Excitation transmission from cell to cell – even from nerve cell to nerve cell – occurs through synapses. These are junctions between two nerve cells or between nerve cell and other tissue cells that are specialized for signal transmission and reception. In most cases, signal transmission occurs via so-called messenger substances (neurotransmitters); only in the case of transmission from muscle cell to muscle cell can excitation transmission also occur via an electrical potential. Excitation transmission is also known as ”’transmission”’.
What is excitation transmission?
Excitation transmission from cell to cell – even from nerve cell to nerve cell – occurs via synapses. The enormous number of cells in the human body must be able to communicate with each other or receive instructions in order to produce a particular behavior of the organism, such as muscle contractions. This multifaceted process occurs via differential excitation transmission or transduction. Most excitatory transmission is relayed at synapses by activation and release of transmitter substances. Thus, this transmission and, if necessary, the distribution of action potentials to multiple recipients usually occurs chemically via chemical synapses where the messenger substances or neurotransmitters are transferred to the recipient cell. In this process, the synapse end-buttons have no direct contact with the target cell, but are separated from it by the synaptic cleft in the order of 20 to 50 nanometers. This offers the possibility of altering or inhibiting the transmitter substances in the synaptic cleft that they have to cross, i.e. converting them into inactive substances. The action potential is then cancelled out again. Muscle cells can also be connected to each other with electrical synapses. In this case, action potentials are transmitted in the form of electrical impulses directly to the next muscle cell or even to many cells simultaneously.
Function and task
Humans have approximately 86 billion nerve cells. A large number of regulatory circuits and many voluntary and purposeful actions, as well as life-sustaining reactions to external threats, must be controlled. The extraordinarily large number of body cells must be made to work together in a coordinated manner to implement the necessary and desired reactions of the entire organism. To accomplish these tasks, the body is crisscrossed by a dense network of nerves that, on the one hand, report sensory information from all regions of the body to the brain and, on the other hand, allow the brain to transmit instructions to organs and muscles. The upright gait alone sets millions of nerve cells into action for coordinated movement, simultaneously and constantly checking, comparing and processing in the brain the position of the limbs, the direction of gravity, forward speed and much more, in order to send contraction and relaxation signals to specific muscle parts in real time. To accomplish these tasks, the body has at its disposal a unique system of excitatory transmissions or transductions. Typically, a signal must be transmitted from nerve cell to nerve cell or from nerve cell to muscle cell or other tissue cell. In some cases, signal transmission between muscle cells is also necessary. In most cases, an electrical action potential is transmitted electrically within a nerve cell and, upon reaching the point of contact (synapse) to the next nerve cell, is again converted into release of specific messenger substances or neurotransmitters. The neurotransmitter must cross the synaptic cleft and, after reception by the recipient cell, is converted back into the electrical impulse and transmitted. The detour of signal transmission via the chemical intermediates is important because specific neurotransmitters can only dock to specific receptors, making the signals selective, which would not be possible with purely electrical signals. A wild chaos of reactions would be triggered. Another important point is that messengers can be altered or even inhibited during passage through the synaptic cleft, which can be equivalent to cancellation of the action potential. Only signal transmission between muscle cells can be purely electrical through electrical synapses.In this case, so-called gap junctions allow electrical signals to be transmitted directly from cytoplasm to cytoplasm. In muscle cells – especially cardiac muscle cells – this has the advantage that many cells can be synchronized over long distances for contraction.
Diseases and disorders
The great advantages of converting electrical action potentials into specific neurotransmitters, which allows simultaneous – and necessary – selective signaling, at the same time carries the risk of harmful interference and attack. Basically, there is a possibility that synapses will be overexcited or inhibited. This means that toxins or drugs can trigger spasms or paralysis at neuromuscular synapses. If synapses in the CNS are affected by toxins or drugs, mild to severe psychological effects set in. Anxiety, pain, fatigue or irritability may be caused without any apparent reason at first. There are several ways to influence transmission. For example, botulinum toxin inhibits vesicle emptying into the synaptic cleft so that no neurotransmitter is transmitted, resulting in muscle paralysis. The opposite effect is produced by black widow’s poison. There is a total emptying of the vesicles, so that the synaptic cleft is literally flooded with neurotransmitters, which leads to severe muscle spasms. Symptoms similar to those of botulinum toxin occur with substances that prevent the reuptake of the neurotransmitters by the recipient cell. There are also other ways to prevent or impair excitation transmission. For example, some substances can occupy the receptors of a particular neurotransmitter, causing paralysis.
