Nerve Impulse

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Nerve Impulse

Introduction

Nerve impulse was discovered by British Scientist Lord Adrian in the 1930s. Owning to the importance of this discovery, he was awarded Noble Prize in 1932. Nerve Impulse is a major mode of signal transmission for the Nervous system. Neurons sense the changes in the environment and as a result, generate nerve impulses to prepare the body against those changes.

Definition

Nerve Impulse is defined as a wave of electrical chemical changes across the neuron that helps in the generation of the action potential in response to the stimulus. This transmission of a nerve impulse across the neuron membrane as a result of a change in membrane potential is known as Nerve impulse conduction.

It is a change in the resting state of the neuron. Due to nerve impulse, the resting potential is changed to an action potential to conduct signals to the target in response to a stimulus. The stimulus can be a chemical, electrical, or mechanical signal. 

The action potential is a result of the movement of ions in and out of the cell. Particularly the ions included in this process are sodium and potassium ions. These ions are propagated inside and outside the cell through specific sodium and potassium pumps present in the neuron membrane. The transmission of a nerve impulse from one neuron to another neuron is achieved by a synaptic connection (synapse) between them. It is thus a mode of communication between different cells.

The speed of nerve impulse propagation varies in different types of cells. The rate of transmission and generation of nerve impulses depends upon the type of cell. Besides, Myelin Sheath also helps in accelerating the rate of signal conduction (about 20 times). Generally, the speed of nerve impulse is 0.1-100 m/s.

Mechanism of Nerve Impulse Conduction

Nerve impulse conduction is a major process occurring in the body responsible for organized functions of the body. So, for conduction of nerve impulse there are two mechanisms:

  1. Continuous conduction
  2. Saltatory conduction

Continuous conduction 

Continuous nerve impulse conduction occurs in non-myelinated axons. The action potential travels along the entire length of the axon. Hence, more time is taken in generating and then transmitting nerve impulses during an action potential.

Continuous conduction requires more energy to transmit impulses and is a slower process (approximately 0.1 m/s). It delays the process of conducting signals because it uses a higher number of ion channels to alter the resting state of the neuron.

Saltatory Conduction 

Saltatory is faster than continuous conduction and occurs in myelinated neurons. In myelinated neurons, myelinated sheaths are present. Between these myelinated sheaths, unmyelinated gaps are presently known as the nodes of Ranvier. Nerve impulse propagates by jumping from one node of Ranvier to the next. This makes the process of nerve impulse faster as the nerve impulse does not travel the entire length of the axon ( this happens in case of continuous conduction). The nerve impulse travels at a speed of 100 m/s in saltatory conduction.

The number of channels utilized in saltatory conduction is less than continuous conduction due to which delay of nerve impulse does not occur. This mode of nerve impulse transmission utilizes less energy as well.

If you consider the axon as an electrical wire or loop, nerve impulse that travels along the axon as current, and the charged particles ( sodium and potassium ions) as the electron particles then the process can be understood quite easily. As the flow of current in a wire occurs at a specific voltage only, similarly the conduction of nerve impulse occurs when a stimulus has a maximum threshold value of -55 millivolts. This is essential for altering the resting membrane state to action membrane potential.

When the voltage has the required number of electron particles it conducts current. Similarly, in the case of nerve impulse conduction, the neurons the stimulus must have a threshold value for causing the movement of ions across the length of axon (for conducting nerve impulse) by opening the voltage-gated ion channels.

Process of transmission of Nerve Impulse

For the transmission of a nerve impulse, the stages are below:

  1. Polarization
  2. Depolarization
  3. Repolarization
  4. Refractory Period
  5. Synapse

Before going into the details of the process of nerve impulse transmission, let’s first discuss action and resting potential states.

Resting Membrane Potential

The resting membrane potential refers to the non-excited state of the nerve cell at rest when no nerve impulse is being conducted. The resting membrane potential of the nerve cell is -70 mV. It is a static state and both the sodium and potassium channels are closed during this state maintaining a high concentration of sodium ions outside and high potassium ions concentration inside the cell.

Action Potential 

An action potential occurs when the nerve cell is in an excited state while conducting nerve impulses. In this situation, sodium channels open and potassium channels are closed. This results in a huge influx of sodium ions inside the cells which trigger the nerve impulse conduction. The action potential is +40 mV.

Polarization 

Polarization is the situation in which the membrane is electrically charged but non-conductive. It means it doesn’t conduct nerve impulses in this state. During polarization, the membrane is in a resting potential state. The concentration of sodium ions is about 16 times more outside the axon than inside. In contrast, the concentration of potassium ions is 25 times more inside the axon than outside.

The polarization state is also known as the “Unstimulated or non-conductive state”. Due to the difference in the concentration of ions inside and outside the membrane, a potential gradient is established ranging between -20-200mV ( in the case of humans, the potential gradient in the polarized state is nearly -70mV). In the polarized state, the axon membrane is more permeable to potassium ions instead of sodium ions and as a result, it causes rapid diffusion of potassium ions.

In the resting state, the membrane potential becomes electro-negatively charged due to the movement of positively charged potassium ions outside the cell and the presence of electro-negative proteins in the intracellular space.

Depolarization

It refers to a graded potential state because a threshold stimulus of about -55mV causes a change in the membrane potential. The threshold stimulus must be strong enough to change the resting membrane potential into action membrane potential.

This results in the alternation in the electro-negativity of the membrane because the stimulus causes the influx of sodium ions (electropositive ions) by 10 times more than in the resting state. For this, sodium voltage-gated channels open. The action potential state is based on the “All or none” method and has two possibilities:

If the stimulus is not more than the threshold value, then there will be no action potential state across the length of the axon.

If the stimulus is more than the threshold value, then it will generate a nerve impulse that will travel across the entire length of the axon.

Repolarization 

It is a condition during which the electrical balance is restored inside and outside the axon membrane. Due to the high concentration of sodium ions inside the axoplasm, the potassium channels will open. During the repolarization state, efflux of potassium ions through the potassium channel occurs. As a result of the opening of potassium voltage-gated channels, sodium voltage-gated channels will be closed. Thus, no sodium ions will move inside the membrane. Therefore, repolarization helps in maintaining or restoring the original membrane potential state.

Until potassium channels close, the number of potassium ions that have moved across the membrane is enough to restore the initial polarized potential state. As a result of this, the membrane becomes hyperpolarized and have a potential difference of -90 mV.

Refractory Period 

The refractory phase is a brief period after the successful transmission of a nerve impulse. During this period, the membrane prepares itself for the conduction of the second stimulus after restoring the original resting state. It persists for only 2 milliseconds.

During this, the sodium ATPase pump allows the re-establishment of the original distribution of sodium and potassium ions. The sodium and potassium ATPase pump, driven by using ATP, helps to restore the resting membrane state for the conduction of a second nerve impulse in response to the other stimulus. It causes the movement of ions both against the concentration gradient. For every two potassium ions that move inside the cell, three sodium ions are transported outside. This process requires ATP because the movement of ions is against the concentration gradient of both ions.

Synapses

The process of transmission of a nerve impulse from one neuron to the other, after reaching the axon’s synaptic terminal, is known as synapses. This transmission of the nerve impulse by synapses involves the interaction between the axon ending of one neuron (Presynaptic neuron) to the dendrite of another neuron (Postsynaptic neuron). There is space between the pre-synaptic neuron and post-synaptic neuron which is known as synaptic cleft or synaptic gap.

After transmitting from one neuron to another, the nerve impulse generates a particular response after reaching the target site. If somehow the synaptic gap doesn’t allow the passage of nerve impulse, the transmission of nerve impulse will not occur and consequently required response too.

Types of synapses 

There are two types of synapses:

  1. Electrical synapses
  2. Chemical synapses

Electrical synapses 

In electrical synapses, two neurons are connected through channel proteins for transmitting a nerve impulse. The nerve impulse travels across the membrane of the axon in the form of an electrical signal. The signal is transmitted in the form of ions and therefore it is much faster than chemical synapses.

In electrical synapses, the synaptic gap is about 0.2nm which also favors faster nerve impulse conduction.

Chemical synapses 

In chemical synapses, the conduction of nerve impulse occurs through chemical signals. These chemical signals are neurotransmitters. In this type of nerve impulse conduction, the synaptic gap is more than electrical synapses and is about 10-20 nm. Due to this, the transmission of nerve impulses is slower than electrical synapses.

CNS and Nerve Impulse

Neurons help in transmitting signals in the form of a nerve impulse from the Central nervous system to the peripheral body parts. Neurons are a complex network of fibers that transmit information from the axon ending of one neuron to the dendrite of another neuron. The signal finally reaches the target cell where it shows a response.

In conducting nerve impulse, the following play a major role:

  • Axon- Helps in the propagation of nerve impulses to the target cell.
  • Dendrites- Receive the signals from the axon ends.
  • Axon Ending- Acts as a transmitter of signals.

Axon plays a major role in the process by transmitting signals in the form of nerve impulses via synapses to the target cells. The neuron is responsible for transferring signals to three target cells:

  • Muscle
  • Gland
  • Another neuron

And this results in the contraction of muscle, secretion by glands and helps neurons to transmit action potential.

Factors Affecting the Speed of Nerve Impulse 

Following are some major factors that affect the speed of nerve impulse:

  1. Myelin Sheath
  2. Axon Diameter
  3. Temperature

Myelin Sheath

Myelin sheath is present around the neuron and functions as an electrical insulator. Due to this sheath, an action potential is not formed on the surface of the neuron. This Myelin sheath has regular gaps, where it is not present, called nodes of Ranvier. An action potential can form at these gaps and impulse will jump from node to node by saltatory conduction. This can be a factor for increasing the speed of nerve impulse from about 30-1 m/ to 90-1 m/s.  

Axon Diameter

As the axon diameter increase, the speed of nerve impulses increases as well. This is because a larger axon diminishes the ion-leakage out of the axon. This helps in maintaining the membrane potential and thus favors faster nerve impulses.

Temperature

Temperature cause changes in the rate of diffusion of ions across the neuron membrane. Temperature directly correlates with the transmission of nerve impulses. If the temperature is higher, the rate of diffusion of sodium and potassium ions will be high and axon will become depolarized quickly which will cause a faster nerve impulse conduction.  

A nerve impulse is thus an important signal transduction mode for triggering a response in major body parts due to a strong stimulus. Any distraction in this process can have drastic effects on the body. 

References

  1.  Lodish, H; Berk, A; Kaiser, C; Krieger, M; Bretscher, A; Ploegh, H; Amon, A (2000). Molecular Cell Biology (7th ed.). New York, NY: W. H. Freeman and Company. p. 695.
  2. Marieb, E. N., & Hoehn, K. (2014). Human anatomy & physiology. San Francisco, CA: Pearson Education Inc.