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
- 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