Conduction of nerve impulse:
The important function of the neuron is to communicate “messages of stimulation” in the form of nerve impulses. Sensory (afferent) neurons come from receptors and go to the brain and motor (efferent) neurons go to muscles and glands. The inter-neurons are the linking neurons. All our behaviours involve the flow of nerve impulses.
There are about 10 billions or more neurons firing in our brain, i.e., sending and receiving various nerve impulses. This is the communicative action of a neuron. Neurons also send nerve impulses when we are asleep. The communicative mechanism is conduction of nerve impulse. When a neuron is adequately stimulated, an electrochemical reaction occurs inside. Like a gun, neurons fire or do not fire; there is no in between stage. The neurons follow an all-or- none law, i.e., they are either on or off.
How does the neuron serve its communicative function? Let us see how nerve impulses or nerve energies are formed. The cell membrane of a neuron is semi-permeable. The neuron contains fluid, which is known as intracellular fluid. The fluid on the outside of the neuron is called the extracellular fluid. The cell membrane is in between the intracellular fluid and extracellular fluid. The fluid contains many dissolved substances.
Many chemical substances are broken to pieces when they dissolve in water or any fluid. Ions are particles formed when a substance is dissolved in fluid. Ions are electrically charged particles when dissolved. The electrical charges carried by ions are of two types, negative and positive. Positive electrical charges repel each other, so also the negative electrical charges. On the other hand, positive and negative electrical charges attract each other. The same thing happens in a neuron. Ions are found in extracellular fluid and in intracellular fluid as well.
When a neuron is in a resting state, there is a negative electrical charge of about -70 mill volts (a mill volt is one-thousandth of a volt) within the neuron. This is called the resting potential of the neuron, which does not come automatically; the neuron works to maintain its resting potential. It pumps out the positively charged ions (electrically charged particles) to its outside and only keeps the negatively charged ions.
The neuron can be compared with a battery with the inside of the neuron representing the negative pole and outside of the neuron representing the positive pole (Koester, 1991). When a message arrives and the neuron is stimulated (by external stimuli such as light, heat, and sound etc. or by messages from other neurons), the positively charged ions outside the neuron rush inside the neuron at rates as high as 100 million ions per second. The sudden arrival of the positive ions inside the neuron causes the charge to change from negative to positive. When the charge reaches a critical level, an electrical nerve impulse known as action potential travels down the axon of the neuron.
The action potential moves from one end of the axon to the other. After the nerve impulse has traveled, the positive ions are pumped out of the axon, and the charge returns to negative. As a result, the neuron returns to its resting state, and becomes ready once more to fire again. The flow of the nerve impulse is 10 to 250 miles per hour depending upon the diameter of the particular neuron. A larger diameter carries nerve impulse speedily. A smaller- diameter axon carries the nerve impulse slowly.
Absolute Refractory Period:
Just after an action potential has passed, the neuron cannot be fired again immediately, no matter how much stimulation it receives. It is almost similar to the act of reloading the gun after each shot. As soon as the action potential is transmitted by the neuron, it takes rest for a brief period of time. This brief period just after carrying action potential during which the neuron is inactive is called the “absolute refractory period.” During the period, the neuron is in “resting potential”. This period of resting time is usually less than 1 /1000th of a second. An action potential cannot be produced during the absolute refractory period. When this short refractory period (i.e., no action) is over the neuron can carry a nerve impulse. The absolute refractory period is followed by a relative refractory period during which a strong stimulus can make the neuron active, i.e., carry a nerve impulse.
The point at which a stimulus triggers an action potential is called the threshold of a neuron. Stimuli especially too weak cannot produce an action potential in a neuron. A weak stimulus, which is too small to produce action potential, does not open the membrane-gate and does not produce nerve impulse. A stimulus of certain strength is needed to produce action potential. Different neurons have different thresholds of excitation. Some require greater stimulation than others to make them fire. Thus weak stimuli evoke few impulses in only a few neurons. Strong stimuli evoke high rates of impulses in many neurons. Generally, threshold point of each neuron is fairly constant.
The rule usually for determining the threshold of a neuron is that if a stimulus causes a nerve impulse 50 percent of the time, that stimulus is said to be at the threshold of that neuron. In other words, the threshold of a neuron is the point at which a stimulus causes a nerve impulse 50% of the time. During the absolute refractory period, the threshold of a neuron becomes very high, as a result of which the neuron does not carry action potential.
All or None Law:
In conducting action potential, the neuron obeys a law called “all-or-none law.” This law states that neurons fire or do not fire; there is no in between stage. It is similar to the action of a gun. If you pull the trigger, the gunfires. Pulling the trigger harder is not going to make the bullet travel faster. Your finger pressure on the trigger must be of certain intensity for the gun to fire. The lesser the pressure than what is required will not result in gunfire and more pressure would not result in a better fire. Similarly, the neurons follow the all-or-none law.
The stimulation must reach a threshold to generate an action potential to be conducted through the axon. Below the threshold, the stimulus cannot excite the nerve. Once the threshold is reached, the stimulus intensity does not have any effect on the amount of nerve impulse conducted. The nerve impulse is an electrochemical stimulation, which does not decrease in its intensity as it travels through the axon. If an axon carries any nerve impulse at all, the impulse continues to maintain the same strength throughout its travel in the axon until it reaches the terminal buttons.
What happens when the stimulus intensity increases beyond the threshold point? As the stimulus intensity increases, the sensory neurons increase the rate of their impulse generation to as many as 200 to 1000 impulses per second.
Furthermore, a stronger stimulus generates impulses in more neurons. On the other hand, weak stimuli evoke low rates of responses and a few impulses in only a few neurons. The axons of the neurons are not of the same size. The speed of a nerve impulse depends on the diameter of the axons. The larger the diameter, the greater is the speed. The strength of the nerve impulse depends on the nature of the axons. The dendrite and the cell body do not obey the all-or-none law. Only the function of the axon is governed by this law.
All our behaviors have a neural base. The neural activity is the biological medium in which all our psychological processes occur. Changes in neural and nervous system activities lead to changes in how people think, perceive, learn, memorize, and behave. All the mental functions stem from biological functions, and in turn also influence the biological activities. It is, therefore, important to understand how neural impulses travel from one part of the biological system to another.
It is not only that the neural impulse travels within a neuron, but also from one neuron to the other. The two major parts of the neural transmission are:
(i) Communication within a neuron (action potential), and
(ii) Communication between neurons (synaptic transmission).
While describing the functions of a neuron, we have already stated how neural impulses travel from one end of the neuron to the other end. The next thing to discuss is how neural impulses travel from one neuron to other neurons. Both these actions constitute the topic of neural transmission.
The synapse is a space between neurons that provides a junction for information transfer. Neurons are not connected with each other. There is a small but important gap between the axon terminals (terminal buttons) of each neuron and the dendrites of the adjacent neurons. This gap is called the synapse. The width of the synaptic gap is about 100 angstroms (one-angstrom unit is one ten-millionth of a millimeter). The synapse is found between the axon terminal of one neuron and the dendrites of another.
A single neuron in the brain may share very large number of synapses with other neurons. It is estimated that billions of neurons in the brain have trillions of synapses. The synapse serves very important and useful functions in making the organism’s behavior adaptive and flexible. The synaptic gap controls the rate of flow of nerve impulses. If the impulse is of high intensity, the gap restricts the flow, and does the opposite when the stimulus is weak. Without the synaptic gap, the man would have been rigid like a machine.
The neural impulse travels through the length of the neuron along the axon, finally arriving at the terminal buttons or axon terminal. There is no direct physical connection to the next neuron. The impulse has to cross the gap called the synapse. Thus a sequence of events called synaptic transmission begins in which the information is relayed from one neuron to another across the synaptic gap.
Pictures taken with electronic microscope revealed the complexities of the synapses. It has been observed that at the axon terminals, there are sacs or synaptic vesicles. As the neural impulse reaches the axon terminal, the synaptic vesicles move from within the cell to the inner membrane of the terminal buttons. Each vesicle contains neurotransmitters, which are biochemical substances that stimulate other neurons. When the synaptic vesicles get fired, they release the neurotransmitters into the synaptic gap. The dendrites of the receiving neuron come in direct contact with these neurotransmitters, and receive the message. If the neurotransmitters are sufficiently stimulated, the receiving neuron will experience a change (either being excited into firing or inhibited from firing). The impulse so received will be conducted within the neuron, and will be relayed from neuron to neuron until the message is completed.
The dendrites of the receiving neuron are not excited electrically no matter how intense is the electrical stimulation. Dendrites are excited only by the neurotransmitters. Thus the impulse crosses the synaptic gap chemically and moves inside the neuron electrically. The process of neural transmission is electrochemical in nature.