From the reflex to plasticity

From the reflex withdrawal of a hand to a childhood memory, everything rests on the same cells. What changes is the organisation of the circuit.

The reflex arc: the shortest circuit

When a hand touches a hot surface it withdraws before the pain is felt. The circuit does not pass through the brain.

   SKIN (receptor)
      |
      |  SENSORY neuron
      v
   SPINAL CORD  --[interneuron]--  (and to the brain, in parallel)
      |
      |  MOTOR neuron
      v
   MUSCLE (effector)  ->  withdrawal

   reflex withdrawal      ~ 50 ms      (via the spinal cord)
   perception of pain     ~ 200 ms     (via the brain)

The movement therefore precedes the sensation. This is not an impression: it is the real order of events, and it limits the damage of a burn.

The knee-jerk reflex — the tap on the tendon that makes the leg kick — uses an even shorter circuit, with only two neurons. Its clinical test reports directly on the integrity of the spinal cord.

Levels of organisation

NEURON        the decision unit
CIRCUIT       a few neurons: a reflex
BRAIN AREA    millions of neurons: vision, language, movement
WHOLE BRAIN   coordination of everything

Areas are specialised but cooperate. Reading a word mobilises the visual area, the language areas and memory — simultaneously, in a few hundred milliseconds.

Plasticity

The brain is not wired once and for all. Connections change with use:

   frequently used synapse   ->  STRENGTHENED
   rarely used synapse       ->  WEAKENED, then eliminated

This is the physical basis of learning, summed up by Hebb's formula (1949): neurons that fire together wire together. Learning is not storing a file: it is modifying the structure of the network.

Three testable consequences:

- repetition strengthens durably (hence the efficacy of spaced review)
- after injury, neighbouring regions can take over a function
- adolescent pruning removes unused connections

This plasticity remains real throughout life, though it declines. The idea of a brain fixed after childhood is false.

What the nervous system does not do like a computer

COMPUTER                          BRAIN
--------------------------------  --------------------------------
memory and computation separate   memory IN the connections
central clock                     no clock, everything asynchronous
very fast, very sequential        slow (ms) but massively parallel
fixed program                     structure modified by use
consumes hundreds of watts        about 20 watts

The last line is striking: the human brain runs on the power of a low-energy light bulb. No artificial system today approaches that energy efficiency — one of the main arguments driving neuromorphic computing research.

Summary

• A reflex bypasses the brain: movement (~50 ms) precedes pain (~200 ms).
• The reflex arc links receptor, sensory neuron, spinal cord, motor neuron, effector.
• The system is organised in levels: neuron, circuit, area, whole brain.
• Plasticity strengthens used synapses and eliminates the others.
• "Neurons that fire together wire together" (Hebb).
• The brain is slow but massively parallel, and uses about 20 watts.