Neurotransmission
Neurotransmission (latin: transmissio = passage, crossing; from transmitto = send, let through), also called synaptic transmission, is an electrical movement within synapses caused by a propagation of nerve impulses. As each nerve cell receives neurotransmitter from the presynaptic neuron, or terminal button, to the postsynaptic neuron, or dendrite, of the second neuron, it sends it back out to several neurons, and they do the same, thus creating a wave of energy until the pulse has made its way across an organ or specific area of neurons.
Nerve impulses are essential for the propagation of signals. These signals are sent to and from the central nervous system via efferent and afferent neurons in order to coordinate smooth, skeletal and cardiac muscles, bodily secretions and organ functions critical for the long-term survival of multicellular vertebrate organisms such as mammals.
Neurons form networks through which nerve impulses travel. Each neuron receives as many as 15,000 connections from other neurons. Neurons do not touch each other; they have contact points 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 cell, either in an inhibitory way or in an excitatory way. The next neuron may be 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.
An example of propagation among neurons is the heart beat. A beat is made when a signal is sent from the Sinoatrial node in a sequence that causes the heart to fully contract emptying all the blood in it and refilling with all new blood. It is important that the pulse is sent out from the SA node because the direction of the pulse between the neurons is what drives the muscle to fully contract. If the pulse comes in from the AV node the heart will stutter and not empty all the blood into the body.
Stages in neurotransmission at the synapse
- Synthesis of the neurotransmitter. This can take place in the cell body, in the axon, or in the axon terminal.
- Storage of the neurotransmitter in storage granules or vesicles in the synapse.
- Release of the neurotransmitter into the synaptic cleft.
- After its release, the transmitter acts on a receptor in the postsynaptic membrane.
- 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]
Cotransmission
Cotransmission is the release of several types of neurotransmitters from one and the same nerve terminal, and functions to multiply effect complexity manifold. Examples include release of NANCs in addition to e.g. noradrenaline and acetylcholine.
See also
- Neurotransmitter
- Serpentine receptor
- Autoreceptor
- neuromuscular transmission
References
- ^ Kolb & Whishaw: Fundamentals of Human Neuropsychology (2003)
- ^ Robert Graham: Reading Guide for Kolb & Whishaw, on: http://core.ecu.edu/psyc/grahamr/DW_3311Site/ReadingGuidesF/RG_Index.html, retrieved April 2007
- ^ http://web.lemoyne.edu/~hevern/psy340/graphics/summation.jpg, retrieved May 2007
- ^ http://www.cameron.edu/~gabrielr/PHYCH4/sld055.htm Retrieved May 2007
