How does the Nervous System work?

TRANSMISSION OF NERVE IMPULSES

The human nervous system contains billions of nerve cells and about 86 billion of them are found in the brain alone. Each neuron has a cell body, from where numerous branch-like projections emerge, which are known as dendrites. The dendrites usually look like the branches of a tree. At the opposite end of the cell body, a long, slender projection can be found, which is known as axon. The dendrites pick up impulses in the form of electrical signals from other neurons, which are then passed down the axon to another neuron or cell.

The axons of most of the neurons are covered by a myelin sheath, which insulates the nerve cells and speeds up the transmission of nerve impulses. Some axons can travel up to one meter or more in human body, before branching off at the end. The branches arising from an axon are slightly swollen at the tip, and these swollen tips are known as, synaptic knobs or terminal button. For transmitting impulses, neurons form a specialized structure called synapse, with other neurons and cells of the body. A synapse typically serves as the junction, where impulses or information can flow from one neuron to another.

There are three elements of a synapse, a presynaptic membrane of the signal-passing neuron (which can be usually found in the synaptic knob of an axon), the postsynaptic membrane located in a dendrite or a target cell, and a synaptic cleft, which is the space between the presynaptic and the post synaptic membranes. There are primarily two types of synapses, chemical synapses and electrical synapses, and both of them vary in the way they send impulses from one neuron to another.

Chemical Synapse:

In a chemical synapse, the transmission of nerve impulses takes place with the help of neurotransmitters. In its resting state, a neuron maintains a small voltage difference across its membrane. The inside of the membrane is negatively charged, while the electrical charge outside the membrane is positive, when the neuron is not stimulated by any impulse. This difference in electrical potential or voltage, between the inside and outside of a cell is known as, membrane potential.

When an impulse reaches the synapse of a presynaptic neuron, it changes the membrane potential of the neuron, which causes calcium channels in the presynaptic membrane to open. The opening of these channels allows calcium ions to enter through the presynaptic membrane and thus, increase the concentration of these ions in the cytoplasm. This induces the synaptic vesicles to fuse with the plasma membrane of the presynaptic neuron, and release neurotransmitter molecules into the synaptic cleft. The neurotransmitter molecules then diffuse and float across the synaptic cleft, to bind to the receptors embedded in the plasma membrane of the postsynaptic cell or neuron.

The process of binding neurotransmitters to the receptors, triggers an electrical response in the postsynaptic neuron and changes its membrane potential, which carry the impulse forward until it reaches its destination. Some neurotransmitters can excite the postsynaptic neuron, while others can inhibit its activity. The neurotransmitter molecules get removed from the receptors due to thermal shaking. They are either broken down, or reabsorbed by the presynaptic cell.

Electrical Synapse:

In electrical synapses, the impulses are transferred from one neuron to another neuron or cell, through a specialized intercellular connection, known as gap junction. Here, a direct connection is established between the cytoplasm of the two cells, so that various ions and molecules can travel easily from one cell to another. Nerve impulses can also flow from one neuron to another through the gap junction. Basically, the process begins when an impulse travels along the presynaptic cell and causes voltage changes in that cell. This allows the charged ions to travel through the gap junction and induce voltage changes in the postsynaptic cell, and thus, carry the nerve impulses.

Sources:
www.buzzle.com/articles/how-does-the-nervous-system-work.html

science.howstuffworks.com/life/human-biology/nerve.htm

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