The fundamental unit of the nervous system is not just neurons but also glia! However, the neurons have features that would allow them to communicate with each other, including long-distance communication. These features are:
- The morphological and functional asymmetry of neurons – dendrites with specialized receptors and the transmitting potential of axon forms the basis of neuronal signaling.
- Neurons can communicate electrically and chemically. This is possible via specialized proteins called ion channels or receptors.
The glia, on the other hand, are less electrically excitable in nature. The function of glia is to reuptake neurotransmitters and ions, regulating neuronal function.
The nervous system of invertebrates such as Drosophila melanogaster has about 100,000 neurons, whereas vertebrates such as mice have 75 million neurons. A human brain has about 86 billion neurons. Despite these differences in neuron-number, neurons and glia communicate with each other to form networks and control basic functions such as reflexes and basic behaviors.
Like other cells, a neuron is a cell comprising the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular organelles. But the unique feature of neurons is receiving and sending electrical signals that make neuron communication possible. Neuron has tree-like structures called dendrites that receive messages from other neurons via connections called synapses.
The lipid bilayer that surrounds a neuron is impermeable to ions. Ions can pass through only via specialized proteins called ion channels or receptors spanning the neuronal membrane. These ion channels allow the passage of ions in and out of the cell. There are different types of ion channels depending on the stimuli they respond to. The ion channels that respond to voltage changes are called voltage-gated ion channels. The difference between the amount of charge between the outside and inside of the cell is called the membrane potential. At rest, a neuron is negatively charged i.e., the inside of the cell is approximately 70 mV more negative than the positive outside. This is called the resting membrane potential of the cell. This membrane potential is created by the ion channels that are selectively permeable to the respective ions. For example, sodium-potassium pumps create a difference between the inside and outside of the cell by allowing two K+ ions inside and three Na+ ions outside. This exchange requires one molecule of ATP. Hence, fifty percent of the neuron’s ATP is utilized in maintaining the resting membrane potential. The concentration of potassium ions is greater inside the cell, hence they have a tendency to move out of the cell creating a net negative charge on the inside. Even though sodium ions are less inside the cell and has a tendency to move in, it is not as freely permeable as potassium ions. Therefore, change in resting membrane potential takes place only in the presence of voltage-change stimuli.
If a neuron receives input from other neurons, the transmission of signal happens through a chemical, called a neurotransmitter, from the axon of the neuron to the dendrite of the other. In order for such chemical signaling to take place, the neurotransmitter from the presynaptic neuron binds with the receptor in the post-synaptic neuron. This allows the opening of voltage-gated sodium ion channels to open leading to the entering of sodium ions and resulting in depolarization. If the signal is strong enough to reach the axon, then depolarization creates a positive feedback loop such that more sodium ions enter the cell as the axon gets further depolarized. In the axon, such a self-propagating reversal of the resting membrane potential is called an action potential. If the initial depolarization is not strong enough to the threshold of excitation, then there will be no action potential due to the all or none property of action potential.
When the resting membrane potential turns into an action potential, the membrane potential reverses from -70 mV to +30 mV. This change triggers the voltage-gated potassium channels to open, allowing the potassium ions to leave the cell, repolarizing the cell. At this point, the voltage-gated sodium channels become inactive. So, there will no more movement of sodium ions. As the potassium ions leave the cell, the resting membrane potential is re-established. This results in the closure of the voltage-gated potassium channels and voltage-gated sodium channels are reset. In this way, depolarization travels as a wave, in terms of the membrane potential, across the axon. Such chemical signaling is mostly unidirectional whereas faster electrical signaling through tight junctions in closely spaced synapses is bidirectional.
Axon allows such signal propagation end in axon terminal which in turn synapses with other downstream neurons or muscle (in case of the neuromuscular junction). Some axons are covered by a specialized structure called the myelin sheath that acts as an insulator, preventing leakage of ions and allowing robust signal transmission. There are also periodic gaps in myelin sheath across the axon. This gap is called as nodes of Ranvier where the electrical impulses are recharged as it travels along the axon. Such an arrangement helps in long-range signal transmission.
Image credits: https://openstax.org/details/books/concepts-biology