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THE NERVOUS SYSTEM 35.2 The body’s communication system Controls and coordinates functions throughout the body. Responds to external and internal messages. NEURONS •Specialized cells of the nervous system • Transmit electrical signals called impulses in only one direction. •3 types – sensory neurons-send signals from sensory receptors brain – motor neurons – send signals from the brain muscles & glands – interneurons –carry impulses between sensory & motor neurons NEURON ANATOMY Nucleus Axon terminals Cell body Myelin sheath Nodes Axon Dendrites FUNCTIONS Dendrites – carry impulses from the environment to the cell body Axons – carry impulses away from the cell body Cell body – where the metabolic activity of the cell takes place Myelin sheath – an insulating membrane surrounding the axon NERVE IMPULSE – RESTING NEURON • A nerve impulse is similar to the flow of electrical current through a wire. • When the neuron is NOT sending an impulse, the inside is negative and the outside is positive Na+ and K+ ions move across the cell Membrane via the sodium-potassium pump The Moving impulse An impulse begins when a neuron is stimulated by another neuron or the environment Action potential - When an impulse is sent the charge reverses inside the cell – it becomes positive Threshold – the amount of stimulus required to activate the neuron The moving impulse An impulse begins when a neuron is stimulated strong enough to reach a threshold. The impulse travels rapidly down the axon towards the terminal end. THE SYNAPSE Direction of impulse axon • Where impulses are transferred from one neuron to another. •Chemicals diffuse across a small gap between the cells •Neurotransmitters- are the chemicals that transfer impulses from one neuron to another. •Receptors of the neighboring neuron pick up the chemical message, and a new impulse begins. vesicle Dendrite of Adjacent neuron receptor Axon terminal Neurotransmitter In a fraction of a second after binding to the receptors, the neur0transmitter molecules are released. They are re-absorbed back into the axon terminal Review questions What are the 4 parts of a neuron? What is a threshold? How does a message move from one neuron to another?