Ch 6. Hearing, Mechanical, and Chemical Senses

Hearing helps us detect important information through periodic compressions in air or other media, sensed as sound waves.

• Amplitude = loudness

• Frequency (Hz) = pitch

• Timbre = tone quality

- Humans hear from ~20 to 20,000 Hz (children hear higher).

- Pitch, loudness, and timbre also convey emotion.

- emotional tone is called prosody

- With age and noise exposure, high-frequency hearing

declines.

The outer ear

- The outer ear includes the pinna, the structure of flesh and cartilage attached to each side of the head.

- Responsible for:

- Altering the reflection of sound waves into the middle ear from the outer ear

- Helping us to locate the source of a sound

The middle ear

- Contains the tympanic membrane, which vibrates at the same rate when struck by sound waves

- Also known as the ear drum

- Connects to three tiny bones (malleus, incus, and stapes) that transform waves into stronger waves to the oval window

- Oval window is a membrane in the inner ear.

- Transmits waves through the viscous fluid of the inner ear

The inner ear

- Contains a snail shaped structure called the cochlea

- Contains three fluid-filled tunnels

- Hair cells are auditory receptors that lie between the basilar membrane in the cochlea.

- When displaced by vibrations in the fluid of the cochlea, they excite the cells of the auditory nerve by opening ion channels.

Frequency theory

The basilar membrane vibrates in sync with sound waves, causing neurons to fire at matching rates (up to ~1000 Hz).

Volley principle

Groups of neurons take turns firing to represent frequencies up to ~4000 Hz.

Place theory

Different parts of the basilar membrane respond to different frequencies.

Updated view

The membrane is stiff at the base (high frequencies) and flexible at the apex (low frequencies).

The Auditory Cortex: Primary auditory cortex (A1) is

located in the superior temporal cortex; receives input mainly from the opposite ear.

• Organized like the visual cortex, with "what" (sound identity) and "where" (sound location) pathways.

• A1 is key for auditory imagery and superior temporal cortex for sound motion detection.

• Tonotopic map: Cells respond to specific tones; some prefer complex over pure sounds.

• A1 is not needed for hearing itself, but for interpreting sound.

• Experience-dependent: A1 develops less in those deaf from birth.

• Damage to A1 may impair processing but does not cause complete deafness unless subcortical regions are also damaged.

Sound Localization

- Depends upon comparing the responses of the two ears

3 cues:

- Time of arrival

- Sound shadow

- Phase difference

- Humans localize low-frequency sound by phase difference and high frequency sound by loudness differences.

Conductive/Middle Ear Deafness

- Occurs if bones of the middle ear fail to transmit sound waves properly to the cochlea

- Can be caused by disease, infections, or tumorous bone growth

- Normal cochlea and auditory nerve allow people to hear their own voice clearly.

- Can be corrected by surgery or hearing aids that amplify the stimulus

Nerve or Inner-Ear Deafness

- Results from damage to the cochlea, the hair cells, or the auditory nerve

- Can vary in degree

- Can be confined to one part of the cochlea

- People can hear only certain frequencies.

- Can be inherited, caused by disease, or noise levels that damage the auditory system

Tinnitus

- Frequent or constant ringing in the ears

- Experienced by many people with nerve deafness

- Sometimes occurs after damage to the cochlea

- Axons representing other part of the body innervate parts of the brain previously responsive to sound.

- Similar to the mechanisms of phantom limb

What evidence suggests that absolute pitch depends on special experiences?

Absolute pitch occurs almost entirely among people who had early musical training and is also more common among people who speak tonal languages, which require greater attention to pitch.

The mechanical senses respond to

pressure, bending, or other distortions of a receptor.

- These include touch, pain, and other body sensations, as well as vestibular sensation, which detects the position and movement of the head.

- Audition is a complex mechanical sense because the hair cells are modified touch receptors.

the vestibular organ is in

the ear and is adjacent to the cochlea.

- Comprises 2 otolith organs (the saccule and utricle) and 3 semicircular canals

- Otoliths are calcium carbonate particles that push against different hair cells and excite them when the head tilts.

- Semicircular canals are filled with a jelly-like substance and hair cells that are activated when the head moves.

Somatosensory system

- The sensation of the body and its movements

- Not one sense but many

- Touch, deep pressure, the position and movement of joints, pain, and temperature

Somatosensory Receptors

- Pacinian corpuscle, which detects vibrations or sudden

displacements on the skin

- Onion-like outer structure resists gradual or constant

pressure.

- Sudden or vibrating stimulus bends the membrane and

increases the flow of sodium ions to trigger an action

potential.

- Receptors that respond to light touch

- Men and women generally have the same number of Merkel disks, but women tend to have smaller fingers (more

sensitive).

Pacinian corpuscle

detects vibrations or sudden displacements on the skin

- Onion-like outer structure resists gradual or constant pressure.

Receptors for Temperature

- Important that humans can regulate temperature as both overheating and overcooling can be fatal

- Cold-sensitive neurons respond to drops in temperature, adapt quickly, and show little response to constant, cold temperatures.

- Heat-sensitive neurons respond to absolute temperature.

- Chemicals can stimulate receptors for heat and cold, for example, capsaicin and menthol.

Tickle

- The sensation of tickle is poorly understood.

- The reason we cannot tickle ourselves is that our brain compares the resulting stimulation to the "expected" stimulation and generates a weaker somatosensory response.

Somatosensation in the Central Nervous System

- Information from touch receptors in the head enters the CNS through the cranial nerves.

- Information from receptors below the head enters the spinal cord and travel through the 31 spinal nerves to the brain.

- Various types of somatosensory information—such as touch, pressure, and pain—travel through the spinal cord in separate pathways toward the thalamus, which then sends impulses to different areas of the primary somatosensory cortex (S1).

Somatosensation in the Spinal Cord

- Each spinal nerve has a sensory component and a motor component and connects to a limited area of the body.

- A dermatome: a body area innervated by a single sensory spinal nerve

- Sensory information entering the spinal cord travels in well-defined and distinct pathways.

- Example: Touch pathway is distinct from pain pathway.

dermatome

a body area innervated by a single sensory spinal nerve

The Somatosensory Cortex

- Various aspects of body sensations remain separate all the way to the cortex.

- Various areas of the somatosensory thalamus send impulses to different areas of the somatosensory cortex located in the parietal lobe.

- Different sub areas of the somatosensory cortex respond to different areas of the body.

- Damage to the somatosensory cortex can result in the impairment of body perceptions (numbsense).

Pain

The experience evoked by a harmful stimulus and directs one's attention toward a danger

- Pain sensation begins with the least specialized of all receptors (bare nerve endings).

- Males and females do not react the same way to pain.

- Activating certain neurons in the midbrain decreases pain sensitivity in male mice, but not in females.

- The pain-relieving effects of opiates and cannabinoids differ between males and females.

Axons carrying pain info have little or no myelin: impulses travel slowly

- However, brain processes pain information rapidly and motor responses are fast.

- Mild pain triggers the release of glutamate in the spinal cord.

- Stronger pain triggers the release of glutamate and releases several neuropeptides including substance P and CGRP (calcitonin gene- related peptide).

Emotional associations of pain

- Activate a path that goes through the reticular formation of the medulla

- And then to several of the central nuclei of the thalamus, the amygdala, hippocampus, prefrontal cortex, and cingulate cortex

- Experimenters monitored people's brain activity and found hurt feelings activate similar pathways as physical pain.

Ways of Relieving Pain

- Opioid system responds to opiates and similar chemicals.

- Opiates bind to receptors mainly in the spinal cord and periaqueductal gray of the midbrain.

- Endorphins are natural brain chemicals that activate these same receptors.

- Different endorphins respond to different types of pain.

- Morphine blocks dull, slow pain but not sharp pain from large-diameter axons.

Opioid system responds to

opiates and similar chemicals

Opiates bind to

receptors mainly in the spinal cord and periaqueductal gray of the midbrain.

Endorphins

are natural brain chemicals that activate these same receptors (in the spinal cord and periaqueductal gray of the midbrain)

Morphine blocks

dull, slow pain but not sharp pain from large-diameter axons.

Gate Theory

- Proposes that the spinal cord areas that receive messages from pain receptors also receive input from touch receptors and from axons descending from the brain

- These other areas that provide input can close the "gates" by releasing endorphins and decrease pain perception.

- Non-pain stimuli around it can modify the intensity of the pain.

Sensitization of Pain

- Mechanisms of the body to increase sensitivity to pain

-Damaged or inflamed tissue releases histamine, nerve growth factor, and other chemicals that increase the responses of nearby pain receptors.

- Certain receptors become potentiated after an intense barrage of painful stimuli.

- Leads to increased sensitivity or chronic pain later

Itch

- The release of histamines by the skin produce itching sensations.

- Activates a distinct pathway in the spinal cord to the brain

- Impulses travel slowly along this pathway (half a meter per second).

- Pain and itch have an inhibitory relationship.

- Opiates, which decrease pain, increase itch.

Chemical Senses

- The first sensory system of the earliest animals was a chemical sensitivity.

- A chemical sense enables a small animal to find food, avoid certain kinds of danger, and even locate mates.

Taste has one simple function—

to tell us whether to swallow something or spit it out.

- Taste results from stimulation of the taste buds, the receptors on the tongue.

- Mammalian taste receptors are in taste buds located in papillae on the surface of the tongue.

- Our perception of flavor is the combination of both taste and smell.

- Taste and smell axons converge onto many of the same cortical cells.

Taste Receptors

- Receptors for taste are modified skin cells.

- Taste receptors have excitable membranes that release neurotransmitters to excite neighboring neurons.

- Taste receptors are replaced every 10-14 days.

- Each papillae may contain up to 10 or more taste buds.

- Each taste bud contains approximately 50 receptors.

- In adult humans, taste buds lie mainly along the edge of the tongue.

Traditional primary tastes:

sweet, sour, salty, and bitter.

- Umami (glutamate/MSG) is a fifth taste—like unsalted broth.

- Possible sixth taste: oleogustus (fat).

- Taste-modifying substances:

- Miracle berries (miraculin) make sour taste sweet.

- Toothpaste (sodium lauryl sulfate) blocks sweet, enhances bitter.

- Gymnema tea dulls sweet taste.

- Adaptation: Reduced sensitivity from receptor fatigue.

- Cross-adaptation: Reduced response to one taste after exposure to another.

- Water can mimic sour by shifting the acid-base balance on the tongue.

Miracle berries (miraculin)

make sour taste sweet

Toothpaste (sodium lauryl sulfate)

blocks sweet, enhances bitter.

Gymnema tea

dulls sweet taste

Most taste receptors are specific, but

some respond to combos (e.g., sweet + salty).

• These receptors connect to neurons that may be specific or broad in response.

• In the insula (primary taste cortex), most cells respond best to one taste, but can react to others.

• The somatosensory cortex handles the texture/touch of food.

• Each brain hemisphere processes taste from the same side

(ipsilateral) of the tongue.

Olfaction

- The sense of smell

- The detection and recognition of chemicals that contact the membranes inside the nose

- Critical in most mammals for finding food and mates, and avoiding danger

- Rats and mice show an immediate, unlearned avoidance of the smells of cats, foxes, and other predators.

The smell of a sweaty female

increases a male's testosterone secretions, especially if the female is near her time of ovulation.

- This effect is stronger for heterosexual males than for homosexual males.

- The smell of a sweaty male does not increase sexual arousal in females. Instead, it increases release of cortisol, a stress hormone.

- Even humans can follow a scent trail to some extent, and we get better with practice.

Olfactory Receptors

- Olfactory cells line the olfactory epithelium in the rear of the nasal passage and are the neurons responsible for smell.

- Olfactory receptors are located on cilia, which extend from the cell body into the mucous surface of the nasal passage.

- Vertebrates have hundreds of olfactory receptors, which are highly responsive to some related chemicals and unresponsive to others.

Proteins in olfactory receptors respond to

chemicals outside the cells and trigger changes in G protein

inside the cell.

- G protein then triggers chemical activities that lead to action potentials.

- Humans have several hundred types of olfactory receptors.

- Unlike your receptors for vision and hearing, which remain with you for a lifetime, an olfactory receptor has an average survival time of just over a month.

Olfactory Coding

- Axons from olfactory receptors carry information to the

olfactory bulb.

- Most receptors respond to either just one chemical or several closely related chemicals.

- All the receptors sensitive to a particular chemical or

combination of chemicals connect to a single cluster in the

olfactory bulb, called a glomerulus.

- The glomeruli send their axons diffusely, almost haphazardly, to the piriform cortex.

- How the cortex recognizes an odor remains a puzzle.

Individual Differences

- Most of the genes controlling olfactory receptors have variant forms.

- 2 people chosen at random probably differ in about 30% of their olfactory receptor genes.

- Odor sensitivity declines with age.

- Many people who were infected by the worldwide pandemic of COVID-19 suffered an impaired sense of smell, with slow recovery.

- Human females detect odors more readily than males, at all ages and in all cultures (female hormones).

Synesthesia

- The experience of one sense in response to

stimulation of a different sense

- An example would be seeing a number or a letter as a specific color.

- Tends to cluster in families that also have perfect pitch

- Genetic predisposition

The different Somatosensory Receptors

- Free nerve ending

- Hair-follicle receptors

- Meissner's corpuscles

- Pacinian corpuscles

- Merkel's disks

- Ruffini endings

- Krause end bulbs

Free nerve ending

Location: Any skin area

Responds to: Pain and temperature

Hair-follicle receptors

Location: Hair-covered skin

Responds to: Movement of hairs, skin stroke

Meissner's corpuscles

Location: Hairless areas, mainly fingertips

Responds to: Discriminative touch and vibration

Pacinian corpuscles

Location: Any skin area

Responds to: Vibration or sudden touch

Merkel's disks

Location: Any skin area

Responds to: Light touch

Ruffini endings

Location: Any skin area, but scarce in humans

Responds to: Skin stretch, roughness

Krause end bulbs

Location: Mostly hairless areas

Responds to: Uncertain