites. Dendrites conduct nerve
impulses towards the nerve cell. The nucleus, which regulates activities in
the cell is labelled 2. Labeled 3, the soma or cell body, is the body of the
neuron. The myelinated sheath, of the structure labeled 4, acts like an
insulator. Not all neurons have myelinated sheaths. In the types that do,
messages to said to 'jump' along the axon. Structure 6 is the axon, which
conducts impulses away from the cell body. Finally, structures labeled 8 are
called terminal branches or synaptic terminals. These transfer impulses toward
the next neuron. (Answer to Neuron Structure) Action potential is what allows
for nerve impulses. The process of action potential begins when there is a
difference in concentration of ions outside and inside of the neuron. Before
this process begins, the neurons are in a state called resting potential. In
this state, neurons are negativelty charged at -70 mv. If an electrical
stimulus is applied, sodium dependent gates open and positive sodium ions to
rush in. Now the neuron is positively charged. The added sodium creates what
is known as a 'spike'. This occurs, on average, at positive 35mv. Following
this, the sodium gates close and potassium gates open. Positive potassium ions
then flow out and the neuron becomes negatively.After this, the neuron becomes
hyperpolarized. More positive potassium ions flow out. This makes the voltage
drop below -70 mv. When this occurs, the K+ gates close. Resting potential
occurs after this at -70 mv (Andrews, Brains and Neurons) Messages travel from
one neuron to another in a process that begins when an action potential
travels down the axon, enters the terminal branches, spreads depolarization.
Following this votage-gated Calcium channels are activated, calcium
permeability increases and Ca influx results. Synaptic vesicles containing
neurotransmitters now fuse with the neuron membrane becuase of increased
intracellular calcium. Neurtrasmittitters are now released into synaptic
clefts. It takes approximetly .3-3 seconds to diffuse. This deceleration is
called the 'synaptic delay'. Depending on the nature of the receptor, either
positive or negative ions will flow into the postsynaptic neuron. If the
neurotransmitter gates a positive channel, sodium will flux and a excitatory
postsynaptic potential occurs. And exitatory postsynaptic potential is a small
depolarizing potential. If the neurotransmitter opens potassium or chlorine
channels, the ions flow in or out (potassium out and chlorine in). This will
cause an inhibitory postsynaptic potential. An inhibitory postsynaptic
potential is a graded hyperpolarization. Reuptake of neurotransmitters by the
presynaptic neuron clears away the neurotransmitter by the presynaptic neuron
clears awy the neurtranmitters in the cleft. Finally, neurotransmitters are
taken back in (Andrews, Nuerons and Nueral Communications) 3 Describe with
examples how cognitive science can be said to have three (separate but
related) levels of description and explanation. Describe briefly this
tri-level hypothesis. Describe examples of research that can naturally be
subsumed under each one of the three levelsIn 1982, David Marr theorized
that in order to to describe cognitive science entirely, one would have to
descibe the three levels of cognitive science. These seperate yet related
descriptions include a computational description, and algorithmic description,
and an implementational description. The computational level of cognitive
science gives a formal desrciption of the functions that an information
processor carries out. The algorithmic description describes procedures
something carries out as well as the tasks involved in accomplishing its task.
An implementational desciption describes what something is physically, how it
was made, and what is does physically (Andrews, The tri-level hypothesis).
Different processes can be described using the tri-level hypothesis. I will
use the example of the turing machine to demonstrate the tri-level hypothesis.
When describing the something one starts off by descibing it's implementation
level, the physical desciption. The turing machine consists of a tape, a
reader and a writer. The next part of a complete description is the
algorithmic level. For a Turing machine an algorithmic description would a
table of instructions detailing what to do if a certain situation occurs and
what steps to follow. Finally, the computational description of a turing
machine would be depend on the exact function that was being carried out. A
set example can't be given becuase a turing machine is a hypothetical machine.
Each of these levels of the tri-level hypothesis contains limitations. The
limitations of the implementational level is that one can't be sure where part
of the machine is necessary for it to function. Limitations on the algorithmic
level occur because a formal account of information and the manner of its
manipulation cannot describe the task very well. Limitations occur at the
computational level because one needs the lower two levels to fully explain a
process.
Word Count: 868
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