Introduction:
The central nervous system uses electric currents to communicate with the rest of the body. Sodium and potassium are responsible for causing the flow of the electric currents throughout the central nervous system. The flow of the electric current is a one-way process, terminating at a pre-synaptic neuron. Another chemical comes into play, namely calcium, which, with sodium and potassium, cause the release of further chemicals called neurotransmitters, which aid in the communication process.
The central nervous system (CNS) is composed of the brain and spinal cord. Signals from the CNS go, via a one-way system, to the motor neurons, which in turn communicate with the somatic (voluntary) muscles and the autonomic (involuntary) muscles to produce movement.
Sensory neurons, send signals to the CNS, via a one-way system, which interprets the information and acts accordingly. For example if person stands on a sharp nail, the information is communicated to the CNS, the brain then informs the foot, to move away from the source of the pain, all done within milliseconds.
But how is this achieved? It is all a question of chemical and electrical processes.
The cell body of the neuron initially receives the signal, or stimulus. This signal is transmitted down the neuron's axon, starting as an electrical impulse at the axon hillock.
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 electric charge between the outside and inside of the neuron, which is called the resting membrane potential. During this stage the inside of the neuron is negative in relation to the outside which is positive. The resting electrical potential is approximately -60 mV.
The main chemical ions involved during the process of communication are Sodium ions (Na+), the main extracellular cations and Potassium ions (K+), the main intracellular cations. Within the cell there is concentration of 140 mM (millimoles per litre) of K+ ions and 15 mM of Na+ ions. Outside the cell the concentration is 5 mM of K+ ions and 150 mM of Na+ ions. This gives rise to two concentration gradients, where ions of a high concentration will try to pass through the plasma membrane of the axon to a low concentration of the same chemical ion. For example, the high concentration of K+ ions in the cell will attempt to pass to the outside of the cell where the concentration is less. This occurs continually via K+ channels, which are constantly open, allowing a constant flow of K+ ions through the channels in the membrane to the outside of the cell. There are voltage-gated Na+ channels (VGC), which are closed during the resting potential, thus preventing the Na+ ions from entering the cell. There are also chemically-gated K+ channels (CGCs), which are also closed during the resting potential. A chemically controlled gate is also called a Ligand gate. The channels are classed as passive transport, as no energy is required to power them, they rely on the concentration gradient to move ions from one area to another. Apart from ion channels there are sodium-potassium (Na+- K+ ) pumps which actively expel Na+ ions from the cell and exchanges them for K+ ions from the outside of 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 classed as active transport as it works against the concentration gradient and requires energy in the form of ATP to power it.
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 ion.
If neurons receive input signals from one or more cells, the neuron will generate an electrical signal or action potential which will travel down the length of the axon, starting at the axon hillock. On receiving the electrical stimulus, the VGCs will open, allowing Na+ ions to flood into the axon, which changes the charge from negative to positive. The CGC's will also open, allowing the K+ ions to flood out. This depolarising current then moves down the axon, opening up more VGCs and CGCs as it goes. However, as the electrical current moves forward, there is no current where the impulse started and so the VGCs and CGCs close, preventing more influx of Na+ ions and expulsion of K+ ions. The Na+- K+ pump then starts to expel the Na+ ions from the cell and exchanges them for K+ ions from the outside of the cell, thus re-instating a resting potential. There is also a refractory period of 1-2 milli seconds, where the VGCs cannot reopen, thus preventing (backup) of the current. It acts as a valve so that the current can only move in one direction. The current moves along the axon like a lit fuse, opening and closing channels as it progresses.
Eventually the current reaches the synaptic knobs, which are bounded by the presynaptic membranes. The synaptic knobs come into close contact with the dendrites and cell body of the post synaptic neuron, but do not touch the surfaces, there is a gap called a synaptic cleft.
The presynaptic neuron has now to communicate with the post synaptic neuron.
As the action potential arrives at the axon terminal, VGC's in the synaptic knob membrane open allowing Na+ ions to flow in to the knob. This depolarisation causes VGC's of Ca2+ ions to open. Ca2+ ions flow into the knob and trigger a fusion of vesicles, containing neurotransmitters, with the cell membrane. The neurotransmitter/s are released across the synaptic cleft and bind to ligand-gated receptor channels on the post-synaptic neuron. The receptors are activated into opening the ligand-gated channels of Na+, K+ and Ca2+ causing these ions to flow in to the post synaptic cell and depolarise it. The depolarisation spreads in the post synaptic membrane, firing an action potential within it.
There are different types of neurotransmitters, which are all chemically 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 arrived at a muscle, the released neurotransmitter would be acetylcholine. The action potential firing in the post synaptic 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 in order for it to move. The brain does this by sending a signal to the muscle. The signal is the result of chemical and electrical processes passing along neurons, which in turn pass that information on to other neurons, for example, muscle neurons.
