Download Action Potentials

Chapter 50
• Sensory and Motor Mechanisms
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Figure 49.1
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Sensory receptors
• Transmit signals to CNS
• Sensations are action potentials
• The brain interprets (integration)  perception
of stimuli
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Integration
• Begins as soon as the information is received
• Occurs at all levels of the nervous system
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Information Processing
• 3 stages
– Sensory input, integration, and motor output
Sensory input
Integration
Sensor
Motor output
Effector
Figure 48.3
Peripheral nervous
system (PNS)
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Central nervous
system (CNS)
Neuron Structure
Dendrites
Cell body
Nucleus
Synapse
Signal
Axon direction
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
Figure 48.5
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Synaptic
terminals
Action Potentials
• At the site where the action potential is
generated an electrical current depolarizes the
neighboring region of the axon membrane
Axon
Action
potential
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+Na +
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K+
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K+
+
Action
potential
–
+
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+
Na+
+
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+
–
K+
Figure 48.14
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K+
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Action
potential
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Na++
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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
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.
is
Conduction Speed
• Increases with the diameter of an axon
• Myelinated axons also faster
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• When action potential reaches a terminal
release of neurotransmitters into the synaptic
cleft
Postsynaptic cell
Presynaptic
cell
Synaptic vesicles
containing
neurotransmitter
5
Presynaptic
membrane
Ligandgated
ion channel
Ca2+ channel
Ca2+
4
2
3
Synaptic cleft
Figure 48.17
Neurotransmitter
Postsynaptic
membrane
Voltage-gated
1
Na+
K+
Ligand-gated
ion channels
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Postsynaptic
membrane
6
Mechanoreceptors
• sense physical deformation
– e.g. pressure, stretch, motion, and sound
Cold
Light touch
Pain
Hair
Heat
Epidermis
Dermis
Nerve
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Connective tissue
Hair movement
Strong pressure
Chemoreceptors
0.1 mm
• Transmit information about solute
concentration
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Electromagnetic Receptors
• Detect light, electricity, EM radiation, and
magnetism
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• Many mammals use the Earth’s magnetic field
lines to orient themselves as they migrate
Figure 49.5b
(b) Some migrating animals, such as these beluga whales, apparently
sense Earth’s magnetic field and use the information, along with
other cues, for orientation.
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Thermoreceptors
• Help regulate body temperature by signaling
both surface and body core temperature
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Hearing and Equilibrium in Mammals
human ear
1
2 The middle ear and inner ear
Overview of ear structure
Incus
Middle
ear
Inner ear
Outer ear
Stapes
Malleus
Skull
bones
Semicircular
canals
Auditory nerve,
to brain
Pinna
Tympanic
membrane
Auditory
canal
Hair cells
Cochlea
Eustachian
tube
Tectorial
membrane
Tympanic
membrane
Oval
window
Eustachian
tube
Round
window
Cochlear duct
Bone
Vestibular canal
Auditory nerve
Basilar
membrane
Figure 49.8
Axons of
sensory neurons
To auditory
nerve
4 The organ of Corti
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Tympanic canal
3 The cochlea
Organ of Corti
Equilibrium
• semicircular canals in the inner ear
semicircular canals
The
, arranged
in three spatial planes, detect angular movements of
the head.
Each canal has at its base a
swelling called an ampulla, containing a
cluster of hair cells.
When the head changes its rate
of rotation, inertia prevents endolymph in
the semicircular canals from moving with
the head, so the endolymph presses
against the cupula, bending the hairs.
Flow
of endolymph
Flow
of endolymph
Vestibular nerve
Cupula
Hairs
Hair
cell
Nerve
fibers
Vestibule
Utricle
Body movement
Saccule
Figure 49.11
The utricle and saccule tell the brain which way is up
and inform it of the body’s position or linear
acceleration.
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The hairs of the hair cells project into
a gelatinous cap called the cupula.
Bending of the hairs increases the
frequency of action potentials in sensory
neurons in direct proportion to the amount
of rotational acceleration.
Taste
• Modified epithelial cells organized into taste
buds
• Five taste perceptions; sweet, sour, salty,
bitter, and umami (elicited by glutamate)
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Smell
• Olfactory receptor cells line the upper portion of
the nasal cavity
Brain
Action potentials
Odorant
Olfactory bulb
Nasal cavity
Bone
Epithelial cell
Odorant
receptors
Chemoreceptor
Plasma
membrane
Figure 49.15
Odorant
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Cilia
Mucus
Vision
• Most invertebrates have some sort of lightdetecting organ
Light
Light shining from
the front is detected
Photoreceptor
Nerve to
brain
Visual pigment
Ocellus
Screening
pigment
Light shining from
behind is blocked
by the screening pigment
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• 2 types of image-forming eyes have evolved in
invertebrates
– compound eye and the single-lens eye
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• Vertebrate eye
Sclera
Choroid
Retina
Ciliary body
Fovea (center
of visual field)
Suspensory
ligament
Cornea
Iris
Optic
nerve
Pupil
Aqueous
humor
Lens
Vitreous humor
Central artery and
vein of the retina
Figure 49.18
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Optic disk
(blind spot)
• Retina contains two types of photoreceptors
– Rods,sensitive to light but do not distinguish
colors
– Cones, distinguish colors but are not as
sensitive
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Animal skeletons
• Support, protection, and movement
• Movement results from muscles working
against some type of skeleton
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Exoskeletons
• e.g. Molluscs and arthropods
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Endoskeletons
• Sponges, echinoderms, and chordates
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• Muscles move skeletal parts by contracting
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Vertebrate Skeletal Muscle
• hierarchy of smaller and smaller units
Muscle
Bundle of
muscle fibers
Nuclei
Single muscle fiber
(cell)
Plasma membrane
Myofibril
Z line
Light
band
Dark band
Sarcomere
0.5 m
TEM
I band
A band
I band
M line
Thick
filaments
(myosin)
Thin
filaments
(actin)
Figure 49.28
Z line
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H zone
Sarcomere
Z line
• Myofibrils are composed to 2 kinds of
myofilaments
– Thin filaments, consisting of actin
– Thick filaments, arrays of myosin molecules
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• Skeletal muscle is striated
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• The Sliding-Filament Model of Muscle
Contraction
(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bands
and H zone are relatively wide.
Z
H
A
Sarcomere
(b) Contracting muscle fiber. During contraction, the thick and
thin filaments slide past each other, reducing the width of the
I bands and H zone and shortening the sarcomere.
(c) Fully contracted muscle fiber. In a fully contracted muscle
fiber, the sarcomere is shorter still. The thin filaments overlap,
eliminating the H zone. The I bands disappear as the ends of
the thick filaments contact the Z lines.
Figure 49.29a–c
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0.5 m
• Myosin-actin interactions underlying muscle
fiber contraction
Thick filament
1 Starting here, the myosin head is
bound to ATP and is in its lowenergy confinguration.
Thin filaments
5 Binding of a new molecule of ATP releases the
myosin head from actin,
and a new cycle begins.
Thin filament
Myosin head (low-
ATP
Thick
filament
Thin filament moves
toward center of sarcomere.
Figure 49.30
+
Cross-bridge
binding site
Actin
ADP
Myosin head (lowenergy configuration)
ADP
2 The myosin head hydrolyzes
ATP to ADP and inorganic
phosphate ( P I ) and is in its
high-energy configuration.
energy configuration)
ATP
Pi
ADP
Pi
4 Releasing ADP and ( P i), myosin
relaxes to its low-energy configuration,
sliding the thin filament.
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Pi
Cross-bridge
Myosin head (highenergy configuration)
13 The myosin head binds to
actin, forming a crossbridge.
The Role of Calcium and Regulatory Proteins
• When a muscle is at rest
– The myosin-binding sites on the thin filament
are blocked by the regulatory protein
tropomyosin
Tropomyosin
Actin
Figure 49.31a
Ca2+-binding sites
(a) Myosin-binding sites blocked
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Troponin complex
• For a muscle fiber to contract
– The myosin-binding sites must be uncovered
• This occurs when calcium ions (Ca2+)
– Bind to another set of regulatory proteins, the
troponin complex
Ca2+
Myosinbinding site
Figure 49.31b
(b) Myosin-binding sites exposed
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• Stimulus leading to the contraction of muscle
fiber is an action potential in a motor neuron
Motor
neuron axon
Mitochondrion
Synaptic
terminal
T tubule
Sarcoplasmic
reticulum
Myofibril
Figure 49.32
Plasma membrane
of muscle fiber
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Ca2+ released
from sarcoplasmic
reticulum
Sarcomere
Review
Synaptic
terminal
of motor
neuron
Acetylcholine
PLASMA MEMBRANE
T TUBULE
Synaptic cleft
1
(
ACh
SR
2
Action potential
Ca2

3
Action potential
triggers Ca2+
release from sarcoplasmic reticulum
(SR).
7
Tropomyosin blockage
4
Ca2
CYTOSOL
6
ADP
P2
Calcium ions bind
Cytosolic Ca2+ is
removed by active
transport into
SR after action
potential ends.
5
Myosin cross-bridges
Figure 49.33
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Other Types of Muscle
• Cardiac muscle, striated cells electrically
connected by intercalated discs
– Can generate action potentials without neural
input
• Smooth muscle, contractions slow and may be
caused by stimulation from autonomic nervous
system
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