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Detecting Environmental Events:
Transduction and Coding
z Sense—The mechanism we use to detect and react to stimuli, transforming environmental stimulation into information the nervous system can use.
z Transduction—The conversion of physical energy into a neural impulse.
z Coding—A process involving a specific pattern of neural activity that contains information about stimuli in the environment.
Components of the Visual System:
The Eye
z Photoreceptor—The receptor cell at the back of the eye that transduces light into a neural impulse. z Cornea—The transparent outer layer of the eyeball. z Aqueous humor—A clear fluid similar to blood plasma which fills the anterior chamber of the eye. z Iris—A structure consisting of bands of muscle covered by colored tissue. z Pupil—An opening in the iris that light passes through (size is regulated by ANS). z Lens—The lens consists of transparent, onion-like layers of tissue whose function is to focus the light passing through the pupil on the retina.
z Ciliary muscles—contract to control the shape of the lens. z Accommodation—change in the shape of the lens to maintain focus of an image on the retina. z Vitreous humor—A clear, jelly-like fluid between the lens and the retina. z Retina—The interior lining at the back of the eye that contains the photoreceptors. z Retinal image—an upside down and reversed image focused on the retina.
Components of the Visual System:
The Eye
Cross-sectional View of the Human Eye
Components of the Visual System:
The Retina
z Composed of several layers of cells of which three are particularly important for vision:
- photoreceptors
- bipolar cells
- ganglion cells
z Bipolar cell—A cell between a photoreceptor and a ganglion cell in the retina. z Ganglion cell—A cell in the third layer of cells in the retina. z The central region of the retina where cones are most concentrated is called the fovea. z Photoreceptors
- A rod is a type of photoreceptor located in the peripheral part of the eye which operates in low light.
- A cone is a type of photoreceptor relatively concentrated in the center of the retina which detects fine details and colors in bright light.
Visual System:
The Retina
z Cones have better acuity; rods more light sensitivity due to a process called convergence.
Components of the Visual System:
The Retina
10.12 An Unobscured View
Transducing Light Into Neural
Messages
z Transduction occurs when the rods and cones change light energy into neural messages which are carried to the cerebral cortex.
The Electromagnetic Spectrum
Transducing Light:
Detection of Visual Stimuli
z Structures of the photoreceptors
- Photopigment—A chemical molecule in the lamellae of the eye that absorbs light.
- Opsin—The protein component of a photopigment.
- Retinal—The lipid component of a photopigment, synthesized from Vitamin A.
- Rod opsin—The form of opsin found in the rods.
- Rhodopsin (rosy color before light exposure)—The photopigment in rods; consists of rod opsin and retinal.
The Visual System:
Lateral Geniculate Nucleus
z Parvocellular layers—composed of a type of neuron with small cell bodies that receive projections from a type of ganglion cell called an X ganglion cell. The X ganglion cells originate from the central portion of the retina (fovea) that contains mostly cones. z Magnocellular layers—composed of neurons with large cell bodies that receive input from a Y ganglion cell. The Y ganglion cells originate mostly from the periphery of the retina which suggests that their input is primarily from rods.
A Hypercolumn
The Visual System:
LGN and the Primary Visual Cortex
z Both the LGN and primary visual cortex are organized spatially like a map of the retina so the representation of an image remains intact as it is sent to the primary visual cortex.
z Primary visual cortex—about 25% of the primary visual cortex receives representation from the fovea (a disproportional amount) which helps to account for the greater acuity of objects seen in bright light.
z About 80% of the optic tract fibers go to the LGN; the remaining 20% to the superior colliculus which is responsible for attention to visual stimuli and eye movement.
The Visual System:
LGN and the Primary Visual Cortex Primary Visual Cortex: Structure
z Primary visual cortex –The area of the cerebral cortex that detects features of the visual environment; also called area V1 or striate cortex.
- The primary visual cortex has six layers like the LGN.
- A cluster of neurons in the primary visual cortex that are sensitive to specific colors are called blobs.
z Visual field deficit—An inability to see objects in a specific part of the visual field, caused by damage to a region of the occipital lobe or the pathways leading to it.
- Blindsight –The ability of an individual to respond to objects in a missing visual field without being conscious of seeing anything.
Primary Visual Cortex: Structure
Pathways to the Secondary Visual
Cortex
z Secondary Visual Cortex—The area of the cerebral cortex that combines visual features into a recognizable visual perception; also called area V2 or prestriate cortex.
Development of the Visual System:
Influence of Experience
z Critical period—Hubel and Wiesel (1963) found a critical period of stimulation required for normal development for cats (about 3 months). This period may last as long as 4 to 5 years or more in humans. z Amblyopia is produced by strabismus and treated by forcing patients to use the “lazy” eye. z Other research indicates that enriched environments are associated with earlier eye opening and precocious development of visual acuity in mice.
The Role of the
Secondary Visual Cortex
z Perceptual problems may be caused by damage within the cerebral cortex even though the sensory pathway is intact.
- Visual agnosia—inability to name an object presented visually (the ability to identify it by another sense, e.g., touch may be present).
- Fusiform face area—The region of the inferotemporal cortex most responsible for recognition of faces.
- Prosopagnosia—An impaired ability to recognize faces visually. A person with this problem may be unable to recognize their own face!
Parts of the Ear
z Pinna—The outer, visible portion of the ear.
- Helps us detect where sounds originate, but it is not essential for hearing. z Tympanic membrane (eardrum)—The membrane that divides the outer and middle parts of the ear. z Malleus (hammer)—The bone of the middle ear attached to the tympanic membrane and the incus. z Incus (anvil)—The bone of the middle ear attached to the malleus and stapes.
Parts of the Ear
z Stapes (stirrup)—The bone of the middle ear attached to the incus and the oval window. z Oval window—The part of the inner ear attached to the stapes. z Eustachian tube—Connects the middle ear with the back of the throat.
- Purpose is to equalize air pressure on either side of the eardrum.
z Cochlea—A snail-shaped structure in the inner ear that contains the auditory receptors.
- Contains three long, fluid-filled chambers: the vestibular canal, the cochlear canal, and the tympanic canal. z Basilar membrane—A membrane in the organ of Corti to which auditory receptors are attached by Deiter’s cells. z Tectorial membrane—A membrane in the organ of Corti in which hair cell cilia are embedded or with which cilia make close contact.
Parts of the Ear
The Transduction of Sound Waves
into Neural Impulses
z The inner hair cells have a resting potential of -60 mV. z When cilia bend in the direction of the longest cilium the membrane depolarizes. z This leads to a rapid influx of Ca^2 +^ ions into the hair cells, which results in the release of glutamate.
Pathways to the Auditory Cortex
z Cochlear nerve—A nerve formed by the axons of bipolar cells in the spiral ganglion that synapse with the hair cells.
z Auditory nerve—Cranial nerve VIII; the nerve that extends from the merging of the cochlear nerve and vestibular nerve.
z Cochlear nucleus—The first neurons in the medulla that receive neural messages from auditory receptors via the auditory nerve.
Pathways to the Auditory Cortex
z Superior olivary nucleus—A group of neurons in the medulla that receives neural messages from the cochlear nuclei. z Inferior colliculus—An area of the tectum of the midbrain that receives neural messages from both the cochlear nucleus and the superior olivary nucleus.
Pathways to the Auditory Cortex
z Medial geniculate nucleus (MGN)—A group of neurons in the thalamus that receives neural impulses from the inferior colliculus. z Secondary auditory cortex—The area of the temporal lobe surrounding the primary auditory cortex, where pitch, loudness, and timbre are perceived and specific sounds are recognized.
The Vestibular Sense
z Vestibular sense—The sense responsible for maintaining balance.
- Enables us to walk on two feet, keep our head upright, and adjust our eye movements to compensate for our head movements.
Skin Receptors
z The functions of the skin include protecting the internal organs from injury; helping regulate body temperature by producing sweat, which cools the body when it becomes too hot; and providing a first line of defense against invading microorganisms.
Skin Receptors
z Pacinian corpuscles—The largest of the somatosensory receptors of the skin
- Approximately 0.5 mm wide by 1.0 mm long
- Have quite large receptive fields
- Sensitive to touch stimulation, especially to high- frequency vibrations (200 to 300 Hz)
z Free nerve endings—Located just below the surface in both hairy and hairless skin
- detects temperature change and pain stimuli (both fast pain and slow pain)
z Meissner’s corpuscle—A type of skin receptor in hairy skin
- located in the elevations of the dermis into the epidermis
- responds to pressure and low-frequency vibrations;
- small receptive fields
Skin Receptors
z Merkel’s disk—A type of skin receptor in the base of the epidermis near the sweat ducts
- sensitive to pressure, but not to vibrations
- small receptive fields z Ruffini’s corpuscle—A type of skin receptor just below the surface
- detects low-frequency vibrations, but not pressure
- large receptive fields
Skin Receptors Somatosensory Pathways
z Once information from the skin reaches the CNS, the neural message travels through one of three somatosensory systems:
- The dorsal column-medial lemniscal system
- The anterolateral system
Three Somatosensory Systems
z Dorsal column-medial lemniscal system—A somatosensory pathway that begins in the spinal cord and transmits information about touch and proprioception to the primary somatosensory cortex.
Three Somatosensory Systems
z Anterolateral - Spinothalamic system—The somatosensory pathway that begins in the spinal cord and transmits information about temperature and pain to the brain stem, reticular formation, and the primary and secondary somatosensory cortices.
Locating Input on the
Somatosensory System
z The somatosensory system is topographically organized – adjacent places on the skin activate adjacent neurons in the primary somatosensory cortex, though the cortical organization is upside down. z Not all body parts are equally represented. The greatest representation is for areas such as the hands, lips, and tongue, which are involved in fine tactile discrimination.
The
Somatosensory
Cortex The Experience and Control of Pain
z Pain has both negative and positive functions:
- Chronic pain can be the bane of a person’s existence.
- However, under ordinary circumstances, pain is extremely useful, warning us of potential injury and inducing us to seek appropriate treatment.
Taste Receptors
z Papilla—A small, visible bump on the tongue that contains taste bumps.
z Taste bud—A cluster of taste receptors that lie either near or within a papilla.
z Three kinds of papillae contain taste buds:
- Foliate
- Circumvallate
- Fungiform
Types of
Papillae and
Distribution
of Taste
Receptors
Genetics of Taste
z People differ in their sensitivity to bitter and some sweet tastes. z These individual differences appear to be partly related to the number of taste buds on the tongue:
- Supertasters (25% of people) have the most taste buds (about 425 per square cm on the tongue tip).
- Medium tasters (50% of people) have about 184 taste buds per square cm.
- Non-tasters (25% of people) have about 96 per square cm.
Mechanisms of Taste Reception
z Mechanism differs for each of the four basic tastes:
- Salty food activates a taste receptor by causing Na +^ ions to move through Na +^ ion channels in the cell membrane.
- H+ ions in sour foods and sugar molecules in sweet foods close the K +^ ion channels in receptor membranes, preventing K +^ ions from leaving the cell.
- In bitter foods, alkaloid compounds trigger the movement of Ca 2+^ ions into the cytoplasm from storage sites in the taste receptor, increasing the release of neurotransmitters.
Gustatory Pathways
z Chorda tympani—A branch of cranial nerve VII that conveys taste information from the posterior tongue and the palate and throat to the nucleus of the solitary tract. z Nucleus of the solitary tract—A group of neurons in the medulla that receives information from taste receptors.
Gustatory Pathways
z Ventral posteromedial thalamic nucleus—A group of neurons that receives taste information from the nucleus of the solitary tract and then transmits it to the primary gustatory cortex. z Primary gustatory cortex—An area located just ventral and rostral to the area representing the tongue in the somatosensory cortex.
Gustatory
Pathways
Olfaction
z Olfactory sense—The sense of smell. z Habituation (a decrease in responding after repeated exposure to an innocuous stimulus) can occur quickly with smells. Whether pleasant or unpleasant, we rapidly “get used to” smells. z This sensory adaptation is caused by decreased responding by receptors when they are exposed to the same stimulus for a continuous period of time.
Olfactory Receptors
z Olfactory epithelium—The mucous membrane in the top rear of the nasal passage; lined by olfactory receptors.
- Humans have approximately 50 million olfactory receptors that detect smell, whereas other species, such as dogs, may have up to 20 times as many, with each cell having more than 10 times as many cilia.
Olfactory Pathways
z Olfactory bulb—A structure at the base of the brain that receives information about odor from olfactory receptors. z Olfactory tract—Axons of olfactory bulb neurons that project to the primary olfactory cortex. z Primary olfactory cortex—An area in the pyriform cortex in the limbic system that gives odors an emotional component.
Olfactory Pathways
z From the primary olfactory cortex, some olfactory messages are transmitted to the hypothalamus, where they become important in motivating approach or avoidance behavior related to food or drink. z Other olfactory messages go to the dorsomedial thalamus and then to the orbitofrontal cortex, which is thought to be responsible for odor identification. z Vomeronasal System
- Pheromones
- Vomeronasal Organ
- Accessory Olfactory Bulb
- Amygdala
Olfactory
Receptors
and
Pathways
