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Transcript
Nervous System Endocrine and nervous systems cooperate to maintain homeostasis Overview • Sensory input • Integration • Motor output CNS • Brain and spinal cord • Integration of sensory input • Association stimuli with appropriate motor output PNS • Network of nerves extending through out body • Carry sensory input to CNS • Carry motor output from CNS Unit of Structure -- neuron • Cell body • Dendrite – signals to cell body • Short, branched (surface area) Axons • • • • • • signals away from cell body Long single process Schwann cells myelin sheath Axon hillock – where impulses generated Telodendria tipped with synaptic terminals Release neurotransmitters Types of neurons • Sensory neurons • Interneurons • Motor neurons Supporting Cells • Reinforce, protect, insulate and assist neurons • Do not conduct impulses • Much greater in number than neurons Glial cells – supporting cells of the CNS • • • • Astrocytes Encircle capillaries in the brain Contribute to blood-brain barrier Restricts passage of most substances into CNS • Oligodentrocytes-forms myelin sheaths in CNS Schwann Cells—supporting cells of PNS • Grow around axon forming concentric layers • Provides electrical insulation • Membranes mostly lipidspoor conductors • Increases speed of nerve impulse propagation • MS-deterioration of myelin sheaths Electrical Membrane Potential • Membrane potential range: -50 to – 100mV • Indicates the charge of cytoplasm side of membrane compared to extracellular fluid • -70mV is a resting neuron Membrane Potential results from: 1. Differences in the ionic composition of the intracellular and extracellular fluids • Principal cation INSIDE cell is K+ • Principal cation OUTSIDE cell is Na+ • Principal anions inside cell: proteins, amino acids, sulfate, phosphate • Principal anions outside cell: Cl- 2. Selective permeability of the plasma membrane • Ions cannot readily diffuse through hydrophobic core of phospholipid bilayer • Ion channels - carrier mediated transport • Large anions inside cell cannot cross membrane Ion Channel • Allows specific ion to cross membrane • Passive - open all the time • Gated – require stimulu to change conformation to open • Selective for specific ion – Na+, K+, Cl• Membrane permeability is a function of TYPE and NUMBER OF ION CHANNELS Maintaining Membrane Potential • • • • • • • K+ diffuses out of cell Down concentration gradient Membrane has high permeability to K+ Transfers positive charges to outside Cell’s interior becomes more negative K+ move into cell down electrical gradient Na+ move into cell down both gradients What prevents weakening of electrical gradient? • Sodium-potassium pump • Uses ATP • Pumps Na+ out gradients of cell against both • Pumps K+ into cell restoring concentration gradient Membrane potential changes • All cells exhibit membrane potential • Only neurons and muscles cells can change membrane potentials in response to stimuli • These are excitable cells • Resting potential – unexcited state, at rest Gated ion channels change the membrane’s permeability • Effect on the neuron depends on type of gated ion channel the stimulus opens • Stimuli that open K+ channels • Hyperpolarize the neuron • K+ effluxes from the cell • Increases the electrical gradient • more negative inside cell Stimuli that open sodium channels • • • • Depolarize the neuron Na+ influxes into the cell Reduce the electrical gradient Inside of cell becomes more positive Graded potentials • Voltage changes caused by stimulation • Depends on strength of stimuls • Threshold stimulus must be above this critical intensity to stimulate the axon • If depolarization reaches threshold • Then cell triggers an action potential Action Potential • Rapid change in membrane potential