The Complex Structure And Structures Of The Mammalian Nervous System

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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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A. Resting membrane potential: Voltage-gated Na+ channels are in the resting state and voltage-gated K+ channels are closed. B. Threshold potential: Stimulus causes depolarisation to threshold. C. Depolarisation: Voltage-gated Na+ channels are open. D. Repolarisation: Na+ channels are inactive and voltage-gated K+ channels are open. E. Hyperpolarisation: Voltage-gated K+ channels are still open and Na+ channels are back to resting state. Adopted and modified from (6).

Neurons can accommodate both short- and long-range signal transmissions. Information is transmitted from one neuron to another across neuronal synapses by means of a chemical neurotransmitter. Neurotransmitters are secreted from the nerve terminals located at the end of the axon upon a triggering
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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

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