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An axon can fire an action potential at a node of Ranvier, but that action potential cannot be conducted through the adjacent patch of axon covered with myelin. The positive charges that flow into the axon at the node, however, spread down the axon. When this spread of current causes the plasma membrane at the next node of Ranvier to depolarize, an action potential is fired at that node. Action potentials therefore appear to jump from node to node down the axon. This form of impulse propagation is called saltatory conduction which is much quicker than propagation down unmyelinated axons. The most remarkable abilities of nervous systems stem from interactions of neurons.
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Cell body- contains the nucleus and most of the cell's organelles Dendrites- projections that sprout from the cell body that bring information from other neurons/sensory cells to neuron's cell body Axon- a projection that is longer than dendrites that carries information away from the cell body. This information is conducted along the axon to the cell that is its target. At the target cell the axon divides into fine nerve endings. At the tip of each ending is a swelling called an axon terminal or synaptic terminal.
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Electrical synapses, or gap junctions, are completely different from chemical synapses because they join neurons electrically The membrane proteins of the two neurons form connexons- molecular tunnels that bridge two cells through which ions and small molecules and readily pass. Electrical transmission across gap junctions is fast and can proceed in either direction.
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How does an action potential move over long distances? Once positive ions are inside the axon, they spread to adjacent regions of the axon making those regions less negative. If this depolarization of the adjacent region of plasma membrane causes the opening of sodium channels, there is an action potential. Neurons are not the only type of cell in the nervous system. There are also glial cells
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If some potassium channels in the plasma membrane opened, K+ ions would diffuse out of the cell. This is due to their higher concentration on the inside of the cell compared to the outside As a result of the exit of K+ ions out of the cell, the plasma membrane would become hyperpolarized- more negative, less positive
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If the postsynaptic neuron's response to a neurotransmitter is depolarization, as at the neuromuscular junction, the synapse is excitatory. If the postsynaptic neuron's response to a neurotransmitter is hyperpolarization, the synapse is inhibitory. The postsynaptic membrane sums excitatory and inhibitory input to "decide" whether or not to fire action potentials. For most neurons the critical area for "decision making" is the axon hillock- the region of the cell body at the base of the axon. The axon hillock is unmyelinated.
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Most neurons have four regions: 1. a cell body 2. dendrites 3. an axon 4. axon terminals
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Neurotransmitter is released from the presynaptic membrane when an action potential arrives at the axon terminal. The action potential causes calcium channels to open and because Ca2+ ions are in greater concentration outside the cell than inside the cell, Ca2+ ions rush into the cell. The increase in Ca2+ inside the cell causes the vesicles full of acetylcholine to fuse with the presynaptic membrane and spill their contents into the synaptic cleft. The acetylcholine molecules diffuse across the cleft and bind to the receptors on the motor end plate. This causes the sodium channels to open and depolarize the postsynaptic membrane.
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Schwann cells- a type of glial cell that wraps around the axons of the peripheral nervous system Oligodendrocytes- glial cells that wrap around axons of the central nervous system The covering produced by Schwann cells and oligodendrocytes is called myelin. These wrappings of myelin are not continuous. There are regularly spaced gaps called nodes of Ranvier where the axon is not covered
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Sudden changes in ion channels generate action potentials (reversals in cell charge) 1. At a normal membrane resting potential sodium channels are mostly closed 2. When sodium channels are caused to open, Na+ from the outside enters the cell and makes the plasma membrane positive What causes the return to resting potential? 1. Sodium channels close so Na+ can no longer enter the cell 2. Potassium channels open so K+ can exit
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Synapses- junctions where one cell influences another cell directly through the transfer of a chemical or an electric message Chemical Synapse The synaptic junctions between neurons and muscles are called neuromuscular junctions. At the very end of the axon terminal is an enlarged knob that contains spherical vesicles filled with chemical messenger molecules called neurotransmitters. Neurotransmitters such as acetylcholine are synthesized in the axon terminal and packed in the vesicles by cell produced enzymes. The portion of the axon terminal plasma membrane that is in closest contact with the muscle is called the presynaptic membrane. The postsynaptic membrane of the neuromuscular junction is a modified part of the muscle cell called a motor end plate. The space between the presynaptic membrane and the postsynaptic membrane is called the synaptic cleft.
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The difference in electric charge across the plasma membrane of a neuron is called its membrane potential (or resting potential). A neuron is sensitive to any chemical or physical factor that causes a change in the resting potential.
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The most extreme change in membrane potential is the nerve impulse- an electric signal that can briefly reverse the charge. This nerve impulse that briefly reverses the charge is also called an action potential. The major ions that carry electric charges across membranes of neurons are sodium (Na+), chloride (Cl-), potassium (K+), and calcium (Ca2+). The plasma membranes of neurons, like those of all other cells, are lipid bilayers that are impermeable to ions. However, these impermeable lipid bilayers contain protein molecules that serve as ion pumps and ion channels.
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A sessile animal can process information by a simple network of neurons A complex animal needs to process information by a cluster of neurons called ganglia. Ganglia that are large and in a special location are called a brain. The brain and spinal cord are called the central nervous system (CNS). Neurons that are outside of the CNS are called the peripheral nervous system. Even though nervous systems vary in structure function, neurons function almost identical in all animals. This is because the plasma membrane of neurons can generate electric signals called nerve impulses.
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A sodium-potassium pump expels three Na+ ions from inside the cell for every two K+ ions that it brings in from outside the cell. Gated ion channels or Ion channels- pores formed by proteins in the lipid bilayer that may be selective- allow only one type of ion to pass through. If some sodium channels in the plasma membrane opened, Na+ ions would diffuse into the cell. This is due to their higher concentration on the outside, and to their attraction to the negative charge in the cell due to proteins. As a result of the entry of Na+ ions into the cell, the plasma membrane would become depolarized- less negative or more positive.
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purpose of the nervous system
to process information
