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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