Transcript Document

Nervous System
2007-2008
Evolution of the Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
eyespot
auricle
ventral nerve
cord with ganglia
cerebral
ganglia
nerve
brain
nerve net
lateral
nerve
cords
transverse
nerves
a. Hydra
b. Planarian
c. Earthworm
2
Evolution of the Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
cerebrum
in forebrain
hindbrain
giant
nerve
fiber
spinal
cord
brain
eye
brain
thoracic
ganglion
d. Crab
tentacle
e. Squid
f. Cat
3
Evolution of the Nervous
System
• Vertebrate Nervous-System Organization
– Central nervous system
• Develops from an embryonic dorsal neural tube
• Cephalization and bilateral symmetry result in
several paired sensory receptors
– Eyes, ears, olfactory structures
– Vertebrate brain is organized into three areas
• Hindbrain
• Midbrain
• Forebrain
4
Evolution of the Nervous
System
• Division of Nervous System:
– The Central nervous system (CNS)
• Includes the brain and spinal cord
– The peripheral nervous system (PNS)
• Consists of all nerves and ganglia that lie outside the
CNS
• Two divisions
– Somatic nervous system
» Sensory and motor functions that control skeletal
muscle
– Autonomic nervous system
» Controls smooth muscle, cardiac, muscle, and gland
» Divided into sympathetic and parasympathetic
divisions
5
Organization of the Human Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
brain
cranial nerves
cervical nerves
thoracic
nerves
spinal cord
radial nerve
median nerve
ulnar nerve
lumbar
nerves
sacral
nerves
sciatic nerve
tibial nerve
common fibular
nerve
a.
6
Organization of the Human Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Central Nervous
System
brain and
spinal cord
Peripheral Nervous
System
somatic sensory
fibers (skin,
special senses)
visceral sensory
fibers (internal
organs)
somatic motor
fibers (to skeletal
muscles)
autonomic motor
fibers (to cardiac
and smooth
muscle, glands)
sympathetic
division
parasympathetic
division
7
b.
37.2 Nervous Tissue
• Neurons (nerve cells)
– Cell body contains nucleus and organelles
– Dendrites receive signals from sensory
receptors or other neurons
– Axon conducts nerve impulses to another
neuron or to other cells
• Covered by myelin sheath
• Any long axon is also called a nerve fiber
8
Nervous Tissue
• Types of Neurons
• Motor (efferent) neurons
– Accept nerve impulses from the CNS
– Transmit them to muscles or glands
• Sensory (afferent) neurons
– Accept impulses from sensory receptors
– Transmit them to the CNS
• Interneurons
– Convey nerve impulses between various parts of the
CNS
9
Neuron Anatomy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
cell body
dendrite
direction
of conduction
myelin
sheath
axon
terminal
node of Ranvier
axon
a. Motor neuron (multipolar)
muscle
10
a: © M.B. Bunge/Biological Photo Service
Neuron Anatomy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
axon
cell body
direction of
conduction
sensory
receptor
axon
b. Sensory neuron (unipolar)
myelin sheath
skin
11
Neuron Anatomy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
axon
cell body
dendrite
c. Interneuron (multipolar)
c: © Manfred Kage/Peter Arnold, Inc.
12
Nervous Tissue
• Transmission of Nerve Impulses
– Resting Potential
• The membrane potential (voltage) when the axon
is not conducting an impulse
– The inside of a neuron is more negative than
the outside, around -70 mV
– Due in part to the activity of the sodiumpotassium pump
13
Nervous Tissue
• An action potential is a rapid change in
polarity across a portion of an axonal
membrane
• An action potential is generated only after
a stimulus larger than the threshold
• Gated channel proteins
– One channel protein suddenly allows sodium
to enter the cell
– Another channel protein allows potassium to
leave the cell
14
Cells: surrounded by charged ions
• Cells live in a sea of charged ions
– anions (negative)
• more concentrated within the cell
• Cl-, charged amino acids (aa-)
– cations (positive)
• more concentrated in the extracellular fluid
• Na+
Na+
Na+
K+
aa-
K+
Na+
aaCl-
Na+
ClK+
Na+
aa-
Na+
K+
aa-
K+
Na+
ClCl-
Na+
aa-
Na+
Na+
Na+
Claa- Cl-
–
K+
+
channel
leaks K+
Cells have voltage!
• Opposite charges on opposite sides of cell
membrane
– membrane is polarized
• negative inside; positive outside
• charge gradient
• stored energy (like a battery)
+ + + + + + + + + + + + + + +
– – – – – – – – – – – – – –
– – – – – – – – – – – – – –
+ + + + + + + + + + + + + + +
Measuring cell voltage
unstimulated neuron = resting potential of -70mV
How does a nerve impulse travel?
• Stimulus: nerve is stimulated
– reaches threshold potential
• open Na+ channels in cell membrane
• Na+ ions diffuse into cell
– charges reverse at that point on neuron
The 1st
domino
goes
down!
• positive inside; negative outside
• cell becomes depolarized
– + + + + + + + + + + + + + +
+ – – – – – – – – – – – – – –
Na+
+ – – – – – – – – – – – – – –
– + + + + + + + + + + + + + +
How does a nerve impulse travel?
• Wave: nerve impulse travels down neuron
– change in charge opens
+ –
+
next Na gates down the line
• “voltage-gated” channels
channel
– Na+ ions continue to diffuse into cell
closed
– “wave” moves down neuron = action potential
Gate
The rest
of the
dominoes
fall!
+
+
channel
open
– – – + + + + + + + + + + + +
+ + + – – – – – – – – – – – –
Na+
+ + + – – – – – – – – – – – –
– – – + + + + + + + + + + + +
wave 
How does a nerve impulse travel?
• Re-set: 2nd wave travels down neuron
– K+ channels open
• K+ channels open up more slowly than Na+ channels
– K+ ions diffuse out of cell
– charges reverse back at that point
• negative inside; positive outside
Set
dominoes
back up
quickly!
K+
+ – – – – + + + + + + + + + +
– + + + + – – – – – – – – – –
Na+
– + + + + – – – – – – – – – –
+ – – – – + + + + + + + + + +
wave 
How does a nerve impulse travel?
• Combined waves travel down neuron
– wave of opening ion channels moves down neuron
– signal moves in one direction     
• flow of K+ out of cell stops activation of Na+ channels in
wrong direction
Ready
for
next time!
K+
+ + + – – – – + + + + + + + +
– – – + + + + – – – – – – – –
Na+
– – – + + + + – – – – – – – –
+ + + – – – – + + + + + + + +
wave 
How does a nerve impulse travel?
• Action potential propagates
– wave = nerve impulse, or action potential
– brain  finger tips in milliseconds!
In the
blink of
an eye!
K+
+ + + + + + + – – – – + + + +
– – – – – – – + + + + – – – –
Na+
– – – – – – – + + + + – – – –
+ + + + + + + – – – – + + + +
wave 
Voltage-gated channels
• Ion channels open & close in response to changes in
charge across membrane
– Na+ channels open quickly in response to depolarization &
close slowly
– K+ channels open slowly in response to depolarization &
close slowly
Structure
& function!
K+
+ + + + + + + + + – – – + + +
– – – – – – – – – + + + – – –
Na+
– – – – – – – – – + + + – – –
+ + + + + + + + + – – – + + +
wave 
How does the nerve re-set itself?
• After firing a neuron has to re-set itself
– Na+ needs to move back out
– K+ needs to move back in
– both are moving against concentration gradients
• need a pump!!
A lot of
work to
do here!
Na+
+
Na+ +
K
K Na+
+
K+
+
Na
Na+
Na+
K+
K
Na+
+Na
+
Na
Na
+ + + + + + + + + + – – – – +
– – +– – – – – – – – + + + + –
Na+
Na
K+
K+
+
+
K
K++ Na
+
+
+
+
Na
K
K
Na K
Na+
Na+
K+
– – – – – – – – – – + + + + –
+ + + + + + + + + + – – – – +
wave 
Na+
+
How does the nerve re-set itself?
• Sodium-Potassium pump
– active transport protein in membrane
• requires ATP
– 3 Na+ pumped out
– 2 K+ pumped in
– re-sets charge
across
membrane
That’s a lot
of ATP !
Feed me some
sugar quick!
ATP
Neuron is ready to fire again
Na+
Na+
Na+
K+
aa-
aaNa+
Na+
Na+
K+
Na+
Na+
K+
Na+
aa-
K+
Na+
Na+
Na+
Na+
K+
aaNa+
Na+
Na+
K+
Na+
Na+
Na+
K+
aa-
aa- K+
K+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
resting potential
+ + + + + + + + + + + + + + +
– – – – – – – – – – – – – – –
– – – – – – – – – – – – – – –
+ + + + + + + + + + + + + + +
Action potential graph
40 mV
4
30 mV
Membrane potential
1. Resting potential
2. Stimulus reaches threshold
potential
3. Depolarization
Na+ channels open;
K+ channels closed
4. Na+ channels close;
K+ channels open
5. Repolarization
reset charge gradient
6. Undershoot
K+ channels close slowly
20 mV
10 mV Depolarization
Na+ flows in
0 mV
–10 mV
3
–20 mV
Repolarization
K+ flows out
5
–30 mV
–40 mV
–50 mV
Threshold
–60 mV
2
–70 mV
–80 mV
1
Resting potential
Hyperpolarization
(undershoot)
6 Resting
Resting and Action Potential of the Axonal
Membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+60
+40
Na+ moves
to inside
axon
K+ moves
to outside
axon
action
potential
+20
Voltage (mV)
0
–20
–40
threshold
–60
resting
potential
0
1
2
3
4
5
6
Time (milliseconds)
d. An action potential can be visualized if voltage changes are
graphed over time.
28
Myelin sheath
 Axon coated with Schwann cells
signal
direction
insulates axon
 speeds signal

 signal hops from node to node
 saltatory conduction

150 m/sec vs. 5 m/sec
(330 mph vs. 11 mph)
myelin sheath
action potential
saltatory
conduction
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Na+
myelin
axon
+
+
+
+
+
–
–
Na+
Multiple Sclerosis
 immune system (T cells)
attack myelin sheath
 loss of signal
Nervous Tissue
• Propagation of Action Potentials
– In nonmyelinated axons, the action potential travels
down an axon one small section at a time
– In myelinated fibers, an action potential at one node
causes an action potential at the next node
• Saltatory (jumping) Conduction
– Conduction of a nerve impulse is an all-or-nothing
event
• Intensity of signal is determined by how many impulses are
generated within a given time span
31
Animation
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which is available at
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32
Animation
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
Animation
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
Nervous Tissue
• Transmission Across a Synapse
– A synapse is a region where neurons nearly touch
– Small gap between neurons is the synaptic cleft
– Transmission across a synapse is carried out by
neurotransmitters
• Sudden rise in calcium in the axon terminal of one neuron
• Calcium stimulates synaptic vesicles to merge with the
presynaptic membrane
• Neurotransmitter molecules are released into the synaptic
cleft
35
Synapse Structure and Function
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
path of action potential
1. After an
action
potential
arrives at an
axon terminal,
Ca2+ enters,
and synaptic
vesicles fuse
with the
presynaptic
membrane.
Ca2+
axon
terminal
synaptic vesicles
enclose neurotransmitter
cell body of
postsynaptic
neuron
synaptic cleft
2. Neurotransmitter
molecules
are released
and bind to
receptors
on the
postsynaptic
membrane.
presynaptic
membrane
neurotransmitter
neurotransmitter
receptor
Na+
postsynaptic
neuron
postsynaptic
membrane
3. When an
excitatory
neurotransmitter
binds to a
receptor,
Na+ diffuses
into the
postsynaptic
neuron, and
an action
potential
begins.
36
Animation
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
What happens at the end of the axon?
Impulse has to jump the synapse!
– junction between neurons
– has to jump quickly from one cell to next
How does
the wave
jump the gap?
Synapse
Chemical synapse
axon terminal
 Events at synapse
action potential

synaptic vesicles
synapse


Ca++

receptor protein
neurotransmitter
acetylcholine (ACh)

muscle cell (fiber)
We switched…
from an electrical signal 
to a chemical signal
action potential
depolarizes membrane
opens Ca++ channels
neurotransmitter vesicles
fuse with membrane
release neurotransmitter
to synapse  diffusion
neurotransmitter binds
with protein receptor
 ion-gated channels open
neurotransmitter
degraded or reabsorbed
Nerve impulse in next neuron
K+
• Post-synaptic neuron
– triggers nerve impulse in next nerve cell
• chemical signal opens ion-gated channels
Na+
+
binding site
• Na diffuses into cell
• K+ diffuses out of cell
Here we
go again!
– switch back to
voltage-gated channel
Na+
ACh
ion channel
K+
K+
Na+
– + + + + + + + + + + + + + +
+ – – – – – – – – – – – – – –
Na+
+ – – – – – – – – – – – – – –
– + + + + + + + + + + + + + +
Nervous Tissue
• Synaptic Integration
– A single neuron is on the receiving end of
• Many excitatory signals, and
• Many inhibitory signals
– Integration
• The summing of excitatory and inhibitory signals
41
Synaptic Integration
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
cell body of the neuron
+20
0
axon terminals
excitatory signal
integration
inhibitory signal
–20
–40
threshold
–70
resting
potential
–80
Time (milliseconds)
42
b.
(a): Courtesy Dr. E.R. Lewis, University of California Berkeley
37.3 The Central Nervous
System
• The Central Nervous System
– Consists of the brain and spinal cord
– Three specific functions:
• Receives sensory input
• Performs integration
• Generates motor output
43
The Central Nervous System
• The Central Nervous System
– Consists of the brain and spinal cord
– Spinal cord and brain are wrapped in three
protective membranes called meninges
• Spaces between meninges are filled with
cerebrospinal fluid
• Fluid is continuous with that of central canal of
spinal cord and the ventricles of the brain
44
The Central Nervous System
• The Spinal Cord
– Two main functions
• Center for many reflex actions
• Means of communication between the brain and
spinal nerves
– Composed of grey matter and white matter
• Cell bodies and short unmyelinated fibers give the
gray mater its color
• Myelinated long fibers of interneurons running in
tracts give white mater its color
45
The Central Nervous System
• The Brain
– The cerebrum is the largest portion of the
brain in humans
• Communicates with, and coordinates the activities
of, the other parts of the brain
• Longitudinal fissure divides the cerebrum into left
and right cerebral hemispheres
46
The Human Brain
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cerebrum
(telencephalon)
opening to lateral
ventricle
third ventricle
skull
meninges
corpus
callosum
pituitary gland
Diencephalon
thalamus
(surrounds the
third ventricle)
hypothalamus
pineal gland
fourth ventricle
Brain stem
midbrain
Cerebellum
pons
medulla
oblongata
a. Parts of brain
spinal cord
b. Cerebral hemispheres
47
The Lobes of a Cerebral
Hemisphere
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
central sulcus
Frontal lobe
primary somatosensory area
primary motor area
premotor area
motor speech
(Broca’s) area
prefrontal
area
Parietal lobe
leg
trunk
arm
hand
somatosensory
association area
primary taste area
general interpretation area
face
tongue
Occipital lobe
primary
visual area
lateral sulcus
Temporal lobe
visual
association
area
auditory association area
primary auditory area
sensory speech (Wernicke’s) area
48
The Central Nervous System
• Cerebral Cortex
• A thin but highly convoluted outer layer of
gray matter
• Covers the cerebral hemispheres
• Contains motor areas and sensory areas as
well as association areas
– Primary motor area is in the frontal lobe, just
ventral to the central sulcus
– Primary somatosensory area is in the parietal lobe,
just dorsal to the central sulcus
49
The Central Nervous System
• Other Parts of the Brain
• Diencephalon
• A region encircling the third ventricle
• Includes three structures
– Hypothalamus
• Forms the floor of the third ventricle
• Integrating center that maintains
homeostasis
• Controls the pituitary gland
50
The Central Nervous System
• Other Parts of the Brain
• Diencephalon (continued)
• Thalamus
• Consists of two masses of gray matter located in the
sides and roof of the third ventricle
• Receives all sensory input except smell
• Integrates sensory information and sends it to the
cerebrum
• Pineal gland
• Secretes melatonin
51
The Central Nervous System
• Other Parts of the Brain
• Cerebellum
• Separated from the brain stem by the fourth ventricle
• Largest portion of the brainstem
• Receives sensory input from the eyes, ears, joints, and
muscles
• Sends motor impulses out the brain stem to the skeletal
muscles
52
The Central Nervous System
• Other Parts of the Brain
• Brain Stem
• Contains the midbrain, pons, and the medulla
oblongata
• Midbrain
• Acts as a relay station for tracts passing between the
cerebrum and the spinal cord or cerebellum
• Pons
• Contains axons that form a bridge between the
cerebellum and the rest of the central nervous
system
• Medulla Oblongata
• Contains reflex centers for vomiting, coughing,
sneezing, hiccupping, and swallowing
53
37.4 The Peripheral Nervous
System
• Somatic system
– Includes cranial nerves and spinal nerves
• Gather information from sensors and conduct decisions to effectors
• Controls the skeletal muscles
– Voluntary
• Autonomic system
–
–
–
–
Controls the smooth muscles, cardiac muscles, and glands
Innervates all internal organs
Usually involuntary
Divided into two divisions
• Sympathetic division
• Parasympathetic division
– Utilizes two neurons and one ganglion for each impulse
54
Cranial and Spinal Nerves
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
spinal cord
gray matter
vertebra
white matter
frontal lobe
olfactory bulb
dorsal root
olfactory tract
dorsal root
ganglion
optic nerve
optic chiasma
spinal
nerve
ventral root
vertebra
b.
temporal lobe
central canal
cerebellum
gray matter
medulla
white matter
a.
c.
c: © Karl E. Deckart/Phototake
55
The Peripheral Nervous System
• Reflex Arc
• Sensory receptors generate a nerve impulse that moves along
sensory axons through a dorsal root ganglion toward the spinal
cord
• Sensory neurons pass signals on to many interneurons in the
gray matter of the spinal cord
• Nerve pulses travel along motor axons to an effector, which
brings about a response to the stimulus
56
A Reflex Arc Showing the
Path of a Spinal Reflex
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
pin
dorsal root ganglion
sensory
receptor
(in skin)
axon of sensory neuron
central canal
white matter
dendrites
Dorsal
gray matter
dorsal
horn
cell body of
sensory neuron
interneuron
dendrites
axon of motor neuron
cell body of
motor neuron
effector
(muscle)
ventral root
ventral horn
Ventral
57
The Peripheral Nervous System
• Sympathetic division
– Especially important during fight or flight
responses
– Accelerates heartbeat and dilates bronchi
• Parasympathetic division
– Promotes all internal responses associated
with a relaxed state
– Promotes digestion and retards heartbeat
58
Autonomic System Structure and Function
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
inhibits tears
stimulates tears
constricts pupils
dilates
pupils
ganglion
inhibits salivation
Sympathetic Division
Parasympathetic Division
stimulates
salivation
cranial
nerves
slows heart
speeds
heart
dilates air
passages
cervical
nerves
constricts
bronchioles
stimulates liver to
release glucose
stimulates gallbladder
to release bile
stimulates
adrenal
secretion
thoracic
nerves
vagus nerve
increases activity
of stomach and
pancreas
inhibits activity
of kidneys,
stomach, and
pancreas
increases
intestinal
activity
decreases
intestinal activity
lumbar
nerves
ganglion
inhibits
urination
stimulates
urination
causes
orgasmic
contractions
sympathetic ganglia
causes
erection
of genitals
sacral
nerves
Acetylcholine is neurotransmitter.
Norepinephrine is neurotransmitter.
59
Comparison of Somatic Motor
and Autonomic Motor Pathways
60