April 22 2015
The Neuron & Nerve Impulses

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Two Types
Neurons
◦ Nerve impulse conduction
◦ AP (action potential)
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Neuroglia
◦ Support & nourish neurons

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Three Main Parts
1. Cell Body
2. Dendrites
3. Axon

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Nucleus
Organelles such as mitochondria, lysozomes

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Many branched extensions off of cell body
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RECEIVE
SIGNALS/INPUTS

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Long & thin extension off cell body
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Conducts nerve impulses toward another neuron, a muscle fiber, or a gland cell (effector cells)
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Axons end by dividing into many fine processes called axon terminals
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Nerve impulses travel from cell body to the axon terminals

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Myelin Sheath
Most axons are surrounded by the myelin sheath
Myelin sheath is made up of lipids & proteins
Insulates the axon of the neuron
Increases the speed of the signals that travel down the axon

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The site where two neurons meet (or a neuron meets an effector cell) is called a
Synapse

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These cells are smaller than neurons
Much more numerous
Do not conduct impulses
Six types of neuroglia cells

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Cells, like neurons, have a difference in the charge within their plasma membrane versus the charge outside
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The voltage across the plasma membrane when the neuron is not conducting an AP is the resting membrane potential
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At rest, the inside of the neuron has a negative charge and the outside of the neuron has a positive charge
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The flow of ions is the current carried between cells in living tissues

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Potassium (K+) and Sodium (Na+)
2 types of Ion Channels
◦ Leakage Channels
◦ Gated Channels
Leakage channels – small, constant stream of ions
Gated channels – open in response to a change in membrane potential

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A stimulus causes a change in the resting membrane potential
If the resting membrane potential reaches a critical level (threshold) an action potential begins
Two Phases
1. Depolarization – the negative membrane potential becomes less negative, passes 0, and become positive
2. Repolarization – the membrane/neuron is returned to its resting potential

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First opens voltage-gated Na+ channels
Na+ rush into the cell
Voltage-gated K+ channels open more slowly
K+ open as Na+ channels close
K+ ions flowing out of the cell allow repolarization

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As long as a stimulus meets the threshold, an action potential will occur
A stronger stimulus does not cause a larger action potential
Size of AP is always the same

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Nerve impulses travel from the area where the axon arises, down the axon, to the axon terminals
At the end of each axon terminal is a synaptic end bulb
The synaptic bulb of one neuron meets (but does not touch) a dendrite of another at the
SYNAPSE

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Depolarization causes a series of events that results in the release of neurotransmitters from the axon terminal into the synaptic cleft
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The neurotransmitters bind to receptors on the dendrites
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Neurotransmitters cause gates to open in the post-synaptic neuron, and the signal continues