2.24.10 - Nervous systems To review: neurons receive...

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Nervous systems To review: neurons receive messages as generator potentials , at synapses on the dendrites or soma (cell body). -- may be excitatory or inhibitory neurons send messages as action potentials , which self-propagate down axons to target cells. -- action potentials are ‘all or nothing’ -- all action potentials on a particular neuron are identical . therefore, a neuron can send different messages only by varying the number or frequency of action potentials. Nervous systems Neurons can send different messages only by varying the number or frequency of action potentials: 0 mV -100 Time Message #1 Message #2 Message #3 0 mV -100 0 mV -100 At any one synapse, these messages are either excitatory or inhibitory Nervous integration How does a nervous system (a bunch of neurons connected together) process information and make ‘decisions?’ Fundamental question is: What causes a neuron to trigger an action potential? this happens when membrane potential at the base of the axon exceeds threshold voltage. -- many neurons spontaneously generate APs, but this output can be modified . the generator potential coming from inputs to dendrites and soma influences membrane potential at the base of the axon: This is the sum (integration) of all the excitatory and inhibitory generator potentials on the dendrites and soma Nervous integration How do generator potentials from dendrites and soma influence action potentials? They affect membrane potential at base of axon Lots of excitatory inputs --> depolarization --> greater probability for an AP Lots of inhibitory inputs --> hyperpolarization --> lower probability for an AP A mix of excitatory and inhibitory inputs produces an intermediate generator potential - - - - - - - + + + + + + +
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Nervous integration What affects the influence of a particular input to dendrites or soma on the probability of action potentials? 1. Total number of synapses: the higher the number, the less influence each one has. 2. Location: the nearer a particular synapse is to the base of the axon, the more influence it has. 3. The number and timing of action potentials at each synapse: dendrites and soma integrate over both time and distance . - - - - - - - + + + + + + + Anatomy of nervous systems Simple radially -symmetrical animals (cnidarians) have a decentralized nerve net extending throughout the body. Bilaterally symmetrical animals usually cephalized, with centralization of the nervous system: bundles of nerve cells called ganglia. ganglia usually distributed around the body (often in each segment in segmented animals) A large ganglion in the head is the brain. Anatomy of nervous systems Simple radially -symmetrical animals (cnidarians) have a decentralized nerve net extending throughout the body. Vertebrate nervous systems contain three kinds of neurons: sensory neurons gather information and bring it into the nervous system interneurons connect one neuron to another (most central processing of information) --> most of brain motor neurons carry information to effector organs Anatomy of nervous systems Central Nervous System (CNS): brain and
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2.24.10 - Nervous systems To review: neurons receive...

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