The human memory - Luke Mastin

NEURONS & SYNAPSES

Diagram of a neuron Picture from Wikipedia (http://en.wikipedia.org/wiki/Neuron)

The core component of the nervous system in general, and the brain in particular, is the neuron or nerve cell, the “brain cells” of popular language. A neuron is an electrically excitable cell that processes and transmits information by electro-chemical signalling. Unlike other cells, neurons never divide, and neither do they die off to be replaced by new ones. By the same token, they usually cannot be replaced after being lost, although there are a few exceptions.

The average human brain has about 100 billion neurons (or nerve cells) and many more neuroglia (or glial cells) which serve to support and protect the neurons (although see the end of this page for more information on glial cells). Each neuron may be connected to up to 10,000 other neurons, passing signals to each other via as many as 1,000 trillion synaptic connections, equivalent by some estimates to a computer with a 1 trillion bit per second processor. Estimates of the human brain’s memory capacity vary wildly from 1 to 1,000 terabytes (for comparison, the 19 million volumes in the US Library of Congress represents about 10 terabytes of data).

Information transmission within the brain, such as takes place during the processes of memory encoding and retrieval, is achieved using a combination of chemicals and electricity. It is a very complex process involving a variety of interrelated steps, but a quick overview can be given here.

A typical neuron possesses a soma (the bulbous cell body which contains the cell nucleus), dendrites (long, feathery filaments attached to the cell body in a complex branching “dendritic tree”) and a single axon (a special, extra-long, branched cellular filament, which may be thousands of times the length of the soma).

Every neuron maintains a voltage gradient across its membrane, due to metabolically-driven differences in ions of sodium, potassium, chloride and calcium within the cell, each of which has a different charge. If the voltage changes significantly, an electrochemical pulse called an action potential (or nerve impulse) is generated. This electrical activity can be measured and displayed as a wave form called brain wave or brain rhythm.

Synaptic transmission Picture from Wikipedia (http://en.wikipedia.org/wiki/Chemical_synapse)

This pulse travels rapidly along the cell's axon, and is transferred across a specialized connection known as a synapse to a neighbouring neuron, which receives it through its feathery dendrites. A synapse is a complex membrane junction or gap (the actual gap, also known as the synaptic cleft, is of the order of 20 nanometres, or 20 millionths of a millimetre) used to transmit signals between cells, and this transfer is therefore known as a synaptic connection. Although axon-dendrite synaptic connections are the norm, other variations (e.g. dendrite-dendrite, axon-axon, dendrite-axon) are also possible. A typical neuron fires 5 - 50 times every second.

Each individual neuron can form thousands of links with other neurons in this way, giving a typical brain well over 100 trillion synapses (up to 1,000 trillion, by some estimates). Functionally related neurons connect to each other to form neural networks (also known as neural nets or assemblies). The connections between neurons are not static, though, they change over time. The more signals sent between two neurons, the stronger the connection grows (technically, the amplitude of the post-synaptic neuron’s response increases), and so, with each new experience and each remembered event or fact, the brain slightly re-wires its physical structure.

The interactions of neurons is not merely electrical, though, but electro-chemical. Each axon terminal contains thousands of membrane-bound sacs called vesicles, which in turn contain thousands of neurotransmitter molecules each. Neurotransmitters are chemical messengers which relay, amplify and modulate signals between neurons and other cells. The two most common neurotransmitters in the brain are the amino acids glutamate and GABA; other important neurotransmitters include acetylcholine, dopamine, adrenaline, histamine, serotonin and melatonin.

When stimulated by an electrical pulse, neurotransmitters of various types are released, and they cross the cell membrane into the synaptic gap between neurons. These chemicals then bind to chemical receptors in the dendrites of the receiving (post-synaptic) neuron. In the process, they cause changes in the permeability of the cell membrane to specific ions, opening up special gates or channels which let in a flood of charged particles (ions of calcium, sodium, potassium and chloride). This affects the potential charge of the receiving neuron, which then starts up a new electrical signal in the receiving neuron. The whole process takes less than one five-hundredth of a second. In this way, a message within the brain is converted, as it moves from one neuron to another, from an electrical signal to a chemical signal and back again, in an ongoing chain of events which is the basis of all brain activity.

The electro-chemical signal released by a particular neurotransmitter may be such as to encourage to the receiving cell to also fire, or to inhibit or prevent it from firing. Different neurotransmitters tend to act as excitatory (e.g. acetylcholine, glutamate, aspartate, noradrenaline, histamine) or inhibitory (e.g. GABA, glycine, seratonin), while some (e.g. dopamine) may be either. Subtle variations in the mechanisms of neurotransmission allow the brain to respond to the various demands made on it, including the encoding, consolidation, storage and retrieval of memories.

As has been mentioned, in addition to neurons, the brain contains about an equal mass of glial cells (neuroglia or simply glia), the most common types being oligodendrocytes, astrocytes and microglia. Because they are so much smaller than neurons, there are up to 10 times as many in number, and different areas of the brain have higher or lower concentrations of glia. It used to be thought that the role of glial cells was limited to the physical support, nutrition and repair of the neurons of the central nervous system. However, more recent research suggests that glia, particularly astrocytes, actually perform a much more active role in brain communication and neuroplasticity, although the extent and mechanics of of this role is still uncertain, and a substantial amount of contemporary brain research is now focused on glials cells.