Chapter 49: Sensory and Motor Mechanism

Sensations are action potentials Once the brain is aware of sensations Sensory receptors transduce stimulus energy and transmit signals to the central nervous system Sensations are action potentials That reach the brain via sensory neurons Once the brain is aware of sensations It interprets them, giving the perception of stimuli

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

Membrane potential (mV) Two types of sensory receptors exhibit these functions A stretch receptor in a crayfish Figure 49.2a (a) Crayfish stretch receptors have dendrites embedded in abdominal muscles. When the abdomen bends, muscles and dendrites 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 produces a larger receptor potential and higher requency of action potentials. Muscle Dendrites Stretch receptor Axon Membrane potential (mV) –50 –70 1 2 3 4 5 6 7 Time (sec) Action potentials Receptor potential Weak muscle stretch Strong muscle stretch

Membrane potential (mV) (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 –50 –70 1 2 3 4 5 6 7 Time (sec) Action potentials No fluid movement Receptor potential Fluid moving in one direction Fluid moving in other direction Membrane potential (mV) “Hairs” of hair cell Neuro- trans- mitter at synapse Axon Less neuro- trans- mitter More neuro- trans- mitter Figure 49.2b A hair cell found in vertebrates 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

Types of Sensory Receptors Based on the energy they transduce, sensory receptors fall into five categories Mechanoreceptors Chemoreceptors Electromagnetic receptors Thermoreceptors Pain receptors

The mammalian sense of touch Relies on mechanoreceptors that are the dendrites of sensory neurons Heat Light touch Pain Cold Hair Nerve Connective tissue Hair movement Strong pressure Dermis Epidermis Figure 49.3

Some snakes have very sensitive infrared receptors That detect body heat of prey against a colder background Figure 49.5a (a) This rattlesnake and other pit vipers have a pair of infrared receptors, one between each eye and nostril. The organs are sensitive enough to 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.

Many mammals appear to 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.

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

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

Exploring the structure of the human ear 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 Figure 49.8

Hearing 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

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 Figure 49.9

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

Segmental muscles of body wall 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 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

Taste in Humans 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)

Transduction in taste receptors Occurs by several mechanisms Taste pore Sugar molecule Sensory receptor cells Sensory neuron Taste bud Tongue G protein Adenylyl cyclase —Ca2+ ATP cAMP Protein kinase A Sugar Sugar receptor SENSORY RECEPTOR CELL Synaptic vesicle K+ Neurotransmitter 1 A sugar molecule binds to a receptor protein on the sensory receptor cell. 2 Binding initiates a signal transduction pathway involving cyclic AMP and protein kinase A. 3 Activated 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. Figure 49.14

When odorant molecules bind to specific olfactory receptors A signal transduction pathway is triggered, sending action potentials to the brain Brain Nasal cavity Odorant Odorant receptors Plasma membrane Cilia Chemoreceptor Epithelial cell Bone Olfactory bulb Action potentials Mucus Figure 49.15

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

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 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. Figure 49.17a–b

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

Structure of the Eye 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

The structure of the vertebrate eye 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 Figure 49.18

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 Rods are sensitive to light but do not distinguish colors Cones distinguish colors but are not as sensitive

Muscles move skeletal parts by contracting The action of a muscle Is always to contract

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 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 Figure 49.28

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

The Sliding-Filament Model of Muscle Contraction 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 (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 Figure 49.29a–c

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 underlying muscle fiber contraction 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 mole- cule 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 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

(a) Myosin-binding sites blocked 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 Figure 49.31a

(b) Myosin-binding sites exposed 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 Figure 49.32

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

Review of 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 propa- gated along plasma membrane and down T tubules. 2 Action potential triggers Ca2+ release from sarco- plasmic 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 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

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 Figure 49.34

Recruitment of multiple motor neurons 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

More rapidly delivered action potentials A twitch Results from a single action potential in a motor neuron More rapidly delivered action potentials Produce a graded contraction by summation Action potential Pair of action potentials Series of action potentials at high frequency Time Tension Single twitch Summation of two twitches Tetanus Figure 49.35

Tetanus is a state of smooth and sustained contraction Produced when motor neurons deliver a volley of action potentials

Types of Muscle Fibers Skeletal muscle fibers are classified as slow oxidative, fast oxidative, and fast glycolytic Based on their contraction speed and major pathway for producing ATP

Types of skeletal muscles

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