Psych 6, Exam 2

Chemical Senses

Some of our oldest senses evolutionarily, and they have evolved to sense chemicals in the environment. To understand the environment and stay nourished.

Tastants

gustation: molecules from environment that we sense through taste

Odorants

Olfaction: molecules from environment that we sense through smell

Nasal Cavity (through Nostrils)

An empty chamber where cells sensitive to odorants sit.

Palate

Upper part of your mouth separates the nasal cavity from the mouth. Very back left open for connection (back door)… both then lead down into the throat.

Epiglottis

Separates the two tubes. Closes when swallowing, so food doesn’t go down the Trachea. 

Taste (Gustatory System)

Taste starts on our tongue, where receptors (Papillae) pick up stuff through infolds. Grooves are in your tongue where Taste Buds actually sit; papillae are the raised bits around them. Food sheds some matter (gustants) that interacts with these taste buds.

• Papilla has grooves containing taste buds

Taste Bud Composition/Components

Taste buds are composed of several cell types

Saliva helps carry molecules to the buds.

Taste Pore is where molecules make contact with the bud.

Most important are Taste Cells, which send Microvilli out to the pore, detecting odorants before sending back to the Gustatory Afferent Axons, which send information further in the system. 

• Taste cells aren’t actually neurons, don’t do APs. They react to stimuli and release NTs in a gradient based on how much they detect (not all or nothing = not an AP). 

  • Send messages to other cells that actually fire APs. 

  • Two-week lifespan because of common damage (acid, spice)

Basal Cells

They produce new taste cells as the old ones die. These assist taste cells and also help regulate messages between the taste cells and the Gustatory afferent axons.

Taste Receptors

Taste Receptors respond preferentially to certain “flavors” of stimuli.

Individual taste cells in a bud have “favorite tastes” that they like responding to. They respond to multiple but prefer some over others (salty, sweet, sour, bitter, umami)

• Have graded depolarizations based on how much is detected. The message will not be the same every time like how neurons and APs function. Thus the connected axons will fire more or less APs based on the reaction gradient of the taste cell. 

• More tastants detected = higher NT release = higher AP frequency in GAAs

Saltiness Transduction Processd

Saltiness is mostly Sodium Chloride. Saliva breaks up molecules; Sodium enters taste cells (open Na+ channels) and depolarizes them.

1) Na+ channels on the apical side are always open

2) Eat something salty, and Na+ develops a high extracellular concentration

3) Na+ diffuses into the cell, causing it to depolarize and propagate through the Soma

4) Voltage-gated Na+ and Ca2+ channels open

5) Taste cell releases serotonin onto gustatory afferent axon

Sour Transduction Process

Similar to saltiness, highly acidic + lots of protons (hydrogen atoms). Acids dissolve in water and generate protons (H+ ions).

1) Outside a cell: H+ can activate proton channels to enter a cell (Inside and Outside: H+ is an antagonist to K+ channels)

2) K+ not being able to leave results in cell depolarization because positive K+ and H+ ions are inside the cell (K+ can no longer leave the cell like it wants to)

3) Voltage-gated Na+ and Ca2+ channels open

4) Taste cells release 5-HT onto the gustatory afferent axon

Bitter, Sweet, and Umami Transduction Process

They work differently, functioning through metabotropic receptors and G proteins. Receptors sensitive to these tastes are made up of two protein subunits (different combos are sensitive to different kinds of taste).

1) Tastant binds to receptors and activates the G-protein

2) G-protein activates Phospholipase C

3) Phospholipase C produces the intracellular messenger IP3.

4) IP3 activates a specialized Na+ channel only found in taste cells (open Na+ channel and depolarize the cell) 

5) IP3 also releases intracellular stores of Ca2+

6) Ca2+ activates a membrane channel to allow ATP efflux (no synaptic vesicles or common neurotransmitters are present in these cells)

Path after Tongue Receives Taste Info

3 cranial nerves coming off the tongue (comprised of GAAs) go to the brain in the Gustatory Nucleus (located in the medulla), where they synapse, then go to the Thalamus VPM Nucleus (Ventral Posterior Medial). From the Thalamus, it’s sent to the Primary Gustatory Cortex (insula), where it can be processed. 

• Tongue → Medulla Gustatory Nucleus → Thalamus VPM → Insula Gustatory Cortex

Conditioned Taste Aversion

Experiments that show animals learn from a single experience whether a food is bad or not, and primarily to avoid poison (rats and NutriGrain bars).

The Garcia Effect

Associates aversions and senses, learning based on sensory information in conjunction with negative effects. Associations between sounds and pain can also be made (like stimuli implying danger in nature).

Olfactory System (Smell) Pathway

1) Inhaled air with molecules that we sense 

2) Air travels through the nasal cavity through the nose or the back of the throat.

3) Molecules in the area make contact with the Olfactory Epithelium lining the top of the nasal cavity that sends Cilia (dendrites of olfactory cells) into the nasal cavity with a Mucus layer.

• Mucus is necessary for trapping odorant molecules so they can make contact with cilia.

Olfactory Epithelium

Specialized tissue in the upper nasal cavity is responsible for the sense of smell. It contains specialized olfactory sensory neurons that detect odor molecules and send signals to the brain, which interprets them as smells.

• Humans have bad smell sensitivity because our olfactory epithelium is extremely small compared to other animals, like dogs.

Olfactory (Smell) System Transduction

1) Odorants bind to receptor proteins and activate G-protein coupled receptors (G_olf)

• Hundreds to thousands of different odorant receptors (depending on species)

2) G-protein activates the adenylyl cyclase enzyme

3) Adenylyl cyclase synthesizes cAMP 

4) cAMP binds to cyclic nucleotide-gated ion channels, allowing influx of Na+ and Ca2+

5) Ca2+ binds to Ca2+ gated Cl- channels

6) Cl- efflux causes cell depolarization (negative Cl- ions leaving the cell) 

• Cl- concentration in these cells must be high to cause efflux through channels (opposite of CNS neurons)

Golf Receptors (Olfactory System)

G_olf Receptors are metabotropic receptors (G protein-coupled) that odorant molecules interact with. They sit on the Cilia (dendrites) of olfactory receptor cells.

• Each is sensitive to a particular kind of odorant. Cells only respond to one odorant with their G_olf receptors.

• About 400 receptors exist in humans, so we respond to 400 types of smells; our perceptions of smells are different combinations of activation among these 400 receptors

Olfactory System (smell) Central Pathway to Brain

Axons off of Olfactory Receptor Cells send signals to Glomeruli on the Olfactory Bulb, where they're organized based on receptor type. Labeling is preserved up this chain, as each “type” of receptor goes to a specific place on the Glomeruli.

• One receptor type, one location. 

• Receptor cells go to the Olfactory Bulb; and from there to both the Olfactory Tubercle (which goes to the Thalamus and the thalamus to the Orbitofrontal Cortex) and PAST by the Thalamus (only sense) directly to the temporal lobe area including the Olfactory Cortex, Hippocampus, and Amygdala. 

  • Receptor cells → olfactory bulb → 2 paths

    • Olfactory tubercle → thalamus → Orbitofrontal cortex

    • Temporal lobe (olfactory cortex, hippocampus, amygdala)

  • The direct pathway hits memory centers which could be related to “nostalgic” kinds of smells. 

Somatosensory System

Made up of touch, proprioception (sensing where our body is in space), pain, and temperature.

Touch

Touch is the ability to sense objects that make contact with our skin. The only sense that can be received throughout our entire body. Everywhere can feel/sense, unlike the limitations of hearing or seeing. 

Hairy hair

½ types of skin - Hair follicles on the skin.

• made up of three layers: Epidermis, Dermis, and Subcutaneous

Glabrous Skin

½ types of skin - skin without hair (palms and feet)

• made up of three layers: Epidermis, Dermis, and Subcutaneous

Skin Layers

Epidermis (top) —>Dermis (middle) —→Subcutaneous (innermost) layers.

• Five different types of neurons can receive touch information, distributed differently throughout the layers. 

Meissner Corpuscles

Skin neuron closest to the outside of our skin found in the Dermis layer.

• These are located in parts of the dermis that push up to get close to the top skin layer (usually where there are “valleys” in the top skin layer itself).

Found ONLY in glabrous skin, perceive fine touch, texture, and sense low-frequency vibrations.

Merkel Cell-Neurite Complexes

Skin neuron found in both types of skin in the epidermis, very sensitive to light touch.

• Good for detecting form and texture (shape), with the highest spatial resolution. Not as good at sensing vibration.

Ruffini Endings

Skin neuron found in both types, down in the Dermis layer, and are sensitive to skin stretching.

Also important for proprioception–understanding where fingers are in relation to each other due to stretching.

Pacinian Corpuscles

Lowest skin neurons that are found between the dermis and subcutaneous layers, most sensitive to deep pressure (found in both skin types).

Also good at sensing high-frequency vibrations (sense slipping when gripping something). 

Somatosensory Mechanoreceptors

The 5 neurons found in the skin that sense mechanical/physical stimuli.

• They’re examples of unipolar neurons (signal goes in and out without going through a soma).

Free Nerve Endings

Mostly for pain detection.

• In all of the sensory nerves, there’s a mechanically gated ion channel that’s opened by touch. Force on the membrane or proteins allows the entrance of Sodium and Calcium.

Skin Receptive Field

Each cell has a receptive field for specific parts of the skin area. The types of cells differ in size of their receptive fields. When something happens in the receptive field, the cell starts firing APs.

• Meissner and Merkel have very small fields (super tiny patches of skin, very precise information)

• Pacinian (largest) and Ruffini have very large fields (a whole finger or section of your palm, general information).

Small receptive fields - sit closer to the surface of skin

Large receptive fields - sit deeper in the skin

Slow Adapting Receptor

Rapidly Adapting Receptors (Somatosensory System)

Are Meissner and Pacinian corpuscles, which fire a lot when something makes initial contact and then stops (alerts when touch happens but not continuously). Also fires when touch leaves. Cares about change. 

• The Corpuscles have onion-like, fluid-filled layers around the tip of the axon. When something touches, the fluid is displaced in the layers and causes a response. Once fluid settles (object still there), the firing will stop. When the object leaves, the fluid will move again, and APs fire. 

Touch Pathway in the PNS

1) Sensory receptors send massive axons to the spinal cord.

2) In the cord, cell bodies called Dorsal Root Ganglion Cells are located right outside the spinal cord.

3) Enters the spinal cord through the dorsal (toward the back) end. The ventral spinal cord has more motor functions, and the dorsal spinal cord is more sensory. 

4) After entering the dorsal part of the spinal cord, the axon splits to Motor Neurons (important for reflexes, which happen immediately without having to go through the brain) and toward the brain.

Spinal Cord Organization

Organized into Dermatomes, different zones of the spinal cord correspond to different parts of the body. 

• Top to bottom is Cervical, Thoracic, Lumbar, Sacral. 

  • Bending over makes the organization clearer. Dermatomes are separated from the left/right sections of someone bent over (touching their toes). 

• Damage higher up in the spinal cord is much worse because info has to travel up, no matter where it comes from.

Dorsal Somatosensory Information

Sensory information

Ventral Somatosensory Information

Motor stuff

Dorsal Column-Medial Lemniscal Pathway to the Brain (Somatosensory)

1) Goes from the dorsal part of the spinal cord to the medulla.

2) Synapses onto the Dorsal column Nuclei still in the Medulla, now info crosses onto the other side of the brain stem.

3) Axon then goes to synapse onto a cell in the Thalamus.

4) From the Thalamus to the cortex (primary somatosensory).

First cell goes from skin to Medulla, then passes to a cell going to the thalamus, last cell carries to the cortex (3 CELLS TOTAL INVOLVED)

• Spinal cord → Medulla Dorsal Column Nuclei → crosses brain stem then goes to Thalamus → Somatosensory Cortex

Face Somatosensory Pathway

Same process as the Dorsal Column-Medial Lemniscal Pathway to the Brain, just no spinal cord involvement needed. Cranial nerves do all the work.

• Brainstem → Crosses the brain stem then goes to Thalamus → Cortex

Primary Somatosensory cortex (S1)

Where sense information is processed in the brain. Sits behind (posterior) to the central sulcus.

• Split up into different sections: Sections 3a and 3b (receive direct input from the thalamus) send information to areas 1 and 2. Area 1 processes texture, Area 2 size and shape.

Somatotopy

Describes the phenomenon of stuff nearby in the body also being nearby in the S1 cortex (finger digits will be close together in the brain). Some form of body shape is represented in the brain as well.

• mapping of the body

Homunculus

Representation of which areas get the most attention in the cortex.

• Areas with more cortex area do not correspond with how big the area is, but rather how dense the area is with sensory cells. 

Somatosensory Cortex Plasticity

Really good musicians can adapt relatively finer or developed sensory areas. Expansion of the finger representation in the cortex can occur.

• S1 has some level of plasticity

Somatosensation

Touch, pain, temperature, and periphery reception

Different somatosensory senses travel at different speeds. Faster conduction with a wider axon diameter and more myelin coating.

• Proprioceptors of skeletal muscle (A Alpha) go really fast

• Mechanoreceptors of skin (A Beta) are a little slower but still very fast.

• The lowest are pain and temperature fibers (A Delta and C Fibers). Pain and temp travel slower than mechanoreceptors and proprioception, with little or no myelination (+smaller diameter) 

Nociception

Pain is also known as Nociception. Pain is mediated through free nerve endings near the surface of the skin. They respond to chemical substances released by damaged cells. Not responding to the contact itself, but the cells’ chemical message. 

• Two types of fibers are responsive, the faster is A Delta, which is more acute and sharp, and the C Fibers are responsible for longer-lasting, dull throbbing types of pain (go much slower and respond to different substances).

  • A Delta = acute, sharp

  • C = long-lasting, throbbing

Thermoreceptors

Detect temperature. 8 types of receptors respond at different temperature levels. Combining info from receptors gives a sense of temperature. 

Temperature and Pain Relation

Extreme temperatures can cause pain.

• Thermoreceptors tell you the temperature

• Nociceptors tell you if it hurts yet. 

• Non-nociceptive thermoreceptors activate more intensely with heat before saturating a little under 45 degrees. 

Around 45 degrees Celsius is when pain cells start to kick in, and temp cells don’t change detection (reach a maximum level of detection). Though thermoreceptors cap around 45 degrees, pain can continue to increase past that limit (temp feels the same temp wise but is more painful)

The Spinothalamic pathway

Where pain and temperature come in.

1) At the right side of the body, axons come in dorsally, Soma is still found in dorsal root ganglion cells.

2) From there, they immediately synapse onto another neuron in the spinal cord and cross over in the spinal cord (a lot earlier than the other pathway).

3) Travels toward the brain on the opposite side from where it entered. 

• Dorsal spinal cord → Synapse in spinal cord → crosses over in spinal cord → travels up to the thalamus in a third cell → S1. 

Note: Does not have to synapse in the brain stem!

Brown-Sequard Syndrome

Since the Spinothalamic crosses midline at a different point than the dorsal column-medial. If one side of the spinal cord is damaged, you can lose touch sense in one side of the body and pain/temperature in the other side

• Damage will cause loss of touch on the left (EX:) and sense of temp/pain on right. Helps locate where spinal damage has happened.

Sound

Compressed air molecules that create a wavelike pattern. Waves are compressed and rarefied air (rarefied = spread out)

Sound properties are frequency and distance.

• Frequency is the time to complete one cycle (longer wavelength = lower frequency). Higher sounds have higher frequencies and lower frequencies for long waves/low sounds

• Intensity (amplitude) is also a factor (volume). The sound will be louder if the pressure difference in air is more significant 

  • Intensity and Frequency determine the components of sound, independent from one another (sound pitch and sound volume).

Most sounds are superimposed sound waves on top of one another

Pinna

What we see on the outside of the ear. It is in a cone/funnel shape that grabs and amplifies sound waves.

Auditory Canal

A part of the outer ear, and the first part where air travels (open space).

Tympanic Membrane (Eardrum)

Sound hits the membrane and causes it to vibrate. The membrane is connected to 3 Ossicles (some of the body’s smallest bones).

• air waves put pressure on the eardrum and ossicles, becoming a mechanical function from bones shaking and then a fluid wave in the cochlear.

• Air wave → Mechanical → Fluid 

Auditory Vestibular Nerve

Sends sound information towards the brain.

Ossicle Bones

Malleus is the first bone, hammers on the Incus bone, and connects to the Stapes bone (the smallest bone in the human body)

• The stapes hammers on the Cochlea in the Oval Window. 

 Eustachian Tube

The tube that connects the air in your sinuses to the ear.

• why your ear pops in altitude changes (the air around you and inside your head are at different pressures).

Tensor Tympani and Stapedius Muscles

Two little muscles in the middle ear protect the ear from really loud sounds.

• When detecting the membrane vibrating painfully hard, muscles tense and dampen the system (protecting your ear from damage). These may also activate when we talk, so we don’t deafen ourselves 

• The middle ear is a very delicate system, these help protect it

• Difficult to study the auditory system because it’s so sensitive and falls apart upon dissection. 

Cochlea

Snail-like structure wound in a spiral shape, receiving disruptive signals from the Stapes bone. Contains three chambers filled with fluid. 

• Scala Vestibuli (Top chamber) and Scala Tympani (Bottom chamber) don’t do much, most stuff happens in the Scala Media. The organ of Corti sits between the top and bottom chambers and receives wave-like vibrations from fluid undulation pushing it up and down.

• Apex is the middle part of the coiled cochlea

• Structured differently at base and apex; different parts are set up to detect different frequencies. Narrow and stiff at the base, good for detecting higher frequency. Getting to the apex, it becomes floppy and wide, which is good for detecting low-frequency waves.

Basilar Membrane

The structure dividing the top and bottom chambers of the Cochlea is the Basilar Membrane, which oscillates from fluid movement in the vestibuli according to the frequency with which the fluid is vibrating. 

Organ of Corti

Between the cochlear chambers is the Organ of Corti. Corti sits right on top of the basilar membrane, pushed and pulled in a wave pattern when the basilar shakes. Above the organ is the tectorial membrane, which doesn’t move. Corti is pushed against the membrane, allowing Hair Cells inside it to detect the frequency of a wave.

Hair Cells in the Organ of Corti

Not Neurons. Hair cells react to deflection of fluid from shaking corti and detect the frequency of a wave. Connected to spiral ganglion cells, which trigger APs.

• Outer hair cells are connected directly through stereocilia that physically connect to the tectorial membrane. Inner membranes also have stereocilia, but don't directly connect; they only sit in the membrane. 

Outer hair cells are different (3 times as many as inner), because one Ganglion cell will receive info from multiple outer hair cells whereas inner hair cells send to multiple Ganglion cells (opposite).

• Inner hair cells - more responsible for detection of pitch or frequency (very important to recognize speech)

• Outer hair cells - more sensitive to intensity or volume.

Spiral Ganglion Cells

Trigger APs (different frequencies, first neurons in chains, sent to the brain through auditory nerve). 

• Connected to the Organ of Corti

Sound reacts to both frequency and intensity; different Ganglion cells are coded to specific frequencies, and will be most active closer to the favorite frequency. 

• Higher intensity sound will cause more activity in the cell. 

Stereocilia

Stereocilia are like toothpicks, very rigid. They deflect relative to the tectorial membrane and bend when the basilar membrane pushes up against them (also happens in inner hair cells, but still move from fluid movement). 

• Connect outer hair cells to the tectorial membrane

Cochlear Implants

It can assist damaged or missing hair cells, pick up on cochlea frequencies, and bypass hair cells, sending signals to the next step in the system. Helps hearing loss caused by a lack of hair cells.

• Later ear damage is usually caused by damage to the middle ear bones, which wouldn’t be helped by something like this (doesn’t make it to cochlea).

Transduction of an Inner Hair Cell

Stereocilia have mechanically gated potassium channels opening based on deflection of stereocilia to the left or right.

1) Much more potassium outside the cell, as opposed to neurons. Potassium enters when channels open (springlike structures connect stereocilia and pull them open synchronously).

• The proportion of channels open changes depending on wave frequency.

2) Potassium depolarizes cells, which opens voltage-gated calcium channels

3) Calcium binds with vesicles that release NT (Glutamate) on the Spiral Ganglion cell (fires AP based on the rate of Glutamate reception/wave frequency). 

Auditory Path to the Brain

1) Axons from Spiral Ganglion Cells travel along the auditory nerve.

2) Synapse first at the base of the Midbrain/top of the Medulla (actually two synapses onto the ventral and dorsal cochlear nuclei, focus more on the ventral). Axons split from there to the Left and Right Superior Olive in the brainstem. Information becomes bilateral in the brainstem.

3) The third cell in the superior olive goes to the Inferior Colliculus.

4) From there goes to the Thalamus (MGN Auditory Nucleus).

5) Finally, the Thalamus axon goes to the Primary Auditory Nucleus (A1) located right below the sylvian fissure (superior temporal lobe area)

Superior Olive Sound Localization

In the superior olive is a slight delay in sound hitting left/right ears, triggering slight differences in APs in the superior olive from the other ear. The brain compares where signals meet to localize where the sound is coming from. 

Tonopy

A1 has Tonotopy, meaning different parts of the cortex respond to sounds of different frequencies. Anterior parts of A1 respond more to low frequencies. As you move posteriorly, higher frequency sounds are accounted for. 

Vestibular System

A sensory system in the inner ear that provides the brain with information about motion, head position, and balance subconsciously.

Vestibular labyrinth and Semicircular Canals

Next to the cochlea is a similar structure called the Vestibular labyrinth with three loops (semicircular canals) filled with fluid reacting to the movement of our heads (three loops because up/down, left/right, side/side).

Within each semicircular canal are Hair Cell systems sticking up into a gelatinous structure that shifts as we move our heads. Has a cap of calcium crystals that pull the jelly and drag with movement. As you move your head, hair cells and gelatinous areas/crystals move back and forth.s

• These detect acceleration as well, responding to change in position of the head relative to the environment. 

• Also helps stabilize eye movements relative to our head

• Motion sickness occurs when there’s a discrepancy between the visual system and the vestibular system. 

Vestibulo-Ocular Reflex

Keep your eyes focused on something even as your head moves, which happens unconsciously.