Whether neurons are sensory or motor, big or small, they all have in common that their activity is both electrical and chemical. Neurons both cooperate and compete with each other in regulating the overall state of the nervous system, rather in the same way that individuals in a society cooperate and compete in decision-making processes.
Chemical signals received in the dendrites from the axons that contact them are transformed into electrical signals, which add to or subtract from electrical signals from all the other synapses, thus making a decision about whether to pass on the signal elsewhere. Electrical potentials then travel down axons to synapses on the dendrites of the next neuron and the process repeats.
The dynamic neuron
A neuron consists of dendrites, a cell body, an axon and synaptic terminals. This structure reflects its functional subdivision into receiving, integrating and transmitting compartments. Roughly speaking, the dendrite receives, the cell-body integrates and the axons transmit – a concept called polarization because the information they process supposedly goes in only one direction.
Like any structure, it has to hold together. The outer membranes of neurons, made of fatty substances, are draped around a cytoskeleton that is built up of rods of tubular and filamentous proteins that extend out into dendrites and axons alike. The structure is a bit like a canvas stretched over the tubular skeleton of a frame tent.
The different parts of a neuron are in constant motion, a process of rearrangement that reflects its own activity and that of its neighbours. The dendrites change shape, sprouting new connections and withdrawing others, and the axons grow new endings as the neuron struggles to talk a bit more loudly, or a bit more softly, to others.
Inside neurons are many inner compartments. These consist of proteins, mostly manufactured in the cell body, that are transported along the cytoskeleton. Tiny protuberances that stick out from the dendrites called dendritic spines. These are where incoming axons make most of their connections.
Proteins transported to the spines are important for creating and maintaining neuronal connectivity. These proteins are constantly turning over, being replaced by new ones when they’ve done their job. All this activity needs fuel and there are energy factories (mitochondria) inside the cell that keep it all working.
The end-points of the axons also respond to molecules called growth factors. These factors are taken up inside and then transported to the cell body where they influence the expression of neuronal genes and hence the manufacture of new proteins. These enable the neuron to grow longer dendrites or make yet other dynamic changes to its shape or function. Information, nutrients and messengers flow to and from the cell body all the time.
Receiving and deciding
On the receiving side of the cell, the dendrites have close contacts with incoming axons of other cells, each of which is separated by a miniscule gap of about 20 billionths of metre. A dendrite may receive contacts from one, a few, or even thousands of other neurons. These junctional spots are named synapses, from classical Greek words that mean “to clasp together”. Most of the synapses on cells in the cerebral cortex are located on the dendritic spines that stick out like little microphones searching for faint signals.
Communication between nerve cells at these contact points is referred to as synaptic transmission and it involves a chemical process. When the dendrite receives one of the chemical messengers that has been fired across the gap separating it from the sending axon, miniature electrical currents are set up inside the receiving dendritic spine. These are usually currents that come into the cell, called excitation, or they may be currents that move out of the cell, called inhibition.
All these positive and negative waves of current are accumulated in the dendrites and they spread down to the cell body. If they don’t add up to very much activity, the currents soon die down and nothing further happens. However, if the currents add up to a value that crosses a threshold, the neuron will send a message on to other neurons.
So a neuron is kind of miniature calculator – constantly adding and subtracting. What it adds and subtracts are the messages it receives from other neurons. Some synapses produce excitation, others inhibition. How these signals constitute the basis of sensation, thought and movement depends very much on the network in which the neurons are embedded.
To communicate from one neuron to another, the neuronal signal has first to travel along the axon.
How do neurons do this?
The answer hinges on harnessing energy locked in physical and chemical gradients, and coupling together these forces in an efficient way. The axons of neurons transmit electrical pulses called action potentials. These travel along nerve fibres rather like a wave travelling down a skipping rope.
This works because the axonal membrane contains ion channels, that can open and close to let through electrically charged ions. Some channels let through sodium ions (Na+), while others let through potassium ions (K+). When channels open, the Na+ or K+ ions flow down opposing chemical and electrical gradients, in and out of the cell, in response to electrical depolarization of the membrane.
When an action potential starts at the cell body, the first channels to open are Na+ channels. A pulse of sodium ions flashes into the cell and a new equilibrium is established within a millisecond. In a trice, the trans-membrane voltage switches by about 100 mV. It flips from an inside membrane voltage that is negative (about -70 mV) to one that is positive (about +30 mV).
This switch opens K+ channels, triggering a pulse of potassium ions to flow out of the cell, almost as rapidly as the Na+ ions that flowed inwards, and this in turn causes the membrane potential to swing back again to its original negative value on the inside.
The action potential is over within less time than it takes to flick a domestic light switch on and immediately off again. Remarkably few ions traverse the cell membrane to do this, and the concentrations of Na+ and K+ ions within the cytoplasm do not change significantly during an action potential.
However, in the long run, these ions are kept in balance by ion pumps whose job is to bail out excess sodium ions. This happens in much the same way that a small leak in the hull of a sailing boat can be coped with by baling out water with a bucket, without impairing the overall ability of the hull to withstand the pressure of the water upon which the boat floats.
The action potential is an electrical event, albeit a complex one. Nerve fibres behave like electrical conductors (although they are much less efficient than insulated wires), and so an action potential generated at one point creates another gradient of voltage between the active and resting membranes adjacent to it. In this way, the action potential is actively propelled in a wave of depolarisation that spreads from one end of the nerve fibre to the other
An analogy that might help you think about the conduction of action potentials is the movement of energy along a firework sparkler after it is lit at one end. The first ignition triggers very rapid local sparks of activity (equivalent to the ions flowing in and out of the axon at the location of the action potential), but the overall progression of the sparkling wave spreads much more slowly. The marvellous feature of nerve fibres is that after a very brief period of silence (the refractory period) the spent membrane recovers its explosive capability, readying the axon membrane for the next action potential.
Much of this has been known for 50 years based on wonderful experiments conducted using the very large neurons and their axons that exist in certain sea-creatures. The large size of these axons enabled scientists to place tiny electrodes inside to measure the changing electrical voltages. Nowadays, a modern electrical recording technique called patch-clamping is enabling neuroscientists to study the movement of ions through individual ion-channels in all sorts of neurons, and so make very accurate measurements of these currents in brains much more like our own.
Insulating the axons
In many axons, action-potentials move along reasonably well, but not very fast. In others, action potentials really do skip along the nerve. This happens because long stretches of the axon are wrapped around with a fatty, insulating blanket, made out of the stretched out glial cell membranes, called a myelin sheath.
New research is telling us about the proteins that make up this myelin sheath. This blanket prevents the ionic currents from leaking out in the wrong place but, every so often the glial cells helpfully leave a little gap. Here the axon concentrates its Na+ and K+ ion channels. These clusters of ion channels function as amplifiers that boost and maintain the action potential as it literally skips along the nerve. This can be very fast. In fact, in myelinated neurons, action-potentials can race along at 100 metres per second!
Action potentials have the distinctive characteristic of being all-or-nothing: they don’t vary in size, only in how often they occur. Thus, the only way that the strength or duration of a stimulus can be encoded in a single cell is by variation of the frequency of action potentials. The most efficient axons can conduct action potentials at frequencies up to 1000 times per second.