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7.3. Nerve impulse

Nerve impulse

The neurons are the cells responsible for transmitting nerve impulses. When a neuron receives a stimulus, electrical changes occur in its membrane that are transmitted from the dendrites to the axon, running through the entire neuron.

This electrical impulse passes from one neuron to another through synapses, connections formed between the end of the axon of one neuron and the dendrite of the adjacent neuron .

In synapses there is no physical contact between neurons, but there is a synaptic cleft that separates them. This is where the axon releases neurotransmitters that will receive the receptors for the dendrites of the postsynaptic neuron.

Nerve impulse propagation

Vesicles, the terminal buttons of the axon , release neurotransmitters into the synaptic cleft, which bind to specific receptors on the dendrite of the postsynaptic neuron. The neuron is excited and spreads the nerve impulse that will be transmitted to the next neuron.

The neurons transmit nerve impulses in the form of electric current. When the stimulus reaches the dendrites of a neuron, electrical changes take place that pass to the neuronal body and continue until ending in the axon. The nerve impulse only propagates in one direction, from the dendrite to the axon.

Transmisión del impulso nervioso

By Laurentaylorj (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Some glial cells, Schwann cells, line the axons of some neurons with an insulating layer called the myelin sheath. This covering means that the nerve impulse is not transmitted at the same speed in the axons covered by myelin sheath as in the uncovered ones. The myelin sheath blocks the passage of the nerve impulse and causes it to “jump” from spaces without myelin sheath to the next space without myelin sheath. These areas of the axons that are not covered by the myelin sheath are called Ranvier's nodules, and this type of nerve impulse propagation is called conduction or saltatory propagation.

According to whether or not they have a myelin sheath, neurons are classified into:

  • Myelinated neurons. Their axons are covered with myelin, and they are thicker. By saltatory conduction, they transmit the nerve impulse more quickly.
  • Unmyelinated neurons. Their axons are not covered by myelin, so they conduct the nerve impulse more slowly.

Propagación saltatoria del impulso nervioso

By Dr. Jana [CC BY 4.0], via Wikimedia Commons

Answer in your notebook

7.5.- What is the relationship between myelin and the speed of nerve impulse transmission?

Who was Ramón y Cajal?

Santiago Ramón y Cajal was a Spanish doctor, born in Petilla de Aragón (Navarra), who studied medicine in Zaragoza, and who obtained the Nobel Prize in Medicine in 1906, shared with Camillo Golgi, for his work on the structure of the Nervous System. Until then it was thought that the cells of the Nervous System were fused to form a network, but his "Doctrine of the neuron" said that neurons are individual cells, with three parts (soma, dendrites and axon), and that they are the structural unit and functional nervous system.

In addition, he discovered that the transmission of the nerve impulse is directional, from the dendrites to the soma, and from the soma, to the axon.

Synapse

The neurons are not connected to one another forming continuous networks, but there is a small space between them, the synapse, which must traverse the nerve impulse to pass from one neuron to another.

The synapse is the area of ​​information transfer from one neuron to another. It is made up of three elements:

  • The anterior neuron (presynaptic component), whose axon releases neurotransmitters into the synaptic space.
  • Space or synaptic cleft.
  • Neuron posterior to the synapse (postsynaptic component), which contains receptors that capture neurotransmitters released from other neurons.

Two adjacent neurons are linked by synapse. When the nerve impulse reaches the end of the axon (presynaptic component), the vesicles containing the neurotransmitters release them into the synaptic cleft, the small space that remains between the two neurons, binding to the specific receptors of the dendrites (postsynaptic component) of the next neuron.

By No machine-readable author provided. DaDez~commonswiki assumed (based on copyright claims). [GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5-2.0-1.0], via Wikimedia Commons

Animation: Synapse interactive.

Animation: The dialogue of neurons.

Answer in your notebook

7.6.- What is the synapse?

Answer in your notebook

7.7.- Where does the nerve impulse enter a neuron? Where does it exit to the next neuron?