PSIO Exam 5 Learning Objectives

List the three overall types of stimuli detected from the special senses

*1. Hearing, Balance* - mechanoreceptors for wave motion and pull of gravity *2. Taste, Smell* - chemoreceptors for tastants and odorants *3. Sight* - photoreceptors for photons of lights

Describe the mechanism of accommodation.

*Accommodation = change of lens shape* 1) Lens becomes more round 2) The entering angle of the light increases 3) The light gets more refracted (bent), so the focused image now falls on the retina

Define the terms receptive field and receptor potential.

*RECEPTIVE FIELD* Discrimination between two similar stimuli (ex. two points on body surface, two sounds of similar pitch) is dependent on the number of receptors within an area of 'sensory surface' - stimulation anywhere within the receptive field results in a signal from the same sensory afferent (sensory nerve) *RECEPTOR POTENTIAL* - these are in sensory cells and cause neurotransmitters to release which causes an EPSP in associated sensory neurons

How is an electrical signal (AP) transmitted from one excitable cell to another?

*Synaptic Tranmission* - same general events occur during neuromuscular transmission

Under resting membrane potential conditions, the movement of which ion across the cell membrane is favored by both it's concentration (chemical) gradient and the cellular electrical gradient?

Sodium

What is the law of electroneutrality?

there are as many positive charges as there are negative charges in any solution - intracellular and extracellular fluids obey this law

List and describe (in order) the physiological events associated with synaptic transmission.

*1)* Action potential arrives at the synaptic end bulb of a presynaptic neuron *2)* The action potential opens voltage gated calcium channels *3)* Calcium entry into the synaptic end bulb triggers the fusion of neurotransmitter containing vesicles (synaptic vesicles) with the pre-synaptic membrane *4)* Neurotransmitter diffuses across the synaptic cleft where it will encounter neurotransmitter receptors (on ligand - gated ion channels) on the post-synaptic membrane *5)* The binding of neurotransmitter to the receptors on ligand - gated ion channels will typically result in the movement of ions across the postsynaptic membrane, generating a graded potential *6)* Depolarization of post-synaptic cells will result if the ion channels opened are for positive ions that enter cells (Na). Resulting graded potential is an Excitatory Post- Synaptic Potential *6)*Hyperpolarization of post-synaptic cells will result if the ion channels opened are for positive ions that exit cells (K+). Resulting graded potential is an Inhibitory Post- Synaptic Potential *7)*Entry of enough positive ions (this is typically Na+) enter the post-synaptic cell, it will cause the initiation of an action potential in the plasmalemma of the post-synaptic cell.

Describe how an image is focused on the retina?

*1)* Parallel rays from long distance *2)* close-up rays no longer parallel *3)* LIght bends (refracts) upon entering new medium *4)* Inverted focuses image falls on plane of retina *5)* Refracted light produces focused image behind retina - a camera would put the image in focus by moving the plane of the 'film' (the retina) - the eye cant do this so it changes shape -ciliary muscles keep tension on the suspensory ligaments *Goes through accommodation of the eye for an image to be focused on the retina*

List and describe the events that result in the propagation of the action potential.

*1.* Entry of Na produces a local current - spreads laterally to depolarize adjacent areas of membrane - if the adjacent membrane area is depolarized to threshold it starts a new cycle of AP in this new region of membrane *2.* Entry of Na in the new region of membrane produces a local current that spreads to adjacent areas -If the new adjacent membrane area is depolarized to threshold it starts a new cycle of action in this 'new' new region of membrane

List (and describe) in order the principal events associated with an action potential.

*1.* Local changes in the cell's membrane potential called graded potentials a) Hyperpolarizing (more negative) b) Depolarizing (less negative) *2.* At the axon hillock, depolarization to a threshold value will induce a population of voltage gated Na channels within that region of the membrane to open (Depolarization Phase) (+30 mV) ( Once step 2 is reached there is no turning back. AP are all or none, once you start them they are designed to travel all the way down the axon to the synaptic terminals.) A) increases Na permeability B) makes inside of cell more positive (increasing depolarization) C) Causes neighboring voltage gated Na channels to open as threshold is reached there D) increasing Na permeability E) perpetuating a POSITIVE FEEDBACK CYCLE *3)* Shortly after voltage gated channels open, they spontaneously close (inactivate) - no more sodium can flow into cell *4)* Depolarization also opens (more slowly) a second population of channels : Voltage gated K+ channels - REPOLARIZATION (-70 to -100) *5)* After hyperpolarizing phase (-70mV)

Describe the Equilibrium pathways

*1.* Vestibular ganglia sensory nerves send APs to the vestibular nuclei in the medulla *2.* Then some vestibular signals travel to the ventral posterior nucleus in the thalamus *3.* Then vestibular signals travel to the vestibular area in the parietal lobe of the cerebral cortex for conscious perception of balance

Describe the following cells and structures related to olfaction and gustation: olfactory epithelium, olfactory receptors, olfactory bulb, taste buds - include more as appropriate.

*5 SPECIAL SENSES* *1. Smell - Olfaction* - in sense organs in nose *2. Taste - gustation* - in sense organs in tongue *3. Balance - equilibrium* - in sense organs in inner ear *4 Hearing - audition* - in sense organ in inner ear *5. Sight - vision* - in sense organ which is the eye

List the steps associated with the transduction of sound stimuli and distinguish between how loudness and pitch of sound are detected.

*Auditory Pathway* *1.* Spiral ganglia sensory nerves send Aps to th cochlear nuclei in the medulla *2.* Then auditry signals travel to the superior olivary nucleus in the pons *3.* Then auditory signals travel to the inferior colliculus in the midbrain *4.* Then auditory signals travel to the medial geniculate nucleus in the thalamus *5.* Then auditor signals travel to the primary auditory are in the temporal lobe of the cerebral cortex for conscious perception of hearing

Define sensory transduction and list the events commonly associated with it

*COMMON STEPS* *1)* Stimulus arrives at receptor and alters membrane potential of receptor - results in a 'graded potential; - graded potentials can be depolarizing (EPSP) or hyperpolarizing (IPSP) *2)* The rate of EPSPs generation in the sensory neuron determines the rate of nerve impulse (AP) production in them *3)* AP's travel to CNS along a specific afferent pathway *4)* CNS interprets/processes these incoming signals - brain assumes that any signals coming in along a sensory afferent from a particular receptor (ex: touch, photoreceptor) reflects stimulation by the appropriate stimulus (ex: pressure; photon) - all other characteristics of the stimulus (ex: intensity, duration) are conveyed by the frequency and pattern of the incoming signals

Define equilibrium potential and explain how the equilibrium potential for different ions relates to the membrane potential

*Changes in Membrane permeability can produce large changes in membrane potential* membrane permeability to an ion (K or Na) = open channels for that ion - regulation of channel-mediated ion permeability allow cells to generate electrical signals 1. Maintain stable Na and K gradients 2. Vary the activity of specific ion channels

Pair different ion channels with the different parts of a neuron

*Dendrites and Cell Body* = Ligand gates/ mechanically-gated ion channels *Axon, axon branches, and synaptic end bulbs* = voltage-gated ion channels ( open/closed by membrane depolarization)

Define the terms depolarization, repolarization, and hyperpolarization.

*Depolarization* - less negative *Hyperpolarization* - more negative *Repolarization* - inactivation of voltage gated Na+ channels, combined with activation of voltage gated K+ channels

Compare and contrast the characteristics of graded vs. action potentials.

*GRADED POTENTIALS* *Origin* - arise mainly in dendrites and cell body (some arise in axons) *Types of channels* - Ligand-gated or mechanically gated ion channels *Conduction* - Not propagated; localized and thus permit communication over a few micrometers *Amplitude* - Depending on strength of stimulus, varies from less than 1 mV to more than 50mV *Duration* - Typically longer, ranging from several msec to several min * Refractory period* - not present, thus spatial and temporal summation can occur *Polarity* - May be hyperpolarizing or depolarizing *ACTION POTENTIALS* *Origin* - Arise at trigger zones and propagate along the axon *Types of channels* - Voltage-gated channels for Na+ and K+ *Conduction* - Propagate and thus permit communication over long distances *Amplitude* - All or none; typically 100mv *Duration* - Shorter, ranging from 0.5-2 msec *Refractory period* - Present, thus summation cannot occur *Polarity* - Always consist of depolarizing phase followed by repolarizing phase and return to resting membrane potential

Differentiate between general and special senses.

*General senses* = itch, oxygen levels, pain, pressure, propriocepion, stretch, temperature, tickle, touch, vibration, etc. *Special Senses* = light, sound, gravity, taste, smell, 'somatic' - require specialized organs

Define graded potential and describe the difference between EPSPs and IPSPs.

*Graded Potentials* - occur in dendrites and cell body of neuron - size varies with strength stimulus - usually generated by chemically and mechanically gated channels TWO TYPES: 1) IPSP = Inhibitory Post-Synaptic Potentials 2) EPSP = Excitatory Post-synaptic potentials

Describe the basic mechanism of hair cells and explain how this relates to the general process of sensory transduction.

*Inner-Ear Receptor Cells:* enable the sensation of hearing and balance *Endolymph*: - unlike any other extracellular fluid in the entire body - contains extremely high potassium and extremely low sodium - every hair cell is in constant contact w body perilymph and endolymph *Hair Cells*: - Have many stereocilia > these contain mechanically gated K+ channels which are linked to those neighboring stereocilia by tip links - re surrounded by endolymph in their stereocilia region, but around perilymph elsewhere - the division between fluids - there is a division between endolymph and perilymph - they respond to mechanical force (mechanically-gated K+ channels open upon mechanical pushing of stereocilia in one direction) - There will be an influx of potassiu from endolymph - Hair cell will depolarize and release more neurotransmitter, triggering more APs in the associated first order sensory neuron *Why do Hair cells act this way?* ENERGY CONSERVATION: hearing and balance do not adapt and must relay signals to the brain 24/7, but how does this setup help? -ANSWER: K+ enters from above the fluid division line (purple line) then exits out below the fluid division line - This is passive (conserves energy), and it maintains electrical and ionic condition gradients for hair cells - Much ATP is expanded by other cell types which maintain endolymph or perilymph

How to manipulate membrane permeability

*Ion Channels* *1.* Integral membrane proteins *2.* Channels can be open or closed *3.* Some channels are routinely open (Leakage channels) *4.* Some channels have their open states regulated

Describe the specific sensory events associated with olfaction (i.e. what cellular events happen when a molecule is detected).

*NOSE: Specialized sense organ* - olfactory epithelium at the roof of each nasal cavity houses sensory ability - no specialized sensory required - Chemoreceptors used to detect odorants - odorants must first dissolve in mucus to bind to receptors - olfactory receptors are neurons and their axons are cranial nerve 1 - olfactory glands produce mucus for chemosensation - supporting cells provide physical an functional support - basal cells are stem cells for olfactory receptors - parts of CN 1 pass through cribifom plate of ethmoid bone - cranial nerve 1 synapses with Mitral cells in olfactory bulb - Mitral cell axons travel along the olfactory tract to deliver sensory info to the brain *OLFACTORY PATHWAY* 1) First order olfactory sensory nerves (cranial nerve 1) send AP's to the Olfactory bulb located below the frontal lobes 2) Some olfactory bulb neuron axons then carry AP's to the primary olfactory area in the temporal lobes of the cerebral cortex for conscious perception of smell

Describe presbyopia, hyperopia, and myopia

*PRESBYOPIA DEFECT* -as lens ages, it loses elasticity even when tension on the suspensory ligaments is relieved, lens retains its elongated shape - Failure to round up means the image of close up objects cannot be focused on the retina - loss of this ability to accommodate means the 'aged' eye cant focus on close-up objects *HYPEROPIA AND MYOPIA* consequences of the shape of the eye

Describe how differences in ionic gradients across the membrane contribute to electrical gradients (i.e. electrochemical gradients).

*RESTING MEMBRANE POTENTIAL* = - is the electrical potential difference across living cells membranes -50mV - -100mV 1. A characteristic of all cells 2. Inside negative 3. K dominated - The only cation with permeability across the membrane, K moves - Its movement is influenced by 2 competing forces: 1) Concentration Gradient -> 2) Electrical Gradient <- *RESULT* - charge separation (build up of PE) across membrane - this is the PE stores across the membrane of a living cell at rest

Describe the physiology of vision - how light influences sodium channel activity, neurotransmitter release, and action potential generation.

*Rod Cell Physiology* - Na channels in outer segment membrane (normally open) - contains integral membrane protein, rhodopsin - 'retinal' absorbs photon which activates 'opsin' - opsin causes the breakdown of 'chemical messengers' that are required to keep the Na channels open RESULT: Hyperpolarization - Rhodopsin activation closes sodium channels Opsin is type of receptor which activates enzymes either synthesize or break down second messengers - cGMP normally binds to ligand-gated Na+ channels in the plasmalemma to keep them open NO MORE EPSPs = hyperpolarization

Describe the principal differences between rods and cones and the basic structure of a rod photoreceptor including the function of rhodopsin.

*Rods* - 125 million - found in the periphery of retina - very sensitive to light - permit excellence of movement - have large receptive fields (monochrome vision) *Cones* - 6 million - found in the center of the retina - in the macula lutea, there are no rods, only cones - the fovea centralis of the macua lutea contains photoreceptors with the smallest receptive fields - less sensitive to light -can discriminate between different wavelengths of lights (cones enable color vision)

Describe the structure of taste buds

*SPECIALIZED SENSE ORGAN: TONGUE* *taste buds* - in papillae house sensory ability - Has specialized sensory cell required: gustatory receptor cells - Types of receptors used: chemoreceptors (tastants) *Three types of taste buds* 1. Vallate 2. Fungiform 3. Filliform *Structure of Taste Bud Structure* - Specialized sensory cells called gustatory receptor cells each extend one long microvillus containing receptors for tastants through taste pore - basal cells are stem cells for supporting cells which then develop into new gustatory receptor cells over time

Compare and contrast 'continuous' and 'saltatory' conduction.

*Saltatory Conduction* STEPS: 1) Na channels at node open 2) Electrotonic conduction 3) Channels at next node open 4) Electronic conduction 5) Next node begins to activate *Continuous Conduction*

Describe the specific sensory events associated with gustation (i.e. what cellular events happen when a molecule is detected).

*Sensory transduction for Gustation* - upon binding to a tastant, a receptor potential is generated in a gustatory receptor leading to release of neurotransmitter - the released neurotransmitter triggers depolarization and AP formation in the first-order sensory neurons for taste *The Gustation Pathway* *1)* First order gustatory sensory nerves in three cranial nerves (Vll, lX, and X) send AP's to the gustatory nucleus in the medulla *2)* Axons then carry taste signals to the limbic system, hypothalamus and thalamus *3)* Some axons carry taste signal from the thalamus to the primary gustatory area in the parietal lobes of the cerebral cortex for conscious perception of taste.

Define spatial and temporal summation and describe how they contribute to neuron activation.

*Spatial SummatIon* - The closer the presynaptic neurons influence is to the trigger zone, the more influential that pre-synaptic neuron is over activation *Temporal Summation* - increased frequency of AP arriving at synapses for one pre-synaptic neuron will cause increased influence - post-synaptic potentials don't immediately fade - temporal summation is possible and it is a strategy that increases a pre-synaptic neurons influence on the post-synaptic neuron

List and describe the structures that light must refract/pass through in order to be detected by the eye.

*Structure and Chambers of the Eye* Cornes, Aqueous humor, Lens, Sclera, Choroid, Vitreous humor, Retina, Optic Nerve *Optics of the eye* - Light rays arrive from an object - focused onto sensory surface of the eye: 'retina' - focusing involves 'bending' of light rays: refraction - refraction occurs when light passes from one medium (air) into another (water) *STRUCTURES WHERE ACTIONS TAKE PLACE* - most refraction occurs at the cornea - lens dos critical 'fine tuning' - iris regulates light entry - pupil constricts as circular muscles of iris contract (parasympathetic) - Pupil dilates as radial muscles of iris contract (sympathetic)

Describe the basis of absolute and relative refractory periods and explain their importance.

*refractory periods* - periods during which it is difficult or impossible to generate a second AP 1) *Absolute Refractory Period* - immediately following inactivation of voltage-gated Na+ channels, membrane cannot be restimulated to produce another AP 2) *Relative Refractory Period* - a period during which a new AP can be produced but doing so requires a larger than normal stimulation *Importance* 1) establishes a maximum rate of AP 2) Influence the characteristics of AP propagation - ensures forward propagation from the axon hillock to axon terminal

At what voltage to the voltage-gated potassium channels open?

+30mV

Explain what the Macula Lutea is

- region in the center of visual field for vision - region contains primarily cone photoreceptors and very few to none of the rod photoreceptors - the center of the macula lutea is the fovea centralis, the central point in our field of vision

Describe how receptive field size is related to sensory resolution/discrimination.

- smaller receptive fields (increased density receptors) permit high resolution discrimination - hand and face have smallest and greatest number of sensory neurons

What is the threshold stimulus for the voltage-gated sodium channel?

-55 mV

Describe the normal concentration gradients for Na and K that exist across the membrane of all cells

Ion gradients require a diff. in concentration on either side of the plasma membrane *POSITIVE IONS* *Sodium (Na)* Intracellular = 10 mM Extracellular = 140mM (14x higher) *Potassium (K)* Intracellular = 140 mM Extracellular = 2.5 mM *NEGATIVE IONS* 1. Chloride (high outside cell) 2. Other (phosphate, proteins, etc.)

Define the phrase 'all-or-none' in the context of the neuronal action potential.

Once step 2 in the events of an AP occur (depolarization phase) is reached there is no turning back. AP are all or none, once you start them they are designed to travel all the way down the axon to the synaptic terminals.

Compare and contrast the characteristics of leak-channels, voltage-gated, chemically-gated, and mechanically-gated channels.

Some channels have their open states regulated 1. *Chemically ligand gated channels* - (open when a signal molecule binds to the channel protein (acH)) 2. *Mechanically gated channels* - (open when membrane gets stretched) 3. *Voltage gated channels* - (open when the membrane potential gets less negative/ depolarized) 4. *Leak Channels* - Always open, K+ leakage channels

Describe the factors that influence the speed of propagation of the action potential.

TWO FACTORS *1. Size (diameter of axon)* - bigger is faster - resistance to current flow in 'axoplasm' decreases as diameter increases *2. Myelination* - certain glial cells (oligodendrocytes and Schwann cells) from insulating layer (sheath) around axons

List the general principles of sensory function

The brain is the organ of sensation & perception - interpretation of sensory info is based on *path travels* and *destination* in the brain - all signals are "sensed" but only 10% is "perceived"

Describe how the activity of the Na+/K+ ATPase maintains these gradients.

The sodium-potassium *ATPase pump* maintains the ion concentration gradients for Na and K 1. For each ATP it uses it pumps out 3 Na ions while at the same time pumping in 2 K ions - as for permeability only one of the two cations can cross through the membrane at rest through potassium leak channels (20x more permeable for potassium than sodium)

What are ion channels?

They are openings in the membrane that determine which electrolytes (charged molecules like ions) can enter/exit (as they go down their gradient

Which proteins must "reset" in order for the relative refractory period to begin?

Voltage-gated sodium channels

Describe the importance of myelination and the function of oligodendrocytes and Schwann cells.

When myelin/insulation is absent, electronic conduction fails to reach and depolarize next node - AP fails

Explain the bioelectricity

When you use the voltmeter inside the neuron *1.* Inside the neuron the voltmeter goes to -70mV *2.* Action Potential - depolarize sufficiently

What does an action potential do when it reaches a synaptic end bulb?

it opens voltage-gated calcium channels

In the region of the dendrites and cell body of a typical interneuron, the plasmalemma contains ligand-gated ion channels which are activated by:

neurotransmitters

Which ion is crossing the plasma membrane during the repolarization phase of the AP?

potassium

What do we call the membrane potential value which, when reached, marks the beginning of the rising (depolarizing) phase of the action potential?

the threshold