The neuron

The nervous system processes information on a staggering scale: about 86 billion neurons in a human, linked by more than a hundred trillion connections. Yet it all rests on a single cell type.

The anatomy of a neuron

      dendrites
       \  |  /
        \ | /              CELL BODY: the nucleus, the metabolism
      ___\|/___                       receives the signals
     (  nucleus )
      ‾‾‾‾|‾‾‾‾‾            AXON: the cable, carries the message
          |                       (up to a metre in humans)
          |  ====  myelin sheath
          |  ====
          |
       __/|\__               TERMINALS: contact the next cell
      /   |   \

DENDRITES    receive messages from other neurons (input)
CELL BODY    integrates the incoming signals
AXON         conducts the message, single and sometimes very long (output)
TERMINALS    pass it on to the next cell

The neuron is therefore a polarised cell: information travels one way only, from dendrites to terminals. This asymmetry is what makes directed information processing possible.

The myelin sheath

Some axons are wrapped in an insulating sheath, myelin, interrupted at regular intervals.

   BARE axon        ————————————————————      1 m/s
   MYELINATED axon  ==[ ]==[ ]==[ ]==[ ]==   up to 100 m/s
                       ^
                   nodes of Ranvier: the message "jumps" from node to node

The gain is a factor of a hundred. It lets a signal travel from foot to brain in hundredths of a second. Destruction of this sheath is exactly what happens in multiple sclerosis: messages slow down or are lost, causing motor and sensory disorders.

Specialised neurons

SENSORY      picks up a stimulus (skin, eye, ear) -> towards the nervous system
MOTOR        from the nervous system -> to a muscle or gland
INTERNEURON  links neurons together; by far the most numerous

Glial cells, long overlooked

Neurons are not alone: glial cells are at least as numerous. They make myelin, feed neurons, clear debris and regulate the chemical environment.

Long regarded as mere "filler" — their name comes from the Greek for "glue" — they are now known to take an active part in transmission and plasticity.

Some orders of magnitude

neurons (human)            ~ 86 billion
connections (synapses)     ~ 100 trillion
maximum axon length        ~ 1 metre (spinal cord -> toe)
brain energy consumption   ~ 20 % of the body's energy for 2 % of its mass

The last figure deserves attention: thinking is expensive. This very high energy cost is one of the major constraints on the evolution of the human brain.

Summary

• A neuron has dendrites (input), cell body, axon (output) and terminals.
• Information travels one way only.
• The myelin sheath speeds conduction by a factor of ~100 (up to 100 m/s).
• Its destruction causes multiple sclerosis.
• Three types: sensory, motor, interneuron.
• The brain uses 20 % of the body's energy for 2 % of its mass.