Nerve cells or neurons are considered to be the basic building blocks of our nervous system. Their basic function is to transmit or receive information (message of sensation) from one neuron to the other neuron in the form of nerve impulses.
The impulse transmission occurs in two ways, one is the communication within one neuron and the other is the transmission among neurons.
COMMUNICATION WITHIN NEURONS
This refers to changes within the same neuron. It is divided into two aspects:
- RESTING POTENTIAL
The cell membrane encloses the axon, dendrites, and cell body of the cell. When micro-electrodes are attached inside or outside the membrane, there is a difference in voltage/ electrical charge called polarized condition. This voltage difference is also called electrical potential. This is because it is a potential source of energy. But as the neuron is not producing a signal currently, it is called resting potential. The size of the resting potential is -70mv towards the inside i.e., the inside of the cell is 70mv more negative than the outside. The neurons work persistently to bring themselves back to the resting potential by firing ions inside and outside the cell.
The difference in concentration of ions inside and outside the cell is because of two specific properties of the cell membrane:
- The cell membrane is selectively permeable (some ions can pass through it freely while others cannot)
- The cell membrane has a sodium(Na)-potassium(k) pump (a pump that draws Na ions outside the neuron and K ions inside the neurons).
This nature of the cell membrane gives rise to two gradients:
- CONCENTRATION GRADIENT: For the Na ion concentration gradient, ions move from high concentration to low concentration. The barrier of the cell membrane is required. Therefore, the result is that Na+ ions are in high concentration pushing to get inside but are being restrained by the cell membrane. For K ions, concentration gradient Na ions are pushed out of the cell. The concentration of each ion operates independently of each other.
- ELECTRICAL GRADIENT: This operates on the principle that opposite electrical charges attract each other while similar electrical charges repel each other. Now, since the inside of the cell is negatively charged, Na+ ions are attracted inside but the membrane retards them which results in an electrical gradient of force that pushes Na ions inside the cell.
For K ions, the two gradients are pushing in opposite directions. The concentration gradient pushes potassium outside the cell while the electrical gradient pushes it in. These two opposite forces are almost exactly equal for potassium ions. For Na ions, both gradients are pushing it inwards apart from the membrane which is resisting its movement. So, if the neural membrane somehow becomes permeable for a brief time, Na ions would rush inside.
- ACTION POTENTIAL
The electrical charge running down from the cell body to the axon is called the action potential. For a very brief time (millionth of a second) a pulse-like charge called the nerve impulse happens in electrical potential across the membrane.
Two charges happen during this:
- Cell membrane becomes permeable to Na ions by opening its gates for Na ions for a very brief period.
- Both concentration and electrical gradients push sodium ions inside, changing the charge inside from – 70mv to around +40mv.
After this brief period, the membrane again becomes impermeable to Na ions but because the inside is still positively charged, the positive K ions are pushed outside and this outflow of K+ ions helps to bring the neurons back to their resting potential. Since after the action potential, too many Na ions are inside and too many K ions are outside the membrane, the Na-k pump helps to restore the cell to its normal concentration. The action potential is triggered when the voltage across the cell membrane changes from -70mv to -60mv but a brief change in charge cannot trigger the action potential. So, the action potential will only be triggered if the change across the membrane reaches the threshold excitation. Therefore, action potential works on the all or none principle which says that if the excitation reaches the threshold, an action potential occurs otherwise it doesn’t.
PROPAGATION OF ACTION POTENTIAL
Action potential propagates through the length of the axon and differs from axons that are covered with myelin sheath and the ones which are not covered by it:
- UNMYELINATED AXONS:
When the action potential occurs, electrical potential becomes positive inside the membrane and this, in turn, makes the nearby point along the membrane also positive resulting in the threshold of excitation that further generates a new action potential. This process is repeated through the length of the axon since action potential depends on the all or none principle.
So the action potential produced throughout the length is equal in size. Therefore, the signal doesn’t weaken from start to end but this process is slow – 2m/sec. So, an alternative is that electrical potential passively spreads down the axon which is extremely fast but since it does not operate on all or none principle, the signal weakens upon the length of the axon.
- MYELINATED AXONS:
The solution to the above is a myelinated axon in which an action potential cannot occur where there is myelin. So, an action potential occurs at the nodes of the ranvier.
Then, the electrical potential passes passively down the axon and is large enough to reach the threshold of excitation and trigger a new action potential. Therefore, in a myelinated axon, action potential jumps from node to node, and also, the speed of impulse generation is very fast in a myelinated axon – 100 m/sec.
When action potential has occurred along the axon, the membrane remains positively charged for a milli-second but the membrane cannot produce another action potential for a period of another milli-second which is called the refractory period. This keeps the neuron from firing action potential continuously after they have started.
COMMUNICATION BETWEEN NEURONS
This involves the following steps:
- SYNAPTIC TRANSMISSION: the transmitting or presynaptic neuron manufactures or synthesizes the neurotransmitter molecules from simpler molecules (derived from the food we eat or other sources)
- STORAGE: manufacture of the neurotransmitter is stored in synaptic vesicles (button like) in the transmitting neuron.
- DISCHARGE / RELEASE: nerve impulse reaching the synaptic vesicles initiates a process that causes some of the vesicles to move towards the synaptic cleft where they discharge their stored neurotransmitter.
- DIFFUSION: the neurotransmitter rapidly diffuses across the narrow synaptic cleft and combines with specialized receptor molecules on the membrane of the postsynaptic or receiving neuron.
- COMBINATION: the combination of neurotransmitter and receptor cell initiates changes in receiving neuron that tends to excitation or inhibition since the function of the neurotransmitter is to either excite or inhibit behaviour.
- DEACTIVATION: the combined neurotransmitter is rapidly deactivated because of excess neurotransmitters in the synaptic cleft to make the post-synaptic cleft ready to receive another neural message. This takes place either by:
- Catalyst: neurotransmitters are broken down by various enzymes.
- Reuptake: neurotransmitters go back to transmitting neurons and are stored in synaptic vesicles.
-Aarushi Saluja
