Introduction:

The central nervous system uses electrical currents to communicate with the rest of the body. Sodium and potassium are responsible for causing the flow of electrical currents through the central nervous system. The flow of electrical current is a unidirectional process that terminates in a presynaptic neuron. Another chemical comes into play, namely calcium, which, along with sodium and potassium, causes the release of other chemicals called neurotransmitters, which aid in the communication process.

The central nervous system (CNS) is made up of the brain and spinal cord. Signals from the CNS travel, via a unidirectional system, to motor neurons, which in turn communicate with somatic (voluntary) muscles and autonomic (involuntary) muscles to produce movement.

Sensory neurons send signals to the CNS, through a unidirectional system, which interprets the information and acts accordingly. For example, if the person steps on a sharp nail, the information is communicated to the CNS, the brain then tells the foot to move away from the source of the pain, all within milliseconds.

But how is this achieved? It is all a matter of chemical and electrical processes.

The cell body of the neuron initially receives the signal or stimulus. This signal is transmitted down the axon of the neuron, beginning as an electrical impulse in the axon mound.

Without any stimulus, the neuron remains in a resting state. Even during this resting state there is a constant flow of chemical ions between the inside and outside of the neuron, due to the ion concentration gradient, which produces a difference in electrical charge between the outside and inside of the neuron, which is called the resting state. potential membrane. During this stage, the interior of the neuron is negative in relation to the exterior, which is positive. The remaining electrical potential is approximately -60 mV.

The main chemical ions involved during the communication process are Sodium ions (Na+), the main extracellular cations, and Potassium ions (K+), the main intracellular cations. Inside the cell there is a concentration of 140 mM (millimoles per liter) of K+ ions and 15 mM of Na+ ions. Outside the cell the concentration is 5 mM K+ ions and 150 mM Na+ ions. This results in two concentration gradients, where ions of a high concentration will attempt to pass through the axon’s plasma membrane to a low concentration of the same chemical ion. For example, the high concentration of K+ ions in the cell will try to pass to the outside of the cell where the concentration is lower. This occurs continuously through K+ channels, which are constantly open, allowing a constant flow of K+ ions through the membrane channels out of the cell. There are voltage-gated Na+ channels (VGCs), which close during the resting potential, preventing Na+ ions from entering the cell. There are also chemically activated K+ channels (CGCs), which are also closed during the resting potential. A chemically controlled gate is also called a ligand gate. Channels are classified as passive transport, since no energy is required to power them, they rely on the concentration gradient to move ions from one area to another. In addition to ion channels, there are sodium-potassium (Na+-K+) pumps that actively expel Na+ ions from the cell and exchange them for K+ ions from outside the cell/plasma membrane. The Na+-K+ pump simultaneously transports 3 Na+ out of the cell and 2 K+ into the cell. The pump is classified as active transport since it works against the concentration gradient and requires energy in the form of ATP to function.

K+ channels are the most common open channels in the plasma membrane of resting neurons, therefore, resting neurons are more permeable to K+ ions than any other ions.

If neurons receive input signals from one or more cells, the neuron will generate an electrical signal or action potential that will travel the length of the axon, beginning at the axon mound. Upon receiving electrical stimulation, the VGCs will open, allowing Na+ ions to flood the axon, changing the charge from negative to positive. The CGCs will also open, allowing K+ ions to flow out. This depolarizing current then travels down the axon, opening more VGCs and CGCs as it goes. However, as the electrical current proceeds, there is no current where the impulse began, and therefore the VGCs and CGCs close, preventing further entry of Na+ ions and expulsion of K+ ions. The Na+-K+ pump begins to expel Na+ ions from the cell and exchanges them for K+ ions from outside the cell, thus restoring a resting potential. There is also a refractory period of 1-2 milliseconds, in which the VGCs cannot reopen, thus preventing (backing up) the current. It acts like a valve so that current can only move in one direction. The current moves along the axon like a lit fuse, opening and closing channels as it goes.

Finally, the current reaches the synaptic knots, which are bounded by the presynaptic membranes. The synaptic knobs come into close contact with the dendrites and cell body of the postsynaptic neuron, but do not touch the surfaces, there is a gap called the synaptic cleft.

The presynaptic neuron now has to communicate with the postsynaptic neuron.

As the action potential reaches the axon terminal, the VGCs in the synaptic button membrane open, allowing Na+ ions to flow into the button. This depolarization causes the VGCs of Ca2+ ions to open. Ca2+ ions flow into the button and trigger a fusion of vesicles, which contain neurotransmitters, with the cell membrane. Neurotransmitters are released across the synaptic cleft and bind to ligand-gated receptor channels on the postsynaptic neuron. The receptors are activated to open channels activated by Na+, K+, and Ca2+ ligands, causing these ions to flow into the postsynaptic cell and depolarize it. Depolarization spreads across the postsynaptic membrane, firing an action potential within it.

There are different types of neurotransmitters, all of them chemical based. They include acetylcholine, monoamines, purines, amino acids, peptides, and gas (nitric oxide). Each neurotransmitter can work alone or in combination with each other, depending on the required effect. For example, if the above action potential were to reach a muscle, the neurotransmitter released would be acetylcholine. Activation of the action potential in the postsynaptic neuron (muscle) would cause movement in that muscle.

Conclusion

In order for muscles to move, voluntarily or involuntarily, the brain must communicate with the muscle to make it move. The brain does this by sending a signal to the muscle. The signal is the result of chemical and electrical processes that pass along the neurons, which in turn transmit that information to other neurons, for example, muscle neurons.