nervous system

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Fig: Nervous system ,[object Object],[object Object],[object Object]

[object Object],The neuron is the functional unit of a nervous system. It connects with many other neurons and receives, conducts and transmits information. This figure illustrates a neuron. Although specialized in shape, it possesses the same basic structures found in other cells: nucleus, mitochondria, cell membrane and so on. Fig: Neuron

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[object Object],Fig: Three types of neurons Neurons can be classified according to how many processes they have: 1. Unipolar neuron 2. Bipolar neuron 3. Multipolar neuron

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[object Object],[object Object],[object Object],[object Object],Why have synapses? ,[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],Fig: Main structures at a synapse Fig: The sequence of events in chemical transmission at a synapse

Structure of the synapse ,[object Object],[object Object],[object Object],[object Object],What happens at the synapse? An action potential arrives at the synaptic bulb. This opens calcium channels in the presynaptic membrane. As the Ca 2+ ion concentration inside the membrane is lower than outside, Ca 2+ ions rush in. As their concentration increases, synaptic vesicles move towards the membrane, fuse with it and release neurotransmitter into the synaptic cleft. The neurotransmitter binds to receptors on the post synaptic cell membrane, some neurotransmitters open sodium channels in the post synaptic membrane causing an inflow of Na + ions. It makes the membrane more receptive to other incoming signals. Other neurotransmitters open chloride and potassium channels causing Cl - ions to flow into the cell and K + ions to flow out. It makes post synaptic membrane less receptive to incoming signals.

[object Object],A reflex action is behavior which a stimulus results in a response which does not have to be learned and which occurs very quickly without conscious thought. For example, a person does not have to learn or even think what to do when his hand accidentally touches a very hot object, he automatically pulls his hand away. Such responses are built into the nervous system from birth. Withdrawal from a painful stimulus is an example of a spinal reflex. The nerve impulses involved in it pass through the spinal cord along spinal nerves. The pathway of these impulses is illustrated very simply in the following figure.

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nervous system

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