No one will argue that the greatest achievement of nature is the human brain. Nerve impulses running along nerve fibers are the quintessence of our essence. The work of the heart, stomach, muscles and the spiritual world - all this is in the hands of a nerve impulse. What is a nerve impulse, how does it arise and where does it disappear, we will consider in this article.
Neuron as a structural unit of the system
The evolution of the nervous system of vertebrates and humans followed the path of the emergence of a complex information network, the processes in which are based on chemical reactions. The most important component of this system is specialized cell neurons. They consist of a body with a core and important organelles. The processes of two types depart from the neuron - several short and branched dendrites and one long axon. Dendrites are the receivers of signals from sensory receptors or other neurons, and the axon transmits signals in the nervous network. To understand the transmission of nerve impulses, it is important to know about the myelin sheath around the axon. These are specific cells, they form the shell of the axon, but not continuous, but intermittently (Ranvier constrictions).
Transmembrane gradient
All living cells, including neurons, have an electrical polarity, which occurs as a result of the potassium-sodium membrane pumps. Its inner surface has a negative charge with respect to the outer. An electrochemical gradient of zero arises and a dynamic equilibrium is established. The resting potential (potential difference inside and outside the membrane) is 70 mV.
How does a nerve impulse arise
When an irritant acts on a nerve fiber, the membrane potential in this place is sharply violated. At the beginning of the onset of excitation, the membrane permeability to potassium ions increases, and they tend to enter the cell. In 0.001 seconds, the inner surface of the neural membrane is positively charged. This is what a nerve impulse is - a short-term recharging of a neuron or an action potential equal to 50-170 mV. A so-called wave of action potential arises, which propagates along the axon, like a stream of potassium ions. The wave depolarizes the axon, and the action potential moves with it.
Axon transmission is another neuron
After reaching the end of the axon, it becomes necessary to transmit a nerve impulse to one or more axons. And here we need a different mechanism, different from the wave of action potential. The end of the axon is the synapse, the place of contact with the synaptic cleft and the presynaptic sacs of the axon. The action potential in this case activates the release of neurotransmitters from presynaptic sacs into the synaptic cleft. Neurotransmitters interact with the membrane of the underlying neurons, causing an imbalance in them. And the story of the sodium-potassium pump is repeated in another neuron. Having fulfilled their function, neurotransmitters either diffuse or are captured back into presynaptic sacs. In this situation, the question of what a nerve impulse is, the answer is: the transmission of excitation through chemical agents (neurotransmitters).
Myelin and pulse rate
In the constrictions of the myelin sheaths, which wrap around the axon like a sleeve, the ion current easily flows into the medium and back. In this case, the membrane is irritated and an action potential is formed. Thus, the nerve impulse moves along the axon in jumps, causing the formation of the action potential only in Ranvier intercepts. It is this jump-like course of the action potential that increases the speed of the nerve impulse many times. For example, in thick myelinated fibers, the pulse speed reaches 70-120 m / s, while in thin nerve fibers without a myelin sheath, the pulse speed is less than 2 m / s.
Electroplating and nerve impulse
In a semi-liquid colloidal protoplasm, the current is galvanic - it is transferred by atoms having an electric charge (ions). But the galvanic current cannot spread over rather large distances, but a nerve impulse can. Why? The answer is simple. When a wave of action potential travels along the axon, it forms a galvanic cell inside the neuron. In the nerve, as in any galvanic cell, there is a positive pole (outer side of the membrane) and a negative pole (inner side of the membrane). Any external action disrupts the equilibrium of these poles, the permeability of a particular section of the membrane changes, a change in permeability in an adjacent section is initiated. That's it, the impulse went further along the length of the axon. And the initial section, from which the excitation began, has already restored its integrity, found its zero gradient and is ready to launch the action potential in the neuron again.
Neuron is not just a conductor
Neurons are living cells, and their protoplasm is even more complex than in cells of other tissues. In addition to the physical processes associated with the initiation and conduction of a nerve impulse, complex processes of metabolism occur in the neuron. It was experimentally established that when a nerve impulse passes through a neuron, the temperature in it rises (albeit by a millionths of a degree). And this means only one thing - all the exchange processes in it are accelerated and proceed more intensively.
Nerve impulses of the same type
The main property of a neuron is the ability to generate a nerve impulse and quickly conduct it. Information about the quality and strength of stimulation is encoded in changes in the frequency of passage of nerve impulses to and from neurons. This frequency varies from 1 to 200 per second. This frequency code assumes different periods of the pulses, combining them into groups with different numbers and patterns of movement. This is what the encephalogram records - a complex spatial and temporal sum of nerve impulses of the brain, its rhythmic electrical activity.
Neuron picks
What makes the neuron “start”, initiate the emergence of the action potential - and today the question is open. For example, brain neurons receive mediators sent by thousands of their neighbors and send thousands of impulses to nerve fibers. In the neuron, the process of processing impulses and the decision are made whether to initiate the action potential or not. The nerve impulse will fade away or be sent on. What makes a neuron make this choice and how does it make a decision? We know almost nothing about this fundamental choice, although it is he who controls the activity of our brain.
So we answered the question of what a nerve impulse is. You will be very surprised, but all the nervous tissue in the human body weighs a little more than a kilogram. But at the same time, these are billions of neurons that are closely connected into a single system. Humanity has learned so much about the work of neurons and the entire system, and at the same time we know almost nothing. We have learned the alphabet, but so far we have difficulty even writing simple words. Let's hope that over time, science will be able to identify those patterns that decipher the dialogue of our brain centers, which makes the biological object Homo Sapiens human.