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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 levels.

In 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.


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