Nerve Impulse

Introduction image

Nerve impulse can define as the generation of action membrane potential beyond the cell membrane in response to the stimulus. The propagation of nerve impulse, as a result of a change in membrane potential beyond the cell membrane commonly, refers to as “Nerve impulse conduction”. When a nerve impulse or action potential reaches the axon terminal, there will be synaptic transmission via an electrical or chemical synapse.

Speed of impulse transmission varies considerably among different kinds of cell. The size of nerve impulse remains the same, but its rate of generation and transmission differs accordingly to the cell type. The layer of a fatty acid substance called Myelin sheath accelerates the rate of signal conduction (up to 20 times faster).

Content: Nerve Impulse

  1. Definition
  2. Mechanisms
  3. Transmission
  4. Functions

Definition of Nerve Impulse

Nerve impulse can define as a signal driven by either electrical, chemical or mechanical stimulus onward the segment of an axon filament. It generates a change in potential gradient of voltage-gated channels across the membrane, resulted from ionic movement in and out of the axolemma. A change in potential difference or the change in phase of resting potential to the action potential leads to the conduction of the signal from one neuron to other.

The transmission of nerve impulse generally speeds at 0.1-100 m/s. Temperature, a diameter of axon, presence or absence of myelin insulating layer influences the rate of impulse transmission. All three factors accelerate the pace of signal transmission.

Mechanisms of Nerve Impulse Conduction

There are two modes of nerve impulse conduction, namely continuous and saltatory conduction.

mechanism of nerve impulse conduction

Continuous conduction

It also refers as non-myelinated conduction, where the flow of nerve-impulse is slower (0.1 m/s). It occurs in unmyelinated axons, where the ions flow throughout the segment of axon via voltage-gated channels.

Saltatory conduction

It also refers as myelinated conduction, where the action potential plunges much faster (100 m/s) from one node to another. It occurs in myelinated axons, where the flow of ions is discontinuous because of the uneven distribution of voltage-gated channels.

The propagation of the signal is faster in saltatory conduction as the nerve impulse plunges from one node to the next, and reaches the target cell more quickly than the continuous conduction. Let us consider, axon as an electrical wire or loop, nerve impulse as a current, and ions (Na+ and K+) as the electron particles.

Like a flow of current requires a specific voltage, the generation of nerve impulse also involves a change of resting membrane potential to the state of action membrane potential, for which the stimulus must reach the threshold value maximum of -55 millivolts.

As the voltage reaches the unit required for the flow of electrons, it conducts the flow of current. Similarly in neurons, a physical, electrical or mechanical stimulus must have the threshold value able to cause the movement of ions beyond the axolemma, by opening the voltage-gated channels.

As the flow of electrons permits the passage of current along the electrical wire, a sudden change in the membrane potential also initiates the conduction of nerve impulse although the axon’s length.

As the flow of current is faster in a broader electrical loop, the conduction of nerve impulse also depends upon the axon’s diameter, like the flow of action potential will be faster on a broader axon, than in narrower.

Transmission of Nerve Impulse

A neuron conducts the transfer of an action potential or nerve impulse through the following stages mentioned below:
Generation of Action Potential

Polarization

It is a state of resting potential, which is electrically charged but non-conductive. The concentration of sodium ions in the extracellular fluid is about 16 times higher than the axon’s cytoplasm or axoplasm. In contrast, the level of potassium ions inside the axoplasm is 25 times higher than the extracellular fluid having sodium ions.

Polarization of membrane also refers to as “Unstimulated state”. The difference in the concentration of ions inside and outside a cell creates a potential difference ranging between -20-200 mV (In human, the potential difference is nearly -70 mV). During this state, the axolemma is more permeable to the potassium ions rather than sodium ions and can cause rapid diffusion of potassium ions than that of sodium.

In the resting-potential state, the membrane potential is electro negatively charged, due to overshooting of positively charged potassium ions outside the cell and presence of more electronegative proteins inside the cell.

Depolarization

It is the graded potential state, where the threshold stimulus having a potential of -55 mV brings a change in the membrane potential. A threshold stimulus must have the potential to convert the resting membrane potential into action membrane potential.

The electronegativity of the membrane-potential changes, when a stimulus increases the influx of sodium electropositive ions more than ten times into the axoplasm by allowing the Na+voltage-gated channels to open. Here the movement of an action potential depends on the mechanism of “All or none” method which is having two possibilities like:

  • If the stimulus does not exceed the threshold value, there will be no movement of an action potential downhill an axon.
  • If the stimulus exceeds the threshold level, then it will initiate the conduction of impulse or an action potential downwards the axon length to reach the axon terminal.

Repolarization

It is the stage of restoring the electrical balance inside and outside the cell membrane. The high concentration of sodium ions inside the axoplasm will trigger the K+voltage-gated channels to open. During repolarization, the potassium ions efflux through the K+ channel beyond the cell membrane. After the opening of potassium voltage-gated channels, the sodium voltage-gated channels will shut down, and cause no influx of sodium ions. Therefore, this step maintains or restores the original membrane potential.

By the time potassium voltage-gated channel closes, more of the potassium ions have moved across the membrane to establish initial polarized potential. As a result of this, a cell membrane becomes hyperpolarized, having a potential difference of -90 mV.

Refractory Period

It is the final stage, where the membrane potential re-establishes the original distribution of sodium and potassium ions through the sodium-potassium ATPase pump. The Na+– K+– ATPase pump facilitates the conversion of cell membrane again to its resting-potential state, where it can respond to the new stimulus. It influx two K+ and efflux three Na+ ions into and outside the cell against the concentration gradient by the ATPase activity. It lasts for two milliseconds.

Synapses

When a nerve impulse reaches the axon’s synaptic terminal, it gets transmitted from one neuron to the next through a phenomenon known as “Synapsis”. The transmission of the signal involves joining of axon terminal of one neuron (Presynaptic neuron) to the dendrite of another neuron (Postsynaptic neuron). There is a space of 0.2 µ in the middle of pre and post-synaptic neuron that refers as “Synaptic cleft”.
Transmission of Nerve Impulse

Functions

The specialized neural network helps in transmitting the signal from CNS to the peripheral body parts and vice versa. Neurons appear as a dense network of long fibres, where the information is passed from the axon terminal of one neuron to the dendrites of another neuron and finally to the target cell.

Therefore, it is quite clear that dendrites function as a receiver; cell body acts as a conductor, axon functions as a propagator and axon terminal functions as a transmitter. The dendritic spines act as a receptor that inflows specific signals into the cell body and can increase their surface area.  A cell body carries the message and conducts it to the filamentous axon through axon hillock.

An axon plays a most critical role by conducting the signal to the target cell via synapses. The conduction of the message can be continuous or saltatory. A neuron terminates on one of the three target cells like muscle, gland and another neuron, and cause a muscle to contract, glands to secrete and neuron to transmit the action potential respectively.

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