Detailed Notes on Hearing, Mechanical, and Chemical Senses

Hearing, Mechanical and Chemical Senses

Physics and Psychology of Sound

• Hearing detects important information via periodic compressions in air or other media, perceived as sound waves.
• Amplitude correlates with loudness.
• Frequency (Hz) determines pitch.
• Timbre represents tone quality.
• Humans can typically hear frequencies ranging from approximately 20 to 20,000 Hz; children can often hear higher frequencies.
• Pitch, loudness, and timbre communicate emotion.
• Emotional tone in speech is referred to as prosody.
• High-frequency hearing tends to decline with age and noise exposure.

Structures of the Ear

Outer Ear

• Anatomists divide the ear into the outer, middle, and inner ear.
• The outer ear consists of the pinna, which is the structure of flesh and cartilage on each side of the head.
• Functions of the outer ear:
  • Modifying sound wave reflection into the middle ear.
  • Aiding in sound localization.

Middle Ear

• The middle ear contains the tympanic membrane, also known as the eardrum, which vibrates in response to sound waves.
• It connects to three small bones: the malleus, incus, and stapes.
• These bones convert sound waves into stronger waves, which are then transmitted to the oval window.
• The oval window is a membrane located in the inner ear.
• It passes waves through the viscous fluid of the inner ear.

Inner Ear

• The inner ear contains the cochlea, a snail-shaped structure.
• The cochlea contains three fluid-filled tunnels.
• Hair cells, which are auditory receptors, are situated between the basilar membrane in the cochlea.
• When vibrations occur in the cochlea's fluid, the hair cells are displaced, exciting auditory nerve cells by opening ion channels.

Pitch Perception

• 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 alternate firing to represent frequencies up to ~4000 Hz.
• Place theory: Different parts of the basilar membrane respond to different frequencies.
• Updated view: The basilar membrane is stiff at the base (responding to high frequencies) and flexible at the apex (responding to low frequencies).

The Auditory Cortex

• The primary auditory cortex (A1) is situated in the superior temporal cortex and mainly receives input from the opposite ear.
• It is organized similarly to the visual cortex, featuring "what" (sound identity) and "where" (sound location) pathways.
• A1 is essential for auditory imagery, while the superior temporal cortex is responsible for sound motion detection.
• Tonotopic map: Cells are tuned to specific tones; some prefer complex sounds over pure tones.
• A1 is not required for hearing itself but for interpreting sounds.
• A1 development is experience-dependent and develops less in individuals deaf from birth.
• Damage to A1 can impair sound processing, but complete deafness only occurs if subcortical regions are also damaged.

Path of Auditory Impulses

• Auditory impulses travel through several structures:
  • Cochlear nucleus
  • Superior olive
  • Inferior colliculus
  • Medial geniculate
  • Primary and secondary auditory cortex

Sound Localization

• Sound localization relies on comparing the responses of the two ears.
• Three primary cues:
  • Time of arrival
  • Sound shadow
  • Phase difference
• Humans localize low-frequency sounds using phase differences and high-frequency sounds using loudness differences.

Phase Differences

• Sound waves can be in phase or out of phase, affecting sound localization.
• Phase differences serve as a cue for sound localization.

Deafness

Conductive/Middle Ear Deafness

• Occurs when the middle ear bones fail to properly transmit sound waves to the cochlea.
• Causes include disease, infections, or tumorous bone growth.
• Individuals can hear their own voice clearly due to a normal cochlea and auditory nerve.
• Correctable via surgery or hearing aids that amplify the stimulus.

Nerve or Inner-Ear Deafness

• Results from damage to the cochlea, hair cells, or the auditory nerve.
• Varies in degree and may be confined to specific parts of the cochlea.
• Individuals may only hear certain frequencies.
• Causes include inheritance, disease, or noise-induced damage.

Tinnitus

• Frequent or constant ringing in the ears.
• Common among individuals with nerve deafness.
• Sometimes occurs after cochlea damage.
• Axons from other body areas may innervate brain regions previously responsive to sound, similar to phantom limb sensations.

Knowledge Check 6-1

• Question: What evidence suggests that absolute pitch depends on special experiences?
• Answer: Absolute pitch is predominantly found in individuals with early musical training and is more common among speakers of tonal languages, which require greater attention to pitch.

Introduction to Mechanical Senses

• Mechanical senses respond to pressure, bending, or other receptor distortions.
• Include touch, pain, body sensations, and vestibular sensation, which detects head position and movement.
• Audition is a mechanical sense as hair cells are modified touch receptors.

Vestibular Sensation

• The vestibular organ resides in the ear, adjacent to the cochlea.
• It comprises two otolith organs (saccule and utricle) and three semicircular canals.
• Otoliths are calcium carbonate particles that stimulate hair cells when the head tilts.
• Semicircular canals are filled with a jelly-like substance and contain hair cells activated by head movements.

Somatosensation

• Encompasses the sensation of the body and its movements.
• Includes touch, deep pressure, joint position and movement, pain, and temperature.

Somatosensory Receptors

• Pacinian corpuscle: Detects vibrations or sudden skin displacements; its onion-like structure resists gradual pressure.

  • Sudden or vibrating stimuli bend the membrane, increasing sodium ion flow and triggering action potentials.

• Receptors for light touch

  • Men and women have a similar number of Merkel disks, but women tend to have smaller, more sensitive fingers.

Somatosensory Receptors and Probable Functions

• Meissner's corpuscle
• Merkel disks
• Ruffini ending
• Pacinian corpuscle
• Pain receptor

Somatosensory Receptors and Probable Functions (Table 6.1)

Receptors for Temperature

• Temperature regulation is crucial; overheating or overcooling can be fatal.
• Cold-sensitive neurons respond to temperature drops adapt quickly, and show little response to constant cold.
• Heat-sensitive neurons respond to absolute temperature.
• Chemicals like capsaicin and menthol can stimulate heat and cold receptors.

Tickle

• The sensation of tickle is poorly understood.
• We cannot tickle ourselves because the brain compares resulting stimulation to “expected” stimulation, generating a weaker somatosensory response.

Somatosensation in the Central Nervous System

• Touch receptor information from the head enters the CNS through cranial nerves.
• Information from receptors below the head enters the spinal cord and travels via 31 spinal nerves to the brain.
• Somatosensory information (touch, pressure, pain) travels through the spinal cord in separate pathways toward the thalamus, which then sends impulses to the primary somatosensory cortex (S1).

Somatosensation in the Spinal Cord

• Each spinal nerve has sensory and motor components and connects to a limited body area.
• A dermatome is a body area innervated by a single sensory spinal nerve.
• Sensory information entering the spinal cord travels in distinct pathways.
• Example: The touch pathway is distinct from the pain pathway.

The Somatosensory Cortex

• Body sensations remain separate up to the cortex.
• Areas of the somatosensory thalamus send impulses to different areas of the somatosensory cortex in the parietal lobe.
• Different sub-areas of the somatosensory cortex respond to different body areas.
• Damage to the somatosensory cortex can impair body perceptions (numbsense).

Pain

• The experience evoked by a harmful stimulus, directing attention to a danger.

• Pain sensation begins with the least specialized receptors (bare nerve endings).

• Males and females react differently to pain.

  • Activating certain midbrain neurons decreases pain sensitivity in male mice but not in females.
  • The pain-relieving effects of opiates and cannabinoids differ between sexes.

• Pain carrying axons have little or no myelin; impulses travel slowly.

• The brain processes pain information rapidly, and motor responses are fast.

• Mild pain triggers glutamate release in the spinal cord.

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

Spinal Pathways for Touch and Pain

• From the medulla to the cerebral cortex, both touch and pain are represented contralaterally.
• In the spinal cord, touch information travels ipsilaterally, while pain information travels contralaterally.
• Discriminative touch involves recognizing shape, size, and texture.
• Other sensations include pain, temperature, and tickle.

Emotional Pain

• Emotional associations of pain activate a path through the reticular formation of the medulla.
• This path extends to the central nuclei of the thalamus, amygdala, hippocampus, prefrontal cortex, and cingulate cortex.
• Hurt feelings activate similar brain pathways as physical pain.

Ways of Relieving Pain

• The 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 receptors.
• Different endorphins respond to different types of pain.
• Morphine blocks dull, slow pain but not sharp pain from large-diameter axons.

Synapses for Pain and Its Inhibition

• Endorphins and Opiate receptors play a role in inhibiting Substance P.
• Substance P is related to pain.

Gate Theory

• Proposes that the spinal cord areas receiving messages from pain receptors also receive input from touch receptors and descending axons from the brain.
• These other areas can close the “gates” by releasing endorphins and decreasing pain perception.
• Non-painful stimuli can modify the intensity of pain.

Sensitization of Pain

• Mechanisms of the body increase sensitivity to pain.
• Damaged or inflamed tissue releases histamine, nerve growth factor, and other chemicals that increase pain receptor responses.
• Certain receptors become potentiated after intense painful stimuli.
• Leads to increased sensitivity or chronic pain later.

Itch

• Histamine release by the skin produces itching sensations.
• Activates a distinct pathway in the spinal cord to the brain.
• Impulses travel slowly (half a meter per second).
• Pain and itch have an inhibitory relationship.
• Opiates, decreasing pain, increase itch.

Chemical Senses

• The first sensory system of the earliest animals was chemical sensitivity.
• A chemical sense enables small animals to find food, avoid danger, and locate mates.

Taste

• Taste functions to determine whether to swallow or spit something out.
• Taste results from stimulation of taste buds, the receptors on the tongue.
• Mammalian taste receptors are in taste buds located in papillae on the tongue’s surface.
• Flavor perception is a combination of taste and smell.
• Taste and smell axons converge onto many of the same cortical cells.

Taste Receptors

• Taste receptors 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 papilla contains 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.

How Many Kinds of Taste Receptors?

• 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.

Taste Coding

• 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.

Major Routes of Impulses Related to Taste

• Taste buds on tongue -> Nucleus of tractus solitarius -> Ventral-posteromedial thalamus -> Insula (primary taste cortex)
• Also connected to Somatosensory cortex, Orbital-prefrontal cortex, Hypothalamus-Amygdala

Variations in Taste Sensitivity

• Taste sensitivity varies among different populations.
• Example: Percentage of supertasters varies across different ethnic groups.

Olfaction

• The sense of smell.
• The detection and recognition of chemicals that contact 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.

Olfaction and Behavior

• The smell of a sweaty female increases a male’s testosterone secretions, especially if the female is near 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 but increases cortisol release (a stress hormone).
• Even humans can follow a scent trail to some extent and improve 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, extending from the cell body into the mucous surface of the nasal passage.
• Vertebrates have hundreds of olfactory receptors, highly responsive to some related chemicals but unresponsive to others.

Olfactory Receptors - Mechanism

• 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 receptors for vision and hearing, 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 one chemical or several closely related chemicals.
• All 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 to the piriform cortex.
• How the cortex recognizes an odor remains a puzzle.

Individual Differences in Olfaction

• Most genes controlling olfactory receptors have variant forms.
• Two randomly chosen people probably differ in about 30% of their olfactory receptor genes.
• Odor sensitivity declines with age.
• Many people infected with 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 is seeing a number or letter as a specific color.
• Tends to cluster in families that also have perfect pitch.
• Genetic predisposition.