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Download Topic 6.5 Neuron and Synapses
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Neurons and Synapses 6.5 The Nervous System • Composed of cells called neurons. • These are typically elongated cells that can carry electrical impulses at very high speeds. • The nervous system can be broken down into two major parts: – Central Nervous System (CNS) • Brain and Spinal cord – Peripheral Nervous System (PNS) • Peripheral nerves that connect all parts of the body to the central nervous system. The Nervous System Nerve Impulse • Neurons transmit messages in the form of nerve impulses. • The impulses are electrical but involve the movement of charged ions, not electrons. • Nerve impulse can travel along a neuron as high as 100 meters per second. Neurons Neurons • Dendrites receive chemical signals and produce an electrical signal • Axon transmits message • Axon covered with a layer of fat called myelin • Myelin does not completely cover the axon. • Gaps are called nodes of ranvier • Speeds up rate of impulse transmission • Saltatory conduction Resting Potential • Neurons pump ions across their plasma membranes by active transport. • Sodium is pumped out of the neuron and potassium is pumped in. • Concentration gradients of both sodium(higher outside) and potassium(lower outside) are established across the membrane. • The inside of the neuron develops a net negative charge compared with the outside. • This is the resting potential across the plasma membrane of a cell that is not conducting an impulse. Action Potentials • An action potential is the reversal and then the restoration of the electrical potential across the plasma membrane of a cell as an electrical impulse passes along it. • An impulse passing along the neuron allows sodium and potassium ions to diffuse across the membrane (through voltage gated ion channels). Action Potential / Net Charges Stage 1 in the Passage of a Nerve Impulse • An action potential in one part of a neuron causes an action potential to develop in the next section of the neuron. • This is from the diffusion of sodium ions between the region with an action potential and the region at the resting potential. • If the potential rises above the threshold level, voltage gated channels open. Stage 2 in the Passage of a Nerve Impulse : Depolarization • Sodium channels open and sodium diffuse down the concentration gradient • This reduces membrane potential and causes more sodium channels to open. • Entry of positively charged sodium ions into the neuron develops a net positive charge. • Depolarization of the membrane occurs reversing the membrane potential Stage 3 in the Passage of a Nerve Impulse : Repolarization • Potassium channels open after a short delay. • Potassium ions diffuse out of the neuron down the concentration gradient through open channels. • Exit of positively charged potassium ions develops a net negative charge inside the cell compared to outside. • Repolarization is the restoration of the membrane potential to its original resting state. Stage 4 in the Passage of a Nerve Impulse (refractory) • Concentration gradients of sodium and potassium are restored. • This is done through the removal of sodium from the neuron by active transport and the addition of potassium through active transport. • This restores the resting potential of that part of the neuron. now again ready to conduct another nerve impulse. • Sodium ions diffuse along inside the neuron from an adjacent region that has already depolarized and initiate depolarization. Action Potential / Net Charges Nerve Synapse • A synapse is a junction between two nerves. • The plasma membranes of either neuron is separated by a narrow fluid gap called the synaptic cleft. • Messages are passed across the synapse in the form of chemicals called neurotransmitters. • Neurotransmitters always pass in the same direction from the pre-synaptic neuron to the post-synaptic neuron. Synaptic Transmission Synaptic Transmission • Stage 1: action potential reaches the axon terminal of the pre-synaptic neuron • Stage 2: calcium channels open and Ca++ ions diffuse into axon terminal • Stage 3: Ca++ influx causes vesicles with neurotransmitters to fuse with the membrane, releasing their contents by exocytosis Synaptic Transmission cont. • Stage 4: neurotransmitters diffuse across the gap (synaptic cleft) and bind to receptors of the post-synaptic neuron • Stage 5: binding of neurotransmitters on receptors cause sodium ion channels to open triggers an action potential • Stage 6: neurotransmitters are either broken down or taken back up by the pre-synaptic neuron Neural Transmission • Both nerve impulses and synaptic transmissions have a threshold potential • If this threshold is not reached, the impulse stops • A typical post-synaptic neuron will be in communication with many pre-synaptic neurons Acetylcholine • Involved with contraction of muscles • If acetylcholine triggers an impulse, the muscle contracts (black widow) • If not enough acetylcholine, muscle does not contract (botox) • Acetylcholinesterase is an enzyme that breaks down the neurotransmitters Insecticide use • Neonicotinoids are synthetic compounds that bind to acetylcholine receptors • End up blocking the action of acetylcholine • Works on insects more so than mammals • Used as an insecticide • Some controversy as to whether it is now disrupting some ecosystems (honeybees)