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Neurons, synapses and the central nervous system

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Your brain contains billions of nerve cells. They communicate using electrical signals and chemical messengers.

Neurons and the CNS

A neuron is a nerve cell that carries information as electrical impulses. Its main parts are dendrites (receive signals), the cell body, the axon (carries the impulse away; often covered by a fatty myelin sheath that speeds it up) and axon terminals.
Sensory neurons carry messages from the senses; relay neurons pass messages within the brain and spinal cord; motor neurons carry messages to muscles.
The central nervous system (CNS) is the brain and spinal cord. Its role is to receive information from the body, process it and decide how to respond, then send instructions out. The spinal cord carries messages between the brain and body and controls simple reflexes.

Synaptic functioning

Neurons do not touch. Between them is a tiny gap called the synapse (synaptic gap).
1. An electrical impulse reaches the end of the pre-synaptic neuron.
2. This causes vesicles to release neurotransmitters into the gap.
3. Neurotransmitters cross the gap and bind to receptors on the post-synaptic neuron.
4. This makes the next neuron more likely (excitatory) or less likely (inhibitory) to fire.
5. Leftover neurotransmitter is broken down or taken back up (reuptake) into the pre-synaptic neuron.

Neurotransmitters

Neurotransmitters are chemical messengers that carry signals across synapses.
Serotonin: linked to mood (low serotonin is linked to depression; SSRIs block its reuptake).
Dopamine: linked to reward and pleasure (involved in addiction) and movement.
This is how neurons and synapses interact: electrical signals travel within a neuron, and chemical signals carry the message between neurons.
Worked example

Explain how SSRIs might affect synaptic functioning.

  1. Normally, serotonin is taken back into the pre-synaptic neuron by reuptake.
  2. SSRIs block this reuptake.
  3. More serotonin stays in the synapse to bind to receptors.

Answer: SSRIs block the reuptake of serotonin, so more remains in the synapse to stimulate the post-synaptic neuron.

Key idea

Neurons carry electrical impulses. At synapses, neurotransmitters are released from vesicles, cross the gap and bind to receptors, making the next neuron more or less likely to fire, then are reabsorbed (reuptake) or broken down. The CNS (brain and spinal cord) processes information and coordinates responses.

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