Olfaction and gustation overlap to generate somatosensory perception of many fundamental sensory signals involving food, territoriality, sexual signals, and fear. Inspired air contains olfactory signals but that air usually crosses the gustatory sensory system during exhalation. Unlike other sensory systems, olfaction and retronasal olfactory/flavor systems activate the hippocampus, amygdala and limbic system directly in the rapid production of strong memories, both conscious and unconscious. The two systems are closely linked with emotion circuits — cravings and fears — in the amygdala, along with hippocampus-based memories. ANOSMIA — inability to smell — is, therefore, a serious condition!

Look at general ear anatomy on your own — Fig 38.29, p783. The key receptor cell for hearing is a MECHANORECEPTOR that deforms according to pressure waves converted by the ear from air pressure to fluid pressure in COCHLEA in the INNER EAR. Fluid waves in vestibular and tympanic canals lead to vibrations of the BASILAR MEMBRANE.

Vestibular canal

Tympanic canal

Middle canal

Organ of Corti

The sound conduction pathway in the mammalian ear. Changes in air pressure lead to vibrations in the tympanum, which are relayed to the malleus, incus and stapes. Their oscillations focus vibrations onto the perilymph-filled cochlea at the oval window. Pressure changes are dampened at the round window. Perilymph vibrations lead to flexions of the basilar membrane, which responds to different vibrational frequencies (pitch) because of different thicknesses. High frequencies generate vibrations at proximal end, while lower frequencies affect the distal end.

The human ear, at best, can hear up to about a 20-fold difference in Hertz to about 18,000 Hertz. Dogs can hear up to around 60,000 Hertz, and cats up to nearly 80,000 Hertz. Bats can hear up to 200,000 Hertz and use sound for echolocation.

Fluid in the vestibular and tympanic membrane is connected at the distal end of the coiled cochlea. The middle canal (called the cochlear duct) has a denser, even less compressible fluid. Endolymph vibrations act like hydraulics and cause the BASILAR MEMBRANE to vibrate, pushing mechanoreceptor HAIR CELLS up and down against the rigid TECTORIAL MEMBRANE. As that happens, cilia on the hair cells deform (bend), generating receptor potentials that leads to the release of neurotransmitter and the generation of action potentials in the AUDITORY NERVE cells. The psycho-acoustic sensation of sound occurs!! Evolved from lateral line vibration receptors in fish!

OVERVIEW. The INNER HAIR CELLS of the cochlea generate signals to the auditory neurons. Bending toward the large kinocilium depolarizes the hair cell while bending away hyperpolarizes the cell. There is no measurable threshold for hair cells. The OUTER HAIR CELLS generate an electromotile response as the cell changes length with every sound wave. They push against tectorial membrane and amplify basilar membrane vibrations, allowing one to hear very quiet sounds.

Vestibular hair cells work in a similar fashion. The kinocilium abuts the tectorial membrane, so all vibrations result in stereocilia movement. A cadherin connects one stereocilium to the channel of the next stereocilium as well as linking to the actin (and myosin 1C) cytoskeleton to result in rapid and coordinated stereocilia movements

VIII cranial nerve

Mechanoreceptors transduce physical vibrations to electrical action potential signals. Stereocilia bend against the tectorial membrane as the basilar membrane vibrates. Each inner ear stereocilium has a TIP LINK protein that opens a K+ channel on the next taller stereocilium. K+ from the endolymph enters and depolarizes membrane, allowing Ca2+ in, which leads to neurotransmitter release.

See Fig 38.21 p 784

K+

PHOTORECEPTION

Photosensitivity involves complex integration from sensory cells that respond to electromagnetic radiation, most often in the range we call “visible light” — for obvious reasons!

The sensory organ itself, the eye, is involved with considerable neuronal integration, as is the CNS, to generate perceptions of color, depth, form, line and motion. Ambiguities in any of those modality characteristics typically lead to misperceptions or no memorable perception at all.

The photoreceptive field, the retina, is a direct outgrowth of the developing CNS and brain. Many eye characteristics make sense only in developmental and evolutionary terms, for example the light-sensitive pigments in vertebrate photoreceptor cells are at the far end of a layer of cells through which light must pass to interact with the receptor cell.

See Fig 38.25, page 786-787

Humans have two types of photoreceptors, rods & cones. Rods react to as little as one photon of light energy! Cones require much more intense light and are effectively non-functional in low light in humans. In the dark, rods and cones are depolarized and TONICALLY release neurotransmitter, inhibiting the BIPOLAR cells. In the light, rods and cones hyperpolarize and neurotransmitter release DECREASES, resulting in a DISINHIBITION of bipolar cells. Once disinhibited, bipolar cells release neurotransmitter and launch an action potential on GANGLION CELLS, the cells of the OPTIC NERVE.

Additional cells — amacrine and horizontal cells — modulate the action of groups of cells within a receptive field. The result is photoreception with finer detail, for example enhancing light-dark contrasts..

Bipolar cell

In the dark, Na+ and K+ channels are open, resulting in a “dark current” — the flow of ions — and a partly depolarized cell (about -35 mV). Na+ flows in and K+ flows out while a Na+-K+ pump moves ions in the opposite direction. Remember that a resting potential in a neuron would be as low as -65 mV, while a neuron’s depolarized potential might reach +30 mV. So a “dark current” potential of -35 mV represents a “partial” depolarization.

When light energy hits a photoreceptor cell, the chemically gated Na+ channels close, mimicking the conditions of a neuron’s resting potential with K+ leak channels still open. The result is a hyperpolarized cell.

In the dark, chemically gated Na+ channels are open as cGMP binds to them. In response to light Na+ channels close. Light is absorbed by the photo-sensitive 11-cis-retinal bound to different opsin proteins, causing an extremely rapid photoisomerization to all-trans-retinal. All-trans-retinal activates a cascade of the G-protein TRANSDUCIN, which then releases its GDP and binds a GTP. With GTP bound, transducin (the G-protein a-subunit) is activated and in turn activates a cGMP phosphodiesterase, which hydrolyzes cGMP. Without cGMP the Na+ chemically gated channel closes. Since K+ channels remain open, the cell hyperpolarizes.

Visual perception pathways. From cranial nerve II — the optic nerve — action potentials travel to the optic chiasm where signals from both left visual fields travel together to right lateral geniculate nucleus and from there to the right primary visual cortex. Signals from both right visual fields travel to the left LG nucleus and to the left visual cortex. This is a major example of DECUSSATION, crossing over by nerves.

During metazoan evolution, diffuse neuronal networks appear more and more organized in “bundles” — ganglia — and along sensorimotor pathways — nerves. The Central Nervous System pathways run to and from more-and-more centralized ganglia. The brain itself comprises many ganglia along with internal connecting pathways.

The PNS and CNS are myelinated somewhat differently: CNS myelination comes from oligodendrocytes while PNS myelination comes from Schwann cells.

The PNS is further subdivided into AUTONOMIC and MOTOR efferent neurons. Autonomic Nervous System (ANS) functions largely below the level of awareness. It innervates internal organs, sweat glands, arteriolar smooth muscles, etc. Motor neurons are the fastest neurons in the body, myelinated a neurons (largest diameter and therefore highest number of Na+ channels along axon.) Except for olfaction, which projects its neurons directly to the amygdala, other sensory input travels to the thalamus first.

Internal brain ganglia and pathways are the current focus of immense research efforts. One collection is known as the LIMBIC SYSTEM, which is often referred to as the ancient “reptilian” brain, but the evidence suggests that all vertebrates have a developed limbic system that significantly resembles the human limbic system structure and functions in a similar way.

Ventral Prefrontal Cortex (VPC) — evaluation of pleasure & beauty

VPC

Anterior Cingulate Cortex — evaluation of motivation/emotion

Dorsal Prefront Cortex — executive function

AMYGDALA — fear & ANS responses

DPC

Striatum — motor skills, reward, implicit memory

Dopaminergic cells— modulation of reward

HYPOTHALAMUS — ANS & visceral functions

Anterior Insular Cortex (INSULA) — conscious awareness of body states, urges, notion of “I am”

Very simplified view of interconnectivity of amygdala and areas involved with emotion, [adapted from Kandel, E. The Age of Insight, 2012, p373]

ACC Insula

NEURAL NETWORKS

Nature, 508, S2-S3, 03 April 2014