Download Chapter 48 PowerPoint 2016 - Spring

Chapter 48+ (sections of 49/50)
Neurons, Synapses, and
Signaling w/in the Nerv. Syst.
Concept 48.1: Neuron organization and structure
reflect function in information transfer
• Nervous systems process information in 3 stages:
sensory input, integration, and motor output
– Sensors detect external stimuli and internal
conditions and transmit information along sensory
neurons
– Sensory information is sent to the brain or ganglia,
where interneurons integrate the information
– Motor output leaves the brain or ganglia via motor
neurons, which trigger muscle or gland activity
• Many animals have a complex nervous system
which consists of:
– central nervous system (CNS) where integration
takes place; this includes the brain and a nerve
cord
Sensory input
– A peripheral nervous system (PNS), which
brings information into and out of the CNS
Motor output
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
Neuron Structure and Function
Fig. 48-4
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock
Presynaptic
cell
Axon
Information is transmitted
from a presynaptic cell
(neuron) to a postsynaptic
cell (neuron, muscle, or
gland cell)
Synapse
Synaptic terminals
Postsynaptic cell
Neurotransmitter
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 48.2: Ion pumps and ion channels
maintain the resting potential of a neuron
• Membrane potential : difference in electrical
charge across plasma membrane
• The resting potential is the membrane
potential of a neuron not sending signals
Microelectrode
–70 mV
Voltage
recorder
Reference
electrode
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Formation of the Resting Potential
Fig. 48-6 The basis of the membrane potential
Key
Na+
K+
OUTSIDE
CELL
OUTSIDE [K+]
CELL
5 mM
INSIDE [K+]
CELL 140 mM
[Na+]
[Cl–]
150 mM 120 mM
[Na+]
15 mM
[Cl–]
10 mM
[A–]
100 mM
INSIDE
CELL
(a)
(b)
1. Na+/K+ pump
2. K+ leak channels
3. Anions
Sodiumpotassium
pump
Potassium
channel
Sodium
channel
Resting potential: membrane potential at rest;
polarized
– Na+ outside, K+ inside cell (Salty Banana)
– Na+/K+ Pump use ATP to maintain these
gradients across the plasma membrane
– Voltage-gated Na+ channel = CLOSED
– Voltage-gated K+ channel = OPEN
Nerve impulse: stimulus causes a change in
membrane potential
– Action potential: neuron membrane
depolarizes
– All-or-nothing response
Na+ channels
open
Na+
enters cell
K+ channels
open
K+ leaves cell
Concept 48.3: Action potentials (nerve impulses)
are the signals conducted by axons
• Neurons contain
gated ion
channels that
open or close in
response to
stimuli
• You’ll want to
review this pic
after you
understand the
action potential
Polarized
Production of Action Potentials
• Voltage-gated Na+ and K+ channels respond
to a change in membrane potential
• When a stimulus depolarizes the membrane,
Na+ channels open, allowing Na+ to diffuse into
the cell
• The movement of Na+ into the cell increases
the depolarization and causes even more Na+
channels to open
• A strong stimulus results in a massive change
in membrane voltage called an action
potential
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• An action potential is a
brief all-or-none
depolarization of a
neuron’s plasma
membrane
• Action potentials are
signals that carry
information along axons
Strong depolarizing stimulus
+50
Action
potential
Membrane potential (mV)
• An action potential
occurs if a stimulus
causes the membrane
voltage to cross a
particular threshold
0
–50 Threshold
Resting
potential
–100
0
(c) Action potential
1 2 3 4 5
Time (msec)
6
Generation of Action Potentials: A Closer Look
• A neuron can produce hundreds of action
potentials per second
• The frequency of action potentials can reflect
the strength of a stimulus
• An action potential can be broken down into a
series of stages
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 48-10-1
Key
Na+
K+
Membrane potential
(mV)
+50
Action
potential
–50
Plasma
membrane
Cytosol
Inactivation loop
1
Resting state
2
–100
Sodium
channel
4
Threshold
1
5
Resting potential
Depolarization
Extracellular fluid
3
0
Time
Potassium
channel
1
Fig. 48-10-2
Key
Na+
K+
Membrane potential
(mV)
+50
Action
potential
–50
2
Plasma
membrane
Cytosol
Inactivation loop
1
Resting state
2
–100
Sodium
channel
4
Threshold
1
5
Resting potential
Depolarization
Extracellular fluid
3
0
Time
Potassium
channel
1
Fig. 48-10-3
Key
Na+
K+
3
Rising phase of the action potential
Membrane potential
(mV)
+50
Action
potential
–50
2
Plasma
membrane
Cytosol
Inactivation loop
1
Resting state
2
–100
Sodium
channel
4
Threshold
1
5
Resting potential
Depolarization
Extracellular fluid
3
0
Time
Potassium
channel
1
Fig. 48-10-4
Key
Na+
K+
3
4
Rising phase of the action potential
Membrane potential
(mV)
+50
Action
potential
–50
2
Plasma
membrane
Cytosol
Inactivation loop
1
Resting state
2
–100
Sodium
channel
4
Threshold
1
5
Resting potential
Depolarization
Extracellular fluid
3
0
Time
Potassium
channel
1
Falling phase of the action potential
Fig. 48-10-5
Key
Na+
K+
3
4
Rising phase of the action potential
Falling phase of the action potential
Membrane potential
(mV)
+50
Action
potential
–50
2
2
4
Threshold
1
1
5
Resting potential
Depolarization
Extracellular fluid
3
0
–100
Sodium
channel
Time
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop
5
1
Resting state
Undershoot
• During the
refractory period
after an action
potential, a second
action potential
cannot be initiated
• The refractory
period is a result of
a temporary
inactivation of the
Na+ channels
Conduction of Action Potentials
• An action potential can travel long distances by
regenerating itself along the axon
• At the site where the action potential is generated,
an electrical current depolarizes the neighboring
region of the axon membrane
• Inactivated Na+ channels behind the zone of
depolarization prevent the action potential from
traveling backwards
• Action potentials travel in only one direction:
toward the synaptic terminals (axon terminals)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Conduction
of an action
potential
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Conduction Speed
• The speed of an action potential increases with
the axon’s diameter
• In vertebrates, axons are insulated by a myelin
sheath, which causes an action potential’s
speed to increase
• Myelin sheaths are made by glia—
oligodendrocytes in the CNS and Schwann
cells in the PNS
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 48-12a
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Nodes of
Myelin sheath Ranvier
Axon
Fig. 48-12b
Schwann
cell
Nucleus of
Schwann cell
Myelinated axon (cross section) 0.1 µm
• Action potentials are formed only at nodes of
Ranvier, gaps in the myelin sheath where voltagegated Na+ channels are found
• Action potentials in myelinated axons jump
between the nodes of Ranvier in a process called
saltatory conduction
Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
Concept 48.4: Neurons communicate with other
cells at synapses
• At electrical synapses, the electrical current
flows from one neuron to another
• At chemical synapses, a chemical
neurotransmitter carries information across the
gap junction
• Most synapses are chemical synapses
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 48-15
5
Synaptic vesicles
containing
neurotransmitter
Voltage-gated
Ca2+ channel
Postsynaptic
membrane
1 Ca2+
4
2
Synaptic
cleft
Presynaptic
membrane
3
Ligand-gated
ion channels
6
K+
Na+
Neurotransmitters
• Chemicals released from vesicles by exocytosis into
synaptic cleft
• Diffuse across synapse
• Bind to receptors on neurons, muscle cells, or gland
cells
• Broken down by enzymes or taken back up into
surrounding cells
• Types of neurotransmitters:
– Excitatory: speed up impulses by causing
depolarization of postsynaptic membrane
– Inhibitory: slow impulses by causing
hyperpolarization of postsynaptic membrane
Summation of Postsynaptic Potentials
• Unlike action potentials, postsynaptic potentials are graded
– A single EPSP is usually too small to trigger an action
potential in a postsynaptic neuron
• Most neurons have many synapses on dendrites and cell body
Terminal branch
of presynaptic
neuron
E2
E1
E2
Membrane potential (mV)
Postsynaptic
neuron
E1
E1
E1
E2
E2
I
I
Axon
hillock
I
I
0
Action
potential
Threshold of axon of
postsynaptic neuron
Action
potential
Resting
potential
–70
E1
E1
(a) Subthreshold, no
summation
E1
E1
(b) Temporal summation
E1 + E2
(c) Spatial summation
E1
I
E1 + I
(d) Spatial summation
of EPSP and IPSP
Table 48-1
Examples of Neurotransmitters
• Acetylcholine (ACh): stimulates muscles, memory
formation, learning
–
Sarin gas; Botox
• Epinephrine: Adrenaline; fight-or-flight
• Norepinephrine: fight-or-flight
• Dopamine: reward, pleasure (“high”)
–
Loss of dopamine  Parkinson’s Disease
–
Excessive amounts  Schizophrenia
–
LSD binds to dopamine receptors  hallucinations
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Examples of Neurotransmitters
• Serotonin: well-being; happiness
–
LSD binds to serotonin receptors  hallucinations
–
Low levels of serotonin  depression
–
Prozac enhances effects of serotonin by inhibiting reuptake
• GABA: inhibitory neurotransmitter
–
Affected by alcohol
–
Valium uses these receptors
• Endorphins: decrease pain perception
–
Morphine/heroine  bind to same receptors
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Regional Specialization of Vertebrate Brain
•
•
•
Forebrain  cerebrum
Midbrain  brainstem
Hindbrain  cerebellum
Human Brain
Structure
Function
Cerebrum
• Information processing (learning, emotion,
memory, perception, voluntary movement)
• Right & Left cerebral hemispheres
• Corpus callosum: connect hemispheres
Brainstem
*Oldest evolutionary part*
•Basic, autonomic survival behaviors
•Medulla oblongata –breathing, heart & blood
vessel activity, digestion, swallowing, vomiting
•Transfer info between PNS & CNS
Cerebellum
• Coordinate movement & balance
• Motor skill learning
Fig. 49-15
Frontal lobe
Parietal lobe
Speech
Frontal
association
area
Somatosensory
association
area
Taste
Reading
Speech
Hearing
Smell
Auditory
association
area
Visual
association
area
Vision
Temporal lobe
Occipital lobe
You should now be able to:
1.
Distinguish among : sensory neurons, interneurons, motor neurons; membrane
potential, resting potential; ungated and gated ion channels; electrical synapse,
chemical synapse; EPSP, IPSP; temporal, spatial summation
2.
Explain the role of the Na+/K+ pump in maintaining the resting potential
3.
Describe the stages of an action potential; explain the role of voltage-gated ion
channels in each stage of this process
4.
Explain why the action potential cannot travel back toward the cell body
5.
Describe the factors that affect the rate of neuronal signal transmission; explain
saltatory conduction
6.
Describe the events that lead to the release of neurotransmitters into the synaptic cleft
7.
Explain the statement: “Unlike action potentials, which are all-or-none events,
postsynaptic potentials are graded”
8.
Describe five categories of neurotransmitters; given information, be able to apply info to
use/blockage of specific signals via simulation or blockage of neurotransmitter
9.
Describe how the brain integrates information.
10.
Describe the different functions of the different regions of the brain.
Fig. 48-UN1
Action potential
Membrane potential (mV)
+50
Falling
phase
0
Rising
phase
Threshold (–55)
–50
–100
Resting
potential
–70
Depolarization
Time (msec)
Undershoot