Edexcel iGCSE Biology-3.39 Role of Neurotransmitters at Synapses- Study Notes- New Syllabus

Role of Neurotransmitters at Synapses

📝 Introduction

• Neurons in the nervous system do not directly touch each other.
• They communicate using small gaps called synapses.
• A synapse allows signals to pass from one neuron to another or to an effector (like a muscle).
• The signal is not electrical across the gap, it is carried by chemicals called neurotransmitters.
• This system allows fast and controlled communication in the body.

🔗 What is a Synapse?

• A synapse is the small gap between:
• Two neurons, OR
• A neuron and an effector (muscle or gland)

Main parts of a synapse:

• Presynaptic neuron → sends the signal
• Synaptic cleft → tiny gap
• Postsynaptic neuron / effector → receives the signal

🧪 What are Neurotransmitters?

• Neurotransmitters are chemical messengers.
• They carry nerve impulses across the synapse.
• They are stored in tiny sacs called vesicles at the end of the neuron.
• Example: acetylcholine, dopamine.

⚙️ How Neurotransmitters Work (Step by Step)

1. Arrival of nerve impulse

• An electrical impulse reaches the end of the presynaptic neuron (axon terminal).
• This triggers a response inside the neuron.

2. Release of neurotransmitters

• Vesicles inside the neuron move towards the membrane.
• They release neurotransmitters into the synaptic cleft.
• This process is called exocytosis.

3. Diffusion across the synapse

• Neurotransmitters move across the gap by diffusion.
• They travel from a high concentration (synaptic cleft) to a low concentration (postsynaptic side).

4. Binding to receptors

• Neurotransmitters fit into specific receptors on the postsynaptic membrane.
• This is like a lock and key mechanism.

5. Formation of new signal

• Binding of neurotransmitters triggers a new electrical impulse in the postsynaptic neuron.
• If the target is a muscle, it causes muscle contraction.
• If the target is a gland, it triggers secretion.

6. Removal of neurotransmitters

• Neurotransmitters are quickly broken down by enzymes.
• Or they are reabsorbed into the presynaptic neuron.
• This stops continuous stimulation.
• Ensures the signal is short and controlled.

🔄 Why Synapses Are Important

• Ensure signals travel in one direction only.
• Prevent confusion of signals in the nervous system.
• Allow fine control of responses (not just simple reactions).
• Help in coordination of complex actions like movement and reflexes.

🧠 Key Features of Synaptic Transmission

• Signal changes from electrical → chemical → electrical.
• Very fast but also controlled.
• Uses small amounts of neurotransmitter for each signal.
• Can be excitatory (starts impulse) or inhibitory (stops impulse).

📊 Components of a Synapse

🌟 Example of Neurotransmitter Action

• Nerve impulse reaches neuron ending.
• Acetylcholine is released.
• It crosses synapse and binds to muscle receptors.
• Muscle contracts.
• Enzyme breaks down acetylcholine to stop contraction.

📌 Quick Recap
Synapse = gap between neurons or neuron & effector.
Neurotransmitters carry the message across the synapse.
Ensures rapid, precise communication between neurons.
Examples: acetylcholine, dopamine.