Neurons and Neural Transmission

1. Neurons and Neural Transmission

2. Neurons – cells responsible for conducting information • Information transmission one-way process • Afferent (sensory neurons) • Efferent (motor neurons) • Nerve composed of axons/dendrites of many neurons + blood vessels, neuroglia cells • Neuroglia (Glia) – responsible for support of neurons

3. Most of a neuron’s organelles are in the cell body • Most neurons have dendrites, highly branched extensions that receive signals from other neurons • The axon is typically a much longer extension that transmits signals to other cells at synapses • An axon joins the cell body at the axon hillock

4. Conduction along axon: electrical ( disruption of chemiosmotic gradient) • Synapses: chemical – involves neurotransmitters

5. Resting Potential: 65mv • More negative ions inside cell than outside • Na ions concentrated outside cell/K ions inside – effect of Na/K pump • Membrane channels regulate movement of Na, K ions • Na-K pump concentrates Na outside membrane, K inside

6. Action potential: Messages transmission as change in membrane potential • Stimulus causes gated channels open causing depolarization of membrane • Sodium ‘gates’ open so Na+ moves into cell = depolarization Followed quickly by: • Potassium moves out of cell = repolarization • Refractory period • Once initiated action potential moves progressively away from cell body towards synaptic terminals

8. The speed of an action potential increases with the axon’s diameter • In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase • Myelin sheaths are made by glia— oligodendrocytesin the CNS and Schwann cells in the PNS • Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found • Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

9. Neurons communicate with other cells at synapses • Most are chemical • The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal • The action potential causes the release of the neurotransmitter • The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell • Excitory neurotransmitter  dopolarization • Inhibitory neurotransmitter  hyperpolarization

10. Fig. 48-14 Postsynaptic neuron Synaptic terminals of pre- synaptic neurons 5 µm

11. Neurotransmitter in synaptic vesicles located in the synaptic terminal of presynaptic neuron released at arrival of action potential • The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell

12. Signal Transmission at Synapse • Electrical Signal is transformed into Chemical Signal = Neurotransmitter • Electrical signal arrives at axon bulb causes opening of Calcium gates and Ca enters cell • Excess Ca causes release of neurotransmitter stored in cell (inside vesicles) into synaptic cleft • Neurotransmitters diffuse across cleft and interact with receptors on next neuron

13. Signal Integration • Many neurons pass signal on to next neuron in the pathway • Some send excitatory signals (cause depolarization of neuron) • others send inhibitory signals (cause hyperpolarization or more negative charge inside neuron) • all or none response: sum of excitatory signals and inhibitory signals determines if an action potential is generated in the next neuron and signal is continued • Type signal depends on type neurotransmitter released

15. Synapses are interaction/integration points • 50+ positive impulses required to fire interneuron • integration involves summing of positive and negative signals • positive signals result in depolarization • negative signals result in hyperpolerization

16. Importance of Neurotransmitters • Many produced ~ 50 • Impact mood, memory, ability • Used in different behavioral systems • Norepinephrine – ‘good’ feeling • Serotonin – well being • Dopamine – emotions

17. Neuortransmitter removed quickly from synapse • Effect of neurotransmitter lasts as long as it is present • Removed by: • Destruction – enzymes • Recycling – returned to axon tip

18. Example of a Neurotransmitter • Acetylcholine • Destroyed by acetylcholinesterase • Stimulates muscle contraction • Pesticides inhibit enzyme – acetylcholinesterase • Result of pesticide poisoning is spasms, death

19. Neurotransmitters – Another Example • Depression: • due to low: • Serotonin • Dopamine • Norepinephrine

20. Depression • Treatment: Drugs that inhibit breakdown of serotonin • Specific action: • Prozac • Zoloft • Paxil

23. The End.