Chemical synapse – transmission, summation and the effects of drugs
BiologyHuman & Animal PhysiologyAges 16–17
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Sign in to playFollow an impulse across a chemical synapse: an action potential reaches the presynaptic knob, Ca²⁺ channels open, vesicles release acetylcholine (or dopamine) into the cleft, the transmitter binds receptors so Na⁺ flows in and causes a postsynaptic potential, and then acetylcholinesterase breaks it down or transporters take it back. Add nicotine, a neonicotinoid, curare, an organophosphate, botulinum toxin or cocaine to see which step each one changes, and watch repeated use reduce receptor numbers, causing tolerance and dependence. A summation screen lets you fire excitatory and inhibitory synapses to reach threshold at the axon hillock, and a reflex-arc screen works out the reflex time.
Lesson: Nervous coordination – synaptic transmission
What it shows
Neurones pass signals to the next cell across a narrow gap, the synaptic cleft. An action potential arriving at the presynaptic knob opens voltage-gated calcium channels; calcium ions make vesicles fuse with the membrane and release a neurotransmitter such as acetylcholine. It diffuses across, binds receptors that are sodium channels, and depolarises the postsynaptic membrane. The transmitter is then removed by an enzyme or by reuptake, so each signal is brief. Many drugs and poisons act on just one of these steps, and synapses also let a neurone add up its many inputs.
How to use
Choose a Synapse type, a Drug or poison and its Dose, then press Send one impulse and compare the three graphs and the table. Set Automatic impulses to send a train and Speed to slow it down. For dopamine, drag Repeated use. In Summation, press E1 + E2 together, E1 three times quickly or Stimulate I. In Reflex arc, pick a reflex and press Stimulate.
Parameters you can change
- Screen Synapse and drugs, Summation, Reflex arc
- Synapse type Cholinergic synapse between two neurones, Neuromuscular junction, Dopamine synapse in the brain
- Drug or poison None, Nicotine, Neonicotinoid (insecticide), Curare, Organophosphate insecticide, Botulinum toxin, Cocaine
- Dose 0–100 %
- Automatic impulse frequency 0–100 Hz
- Simulation speed Very slow (2 ms per second), Slow (10 ms per second), Faster (50 ms per second)
- Days of repeated drug use 0–30 days
- Excitatory synapse E1 frequency 0–100 Hz
- Excitatory synapse E2 frequency 0–100 Hz
- Inhibitory synapse I frequency 0–100 Hz
- Reflex Withdrawal from a pin prick (3 neurones), Knee-jerk reflex (2 neurones)
Questions to explore
- Why does an organophosphate make muscles stay contracted while curare leaves them limp?
- Why does a single impulse at one excitatory synapse usually not make the postsynaptic neurone fire?
- Why do people who use cocaine for a long time need larger doses and feel low when they stop?
