CLASS #5
Dr. Charles-Etienne Benoit
Today’s Lecture
The 5 senses Organs
Receptors and sensory coding
Integrative pathway to the nervous system
5 senses
A sense is a physiological capacity of organisms that provides data for perception.
Share processing
Each system begins with anatomical structures for collecting, filtering, and amplifying information from the environment.
They have specialized receptor cells that transduce the environmental stimulus into neural signals.
Specialized receptors
These receptors transduce a specific type of stimulus energy into electrical signals.
Sensory coding
The major sensory modalities in humans
are mediated by distinct classes of receptor neurons
located in specific sense organs.
Classification of sensory receptors
Intensity of the stimululation
The firing rates of sensory nerves encode the stimulus magnitude.
Anatomy of senses
These neural signals are passed along their specific sensory nerve pathways and travel up the spinal cord and enter the brain to terminate in different parts of the thalamus.
Thalamus
The thalamus is a large mass of gray matter in the dorsal part of the diencephalon of the brain with several functions such as relaying of sensory signals to the cerebral cortex, but also the regulation of consciousness, sleep, and alertness.
Anatomy of senses
Sensory inputs about taste, touch, smell, hearing, and
seeing travel to specific regions of the brain for initial
processing.
Primary sensory cortices
From the thalamus, neural connections these pathways travel first to primary sensory cortex.
Approximate location of the five primary sensory areas and motor cortex
Further processing
Approximate locations of the secondary and tertiary sensory and motor cortices
Audition
Hearing, or auditory perception, is the ability to perceive sounds by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ such as the ear.
The sounds we hear can also be classified as either pure tones or
complex sounds.
Pure tones have waveforms that are a very regular shape, called a sine wave.
Complex sounds are a mixture of several frequencies.
Sound waves
In air the disturbances travels with the 343 m/s, the speed of sound.
Entering the ear
Sound waves arriving at the ear enter the auditory canal where the sound waves are amplified
It travel to the far end of the canal, where they hit the tympanic membrane (eardrum), and make it vibrate.
These low-pressure vibrations then travel through the air-filled middle ear and rattle three tiny bones, the malleus , incus , and stapes , which cause a second membrane, the oval window, to vibrate.
Transduction along the cochlea.
The oval window is the “door” to the fluid-filled cochlea.
Within the cochlea are tiny hair cells located along the inner surface of the basilar membrane . The hair cells are the sensory receptors of the auditory system.
The location of a hair cell on the basilar membrane determines its frequency tuning , the sound frequency that it responds to.
Hair cells (Cilia)
Cilia tips are joined by a fiber link.
Their movement produces tension of the link which opens an ion channel in the adjacent tip.
Calcium and potassium ions flow into the cilia and produce a depolarization.
Auditory integration
The output from the auditory nerve projects to the cochlear nuclei in the brainstem.
Ascending fibers reach the auditory cortex following synapses in the inferior colliculus and medial geniculate nucleus.
Auditory integration
The tuning curves for auditory cells can be quite broad.
Cochlear implants
Cochlear implant is a surgically implanted device that bypass the normal acoustic hearing process.
The sound sensation comes from the sound that is converted to electric signals which directly stimulate the auditory nerve.
The brain adapts to the new mode of hearing, and eventually can interpret the electric signals as sound and speech.
Olfaction
Smell is the sensory experience that results from the transduction of neural signals triggered by odor molecules, or odorants detected by receptors in the olfactory epithelium.
Odorant processing
The axons of these neurons project to the olfactory bulb where they terminate on mitral and relay neurons within glomeruli.
The relay neuron axons project to the olfactory cortex.
Olfactory receptors
Humans have approximately 350 different odorant receptors.
Opening of cyclic nucleotide-gated cation channels, causing cation in flux and a change in membrane potential in the ciliary membrane
Odorants
Each odorant is recognized by a unique combination of receptors which explains how mammals can distinguish between their similar chemical structures.
Odorants flow rate
The response across the nostrils will be different because of variation in flow rates.
One nostril always has a greater input airflow than the other, and the nostrils switch between the two rates every few hours.
Olfactory epithelium cells
The olfactory epithelium contains sensory neurons interspersed with supporting cells as well as a basal layer of stem cells.
Cilia extend from the dendrite of each neuron into the mucus lining the nasal cavity. An axon extends from the basal end of each neuron to the olfactory bulb.
Olfaction central integration
The information is send to the glomeruli in the olfactory bulb, the axons of which form the olfactory nerve that relays information to the primary olfactory cortex.
The orbitofrontal cortex is a secondary olfactory processing area.
Olfaction central integration
Targets include frontal and orbitofrontal areas of the neocortex, which are thought to be important for odor discrimination, and the amygdala and hypothalamus, which
may be involved in emotional and physiological responses to odors.
Gustation
The sense of taste depends greatly on the sense of smell since both begin with a chemical stimulus.
They are referred to as the chemical senses.
The primary function of the gustatory system is nutritional.
Taste processing
Three different types of taste papillae span the surface of the tongue.
Each cell is sensitive to one of five basic tastes.
Taste qualities localisation
Taste sensitivity shows significant individual differences.
The number of taste buds declines with age.
Taste receptors
Different taste qualities involve different detection mechanisms and receptors activations in the microvilli in the taste cells.
Humans can distinguish five different taste qualities:
Sweet
Bitter
Salty
Sour
Umami (associated with amino acids / glutamate)
Taste integration in the CNS
The taste pathway projects to the ventral posterior medial nucleus of the thalamus and information is then relayed to the gustatory
cortex in the insula.
The neural correlates of satiation
Participants use a 10-point scale to rate the motivation and pleasantness of chocolate when offered a morsel seven times during the PET session.
Desire and enjoyment declined over time.
The neural correlates of satiation
Across presentations, activity dropped in primary gustatory cortex (left) and increased in orbitofrontal cortex (right). The former could indicate an attenuated response to the chocolate sensation as the person habituates to the taste. The latter might correspond to a change in the participants’ desire (or aversion) to chocolate.
Activation as measured during PET 15O – H2O
scanning during repeated presentations of chocolate
(red). Water was also presented (blue).
Somatosensory system
Proprioception is the sense of oneself. Receptors in skeletal muscle, joint capsules, and the skin enable us to have conscious awareness of the posture and movements of our own body, particularly the four limbs and the head.
Exteroception is the sense of direct interaction with the external world as it impacts on the body. The principal mode of exteroception is the sense of touch, which includes sensations of contact, pressure, stroking, motion, and vibration, and is used to identify objects.
Epidermis
Epidermis (Thin outer layer)
Dermis (Thick inner layer)
Humans loose 50 million epidermal cells per day
Most somatosensory receptors are mechanoreceptors
Mechanoreceptors
4 types of mechanoreceptor
Receptive field
The receptive field of a touch-sensitive neuron denotes the region of skin where gentle tactile stimuli evoke action potentials in that neuron.
Receptive field
RF size: 2-10 mm RF size: several cm
Bipolar neurons
Physical interaction at the nerve terminus forces Na+ ion channels to open.
Influx of sodium results in rapid depolarization triggers an action potential.
Na+ ion influx (depolarization)
Primary afferent axons
Aa, Ab, Ad, C
C fibers mediate pain and temperature
Ab mediates touch sensations
Ad mediates acute, early pain
Axon size and propagation
Large-diameter fibers conduct action potentials more rapidly
because the internal resistance to current flow
along the axon is low, and the nodes of Ranvier are widely
spaced along its length
Peripheral nerves innervating
The graphs illustrate the distribution of four groups of sensory nerve fibers innervating skeletal muscle and the skin.
Each group has a characteristic axon diameter and conduction velocity.
Myelinated peripheral nerve fibers is approximately six times the fiber diameter.
Major somatosensory pathway
Somatosensory map
Cortical volume changes with mechanoreceptor density
variation across skin regions
(so-called “homunculus”)
Homunculus
This model shows what a man’s body would look like if each
part grew in proportion to the area of the cortex of the brain concerned with its
sensory perception.
Temperature
Transient receptor potential (TRP) channels are membrane proteins with six transmembrane domains.
TRP channels are gated by temperature and various ligands.
Different types respond to different temperature and have different thresholds.
Pain
Propagation of action potentials in different classes of nociceptive fibers.
Visual perception
Visual perception is the ability to interpret the surrounding environment using light in the visible spectrum reflected by the objects in the environment.
Visual information is contained in the light reflected from objects. To perceive objects, we need sensory detectors that respond to the reflected light.
Electromagnetic radiation
Structure of the retina
Light activates the receptor cells of the retina.
There are two types of receptor cells: rods and cones.
The output of the receptor cells is processed and then relayed to the
central nervous system via the optic nerve, the axons of the ganglion cells.
Structure of the retina
The retina comprises five
distinct layers of neurons and synapses.
Rod & cone
Rod and cone photoreceptors have similar structures.
The outer segment consists of a stack of membranous discs that contain the light-absorbing photopigments.
Rod
The rod cell responds to light. Rhodopsin molecules in the outer-segment.
The discs absorb photons, which leads to the closure of cGMP-gated channels in the plasma membrane.
Visual photoreceptors
Human perception of colors results from the simultaneous activation of different classes of photoreceptors in the retina.
Visual system
The input from each visual field is projected to the primary visual cortex in the contralateral hemisphere.
A small percentage of visual fibers of the optic nerve terminate in the superior colliculus and pulvinar nucleus.
A visual scene analyzis
Simple attributes of the visual environment are analyzed to parse the visual scene.
Local visual features are assembled into surfaces, objects are segregated from background, local orientation is integrated into global contours and surface shape is identified from shading and kinematic cues.
Finally, surfaces and contours are used to identify the object
Retinal implant
The tiny implant chip contains 1,500 light-sensitive microphotodiodes.
Color blindness
It is carried by the X chromosome recessively
Color blindness test
It is a color perception test for red-green color deficiencies.
Next class
Effect of music on the brain
Music therapy
Rhythm and Parkinson’s disease
That’s it for today!
Perceiving the world to understand who you are.
