A neurotransmitter is a chemical released from a presynaptic neurone that diffuses across a synaptic cleft and binds to receptors on a postsynaptic cell, changing its membrane potential.Examples include:• Acetylcholine (ACh): used at neuromuscular junctions, in the parasympathetic nervous system and in parts of the brain. Synapses using ACh are cholinergic.
• Noradrenaline: used in the sympathetic nervous system and the brain. Synapses using it are adrenergic.
• Dopamine: used in the brain in movement, motivation and the reward pathway. A shortage in part of the brain causes Parkinson's disease.
• Serotonin: used in the brain in mood and sleep. Low levels are linked to depression.
• GABA: the main inhibitory neurotransmitter in the brain.A neurotransmitter can be excitatory, depolarising the postsynaptic membrane and making an action potential more likely, or inhibitory, hyperpolarising it and making an action potential less likely. The effect depends on the type of receptor on the postsynaptic membrane, so the same neurotransmitter can be excitatory at one synapse and inhibitory at another. For example, ACh stimulates skeletal muscle but slows the heart.To keep transmission under control, the neurotransmitter must be removed from the cleft quickly. This happens in two main ways:• enzyme breakdown: acetylcholinesterase hydrolyses ACh
• reuptake: transporter proteins in the presynaptic membrane take neurotransmitters such as dopamine, serotonin and noradrenaline back into the synaptic knob, to be reused or broken downAll neurotransmitters are made in the presynaptic neurone, stored in vesicles and released by exocytosis when calcium ions enter the synaptic knob.
• Noradrenaline: used in the sympathetic nervous system and the brain. Synapses using it are adrenergic.
• Dopamine: used in the brain in movement, motivation and the reward pathway. A shortage in part of the brain causes Parkinson's disease.
• Serotonin: used in the brain in mood and sleep. Low levels are linked to depression.
• GABA: the main inhibitory neurotransmitter in the brain.A neurotransmitter can be excitatory, depolarising the postsynaptic membrane and making an action potential more likely, or inhibitory, hyperpolarising it and making an action potential less likely. The effect depends on the type of receptor on the postsynaptic membrane, so the same neurotransmitter can be excitatory at one synapse and inhibitory at another. For example, ACh stimulates skeletal muscle but slows the heart.To keep transmission under control, the neurotransmitter must be removed from the cleft quickly. This happens in two main ways:• enzyme breakdown: acetylcholinesterase hydrolyses ACh
• reuptake: transporter proteins in the presynaptic membrane take neurotransmitters such as dopamine, serotonin and noradrenaline back into the synaptic knob, to be reused or broken downAll neurotransmitters are made in the presynaptic neurone, stored in vesicles and released by exocytosis when calcium ions enter the synaptic knob.