Hello, my dear readers. Thank you all for the motivation and encouragement. On my blog today, I will be discussing the roles of neurotoxins, neurons, nerves and impulses. I will introduce the topic by explaining how the Amazonian Indians hunt animals with the use of curare, a very potent neurotoxin. Kindly follow me as I take you through the entire journey......
Neurotoxins on the arrow tip
Some Amazonian Indians have a very effective way of hunting monkeys and other animals. They tip their blowpipe arrows in a preparation made from the bark of one of the local trees. The substance, known as curare, is a powerful neurotoxin.
Ben Brahim Mohammed, CC BY 3.0
The Amazonian Indians, not being analytical chemists, assess the strength of their preparation by how long it takes a monkey to fall out of the tree. ‘One-tree curare’ is the most powerful; the monkey only escapes to the next tree before the paralysing toxin brings it tumbling to the forest floor. ‘Two-tree curare’ is obviously weaker and Three-tree curare’ is the weakest preparation that is useful. After that, the monkey travels too far and, although it still dies from the action of the poison, it’s usually impossible to find.
For many centuries, the exact nature of curare was a mystery. In 1814, in an attempt to confirm his suspicions, the explorer Charles Waterton injected a donkey with curare. Within a few minutes, the donkey appeared dead. Waterton cut a small hole in her throat and inflated her lungs with a pair of bellows. According to his account, the donkey regained consciousness and looked around. This crude method of artificial respiration was continued for a couple of hours until the effects of the curare had worn off.
Analysis of curare has since shown that it prevents nerve impulses getting through to the muscles, causing paralysis. In addition to affecting the movement of skeletal muscles, curare interferes with heartbeat and breathing, although the exact effects depend on the dosage and whether the curare is taken orally or by injection. If heartbeat and breathing are severely affected, the result is usually fatal.
Rama, wikimedia common
In 1939, the active ingredient in curare was isolated, and in 1943 doctors started to use it as an anaesthetic. It was later used as a muscle relaxant and is now a vital tool in surgery, relaxing muscles and generally making the surgeon's job a lot easier. It is also useful for treating conditions in which the muscles go into spasm - polio, tetanus, epilepsy and cholera. Today, synthetic analogues of the active ingredient in curare, such as d-tubocurarine, are used widely in medicine.
IT’S ALL ABOUT COMMUNICATION
The body of a human is incredibly complex. The human body consists of four basic types of tissue namely epithelial, connective,muscular and nervous tissues. Epithelial tissue forms coverings and linings, connective tissue holds other tissues together, muscle tissue can contract and nervous tissue is excitable - it can transmit impulses. All of our organs are formed from these four basic tissue types. To ensure that the different parts work together effectively, there needs to be communication to register changes in internal and external conditions. In this series of post, I will look at the role of hormonal and nervous communication.
The nervous system is made up of specialised cells that allow the different parts of the body to communicate. These cells are called neurons. They carry information from one part of the body to another. I will concentrate on the structure and function of these remarkable cells in this post.
BruceBlaus, CC BY 3.0
OVERVIEW OF THE NERVOUS SYSTEM
The nervous system as a whole ensures that the body responds appropriately to the external conditions at any given time. It means we can do the following:
- Gather information. Sense organs called receptors detect stimuli from the internal and the external environment.
- Transmit sensory information to the central nervous system by means of the sensory nerves.
- Co-ordinate information. Incoming information travels to the brain via the spinal cord. The brain then decides what to do. Decisions are often based on memory, the result of our past experience.
- Transmit the information to the effectors: the muscles and glands. Impulses pass from the central nervous system to the effectors via motor nerves.
HOW INFORMATION TRAVELS AROUND THE BODY
Information passes along neurons in the form of electrical signals called nerve impulses. A nerve impulse, known as an action potential, is not a message, nor is it an electrical current (a flow of electrons). It is more a change in ion balance in the nerve cell, which spreads rapidly from one end to the other, like the fire travelling along a burning fuse. It is little more than an electrical ‘blip – but the brain can make sense of the blips because they vary in frequency and arrive down specific nerves.
So what happens when the nerve impulse reaches the end of the neuron? It connects with other neurons at junctions called synapses. A nerve impulse crosses a synapse usually by means of a chemical transmitter.
The whole of the nervous system therefore communicates by a mixture of electrical and chemical signals. This allows information to travel around an organism with far greater speed and precision than if only chemical signals were used.
Neurons rarely act alone. They are bundled together into larger, visible structures called nerves. Nerves form a complex network throughout the body. Sensory nerves take information received from receptors, or sense organs, on the outer parts of the body the eyes, ears, tongue and nose and touch receptors in the skin – into the central nervous system.
Processing of that information happens in the brain, and then motor nerves take the information from the brain to effectors such as muscles and glands to cause the body to take some action. Think what happens if someone puts, say, a glass of water in front of you. Sensory nerves take the information from the eyes to the brain. If you are thirsty, the brain may then send information down motor nerves to your arm and hand and you would reach for the glass, pick it up and drink. If you aren’t thirsty, information might travel down motor nerves to the muscles in your face and throat for you to say ‘No thanks’.
In the reflex arc, the sensory and motor nerves are linked by a neuron in the spinal cord, rather than by neurons in the brain.
HOW DO NEURONS CARRY INFORMATION?
Neurons have two properties that enable them to carry information. They are excitable – they can detect and respond to stimuli – and conductive - they can transmit a signal from one end to the other. Let’s find out what is going on in the neuron before information arrives.
THE NEURON AT REST
At any given moment a neuron needs to be ready to conduct impulses. This state of readiness is called the resting potential. At this point, the axon membrane is polarised: the fluid on the inside is negatively charged with respect to the outside. This difference in charge, about -70 mV, results from an unequal distribution of ions known as an electrochemical gradient.
THE RESTING POTENTIAL
The resting potential results from an unequal distribution of ions, brought about by two processes; active transport and facilitated diffusion:
- Active transport. All animal cell membranes contain a protein pump called Na+ K+ATPase. This splits ATP to gain the energy to pump ions. Three sodium ions move out of the cell at the same time as two potassium move in. It is an unequal exchange: more positive ions leave than enter the cell. The Na+K+ATPase protein is thought to have evolved as an osmoregulator to keep the internal water potential high to prevent water entering animal cells and bursting them. Plant cells don’t need this as they have strong cell walls that stop them bursting.
- Facilitated diffusion. There are also sodium and potassium ion channels in the membrane. These channels are normally closed, but they 'leak’, allowing sodium ions to diffuse into the cell and potassium ions to leak out, down their respective concentration gradients. Generally, the potassium channels are more leaky than the sodium channels, so more potassium diffuses out. The potassium ions join the sodium ions that have been pumped out of the cell by the Na+K+ATPase pump.
The overall effect of the two processes is to cause an imbalance of sodium and potassium ions across the membrane: there are more positive ions outside the axon than inside, so the inside of the cell is negatively charged with respect to the outside. This results in a potential difference across all animal cell membranes, called the resting potential or the membrane potential. The value of this potential varies from -20 to -200 mV in different cells and species, but is typically about -70mV.
Working out how neurons carry information
It is difficult to study mammalian nerves because they are so small. Hodgkin, Huxley and Eccles did much of the early pioneering work on the nature of the nerve impulse in the 1940s and 1950s using the giant axons found in squid. These axons are 1 mm across, making it far easier to insert electrodes into them.
When they measured the concentration of different ions, both in and out of the axon the results were shown that an approximate concentratioms of sodium ions inside and outside the axon membrane in mol/kg are 50 and 460 respectively, while that of potassium ions are 400 and 30 and chlorine ions are 100 and 560 mol/kg respectively.
scientificanimations.com, CC BY-SA 4.0
There are two opposing forces acting across the axon membrane:
- Ions will diffuse down their concentration gradient as long as the membrane is permeable to them. Potassium ions diffuse out because the membrane is more permeable to them than to sodium ions.
- This movement is opposed by the electrochemical gradient – the balance of positive and negative ions either side of the membrane. Generally, positive ions will move towards areas of negative charge, and vice versa, until things are neutral. There are already more positive ions outside the membrane, and lots of negative ions inside the membrane, so the electrochemical gradient opposes the movement of potassium. At -70 mV these two processes are balanced, hence the value of the resting potential.
The experiments on squid axons also demonstrated why the trace of an action potential has its distinctive shape. In one experiment, two electrodes were placed inside the large Squid axon, one at each end. A short pulse of electrical current was applied through the stimulating electrode to mimic a nerve impulse. The recording electrode at the other end of the axon recorded a change of about 90 mV from -70 mV (the resting potential) to +20 mV. This was only very short-lived – the reading very quickly returned to -70 mV.
For now, I will be stopping here and in my next post, I will continue by discussing the impact of the action potential of a neuron and some other interesting features and roles of neurons. Kindly look forward to it.
Thanks for reading.