Nerve impulse conduction -mechanism and characteristics

Nervous system consists of  interconnecting fibers of communication network. In the ‘hard-wiring’ of the nerves, the signals travel in the form of a flow of electrical current called nerve impulses. Irritability is the universal property of life which means the capacity of organisms to respond to changes in the environmental conditions. The specific environmental stimulus elicits a change in an organism is termed a response. The stimulus-response reactions afford internal constancy in the face of environmental changes.

Discovery of neuron

Camillo Golgi (1843-1926) invented a specific    staining technique for neurons. Cajal in 1888 identified the networks of nerve cells. Golgi and Cajal received the Nobel Prize in 1906 for Medicine and Physiology. Wilhelm His in 1886 showed that the dendrites and axons grow out progressively from the immature neurons in the brains of embryos. Henri Forel (1848-1931) in 1886,observed that when the  cell body dies or an axon is cut, degeneration  of the neuron stop at the junction to another neuron, thus giving evidence that they are separate.

Description of a neuron

A neuron consists of a cell body and two kinds of processes, the dendrites and the axon. The cell body has neuroplasm, a nucleus, nissl bodies, neurofibrils and a cell membrane. The dendrites carry impulses towards the cell body. The axon carries impulses away from the cell body. The axon originates from axon hillock of the cell body. The axon is surrounded by two coverings: myelin sheath and Schwann sheath. These two coverings are interrupted at intervals by nodes of Ranvier. The fine branches at the end of axon are called axon terminals.

Properties of neuron

1.    Excitability –a nerve can be stimulated by suitable stimulus-mechanical, thermal, chemical, electrical.

2.    Conductivity – impulse is conducted similar to cable conduction and digital in character.

3.    All or none law – the stimulus should be in adequate threshold strength.

4.    Refractory period – when the nerve fiber is once excited, it will not respond to a second stimulus for a brief period.

5.    Indefatigability – nerve is normally not fatigued.

6.    Adaptation –the nerve quickly adapts itself.

7.    Accommodation – slowly applied stimulus is accommodated.

Definition of nerve impulse

•         A nerve impulse is the sum total of physical and chemical events associated with the transmission of a signal along an axon.

•         A wave of physiological activity- primarily an electrical phenomenon.

Stimulus is defined as a sudden change in the environment which is strong enough to cause a response in the living organism. All or none law indicates the relation between the stimulus and response. A stimulus, if it is capable of causing a response, causes a maximum response. If it is below the capacity, it will not cause any response. The lowest strength of stimulus required to give rise to an action potential is the threshold stimulus. A stimulus which is less than the threshold fails to induce any response-sub-threshold stimulus. A stimulus which is greater than optimum is supra threshold stimulus.

Stimulation can be affected by strength and duration. The weaker the stimulus, the longer it will have to be applied to produce a response. The nerve takes lesser time to respond for a stronger stimulus.

Velocity of impulse conduction

Johannes Muller believed that nerve impulse travelled at a speed of light-186,000 miles per sec. Helmholtz showed that velocity of conduction was 100m per sec. (about 10 times faster than a man can run). The velocity varies from 100 m per sec in large fibers to 0.5 m per sec in small non-myelinated fibers.

Refractory period of impulse conduction

Once an impulse has passed over any part of the neuron, for a short time it is unable to conduct any other stimulus. This brief period of non-conductivity is called refractory period. Under good physiological conditions, the nerve fiber is indefatigable.

                   Mechanism of impulse conduction

Bioelectricity

L. Galvani in 1786 discovered the presence of electrical current in nerves and muscles.

Electrophysiology

Du Bois Raymond in 1848 concluded that impulse transmission was electrical by a wave of relative negativity.

Electrochemical basis of bioelectricity

Hodgkin and Huxley in 1939 demonstrated the electrical and chemical processes involved in bioelectricity.

Demonstration of electrogenesis – microelectrode studies

In order to confirm the generation of electricity, one microelectrode is placed on the outer surface of the nerve membrane and the other placed inside the nerve cell. When the terminals are connected to a galvanometer, the needle show a deflection indicating the flow of electrical current (-70mV). In a resting neuron the electrical potential ranges from 20 to 100 mV.

Resting potential

According to the membrane potential hypothesis of Bernstein (1902), the differential concentration of ions between the inside and outside of the nerve cell is the basis of resting potential. The inner side of the nerve contains large negatively charged non-diffusible protein ions and smaller diffusible K⁺ and Cl^- ions. Na⁺ ions are more concentrated on outside of the nerve cell. Radio-isotopic studies showed that potassium and sodium ions readily diffuse through the nerve cell membrane.

In a state of physiological rest, the inner side of a neuron is electrically negative to outside. This difference in electrical charge is called resting potential. The resting potential is maintained as long as the cell is alive and active. The resting potential depends upon the selectivity and variable permeability of cell membranes. The unequal distribution of ions maintains the resting potential.

Factors influencing ionic imbalance in a nerve cell

·       Active transport

·       Concentration gradient

·       Membrane permeability

·       Electrostatic attraction

Ion-exchange pump - This Na⁺ - K⁺ pump maintains unequal concentration of ions in the nerve fibers by the process of active transport. This pump changes the electrical character of the nerve fibers. ATP is used as energy for the process.

Concentration gradient - The active transport establishes the concentration gradient. The extrusion of sodium ions is linked with active uptake of potassium ions. The rates of diffusion of ions depend upon gradients and membrane permeability.

Donnan membrane equilibrium - “at equilibrium, the product of the concentration of the diffusible ions inside the membrane equals the product of the concentration of the diffusible ions outside.” (Donnan 1928).

Selective membrane permeability - All the plasma membranes are selectively permeable to ions. This membrane selectively allows inward diffusion of K⁺ and prevents inward diffusion of Na⁺ ions.

Electrical gradient - The inner side of the nerve cell is negative to the outside. There is a growing attraction between ions of the same charges. The parallel diffusion of ions restores the resting potential.

Action potential (Hodgkin 1949, Huxley 1952 and Katz 1959)

When a nerve is stimulated, Na+ ions suddenly move into the cell and causing a positive potential. The influx of Na + ions reaches its peak in 100msec.Sodium permeability causes depolarization. The inward diffusion of Na + ions halted near the peak of action potential. The diffusion K+ ions restore the membrane potential –called repolarization. By using radioactive sodium (Na ²⁴) and potassium (K⁴²) Hodgkin and Keynes (1955) showed that inward flux of sodium was increased 20 times and outward flux of potassium 3-4 times after stimulation of nerve.

             Properties of Impulse conduction

The conduction of nerve impulse causes two phases in the moving wave called biphasic action potential. The conduction of nerve impulse in a myelinated fiber is so rapid because the action potential skips from node to node and this phenomenon is called saltatory conduction. The magnitude of nerve impulse transmission occurs without decrement is termed Non-decremental conduction. The velocity of conduction in a myelinated nerve is directly proportional to its diameter. The property of myelinization speeded up the conduction velocity and consumption of energy.