Download - Chapter 50
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint® Lecture Presentations for
Biology Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Chapter 50Chapter 50
Sensory and Motor Mechanisms
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Sensing and Acting
• Bats use sonar to detect their prey
• Moths, a common prey for bats, can detect the bat’s sonar and attempt to flee
• Both organisms have complex sensory systems that facilitate survival
• These systems include diverse mechanisms that sense stimuli and generate appropriate movement
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Fig. 50-1
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Concept 50.1: Sensory receptors transduce stimulus energy and transmit signals to the central nervous
system
• All stimuli represent forms of energy
• Sensation involves converting energy into a change in the membrane potential of sensory receptors
• Sensations are action potentials that reach the brain via sensory neurons
• The brain interprets sensations, giving the perception of stimuli
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Sensory Pathways
• Functions of sensory pathways: sensory reception, transduction, transmission, and integration
• For example, stimulation of a stretch receptor in a crayfish is the first step in a sensory pathway
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Fig. 50-2
Slight bend:weakstimulus Stretch
receptor
Mem
bra
ne
po
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tial
(m
V)
Axon
Dendrites
Strong receptorpotential
Weakreceptorpotential
Muscle
–50
–70M
emb
ran
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mV
)
–50
–70
Action potentials
Action potentials
Mem
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Large bend:strongstimulus
Reception
Transduction
0
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1 2 3 4 5 6 7
Mem
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Time (sec)
1 2 3 4 5 6 7Time (sec)
Transmission Perception
Brain
Brain perceiveslarge bend.
Brain perceivesslight bend.
1 2
34
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2 3 4
0
0
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Sensory Reception and Transduction
• Sensations and perceptions begin with sensory reception, detection of stimuli by sensory receptors
• Sensory receptors can detect stimuli outside and inside the body
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• Sensory transduction is the conversion of stimulus energy into a change in the membrane potential of a sensory receptor
• This change in membrane potential is called a receptor potential
• Many sensory receptors are very sensitive: they are able to detect the smallest physical unit of stimulus
– For example, most light receptors can detect a photon of light
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Transmission
• After energy has been transduced into a receptor potential, some sensory cells generate the transmission of action potentials to the CNS
• Sensory cells without axons release neurotransmitters at synapses with sensory neurons
• Larger receptor potentials generate more rapid action potentials
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• Integration of sensory information begins when information is received
• Some receptor potentials are integrated through summation
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Perception
• Perceptions are the brain’s construction of stimuli
• Stimuli from different sensory receptors travel as action potentials along different neural pathways
• The brain distinguishes stimuli from different receptors by the area in the brain where the action potentials arrive
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Amplification and Adaptation
• Amplification is the strengthening of stimulus energy by cells in sensory pathways
• Sensory adaptation is a decrease in responsiveness to continued stimulation
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Types of Sensory Receptors
• Based on energy transduced, sensory receptors fall into five categories:
– Mechanoreceptors
– Chemoreceptors
– Electromagnetic receptors
– Thermoreceptors
– Pain receptors
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Mechanoreceptors
• Mechanoreceptors sense physical deformation caused by stimuli such as pressure, stretch, motion, and sound
• The sense of touch in mammals relies on mechanoreceptors that are dendrites of sensory neurons
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Fig. 50-3
Connectivetissue
Heat
Strongpressure
Hairmovement
Nerve
Dermis
Epidermis
Hypodermis
Gentletouch
Pain Cold Hair
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Chemoreceptors
• General chemoreceptors transmit information about the total solute concentration of a solution
• Specific chemoreceptors respond to individual kinds of molecules
• When a stimulus molecule binds to a chemoreceptor, the chemoreceptor becomes more or less permeable to ions
• The antennae of the male silkworm moth have very sensitive specific chemoreceptors
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Fig. 50-4
0.1
mm
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Electromagnetic Receptors
• Electromagnetic receptors detect electromagnetic energy such as light, electricity, and magnetism
• Photoreceptors are electromagnetic receptors that detect light
• Some snakes have very sensitive infrared receptors that detect body heat of prey against a colder background
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Fig. 50-5
(a) Rattlesnake
(b) Beluga whales
Eye
Infraredreceptor
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Fig. 50-5a
(a) Rattlesnake
Eye
Infraredreceptor
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• Many mammals appear to use Earth’s magnetic field lines to orient themselves as they migrate
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Fig. 50-5b
(b) Beluga whales
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Thermoreceptors
• Thermoreceptors, which respond to heat or cold, help regulate body temperature by signaling both surface and body core temperature
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Pain Receptors
• In humans, pain receptors, or nociceptors, are a class of naked dendrites in the epidermis
• They respond to excess heat, pressure, or chemicals released from damaged or inflamed tissues
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Concept 50.2: The mechanoreceptors responsible for hearing and equilibrium detect moving fluid or settling particles
• Hearing and perception of body equilibrium are related in most animals
• Settling particles or moving fluid are detected by mechanoreceptors
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Sensing Gravity and Sound in Invertebrates
• Most invertebrates maintain equilibrium using sensory organs called statocysts
• Statocysts contain mechanoreceptors that detect the movement of granules called statoliths
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Fig. 50-6
Sensory axons
Statolith
Cilia
Ciliatedreceptor cells
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• Many arthropods sense sounds with body hairs that vibrate or with localized “ears” consisting of a tympanic membrane and receptor cells
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Fig. 50-7
1 mm
Tympanicmembrane
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Hearing and Equilibrium in Mammals
• In most terrestrial vertebrates, sensory organs for hearing and equilibrium are closely associated in the ear
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Fig. 50-8
Hair cell bundle froma bullfrog; the longestcilia shown areabout 8 µm (SEM).
Auditorycanal
EustachiantubePinna
Tympanicmembrane
Ovalwindow
Roundwindow
Stapes
Cochlea
Tectorialmembrane
Incus
Malleus
Semicircularcanals
Auditory nerveto brain
Skullbone
Outer earMiddle
ear Inner ear
Cochlearduct
Vestibularcanal
Bone
Tympaniccanal
Auditorynerve
Organ of Corti
To auditorynerve
Axons ofsensory neurons
Basilarmembrane
Hair cells
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Fig. 50-8a
Auditorycanal
EustachiantubePinna
Tympanicmembrane
Ovalwindow
Roundwindow
Stapes
Cochlea
Incus
Malleus
Semicircularcanals
Auditory nerveto brain
Skullbone
Outer earMiddle
ear Inner ear
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Fig. 50-8b
Cochlearduct
Vestibularcanal
Bone
Tympaniccanal
Auditorynerve
Organ of Corti
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Fig. 50-8c
Tectorialmembrane
To auditorynerve
Axons ofsensory neurons
Basilarmembrane
Hair cells
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Fig. 50-8d
Hair cell bundle froma bullfrog; the longestcilia shown areabout 8 µm (SEM).
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Hearing
• Vibrating objects create percussion waves in the air that cause the tympanic membrane to vibrate
• Hearing is the perception of sound in the brain from the vibration of air waves
• The three bones of the middle ear transmit the vibrations of moving air to the oval window on the cochlea
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• These vibrations create pressure waves in the fluid in the cochlea that travel through the vestibular canal
• Pressure waves in the canal cause the basilar membrane to vibrate, bending its hair cells
• This bending of hair cells depolarizes the membranes of mechanoreceptors and sends action potentials to the brain via the auditory nerve
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Fig. 50-9
“Hairs” ofhair cell
Neuro-trans-mitter atsynapse
Sensoryneuron
Moreneuro-trans-mitter
(a) No bending of hairs (b) Bending of hairs in one direction (c) Bending of hairs in other direction
Lessneuro-trans-mitter
Action potentials
Mem
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po
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(m
V)
0
–70
0 1 2 3 4 5 6 7
Time (sec)
Sig
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Sig
nal
–70
–50 Receptor potential
Mem
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(m
V)
0
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0 1 2 3 4 5 6 7Time (sec)
–70
–50
Mem
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0
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–50
Sig
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Fig. 50-9a
“Hairs” ofhair cell
Neuro-trans-mitter atsynapse
Sensoryneuron
(a) No bending of hairs
Action potentialsM
emb
ran
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ote
nti
al (
mV
)
0
–70
Time (sec)
Sig
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–70
–50
0 1 2 3 4 5 6 7
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Fig. 50-9b
Moreneuro-trans-mitter
(b) Bending of hairs in one direction
Receptor potential
Mem
bra
ne
po
ten
tial
(m
V)
0
–70
Time (sec)
Sig
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–70
–50
0 1 2 3 4 5 6 7
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Fig. 50-9c
Lessneuro-trans-mitter
(c) Bending of hairs in other direction
Mem
bra
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(m
V)
0
–70
Time (sec)
Sig
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0 1 2 3 4 5 6 7
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• The fluid waves dissipate when they strike the round window at the end of the tympanic canal
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Fig. 50-10
Axons ofsensory neurons
Vibration
Basilar membrane
Basilar membrane
Apex
Apex
Ovalwindow
Flexible end ofbasilar membrane
Vestibularcanal
500 Hz(low pitch)
16 kHz(high pitch)
Stapes
BaseRoundwindow
Tympaniccanal Fluid
(perilymph)Base(stiff)
8 kHz
4 kHz
2 kHz
1 kHz
{
{
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Fig. 50-10a
Axons ofsensory neurons
Vibration
Basilar membrane
Apex
Ovalwindow Vestibular
canal
Stapes
BaseRoundwindow
Tympaniccanal Fluid
(perilymph)
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• The ear conveys information about:
– Volume, the amplitude of the sound wave
– Pitch, the frequency of the sound wave
• The cochlea can distinguish pitch because the basilar membrane is not uniform along its length
• Each region vibrates most vigorously at a particular frequency and leads to excitation of a specific auditory area of the cerebral cortex
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Fig. 50-10b
Basilar membrane
Apex
Flexible end ofbasilar membrane
500 Hz(low pitch)
16 kHz(high pitch)
Base(stiff)
8 kHz
4 kHz
2 kHz
1 kHz
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Equilibrium
• Several organs of the inner ear detect body position and balance:
– The utricle and saccule contain granules called otoliths that allow us to detect gravity and linear movement
– Three semicircular canals contain fluid and allow us to detect angular acceleration such as the turning of the head
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Fig. 50-11
Vestibular nerve
Semicircular canals
Saccule
Utricle Body movement
Hairs
Cupula
Flow of fluid
Axons
Haircells
Vestibule
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Hearing and Equilibrium in Other Vertebrates
• Unlike mammals, fishes have only a pair of inner ears near the brain
• Most fishes and aquatic amphibians also have a lateral line system along both sides of their body
• The lateral line system contains mechanoreceptors with hair cells that detect and respond to water movement
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Fig. 50-12
Surrounding water
Lateral line
Lateral line canal
Epidermis
Hair cell
Cupula
Axon
Sensoryhairs
Scale
Lateral nerve
Opening oflateral line canal
Segmental muscles
Fish body wall
Supportingcell
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Concept 50.3: The senses of taste and smell rely on similar sets of sensory receptors
• In terrestrial animals:
– Gustation (taste) is dependent on the detection of chemicals called tastants
– Olfaction (smell) is dependent on the detection of odorant molecules
• In aquatic animals there is no distinction between taste and smell
• Taste receptors of insects are in sensory hairs called sensilla, located on feet and in mouth parts
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Taste in Mammals
• In humans, receptor cells for taste are modified epithelial cells organized into taste buds
• There are five taste perceptions: sweet, sour, salty, bitter, and umami (elicited by glutamate)
• Each type of taste can be detected in any region of the tongue
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Fig. 50-13
G proteinSugar molecule
Phospholipase C
Tongue
Sodiumchannel
PIP2
Na+
IP3
(secondmessenger)
Sweetreceptor
ER
Nucleus
Taste pore
SENSORYRECEPTORCELL
Ca2+
(secondmessenger)
IP3-gatedcalciumchannel
Sensoryreceptorcells
Tastebud
Sugarmolecule
Sensoryneuron
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• When a taste receptor is stimulated, the signal is transduced to a sensory neuron
• Each taste cell has only one type of receptor
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Fig. 50-14
PBDG receptorexpression in cellsfor sweet taste
Re
lati
ve
co
ns
um
pti
on
(%
)
Concentration of PBDG (mM); log scale
No PBDGreceptor gene
PBDG receptorexpression in cellsfor bitter taste
0.1 1 10
80
60
40
20
RESULTS
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Smell in Humans
• Olfactory receptor cells are neurons that line the upper portion of the nasal cavity
• Binding of odorant molecules to receptors triggers a signal transduction pathway, sending action potentials to the brain
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Fig. 50-15
Olfactorybulb
Odorants
Bone
Epithelialcell
Plasmamembrane
Odorantreceptors
Odorants
Nasal cavity
Brain
Chemo-receptor
Cilia
Mucus
Action potentials
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Concept 50.4: Similar mechanisms underlie vision throughout the animal kingdom
• Many types of light detectors have evolved in the animal kingdom
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Vision in Invertebrates
• Most invertebrates have a light-detecting organ
• One of the simplest is the eye cup of planarians, which provides information about light intensity and direction but does not form images
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Fig. 50-16
Nerve tobrain
Ocellus
Screeningpigment
Light
Ocellus
Visual pigment
Photoreceptor
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• Two major types of image-forming eyes have evolved in invertebrates: the compound eye and the single-lens eye
• Compound eyes are found in insects and crustaceans and consist of up to several thousand light detectors called ommatidia
• Compound eyes are very effective at detecting movement
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Fig. 50-17
Rhabdom
(a) Fly eyes
Crystallinecone
Lens
(b) OmmatidiaOmmatidium
PhotoreceptorAxons
Cornea
2 m
m
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Fig. 50-17a
(a) Fly eyes
2 m
m
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Fig. 50-17b
Rhabdom
Crystallinecone
Lens
(b) OmmatidiaOmmatidium
PhotoreceptorAxons
Cornea
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• Single-lens eyes are found in some jellies, polychaetes, spiders, and many molluscs
• They work on a camera-like principle: the iris changes the diameter of the pupil to control how much light enters
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The Vertebrate Visual System
• In vertebrates the eye detects color and light, but the brain assembles the information and perceives the image
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Structure of the Eye
• Main parts of the vertebrate eye:
– The sclera: white outer layer, including cornea
– The choroid: pigmented layer
– The iris: regulates the size of the pupil
– The retina: contains photoreceptors
– The lens: focuses light on the retina
– The optic disk: a blind spot in the retina where the optic nerve attaches to the eye
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• The eye is divided into two cavities separated by the lens and ciliary body:
– The anterior cavity is filled with watery aqueous humor
– The posterior cavity is filled with jellylike vitreous humor
• The ciliary body produces the aqueous humor
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Fig. 50-18
Opticnerve
Fovea (centerof visual field)
Lens
Vitreous humorOptic disk(blind spot)
Central artery andvein of the retina
Iris
RetinaChoroidSclera
Ciliary body
Suspensoryligament
Cornea
Pupil
Aqueoushumor
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• Humans and other mammals focus light by changing the shape of the lens
Animation: Near and Distance VisionAnimation: Near and Distance Vision
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Fig. 50-19
Ciliary musclesrelax.
Retina
Choroid
(b) Distance vision(a) Near vision (accommodation)
Suspensoryligaments pullagainst lens.
Lens becomesflatter.
Lens becomesthicker androunder.
Ciliary musclescontract.
Suspensoryligaments relax.
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• The human retina contains two types of photoreceptors: rods and cones
• Rods are light-sensitive but don’t distinguish colors
• Cones distinguish colors but are not as sensitive to light
• In humans, cones are concentrated in the fovea, the center of the visual field, and rods are more concentrated around the periphery of the retina
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Sensory Transduction in the Eye
• Each rod or cone contains visual pigments consisting of a light-absorbing molecule called retinal bonded to a protein called an opsin
• Rods contain the pigment rhodopsin (retinal combined with a specific opsin), which changes shape when absorbing light
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Fig. 50-20
Rod
Outersegment
Rhodopsin
Disks
Synapticterminal
Cell body
trans isomerRetinal
Opsin
Light
cis isomer
Enzymes
CYTOSOL
INSIDEOF DISK
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• Once light activates rhodopsin, cyclic GMP breaks down, and Na+ channels close
• This hyperpolarizes the cell
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Fig. 50-21
Light
Sodiumchannel
Inactive rhodopsin
Active rhodopsin Phosphodiesterase
Diskmembrane
INSIDE OF DISK
Plasmamembrane
EXTRACELLULARFLUID
Light
Transducin
CYTOSOL
GMP
cGMP
Na+
Na+
Dark
Time
Hyper-polarization
0
Mem
bra
ne
po
ten
tial
(m
V)
–40
–70
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• In humans, three pigments called photopsins detect light of different wave lengths: red, green, or blue
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Processing of Visual Information
• Processing of visual information begins in the retina
• Absorption of light by retinal triggers a signal transduction pathway
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Fig. 50-22
Light Responses
Rod depolarized
Rhodopsin inactive Rhodopsin active
Dark Responses
Na+ channels open Na+ channels closed
Glutamatereleased
Bipolar cell eitherdepolarized orhyperpolarized
Rod hyperpolarized
No glutamatereleased
Bipolar cell eitherhyperpolarized ordepolarized
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• In the dark, rods and cones release the neurotransmitter glutamate into synapses with neurons called bipolar cells
• Bipolar cells are either hyperpolarized or depolarized in response to glutamate
• In the light, rods and cones hyperpolarize, shutting off release of glutamate
• The bipolar cells are then either depolarized or hyperpolarized
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• Three other types of neurons contribute to information processing in the retina
– Ganglion cells transmit signals from bipolar cells to the brain; these signals travel along the optic nerves, which are made of ganglion cell axons
– Horizontal cells and amacrine cells help integrate visual information before it is sent to the brain
• Interaction among different cells results in lateral inhibition, a greater contrast in image
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Fig. 50-23
Retina
Retina
Photoreceptors
Light
Optic nerve
Light
Tobrain
Choroid
NeuronsCone Rod
Ganglioncell
Opticnerveaxons
Amacrinecell Horizontal
cell
Bipolarcell
Pigmentedepithelium
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• The optic nerves meet at the optic chiasm near the cerebral cortex
• Here, axons from the left visual field (from both the left and right eye) converge and travel to the right side of the brain
• Likewise, axons from the right visual field travel to the left side of the brain
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• Most ganglion cell axons lead to the lateral geniculate nuclei
• The lateral geniculate nuclei relay information to the primary visual cortex in the cerebrum
• Several integrating centers in the cerebral cortex are active in creating visual perceptions
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Fig. 50-24
Rightvisualfield
Righteye
Leftvisualfield
Lefteye
Opticchiasm
Primaryvisual cortex
Lateralgeniculatenucleus
Optic nerve
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Evolution of Visual Perception
• Photoreceptors in diverse animals likely originated in the earliest bilateral animals
• Melanopsin, a pigment in ganglion cells, may play a role in circadian rhythms in humans
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Concept 50.5: The physical interaction of protein filaments is required for muscle function
• Muscle activity is a response to input from the nervous system
• The action of a muscle is always to contract
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Vertebrate Skeletal Muscle
• Vertebrate skeletal muscle is characterized by a hierarchy of smaller and smaller units
• A skeletal muscle consists of a bundle of long fibers, each a single cell, running parallel to the length of the muscle
• Each muscle fiber is itself a bundle of smaller myofibrils arranged longitudinally
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• The myofibrils are composed to two kinds of myofilaments:
– Thin filaments consist of two strands of actin and one strand of regulatory protein
– Thick filaments are staggered arrays of myosin molecules
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• Skeletal muscle is also called striated muscle because the regular arrangement of myofilaments creates a pattern of light and dark bands
• The functional unit of a muscle is called a sarcomere, and is bordered by Z lines
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Fig. 50-25
Bundle ofmuscle fibers
TEM
Muscle
Thickfilaments(myosin)
M line
Single muscle fiber(cell)
Nuclei
Z lines
Plasma membrane
Myofibril
Sarcomere
Z line Z line
Thinfilaments(actin)
Sarcomere
0.5 µm
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Fig. 50-25a
Bundle ofmuscle fibers
Muscle
Single muscle fiber(cell)
Nuclei
Z lines
Plasma membrane
Myofibril
Sarcomere
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Fig. 50-25b
TEM
Thickfilaments(myosin)
M line
Z line Z line
Thinfilaments(actin)
Sarcomere
0.5 µm
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The Sliding-Filament Model of Muscle Contraction
• According to the sliding-filament model, filaments slide past each other longitudinally, producing more overlap between thin and thick filaments
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Fig. 50-26
Z
Relaxedmuscle
M Z
Fully contractedmuscle
Contractingmuscle
Sarcomere0.5 µm
ContractedSarcomere
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• The sliding of filaments is based on interaction between actin of the thin filaments and myosin of the thick filaments
• The “head” of a myosin molecule binds to an actin filament, forming a cross-bridge and pulling the thin filament toward the center of the sarcomere
• Glycolysis and aerobic respiration generate the ATP needed to sustain muscle contraction
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Fig. 50-27-1
Thinfilaments
ATP Myosin head (low-energy configuration
Thick filament
Thin filament
Thickfilament
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Fig. 50-27-2
Thinfilaments
ATP Myosin head (low-energy configuration
Thick filament
Thin filament
Thickfilament
Actin
Myosin head (high-energy configuration
Myosin binding sites
ADP
P i
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Fig. 50-27-3
Thinfilaments
ATP Myosin head (low-energy configuration
Thick filament
Thin filament
Thickfilament
Actin
Myosin head (high-energy configuration
Myosin binding sites
ADP
P i
Cross-bridgeADP
P i
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Fig. 50-27-4
Thinfilaments
ATP Myosin head (low-energy configuration
Thick filament
Thin filament
Thickfilament
Actin
Myosin head (high-energy configuration
Myosin binding sites
ADP
P i
Cross-bridgeADP
P i
Myosin head (low-energy configuration
Thin filament movestoward center of sarcomere.
ATP
ADP P i+
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The Role of Calcium and Regulatory Proteins
• A skeletal muscle fiber contracts only when stimulated by a motor neuron
• When a muscle is at rest, myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin
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Fig. 50-28
Myosin-binding site
Tropomyosin
(a) Myosin-binding sites blocked
(b) Myosin-binding sites exposed
Ca2+
Ca2+-binding sites
Troponin complexActin
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• For a muscle fiber to contract, myosin-binding sites must be uncovered
• This occurs when calcium ions (Ca2+) bind to a set of regulatory proteins, the troponin complex
• Muscle fiber contracts when the concentration of Ca2+ is high; muscle fiber contraction stops when the concentration of Ca2+ is low
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• The stimulus leading to contraction of a muscle fiber is an action potential in a motor neuron that makes a synapse with the muscle fiber
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Fig. 50-29
Sarcomere
Ca2+
ATPasepump
Ca2+ released from SR
Synapticterminal
T tubule
Motorneuron axon
Plasma membraneof muscle fiber
Sarcoplasmicreticulum (SR)
Myofibril
Synaptic terminalof motor neuron
Mitochondrion
Synaptic cleft T Tubule Plasma membrane
Ca2+
Ca2+
CYTOSOL
SR
ATP
ADPP i
ACh
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Fig. 50-29a
SarcomereCa2+ released from SR
Synapticterminal
T tubule
Motorneuron axon
Plasma membraneof muscle fiber
Sarcoplasmicreticulum (SR)
Myofibril
Mitochondrion
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• The synaptic terminal of the motor neuron releases the neurotransmitter acetylcholine
• Acetylcholine depolarizes the muscle, causing it to produce an action potential
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Fig. 50-29b
Ca2+
ATPasepump
Synaptic terminalof motor neuron
Synaptic cleft T Tubule Plasma membrane
Ca2+
Ca2+
CYTOSOL
SR
ATP
ADPP i
ACh
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• Action potentials travel to the interior of the muscle fiber along transverse (T) tubules
• The action potential along T tubules causes the sarcoplasmic reticulum (SR) to release Ca2+
• The Ca2+ binds to the troponin complex on the thin filaments
• This binding exposes myosin-binding sites and allows the cross-bridge cycle to proceed
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• Amyotrophic lateral sclerosis (ALS), formerly called Lou Gehrig’s disease, interferes with the excitation of skeletal muscle fibers; this disease is usually fatal
• Myasthenia gravis is an autoimmune disease that attacks acetylcholine receptors on muscle fibers; treatments exist for this disease
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Nervous Control of Muscle Tension
• Contraction of a whole muscle is graded, which means that the extent and strength of its contraction can be voluntarily altered
• There are two basic mechanisms by which the nervous system produces graded contractions:
– Varying the number of fibers that contract
– Varying the rate at which fibers are stimulated
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• In a vertebrate skeletal muscle, each branched muscle fiber is innervated by one motor neuron
• Each motor neuron may synapse with multiple muscle fibers
• A motor unit consists of a single motor neuron and all the muscle fibers it controls
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Fig. 50-30Spinal cord
Motor neuroncell body
Motor neuronaxon
Nerve
Muscle
Muscle fibers
Synaptic terminals
Tendon
Motorunit 1
Motorunit 2
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• Recruitment of multiple motor neurons results in stronger contractions
• A twitch results from a single action potential in a motor neuron
• More rapidly delivered action potentials produce a graded contraction by summation
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Fig. 50-31
Summation oftwo twitches
Tetanus
Singletwitch
Time
Ten
sio
n
Pair ofaction
potentials
Actionpotential Series of action
potentials athigh frequency
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• Tetanus is a state of smooth and sustained contraction produced when motor neurons deliver a volley of action potentials
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Types of Skeletal Muscle Fibers
• Skeletal muscle fibers can be classified
– As oxidative or glycolytic fibers, by the source of ATP
– As fast-twitch or slow-twitch fibers, by the speed of muscle contraction
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Oxidative and Glycolytic Fibers
• Oxidative fibers rely on aerobic respiration to generate ATP
• These fibers have many mitochondria, a rich blood supply, and much myoglobin
• Myoglobin is a protein that binds oxygen more tightly than hemoglobin does
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• Glycolytic fibers use glycolysis as their primary source of ATP
• Glycolytic fibers have less myoglobin than oxidative fibers, and tire more easily
• In poultry and fish, light meat is composed of glycolytic fibers, while dark meat is composed of oxidative fibers
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Fast-Twitch and Slow-Twitch Fibers
• Slow-twitch fibers contract more slowly, but sustain longer contractions
• All slow twitch fibers are oxidative
• Fast-twitch fibers contract more rapidly, but sustain shorter contractions
• Fast-twitch fibers can be either glycolytic or oxidative
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• Most skeletal muscles contain both slow-twitch and fast-twitch muscles in varying ratios
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Other Types of Muscle
• In addition to skeletal muscle, vertebrates have cardiac muscle and smooth muscle
• Cardiac muscle, found only in the heart, consists of striated cells electrically connected by intercalated disks
• Cardiac muscle can generate action potentials without neural input
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• In smooth muscle, found mainly in walls of hollow organs, contractions are relatively slow and may be initiated by the muscles themselves
• Contractions may also be caused by stimulation from neurons in the autonomic nervous system
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Concept 50.6: Skeletal systems transform muscle contraction into locomotion
• Skeletal muscles are attached in antagonistic pairs, with each member of the pair working against the other
• The skeleton provides a rigid structure to which muscles attach
• Skeletons function in support, protection, and movement
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Fig. 50-32
GrasshopperHuman
Bicepscontracts
Tricepscontracts
Forearmextends
Bicepsrelaxes
Tricepsrelaxes
Forearmflexes
Tibiaflexes
Tibiaextends
Flexormusclerelaxes
Flexormusclecontracts
Extensormusclecontracts
Extensormusclerelaxes
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Types of Skeletal Systems
• The three main types of skeletons are:
– Hydrostatic skeletons (lack hard parts)
– Exoskeletons (external hard parts)
– Endoskeletons (internal hard parts)
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Hydrostatic Skeletons
• A hydrostatic skeleton consists of fluid held under pressure in a closed body compartment
• This is the main type of skeleton in most cnidarians, flatworms, nematodes, and annelids
• Annelids use their hydrostatic skeleton for peristalsis, a type of movement on land produced by rhythmic waves of muscle contractions
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Fig. 50-33Circularmusclecontracted
Circularmusclerelaxed
Longitudinalmuscle relaxed(extended)
Longitudinalmusclecontracted
BristlesHead end
Head end
Head end
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Exoskeletons
• An exoskeleton is a hard encasement deposited on the surface of an animal
• Exoskeletons are found in most molluscs and arthropods
• Arthropod exoskeletons are made of cuticle and can be both strong and flexible
• The polysaccharide chitin is often found in arthropod cuticle
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Endoskeletons
• An endoskeleton consists of hard supporting elements, such as bones, buried in soft tissue
• Endoskeletons are found in sponges, echinoderms, and chordates
• A mammalian skeleton has more than 200 bones
• Some bones are fused; others are connected at joints by ligaments that allow freedom of movement
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Fig. 50-34
Examplesof joints
Humerus
Ball-and-socket joint
Radius
Scapula
Head ofhumerus
Ulna
Hinge joint
Ulna
Pivot joint
Skull
Shouldergirdle
Rib
Sternum
Clavicle
Scapula
Vertebra
Humerus
Phalanges
Radius
Pelvicgirdle
Ulna
Carpals
Metacarpals
Femur
Patella
Tibia
Fibula
TarsalsMetatarsalsPhalanges
1
1
2
3
3
2
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Fig. 50-34a
Examplesof jointsSkull
Shouldergirdle
RibSternum
ClavicleScapula
VertebraHumerus
Phalanges
Radius
Pelvic girdle
Ulna
Carpals
Metacarpals
FemurPatella
Tibia
Fibula
Tarsals
Metatarsals
Phalanges
1
2
3
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Fig. 50-34b
Ball-and-socket joint
Scapula
Head ofhumerus
1
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Fig. 50-34c
Hinge joint
Ulna
Humerus
2
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Fig. 50-34d
Pivot joint
UlnaRadius
3
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Size and Scale of Skeletons
• An animal’s body structure must support its size
• The size of an animal’s body scales with volume (a function of n3), while the support for that body scales with cross-sectional area of the legs (a function of n2)
• As objects get larger, size (n3) increases faster than cross-sectional area (n2); this is the principle of scaling
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• The skeletons of small and large animals have different proportions because of the principle of scaling
• In mammals and birds, the position of legs relative to the body is very important in determining how much weight the legs can bear
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Types of Locomotion
• Most animals are capable of locomotion, or active travel from place to place
• In locomotion, energy is expended to overcome friction and gravity
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Swimming
• In water, friction is a bigger problem than gravity
• Fast swimmers usually have a streamlined shape to minimize friction
• Animals swim in diverse ways
– Paddling with their legs as oars
– Jet propulsion
– Undulating their body and tail from side to side, or up and down
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Locomotion on Land
• Walking, running, hopping, or crawling on land requires an animal to support itself and move against gravity
• Diverse adaptations for locomotion on land have evolved in vertebrates
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Fig. 50-35
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Flying
• Flight requires that wings develop enough lift to overcome the downward force of gravity
• Many flying animals have adaptations that reduce body mass
– For example, birds lack teeth and a urinary bladder
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Energy Costs of Locomotion
• The energy cost of locomotion
– Depends on the mode of locomotion and the environment
– Can be estimated by the rate of oxygen consumption or carbon dioxide production
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Fig. 50-36
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• Animals specialized for swimming expend less energy per meter traveled than equivalently sized animals specialized for flying or running
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Fig. 50-37
Body mass (g)
Running
Swimming
Flying
En
erg
y co
st (
cal/
kg•m
)
102
103
10
1
10–1
10–3 1061
RESULTS
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Fig. 50-UN1
Stimulus
Sensoryreceptor
Stimulus
Afferentneuron
Sensoryreceptorcell
Afferentneuron
To CNS To CNS
Neuro-transmitter
Receptorprotein forneuro-transmitter
(a) Receptor is afferent neuron. (b) Receptor regulates afferent neuron.
Stimulusleads toneuro-transmitterrelease
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Fig. 50-UN2
Sensoryreceptors
More receptorsactivated
Low frequency of action potentials
(b) Multiple receptors activated
Fewer receptorsactivated
(a) Single sensory receptor activated High frequency of action potentials
Gentlepressure
Morepressure
More pressure
Gentle pressure
Sensoryreceptor
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Fig. 50-UN3
FoveaPosition along retina (in degrees away from fovea)
Opticdisk
Nu
mb
er o
f p
ho
tore
cep
tors
90°45°0°–45°–90°
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Fig. 50-UN4
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You should now be able to:
1. Distinguish between the following pairs of terms: sensation and perception; sensory transduction and receptor potential; tastants and odorants; rod and cone cells; oxidative and glycolytic muscle fibers; slow-twitch and fast-twitch muscle fibers; endoskeleton and exoskeleton.
2. List the five categories of sensory receptors and explain the energy transduced by each type.
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3. Explain the role of mechanoreceptors in hearing and balance.
4. Give the function of each structure using a diagram of the human ear.
5. Explain the basis of the sensory discrimination of human smell.
6. Identify and give the function of each structure using a diagram of the vertebrate eye.
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7. Identify the components of a skeletal muscle cell using a diagram.
8. Explain the sliding-filament model of muscle contraction.
9. Explain how a skeleton combines with an antagonistic muscle arrangement to provide a mechanism for movement.