Transcript Lecture #13 – Animal Nervous Systems

Lecture #13 – Animal Nervous Systems
1
Key Concepts:
• Evolution of organization in nervous
systems
• Neuron structure and function
• Neuron communication at synapses
• Organization of the vertebrate nervous
systems
• Brain structure and function
• The cerebral cortex
• Nervous system injuries and diseases??? 2
All animals except sponges have some kind
of nervous system
• Increasing complexity accompanied increasingly
complex motion and activities
• Nets of neurons  bundles of neurons  cephalization
3
First split
was tissues;
next was
body
symmetry;
echinoderms
“went back”
to radial
symmetry
4
Derived radial symmetry and nerve network
5
Cephalization
• The development of a brain
• Associated with the development of
bilateral symmetry
• Complex, cephalized nervous systems are
usually divided into 2 sections
Central nervous system (CNS) integrates
information, exerts most control
Peripheral nervous system (PNS) connects
CNS to the rest of the body
6
7
Critical Thinking
• What is the functional advantage of
cephalization???
8
Critical Thinking
• What is the functional advantage of
cephalization???
9
Cephalization
• The development of a brain
• Associated with the development of
bilateral symmetry
• Complex, cephalized nervous systems are
usually divided into 2 sections
Central nervous system (CNS) integrates
information, exerts most control
Peripheral nervous system (PNS) connects
CNS to the rest of the body
10
PNS  CNS  PNS
11
Specialized neurons support
different sections
• Sensory
Transmit information from the sensory
structures that detect the both external and
internal conditions
• Interneurons
Analyze and interpret sensory information,
formulate response
• Motor
Transmit information to effector cells – the
muscle or endocrine cells that respond to input
12
Critical Thinking
• Which type of neuron would have the most
branched structure???
Sensory neurons
Interneurons
Motor neurons
13
Critical Thinking
• Which type of neuron would have the most
branched structure???
Sensory neurons
Interneurons
Motor neurons
14
Neuron structure is complex
100 billion
nerve cells in
the human
brain!
15
16
17
Basic Neuron Structure
• Cell body
• Dendrites
• Axons
• Axon hillock
• Myelin sheath
• Synaptic terminal
18
Cell Body
• Contains most cytoplasm and organelles
• Extensions branch off cell body
19
Dendrites
• Highly branched extensions
• Receive signals from other neurons
20
Axons
• Usually longer extension, unbranched til end
• Transmits signals to other cells
21
Axon Hillock
• Enlarged region at base of axon
• Site where axon signals are generated
Signal is sent after summation
22
Myelin Sheath
• Insulating sheath around axon
• Also speeds up signal transmission
23
Synaptic Terminal
• End of axon branches
• Each branch ends in a synaptic terminal
Actual site of between-cell signal generation
24
Synapse
• Site of signal transmission between cells
• More later…
25
Supporting Cells - Glia
• Maintain structural integrity and function of
neurons
• 10 – 50 x more glia than neurons in
mammals
• Major categories
Astrocytes
Radial glia
Oligodendrocytes and Schwann cells
26
Glia – Astrocytes
• Structural support for neurons
• Regulate extracellular ion and
neurotransmitter concentrations
• Facilitate synaptic transfers
• Induce the formation of the blood-brain
barrier
Tight junctions in capillaries allow more control
over the extracellular chemical environment in
the brain and spinal cord
27
Glia – Radial Glia
• Function mostly during embryonic
development
• Form tracks to guide new neurons
out from the neural tube (neural
tube develops into the CNS)
• Can also function as stem cells to
replace glia and neurons (so can
astrocytes)
This function is limited in nature;
major line of research
28
Glia – Oligodendrocytes (CNS) and
Schwann Cells (PNS)
• Form the myelin sheath around axons
• Cells are rectangular and tile-shaped,
wrapped spirally around the axons
• High lipid content insulates the axon –
prevents electrical signals from escaping
• Gaps between the cells (Nodes of
Ranvier) speed up signal transmission
29
The nerve signal is electrical!
• To understand signaling process, must
understand the difference between resting
potential and action potential
30
Resting Potential
• All cells have a resting potential
Electrical potential energy – the separation of
opposite charges
Due to the unequal distribution of anions and
cations on opposite sides of the membrane
Maintained by selectively permeable
membranes and by active membrane pumps
Charge difference = one component of the
electrochemical gradient that drives the
diffusion of all ions across cell membranes
31
Neuron Function – Resting Potential
• Neuron resting potential is ~ -70mV
At resting potential the neuron is NOT actively
transmitting signals
Maintained largely because cell membranes
are more permeable to K+ than to Na+; more
K+ leaves the cell than Na+ enters
An ATP powered K+/Na+ pump continually
restores the concentration gradients; this also
helps to maintain the charge gradient
32
Resting Potential Ion Concentrations
1. Cell membranes are
more permeable to
K+ than to Na+
2. There is more K+
inside the cell than
outside
3. There is more Na+
outside the cell than
inside
• Both ions follow their
[diffusion] gradients
33
Critical Thinking
• If both ions follow their
diffusion gradients, what
is the predictable
consequence???
34
Critical Thinking
• If both ions follow their
diffusion gradients, what
is the predictable
consequence???
35
Resting Potential Ion Concentrations
• A dynamic equilibrium
is predictable, but is
prevented by an ATP
powered K+/Na+ pump
36
Neuron Function – Resting Potential
• Neuron resting potential is ~ -70mV
At resting potential the neuron is NOT actively
transmitting signals
Maintained largely because cell membranes
are more permeable to K+ than to Na+; more
K+ leaves the cell than Na+ enters
An ATP powered K+/Na+ pump continually
restores the concentration gradients; this also
helps to maintain the charge gradient
37
Resting Potential Ion
Concentrations
• ATP powered pump
continually transfers 3
Na+ ions out of the
cytoplasm for every 2 K+
ions it moves back in to
the cytoplasm
• This means that there is
a net transfer of + charge
OUT of the cell
38
Resting Potential Ion Concentrations
• Thus, the membrane
potential is maintained
• Cl- and large anions
also contribute to the
net negative charge
inside the cell
39
REVIEW
Neuron Function – Resting Potential
• Neuron resting potential is ~ -70mV
At resting potential the neuron is NOT actively
transmitting signals
Maintained largely because cell membranes
are more permeable to K+ than to Na+; more K+
leaves the cell than Na+ enters
An ATP powered K+/Na+ pump continually
restores the concentration gradients; this also
helps to maintain the charge gradient
Cl-, other anions, and Ca++ also affect resting
potential
40
Gated Ion Channels
Why Neurons are Different
• All cells have a membrane potential
• Neurons can change their membrane
potential in response to a stimulus
• The ability of neurons to open and close
ion gates allows them to send electrical
signals along the extensions (dendrites
and axons)
Gates open and close in response to stimuli
Only neurons can do this!
41
Gated Ion Channels
Why Neurons are Different
• Gated ion channels manage membrane
potential
Stretch gates – respond when membrane is
stretched
Ligand gates – respond when a molecule
binds (eg: a neurotransmitter)
Voltage gates – respond when membrane
potential changes
42
Gated Ion Channels
Why Neurons are Different
• Hyperpolarization = inside of neuron
becomes more negative
• Depolarization = inside of neuron becomes
more positive
Either can occur, depending on stimulus
Either can be graded – more stimulus = more
change in membrane potential
• Depolarization eventually triggers an
action potential = NOT graded
43
Depolarization eventually triggers an
action potential – action potentials
are NOT graded
44
Action Potentials ARE the Nerve Signal
• Triggered whenever depolarization reaches
a set threshold potential
• Action potentials are all-or-none responses
of a fixed magnitude
Once triggered, they can’t be stopped
There is no gradation once an action potential
is triggered
• Action potentials are brief depolarizations
1 – 2 milliseconds
• Voltage gated ion channels control signal
45
Critical Thinking
• If the action potential is of a fixed
magnitude, how do we sense different
levels of a stimulus???
46
Critical Thinking
• If the action potential is of a fixed
magnitude, how do we sense different
levels of a stimulus???
47
Action Potentials ARE the Nerve Signal
• Triggered whenever depolarization reaches
a set threshold potential
• Action potentials are all-or-none responses
of a fixed magnitude
Once triggered, they can’t be stopped
There is no gradation once an action potential
is triggered
• Action potentials are brief depolarizations
1 – 2 milliseconds
• Voltage gated ion channels control signal
48
Fig. 48.13; p. 1019, 7th Ed.
49
Voltage Gate Activity
1. Resting Potential – Na+ and K+ activation
gates closed; Na+ inactivation gate open
on most channels
2. Depolarization – Na+ activation gates
begin to open – Na+ begins to enter cell
3. Rising Phase – threshold is crossed, Na+
floods into the cell, raising the membrane
potential to ~ +35mV
50
51
1. Resting Potential – Na+ and K+
activation gates closed; Na+
inactivation gate open on most
channels
52
Voltage Gate Activity
1. Resting Potential – Na+ and K+ activation
gates closed; Na+ inactivation gate open
on most channels
2. Depolarization – Na+ activation gates
begin to open – Na+ begins to enter cell
3. Rising Phase – threshold is crossed, Na+
floods into the cell, raising the membrane
potential to ~ +35mV
53
54
2. Depolarization – Na+ activation gates
begin to open – Na+ begins to enter cell
55
Voltage Gate Activity
1. Resting Potential – Na+ and K+ activation
gates closed; Na+ inactivation gate open
on most channels
2. Depolarization – Na+ activation gates
begin to open – Na+ begins to enter cell
3. Rising Phase – threshold is crossed, Na+
floods into the cell, raising the membrane
potential to ~ +35mV
56
57
3. Rising Phase – threshold is crossed,
Na+ floods into the cell, raising the
membrane potential to ~ +35mV
58
Voltage Gate Activity
4. Falling Phase – Na+ inactivation gates
close, K+ activation gates open – Na+
influx stops, K+ efflux is rapid
5. Undershoot – K+ activation gates close,
but not until membrane potential has gone
a little bit below resting potential
6. Refractory Period – the Na+ inactivation
gates remain closed during stages 4 and
5, limiting the maximum frequency of
59
action potentials
Membrane
repolarizes
60
4. Falling Phase – Na+ inactivation gates
close, K+ activation gates open – Na+
influx stops, K+ efflux is rapid
61
Voltage Gate Activity
4. Falling Phase – Na+ inactivation gates
close, K+ activation gates open – Na+
influx stops, K+ efflux is rapid
5. Undershoot – K+ activation gates close,
but not until membrane potential has gone
a little bit below resting potential
6. Refractory Period – the Na+ inactivation
gates remain closed during stages 4 and
5, limiting the maximum frequency of
62
action potentials
63
5. Undershoot – K+ activation gates close,
but not until membrane potential has
gone a little bit below resting potential
64
Voltage Gate Activity
4. Falling Phase – Na+ inactivation gates
close, K+ activation gates open – Na+
influx stops, K+ efflux is rapid
5. Undershoot – K+ activation gates close,
but not until membrane potential has gone
a little bit below resting potential
6. Refractory Period – the Na+ inactivation
gates remain closed during stages 4 and
5, limiting the maximum frequency of
65
action potentials
6. Refractory Period – the Na+ inactivation
gates remain closed during stages 4
and 5, limiting the maximum frequency
of action potentials
66
67
Fig. 48.13, 7th Ed.
Conduction of
Action Potential
• Electrical signal moves
along the axon by
depolarizing adjacent
regions of the membrane
past the threshold
• The depolarization effect
is NOT directional – the
cytoplasm becomes more
+ in both directions
68
Critical Thinking
• If the depolarizing effect is bilateral, why
does the signal travel in one direction
only???
69
Critical Thinking
• If the depolarizing effect is bilateral, why
does the signal travel in one direction
only???
70
Conduction of
Action Potential
• Electrical signal moves
along the axon by
depolarizing adjacent
regions of the membrane
past the threshold
• Depolarization zone
travels in one direction
only due to the refractory
period (Na+ gates locked)
71
Speed!
• Diameter of axon
Larger = less resistance  faster signal
Found in invertebrates
Max speed ~ 100 m/second
• Nodes of Ranvier
Signal jumps from node to node
Found in vertebrates
Saves space – 2,000 myelinated axons can fit
in the same space as one giant axon
Max speed ~ 120 m/second
72
Synapses – the gaps between cells
• Electrical synapses occur at gap junctions
Action potential is transmitted directly from cell
to cell
Especially important in rapid responses such as
escape movements
 Also with controlling heart beat (but with specialized muscle tissue)
• Most synapses are chemical
The signal is converted from electrical 
chemical  electrical
Neurotransmitters cross the synapse and carry
73
the signal to the receiving cell
Chemical Synapses
• A multi-stage process
Neurons synthesize neurotransmitters, isolated
into synaptic vesicles located at the synaptic
terminal
The action potential triggers the release of
neurotransmitters into the synapse
Neurotransmitters diffuse across the synapse
Neurotransmitter binds to a receptor,
stimulating a response (more later)
74
Chemical Synapses
1. Action potential depolarizes membrane at
synaptic terminal
2. Depolarization in this region opens Ca++
channels
3. Influx of Ca++ stimulates synaptic vesicles
to fuse with neuron cell membrane
4. Neurotransmitters are released by
exocytosis
5. Neurotransmitters bind to the receiving cell
75
membrane
Chemical Synapses
76
Chemical Synapses
REVIEW
1. Action potential depolarizes membrane at
synaptic terminal
2. Depolarization in this region opens Ca++
channels
3. Influx of stimulates synaptic vesicles to
fuse with neuron cell membrane
4. Neurotransmitters are released by
exocytosis
5. Neurotransmitters bind to the receiving cell
77
membrane
Chemical Synapses
• Direct synaptic transmission
Neurotransmitter binds directly to ligand-gated
channels
Channel opens for Na+, K+ or both
• Indirect synaptic transmission
Neurotransmitter binds to a receptor on the
membrane (not to a channel protein)
Signal transduction pathway is initiated
Second messengers eventually open channels
Slower but amplified response
78
Chemical synapses allow more
complicated signals
• Electrical signals pass unmodified at
electrical synapses
• Chemical signals are modified during
transmission
Type of neurotransmitter varies
Amount of neurotransmitter released varies
Some receptors promote depolarization; some
promote hyperpolarization
Signals are summed over both time and space
Remember that many, many neurons are
79
responding to any given stimulus
Chemical synapses allow more
complicated signals
• Responses are summed at the axon hillock
Action potential is generated and sent down
axon; or not
80
Chemical synapses allow more
complicated signals
• Summation is over both time and space
• Excitory and inhibitory signals can “cancel”
each other
81
Neurotransmitters – review text and
table, but don’t memorize
Table 48.1, 7th ed.
82
CNS Organization in Vertebrates
• Brain – integrates
• Spinal cord – 1o transmits
• Both derived from hollow, dorsal embryonic
nerve cord
Hollow remnants remain in ventricles of brain
and central canal of spinal cord
Spaces are filled with cerebrospinal fluid that
helps circulate nutrients, hormones, wastes, etc
Fluid also cushions CNS
• Axons are aggregated = white matter
83
84
PNS Organization in Vertebrates
• Major role – transmitting
information from
sensory structures to the
CNS; and from the CNS
to effector structures
Nerves always in left/right
pairs that serve both
sides of the body
85
PNS Organization in Vertebrates
• Cranial nerves originate in
brain and connect to the
head and upper body
Some have only sensory
neurons (eyes, nose)
• Spinal nerves originate in
spinal cord and connect to
the rest of the body
Contain both sensory and
motor neurons
86
Critical Thinking
• Can the eyes do anything besides see???
• Can the nose do anything besides smell???
• Can the ears do anything besides hear???
87
Critical Thinking
• Can the eyes do anything besides see???
• Can the nose do anything besides smell???
• Can the ears do anything besides hear???
88
PNS Organization in Vertebrates
• Cranial nerves originate in
brain and connect to the
head and upper body
Some have only sensory
neurons (eyes, nose)
• Spinal nerves originate in
spinal cord and connect to
the rest of the body
Contain both sensory and
motor neurons
89
PNS – Sub-divisions
All work together to
maintain
homeostasis and
respond to external
stimuli
90
PNS - Somatic
• Nerves that transmit signals to and from
skeletal muscles
• Respond primarily to external stimuli
• Largely under voluntary control
91
PNS - Autonomic
• Nerves that control the internal environment
• Respond to both internal and external
signals
• Largely under involuntary control
• Three sub-divisions
Sympathetic – stress responses
Parasympathetic – opposes sympathetic
Enteric – controls digestive system
92
PNS – Autonomic
93
Autonomic - Sympathetic
• Activates flight or fight
responses
• Promotes functions that
increase sensory
perception and ATP
levels
• Inhibits non-essential
functions such as
digestion and urination
94
Autonomic – Parasympathetic
• Returns body systems to
base-line function
• Promotes digestion and
other normal functions
• Usually antagonistic to
sympathetic division
95
Autonomic – Enteric
• Specifically controls the digestive system
• Regulated by both the sympathetic and
parasympathetic divisions
96
Brain Development
97