Neuron Structure and the Nerve Impulse

What you do

• Trace one message: dendrite, cyton, axon, nerve ending, then across the synapse into the next neuron.
• In the resting or polarised state the outside of the fibre is positive because sodium ions sit outside. Stimulation lets them rush in, and the sodium pump later drives them back out using ATP.

The parts you label

• Dendrite — Branched cytoplasmic projections of the cell body. They conduct nerve impulses towards the cyton.
• Cyton (cell body) — Also called the perikaryon. It holds all the usual cell organelles except the centrosome, because nerve cells have lost the ability to divide.
• Nucleus — A well-defined nucleus surrounded by the granular cytoplasm of the cell body.
• Myelin sheath — A white insulating sheath around the axon, covered outside by the neurolemma and interrupted at intervals by gaps called the Nodes of Ranvier.
• Axon — A single long process carrying the impulse away from the cell body. It ranges from a few millimetres to more than a metre in length.
• Nerve ending — Swollen bulb-like terminals that store neurotransmitters. They lie very close to the dendrites of the next neuron but never actually join them.

Challenges to clear

• Tap the insulating layer that stops impulses in neighbouring fibres from mixing.
• Tap the swollen ends that store the neurotransmitter.
• Label the whole neuron.

Hint

An impulse is a wave of depolarisation followed by repolarisation. Nothing actually travels along the fibre the way electrons travel along a wire, which is why it manages only about 100 metres per second while electricity manages 150,000 km per second.