1 3 3 Neurons Chemical Synapses BIO 361T Fall 2014 1 In the following diagram is the grey neuron pre synaptic or post synaptic Explain your answer 2 both Cnidarians sea anemones jellyfish have neurons that can depolarize in any direction unlike those in the figure above Propose an evolutionary hypothesis to explain this observation They are radially symmetric so neural input needs to go in all directions as opposed to most animals which are bilaterally symmetric and have a head 3 As nervous systems become more complex and body size remains the same we find animals with more myelinated axons instead of large diameter axons like in the giant squid Propose a hypothesis to explain this observation More neurons in same space To increase conduction speed but not use up a lot of space myelinate axons instead of making them larger 4 All animals Metazoa have voltage gated Na channels with one exception the tiny 1 mm long nematode Caenorabdhitis elegans Propose an evolutionary hypothesis to explain this observation They are small enough that passive potential spread via electrotonic conduction is sufficient 5 Applying acetylcholine ACh to a skeletal muscle cell will cause it to contract Applying ACh to a cardiac heart muscle cell will cause it to relax Explain how ACh can have opposite effects on different cells Different receptors skeletal muscle has nicotinic cholinergic receptors which are excitatory cardiac muscle has muscarinic cholinergic receptors that are inhibitory not all muscarinic cholinergic receptors are inhibitory 6 If ACh is applied to a skeletal muscle and cardiac muscle simultaneously the skeletal muscle will contract milliseconds before the cardiac muscle relaxes Explain this relative latency in response Ionotropic receptors in skeletal muscle and metabotropic receptors in cardiac muscle 2 7 Chemicals that mimic the effect of a ligand binding a receptor are agonists of that receptor If they bind a receptor and have no effect except for blocking it they are referred to as antagonists Atropine is an antagonist for an inhibitory cholinergic receptor expressed by cardiac muscle Why is atropine administered during cardiac arrest Atropine blocks ACh from binding cholinergic receptors on the heart which would have an inhibitory effect slowing heart rate By preventing this inhibitory input it allows the heart rate to increase 8 Myasthenia gravis is a human auto immune disorder in which ACh receptors in skeletal muscle cells are destroyed Symptoms include muscle fatigue and weakness but these can be alleviated by an acetylcholinesterase ACE inhibitor a Explain how ACE inhibitors alleviate the symptoms of myasthenia gravis By preventing ACh degradation in the synapse it extends the life of ACh allowing it to bind more of the receptors present and strengthen chemical signals between the neuron and the muscle b Some chemical weapons and pesticides are also ACE inhibitors but at much higher concentrations What do you think the physiological effects of a high dose of ACE inhibitor are If the signal for muscles to contract is too strong they will contract uncontrollably causing twitching and paralysis
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