Chapter 3: Senses and Perception

Objectives of this Chapter:

• I will be able to know about vision
• I will be able to know about hearing
• I will be able to know about taste and smell
• I will be able to know about touch and pain

Vision

• Vision involves around 30% of the human brain

It all starts with light

• Vision begins with light going through the cornea   * The cornea does around 75% of the focusing
• The lens then adjusts the focus   * The shape of the lens is altered by muscles behind the iris so near or far objects can be brought into focus on the retina
• $Retina:$ a sheet of photoreceptors in the back of the eye   * The cornea & lens combine to produce image onto the retina
• Signals are sent via the optic nerve to parts of the brain that process images and allow us to see   * Retinal image is reversed in the brain to look right-side up
• The size of the pupil controls how much light enters the eye   * $Pupil$: a hole in the eye that allows light to enter   * $Iris$: the muscled ring around the eye that controls the size of the pupil
• $Binocular vision$: vision that utilizes two eyes   * This is what primates have
• $Optic Chiasm$: the X-shaped structure formed at the point below the brain where the two optic nerves cross over each other   * Visual signals pass from the optic nerve to the optic chiasm
• Each half of the cerebrum is responsible for processing information from the opposite half of the body (left hemisphere to right side, vice versa)
• $Photoreceptors:$ neurons specialized to turn light into electrical signals   * There are approximately 125 million photoreceptors in each eye   * 2 major types: rods and cones     * $Rods$: extremely sensitive to light & allow us to see in dim light but do not convey color     * $Cones$: need a bright enough light but give acute details and convey color       * 3 cones: red, green, blue         * Each sensitive to a different range of colors       * Cones work in combination to convey information about all visible colors
• $Fovea$: area of the retina where the light is focused
• $Macula$: the area in the retina around the fovea critical for reading and driving
• The retina has 3 layers   * 1st layer: rods and cones   * 2nd layer: interneurons that relay information   * 3rd layer: ganglia that make optic nerve
• $Receptive Field$: region of visual space providing input to neuron   * Center of the retina is most receptive area while the sides are less receptive

How Visual Information is processed

• A visual cell’s receptive field is activated when light hits the center and not at the sides   * If light hits all the parts, the cell responds weakly
• Visual processing starts by comparing the amount of light striking any tiny region of the retina with the amount of surrounding light
• Visual information goes in this pathway:   * retina → lateral geniculate nucleus in the thalamus → primary visual cortex (PVC)
• Cells above and below the middle layer of PVC respond differently than the middle layer   * Cells in the layers above and below prefer stimuli in shape of bars or edges and those at a particular angle
• Signals are fed into at least 3 processing systems   * First system processes information about shape   * Second system processes color information   * Third system processes information about movement, location, spatial organization
• Perception of movement, depth, perspective, relative size and movement, shading, and gradations in texture primarily depend on contrasts in light intensity rather than color

Research leads to more effective treatment

• $Strabismus$: a condition where the eyes are not properly aligned
• Extensive genetic studies and the use of model organisms make it possible to make gene/stem cell therapy or discover new drugs for treatments

Hearing

• Hearing allows for communication and information for survival
• $External ear$: the collective name for the visible portion of the ear (pinna) and the auditory canal   * This is the initial collector of sound waves
• $Tympanic membrane/Eardrum$: thin tissue that produces and sends sound vibrations to the middle ear   * Eardrum makes the ossicles vibrate and amplify its vibration     * $Ossicles$: three bones in the middle ear (malleus, incus, stapes) that amplifies the vibrations produced by the eardrum
• The stapes pushes on a part called the oval window to send pressure waves to cochlea   * $Cochlea$: snail-shaped organ in the inner ear that converts mechanical vibrations from the eardrum and ossicles to electrical signals to be sent to the brain     * An important part of the cochlea is the basilar membrane       * $basilar membrane$: a membrane containing cells called hair cells that react to different frequencies/pitches         * Hair cells are topped with stereocilia that are deflected by the overlying tectorial membrane         * Hair cells convert mechanical vibration to electrical signals and excite the auditory nerve
• $auditory nerve:$ one of the 12 cranial nerves that is responsible for carrying auditory information from the cochlea to the brain   * Each nerve fiber of the auditory nerve contains information about a different frequency to the brain
• $Superior temporal gyrus/auditory cortex:$ the part of the brain that analyzes auditory information   * In the auditory cortex, adjacent neurons respond to tones of similar frequency     * The neurons each specialize in different combos of tones   * Other neurons combine information to recognize the sound   * The left auditory cortex is specialized for speech

Taste and smell

Taste

• Taste is focused on distinguishing chemicals that have sweet, salty, sour, bitter, or umami (savory) taste
• Tastants: chemicals present in foods that give them flavor   * Tastants are detected by taste buds     * $taste buds:$ the sensory organs responsible for obtaining information about taste       * Taste buds are embedded in papillae       * Taste buds are found on the tongue, the back of the mouth, and on the palate       * 1 taste bud= 50-100 sensory cells
• These sensory cells are stimulated by sugars, salts, acids
• When stimulated, sensory cells send impulses along the cranial nerves → taste regions in brain → thalamus   * The thalamus sends it to a specific area of cerebral cortex which makes us conscious of taste

Smell

• Odorants are detected by sensory neurons in a small patch of mucus membrane on the roof of the nose
• Axons of the cells pass through holes in the bone and enter 2 olfactory bulbs against the underside of the brain’s frontal lobe   * $Olfactory bulbs$: a rounded structure that contains neurons receiving information about odors detected by sensory neurons on the roof of the nose
• Odorants stimulate receptors and initiate a neural response   * Odorants can act on more than 1 receptor but to varying degrees
• The pattern of activity is sent to the olfactory bulb where other neurons are activated to form a spatial map of odor
• Neural activity passes to the primary olfactory cortex at the back of the underside of the frontal lobe   * This information then passes to the orbital cortex to combine with taste information to make the perception of flavor

Touch and Pain

Touch

• $Touch$: the sense by which we determine the characteristics of objects (size, shape, texture)
• In areas with hairy skin, some touch receptors consist of webs of neuron endings wrapped around the base of hair   * Signals from these receptors pass through sensory nerves to the spinal cord
• The spinal cord passes information about touch to the thalamus and to the sensory cortex
• The transmission of information about touch is highly topographic   * $topographic$: meaning the body is represented in an orderly fashion based on sensory requirements at different levels of the nervous system
• Larger areas of the cortex are made for more sensitive areas like the hands and lips while smaller cortex areas represent less sensitive parts of the body
• Different parts of the body vary in sensitivity to tactile and painful stimuli   * This is largely based on the number and distribution of receptors     * E.g: the cornea is several hundreds of times more sensitive to painful stimuli than the soles of feet   * The fingertips are good at touch discrimination but the torso is not
• $Two-point threshold$: distance between 2 points of skin in order for the person to distinguish 2 stimuli from one   * Neurologists measure sensitivity by determining the two-point threshold   * The acuity of the two-point threshold is greatest where there are most nerves

Pain

• $Nociceptors$: sensory fibers that respond to tissue-damaging stimuli and cause pain   * Different subsets of nociceptors make molecules that are responsible for responses to painful, thermal, mechanical, or chemical stimulation
• Tissue injury releases many different chemicals at the site of damage/inflammation
• $Prostaglandins$: enhance sensitivity of receptors to tissue damage and induces more pain   * also contributes to allodynia
• $Allodynia$: triggering of pain response from stimuli which doesn’t usually provoke pain
• Persistent pain leads to changes in the nervous system that amplify and prolong pain

Sending and Receiving Pain and Itch Messages

• Pain and itch messages are transmitted to the spinal cord through small myelinated and unmyelinated ( C ) fibers   * Myelinated fibers are pain sensitive and evoke sharp and fast pain   * Unmyelinated/C fibers are slower in onset and cause more dull, diffuse pain
• Impulses relayed to several brain structures including the thalamus and cerebral cortex   * Thalamus and cerebral cortex involved in making the pain/itch message into a conscious experience
• Factors like the setting and emotional impact contribute to the overall response to a painful experience
• Pain messages can be suppressed by neurons originating from gray matter in the brainstem   * They suppress pain by inhibiting the transmission of pain signals from the dorsal horn of the spinal cord to higher brain areas
• Some systems use natural chemicals: endogenous opioids or endorphins   * Endorphins very similar to morphine   * After a technique for putting opioids in the spine was successfully performed in animals, this treatment began in humans.     * Now, this technique is common in treating pain after surgery.
• There is no area in the brain specifically for pain   * Emotional and sensory components constitute a mosaic of activity leading to pain

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