Edexcel A Level (IAL) Biology -8.6 Synapses & Neurotransmitters- Study Notes- New Syllabus

Synapses and Nerve Impulse Transmission

🌱 (i) Structure and Function of Synapses

What is a Synapse?

• A junction between two neurones (pre-synaptic → post-synaptic) or a neurone and effector.
• Allows one-way transmission of impulses.

Structure of a Chemical Synapse

• Pre-synaptic neurone terminal: contains synaptic vesicles filled with neurotransmitters (e.g., acetylcholine).
• Synaptic cleft: tiny gap between pre- and post-synaptic membranes.
• Post-synaptic membrane: has receptor proteins for neurotransmitters.

How Transmission Happens

• Action potential reaches pre-synaptic terminal.
• Voltage-gated Ca²⁺ channels open → Ca²⁺ enters.
• Vesicles release acetylcholine (ACh) into the synaptic cleft.
• ACh binds to receptors on the post-synaptic membrane.
• Sodium channels open → depolarisation occurs in post-synaptic neurone.
• Impulse continues if threshold is reached.
• ACh is broken down by acetylcholinesterase → stops signal.

Key Points

• Synapses ensure unidirectional flow.
• Allow amplification or inhibition of signals.
• Acetylcholine is a common neurotransmitter at neuromuscular junctions.

👁️ (ii) Pupil Reflex: Dilation and Contraction

What is the Pupil Reflex?

• Iris controls pupil size in response to light intensity.
• Regulated by the autonomic nervous system.

Contraction (Pupillary Light Reflex)

• Bright light → circular muscles of iris contract → pupil gets smaller (constriction).
• Parasympathetic nervous system controls this.

Dilation

• Dim light → radial muscles of iris contract → pupil gets larger (dilation).
• Sympathetic nervous system controls this.

🔹 Summary Table