Chapter 49. Sensory and Motor Mechanisms. 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. Figure 49.1. Both of these organisms Have complex sensory systems that facilitate their survival
Sensory and Motor Mechanisms
Weak and transmit signals to the central nervous systemmuscle stretch
stretch, producing a receptor potential in the stretch receptor. The receptor potential triggers action potentials in the axon of the stretch
receptor. A stronger stretch producesa larger receptor potential and higherrequency of action potentials.
(a) Crayfish stretch receptors have dendrites embedded in abdominal muscles. When the abdomen bends, muscles and dendrites
Fluid moving in and transmit signals to the central nervous systemone direction
Fluid moving in other direction
“Hairs” ofhair cell
Neuro-trans-mitter at synapse
(b) Vertebrate hair cells have specialized cilia or microvilli (“hairs”) that bend when sur-rounding fluid moves. Each hair cell releases an excitatory neurotransmitter at a synapse
with a sensory neuron, which conducts action potentials to the CNS. Bending in one direction depolarizes the hair cell, causing it to release more neurotransmitter and increasing frequency
of action potentials in the sensory neuron. Bending in the other direction has the opposite effects. Thus, hair cells respond to the direction of motion as well as to its strength and speed.s
Cold and transmit signals to the central nervous system
0.1 mm and transmit signals to the central nervous system
(a) This rattlesnake and other pit vipers have a pair of infrared receptors,one between each eye and nostril. The organs are sensitive enoughto detect the infrared radiation emitted by a warm mouse a meter away. The snake moves its head from side to side until the radiation is detected equally by the two receptors, indicating that the mouse is straight ahead.
(b) Some migrating animals, such as these beluga whales, apparentlysense Earth’s magnetic field and use the information, along with other cues, for orientation.
Ciliated equilibrium detect settling particles or moving fluidreceptor cells
Sensory nerve fibersSensing Gravity and Sound in Invertebrates
Tympanic equilibrium detect settling particles or moving fluidmembrane
1 equilibrium detect settling particles or moving fluid
Overview of ear structure
The middle ear and inner ear
Auditory nerve,to brain
Axons of sensory neurons
Organ of Corti
The organ of Corti
Cochlea equilibrium detect settling particles or moving fluid
Cochlea equilibrium detect settling particles or moving fluid(uncoiled)
Apex(wide and flexible)
500 Hz(low pitch)
16 kHz(high pitch)
Frequency producing maximum vibration
Base(narrow and stiff)
Each canal has at its base a equilibrium detect settling particles or moving fluidswelling called an ampulla, containing a cluster of hair cells.
The semicircular canals, arranged in three spatial planes, detect angular movements of the head.
When the head changes its rateof rotation, inertia prevents endolymph in the semicircular canals from moving with the head, so the endolymph presses against the cupula, bending the hairs.
The hairs of the hair cells project into a gelatinous cap called the cupula.
The utricle and saccule tell the brain which way is up and inform it of the body’s position or linear acceleration.
Bending of the hairs increases the frequency of action potentials in sensory neurons in direct proportion to the amount of rotational acceleration.
Lateral equilibrium detect settling particles or moving fluidline
Opening of lateralline canal
Lateral line canal
Segmental muscles of body wall
EXPERIMENT hairs called sensilla Insects taste using gustatory sensilla (hairs) on their feet and mouthparts. Each sensillum contains four chemoreceptors with dendrites that extend to a pore at the tip of the sensillum. To study the sensitivity of each chemoreceptor, researchers immobilized a blowfly (Phormia regina) by attaching it to a rod with wax. They then inserted the tip of a microelectrode into one sensillum to record action potentials in the chemoreceptors, while they used a pipette to touch the pore with various test substances.
RESULTS Each chemoreceptor is especially sensitive to a particular class of substance, but this specificity is relative; each cell can respond to some extent to a broad range of different chemical stimuli.
Pore at tip
Pipette containingtest substance
Number of action potentials
in first second of response
CONCLUSION Any natural food probably stimulates multiple chemoreceptors. By integrating sensations, the insect’s brain can apparently distinguish a very large number of tastes.
Taste pore hairs called sensilla
1 A sugar molecule binds to a receptor protein on the sensory receptor cell.
2Binding initiates a signal transduction pathway involving cyclic AMP and protein kinase A.
3Activated protein kinase A closes K+ channels in the membrane.
4 The decrease in the membrane’s permeability to K+ depolarizes the membrane.
5 Depolarization opens voltage-gated calcium ion (Ca2+) channels, and Ca2+ diffuses into the receptor cell.
6 The increased Ca2+ concentration causes synaptic vesicles to release neurotransmitter.
Brain hairs called sensilla
Light the animal kingdom
Light shining from the front is detected
Nerve to brain
Light shining from behind is blockedby the screening pigment
(a) invertebrates The faceted eyes on the head of a fly,
photographed with a stereomicroscope.
(b) The cornea and crystalline cone of each ommatidium function as
a lens that focuses light on the rhabdom, a stack of pigmented
plates inside a circle of photoreceptors. The rhabdom traps light and guides it to photoreceptors. The image formed by a compound eye is a mosaic of dots produced by different intensities of light entering the many ommatidia from different angles.
Fovea (centerof visual field)
Central artery and
vein of the retina
Optic disk(blind spot)
Front view of lens invertebratesand ciliary muscle
Ciliary muscles contract, pullingborder of choroid toward lens
Suspensory ligaments relax
Lens becomes thicker and rounder, focusing on near objects
(a) Near vision (accommodation)
Ciliary muscles relax, and border of choroid moves away from lens
Suspensory ligaments pull against lens
Lens becomes flatter, focusing on distant objects
(b) Distance vision
(b) Retinal exists as two isomers. Absorption of light converts the cis isomer to the trans isomer, which causes opsin to change its conformation (shape). After a few minutes, retinal detaches from opsin. In the dark, enzymes convert retinal back to its cis form, which recombines with opsin to form rhodopsin.
(a) Rods contain the visual pigment rhodopsin, which is embedded in a stack of membranous disks in the rod’s outer segment. Rhodopsin consists of the light-absorbing molecule retinal bonded to opsin, a protein. Opsin has seven helices that span the disk membrane.
Figure 49.20a, b
INSIDE OF DISK
– Hyper- polarization
3Transducinactivates theenzyme phos-phodiesterae(PDE).
4 Activated PDE detaches cyclic guanosine monophosphate (cGMP) from Na+ channels in the plasma membrane by hydrolyzing cGMP to GMP.
2Active rhodopsin in turn activates a G protein called transducin.
1Light isomerizes retinal, which activates rhodopsin.
5The Na+ channels close when cGMP detaches. The membrane’s permeability to Na+ decreases, and the rod hyperpolarizes.
Dark Responses invertebrates
Na+ channels closed
Na+ channels open
Bipolar cell either
depolarized, depending on glutamate receptors
Bipolar cell either
Circular protection, and movement
(a) Body segments at the head and just in front of the rear are short and thick (longitudinal muscles contracted; circular muscles relaxed) and anchored to the ground by bristles. The other segments are thin and elongated (circular muscles contracted; longitudinal muscles relaxed.)
(b) The head has moved forward because circular muscles in the head segments have contracted. Segments behind the head and at the rear are now thick and anchored, thus preventing the worm from slipping backward.
(c) The head segments are thick again and anchored in their new positions. The rear segments have released their hold on the ground and have been pulled forward.
Head of protection, and movementhumerus
1Ball-and-socket joints, where the humerus contactsthe shoulder girdle and where the femur contacts thepelvic girdle, enable us to rotate our arms andlegs and move them in several planes.
2Hinge joints, such as between the humerus andthe head of the ulna, restrict movement to a singleplane.
3Pivot joints allow us to rotate our forearm at theelbow and to move our head from side to side.
Human protection, and movement
Muscle protection, and movement
Bundle ofmuscle fibers
Single muscle fiber
SarcomereVertebrate Skeletal Muscle
0.5 m protection, and movement
(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bandsand H zone are relatively wide.
(b) Contracting muscle fiber. During contraction, the thick andthin filaments slide past each other, reducing the width of theI bands and H zone and shortening the sarcomere.
(c) Fully contracted muscle fiber. In a fully contracted musclefiber, the sarcomere is shorter still. The thin filaments overlap,eliminating the H zone. The I bands disappear as the ends ofthe thick filaments contact the Z lines.
Thick filament protection, and movement
1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration.
5 Binding of a new mole-
cule of ATP releases the
myosin head from actin,
and a new cycle begins.
Myosin head (low-energy configuration)
The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration.
Cross-bridge binding site
Thin filament moves toward center of sarcomere.
Myosin head (high-energy configuration)
Myosin head (low-energy configuration)
1 The myosin head binds toactin, forming a cross-bridge.
Releasing ADP and ( i), myosinrelaxes to its low-energy configuration, sliding the thin filament.
Tropomyosin protection, and movement
(a) Myosin-binding sites blocked
Ca protection, and movement2+
(b) Myosin-binding sites exposed
Motor protection, and movementneuron axon
Ca2+ releasedfrom sarcoplasmicreticulum
Plasma membraneof muscle fiber
Acetylcholine (ACh) released by synaptic terminal diffuses across synapticcleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber.
Action potential is propa-
gated along plasma
membrane and down
release from sarco-
Calcium ions bind to troponin;
troponin changes shape,
removing blocking action
of tropomyosin; myosin-binding
Tropomyosin blockage of myosin-
binding sites is restored; contraction
ends, and muscle fiber relaxes.
Cytosolic Ca2+ is
removed by active
SR after action
Myosin cross-bridges alternately attach
to actin and detach, pulling actin
filaments toward center of sarcomere;
ATP powers sliding of filaments.
Motor produces graded contractions of whole musclesunit 1
Motor neuroncell body
Tetanus produces graded contractions of whole muscles
Summation of two twitches
Series of action potentials at high frequency
EXPERIMENT friction and gravity
Physiologists typically determine an animal’s rate of energy use during locomotion by measuring its oxygen consumption or carbon dioxide production while it swims in a water flume, runs on a treadmill, or flies in a wind tunnel. For example, the trained parakeet shown below is wearing a plastic face mask connected to a tube that collects the air the bird exhales as it flies.
This graph compares the energy cost, in joules per kilogram of body mass per meter traveled, for animals specialized for running, flying, and swimming (1 J = 0.24 cal). Notice that both axes are plotted on logarithmic scales.
Energy cost (J/Kg/m)
Comparing Costs of Locomotion
For animals of a given body mass, swimming is the most energy-efficient and running the least energy-efficient mode of locomotion. In any mode, a small animal expends more energy per kilogram of body mass than a large animal.