Neurons carry signals from one place to another, around the many parts of the nervous system. They connect sense receptors to the central nervous system and also connect one part of the nervous system to another, for example in the brain and spine. They also carry signals from the nervous system to effector organs, such as muscles and glands.
When neurons are stimulated they transmit an electrical impulse.
There are three different types of neurones, each with a slightly different function.
- Sensory neurons carry signals from receptors to the spinal cord and brain.
- Relay neurons carry messages from one part of the CNS to another.
- Motor neurons carry signals from the CNS to effectors.
The diagram below shows a motor neuron. It has a nucleus surrounded bycytoplasm. The cytoplasm forms a long fibre that is surrounded by a cell membrane. This is called an axon. The axon carries the electrical impulse and is protected by a fatty sheath – a bit like the plastic coating around an electrical wire. The fatty sheath increases the speed at which the nerve impulse is transmitted. The nerve ending is branched to make good contact with other neurons or the effector organ.
To test yourself of the structure of a neuron, try this animated quiz here.
Two neurons do not make direct contact. Where they meet, there is a very small gap called a synapse. The signal needs to cross this gap to continue on its journey to, or from, the CNS. This is done by means of chemicals which diffuse across the gap between the two neurons.
Each neuron has approximately 100-10,000 synapses. At the synaptic terminal, an electrical impulse will trigger the release of vesicles containing neurotransmitters toward the presynaptic membrane. The vesicle membrane will fuse with the presynaptic membrane releasing the neurotransmitters into the synaptic cleft. Until recently, it was thought that a neuron produced and released only one type of neurotransmitter. This was called “Dale’s Law.” However, there is now evidence that neurons can contain and release more than one kind of neurotransmitter.
Below is the process of synaptic transmission:
- An electrical impulse travels along an axon.
- This triggers the nerve-ending of a neuron to release chemical messengers called neurotransmitters.
- These chemicals diffuse across the synapse (the gap) and bind with receptor molecules on the membrane of the next neuron.
- The receptor molecules on the second neuron bind only to the specific chemicals released from the first neuron. This stimulates the second neuron to transmit the electrical impulse.
Excitation vs Inhibition
Neurons can be either excitatory or inhibitory.
Excitatory neurons release neurotransmitters, such as dopamine, which bind to receptors on the post-synaptic neurons and trigger a positive change in the membrane potential of that cell. They excite the messaging pathway.
Inhibitory neurons, on the other hand, release neurotransmitters such as GABA, which bind to receptors on the post-synaptic neurons and trigger a negative change in the membrane potential of the cell. They inhibit the messaging pathway. Low levels of GABA have been linked to anxiety.
Drugs and synapses
Some drugs stop the impulse from passing across the synapse. Drugs such as curare – the South American plant toxin used in arrow poison – do this. They cause complete paralysis, and even stop an animal or person breathing.
Other drugs stimulate the synapse so that once an impulse crosses the gap the impulse is repeated over and over again. Drugs such as strychnine do this. They cause all the muscles in the body to go into a continuous spasm of contraction. This also stops the person from breathing.
Drugs can be agonists, which means they mimic the effects of a particular neurotransmitter. Morphine and opium are agonists, as they mimic the brain’s natural opiates. These ‘fake’ neurotransmitters don’t activate neurons in the same way as the body’s natural neurotrasmitters and can lead to abnormal messages being sent throughout the brain.
Antagonists, on the other hand, block the affect of agonists, slowing down the brain’s natural response.
Usually, once neurotransmitters have served their purpose, they are taken back up into the pre-synaptic neuron via re-uptake channels. Some drugs block these re-uptake channels, causing the affected neurotrasmitters to remain in the synaptic gap and continue trigging an effect.
Most drugs, however, directly or indirectly target the brain’s reward system and flood synapses with dopamine. This causes feelings of pleasure, but also affects movement, emotion and motivation. Anything which stimulates dopamine release can lead to addiction, with the strength of impact and frequency of stimulation affecting how addictive a substance has the potential to be. With heavy use of a drug the brain’s reward centre lowers its own dopamine production (called down-regulation), causing the individual to have lower than normal levels of dopamine when they are not taking the drug. The user then needs to take increasing amounts of the drug to feel the same initial effects. This is called tolerance. This perpetuates the cycle of addiction.
Decreased dopamine transporters in a met-amphetamine abuser
- Mouse Party is an amazing resource created by the University of Utah. Zoom in to the brains of mice who have taken different drugs, to see the effects on synaptic transmission and the brain.
- A great Ted Talk by V. S. Ramachandran on neurons: