Neurons are not just reams of wire coursing through your body. The average length of a neuron is approximately 0.1 millimetre, although some can stretch to over a metre long! Each neuron is connected to its neighbour via a synapse- the meeting point between two neurons.
However, there isn’t just one type of synapse. There are two: electrical and chemical.
As you can see from the diagram, the pre-synaptic and the post-synaptic membrane are incredibly close in an electrical synapse. The ‘gap junctions’ link the two neurons.These are composed of a pair of precisely aligned ion channels, forming a pore. Gap junctions are relatively large, allowing a wide variety of substances access via diffusion. This permits molecules with large molecular weights, such as ATP, to traverse the synapse.
Electrical synapses are much faster at transmitting impulses than a chemical synapse, since an ion current is sent directly from the cytoplasm of one neuron to the next. The impulse travels through the gap junction, as you can see on the diagram. The impulse is never converted from its pure electrical state, which explains their speed.
You might be thinking, why are all synapses not electrical? Well, due to the nature of these synapses, an action potential in one neuron will inevitably create a second in the next neuron. This is beneficial for certain functions, but not all. For example, the hypothalamus is riddled with electrical synapses to ensure the synchronized release of hormones into the blood, or, how ATP can diffuse through gap junction pores, thus coordinating cell signalling.
2) Chemical synapses
Chemical synapses are much slower, yet are considerably more abundant. Unlike electrical synapses, these are more selective in terms of the chemicals permitted to cross the synapse, which has the added benefit of being exceedingly more precise.
Why are they slower? Contrary to electrical synapses, once the electrical impulse propagating down the axon reaches the synapse, it is converted into a chemical signal. This takes time.
So how does it work? Firstly, an action potential flows down the pre-synaptic neuron (as described in a previous page). Upon meeting the synapse, voltage gated calcium channels open. This causes an influx of Ca2+ ions. As you can see in the right diagram, chemical synapses have bags of neurotransmitter called vesicles. The flow of calcium ions forces the vesicles to migrate towards the cell surface membrane, fusing and releasing neurotransmitters into the extracellular space in the ‘synaptic cleft’, such as noradrenaline or acetylcholine. The neurotransmitters diffuse to the post-synaptic neuron, binding to its plasma membrane. This binding sets up a second electrical signal, once again sending an action potential rushing down the axon of this neuron.
What happens to the neurotransmitters once they’ve bound to the post-synaptic membrane? There are a variety of conclusions for neurotransmitters. Either they are reabsorbed to be used again, and some are broken down by enzymes (i.e. acetylcholinesterase enzyme breaks down [hydrolyses] acetylcholine).
Many legal, and illegal, drugs work by influencing chemical synapses, for example cocaine. This drug blocks the re-uptake of neurotransmitters, leading to excessive stimulation of the post-synaptic neuron. The re-uptake of the neurotransmitter serotonin is halted when taking cocaine, which is usually involved with mood regulation, which explains why some people choose to cake cocaine. It also affects dopamine reuptake, which is routinely responsible for emotion and attention. Furthermore, cocaine affects norepinephrine re-uptake. This neurotransmitter is involved with the fight or flight response, explaining the increase in heart rate associated with cocaine use.
The enzyme which breaks down neurotransmitters is also a target for certain drugs. The enzyme acetylcholinesterase is inhibited by a class of pesticides, resulting in rapid firing of neurons, so much so that it leads to death, an effective way of killing aggravating bugs.
So, now you know what connects your neurons and why ingesting pesticide is not a good idea…