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Types of Sensory Neurons • PHYSICAL: • Photoreceptors (for vision) • Mechanoreceptors (for touch & hearing) • CHEMICAL: • Also known as “Chemoreceptors”: • Odor receptors (for smell) • Gustatory receptors (for taste) Taste buds contain sensory neurons w/ taste receptors Hearing • Exploring the structure of the human ear 1 2 The middle ear and inner ear Overview of ear structure Incus Middle ear Inner ear Outer ear Skull bones Semicircular canals Stapes Malleus Auditory nerve, to brain Pinna Tympanic membrane Auditory canal Hair cells Cochlea Eustachian tube Tectorial membrane Tympanic membrane Oval window Eustachian tube Round window Cochlear duct Bone Vestibular canal Auditory nerve Basilar membrane Figure 49.8 Axons of sensory neurons 4 The organ of Corti To auditory nerve Tympanic canal 3 The cochlea Organ of Corti Hair cells containing mechanoreceptors are inside the cochlea Cochlea Stapes Axons of sensory neurons Oval window Vestibular canal Perilymph Apex Base Round window Tympanic canal Basilar membrane Cochlea (uncoiled) Basilar membrane Apex (wide and flexible) 500 Hz 1 kHz (low pitch) 2 kHz 4 kHz 8 kHz 16 kHz (high pitch) Base (narrow and stiff) Frequency producing maximum vibration The Vertebrate Eye • The structure of the vertebrate eye Sclera Choroid Retina Ciliary body Fovea (center of visual field) Suspensory ligament Cornea Iris Optic nerve Pupil Aqueous humor Lens Vitreous humor Central artery and vein of the retina Figure 49.18 Optic disk (blind spot) • Humans and other mammals – Focus light by changing the shape of the lens Front view of lens and ciliary muscle Lens (rounder) Ciliary muscles contract, pulling border of choroid toward lens Choroid Suspensory ligaments relax Retina Ciliary muscle Lens becomes thicker and rounder, focusing on near objects Suspensory ligaments (a) Near vision (accommodation) 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 (b) Distance vision Lens (flatter) Lateral view of the vertebrate eye 2 Types of Photoreceptors: • • • • • • RODS: More numerous (125 x 106) More sensitive to light Responsible for night vision Absent from the fovea Lower resolution • • • • • CONES: Less numerous Sensitive to (wavelength) Responsible for color vision Come in 3 types: red, green, blue (photopsins) • Higher resolution Photoreceptors in the retina Looking inside a photoreceptor Retinal pigment changes shape when light strikes it Signal Transduction in Rods Light EXTRACELLULAR FLUID INSIDE OF DISK Active rhodopsin PDE • Absorption of light by retinal Membrane potential (mV) – Triggers a signal transduction pathway CYTOSOL Plasma membrane 0 Dark Light Inactive rhodopsin Transducin cGMP Disk membrane – 40 GMP Na+ 1 Light isomerizes retinal, which activates rhodopsin. Figure 49.21 2 Active rhodopsin in turn activates a G protein called transducin. 3 Transducin activates the enzyme phosphodiesterae( PDE). 4 Activated PDE detaches cyclic guanosine monophosphate (cGMP) from Na+ channels in the plasma membrane by hydrolyzing cGMP to GMP. – 70 – Hyperpolarization Time Na+ 5 The Na+ channels close when cGMP detaches. The membrane’s permeability to Na+ decreases, and the rod hyperpolarizes. • Three other types of neurons contribute to information processing in the retina – Ganglion cells, horizontal cells, and amacrine cells Retina Optic nerve To brain Retina Photoreceptors Neurons Cone Rod Amacrine cell Figure 49.23 Optic nerve fibers Ganglion cell Horizontal cell Bipolar cell Pigmented epithelium A bird’s eye view… An overview of visual processing Message travels from RETINA to VISUAL CORTEX (oc.lobe) The brain tries to make sense of missing visual information Which yellow line is longer? The brain applies perspective to understand different sizes