Nerves Chapter 48

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NervesChapter 48

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General Nervous System Functions

• Control of the internal environment
• Nervous system works with endocrine system
• Difference?
• Voluntary control of movement
• Spinal cord reflexes
• Assimilation of experiences necessary for memory
  and learning

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Organization of the Nervous System

• Central nervous system (CNS)
• Brain and spinal cord
• Peripheral nervous system (PNS)
• Neurons outside the CNS
• Sensory division
• Afferent fibers transmit impulses from receptors
  to CNS
• Motor division
• Efferent fibers transmit impulses from CNS to
  effector organs
• Somatic system to muscle
• Autonomic system

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Structure of a Neuron

• Cell body
• Dendrites
• Conduct impulses toward cell body
• Axon
• Carries electrical impulse away from cell body
• May be covered by Schwann cells
• Form discontinuous myelin sheath along length of
  axon

• Synapse
• Contact points between axon of one neuron and
  dendrite of another neuron

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Figure 48.5 Schwann cells
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Neuronal diversity
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Squid Giant axon
1mm diameter Many cm long
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Electrical Activity in Neurons

• Resting membrane potential
• At rest, neurons are negatively charged
• Action potential
• Potential across membrane changes

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Figure 48.6 Measuring membrane potentials
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Sodium (Na) Potassium (K) pumpA reminder!

• Cells maintain low intracellular Na
• 440mM outside, 50 mM inside
• Cells maintain high intracellular K
• 560mM inside, 90mM outside
• Ions cannot diffuse through lipid bilayer
• Sodium-Potassium dependent ATPase

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Figure 8.15 The sodium-potassium pump a
specific case of active transport
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Resting Potential
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Resting Potential

• Resting potential
• Donnan Equilibrium
• -70mV
• Determined by balance between sodium and
  potassium concentrations
• Sodium/Potassium pumps
• Separate sodium and potassium channel proteins

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Resting Potential
Negative charge
Positive charge
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Figure 48.10 The basis of the membrane potential
Resting Potential
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Figure 48.11 Modeling a mammalian neuron
Donnan Equilibrium
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Action potential
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Figure 48.12 Graded potentials and the action
potential in a neuron
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Figure 48.13 The role of voltage-gated ion
channels in the action potential (Layer 5)
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Figure 48.13 The role of voltage-gated ion
channels in the action potential (Layer 1)
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Figure 48.13 The role of voltage-gated ion
channels in the action potential (Layer 2)
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Figure 48.13 The role of voltage-gated ion
channels in the action potential (Layer 3)
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Figure 48.13 The role of voltage-gated ion
channels in the action potential (Layer 4)
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Figure 48.13 The role of voltage-gated ion
channels in the action potential (Layer 5)
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Figure 48.13 The role of voltage-gated ion
channels in the action potential (Layer 5)
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• Propagation of impulse
• Non-myelinated neurons
• Myelinated neurons

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Figure 48.14 Propagation of the action
potential-non myelinated neurons
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Figure 48.11 Saltatory conduction
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Synapses

• Electrical
• Gap junctions
• fast
• Chemical
• Neurotransmitters
• slow

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Figure 48.17 A chemical synapse

• Synaptic cleft approximately 20nm across
• Mucopolysaccharide glue
• Signal passes from pre-synaptic to post-synaptic
  cell

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Neurotransmitters and Synaptic Transmission

• Neurons communicate across synapses using
  neurotransmitters
• Released from presynaptic membrane
• Binds to receptor on post synaptic membrane
• Excitatory transmitters
• Cause depolarization (EPSP)
• Inhibitory transmitters
• Cause hyperpolarization (IPSP)

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Figure 48.12 Graded potentials and the action
potential in a neuron
IPSP EPSP
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Integration of excitatory and inhibitory inputs

• Spatial summation
• Excitatory and Inhibitory input

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Integration of multiple synaptic inputs
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Figure 48.12 Graded potentials and the action
potential in a neuron
Threshold exists at the Hillock
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Figure 48.18 Summation of postsynaptic potentials

• Excitatory transmitters
• Cause depolarization (EPSP)
• Inhibitory transmitters
• Cause hyperpolarization (IPSP)

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Neurotransmitters

• Must be released by pre-synaptic cell
• Must elicit effect on post-synaptic cell
• Agonists should work the same way
• Must be removed after action
• Can be small molecules or large peptides

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Table 48.1 The Major Known Neurotransmitters
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Elimination of Neurotransmitters

• Essential for transient signal
• Chemical breakdown
• Acetylcholinesterase
• Reabsorption
• Serotonin
• Dopamine

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