Sensory and Motor Mechanisms-2

SENSORY AND MOTOR MECHANISMS Sensations begins as different forms of energy such as light, heat, sound, and smells are detected by specialized sensory receptor cells and ultimately converted to action potentials that travel to the brain. Sensory signals go first to the thalamus. Information is sorted out in the thalamus using instructions from the cerebral cortex. Information is then sent to different parts of the brain that contribute in the formation of perceptions. Memories influence the final perception, so that in some cases we perceive what we expect rather that what is there. The brain chooses the appropriate motor behavior. INTRODUCTION TO SENSORY RECEPTION Sensory receptors are structures specialized to respond to stimuli and changes in the external environment. Sensory receptors consists of Neuron endings. Specialized cells in close contact with neurons. These receptors transduce (convert) the energy of the stimulus into electrical signals that are then transmitted by the neurons. Action potentials transmitted by sensory neurons are called sensations. Once the brain receives the sensations, it interprets it giving us the perception of the stimulus. Sensory receptors and other types of cells make the sense organs: eyes, ears, nose, and taste buds. There are six senses recognized by biologists: sight, hearing, smell, taste, touch and balance. Receptors are classified according to the source or type of stimulus. According to location: Exteroceptors receive stimuli from outside the body. Interoceptors are located within body organs and detect physiological changes, e.g. pH, temperature, chemicals in blood. Sensory transduction Sensory transduction occurs when the stimulus energy causes a change in the membrane potential of a receptor cell: energy ? receptor potential. First, there is a change in membrane permeability, resulting in a graded change in membrane potential called a receptor potential. Specific receptor molecules of the membrane of a receptor cell open or close ion channels Amplification Stimulus energy that is too weak to be carried by the nervous system is strengthened or amplified. Amplification of the signal may occur in accessory structures of a complex sense organ. Signal transduction in the pathway can contribute to amplification: in diverging circuits one neuron triggers responses of several neurons, each of which in turn trigger responses on several other neurons in an ever increasing number of neurons farther and farther along the circuit. E. g. one neuron from the brain activates a hundred or more motor neurons in the spinal cord and then thousands of skeletal muscle fibers. Transmission After transduction into a receptor potential, the signal is transmitted to the CNS. In some cases the receptor itself is a sensory neuron, like in the case of "pain cells." For some receptor cells, the strength of the stimulus and receptor potential affect the amount of neurotransmitter released by the receptor at its synapse with the sensory neuron. The amount of neurotransmitter determines the frequency of action potentials generated by the sensory neuron. Integration Integration is the processing and interpretation of information. Through integration, the nervous system decides what to do at every minute. Sensory adaptation is a decrease in responsiveness during continued stimulation. The sensitivity of the receptors may vary. The threshold for transduction by receptor cells varies with conditions. E. g. we can detect different levels of concentration of a substance in food, too salty, too sweet. CLASSIFICATION OF RECEPTORS According to types of stimuli to which they respond (the energy they transduce). Mechanoreceptors respond to mechanical energy, e.g. pressure, touch, and gravity. The muscle spindle is an interoreceptor stimulated by mechanical distortion of the muscles. Hair cells detect motion. They are specialized cilia or microvilli found in the vertebrate ear and in the lateral line organs of fishes and amphibians. Pain receptors are naked dendrites in the epidermis of the skin called nociceptors. The stimulus is translated into defensive reaction. Chemoreceptors respond to chemicals, e.g. odors. They respond to concentration or kind of molecules present. Gustatory and olfactory receptors. Electromagnetic receptors respond to light, electricity and magnetism. Photoreceptors respond to light. Electroreceptors detect electrical energy. Thermoreceptors detect changes in temperature. There is a debate about the identity of thermoreceptors. They may be modified pressure receptors or dendrites of neurons. They are located in the skin, and the interoreceptor in the hypothalamus. Pain receptors are also called nociceptors. They consist of naked dendrites. Pain leads to a defensive reaction and withdrawal from danger. Different groups of pain receptors respond to excess heat, pressure, or specific classes of chemicals released by damaged tissues. MECHANORECEPTORS GRAVITY AND SOUND SENSORS IN INVERTEBRATES Many invertebrates have gravity receptors called statocysts. Statocysts function in the sense of equilibrium. Infolding of the skin lined with cells that have hairs. Statoliths are tiny granules of sand or CaCO3 located in the infolding of the skin. Statocysts in invertebrates have various locations. Many insects have hairs of different thickness that vibrate with sound waves at different frequencies. Some insects also have a tympanic membrane stretched over and internal air chamber. Vibrations of the membrane result in a nerve impulse. HEARING AND EQUILIBRIUM IN MAMMALS Functions in hearing and maintaining equilibrium or balance. Three regions: 1. External ear: pinna and auditory canal in some vertebrates. 2. Middle ear: tympanic membrane and auditory bones. 3. Inner ear: semicircular canals, vestibule and cochlea. The Eustachian tube connects the middle ear with the pharynx and equalizes the pressure between the middle ear and the atmosphere. All vertebrates have inner ears. Outer and middle ears may be absent in some groups. Auditory bones are the malleus, incus and stapes. The vestibule consists of two chambers, the saccule and the utricle. The vestibule and semicircular canals are also known as the labyrinth. The inner ear is made of a membrane that fits inside the skull bone AUDITORY RECEPTION Auditory receptors are located in the cochlea. The cochlea transduces the energy of a vibrating fluid into action potentials. A spiral tube consisting of three canals separated by membranes. Canals are filled with the perilymph. Vestibular canal and tympanic canal are connected at the apex of the cochlea. The cochlear or middle canal is filled with endolymph and contains the organ of Corti. Basilar membrane separates the tympanic canal from the cochlear or middle canal. Above the organ of Corti is the tectorial membrane. HEARING Pinna and auditory canal collect sound waves and transmit them to the tympanic membrane. Sound waves create vibrations in the tympanic membrane. These vibrations in turn are transmitted to the ear ossicles. The stapes transmits the vibration to oval window, which creates a traveling pressure wave in the fluid of the cochlea. These waves pass into the vestibular canal and around the tip of the cochlea to the tympanic canal. The wave dissipates when it strikes the round window. The waves in the vestibular canal distort the cochlear duct and the basilar membrane. Distortion of the basilar membrane causes the organ of Corti to alternately rub against the tectorial membrane. The basilar membrane vibrates up and down with the waves and its hair cells brush against and are withdrawn from the tectorial membrane. Deflections of the hairs opens ion channel in the plasma membrane of the hair cells, and potassium ions enter. The hair cells depolarize and release neurotransmitters that trigger an action potential in the sensory neuron. The amplitude or height of the sound wave determines the volume or loudness of sound. Loud sounds cause waves of greater amplitude resulting in greater stimulation of hair cells and transmission of greater number of impulses per second. Pitch depends on the frequency of the sound waves; e.g., high frequency results in high pitch. Different regions of the basilar membrane are affected by different frequencies. The sensory neurons associated with each vibrating region send the most action potentials along the auditory nerve. Sensory neurons connect to specific auditory areas of the cerebral cortex, which interprets the pitch of the sound. EQUILIBRIUM Equilibrium in humans depends on the proper functioning of the labyrinth and proprioceptors, the sense of vision, and stimulus coming from the soles of the feet. Behind the oval window of the inner ear is a vestibule that contains two chambers, the utricle and the saccule. The utricle opens into three semicircular canals. The saccule and utricle of vertebrates contain otoliths (CaCO3) that change position when the head is tilted or when the body is moving in straight line. Hair cells are located in the saccule and utricle. Hair cells are surrounded at their tips by a gelatinous cupula. Hair cells send information to the brain about the direction of gravity. The semicircular canals inform the brain about turning movements (linear acceleration). They are arranged in the three planes. Each canal is hollow, connected to the utricle and at right angle to the other two. Filled with endolymph. At one of the openings of each semicircular canal there is a bulb-like enlargement, the ampulla. Endolymph movement stimulates the cristae. No otoliths are present in the ampulla. HEARING IN FISH AND AQUATIC AMPHIBIANS Fishes and aquatic amphibians have an inner ear with a saccule, utricle and semicircular canals. The cochlea is absent. Within these chambers there are hair cells that are stimulated by otoliths. The hearing apparatus of fish does not open to the outside. Vibrations are transmitted through the skeleton of the head to the inner ears, setting the otoliths in motion and stimulating the hair cells. The swim bladder filled with gas also vibrates and contributes to the stimulus of the inner ear. Some fishes have a series of bones that transmit vibrations from the swim bladder to the inner ear. Lateral line organs are found in fish and in aquatic amphibians. Long canal running the length of the body and head Line with sensory cells with hairs. Tips of hairs have a cupula, a mass of gelatinous material. Respond to waves and currents in the water flowing through the system. Complement vision. Some amphibians have lateral line in the larva stage but not as adults, e. g. tadpoles and frogs. Birds have a cochlea and sound is transmitted from the tympanic membrane to the inner ear, like in amphibians and reptiles, by a single bone, the stapes. CHEMORECEPTORS The senses of smell and taste use chemoreceptors. Taste detects chemicals in solution, and smell detects airborne chemicals. 1. TASTE (gustation) Taste receptors are specialized epithelial cells in the taste buds located in the tongue and mouth. In humans, taste buds are located on the tongue, in tiny elevations or papillae. There are about 3,000 papillae on the human tongue. There are four basic tastes: salty, sweet, sour and bitter. Each receptor cell is more responsive to a particular type of substance, it can be s stimulated by a broad range of chemicals. The brain integrates the differential input from the taste buds and complex flavor is perceived. Each taste bud is an epithelial capsule containing about 100 taste receptor cells interspersed with supporting cells. Tips of the taste receptor cells have microvilli that extend into the taste pore on the tongue's surface. These receptors detect food molecules dissolved in saliva. Flavor depends on the four tastes in combination with smell, texture, and temperature. The ability to taste certain chemicals is inherited. 2. SMELL (olfaction) In humans, the olfactory epithelium is found on the roof of the nasal cavity. It contains about 100 million specialized olfactory cells with ciliated tips. The cilia extend into the layer of mucus on the epithelial surface of the nasal passageway. Receptor molecules on the cilia bind to compounds that dissolve in the mucus. The other end of each olfactory cell is an axon that extends into the olfactory bulb of the brain. These axons make the first cranial nerve. Messages travel from the olfactory bulb then to the olfactory cortex, to the limbic system and finally to other areas of the cortex by way of the thalamus. The number of odorous molecules determines the intensity of the receptor potential. Humans can detect seven main groups of odors. Each odor is made of several components and each component may bind with a particular type of receptor. The combination of receptors activated determines the odor we perceive. Olfactory sense adapts very quickly. PHOTORECEPTORS AND VISION Most animals have photoreceptors that use a group of the pigments called rhodopsins to absorb light. Invertebrates have eyespots or eye cups, simple eyes and compound eyes. Simplest are found in some cnidarians and flatworms. Eyespots are called ocelli, a bowl shaped cluster of light sensitive cells within the epidermis. They detect light intensity and direction but no images. Effective image formation requires a lens that concentrates light on photoreceptors. The brain interprets the message of the photoreceptors - VISION. It integrates information about brightness, location, position and shape of the stimulus. THE COMPOUND EYE Compound eyes are found in crustaceans and insects. They consist of ommatidia, which collectively produce a mosaic image. Some crustaceans have 20 ommatidia and dragonflies have 28,000. Each ommatidium has a convex lens and a crystalline cone. Compound eyes form a mosaic image based on the message sent by each ommatidium. The eye is sensitive to flickers of high frequencies; e.g. a fly can follow flickers of about 265 flickers/second. Compound eyes are sensitive to wavelengths from red to UV. SINGLE-LENS EYE Single-lens eyes are found in jellyfishes, polychaetes, spiders and many mollusks. It has a small opening the pupil, through which light enters; behind the pupil, a single lens focuses the image on a retina, which is the photoreceptor. THE VERTEBRATE EYE Position of the eye offers different advantages; e.g. lateral eyes of grazers allow them to detect predators. Humans have binocular vision useful in judging distance and depth. Two layers of tissue protect the eye: Sclera, the outer coat of the eye, is a tough layer of connective tissue that protects and helps maintain the rigidity of the eyeball. Choroid, cells contain black pigment that absorbs extra light and prevents internally reflected light from blurring the image. The conjunctiva is delicate layer of epithelial cells that covers the sclera and keeps the eye moist. The thin, transparent cornea is the continuation of the sclera on the front of the eye. The conjunctiva does not cover the cornea. The iris is formed by the anterior choroid. It controls the amount of light entering the eye. The pupil is the whole in the center of the iris. The retina is the innermost layer of the eyeball and contains the photoreceptor cells. The lens of the eye is a transparent, elastic protein ball immediately behind the iris. The ciliary body is a gland-like processes that constantly secretes the clear, watery aqueous humor that fills the anterior cavity of the eye. The lens and the ciliary body divide the eye into two cavities The anterior cavity between the cornea and the lens is filled with a watery substance, the aqueous humor. The larger posterior cavity between the lens and the retina is filled with viscous fluid called the vitreous humor. The aqueous and vitreous humors function as liquid lenses that help focus the image on the retina. Ciliary muscles adjust the lens to focus for near or far vision. Squid, octopuses and many fishes focus the image by moving the lens forward or backward. Humans and other mammals focus by changing the shape of the lens. This is called accommodation. The retina contains light-sensitive rods (125 million in humans) and cones (6.5 million). Rods are more sensitive to light but cannot detect color. Rods are for dim-light vision and allow detecting shape and movement. Rods are more numerous in the periphery of the retina. Cones are responsible for color, fine detail and bright-light vision. Cones are concentrated in the fovea, a small depressed area in the center of the retina. Light must pass through several layers of connecting neurons in the retina to reach the rods and cones. Rhodopsin in the rod cells and other related pigments in the cones are responsible for the ability to see. Rhodopsin is the visual pigment. A chemical change in rhodopsin leads to the response of a rod to light. Rhodopsin is made of opsin (polypeptide) and retinal (pigment from vitamin A). Opsins vary in structure from one type of photoreceptor to another. The rod cells' signal transduction pathway. Rod cells synapse with bipolar cells. Bipolar cells have two kinds of receptors. The receptors are either inhibited or excited by the neurotransmitter glutamate. Two isomers of retinal exist: cis and trans forms. In the dark, the photoreceptors have the Na+ channels open and are depolarized. The membrane potential is about -40 mV. This depolarization allows calcium channels to remain open and cause a continuous release of the neurotransmitter glutamate at the synapse with the bipolar cell. The photoreceptors are releasing glutamate, inhibitory neurotransmitters. Retinal binds to opsin in the cis form to make rhodopsin. Cyclic GMP, guanosine monophosphate, maintains the Na+ open. The release of neurotransmitter is graded according to the degree of depolarization. When light strikes rhodopsin, rhodopsin breaks down into opsin and retinal. Cis retinal changes to trans-retinal. This is called "bleaching of rhodopsin". Opsin then becomes activated as an enzyme. The opsin molecule activates a G protein called transducin. In turn transducin activates an enzyme, phosphodiesterase (PDE) that converts cGMP to GMP (cyclogaunosine monophosphate to guanosine monophosphate). The molecule cGMP is a second messenger. Sodium channels must be bound to cGMP to remain open. PDE detaches cGMP from the sodium channels by hydrolyzing it to GMP. When cGMP decreases and GMP increases, the Na+ channels begin to close and the cell becomes hyperpolarized to -70 mV. Hyperpolarization slows the rod cell's release of neurotransmitters at the synapse of the rod cell with the bipolar cell. Bipolar cells detect the change and become depolarized. Depolarized bipolar cells release neurotransmitters that stimulate the ganglion cell, which sends its axon to the brain in the optic nerve. In summary: Light turns off sodium channels, the inhibitory neurotransmitter glutamate is no longer released, and the rod becomes hyperpolarized. Hyperpolarization causes either excitation by removing the inhibitor or inhibition by removing the neurotransmitter. The bipolar cell becomes depolarized and then sends an action potential. COLOR VISION There are three types of cones: blue, red and green cones, named according to the wavelength that its pigment responds more strongly. Each type has a different photopigment collectively called photopsins. The retinal is the same as in rhodopsin but the opsin is slightly different in each type. All three types respond to a wide range of wavelengths. Their absorption spectra overlap. Different colors depend on how the brain interprets the differential stimulation of the two or three types of cones, and strongly each type is stimulated. THE RETINA The retina has five main types of neurons: Photoreceptors: rods and cones. Bipolar cells, which make synaptic contact with photoreceptors and ganglion cells. Ganglion cells. Their axons form the optic nerve. Horizontal cells receive information from photoreceptors and convey them to bipolar cells. Amacrine cells receive messages from bipolar cells and send signals to ganglion cells. In the so called vertical pathway, the stimulus passes directly from the photoreceptors to the bipolar cells and then to the ganglion cells. Vision events: Light passes through... Cornea aqueous fluid lens vitreous body image forms on the retina (rods, cones) impulses in bipolar cells impulses in ganglion cells optic nerve transmits nerve impulses to thalamus integration by visual areas of cerebral cortex. In the lateral pathway, the information passes through the amacrine and horizontal cells. The horizontal cells pass the information to several bipolar cells or to other photoreceptor cells The amacrine cells relay the information from one bipolar cell to several ganglion cells. Horizontal cells inhibit bipolar cells and photoreceptors that are not being stimulated light increasing the contrast between the illuminated and not illuminated areas. This is called lateral inhibition. It sharpens the edges and enhances the contrast in the image. NEURAL PATHWAYS Ganglion cells transmit specific types of visual stimuli such as color, brightness and motion. The optic nerves cross the floor of the hypothalamus and form the optic chiasm. Some axons crossover to the other side of the brain. Axons end and transmit information to the lateral geniculate nuclei in the thalamus. From there neurons bring information to the primary visual cortex in the occipital lobe of the cerebrum. Information is then transmitted to other cortical areas for further integration. The mechanism involved in the integration of visual information is not well understood. MOVEMENT AND LOCOMOTION SKELETONS Skeletons support and protect the animal body and are essential to movement. Main functions of the human skeleton are: Transmit mechanical forces created by muscles (levers). Support internal organs and tissues. Protect internal organs. Storage of calcium salts. Blood cell production (hematopoiesis). Coelomates have a hydrostatic skeleton used to transmit forces created by muscles. It consists of fluid held under pressure in a closed compartment. It is the skeleton of cnidarians, nematodes, flatworms and annelids. Peristaltic movements are rhythmic waves created by the contraction of longitudinal and circular muscles that pass from the anterior to the posterior of the animal. Exoskeleton of invertebrates is a non-living deposit on top of the epidermis. Arthropod exoskeleton is made of chitin; jointed; need to molt. Chitin is nitrogen containing polysaccharide. Fibrils of chitin are embedded in a matrix of protein giving flexibility and strength. Mollusks produce a calcium containing shell. It varies in thickness and flexibility throughout the body. Need to molt in order to grow. Endoskeleton of echinoderms and chordates is composed of living tissue and capable of growth. The endoskeleton is embedded in tissues and organs. Sponges have an endoskeleton of spicules made of protein, silica and/or calcium carbonate. Echinoderm endoskeleton is made calcium plates and spines. The endoskeleton of chordates consists of cartilage, bone or a combination of the two. GENERAL STRUCTURE OF THE SKELETON. Vertebrate endoskeleton consists of 206 bones divided in two portions: Axial skeleton: skull, vertebrae, ribs and sternum. Appendicular skeleton: bones of arms, legs, pectoral girdle and pelvic girdle. Skull consists of 8 cranial bones and 14 facial bones. Vertebral column is made of 24 vertebrae and two fused bones, the sacrum and coccyx. Cervical region: 7 vertebrae. Atlas is the first cervical vertebra and supports the skull. Axis is the second cervical vertebra and allows the head to rotate. Thoracic region: 12 vertebrae. Lumbar region: 5 vertebrae. Sacral region: 5 fused vertebrae. Coccygeal region: 4 fused rudimentary vertebrae. The mammalian rib cage consists of the sternum and 12 pairs of ribs. Seven pairs are attached directly to the sternum. Three pairs are attached indirectly by means of cartilage. Two pairs have no attachment, the "floating ribs". Pectoral girdle consists of the scapulas (shoulder blades) and clavicles (collarbones) Pelvic girdle is made of two large bones made in turn by three fused bones. Arms and legs of humans are made of 30 bones and end in five digits. Pigs have four digits; rhinoceros have three, two in camel and one in horses. Apes and humans have an opposable thumb useful in grasping and manipulating objects. Apes also have an opposable big toe. MUSCLE Prefixes myo or mys = muscle; the prefix sarco = flesh. A muscle is an organ made of contractile cells that allow movement. FUNCTIONS OF MUSCLES Produce movement of body parts, circulation of blood, passing of food through the digestive system, and manipulation of objects. Maintain posture. Generate heat. There are three types of muscles: striated, smooth and cardiac. MUSCLE STRUCTURE In vertebrates, muscles are organs. Muscle cells are called fibers. Muscle fibers are huge cells ranging between 10 and 100 ?m (10-6 m), up to 10x that of an average body cell; their length could reach 30 cm (12 inches). Muscle fibers are multinucleate. Muscle fibers originate from the fusion of hundreds of embryonic cells. Actin is a contractile protein found in all eukaryotic cells. In most cells, myosin is associated with actin. Fibers are grouped into bundles called fascicles. Fascicles are wrapped by connective tissue making the muscle. Plasma membrane or sarcolemma has many inward extensions called T tubules (transverse tubules). Cytoplasm or sarcoplasm. ER of sarcoplasmic reticulum. Myofibrils run the length of the muscle fiber. Myofibrils are made of two kinds of myofilaments: Thick myofilaments made of myosin. Thin myofilaments made of two strands of actin and one of a regulatory protein coiled around one another. The proteins tropomyosin and troponin complex are also present. Myofilaments are organized into repeating and contractile units called sarcomeres. Sarcomeres are joined end to end at the Z line. The Z line is made of a protein that anchors the thin filaments and connects each myofibril to the next. The regular arrangement of microfilaments creates a repeating pattern of light and dark bands, striation. Hundreds of sarcomeres connected end-to-end make up the myofibril. Contraction occurs when actin and myosin filaments slide past each other. Muscles are made of fascicles; fascicles are made of fibers; fibers are made of myofibrils; myofibrils are made of two kinds of microfilaments. Structure of the sarcomere See figure 49.31. The A band is the broad region that corresponds to the length of the thick filaments. The H zone is in the center of the A band and contains only thick filaments; the thin filaments do not extend completely across the sarcomere. The I band is near the Z band and contains only thin filaments. The Z lines mark the borders of the sarcomere MUSCLE CONTRACTION During muscle contraction the thin actin filaments are pulled between the myosin filaments toward the center of the sarcomere. The distance from on Z line to the next becomes shorter and the H zone disappears. Muscle contraction involves the activity of five molecules: actin, myosin, tropomyosin, troponin and ATP. Ca2+ are also involved. Sequence of events: Motor neuron releases acetylcholine into the cleft between the neuron and muscle fiber. Acetylcholine causes the depolarization of the sarcolemma and the transmission of an action potential. The impulse spreads through the T tubules and stimulates Ca2+ ion release from the sarcoplasmic reticulum. Ca2+ ions initiate a process that uncovers the active sites of the actin filaments. Ca2+ bind to troponin causing a change in shape. Troponin pushes tropomyosin away exposing the active sites on actin filaments. Myosin molecules are made of a folded into two globular structures called heads and a long tail. ATP is bound to myosin when the fiber is at rest. Myosin heads have the ability to breakdown ATP in the presence of Ca2+. Cross bridges form, linking the myosin and actin filaments. ADP and Pi are released. Cross bridges flex utilizing the energy released by ATP, and the filaments are pulled past one another. The muscle shortens. The actin-myosin complex binds to ATP again and myosin separates from actin. This series of events takes milliseconds. VARIATION IN MUSCLE ACTIVITY We can voluntarily change the extent and strength of contraction. Stimulus that depolarizes a muscle fiber triggers an all-or-none contraction. Muscle contractions are graded. How can this be? A single contraction produces an increase in muscle tension lasting about 100 msec or less, a single twitch. If a second action potential arrives before the first response is over, there will be a summation and a greater response. Overlapping series of action potentials will add and the level of tension will depend on the rate of stimulation. If the rate of stimulation is fast enough, the twitches will blur into one smooth and sustained contraction called tetanus. Motor neurons usually deliver their action potentials in rapid-fire volleys, and the resulting summation of tension results in smooth contraction typical of tetanus Muscle cells are organized into motor units. In vertebrates, each muscle cell is enervated by a single neuron, but each branched motor neuron may enervate many muscle cells. A motor unit consists of a single motor neuron and the many muscle fibers it controls. When a motor neuron fires, all the muscle fibers it controls contract as a unit. The strength of the contraction depends on how many muscle fibers the motor neuron controls. The activation of more and more motor neurons controlling a muscle results in progressively greater tension. This is called recruitment. In muscles that are always partially contracted, fatigue is avoided by the nervous system activating alternately different motor units, so they take turn in being contracted. This happens in muscles involved in maintaining the upright position. Fast and slow muscle fibers The duration of the contraction is controlled by how long the calcium concentration in the cytosol remains elevated. Fast-twitching fibers specialized for quick response, and slow-twitching fibers specialized for slow response, and sustain long contractions. Slow fibers have less sarcoplasmic reticulum and slower calcium pumps than fast fibers, so calcium remains in the cytosol longer. Slow fibers make use of a steady supply of energy. They have many mitochondria, a rich blood supply, and an oxygen-storing protein called myoglobin. Myoglobin is a protein that stores oxygen within the muscle fiber; it is similar to hemoglobin. Myoglobin is a brownish-red pigment that makes the meat of poultry and fish dark. It binds more tightly to oxygen, and can effectively remove oxygen from the hemoglobin in the blood. Cardiac muscles found only in the heart consist of striated, branching cells that are electrically connected by intercalated discs. Gap junctions provide direct electrical coupling among cells. Cardiac cells can generate action potentials on their own, without neural input. The action potential of cardiac cells last 20 times longer than those of skeletal muscles. They control the duration of the contraction. In smooth muscle, contractions are slow but can be sustained over long periods of time. The actin and myosin filaments have a spiral arrangement within the muscle fiber. They lack T tubules and have less sarcoplasmic reticulum. Calcium ions must enter the cytosol via the plasma membrane during action potential, and the amount reaching the filaments is rather small. Contractions are slow and contract over a greater length than striated muscles. LOCOMOTION REQUIRES ENERGY TO OVERCOME FRICTION AND GRAVITY Animals spend a significant amount of time searching for food. Movement is characteristic of animals. Locomotion is the displacement from place to place. Locomotion requires energy to overcome friction and gravity. Flying animals spend more than animals swimming or walking per meter traveled. Swimming Buoyancy is greater in water than in air but so is resistance. The fusiform shape is an adaptation to decrease resistance. Animals may move by squirting water in one direction (e. g. squids, some cnidarians), by moving their tails side to side (w. g. fishes) or by moving their tails up and down in an undulating movement (e g. whales). Swimming tends to be the most energy efficient method of locomotion. Locomotion on land Air poses relatively little resistance but gravity becomes a significant factor. When walking, running or hopping, leg muscles spend energy to propel the animal and to keep it from falling down. Inertia must be overcome by accelerating a leg from a standing start. Powerful leg muscles and strong skeletal support are more important than a streamlined shape. Maintaining a balance is also important. Many animals keep at least one foot on the ground when running. The momentum during running may permit a momentarily loss of contact with the ground. Crawling animals most overcome considerable friction. Flying Gravity is a major obstacle to flying. The key to flying is the shape of the wing. All types of wings are designed to alter air currents in a way that create lift. Cellular and skeletal underpinnings of locomotion At the cellular level, animal movement is based on the microtubules responsible for the rating of cilia and the undulations of flagella. the microfilaments involved in amoeboid movement resulting from the movement of protein units.