Chapter 2: Cellular Mechanisms and Cognition

Neurons

The major parts of the neuron are the soma, dendrites and axons. Dendrites receive information from other neurons and axons pass the information on. The soma is the cell body. There are different forms of neurons. They are: unipolar, bipolar, pseudounipolar and multipolar. Unipolar neurons have only one process that extends from the cell body. Bipolar neurons have two processes that extends from the cell body. Pseudounipolar neurons look like unipolar neurons but were bipolar neurons. Their dendrites and axons have merged together. Multipolar have only one axon and have many dendrites extending from the cell body.

Resting Membrane Potential

There are ion channels that allow sodium, potassium and chloride either in or out of the cell. There is a higher concentration of sodium and chloride outside than there is inside. This is opposite for potassium, there is a lower concentration outside than inside. Because of the concentration differences potassium will enter the cell while sodium and chloride are pushed out. However, the cell would be a completely different cell if there were nothing to maintain the concentration difference. The pomp helps do this. Typically, the voltage for resting potential is about -70 millivolts. Since the cell has a concentration difference in ions the voltage changes, this is called the action potential. During action potential sodium enters into the cell increasing the voltage. It seems as if it is reaching the sodium equilibrium. However, when the voltage gets too high the pump kicks in and works to lower the voltage to its normal state.

Synaptic Transmission

The important function a neuron serves is communicating with other neurons. The action potential leads to the depolarization of the neuron, which initiates an increase in calcium. This causes small vesicles to fuse with the membranes at the synapse; in turn the transmitter is released into the synaptic cleft.  The chemical interaction can lead to either hyperpolarization or depolarization. Hyperpolarization (inhibition) is very important. If the neuron would fire continually, this would produce a seizure. If the postsynaptic cell is a neuron this will lead to excitatory postsynaptic potential (EPSP). However, if the neurotransmitter has an inhibitory action this will lead to a inhibitory postsynaptic potential (IPSP). Hyperpolarization will cause the membrane potential to become negative therefore farther away from threshold. This would decrease the chances of the neuron to produce an action potential. Sometimes inhibitory synaptic inputs lead to depolarization. This is because the equilibrium potential for chloride is about -60 millivolts. The resting potential is about -70 millivolts, so opening the chloride channels would cause the membrane potential to move towards -60 milllivolts. The chloride would force the potential to remain around -60 millivolts. The threshold to create an action potential is about -40 millivolts. So, depolarization to -60 millivolts would still lead to inhibition of the neuron.