Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings The Synapse Key Stage 5 Biology

Learning Objectives  Be able to describe how an action potential causes activation of a synapse  Be able to explain how signals cross a synapse  Be aware of the different types of neurotransmitters  Be aware of the effect of drugs on synapses

What is a synapse  Synapses are the point where neurones connect to other neurones, sensory cells or effector cells (E.g. muscle)  A signal travels along a neurone as an…  …action potential  When that action potential reaches the synapse the signal becomes a chemical message

Synapses Layout  The synapse is made of several parts.  The Pre-Synaptic membrane in on the neurone where the signal is coming from  The Post-Synaptic membrane is on the neurone where the signal is going to  Between these two membrane is the synaptic cleft

Cell to Cell Conduction: the Synapse

Synapse Depolarisation  On the pre synaptic membrane are Calcium pumps which pump calcium out of the cell  When the action potential arrives at the synapse is causes a voltage gates calcium channel to open  This causes calcium to flood into the cell

Neurotransmitter Secretion  As the intracellular levels of calcium rise this triggers special synaptic vesicles containing neurotransmitters to fuse with the pre-synaptic membrane  This causes neurotransmitters to be released (by exocytosis) into the synaptic cleft  This exocytosis process uses ATP derived from mitochondria

Synapse Mechanism

Examples of Neurotransmitters  Acetylcholine  GABA  Dopamine  Serotonin  Glutamate  Glycine  Noradrenalin  Nitrous Oxide

Examples of Neurotransmitters

Neurotransmitter Receptors  Receptors on the post-synaptic membrane detect the neurotransmitters  Sodium channels are then opened which cause a sodium to enter the cell creating a threshold potential  This then sets off an action potential along the next membrane  To stop the neurotransmitters effects, enzymes (e.g. acetylcholinase) break them down. This also stop neurone cross firing

Acetylcholine synthesis

Types of Neurotransmitters  Excitatory Neurotransmitters cause the generation of an action potential.  E.g. Acetylcholine binds to cholinergenic receptors and is Excitatory  Inhibitory Neurotransmitters cause hyper- polarisation of the post synaptic membrane. This makes them harder to excite as the PD is reduced below -65mV  E.g. Noradrenalin binds to Adrenergic receptors and is inhibitory

Drugs and Poisons  Some drugs and poisons can effect the mechanism of neurotransmitters  Agonists are chemicals that mimic the effects of neurotransmitters.  E.g. Nicotine is a cholinergic agonist  Antagonists block the effect of neurotransmitters.  E.g. Curare is a poison which blocks cholinergic receptors causing muscle paralysis