Sensory and Motor Mechanisms

Sensory and Motor Mechanisms
Chapter 50 Sensory and Motor Mechanisms

Sensations and Perceptions
Begin with sensory reception, the detection of stimuli by sensory receptors Exteroreceptors Detect stimuli coming from the outside of the body Interoreceptors Detect internal stimuli

Functions Performed by Sensory Receptors
All stimuli represent forms of energy Sensation involves converting this energy Into a change in the membrane potential of sensory receptors Sensory receptors perform four functions in this process Sensory transduction, amplification, transmission, and integration Sensory transduction is the conversion of stimulus energy into a change in the membrane potential of a sensory receptor. This change in the membrane potential is known as a receptor potential. Many sensory receptors are extremely sensitive with the ability to detect the smallest physical unit of stimulus possible. Amplification is the strengthening of stimulus energy by cells in sensory pathways. After energy in a stimulus has been transduced into a receptor potential, some sensory cells generate action potentials, which are transmitted to the CNS. Sensory cells without axons release neurotransmitters at synapses with sensory neurons. The integration of sensory information begins as soon as the information is received. It occurs at all levels of the nervous system. Some receptor potentials are integrated through summation. Another type of integration is sensory adaptation (a decrease in responsiveness during continued stimulation).

Types of Sensory Receptors
Based on the energy they transduce, sensory receptors fall into five categories Mechanoreceptors Chemoreceptors Electromagnetic receptors Thermoreceptors Pain receptors Mechanoreceptors sense physical deformation caused by stimuli such as pressure, stretch, motion, and sound. The mammalian sense of touch relies on mechanoreceptors that are the dendrites of sensory neurons. Chemoreceptors include general receptors that transmit information about the total solute concentration of a solution. Specific receptors that respond to individual kinds of molecules. Electromagnetic receptors detect various forms of electromagnetic energy such as visible light, electricity, and magnetism. Thermoreceptors, which respond to heat or cold. They help regulate body temperature by signaling both surface and body core temperature. In humans, pain receptors, also called nociceptors, are a class of naked dendrites in the epidermis. They respond to excess heat, pressure, or specific classes of chemicals released from damaged or inflamed tissues.

Hearing and the perception of body equilibrium
The mechanoreceptors involved with hearing and equilibrium detect settling particles or moving fluid Hearing and the perception of body equilibrium Are related in most animals

Sensing Gravity and Sound in Invertebrates
Most invertebrates have sensory organs called statocysts that contain mechanoreceptors and function in their sense of equilibrium Ciliated receptor cells Cilia Statolith Sensory nerve fibers Figure 49.6

Sensing Sound in Arthropods
Many arthropods sense sounds with body hairs that vibrate Or with localized “ears” consisting of a tympanic membrane and receptor cells 1 mm Tympanic membrane Figure 49.7

Hearing and Equilibrium in Mammals
Pinna Auditory canal Eustachian tube Tympanic membrane Stapes Incus Malleus Skull bones Semicircular canals Auditory nerve, to brain Cochlea Oval window Round window Vestibular canal Tympanic canal Auditory nerve Bone Cochlear duct Hair cells Tectorial membrane Basilar membrane To auditory nerve Axons of sensory neurons 1 Overview of ear structure 2 The middle ear and inner ear 4 The organ of Corti 3 The cochlea Organ of Corti Outer ear Middle ear Inner ear In most terrestrial vertebrates the sensory organs for hearing and equilibrium are closely associated in the ear. Vibrating objects create percussion waves in the air that cause the tympanic membrane to vibrate. The three bones of the middle ear transmit the vibrations to the oval window on the cochlea. Figure 49.8

Hearing and Equilibrium in Mammals
These vibrations create pressure waves in the fluid in the cochlea That travel through the vestibular canal and ultimately strike the round window Cochlea Stapes Oval window Apex Axons of sensory neurons Round window Basilar membrane Tympanic canal Base Vestibular canal Perilymph The pressure waves in the vestibular canal cause the basilar membrane to vibrate up and down causing its hair cells to bend. The bending of the hair cells depolarizes their membranes, sending action potentials that travel via the auditory nerve to the brain. Figure 49.9

Equilibrium The utricle, saccule, and semicircular canals in the inner ear function in balance and equilibrium The semicircular canals, arranged in three spatial planes, detect angular movements of the head. Body movement Nerve fibers 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. The utricle and saccule tell the brain which way is up and inform it of the body’s position or linear acceleration. 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. Vestibule Utricle Saccule Vestibular nerve Flow of endolymph Cupula Hairs Hair cell Several of the organs of the inner ear detect body position and balance. Figure 49.11

Hearing and Equilibrium in Other Vertebrates
The lateral line system contains mechanoreceptors with hair cells that respond to water movement Nerve fiber Supporting cell Cupula Sensory hairs Hair cell Segmental muscles of body wall Lateral nerve Scale Epidermis Lateral line canal Neuromast Opening of lateral line canal Lateral line Like other vertebrates, fishes and amphibians also have inner ears located near the brain Most fishes and aquatic amphibians also have a lateral line system along both sides of their body Figure 49.12

The senses of taste and smell are closely related in most animals
The perceptions of gustation (taste) and olfaction (smell) Are both dependent on chemoreceptors that detect specific chemicals in the environment The taste receptors of insects are located within sensory hairs called sensilla Which are located on the feet and in mouthparts

The receptor cells for taste in humans
Are modified epithelial cells organized into taste buds Five taste perceptions involve several signal transduction mechanisms Sweet, sour, salty, bitter, and umami (elicited by glutamate)

Smell in Humans Olfactory receptor cells are neurons that line the upper portion of the nasal cavity Brain Nasal cavity Odorant Odorant receptors Plasma membrane Cilia Chemoreceptor Epithelial cell Bone Olfactory bulb Action potentials Mucus Figure 49.15 When odorant molecules bind to specific receptors A signal transduction pathway is triggered, sending action potentials to the brain

Vision in the Animal Kingdom
Similar mechanisms underlie vision throughout the animal kingdom Many types of light detectors Have evolved in the animal kingdom and may be homologous

Vision in Invertebrates
One of the simplest is the eye cup of planarians, which provides information about light intensity and direction but does not form images Light Light shining from the front is detected Photoreceptor Visual pigment Ocellus Nerve to brain Screening pigment Light shining from behind is blocked by the screening pigment Most invertebrates Have some sort of light-detecting organ Figure 49.16

Compound Eyes Compound eyes are found in insects and crustaceans, and consist of up to several thousand light detectors called ommatidia Cornea Crystalline cone Rhabdom Photoreceptor Axons Ommatidium Lens 2 mm (a) 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. Two major types of image-forming eyes have evolved in invertebrates The compound eye and the single-lens eye Figure 49.17a–b

Single-Lens Eyes Single-lens eyes
Are found in some jellies, polychaetes, spiders, and many molluscs Work on a camera-like principle

The Vertebrate Visual System
Ciliary body Iris Suspensory ligament Cornea Pupil Aqueous humor Lens Vitreous humor Optic disk (blind spot) Central artery and vein of the retina Optic nerve Fovea (center of visual field) Retina Choroid Sclera The eyes of vertebrates are camera-like But they evolved independently and differ from the single-lens eyes of invertebrates The main parts of the vertebrate eye are The sclera, which includes the cornea The choroid, a pigmented layer The conjunctiva, that covers the outer surface of the sclera The iris, which regulates the pupil The retina, which contains photoreceptors The lens, which focuses light on the retina Figure 49.18

Humans and other mammals focus light by changing the shape of the lens
Focusing the Eye Humans and other mammals focus light by changing the shape of the lens Lens (flatter) Lens (rounder) Ciliary muscle Suspensory ligaments Choroid Retina Front view of lens and ciliary muscle Ciliary muscles contract, pulling border of choroid toward lens Suspensory ligaments relax Lens becomes thicker and rounder, focusing on near objects (a) Near vision (accommodation) (b) Distance vision Ciliary muscles relax, and border of choroid moves away from lens Suspensory ligaments pull against lens Lens becomes flatter, focusing on distant objects Figure 49.19a–b

The human retina contains two types of photoreceptors
The Retina The human retina contains two types of photoreceptors Rods are sensitive to light but do not distinguish colors Cones distinguish colors but are not as sensitive Each rod or cone in the vertebrate retina Contains visual pigments that consist of a light-absorbing molecule called retinal bonded to a protein called opsin

Rods in the Eye Rods contain the pigment rhodopsin which changes shape when it absorbs light Figure 49.20a, b Rod Outer segment Cell body Synaptic terminal Disks Inside of disk (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. (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. Retinal Opsin Rhodopsin Cytosol H C H2C CH3 O H3C trans isomer cis isomer Enzymes Light

Processing Visual Information
The processing of visual information Begins in the retina itself Absorption of light by retinal Triggers a signal transduction pathway In the dark, both rods and cones Release the neurotransmitter glutamate into the synapses with neurons called bipolar cells, which are either hyperpolarized or depolarized In the light, rods and cones hyperpolarize Shutting off their release of glutamate The bipolar cells Are then either depolarized or hyperpolarized

Animal skeletons function in support, protection, and movement
The various types of animal movements All result from muscles working against some type of skeleton The three main functions of a skeleton are Support, protection, and movement The three main types of skeletons are Hydrostatic skeletons, exoskeletons, and endoskeletons

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

Exoskeletons & Endoskeletons
An exoskeleton is a hard encasement Deposited on the surface of an animal Exoskeletons Are found in most molluscs and arthropods An endoskeleton consists of hard supporting elements Such as bones, buried within the soft tissue of an animal Endoskeletons Are found in sponges, echinoderms, and chordates

The Human Skeleton Figure 49.26
1 Ball-and-socket joints, where the humerus contacts the shoulder girdle and where the femur contacts the pelvic girdle, enable us to rotate our arms and legs and move them in several planes. 2 Hinge joints, such as between the humerus and the head of the ulna, restrict movement to a single plane. 3 Pivot joints allow us to rotate our forearm at the elbow and to move our head from side to side. key Axial skeleton Appendicular skeleton Skull Shoulder girdle Clavicle Scapula Sternum Rib Humerus Vertebra Radius Ulna Pelvic girdle Carpals Phalanges Metacarpals Femur Patella Tibia Fibula Tarsals Metatarsals 1 Examples of joints 2 3 Head of humerus The mammalian skeleton is built from more than 200 bones Some fused together and others connected at joints by ligaments that allow freedom of movement Figure 49.26

Physical Support on Land
In addition to the skeleton Muscles and tendons help support large land vertebrates Muscles move skeletal parts by contracting The action of a muscle Is always to contract

Skeletal Muscle Skeletal muscles are attached to the skeleton in antagonistic pairs, with each member of the pair working against each other Human Grasshopper Biceps contracts Triceps relaxes Forearm flexes Biceps relaxes Triceps contracts Forearm extends Extensor muscle relaxes Flexor muscle contracts Tibia flexes Extensor muscle contracts Flexor muscle relaxes Tibia extends Muscles actively contract, but they elongate only when passively stretched. Back and forth motion is accomplished by antagonistic pairs – each one working against the other. Figure 49.27

Vertebrate Skeletal Muscle
Bundle of muscle fibers Single muscle fiber (cell) Plasma membrane Myofibril Light band Dark band Z line Sarcomere TEM 0.5 m I band A band M line Thick filaments (myosin) Thin filaments (actin) H zone Nuclei Vertebrate skeletal muscle is characterized by a hierarchy of smaller and smaller units. A skeletal muscle consists of a bundle of long fibers running parallel to the length of the muscle A muscle fiber is itself a bundle of smaller myofibrils arranged longitudinally The myofibrils are composed to two kinds of myofilaments Thin filaments, consisting of two strands of actin and one strand of regulatory protein Thick filaments, staggered arrays of myosin molecules Skeletal muscle is also called striated muscle Because the regular arrangement of the myofilaments creates a pattern of light and dark bands Each repeating unit is a sarcomere, bordered by Z lines The areas that contain the myofilments are the I band, A band, and H zone Figure 49.28

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. (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. 0.5 m Z H A Sarcomere According to the sliding-filament model of muscle contraction The filaments slide past each other longitudinally, producing more overlap between the thin and thick filaments As a result of this sliding The I band and the H zone shrink Figure 49.29a–c

The Sliding-Filament Model of Muscle Contraction
The sliding of filaments is based on The interaction between the actin and myosin molecules of the thick and thin 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

Myosin-Actin Interactions
Thick filament Thin filaments Thin filament ATP ADP P i Cross-bridge Myosin head (low- energy configuration) Myosin head (high- energy configuration) + Thin filament moves toward center of sarcomere. Thick filament Actin Cross-bridge binding site 1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration. 5 Binding of a new molecule of ATP releases the myosin head from actin, and a new cycle begins. 2 The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration. P Myosin-actin interactions underlying muscle fiber contraction: Starting here, the myosin head is bound to ATP and is in its low-energy confinguration. The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration. The myosin head binds to actin, forming a cross-bridge. Releasing ADP and ( i), myosin relaxes to its low-energy configuration, sliding the thin filament. Binding of a new molecule of ATP releases the myosin head from actin, and a new cycle begins. 1 The myosin head binds to actin, forming a cross- bridge. 3 4 Releasing ADP and ( i), myosin relaxes to its low-energy configuration, sliding the thin filament. P Figure 49.30

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, the myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin Actin Tropomyosin Ca2+-binding sites Troponin complex (a) Myosin-binding sites blocked The thin filament has two strands of actin twisted into a helix. When the muscle is at rest, a long, rodlike tropomyosin molecule blocks the myosin binding sites that are instrumental in forming cross-bridges. When another protein complex, troponin, binds calcium ions, the binding sites on actin are exposed, cross-bridges with myosin can form, and the muscle contracts. Figure 49.31a

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+ Myosin- binding site (b) Myosin-binding sites exposed Figure 49.31b

The stimulus leading to the contraction of a skeletal muscle fiber is an action potential in a motor neuron that makes a synapse with the muscle fiber Motor neuron axon Mitochondrion Synaptic terminal T tubule Sarcoplasmic reticulum Myofibril Plasma membrane of muscle fiber Sarcomere Ca2+ released from sarcoplasmic reticulum The synaptic terminal of the motor neuron Releases the neurotransmitter acetylcholine, depolarizing the muscle and causing it to produce an action potential Action potentials travel to the interior of the muscle fiber Along infoldings of the plasma membrane called transverse (T) tubules The action potential along the T tubules Causes the sarcoplasmic reticulum to release Ca2+ The Ca2+ binds to the troponin-tropomyosin complex on the thin filaments Exposing the myosin-binding sites and allowing the cross-bridge cycle to proceed Figure 49.32

Contraction in a Skeletal Muscle Fiber
ACh Synaptic terminal of motor neuron Synaptic cleft T TUBULE PLASMA MEMBRANE SR ADP CYTOSOL Ca2 P2 Acetylcholine (ACh) released by synaptic terminal diffuses across synaptic cleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber. 1 Action potential is propagated along plasma membrane and down T tubules. 2 Action potential triggers Ca2+ release from sarcoplasmic reticulum (SR). 3 Tropomyosin blockage of myosin- binding sites is restored; contraction ends, and muscle fiber relaxes. 7 Calcium ions bind to troponin; troponin changes shape, removing blocking action of tropomyosin; myosin-binding sites exposed. 4 Acetylcholine (ACh) released by synaptic terminal diffuses across synaptic cleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber. 2) Action potential is propagated along plasma membrane and down T tubules. 3) Action potential triggers Ca2+ release from sarcoplasmic reticulum (SR). 4) Calcium ions bind to troponin; troponin changes shape, removing blocking action of tropomyosin; myosin-binding sites exposed. 5) Myosin cross-bridges alternately attach to actin and detach, pulling actin filaments toward center of sarcomere; ATP powers sliding of filaments. 6) Cytosolic Ca2+ is removed by active transport into SR after action potential ends. 7) Tropomyosin blockage of myosin-binding sites is restored; contraction ends, and muscle fiber relaxes. Cytosolic Ca2+ is removed by active transport into SR after action potential ends. 6 Myosin cross-bridges alternately attach to actin and detach, pulling actin filaments toward center of sarcomere; ATP powers sliding of filaments. 5 Figure 49.33

Neural Control of Muscle Tension
Contraction of a whole muscle is graded Which means that we can voluntarily alter the extent and strength of its contraction There are two basic mechanisms by which the nervous system produces graded contractions of whole muscles By varying the number of fibers that contract By varying the rate at which muscle fibers are stimulated

Neural Control of Muscle Tension
In a vertebrate skeletal muscle each branched muscle fiber is innervated by only one motor neuron Each motor neuron may synapse with multiple muscle fibers Spinal cord Nerve Motor neuron cell body Motor unit 1 Motor unit 2 Motor neuron axon Muscle Tendon Synaptic terminals Muscle fibers A motor unit Consists of a single motor neuron and all the muscle fibers it controls Recruitment of multiple motor neurons Results in stronger contractions Figure 49.34

Twitch & Summation Figure 49.35
Action potential Pair of action potentials Series of action potentials at high frequency Time Tension Single twitch Summation of two twitches Tetanus A single action potential in a muscle will cause the muscle to contract locally and minutely for a few milliseconds and then to relax. This is called a twitch. If a second action potential arrives before the first response is over, there will be a summation effect and the contraction will be larger. If a muscle receives a series of overlapping action potentials, even further summation will occur. If the rate of stimulation is fast enough, the twitches will blur into one smooth, sustained contraction called tetanus. This is what occurs when a large muscle such as a bicep contracts. If the muscle continues to be stimulated without respite, it will eventually fatigue and relax. Figure 49.35

Other Types of Muscle Cardiac muscle, found only in the heart
Consists of striated cells that are electrically connected by intercalated discs Can generate action potentials without neural input In smooth muscle, found mainly in the walls of hollow organs The contractions are relatively slow and may be initiated by the muscles themselves In addition, contractions may be caused by Stimulation from neurons in the autonomic nervous system

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