Chapter 7 - Nonvisual Sensation and Perception

Question 1

AUDITION - SOUND as a STIMULUS

Answer 1

Sound as a physical stimulus begins with the movement of an object, which sets off waves of collisions between adjacent molecules that produce bands of high and low pressure. Sound stimuli have three characteristics.

1) AMPLITUDE, the amount of vibration produced by the sound, which in turn is perceived as loudness.
2) FREQUENCY, the number of wave cycles per unit of time, which is perceived as pitch.
3) TIMBRE, the specific combination of a fundamental frequency with its harmonics, which is perceived as “sound color”.

Question 2

the AUDITORY SYSTEM

Answer 2

the AUDITORY SYSTEM can be divided in:

• the OUTER EAR;
• the MIDDLE EAR;
• the INNER EAR.

Question 3

the OUTER EAR

Answer 3

The OUTER EAR consists of the structures visible outside the body:

1) the PINNA, which serves to (A) collect and focus sounds and (B) localize their source.
2) the AUDITORY CANAL, which connects to the middle ear.

Question 4

the MIDDLE EAR

Answer 4

The MIDDLE EAR consists of:

1) the TYMPANIC MEMBRANE - or eardrum - which forms the boundary between the outer ear and middle ear.
2) the OVAL WINDOW, which forms the boundary between the middle ear and inner ear.
3) 3 ossicles - MALLEUS, INCUS, STAPLES - which connect the two aforementioned membranes and transfer sound energy from the outside air to the fluid in the inner ear. Auditory stimulus are actually amplified by these ossicles, for their connections are hinged and create a lever action that increases the force of the vibration. Furthermore, force applied to the much smaller oval window produces much more pressure than the same force applied to the larger tympanic membrane.

Question 5

the INNER EAR

Answer 5

The INNER EAR consists of:

1) the OTOLITH ORGANS and the SEMICIRCULAR CANALS, which are part of the vestibular system and not of the auditory system;
2) the COCHLEA, a spiral-shaped, fluid-filled cavity.

The COCHLEA is divided in three canals:

1) the VESTIBULAR CANAL, closed on one end by the OVAL WINDOW;
2) the TYMPANIC CANAL, closed on one end by the ROUND WINDOW;

The two canals are connected at the APEX, or the inner point of the cochlea - because of this connection, pressure applied to the oval window by the stapes travels around the apex and pushes the round window out into the middle ear.

3) the COCHLEAR DUCT, found amid the other two canals, is filled with a fluid known as ENDOLYMPH, rich in potassium and low in sodium. It contains the ORGAN OF CORTI, responsible for translating vibrations in the inner ear into neural messages, which rests on the BASILAR MEMBRANE, which separates the cochlear duct from the tympanic canal. The basilar membrane responds to vibration of the oval window with a wave­ like motion - its base responds to high-frequency stimuli, whereas low-frequency stimuli produce vibration toward the apex.
Movement of the basilar membrane - which displaces endolymph - is sensed by HAIR CELLS attached to the organ of Corti, which are connected to fibers of the AUDITORY NERVE (cranial nerve 8).

Question 6

DEPOLARIZATION of HAIR CELLS

Answer 6

Movement of the cilia back and forth within the ENDOLYMPH hyperpolarizes and depolarizes hair cells - these changes result from the opening and closing of POTASSIUM CHANNELS located in the tips of the cilia. Unlike most extracellular fluid, endolymph is rich in potassium, thus opening of potassium channels leads to flow of potassium ions into the cell for both diffusion and electrostatic pressure.

• in the resting hair cell, approximately 15% of potassium channels are open;
• when the moving endolymph displaces the cilia of the hair cell toward their TALLEST member, many more potassium channels open, leading to the opening of CALCIUM CHANNEL and consequent firing of the cell.
• when the moving endo­lymph displaces the cilia of the hair cell toward their SHORTEST member, potassium channels close, and the cell hyperpolarizes.

Question 7

AUDITORY PATHWAYS

Answer 7

There are two different auditory pathways:

A) the IPSILATERAL PATHWAY:
1 - HAIR CELLS in the cochlea;
2 - AUDITORY NERVE;
3 - DORSAL COCHLEAR NUCLEUS of the MEDULLA;
4 - INFERIOR COLLICULUS in the midbrain;
5 - MEDIAL GENICULATE NUCLEUS (MGN) of the THALAMUS;
6 - PRIMARY AUDITORY CORTEX.

B) the IPSILATERAL -CONTRALATERAL PATHWAY:
1 - hair cells in the cochlea;
2 - auditory nerve;
3 - VENTRAL COCHLEAR NUCLEUS in the medulla - from here the pathway divides in contralateral and ipsilateral pathways;
4 - CONTRALATERAL and IPSILATERAL SUPERIOR OLIVE of the pons;
5 - inferior colliculus in the midbrain;
6 - medial geniculate nucleus (MGN) of the thalamus;
7 - primary auditory cortex.

The PRIMARY AUDITORY CORTEX is organised in columns that respond to single frequencies according through TONOTOPIC ORGANIZATION - neurons responding to one frequency are located next to neurons responding to similar frequencies.

Question 8

LOCALIZATION of SOUND

Answer 8

We localise sounds through different strategies.

1) to localise sound sources in the HORIZONTAL PLANE, we mainly compare the arrival times of a sound at each ear - this is why sounds immediately in front or behind of an individual are difficult to localise, for there’s no difference in arrival times. This comparison is carried out by neurons in the SUPERIOR OLIVE. These neurons are known as BINAURAL NEURONS because they receive input from both ears - they respond most vigorously when input from both ears reaches them simultaneously. Another means by which we localise sounds by comparing the intensities of sound reaching each year, for the head casts a “shadow” on the ear farthest from the sound source by blocking some waves - this is not possible with low-pitch stimuli, which freely move around the head and are not blocked by it.
2) to localise sound sources in the VERTICAL PLANE, we make use of the PINNA (when different-shaped false pinnas were attached to human participants, sound localization was impaired).

Question 9

HEARING DISORDERS

Answer 9

Hearing disorders result from either:

1) CONDUCTION LOSS, or hearing loss resulting from problems in the outer or middle ear;
2) SENSORINEURAL LOSS, or hearing loss due to damage to the inner ear, the auditory pathways, or the auditory cortex.

Question 10

the VESTIBULAR SYSTEM

Answer 10

The VESTIBULAR SYSTEM provides information about the posi­tion and movements of our heads, which contribute to our sense of balance.
Its sensory organs are found in the inner ear and can be divided in:

1) the OTOLITH ORGANS, which provide information about the angle of the head relative to the ground, as well as information about LINEAR ACCELERATION. They consist of two separate structures, the SACCULE and the UTRICLE, which both contain HAIR CELLS with cilia that extend into a gelatinous layer. Covering the gelatinous layer are OTOLITHS, which displace the gelatinous layer when they move due to the acceleration of the head - the hair cells either depolarize or hyperpolarize in response to this movement, which in turn affects the firing rates of fibers in the AUDITORY NERVE. The hair cells in the saccule are arranged along a VERTICAL membrane, whereas the hair cells in the utricle are arranged along a HORIZONTAL membrane.
2) the SEMICIRCULAR CANALS, 3 looping chambers which provide information about rotational movements of the head. Rotating the head causes the endolymph within the canals to bend hair cells.

Question 11

TOUCH RECEPTORS

Answer 11

Receptor cells for touch are MECHANORECEPTORS, cells that respond to physical displacement such as bending or stretching. Mechanoreceptors are categorized according to:

• whether they are ENCAPSULATED or not, that is if their axon terminal is surrounded by a capsule;
• the size of their RECEPTIVE FIELDS, namely the area of skin or other tissue that provides information to a particular receptor;
• their rate of ADAPTATION, or the length of time it will continue to respond to unchanging stimuli.
• the TYPE OF INFORMATION they process.

The 4 most common types of mechanoreceptors are:

1) PACINIAN CORPUSCLES;
2) MEISSNER’S CORPUSCLES;
3) MERKEL’S DISKS;
4) RUFFINI’S ENDINGS.

Variations in sensitivity from one part of the body to the next result from:
1) DENSITY of mechanoreceptors;
2) RECEPTIVE FIELD AREAS of mechanoreceptors.
Fingers and lips have a greater sensitivity than the rest of the body.

Question 12

TOUCH PATHWAYS

Answer 12

1) MECHANORECEPTORS;
2) IPSILATERAL DORSAL ROOT OF THE SPINAL CORD;
3) first synapse in the DORSAL NUCLEUS of the MEDULLA - from now on, information is processed contralaterally.
4) CONTRALATERAL VENTRAL POSTERIOR NUCLEUS of the THALAMUS;
5) PRIMARY SOMATOSENSORY CORTEX.

Areas of the body receive cortical representation in the primary somatosensory cortex according to their need for precise sensory feedback.

Question 13

PAIN RECEPTORS

Answer 13

Receptor cells that sense pain are called NOCICEPTORS. Nociceptors respond to a variety of stimuli associated with tissue damage - mechanic stimulation, harmful temperatures and chemicals. There are two main pain pathways:

• ASCENDING PATHWAYS, from sensation to perception of pain;
• DESCENDING PATHWAYS, which modulate pain perception.

Question 14

ASCENDING PAIN PATHWAYS

Answer 14

Pain sensed below the neck:
1) NOCICEPTORS
2) IPSILATERAL DORSAL ROOT OF THE SPINAL CORD;
3) CONTRALATERAL VENTRAL COLUMN OF THE SPINAL CORD;
4) first synapse in the INTRALAMINAR NUCLEI of the THALAMUS;
5A) ANTERIOR CINGULATE CORTEX, which provides the emotional experience of pain;
5B) SOMATOSENSORY CORTEX, which provides the raw sensation of pain.

Pain from head and neck:
1) NOCICEPTORS;
2) TRIGEMINAL NERVE (cranial nerve 5);
3) first synapse in the BRAINSTEM;
4) INTRALAMINAR NUCLEI of the THALAMUS;
5A) ANTERIOR CINGULATE CORTEX, which provides the emotional experience of pain;
5B) SOMATOSENSORY CORTEX, which provides the raw sensation of pain.

Question 15

DESCENDING PAIN PATHWAYS

Answer 15

Descending control from the brain has a dramatic influence on our perception of pain. Many brain structures of the forebrain - the cortex, thalamus, hypothalamus and amygdala - send descending input that modulate pain perception.

1) FOREBRAIN;
2) PERIAQUEDUCTAL GRAY (PAG);
3) RAPHE NUCLEI in the MEDULLA;
4) DORSAL HORN of the SPINAL CORD.

Question 16

OLFACTION

Answer 16

OLFACTION begins with the detection of molecules suspended in the air. OLFACTORY RECEPTORS are contained in the OLFACTORY EPITHELIUM of the nasal cavity alongside SUPPORTING CELLS, glia cells that produce mucus that covers the epithelium. Olfactory receptors are bipolar neurons with cilia that extend into the mucus and synapse with the OLFACTORY NERVE (cranial nerve 1) which is contained in the OLFACTORY BULB. Olfactory receptors, unlike most of other neurons, regularly die and are replaced in a cycle lasting four to six weeks.

Unlike other major sense systems, olfactory information travels to the cere­bral cortex without synapsing in the thalamus first.

Question 17

THEORIES of OLFACTORY ENCODING

Answer 17

Humans only have 350 to 400 types of olfactory receptors - still they must catalog the many thousands of different smells that we are able to discriminate. There are 3 main theories that try to explain how such a small number of receptors encode information about such a wide array of ODORANTS:

1) SHAPE-PATTERN THEORY:
odorant molecules interact with receptors like keys fitting into locks;
2) COMBINED SHAPE-PATTERN THEORY:
a modification of the original theory, it maintains that odorants activate specific combinations of receptors. This theory accounts for the difference between the number of odors and the number of receptors.
3) VIBRATION THEORY:
the olfactory system responds to the vibration of odorant molecules rather than to their shapes. Axons from the olfactory bulbs synapse in the OLFACTORY CORTEX, which is located at the base of the frontal lobe extending onto the medial surface of the temporal lobe.

Question 18

GUSTATION

Answer 18

Gustation begins with the dissolving of molecules in the saliva of the mouth. Each bump on the surface of the tongue, known as a PAPILLA, contains several TASTE BUDS, structures that contain several TASTE RECEPTORS. Each taste receptor has a set of fibers, known as MICROVILLI, that extend into the saliva and interact with substances to be sensed.
The 5 major categories of taste - sweet, salty, sour, bitter and umami - can be mapped on the tongue based on the location of the majority of the receptors that sense them.

Question 19

GUSTATION PATHWAYS

Answer 19

1) TASTE RECEPTORS synapse with fibers of
2) CRANIAL NERVES 7, 9 and 10;
3) GUSTATORY NUCLEUS of the MEDULLA;
4) VENTRAL POSTERIOR MEDIAL nucleus of the THALAMUS;
5) GUSTATORY CORTEX, which is located at the junction of the frontal and parietal lobes.