Download 53 XIX BLY 122 Lecture Notes (O`Brien)

XIX BLY 122 Lecture Notes (O’Brien) 2009
Chapter 45—Electrical Signals in Animals
I.
Key Concepts
A. Neuron
1. Cell that transmits electrical signals
2. Membrane potential = electrical potential due to differences in concentrations of ions
on either side of a neuron’s plasma membrane.
3. Action potential = electrical signal
a. All-or-none change in membrane voltage at plasma membrane
b. Inflow of sodium ions (Na+) is followed by outflow of potassium ions (K+)
B. Synapse
1. Connection between two neurons
2. Electrical signal from one neuron is converted to a chemical signal = neurotransmitter
3. Neurotransmitters cross space between neurons & cause changes in 2nd cell’s
membrane potential
C. Organization of vertebrate nervous system
1. Central nervous system (CNS) contains brain and spinal cord
2. Peripheral nervous system (PNS) receives sensory information that it sends to CNS
3. CNS makes “decisions” and sends signals to body along PNS neurons
II.
Principles of Electrical Signaling (45.1)
A. The anatomy of a neuron
1. Neurons consist of a cell body, dendrites, and one or more axons. Fig 45.3a
2. Neurons transmit information via electrical impulses. Fig 45.1a
a. Sensory receptors transmit information about the internal or external
environment to sensory neurons.
b. Sensory neurons connect to neurons in the central nervous system (CNS).
c. The central nervous system integrates information and activates motor
neurons.
d. Motor neurons transmit information to effectors in glands or muscles.
e. Reflexes are rapid responses that connect sensory neurons to motor
neurons in a way that bypasses the brain. Fig 45.1b
B. An introduction to membrane potentials and Resting Potential
1. An imbalance of ions across a cell membrane creates an electrical potential.
a. Voltage is a measure of the charge difference across a membrane.
b. Ions move across membranes in response to electrochemical gradients
2. The Resting potential is the membrane potential when sodium (Na+), potassium
(K+), and Chloride (Cl–) ions are at equilibrium
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For many neurons this is –70 mV. Fig 45.4
III. Dissecting the Action Potential (45.2)
A. Distinct ion currents are responsible for depolarization and repolarization Figs 45.5 &
45.6
1. Each action potential in a neuron has the same magnitude and duration.
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2. The frequency of action potentials, not their duration, is important.
3. Depolarization is caused by an influx of sodium ions through a channel.
4. Repolarization results from an efflux (= exit) of potassium ions from a channel.
B. How Do Voltage-Gated Channels Work?
1. Na+ and K+ channels open and close in response to changes in voltage.
2. Changes in charges at the membrane surface alter the conformation of the
channel proteins.
a. Voltage-clamping studies—depolarization triggers the rapid opening of Na+
channels and the slower opening of K+ channels. Figs 45.7, 45.8
b. Outward flow of K+ occurs after Na+ entry has dissipated the Na+ gradient.
3. Patch clamping and Studies of Single Channels
a. Three Na+ ions are transported out for every two K+ ions transported in. Fig
45.9
b. The pump prevents dissipation of ion gradients due to action potentials.
C. How is the Action Potential Propagated?
1. Depolarizing voltages open Na+ channels, which causes more depolarization.
2. More depolarization opens more Na+ channels.
3. The influx of Na+ causes positive charges to spread away from Na+ channels.
4. Positive feedback occurs, which causes more Na+ channels to open.
5. The action potential does not propagate back up the axon. Fig 45.11
6. Speed of propagation is enhanced by either large axon size or myelination. Fig
42.12
IV.
The Synapse (45.3)
A. Neurotransmitters and the Synapse
1. Neurotransmitters are stored in synaptic vesicles at the ends of axons. Fig.
45.14
2. The action potential causes calcium channels to open on pre-synaptic
membranes.
3. Calcium triggers exocytosis of neurotransmitters into the synapse. Fig 45.15
4. Receptors for neurotransmitters are on post-synaptic membrane.
5. An action potential can only cross in 1 direction at a synapse
B. Postsynaptic Potentials and Summation
1. Changes in membrane potential that INCREASE the probability of an action
potential are excitatory postsynaptic potentials (EPSPs).
2. Changes in membrane potential that DECREASE the probability of an action
potential are inhibitory postsynaptic potentials (IPSPs).
3. EPSPs and IPSPs are not all-or-none events. Fig 45.16
4. EPSPs and IPSPs are summed at the axon hillock. Fig 45.17
V. The Vertebrate Nervous System (45.4)
A. What Does the Peripheral Nervous System (PNS) Do? Fig. 45.18
1. The central nervous system (CNS) is made up of the brain and spinal cord
2. The PNS is made up of neurons outside the CNS
3. The autonomic system of the PNS controls involuntary responses. Fig 45.19
a. Parasympathetic Nerves slow activities, "rest and digest" responses
b. Sympathetic Nerves speed up activities, “fight or flight” responses
c. The somatic system controls voluntary muscle activity
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B.
Functional anatomy of the CNS.
1.
The brain is the most complex organ known. Fig 45.20
a.
Pons: relays messages to cerebellum
b.
Medulla: Autonomic center for regulating heart, lungs & digestive system
c.
Cerebellum: coordinates complex motor activities
d.
Cerebrum: Conscious thought & memory
(1) Two hemispheres
(2) Corpus callosum: neurons that connect 2 hemispheres
2.
Mapping functional areas: Lesion studies. Fig 45.21
a. Right hemisphere controls motor & sensory functions of left side of body &
vice versa
b. Math & language on left side
c. Spatial visualization & analysis on right side
B. How does memory work?
1. Hypothesis—During learning and memory, neurons must either change the
synaptic connections they make or the type of neurotransmitters they secrete.
2. Touching experiments with the sea slug Aplysia show that it can learn to
withdraw its gill in response to light touching of its siphon. Fig 45.23
3. In Aplysia, learning and memory are correlated with changes in the nature of the
synapse. Fig 45.24
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