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Transcript
Synaptic Transmission Impulses Along a Neuron • Dendrites receive the nerve impulse and carry it toward the cell body, which contains the nucleus. • The axon carries the impulse from the cell body toward the synaptic knobs where it will be transferred to other neurons. Transmission at the Synapse • There is a tiny gap between the synaptic knobs of one neuron and the dendrites of the next one. • This gap is called the synapse or synaptic cleft. • The nerve impulse needs to cross this gap and it does so by the release of special chemicals called neurotransmitters. The Synapse Neurotransmitters 1. Are released from the presynaptic knobs by exocytosis (active) 2. Diffuse across the synapse (passive) 3. React with special receptors in the dendrites of the postsynaptic neuron 4. This causes sodium gates to open, an action potential starts and the nerve impulse continues 5. Once the neurotransmitters have passed along the impulse, the “extra” neurotransmitters still remaining in the synapse are either: a) broken down by enzymes or b) transported back into vesicles in the synaptic knob by endocytosis. Synapse animation Structure of the Synapse Diagram 1. 2. 3. 4. axon mitochondria synaptic knob vesicles containing neurotransmitters 5. synapse 6. neurotransmitter 7. receptors Diagram of Synapse 1.________________ 2.________________ 3.________________ 4.________________ 5.________________ 6.________________ 7.________________ Neurotransmitters, cont’d • • There are excitatory neurotransmitters which start impulses in neighboring neurons Ex: acetylcholine (Ach) – opens sodium ion channels on postsynaptic neurons, broken down by cholinesterase Neurotransmitters, cont’d • Inhibitory neurotransmitters which prevent impulses (ex. serotonin, dopamine). They make the postsynaptic membrane more permeable to K+ so that the membrane becomes even more negative in its resting state (called hyperpolarized) Neuron Impulse and Synaptic Transmission Neurotoxins Neurotoxins • Many animals (snakes, spiders, pufferfish, scorpion) use neurotoxins as a form of defence. • Most act to block or open Na+ or K+ channels or block the release of acetylcholine.
