Sensory and Motor Mechanisms

SENSORY AND MOTOR MECHANISMS A. Sensing, Acting, and Brains 1. The brain’s processing of sensory input and motor output is cyclical rather than linear The way it ISN’T: sensing brain analysis action. The way it is: sensing, analysis, and action are ongoing and overlapping processes. Sensations begin as different forms of energy that are detected by sensory receptors. This energy is converted to action potentials that travel to appropriate regions of the brain. The limbic region plays a major role in determining the importance of a particular sensory input. B. Introduction To Sensory Reception Sensations are action potentials that reach the brain via sensory neurons. Perception is the awareness and interpretation of the sensation. 1. Sensory receptors transduce stimulus energy and transmit signals to the nervous system Sensory reception begins with the detection of stimulus energy by sensory receptors. Exteroreceptors detect stimuli originating outside the body. Interoreceptors detect stimuli originating inside the body. Sensory receptors convey the energy of stimuli into membrane potentials and the transmit signals to the nervous system. This involves: sensory transduction, amplification, transmission, and integration. Sensory Transduction. The conversion of stimulus energy into a change in membrane potential. Receptor potential Amplification. The strengthening of stimulus energy that is can be detected by the nervous system. May be a part of, or occur apart from, sensory transduction. Transmission. The conduction of sensory impulses to the CNS. Some sensory receptors must transmit chemical signals to sensory neurons. The strength of the stimulus and receptor potential affects the amount of neurotransmitter released by the sensory receptor. Some sensory receptors are sensory neurons. The intensity of the receptor potential affects the frequency of action potentials. Integration. The processing of sensory information. Begins at the sensory receptor. For example, sensory adaptation is a decrease in responsiveness to continued stimulation. For example, the sensitivity of a receptor to a stimulus will vary with environmental conditions. 2. Sensory receptors are categorized by the type of energy they transduce Mechanoreceptors respond to mechanical energy. For example, muscle spindles are an interoreceptor that responds to the stretching of skeletal muscle. For example, hair cells detect motion. Pain receptors = nocioceptors. Different types of pain receptors respond to different types of pain. Prostaglandins increase pain by decreasing a pain receptors threshold. Anti-inflammatories work by inhibiting prostaglandin synthesis. Thermoreceptors respond to heat or cold. Respond to both surface and body core temperature. Chemoreceptors respond to chemical stimuli. General chemoreceptors transmit information about total solute concentration. Specific chemoreceptors respond to specific types of molecules. Internal chemoreceptors respond to glucose, O2, CO2, amino acids, etc. External chemoreceptors are gustatory receptors and olfactory receptors. Electromagnetic receptors respond to electromagnetic energy. Photoreceptors respond to the radiation we know as visible light. Electroreceptors: some fish use electric currents to locate objects. C. Photoreceptors And Vision Most, if not all, animal photoreceptors may be homologous. 1. A diversity of photoreceptors has evolved among invertebrates Eye cups are among the simplest photoreceptors Detect light intensity and direction, but no image formation. The movement of a planarian is integrated with photoreception. Image-forming eyes. Compound eyes of insects and crustaceans. Each eye consists of ommatidia, each with its own light-focusing lens. This type of eye is very good at detecting movement. Single-lens eyes of invertebrates such as jellies, polychaetes, spiders, and mollusks. The eye of an octopus works much like a camera and is similar to the vertebrate eye. 2. Vertebrates have single-lens eyes Is structurally analogous to the invertebrate single-lens eye. Sclera: a tough white layer of connective tissue that covers all of the eyeball except the cornea. Conjunctiva: external cover of the sclera that keeps the eye moist. Cornea: transparent covering of the front of the eye. Allows for the passage of light into the eye and functions as a fixed lens. Choroid: thin, pigmented layer lining the interior surface of the sclera. Prevents light rays from scattering and distorting the image. Anteriorly it forms the iris. The iris regulates the size of the pupil. Retina: lines the interior surface of the choroid. Contains photoreceptors. Except at the optic disk (where the optic nerve attaches). The lens and ciliary body divide the eye into two cavities. The anterior cavity is filled with aqueous humor produced by the ciliary body. Glaucoma results when the ducts that drain aqueous humor are blocked. The posterior cavity is filled with vitreous humor. The lens, the aqueous humor, and the vitreous humor all play a role in focusing light onto the retina. Accommodation is the focusing of light in the retina. In squid, octopuses, and many fish this is accomplished by moving the lens forward and backward. In mammals accommodation is accomplished by changing the shape of the lens. The lens is flattened for focusing on distant objects. The lens is rounded for focusing on near objects. Photoreceptors of the retina. About 125 million rod cells. Rod cells are light sensitive but do not distinguish colors. About 6 million cone cells. Not as light sensitive as rods but provide color vision. Most highly concentrated on the fovea, an area of the retina that lacks rods. 3. The light-absorbing pigment rhodopsin triggers a signal-transduction pathway Rhodopsin (retinal + opsin) is the visual pigment of rods. The absorption of light by rhodopsin initiates a signal-transduction pathway. Color reception is more complex than the rhodopsin mechanism. There are three subclasses of cone cells, each with its own type of photopsin. Color perception is based on the brain’s analysis of the relative responses of each type of cone. In humans, colorblindness is due to a deficiency, or absence, of one or more photopsins. Inherited as an X-linked trait. 4. The retina assists the cerebral cortex in processing visual information Visual processing begins with rods and cones synapsing with bipolar cells. Bipolar cells synapse with ganglion cells. Visual processing in the retina also involves horizontal cells and amacrine cells. Vertical pathway: photoreceptors bipolar cells ganglion cells axons. Lateral pathways: Photoreceptors horizontal cells other photoreceptors. Results in lateral inhibition. More distant photoreceptors and bipolar cells are inhibited, which sharpens edges and enhances contrast in the image. Photoreceptors bipolar cells amacrine cells ganglion cells. Also results in lateral inhibition, this time of the ganglion cells. The optic nerves of the two eyes meet at the optic chiasm. Where the nasal half of each tract crosses to the opposite side. Ganglion cell axons make up the optic tract. Most synapses in the lateral geniculate nuclei of the thalamus. Neurons then convey information to the primary visual cortex of the optic lobe. D. Hearing And Equilibrium 1. The mammalian hearing organ is within the ear The outer ear includes the external pinna and the auditory canal. Collects sound waves and channels them to the tympanic membrane. From the tympanic membrane sound waves are transmitted through the middle ear. Malleus incus stapes. From the stapes the sound wave is transmitted to the oval window and on to the inner ear. The eustachian tube connects the middle ear with the pharynx. The inner ear consists of a labyrinth of channels housed within the temporal bone. The cochlea is the part of the inner ear concerned with hearing. Structurally it consists of the upper vestibular canal and the lower tympanic canal, which are separated by the cochlear duct. The vestibular and tympanic canals are filled with perilymph. The cochlear duct is filled with endolymph. The organ of Corti rests on the basilar membrane. The tectorial membrane rests atop the hair cells of the organ of Corti. From inner ear structure to a sensory impulse: follow the vibrations. The round window functions to dissipate the vibrations. Vibrations in the cochlear fluid basilar membrane vibrates hair cells brush against the tectorial membrane generation of an action potential in a sensory neuron. Pitch is based on the location of the hair cells that depolarize. Volume is determined by the amplitude of the sound wave. 2. The inner ear also contains the organs of equilibrium Behind the oval window is a vestibule that contains the utricle and saccule. The utricle opens into three semicircular canals. The utricle and saccule respond to changes in head position relative to gravity and movement in one direction. Hair cells are projected into a gelatinous material containing otoliths. When the head’s orientation changes the hair cells are tugged on, sending nerve impulse along a sensory neuron. The semicircular canals respond to rotational movements of the head. The mechanism is similar to that associated with the utricle and saccule. 3. A lateral line system and inner ear detect pressure waves in most fishes and aquatic amphibians Fishes and amphibians lack cochleae, eardrums, and openings to the outside. However, they have saccules, utricles, and semicircular canals. Most fish and amphibians have a lateral line system along both sides of their body. Contains mechanoreceptors that function similarly to the mammalian inner ear. Provides a fish with information concerning its movement through water or the direction and velocity of water flowing over its body. 4. Many invertebrates have gravity sensors and are sound-sensitive Statocysts are mechanoreceptors that function in an invertebrates sense of equilibrium. Statocyst function is similar to that of the mammalian utricle and saccule. Sound sensitivity in insects depends on body hairs that vibrate in response to sound waves. Different hairs respond to different frequencies. Many insects have a tympanic membrane stretched over a hollow chamber. E. Chemoreception – Taste And Smell 1. Perceptions of taste and smell are usually interrelated Taste receptors in insects are located on their feet. In mammals, taste receptors are located in taste buds, most of which are on the surface of the tongue. Each taste receptor responds to a wide array of chemicals. It is the pattern of taste receptor response that determines perceived flavor. In mammals, olfactory receptors line the upper portion of the nasal cavity. The binding of odor molecules to olfactory receptors initiates signal-transduction pathways involving a G-protein-signaling pathway and, often, adenylyl cyclase and cyclic AMP. F. Movement And Locomotion Locomotion is active movement from one place to another. 1. Locomotion requires energy to overcome friction and gravity A comparison of the energy costs of various modes of locomotion. Swimming. Since water is buoyant, gravity poses less of a problem for swimming than for other modes of locomotion. However, since water is dense, friction is more of a problem. Fast swimmers have fusiform bodies. For locomotion on land, powerful muscles and skeletal support are more important than a streamlined shape. When hopping, the tendons in a kangaroo’s legs store and release energy like a spring that is compressed and released – the tail helps in the maintenance of balance. When walking, having one food on the ground helps in the maintenance of balance. When running, momentum helps in the maintenance of balance. Crawling requires a considerable expenditure of energy to overcome friction – but maintaining balance is not a problem. Gravity poses a major problem when flying. The key to flight is the aerodynamic structure of wings. Cellular and Skeletal Underpinning of Locomotion. On a cellular level, all movement is based on contraction. Either the contraction of microtubules or the contraction of microfilaments. 2. Skeletons support and protect the animal body and are essential to movement Hydrostatic skeleton: consists of fluid held under pressure in a closed body compartment. Form and movement is controlled by changing the shape of this compartment. The hydrostatic skeleton of earthworms allows them to move by peristalsis. Advantageous in aquatic environments and can support crawling and burrowing. Does not allow for running or walking. Exoskeletons: hard encasements deposited on the surface of an animal. Mollusks are enclosed in a calcareous exoskeleton. The jointed exoskeleton of arthropods is composed of a cuticle. Regions of the cuticle can vary in hardness and degree of flexibility. About 30 – 50% of the cuticle consists of chitin. Muscles are attached to the interior surface of the cuticle. This type of exoskeleton must be molted to allow for growth. Endoskeletons consist of hard supporting elements within soft tissues. Sponges have spicules. Echinoderms have plates composed of magnesium carbonate and calcium carbonate. Chordate endoskeletons are composed of cartilage and bone. The bones of the mammalian skeleton are connected at joints by ligaments. 3. Physical support on land depends on adaptations of body proportions and posture In the support of body weight, posture is more important than body proportions. 4. Muscles move skeletal parts by contracting Muscles come in antagonistic pairs. Structure and Function of Vertebrate Skeletal Muscle. The sarcomere is the functional unit of muscle contraction. Thin filaments consist of two strands of actin and one tropomyosin coiled about each other. Thick filaments consist of myosin molecules. 5. Interactions between myosin and actin generate force during muscle contractions The sliding-filament model of muscle contraction. 6. Calcium ions and regulatory proteins control muscle contraction At rest, tropomyosin blocks the myosin binding sites on actin. When calcium binds to the troponin complex, a conformational change results in the movement of the tropomyosin-tropinin complex and exposure of actin’s myosin binding sites. But, wherefore the calcium ions? Follow the action potential. When an action potential meets the muscle cell’s sarcoplasmic reticulum (SR) stored Ca2+ is released. 7. Diverse body movements require variation in muscle activity An individual muscle cell either contracts completely or not all. Individual muscles, composed of many individual muscle fibers, can contract to varying degrees. One way variation is accomplished is by varying the frequency of action potentials that reach the muscle from a single motor neuron. Graded muscle contraction can also be controlled by regulating the number of motor units involved in the contraction. Recruitment of motor neurons increases the number of muscle cells involved in a contraction. Some muscles, such as those involved in posture, are always at least partially contracted. Fatigue is avoided by rotating among motor units. Fast and Slow Muscle Fibers. Fast muscle fibers are adapted for rapid, powerful contractions. Fatigue relatively quickly. Slow muscle fibers are adapted for sustained contraction. Relative to fast fibers, slow fibers have. Less SR, so Ca2+ remains in the cytosol longer. More mitochondria, a better blood supply, and myoglobin. • Other Types of Muscle. In addition to skeletal muscle, vertebrates have cardiac and smooth muscle. Cardiac muscle: similar to skeletal muscle. Intercalated discs facilitate the coordinated contraction of cardiac muscle cells. Can generate their own action potentials. Action potentials of long duration. Smooth muscle: lacks the striations seen in both skeletal and cardiac muscle. Contracts with less tension, but over a greater range of lengths, than skeletal muscle. No T tubules and no SR. Ca2+ enters the cytosol via the plasma membrane. Slow contractions, but with more control over contraction strength than with skeletal muscle. Found lining the walls of hollow organs. Invertebrate muscle cells are similar to vertebrate skeletal and smooth muscle cells.