Neurons and Synapses Lecture Note

Topic 6.5: nEuROnS & SynAPSES Nervous System Structure of a Motor Neuron The nervous system consists of two main divisions: • Central nervous system (CNS) = brain and spinal cord • Peripheral nervous system (PNS) = peripheral nerves Dendrite Soma (cell body) Axon The nervous system is composed of specialised cells called neurons that function to transmit electrical signals The CNS coordinates sensory & motor signals from the PNS • Sensory neurons send signals to the CNS (afferent pathway) • Motor neurons send signals from the CNS (efferent pathway) • Relay neurons (interneurons) send signals within the CNS Axon terminal Myelin sheath Direction electrical impulse travels Membrane Potentials Myelination Neurons have a difference in charge across their membranes due to the distribution of positively-charged ions (Na+ / K+) Nerve impulses are action potentials propagated via axons • Action potentials are ‘all or none’ and are only propagated if a certain threshold potential is reached (~ -55mV) Electrical signals are created by changing membrane polarity • Polarity of a neuron at rest is the resting potential (-70mV) • Polarity of a firing neuron is the action potential (+30mV) Nerve Impulses The resting potential is maintained by a Na+/K+ pump • It exchange sodium ions (3 out) and potassium ions (2 in) so that the membrane potential becomes slightly negative An action potential changes the resting membrane potential • The opening of sodium channels causes a sodium influx • This creates a positive membrane potential (depolarisation) • Opening potassium channels causes a potassium efflux • This restores a negative membrane potential (repolarisation) Membrane Potential (mV) The ion distribution must be restored to original conditions before a neuron can fire again (this is the refractory period) 30 20 10 0 - 10 - 20 - 30 - 40 - 50 - 60 - 70 - 80 In certain neurons, the axon is covered by a myelin sheath • This enables saltatory conduction (⬆ transmission speed) • The action potential ‘hops’ between gaps in the myelin sheath (called nodes of Ranvier) for faster transmission Synaptic Transfer Synapses are the physical junctions between two neurons • Electrical impulses cannot cross these physical gaps Neurons release neurotransmitters into the synapse cleft • Depolarisation in axon terminals opens Ca2+ channels • Ca2+ influx causes vesicles containing neurotransmitters to release their contents into the synapse (via exocytosis) • Neurotransmitters bind receptors on post-synaptic cells and generate graded potentials (excitatory or inhibitory) • The summation of these graded potentials determines if the post-synaptic neuron (or effector cell) is activated Neonicotinoid Pesticides Threshold Potential Resting Potential Depolarisation Repolarisation Refractory Period Acetylcholine is a neurotransmitter used in CNS and PNS • It is broken down in synapses by acetylcholinesterase • This prevents the overstimulation of the receptors Neonicotinoid pesticides irreversibly bind to acetylcholine receptors and cannot be digested by acetylcholinesterase • Insects have higher levels of these types of receptors • This makes neonicotinoids highly effective pesticides