09 Control and Coordination - part 05 - Transmission of nerve impulse

 

09 Control and Coordination - part 05 - Transmission of nerve impulse

Transmission of nerve impulse :

• Neurons are cells with some specials features. 
• Cells can be excited. 
• Nerve impulse is a wave of bioelectrical or electrochemical disturbances passing along a neuron. 
• Transmission of the nerve impulse along the long nerve fibre/axon tube is a result of electrical charges across the neuronal membrane during conduction of an excitation.
• Each neuron has a charged cellular membrane with a voltage which is different on the outer and inner side of the membrane. 
• Plasma membrane separates the outer and inner solutions of different chemical compounds but having approximately the same total number of ions. 
• The external tissue fluid has both Na⁺ and K⁺ but there is predominance of Na⁺ and Cl^-.
• While K⁺ is predominant within the fibre or in the intracellular fluid. 
• This condition of resting nerve is also called a polarised state and it is established by maintaining an excess of Na⁺ on the outerside. 
  
• On the inside there is an excess of K⁺ along with large negatively charged protein molecules and nucleic acid.
• Some amount of Na⁺ and K⁺ is always leaks across the membrane. 
• The Na⁺/K⁺ pump in the membrane actively restores the ions to their appropriate side. 
• Against the concentration and electrochemical gradient, Na⁺ is being forced out and K⁺ is being forced inside the membrane. This process is called sodium pump or Na-K exchange pump. 
• This active process requires ATP energy. 
• The difference in distribuiton of Na+ and K⁺ on the two sides of the membrane produces a potential difference of -50 to -100 millivolts (average is -70 millivolts).
• This potential difference seen in a resting nerve is thus called resting potential. (-70 millivolts) 
• It is mainly due to differential permeability of the resting membrane which is much more permeable to K+ than to Na+.
• This results in slightly more K⁺ diffusing out than Na⁺ moving inside and causing slight difference in polarity. 
• Also ions like negatively charged proteins and nucleic acids inside the cell make the overall charge negative on the inside and positive charge on the outside.
• The nerve membrane not only has leakage channels but also has many gated channels for Na⁺/K⁺. These are also called voltage gated channels. 
• These channels enable the neuron  to change it membrane potential to active potential in response to a stimuli. 
• The Na⁺/K⁺ gated channels are separate so transport of both these ions is separately done. 
• During resting potential, both these gates are closed and the membrane resting potential is maintained.

Generation of nerve impulse :
1. Depolarization : 

• The origin and maintenance of resting potential depends on the original perfect state. 
• Any change or disturbance to the membrane will cause Na+ to enter into the membrane and lower the potential difference (lesser than -70 millivolts). 
• This makes the membrane more permeable to Na+, so there will be rapid influx of Na⁺. This property is peculiar to a nerve membrane.
• The voltage gated Na⁺/K⁺ channels are special in 2 ways : 

1. They can change the potential difference of the membrane as per the stimulus received and 
2. Also the gates operate separately and are self closing.

• During resting potential, both gates are closed and resting potential is maintained.
• However during depolarization the Na⁺ gates open but not the K⁺ gates. 
• This causes Na+ to rush into the axon and bring about a depolarisation (opposite of polarity). 
• Extra cellular fluid (ECF) becomes electronegative with respect to the inner membrane which becomes electropositive. 
• The value of action potential is +30 millivolts to +60 millivolts.
• This triggers depolarisation in the next part while it itself starts going to repolarisation.

2. Re-polarization : 

• Change in the polarity from depolarized, back to the original state is done by the process of repolarization. 
• It occurs after a short interval called refractory period.
• The large number of Na⁺ on the inside causes a drop in the permeability of membrane to Na⁺ 
• At the same time making it more permeable to K⁺ ions by opening the K⁺ voltage gates and slowly closing the Na⁺ gates. 
• This action is a localized activity. 
• K⁺ ions passes out very rapidly as compared to slow entry of Na+.
• In this period, Na+ gates are closed, K+ gates are open and Na⁺ - K⁺ pumps becomes operational. 
• This process of producing a wave of stimulation → causing depolarization → repolarization is repeated continuously upto the end of axon terminal. 
• It is a self propagating process.
• In medullated nerves, the insulating fatty myelin sheath prevents flow of ions between the axoplasm and ECF. 
• The transport pump and gated channels can operate only in the region of nodes of Ranvier, where myelin sheath is absent.
• The action potential cannot travel as a wave of membrane depolarization it has to jump from node to node. This process called saltatory conduction, is at the rate of 120 m/second. 
• It is faster than the continuous conduction in non-medulated fibre (50 :1).

Do you know ?

• Na-K pump operates actively and by use of carrier.
• For every 3 Na⁺ pumped out 2K+ are pumped in. 
• It is electrogenic.

Always Remember

• The resting potential of axon is -70mV. 
• Na⁺- K⁺ Pump pumps out 3 Na⁺ ions for every 2 K⁺ ions they pump into the cell.