Bio 215 Chapter 49: Sensory and Motor Mechanisms
Ch 49: Sensory and Motor Mechanisms
1 Sensory receptors detect changes in the external and internal environment: an overview
An animal’s interaction with its environment depends on the processing of sensory information and the generation of
1) Sensory receptors receive information form the environment.
2) Motor effectors carry out the movement in response to the sensory information
A) sensation and perception Sensations = action potentials traveling along sensory neurons that are interpreted by
the brain as perceptions; what one perceives depends on what part of the brain receives the impulse
B. The general function of sensory receptors = structures that transmit information about changes in an animal’s
internal and external environment
a) are usually modified neurons occurring singly or within groups in sensory organs
b) specialized to respond to specific stimuli and convert stimuli energy into electrochemical energy of action
c) All receptor cells have the same five functions: reception, transduction, amplification, transmission, and
Reception = ability of a cell to absorb the energy of a stimulus is specific to the receptor
Transduction = conversion of stimulus energy into electrochemical activity of nerve impulses (action
potentials) stimulus energy may change membrane permeability of the receptor cell, open of close ion
channel gates, or increase ion flow by stretching the receptor cell membrane
Amplification= of stimulus energy which is too week to be carried into the nervous system often occurs
Initial responses to stimuli during transmission are graded changes in membrane potential (receptor
potential); the stimulus modulates action potential frequency.
Transmission= the receptor cell can double as a sensory neuron (e.g. pain cell) so the intensity of the
receptor potential will affect the frequency of action potentials that convey sensations to the CNS. )O the
receptor can transmit a chemical signal (neurotransmitter) across a synapse to a secondary neuron. IN this
case the amount of neurotransmitter that is released influences the frequency of the action potential.
Integration= receptor signals are integrated through summation of graded potentials
sensory adaptation is a decrease in sensitivity during continued stimulation; a type of integration which
results in selective information being sent to the CNS.
The threshold for firing in receptor cells varies with conditions resulting in a change in receptor
Sensory information integration occurs at all levels in the nervous system
C. Types of Receptors: Exteroreceptors monitor the external environment while interoreceptors monitor changes in
the bodies internal environment. These are separated into 5 types
1) Mechanoreceptors are stimulated by their physical deformation caused by pressure, stretch, motion, sound
a) bending of the plasma membrane increases its permeability to Na+ and K+ resulting in a receptor
b) In human skin, Pacinian corpuscles deep in the skin respond to strong pressure, while Meissner’s
corpuscles and Merkel’s discs, closer to the surface, detect light touch.
c) Muscle spindles are stretch receptors ( a type of interoreceptors) that monitor the length of skeletal
muscles, as in the reflex arc.
d) Hair cells detect motion.
2) Chemoreceptors include general receptors that sense total solution concentration, receptors can respond to
individual molecules and to whole categories of related chemicals (salt etc)
3) Electromagnetic receptors respond to electromagnetic radiation such as light and magnetic fields, and
electricity. Evidence indicates that the photoreceptors are homologous.
4) Thermoreceptors respond to heat or cold and help regulate body temperature
a) in human skin, ruffini’s end organs may be heat receptors, and end-bulbs of Krause may be cold
b) The interothermoreceptors in the hypothalamus function as the primary temperature control of the
5) Nociceptors = A class of naked dendrites that function as pain receptors
a) different groups respond to excess heat, pressure, or specific chemicals released form damaged or
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Bio 215 Chapter 49: Sensory and Motor Mechanisms
b) prostaglandins increase pain by lowering receptor thresholds; aspirin reduces pain by inhibiting
2. Photoreceptors contain light-absorbing visual pigments: a closer look
1) the eye cup of planarians is a simple light receptor that responds to light intensity and direction without
forming an image
a) an opening on one side of the cup permits light to enter; the opening of one cup faces left and slightly
forward, the other cup opens right and slightly forward
b) light enters the opening and stimulates photoreceptors that contain light absorbing pigments.
c) Planaris move away form light sources to avoid predators
d) The proper direction is determined by the brain, which compares the rate of nerve impulses coming
from the two cups; the animal turns until the impulses form each cup are equal and minimal
2) two types of image forming eyes evolved in invertebrates:
a) a compound eye contains thousands of light detectors called ommatidia each with its own cornea and
Found in insects, crustaceans, and some polychaete worms
Results in a mosaic image
More acute at detecting movement partly due to rapid recovery of photoreceptors
Superimposition eyes have ommatidia with lens that work like prisms and parabolic mirrors,
focusing light entering several ommatidia onto photoreceptor
b) in a single lens eye, one lens focuses light onto the retina a bilayer of photosensitive receptor cells;
found in some jellyfish, polychaetes, spiders and many mollusks
B. structure and function of the vertebrate Eye
1) The parts of the vertebrate eye are structurally and functionally diverse
a) tough outer layer of connective tissue (sclera), a thin inner pigmented layer (choroid) and a thin layer of
cells (conjunctiva) covers the sclera and keeps the eye moist.
b) the cornea located in front and is transparent area of the sclera, allows light to enter the eye and acts as a
c) The anterior choroid forms the iris, that regulates the amount of light entering the pupil.
d) the retina is the innermost layer and contains the photoreceptor cells which transmit signals for the optic
disc to the optic nerve.
e) the aqueous and vitreous humor are liquids that help focus light onto the retina.
f) The lens is a circular protein structure that can focus an image on the retina by changing shape which is
controlled by the ciliary muscle (accommodation)
g) photoreceptors are rod and cone cells (rod sensitive to light b/w) cone are color but less sensitive.
2) the photoreceptors convert light energy into a signal and transude this stimuli into an action potential.
a) the visual pigments consist of light absorbing retinal which is synthesized form vitamin A bonded to a
membrane protein opsin.
b) rods contain rhodopsin. When light is absorbed the retinal component changes shape. this triggers a
chain of events that hyperpolarizes the photoreceptor cell membrane. In bright light this is continuously
hyperpolarized and is therefor ineffective.
3) processing and vision are functions performed in the cerebral cortex.
a) The optic nerve form each eye meets at the optic chiasm.
3. Hearing and equilibrium: are elated in most animals and involve mechanoreceptors.
A. The mammalian hearing organ is within the inner ear.
B. The inner ear also contains the organs of equilibrium.
C. A lateral line system and inner ear detect pressure waves in most fishes and aquatic amphibians.
1) Fish tend to hear higher frequencies due to their inner ears.
D. Many invertebrates have gravity sensors and are sound-sensitive
1) most invertebrates have mechanoreceptors called statocysts that function in their sense of equilibrium..
basically dense particles settle to the low point in a chamber.
2) most have sensors for specific sound , like male mosquitoes being able to detect the hum produced by a
females wings. or vibrating body hairs that can detect a predator.
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Bio 215 Chapter 49: Sensory and Motor Mechanisms
4. Chemoreceptors--Taste and Smell. Animals rely on chemoreception for many purposes, including locating food and
mates recognizing territories, and to assist with navigation.
A. perception of taste and smell are usually interrelated.
1) insects taste receptors are within sensory hairs called sensillae on the feet and mouthparts. olfactory
sensillae are located on antennae.
B. in mammals receptors cells for taste are organized into taste buds. olfactory receptor cells line the upper nasal
5. Movement and Locomotion: movement is a hallmark of animals. to catch food, an animal must move through the
environment or move the surrounding medium (water or air) past itself. While some animals are sessile, most are
mobile and rely on locomotion to acquire food or to escape form becoming food and to locate mates.
A. Locomotion requires energy to overcome friction and gravity
2) locomotion on land. walking or running
B. skeletons support and protect he animal body and are essential to movement
help maintain the shape of aquatic animals.
hard skeletons protect soft body tissues.
skeletons provide a firm attachment against which muscles can work during movement.
1) hydrostatic skeletons
C. Muscles move skeletal parts by contracting
1) Skeletal muscle
thin filaments: two strands of actin and one strand or regulatory protein coiled together.
thick filaments: are staggered arrays of myosin molecules
2) the sarcomere is the unit of organization of the skeletal muscle
z ones are borders of the sarcomere; aligned in adjacent myofibrils.
I bands are areas near the edge of the sarcomere containing only thin filaments
A bands are regions where thick and thin filaments overlap and correspond to the length of the thick
H zones are areas in the center of the A bands containing only thick filaments.
D. Interactions between myosin and actin underlie muscle contractions
E. Calcium ions and regulatory proteins control muscle contraction
F. diverse body movements require variation in muscle activity
1) fast and slow muscle fibers
2) other types
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