Pain, Taste, Smell, and Audition

Goals

• To discuss the pain subsystem of the somatosensory system
• To discuss the chemical senses: taste and olfaction
• To discuss the perception of disgust
• To review the organization of the auditory system.

Topic Slide

Richard Axel and Linda Buck won the Nobel Prize in 2004 for their work on the olfactory system and odorant receptors. You can read about their discoveries here. One of the more interesting conclusions from their work is that thousands of our genes (~3% of all of our genes!) code for different odorant receptors.

Reading

• PN6 Chapters 10 (Pain)
• PN6 Chapter 13 (Audition)
• PN6 Chapter 15, pp. 345-353 (Taste)

Pain

Injury to tissue release chemicals that can depolarize the nociceptors (free nerve endings) that transduce pain into neural signals. These receptors are separate from those that transduce other somatic senses (such as light touch, pressure, heat/cold, etc). Pain sensation is not the result of overstimulating mechanoreceptors or thermal receptors. The cell bodies for nociceptors are located in the dorsal root ganglia (like those for mechanoreceptors and proprioceptors). The nociceptive axons cross the spinal cord upon entry through the dorsal roots and ascend in the contralateral anterolateral tract. The nociceptive axons are small diameter and either unmyelinated (C fibers) or incompletely myelinated (A delta fibers). That means they transmit very slowly relatively to mechanoreceptors and proprioceptors.

As a side note, the sensory neurons in the dorsal root ganglia (DRG) are called pseudounipolar. The neuron has one axon, but with two branches: central and peripheral. The peripheral branch goes from the cell body to the location where the transduction of the somatic sense occurs. The trigger zone that generates and action potential is at the end of the peripheral segment. The central branch of the axon exits the cell body in the DRG and enters the spinal cord.

Pain signals have widespread distribution in the brain. Some projections from the anterolateral spinal cord tract ascend to the ventral posterior lateral (VPL) thalamus and are projected to primary sensory cortex. These projections are thought to represent the discriminative aspects of pain processing – i.e., where is the location of the painful stimulus?

However, there are other projections from the anterolateral system that appear to be related to the emotional/affective/motivational aspects of pain. These include the hippocampus and amygdala, which we will learn later in the semester and involved in stress and the activation of 'fight or flight' sympathetic nervous activity. The amygdala, we will learn, is involved in emotional responses (among others) with an emphasis upon fear. Another region strongly activated by painful stimulus is the insula. As we will see below, the insula is also activated by the chemical senses and is activated when one perceives disgust.

Perhaps one of the more interesting recent finding derived from fMRI studies of pain, is that observing pain in others and experiencing pain in ourselves activate many of the same brain regions. I presented an example of this finding in class, and will return later in the semester to the topic in my lectures on empathy and social neuroscience.

Taste

Taste is conveyed through the following cranial nerves:

• Cranial nerve VII (Facial Nerve) carries information from the anterior 2/3 of the tongue.
• Cranial nerve IX (Glossopharyngeal Nerve) carries information from the posterior 1/3 of the tongue.
• Cranial Nerve X (Vagus) has a minor role in taste perception.

The distribution of taste buds conveying the five basic tastes (sweet, sour, salty, bitter, umami) is not uniform across the tongue.

Taste input is transmitted to the nucleus of the solitary tract to the thalamus (VPM – ventral posterior medial nucleus). From the VPM thalamus, it is projected to specific regions of the insula. The insula appears to be ‘primary sensory cortex’ for taste.

The cortical 'feature space' of taste is not well understood. There is fMRI evidence that the primary tastes activate distinct (though overlapping) regions of insula cortex.

Lesions of the insula can lead to altered perceptions of taste. I discussed a patient with an insula stroke who reported that ‘food tasted like dirt’. Such strokes can lead to dangerous weight loss if not detected.

Olfaction

Olfaction is (perhaps) the oldest sense. Compared to other species, humans have a smaller extent of cortex devoted to olfaction that many other species in both relative and absolute terms. There are also many fewer olfactory receptive neurons (ORNs) in humans than many other species. Nevetheless, humans are quite good at detecting minute concentrations of odorants. As illustrated in PN, humans can also track a trail using odor cues.

Olfaction is the one sense that does not project to a thalamic relay nucleus. Rather, olfactory input is transduced in the nasal epithelium and carried to cell bodies in the olfactory bulb.

The transducer neuron is called the olfactory receptor neuron. Follow this link for a good description. The ORN synapses with neurons in the olfactory bulb which give rise to the olfactory nerve (cranial nerve I).

There are several target of the olfactory nerve including the pyriform cortex (archicortex), entorhinal cortex, and amygdala. These, in turn, project to the orbitofrontal cortex, thalamus, and hypothalamus. The entorhinal cortex is the main input to the hippocampus.

Like with taste, the cortical 'feature space' of olfaction is not well understood. There is fMRI evidence that the pattern of activation in piriform cortex is distinct and discriminable among different odorants.

Recent fMRI data have shown that the hedonic aspects of smells (whether pleasant or unpleasant strongly influence the activity in the amygdala and insula.

Insula and disgust

The insula is a cortical region hidden from direct view by the overhanging frontal and temporal cortex. It is composed of several oblique gyri – looks to me like an array of fingers pointing upward.

A major general function of the insula may be Interoception – awareness of bodily states (and emotion)

Anterior Insular cortex:

• Strong reciprocal connections between the anterior insular cortex and the amygdala
• Orbitofrontal cortex, temporal, occipital lobe
• Gustation, olfaction
• emotion, sense of self, orgasm

Posterior Insular cortex:

• Projections from VPI thalamus and S2
• Pain, temperature, touch, bowel distension, vomiting

The Auditory System

The auditory system localizes sounds in space and identifies auditory objects.

What are auditory objects?

• Environmental sounds
• Music – melody and rhythm (dissociable components)
• Language

Transduction of sound waves

Sounds are longitudinal pressure waves that propagate through the medium of air.

Sound pressure waves are transduced by the ear and cochlea.

• Pressure waves enter outer ear (pinna).
• The pinna is particularly sensitive to sound waves in the speech frequency range. Creates an 'auditory fovea'.
• The pressure waves encounter the tympanic membrane in the middle and transfers vibration to the three small bones (ossicles) of the middle ear.
• The small bones of the middle ear (malleus, incus, stapes, or in English, hammer, anvil, and stirrup) perform an impedance matching process between pressure waves in the air, and in the viscous medium of the cochlea.
• The stapes (stirrup) directly contacts the oval window of the cochlea and creates traveling waves in the viscous medium of the cochlea
• The traveling waves (think of snapping a rope) course along the basilar membrane and vibrate this membrane in a (frequency dependent) tonotopic manner. High frequency sounds maximally vibrate the base end (near the round window) while low frequency sounds maximally vibrate the far end (the apex).
• The hair cells mechanically transduce the vibrations into neural signals. This process bears similarity to mechanoreceptors in that the physical shearing of the hair cells open channels and depolarize the receptors.

A particularly good video on the transduction of sound waves in the inner ear and cochlea can be found embedded below.

Cochlear implant

A cochlear implant is now a fairly routine procedure in which an array of electrodes are implanted within the cochlea and which directly stimulate the basilar membrane. About 500,000 people have received cochlear implants. I illustrated the cochlear implant with YouTube videos

Auditory signal path

The auditory pathway is similar to the visual and somatosensory systems we have already studied, in that the signals from the receptors project to thalamus (the medial geniculate nucleus of the thalamus) and then to primary auditory cortex (A1) that maintains a tonotopic map.

However, there are numerous intermediary nuclei and alternative projections in the auditory system. A good representation of the auditory pathways can be found here.

All fibers from the cochlea synapse in either the dorsal or ventral cochlear nuclei.

The most straightforward pathway is from the dorsal cochlear nucleus:

• hair cell receptors through 8th nerve to dorsal cochlear nucleus (DCN)
• from DCN to contralateral inferior colliculus (part of the tectum)
• from inferior colliculus to medial geniculate nucleus (thalamus)

The dorsal cochlear nucleus projections may be involved in a fast alerting system. It is interesting to note that the DCN receives efferent projections from other auditory nuclei as well as from somatosensory nuclei.

The pathways from ventral cochlear nucleus are a bit more complicated. Again, I refer you to here, where good diagram can be found.

• Axons from the ventral cochlear nucleus (VCN) synapse in the ipsilateral and contralateral superior olivary nucleus.
  • Some VCN axons cross the midline in a bundle called the trapezoid body. This provides the superior olive with inputs from both ears and thus binaural processing can take place.
• The output from the superior olive travels in a bundle called the lateral lemniscus.
  • There are nuclei within the lateral lemniscus that further process the sound.
• Most axons from the superior olive synapse in the inferior colliculus.
• All afferents then synapse in the medial geniculate body of the thalamus.

Localizing sounds in space

Considerable processing of auditory information occurs in the superior olive. The olive is not a unitary structure, but a collection of nuclei that include the lateral superior olive and the medial superior olive. The fiber (axon) bundle that crosses the midline and carries information from both ears is called the trapezoid body. There are nuclei within the trapezoid body that also participate in auditory processing. Chief among them is the medial nucleus of the trapezoid body (MNTB). One important function carried out in these nuclei is the localization of sound in space.

The relative timing and the relative intensity of pressure waves entering the left and right ear are used to localize sounds in space. I discussed two neural mechanisms:

• Delay line neuronal mechanism (timing).
• Intensity differences.

Decussations.

Each ear projects to each hemisphere, but the contralateral projection is dominant. We will return to this fact when we discuss hemispheric differences in auditory processing.

Auditory Cortex

Auditory cortex is located in the temporal lobe. Primary auditory cortex (A1) is located on Heschl's gyrus and is organized tonotopically; i.e., а just like the basilar membrane of the cochlea. A1 is surrounded by auditory 'belt cortex' i.e., higher level auditory cortex.

Hemisphere differences in processing acoustic stimuli

There are hemispheric differences in auditory processing at the level of primary and belt auditory cortex. In general, the left hemisphere is differentially sensitive to speech sounds, while the right hemisphere is more sensitive to music and environmental sounds.

I presented a case study of patient with acquired amusia (inability to perceive music as music а e.g., singers seem like they are talking).

Videos

Prerecorded videos for 2020

Live lectures from prior semesters

The video below was recorded in fall, 2018, and covered by these notes. However, in that semester, it was presented as Lecture 10 and it preceded the lectures on the motor system.

The video below was recorded in fall, 2019.