Rats raised in darkness grow fewer spines on their visual neurons
Young rats kept in the dark develop brain cells in their visual cortex with fewer tiny spines on their branches, and with branches laid out differently. Those branches are dendrites, the tree-like receiving antennae of nerve cells. Their shape is not fixed by genes alone; what an animal experiences helps prune and grow them.
Dendron is Greek for tree, and the resemblance is apt. A neuron has two kinds of outgrowth: typically a single axon, which is long, even in width and carries signals away, and dendrites, which are shorter, taper as they branch and gather incoming messages. Most connections are axon to dendrite, though dendrites can also signal each other, and a neuron can even form a synapse onto itself.
Each input nudges the voltage across the dendrite's membrane, but the change fades as it spreads towards the cell body. One lonely signal accomplishes little. Only when many excitatory synapses fire together is the combined jolt strong enough to trigger an action potential, which usually starts where the axon leaves the cell body. Dendrites therefore do more than collect: they add up and weigh their inputs, deciding whether the neuron fires. Signals can also travel backwards into them, a kind of feedback.
Branching patterns define neuron types. Pyramidal cells in the cortex have one axon and many dendritic trees, including a long branch reaching up towards the brain's surface. Bipolar cells have two main dendrites at opposite ends, a shape common among inhibitory neurons, while insects favour a stalked unipolar design. Form matters so much that faulty branching is closely tied to impaired nerve function, and small molecular changes can contribute to developmental disorders.
How neurons sculpt such specific trees is only partly understood. One idea, the synaptotropic hypothesis, holds that forming synapses steer growth, with branches extending towards incoming connections. Size probably reflects a trade-off between the energy cost of building branches and the need to cover enough territory. The anatomist Wilhelm His coined the term dendrite in 1889.
Source: Dendrite
