10 minutes of exercise enough to boost brain
Consciousness Doesn’t Reside in the Brain: New Theory or Old News?
Signaling in the brain: getting connected
The problem of connection, the sending of information effectively around the nervous system, arises because signals must be communicated undistorted over the length of the body, which might be a very large distance indeed, in the case of the blue whale for example. Coupled to this is the fact that, in an unforgiving world, animals must react quickly to be an effective predator or so as to avoid being eaten. So the basic requirements of signaling coded information in the nervous system are that the signals have to be routed correctly and sent reliably over long distances as rapidly as possible.
In order to achieve this neurons convey and encode information electrically. Brief electrical pulses (lasting a few thousandths of a second), known as action potentials or nerve impulses, travel along biological cables (axons) that extend from the cell bodies of neurons to connect their input to their outputs with other neurons.
Compared to the speed of electrical information traffic along the wires in a computer (close to the speed of light), conduction velocities of impulses in the brain are slow, about 120 meters per second in the fastest conducting axons. When they reach the terminals of axons, impulses trigger the release of chemical signals that are able to initiate or suppress electrical signals in other neurons. In this way neurons transmit information from one to another by an alternating chain of electrical and chemical signals. The chemical signals are released at specialized sites called synapses, at which the chemical signals (neurotransmitters) pass across a very narrow gap separating two neurons. Released neurotransmitter molecules work by binding to and thereby activating specialized receptor molecules located on the surface of the receiving neuron on the other side of the synapse. An activated receptor causes a brief electrical response, called a synaptic potential, in the receiving neuron. These potentials may be either inhibitory or excitatory depending on whether the voltage in the receiving neuron becomes more negative (inhibitory or hyperpolarizing) or less negative (excitatory or depolarizing).
Inhibitory potentials make the receiving neuron less likely to fire a nerve impulse. Excitatory potentials increase that probability. A ‘decision’ to produce nerve impulses is therefore made through the summation of all of the inhibitory and excitatory potentials impinging on a neuron. Once a critical threshold voltage is reached by this summation, nerve impulses will be generated. The more the excitation, the higher will be the frequency of the impulse train. An important way that information is coded in the brain is by the impulse frequency (number of impulses or action potentials per second) and by the pattern of impulses. Nerve impulses travel rapidly along the axon, feeding information to many other neurons where the process of neurotransmitter release and chemical communication is repeated.
Neurons may receive chemical signals from hundreds of other neurons through a thousand or more synapses on their surfaces, each having some influence on the ‘decision’ to fire a nerve impulse and on the firing rate. The complexity of the resulting signaling network in the brain is almost unimaginable: one hundred billion neurons each with one thousand synapses, producing a machine with one hundred trillion interconnections! If you started to count them at one per second you would still be counting 30 million years from now!
Physics and the problem of electrical signaling when a neuron is inactive or at rest there exists a stable negative voltage across the membrane of about −70mv, known as the resting potential. When excited by another neuron or in the case of a sensory receptor cell by a sensory stimulus, the neuron may generate a train of action potentials. Nerve impulses attain a positive voltage of about +50mv before returning to the resting potential. So the total voltage excursion of a nerve impulse is about 120mv or 0.12 volts.
We need now to understand something about how these electrical impulses are generated and propagated along axons in the wet, salty, and gelatinous medium that is the brain: a very unsuitable environment for an electrical signaling system. The problem is made even more difficult by the dreadful electrical properties of axons. Axons are very poor conductors of electricity, so bad in fact that over relatively short distances, far less than a typical axon’s length, most of the original signal will leak away into the salty surroundings. This inescapable problem is a consequence of the 3. Neuron-to-neuron communication. An electrical action potential or nerve impulse travels at speeds up to 120 meters per second along the axon of the presynaptic neuron. When it reaches the synapse the impulse causes neurotransmitter molecules to be released. Receptor molecules react to the neurotransmitter molecules causing the postsynaptic neuron to be either excited (illustrated) or inhibited. An inhibitory synaptic potential would dip below the resting potential, making the postsynaptic neuron less likely to fire an action potential way the laws of physics apply to the flow of electricity in electrical cables immersed in salty water.
These laws were first formulated by the British scientist Lord Kelvin (1824–1907) who figured out how to send telegraphic information across the Atlantic Ocean through a submarine cable. Lord Kelvin defined a parameter called the ‘length constant’, which allows us to compare how good different types of cable are at transmitting electrical signals over a distance. A length constant is the distance over which about two-thirds of the electrical signal’s amplitude will be lost and its value can vary enormously. For example, the length constant of a submarine cable is a few tens of miles. This means it is not possible simply to lay a cable across the Atlantic and expect an electrical signal injected at one end to appear at the other end undiminished, several thousands of kilometers away.
For a submarine cable, the length constant is a small fraction of the distance over which information must be sent and the same is true for biological cables, axons. So in a similarly salty environment both submarine cables and axons must detect a failing electrical signal and boost it back to its original strength before sending it on its way again. In submarine cables booster amplifiers placed at regular intervals achieve this, and axons solve the problem in a rather similar way. But how, using the unlikely ingredients of a few proteins, fats, some smaller organic molecules, and plenty of salty water, can nerve cells make a battery-powered amplifier?
