The Mammalian Nervous System: Neurons

The mammalian nervous system consists of highly specialised cells called neurons, alongside supporting cells. The human brain contains 1010 to 1012 neurons (1). Neurons have a distinct cell shape and range from microns up to a millimeter in length. Neurons’ unique structure allows for rapid and specific transmission of signals along a neuron, and from one neuron to another.

Neurons transmit nerve impulses over long and thin axons, and receive information through branches of dendrites. Although neurons are a diverse set of cells, most neurons share certain features in their structural form - the cell body, the axons and the dendrites (2).

The cell body contains the nucleus of the neuron and other intracellular organelles. This spherical-shaped
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The synapse consists of a pre-synaptic terminal, the synaptic cleft and a post-synaptic terminal. The pre-synaptic axon terminal is separated from the post-synaptic membrane of another cell’s dendrite by a narrow synaptic cleft, a distance of 20 to 30 nanometers across (1). Fig. 2-2 illustrates the series of events that occur at the synapse. At the synapse, the axons form the information-delivering terminal. The axon terminal contains tiny spherical structures called synaptic vesicles. APs travel upon cell membrane depolarisation beyond the threshold (-55 mV), allowing Na+ to flood into the cells through Na+ channels. Due to depolarisation in the pre-synaptic terminal, Ca2+ ion channels open, allowing the transport of Ca2+ into the cell. Entry of Ca2+ promotes the fusion of synaptic vesicles to the pre-synaptic membrane. Each synaptic vesicle holds thousands of chemical neurotransmitter molecules. The vesicles discharge their contents (chemical neurotransmitters) into the synaptic cleft (7, 8), the area between pre-synaptic membrane and postsynaptic membrane of the subsequent neuron. Chemical neurotransmitters diffuses across the synaptic cleft and travel towards the post-synaptic membrane of the target cell in less than a millisecond. Neurotransmitter then binds to the highly specific receptor sites on the post-synaptic membrane. The binding changes the potential or electrical …show more content…
A. AP arrives at the pre-synaptic terminal. B. Depolarisation of the pre-synaptic terminal membrane opens voltage-gated Ca channels. Ca2+ enters the neuron terminal. C. The influx of Ca2+ drives the fusion of synaptic vesicles and pre-synaptic membrane. D. Neurotransmitters release from the vesicles and discharge into the synaptic cleft. E. Neurotransmitters diffuse across the synaptic cleft and bind to the specific receptor sites on the post-synaptic membrane. F. Membrane channel changes shape. Opening and closing of ion channels change post-synaptic membrane potential and electrical state. AP continues to propagate through the next cell. Adopted from (6).

Cells in most higher organisms communicate primarily through, but not limited to, the release of chemical substances. Communication between cells can be generally classified into neuronal or hormonal communication. Complex organisms require sophisticated and long-range signaling to coordinate communication between

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