nervous system function: senses, processes, interprets, and determines the response to stimuli from...
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Nervous System
FUNCTION: Senses, processes, interprets, and determines the response to stimuli from the environment
• Central Nervous System (CNS) - made of the brain and spinal chord
• Peripheral Nervous System (PNS) - all nerve cells outside of the CNS
Divisions of the Nervous System (see pages 388-89 for more detail on each)
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“Fight or flight” “rest & digest”
voluntaryinvoluntary
Motor division - relaysinformation to organs/glands
Sensory division - receivesInformation (senses)
Cells of the Nervous System
• Neurons - cells that conduct nerve impulses (action potentials) to communicate with organs and glands
• Neuroglia (glial cells) - support, protect and nourish neurons (do not send nerve impulses
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Structure of a Neuron
• Axon - transmits nerve impulses to communicate with other cells and organs
• Dendrites - receive signals from other neurons
• Myelin sheath - fatty coating on axon that speeds up action potential
• Nodes of Ranvier - gaps in the myelin sheath where the axon is exposed
• Cell body - part of neuron from which dendrites arise (also contains nucleus of cell)
• Axon terminals - end of axon/part that releases neurotransmitters to communicate with other neurons
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Functional Regions of the Neuron
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Receptive zone-Receivesinput from other neurons
Conducting zone-generates action potential(nerve impulse)
Secretory zone-Releasesneurotransmitters
Functional Classification of Neurons
Sensory (afferent) neurons• Receive information from the
environment (senses)Motor (efferent) neurons• Send signals to
muscles/glands/organs to carry out response
Interneurons• Relay signals between sensory
and motor neurons
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Nerves vs. Neurons
Nerves are bundles
of neurons
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Synapse - area where two or more neurons communicate with each other
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Action Potentials
• A brief reversal in charge across a membrane
• Happens in the axon membrane at Nodes of Ranvier (Saltatory conduction)
• Voltage gated ion channels for Na+ and K+ open and close in response to changes in membrane potential (charge)
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Action potentialis initiated by the axon hillock
Stages of an Action Potential
1) More Na+ outside cell/more K+ inside
2) Na+ enters the axon (charge becomes more positive - depolarization)
3) K+ leaves axon (charge becomes more negative)
4) Too much K+ has left (hyperpolarization - more negative than resting)
5) Na/K pump restores original conditions
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View action potential stages in action
Neurotransmitters
• Chemical messengers that cross the synapse allowing one neuron to communicate with another
• Can be excitatory (cause post-synaptic neuron to depolarize (become more positive))
• Can be inhibitory (cause post synaptic membrane to hyperpolarize (become more negative))
Neurotransmitters (cont.)
• Stored in vesicles in axon terminals
• Ca2+ rushes into the terminal in response to arriving action potentials
• Ca2+ causes vesicles to release neurotransmitters into synaptic cleft (example)
• Neurotransmitters bind to their receptors on the post-synaptic membrane
• Neurotransmitters are broken down by enzymes in the synaptic cleft or are taken back up by the pre-synaptic neuron via transporter proteins
Neurotransmitters (cont)
Examples:
• GABA (inhibitory)
• Glutamate (excitatory)
• Dopamine, serotonin, norepinephrine (excitatory or inhibitory depending on the nature of the synapse)
• Over 50 identified
IPSP vs. EPSP
• Inhibitory post-synaptic potentials (IPSP) decrease the likelihood of the post-synaptic neuron sending an action potential (hyperpolarizes post-synaptic neuron: lets Cl- in or lets K+ out)
• Excitatory post-synaptic potentials (EPSP) increase the likelihood of the post-synaptic neuron sending an action potential (depolarizes post-synaptic neuron: lets Na+ in)
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Summation
• Additive effect of the inputs of all pre-synaptic neurons
• If there are more excitatory than inhibitory signals, then depolarization may occur and an action potential may be sent by the post-synaptic neuron
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Long-term Potentiation (LTP)
• LTP is the long-lasting strengthening of synapses between two neurons
• Post-synaptic neurons become more sensitive to neurotransmitters coming from the pre-synaptic neuron(s) by:
1) making more receptors
2) increased sensitivity of existing receptors
• Involved in learning and memory formation
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BEFORE:Glutamate (excitatory neurotransmitter) stimulates NMDA receptors (in green) at a high frequency
AFTER:Because of the frequency of stimulation, there is an increase in the number and sensitivityof receptors on the post-synaptic neuron (increasing the strength of the synapse)
What causes this to happen?
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