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 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

Hair cells containing mechanoreceptors are inside the cochlea

Axons of sensory neurons Cochlea Stapes Oval window Apex Axons of sensory neurons Round window Basilar membrane Tympanic canal Base Vestibular canal Perilymph Cochlea (uncoiled) Basilar membrane Apex (wide and flexible) Base (narrow and stiff) 500 Hz (low pitch) 1 kHz 2 kHz 4 kHz 8 kHz 16 kHz (high pitch) Frequency producing maximum vibration

The Vertebrate Eye

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

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 EXTRACELLULAR FLUID Membrane potential (mV) – 40 – 70 Dark Light – Hyper- polarization Time Na+ cGMP CYTOSOL GMP Plasma membrane INSIDE OF DISK PDE Active rhodopsin Light Inactive rhodopsin Transducin Disk membrane 2 Active rhodopsin in turn activates a G protein called transducin. 3 Transducin activates the enzyme phos-phodiesterae(PDE). 4 Activated PDE detaches cyclic guanosine monophosphate (cGMP) from Na+ channels in the plasma membrane by hydrolyzing cGMP to GMP. 5 The Na+ channels close when cGMP detaches. The membrane’s permeability to Na+ decreases, and the rod hyperpolarizes. 1 Light isomerizes retinal, which activates rhodopsin. Absorption of light by retinal Triggers a signal transduction pathway Figure 49.21

Three other types of neurons contribute to information processing in the retina Ganglion cells, horizontal cells, and amacrine cells Optic nerve fibers Ganglion cell Bipolar cell Horizontal cell Amacrine cell Pigmented epithelium Neurons Cone Rod Photoreceptors Retina Optic nerve To brain Figure 49.23

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 You see a white triangle even though no triangle exists.

Which yellow line is longer? The brain applies perspective to understand different sizes