Neurotransmission

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Neurotransmission (latin: transmissio = passage, crossing; from transmitto = send, let through), also called synaptic transmission, is the transfer of signals between neurons.

All experiences, such as thoughts and feelings, and all actions, are the results of neurons generating nerve impulses. Without nerve impulses an organism is clinically dead, so they are essential for the organism's existence.
Neurons form networks through which nerve impulses travel. Each neuron receives as many as 15000 connections from other neurons. Neurons don't touch each other, but they have contact points, that are called synapses. A neuron transports its information by way of a nerve impulse. When a nerve impulse arrives at the synapse, it releases neurotransmitters, which influence another neuron, either in an inhibitory way or in an excitatory way. This next neuron is connected to many more neurons, and if the total of excitatory influences is more than the inhibitory influences, it will also "fire", that is, it will create a new action potential at its axon hillock, in this way passing on the information to yet another next neuron, or resulting in an experience or an action.

Stages in neurotransmission at the synapse

  1. Synthesis of the neurotransmitter. This can take place in the cell body, in the axon, or in the axon terminal.
  2. Storage of the neurotransmitter in storage granules or vesicles in the synapse.
  3. Release of the neurotransmitter into the synaptic cleft.
  4. After its release, the transmitter acts on a receptor in the postsynaptic membrane.
  5. Deactivation of the neurotransmitter. The neurotransmitter is either destroyed, or taken back into the terminal from which it came, where it can be reused, or degraded and removed.[1]

Summation

Each neuron is connected with numerous other neurons, receiving numerous impulses from them. Summation is the adding together of these impulses at the axon hillock. If the neuron only gets excitatory impulses, it will also generate an action potential; but if the neuron gets as many inhibitory as excitatory impulses, the inhibition cancels out the excitation and the nerve impulse will stop there. Summation takes place at the axon hillock.[2]

Spatial summation means several firings on different places of the neuron, that in themselves are not strong enough to cause a neuron to fire. However, if they fire simultaneously, their combined effects will cause an action potential.

Temporal summation means several firings at the same place, that won't cause an action potential if they have a pause in between, but when there are several firings in rapid succession, they will cause the neuron to reach the threshold for excitation.[3]

Convergence and divergence

Neurotransmission implies both a convergence and a divergence of information. First one neuron is influenced by many others, resulting in a convergence of input. When the neuron fires, the signal is sent to many other neurons, resulting in a divergence of output. Many other neurons are influenced by this neuron.[4]

References

  1. Kolb & Whishaw: Fundamentals of Human Neuropsychology (2003)
  2. Robert Graham: Reading Guide for Kolb & Whishaw, on: http://core.ecu.edu/psyc/grahamr/DW_3311Site/ReadingGuidesF/RG_Index.html, retrieved April 2007
  3. http://web.lemoyne.edu/~hevern/psy340/graphics/summation.jpg, retrieved May 2007
  4. http://www.cameron.edu/~gabrielr/PHYCH4/sld055.htm Retrieved May 2007




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