Transcript 04-21-06

Nervous Systems
Chapt 48 (pp 1011-1025)
4/21/06
IB-202-14-06
Complex Brain-Human!
• Overview: Command and Control Center
• The human brain contains an estimated 100
billion nerve cells, or neurons
• Each neuron may communicate with thousands
of other neurons
• Functional magnetic resonance imaging
– Is a technology that can reconstruct a threedimensional map of brain activity
Colored areas of
brain active
during language
processing.
The results of brain
imaging and other
research methods
reveal that groups of
neurons function in
specialized circuits
dedicated to different
tasks
Figure 48.1
Simple Nervous Systems
• Concept 48.1: Nervous systems consist of
circuits of neurons and supporting cells
• All animals except sponges have some type of
nervous system
• What distinguishes the nervous systems of
different animal groups is how the neurons are
organized into circuits
• Most invertebrate nervous systems are simple
Organization of Nervous Systems
• The simplest animals with nervous systems,
the cnidarians have neurons arranged in nerve
nets
When prey touch a
tentacle, the hydra can
contract its tentacle to its
mouth and engulf the
prey item.
Figure 48.2a
Nerve net
(a) Hydra (cnidarian)
Star fish
• Sea stars have a nerve net in each arm connected by radial
nerves to a central nerve ring. No Photosensitive Organs
Radial
nerve
Each radial
nerve would
have smaller
nerves sending
signals to the
water vascular
system as well
as muscles.
Nerve
ring
Figure 48.2b
(b) Sea star (echinoderm)
Appearance of cephalization and
centralization of nervous system
• In relatively simple cephalized animals, such
as flatworms a central nervous system (CNS)
is evident
Eyespot
1st appearance
of eye spots at
head end!
Allow it to turn
away from
light!
Brain
Nerve
cord
Transverse
nerve
Two ventral nerve cords
(interconnected so
communicate with each
other)!
Figure 48.2c
(c) Planarian (flatworm)
Segmented invertebrates
• Annelids and arthropods
– Have segmentally arranged clusters of neurons
called ganglia
• These ganglia connect to the CNS and make up
a peripheral nervous system (PNS)
Brain
Brain
Ventral
nerve
cord
Segmental
ganglion
Figure 48.2d, e
Ventral
nerve cord
Segmental
ganglia
(d) Leech (annelid)
(e) Insect (arthropod)
Molluscs
• Nervous systems in molluscs
– Correlate with the animals’ lifestyles
• Sessile molluscs (clams sitting in the mud) have
simple systems while more complex molluscs have
more sophisticated systems like the squid and octopus
both which have eyes and are capable of complex
behavior, including learning.
Anterior
nerve ring
Well developed
brain and eyes in
squid!
Ganglia
Brain
Longitudinal
nerve cords
Figure 48.2f, g
(f) Chiton (mollusc)
Ganglia
(g) Squid (mollusc)
Can “grasp” items
with tentacles and
manipulate them!
Vertebrates have a brain encased in a
skull for protection.
• In vertebrates
– The central nervous system consists of a brain and dorsal
spinal cord
– The periferal nerves system connects to the CNS
Brain
Spinal
cord
(dorsal
nerve
cord)
Figure 48.2h
Dorsal sensory
ganglion
(h) Salamander (chordate)
What does the nervous system do? Gathers information
about its surroundings, processes it and acts on it with
some sort of output.
• Nervous systems process information in three
stages--sensory input, integration, and motor
output
Sensory input
Integration
Sensor
Motor output
Effector
Figure 48.3
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
Knee jerk as an example of info processing
outside of the brain
• The three stages of information processing
2 Sensors detect
a sudden stretch in
the quadriceps.
3 Sensory neurons
convey the information
to the spinal cord.
Cell body of
sensory neuron
in dorsal
root ganglion
4 The sensory neurons communicate with
motor neurons that supply the quadriceps. The
motor neurons convey signals to the quadriceps,
causing it to contract and jerking the lower leg forward.
Gray matter
5 Sensory neurons
from the quadriceps
also communicate
with interneurons
in the spinal cord.
Quadriceps
muscle
White
matter
Stretching of
quadriceps when
leaning forward!
Information
is in the form
1
is
of electrical initiatedTheby reflex
tapping
the tendon connected
signals.
to the quadriceps
Figure 48.4
(extensor) muscle.
Hamstring
muscle
Spinal cord
(cross section)
Sensory neuron
Motor neuron
Interneuron
6 The interneurons
inhibit motor neurons
that supply the
hamstring (flexor)
muscle. This inhibition
prevents the hamstring
from contracting,
which would resist
the action of
the quadriceps.
Neuron Structure
• Most of a neuron’s organelles are located in the cell
body. Axons conduct impulse away from cell body!
Dendrites
Cell body
Nucleus
Synapse
Signal
Axon direction
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
Figure 48.5
Synaptic
terminals
• Most neurons have dendrites
– Highly branched extensions that receive signals
from other neurons
• The axon is typically a much longer extension
– That transmits signals to other cells at synapses
– That may be covered with a myelin sheath
• Neurons have a wide variety of shapes
– That reflect their input and output interactions
Dendrites
Axon
Cell
body
Figure 48.6a–c (a) Sensory neuron
(b) Interneurons
(c) Motor neuron
• Oligodendrocytes (in the CNS) and Schwann
cells (in the PNS) are supporting cells that
form the myelin sheaths around the axons of
many vertebrate neurons
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Figure 48.8
Myelin sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
0.1 µm
Basis for generation of anelectrical
signal is the alteration of the resting
membrane potential of excitable
cells!
1. Every cell has a voltage, or membrane
potential, across its plasma membrane
• A membrane potential is a localized electrical
gradient across membrane. The basis for the
gradient is the disproportionate distribution of
charged ions.
– Anions are more concentrated within a cell.
– Cations are more concentrated in the extracellular
fluid.
– A greater number of negative charges within the cell
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The resting membrane potential of a cell
can be measured
APPLICATION Electrophysiologists use intracellular recording to measure the membrane potential of
neurons and other cells.
A microelectrode is made from a glass capillary tube filled with an electrically conductive
TECHNIQUE
salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a
microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A
voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the
microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell.
Microelectrode
–70 mV
Voltage
recorder
Figure 48.9
Reference
electrode
• Concept 48.2: Ion pumps and ion channels
maintain the resting potential of all cells
including neurons
• For a neuron the resting potential is the
membrane potential of a cell that is not
transmitting signals
• Cells that can transmit signals are called
excitable cells (nerves and muscles)
• How a Cell Maintains a Membrane Potential.
– Cations.
• K+ the principal intracellular cation.
• Na+ is the principal extracellular cation.
– Anions.
• Proteins, amino acids, sulfate, and phosphate are the
principal intracellular anions.
• Cl– is principal extracellular anion.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Ungated ion channels allow ions to diffuse
across the plasma membrane.
– These channels are always open.
• This diffusion does not achieve an equilibrium
since sodium-potassium pump transports these
ions against their concentration gradients. If
poison the pump they will.
Fig. 48.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Size of arrow represents the
rate of diffusion. Faster for
K+ than Na+
Excitable cells have the ability to generate large
changes in their membrane potentials because
they have gated ion channels.
– Gated ion channels open or close in response to
stimuli. (These are separate and different from the ion
channels in the former slide)
• The subsequent movement of ions across the membrane
leads to a change in the membrane potential.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Gated Ion Channels
• Gated ion channels open or close
– In response to membrane stretch or the
binding of a specific ligand
– In response to a change in the membrane
potential
Production of Action Potentials
• In most neurons, depolarizations
– Are graded only up to a certain membrane
voltage, called the threshold
• Some stimuli trigger a hyperpolarization
– An increase in the magnitude of the membrane
potential
Stimuli
Membrane potential (mV)
+50
0
–50
Threshold
Resting
potential Hyperpolarizations
–100
0 1 2 3 4 5
Time (msec)
(a) Graded hyperpolarizations
produced by two stimuli that
increase membrane permeability
to K+. The larger stimulus produces
Figure 48.12a a larger hyperpolarization.
• Other stimuli trigger a depolarization
– A reduction in the magnitude of the membrane
potential
Stimuli
Membrane potential (mV)
+50
0
–50
Threshold
Resting Depolarizations
potential
–100
0 1 2 3 4 5
Time (msec)
(b) Graded depolarizations produced
by two stimuli that increase
membrane permeability to Na+.
The larger stimulus produces a
Figure 48.12b larger depolarization.
• Hyperpolarization and depolarization
– Are both called graded potentials because the
magnitude of the change in membrane
potential varies with the strength of the
stimulus
• A stimulus strong enough to produce a depolarization that
reaches the threshold of -55mV triggers a different type of
response, called an action potential
Stronger depolarizing stimulus
Membrane potential (mV)
+50
Action
potential
0
–50
Threshold
Resting
potential
–100
Figure 48.12c
0 1 2 3 4 5 6
Time (msec)
(c) Action potential triggered by a
depolarization that reaches the
threshold.
• An action potential
– Is a brief all-or-none depolarization of a
neuron’s plasma membrane
– Is the type of signal that carries information
along axons
• Both voltage-gated Na+ channels and voltagegated K+ channels
– Are involved in the production of an action
potential
• When a stimulus depolarizes the membrane
– Na+ channels open, allowing Na+ to diffuse into the
cell changing the potential to a positive value
• As the action potential subsides
– K+ channels open, and K+ flows out of the cell
• A refractory period follows the action
potential
– During which a second action potential cannot
be initiated
• An action potential can travel long distances
– By regenerating itself along the axon
• The generation of an action potential
Na+
Na+
– –
– –
– –
– –
+ +
+ +
+ +
+ +
K+
Rising phase of the action potential
Depolarization opens the activation
gates on most Na+ channels, while the
K+ channels’ activation gates remain
closed. Na+ influx makes the inside of
the membrane positive with respect
to the outside.
Na+
+ +
+ +
– –
– –
+50
+ +
– –
K+
– –
–50
Na+
+ + + + + + + +
+ +
– –
– –
– –
– –
3
2
4
Threshold
5
1
1
Resting potential
Na+
Potassium
channel
+ +
Activation
gates
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
K+
– – – – – – – –
Cytosol
– –
Sodium
channel
1
Na+
+ +
Plasma membrane
Figure 48.13
Falling phase of the action potential
The inactivation gates on
most Na+ channels close,
blocking Na+ influx. The
activation gates on most
K+ channels open,
permitting K+ efflux
which again makes
the inside of the cell
negative.
Time
Depolarization A stimulus opens the
activation gates on some Na+ channels. Na+
influx through those channels depolarizes the
membrane. If the depolarization reaches the
threshold, it triggers an action potential.
Extracellular fluid
+ +
Action
potential
0
–100
2
+ +
4
Na+
+ +
+ +
K+
Membrane potential
(mV)
3
Na+
Na+
– –
K+
– –
Inactivation
gate
Resting state
The activation gates on the Na+ and K+ channels
are closed, and the membrane’s resting potential is maintained.
5
Undershoot
Both gates of the Na+ channels
are closed, but the activation gates on some K+
channels are still open. As these gates close on
most K+ channels, and the inactivation gates
open on Na+ channels, the membrane returns to
its resting state.
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, usually the axon hillock
– An electrical current depolarizes the
neighboring region of the axon membrane
Axon
Action
potential
– –
+ ++
Na
+ +
– –
K+
+ +
– –
– –
+ +
K+
Figure 48.14
+
–
–
+
+
–
–
+
+
+
+
+
+
+
–
–
+
–
–
+
–
–
+
–
–
+
–
–
+
–
–
+
+
–
–
+
+
–
–
+
Action
potential
–
+
Na
–
+
+
+ +
–
–
K+
+ +
– –
– –
+ +
K+
+
–
–
+
Action
potential
– –
+ ++
Na
+ +
– –
–
+
+
–
1
An action potential is generated
as Na+ flows inward across the
membrane at one location.
2
The depolarization of the action
potential spreads to the neighboring
region of the membrane, re-initiating
the action potential there. To the left
of this region, the membrane is
repolarizing as K+ flows outward.
3
The depolarization-repolarization process is
repeated in the next region of the
membrane. In this way, local currents
of ions across the plasma membrane
cause the action potential to be propagated
along the length of the axon.
+
–
–
+
–
+
+
–
Conduction Speed
• The speed of an action potential
– Increases with the diameter of an axon
• In vertebrates, axons are myelinated
– Also causing the speed of an action potential
to increase
Myelinated axons conduct impulses
faster than non-myelinated
• Action potentials in myelinated axons jump between the
nodes of Ranvier in a process called saltatory conduction
Schwann cell
Depolarized region
(node of Ranvier)
Myelin
sheath
––
–
Cell body
+
++
+
++
–––
––
–
+
+
Axon
+
++
––
–
Figure 48.15
• Concept 48.4: Neurons communicate with other
cells at synapses
• In an electrical synapse
– Electrical current flows directly from one cell to
another via a gap junction (tail flick escape
response in lobster uses electrical connection
because it must be as fast as possible).
• The vast majority of synapses
– Are chemical synapses
• In a chemical synapse, a presynaptic neuron
releases chemical neurotransmitters, which
are stored in the synaptic terminal
Postsynaptic
neuron body
5 µm
Synaptic
terminal
of presynaptic
neurons
Figure 48.16
• When an action potential reaches a terminal
– The final result is the release of
neurotransmitters into the synaptic cleft
Postsynaptic cell
Presynaptic
cell
Action potential
results in influx
of calcium!
Calcium causes
vesicles to fuse
with presynaptic
membrane
releasing
neurotransmitter
Figure 48.17
Synaptic vesicles
containing
neurotransmitter
5
Presynaptic
membrane
Na+
K+
Neurotransmitter
Postsynaptic
membrane
Ligandgated
ion channel
Voltage-gated
Ca2+ channel
1 Ca2+
4
2
Synaptic cleft
3
Ligand-gated
ion channels
Postsynaptic
membrane
6