Overview of Sensory Receptors

Transcription

1 Sensory Systems Chapter 45 Overview of Sensory Receptors Sensory receptors provide information from our internal and external environments that is crucial for survival and success -Exteroceptors sense external stimuli -Some function well on land but not in water, and vice versa -Interoceptors sense internal stimuli -Usually simpler than exteroceptors 2 Overview of Sensory Receptors Receptors can be grouped into three classes 1. Mechanoreceptors are stimulated by mechanical forces such as pressure 2. Chemoreceptors detect chemicals or chemical changes 3. Energy-detecting receptors react to electromagnetic and thermal energy 3 1

2 Mechanoreceptors Cutaneous receptors respond to stimuli in between external and internal environments Nociceptors transmit impulses perceived as pain -Most consist of free nerve endings located throughout the body, especially where damage is most likely to occur 4 Mechanoreceptors Thermoreceptors are naked dendritic endings of sensory neurons that are sensitive to changes in temperature -Contain TRP ion channels that are responsive to hot and cold -Cold receptors are located higher in the skin, and are much more numerous, than warm receptors 5 Mechanoreceptors Several types of mechanoreceptors in the skin detect the sense of touch 6 2

3 Mechanoreceptors Proprioceptors monitor muscle length and tension -Provide information about the relative position or movement of animal s body parts -Examples: -Muscle spindles -Golgi tendon organs 7 Mechanoreceptors Baroreceptors monitor blood pressure -A highly branched network of afferent neurons located in the carotid sinus and aortic arch -Detect tension or stretch in the walls of these blood vessels -When blood pressure decreases, the frequency of impulses produced by baroreceptors decreases 8 Hearing Hearing is the detection of sound waves -Sound is the result of vibration, or waves, traveling through a medium Auditory stimuli travel farther and more quickly than chemical ones Auditory receptors provide better directional information than chemoreceptors 9 3

4 Lateral Line System in Fish Consists of canals running the length of the fish s body beneath the skin surface -Innervated by sensory neurons that transmit impulses to the brain 10 Hearing Structure in Fish Hearing structures in fish are called otoliths -Composed of calcium carbonate crystals -Contained in the otoliths organs of the membranous labyrinth Otoliths vibrate against stereocilia-containing hair cells -Thus producing action potentials 11 Ear Structure of Land Vertebrates Air vibrations are channeled through the auditory canal of the outer ear -Vibrations reach the tympanic membrane causing movement of three small bones (ossicles) in the middle ear -Malleus (hammer), incus (anvil) and stapes (stirrup) -The stapes vibrates against the oval window, which leads into the inner ear 12 4

5 Ear Structure of Land Vertebrates The inner ear consists of the cochlea, a bony structure containing part of the cochlear duct 13 Ear Structure of Land Vertebrates 14 Ear Structure of Land Vertebrates The organ of Corti, which transduces sound in the cochlea, consists of: -Basilar membrane: Bottom of cochlear duct -Hair cells with associated sensory neurons -Tectorial membrane: Overhanging, gelatinous membrane Stereocilia of hair cells bend in response to vibrations of the basilar membrane -Send nerve impulses to brain, where they are interpreted as sound 15 5

6 Navigation by Sound A few mammals have the ability to perceive presence and distance of objects by sound -Bats, shrews, whales, dolphins -They emit sounds and then determine the time it takes these sounds to return -This process is called echolocation The invention of sonar (sound navigation and ranging) relied on echolocation principles 16 Detection of Body Position Most invertebrates can orient themselves with respect to gravity due to a sensory structure called a statocyst -Consists of ciliated hair cells embedded in calcium carbonate stones called statoliths In vertebrates, the gravity receptors consist of two chambers in the membranous labyrinth -Utricle and saccule 17 Detection of Body Position Within the utricle and saccule are hair cells with stereocilia and a kinocilium -Embedded in the calcium carbonate-rich otolith membrane Utricle more sensitive to horizontal acceleration Saccule more sensitive to vertical acceleration -Both types of accelerations cause cilia to bend, thus producing an action potential in 18 an associated sensory neuron 6

7 Detection of Body Position The utricle and saccule are continuous with three semicircular canals that detect angular acceleration in any direction 19 Detection of Body Position When the head rotates, the semicircular canal fluid pushes against the cupula, causing the cilia to bend -Bending in the direction of the kinocilium causes a receptor potential -Stimulates an action potential in the associated sensory neuron Saccule, utricle and semicircular canals are collectively called the vestibular apparatus 20 Chemoreceptors Chemoreceptors can bind to particular chemicals in the extracellular fluid -Membrane of sensory neuron becomes depolarized and produces action potentials Chemoreceptors are used in the senses of taste and smell -Are also important in monitoring the chemical composition of blood 21 7

8 Taste The perception of taste (gustation) is a mixture of physical & psychological factors -Broken down into five categories: -Sweet, sour, salty, bitter, and umami (perception of glutamate and other amino acids) Taste buds are collections of chemosensitive cells associated with afferent neurons 22 Taste In fish, taste buds are scattered all over the body surface In land vertebrates, taste buds are located in the epithelium of the tongue and oral cavity within raised areas called papillae 23 Taste Many arthropods have taste chemoreceptors -Flies have them in sensory hairs located on their feet 24 8

9 Smell In land vertebrates, the sense of smell (olfaction) involves neurons located in the upper portion of the nasal passages -These neurons transmit impulses directly to the brain via the olfactory nerve Humans can discern thousands of different smells 25 Vision Vision begins with the capture of light energy by photoreceptors -Visual information is used to determine both the direction and distance of an object Invertebrates have simple visual systems with photoreceptors clustered in an eyespot -Flatworms can perceive the direction of light but cannot construct a visual image 26 Vision The members of four phyla have evolved well-developed, image-forming eyes -Annelids, mollusks, arthropods, and chordates Although these eyes are similar in structure, they have evolved independently -An example of convergent evolution 27 9

10 Vision 28 Structure of the Vertebrate Eye Sclera = White portion of the eye, formed of tough connective tissue Cornea = Transparent portion through which light enters; begins to focus light Iris = Colored portion of the eye -Contraction of iris muscles in bright light decreases the size of its opening, the pupil Lens = A transparent structure that completes focusing of light onto the retina 29 Structure of the Vertebrate Eye The lens is attached to the ciliary muscles by the suspensory ligament -In near vision, ciliary muscles contract, -Lens becomes more rounded and bends light more strongly -In distance vision, ciliary muscles relax -Lens becomes more flattened and bends light less 30 10

11 31 Structure of the Vertebrate Eye The vertebrate retina contains two types of photoreceptors: -Rods = Responsible for black-and-white vision when illumination is dim -Cones = Responsible for color vision and high visual acuity (sharpness) -Most are located in the central region of the retina known as the fovea -Sharpest image is formed 32 Structure of the Vertebrate Eye The retina consists of three layers of cells -External layer contains the rods and cones -Middle layer contain bipolar cells -Layer closest to eye cavity contains ganglion cells Once photoreceptors are activated, they stimulate bipolar cells, which in turn stimulate ganglion cells -Transmit impulses to brain via optic nerve 33 11

12 Visual Processing Primates and most predators have two eyes, one located on each side of the face -The two fields of vision overlap, permitting binocular vision -Ability to perceive 3-D images and depth In contrast, prey animals generally have eyes located to the sides of the head -This prevents binocular vision, but enlarges the overall receptive field 34 Diversity of Sensory Experiences The only vertebrates that can sense infrared radiation are pit viper snakes -Have a pair of heat-detecting pit organs on either side of the head between the eye and nostril -Locate heat sources in the environment, including prey in darkness 35 Diversity of Sensory Experiences 36 12

13 Diversity of Sensory Experiences The elasmobranchs (sharks, rays and skates) have electroreceptors called the ampullae of Lorenzini -Can sense electrical currents generated by the muscle contractions of their prey Eels, sharks, bees and many birds appear to navigate along the magnetic field lines of the Earth 37 13