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In the nervous system, a synapse [1] is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending on the mechanism of signal transmission between neurons.
The number of synapses in the human cerebral cortex has separately been estimated at 0.15 quadrillion (150 trillion) [3] The word "synapse" was introduced by Sir Charles Scott Sherrington in 1897. [4] Chemical synapses are not the only type of biological synapse: electrical and immunological synapses also exist. Without a qualifier, however ...
An electrical synapse, or gap junction, is a mechanical and electrically conductive synapse, a functional junction between two neighboring neurons. The synapse is formed at a narrow gap between the pre- and postsynaptic neurons known as a gap junction .
A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. [1] It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. [2] Muscles require innervation to function—and even just to maintain muscle tone, avoiding atrophy.
An autapse is a synapse in which a neuron's axon connects to its dendrites. The human brain has some 8.6 x 10 10 (eighty six billion) neurons. [31] [32] Each neuron has on average 7,000 synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about 10 15 synapses (1 quadrillion).
The special structure found in the CNS that allows for multiple inputs is the dendritic spine, the highly dynamic site of excitatory synapses. This morphological dynamism is due to the specific regulation of the actin cytoskeleton, which in turn allows for regulation of synapse formation. [ 42 ]
Axon terminals are specialized to release neurotransmitters very rapidly by exocytosis. [1] Neurotransmitter molecules are packaged into synaptic vesicles called quanta that cluster beneath the axon terminal membrane on the presynaptic side (A) of a synapse.
There are also dendrodendritic synapses, signaling from one dendrite to another. [6] An autapse is a synapse in which the axon of one neuron transmits signals to its own dendrite. The general structure of the dendrite is used to classify neurons into multipolar, bipolar and unipolar types. Multipolar neurons are composed of one axon and many ...