Human Physiology Study Guide sensory physiology

What are the five types of sensory receptors?

Chemoreceptor (O2, pH, glucose, etc.)

mechanoreceptors (pressure, cell stretch, vibration, acceleration, sound).

Photoreceptors (light),

thermoreceptors (heat),

nociceptors (excess heat, pressure, chemicals released from damaged or inflamed tissues)

Where are the primary sensory cortices for vision, hearing, and somatic senses located?

Vision: visual cortex at the back of the occipital lobe;

Hearing: auditory cortex in the temporal lobe;

Somatic senses: primary somatic sensory cortex at the front of the parietal lobe.

What is receptor potential?

a.     The graded potential in sensory receptor caused by a stimulus

b.     Receptor potential strength and duration vary with the stimulus > receptor potential is integrated at the trigger zone > frequency of action potentials is proportional to stimulus intensity. Duration of a series of action potentials is proportional to stimulus duration > neurotransmitter release varies with the pattern of action potentials arriving at the axon terminal

What is the difference in sensory transduction between neuron receptors and non-neuron receptors? how do they code for stimulus intensity?

a.     Stimulus intensity is coded by both the number of receptors activated and the response in one receptor

b.     The response of a sensory receptor, receptor potential, varies with intensity of stimuli

c.     If the receptor is not a neuron (e.g. taste receptor), a larger receptor potential causes more neurotransmitters to be released

d.     If the receptor is a neuron (e.g. olfactory receptor), a larger receptor potential results in more frequent action potentials, which in turn will release more neurotransmitters at synapse

e.     Stronger stimuli sometimes (e.g. touch) induce more receptors to respond

f.      Neurons: More number and more frequent action potentials, and more neurotransmitter-release, Non-neurons: more neurotransmitter release

Where are the most sensitive regions in the human body to touch?

The fingertips and lips have a higher density of mechanoreceptors in the skin, more neurons in the brain devoted to the region

two point touch discrimination test

What is lateral inhibition and its benefit?

a.     Lateral inhibition allows the stimulus to “stand out” from the neighboring regions, thus “sharpens contrast” in the pattern of action potentials received by the CNS, allowing a finer resolution of stimulus location

b.     General Principle: this applies to somatic senses, hearing and vision

What is a receptive field?

a.     The area of the body that, when stimulated, lead to activity in a particular afferent neuron is called the receptive field

                                               i.     Receptive field of adjacent receptors usually overlap

b.     Convergence creates large receptive fields: receptive fields of three primary sensory neurons overlap to form one large secondary receptive field

c.     Divergence: one to many, convergence: many to one

d.     If two receptors converge on only one/same neuron, brain cannot differentiate between two

e.     Referred pain due to principle of convergence

What is the neurological basis for phantom sensations?

a.     could be due to this reorganization in the somatosensory cortex, which is located in the postcentral gyrus, and which receives input from the limbs and body.

b.     Pathway not being used, so some neurons from neighboring pathway grow into abandoned pathway

                                               i.     i.e. face cortex region unaffected, grows into arm cortex region (phantom limb); so when face cortex region activated also activates cortex region for arm

Describe the properties of TRP channels.

Transient receptor potential, ion channels sensing range of temperature and chemicals (i.e. spicy/hot)

How can mint (menthol) and chili pepper (capsaicin) make us feel cold and hot, respectively?

a.     Capsaicin (“hot” in pepper) activates ligand-gated channels on nociceptors, these channels are also opened by heating the tissue to ~43ºC

b.     Menthol also opens trpm8 channels that give cold feeling

c.     Generally: chemicals open the same ion channels as temperature

What is referred pain?

a.     Visceral (internal sensation) and somatic afferent neurons often converge on the same neuron in the spinal cord leading to referred pain

b.     Pain receptors on the skin and internal, visceral organs

                  i.     Both go into CNS

1.     Synapse on secondary neuron

2.     Convergence here, meaning they share the same secondary neuron

What neurotransmitters are released by nociceptors? how do opioids and endorphins suppress pain?

a.     Some neurons release endogenous opioids to inhibit the axon activity in the pain pathway at brain stem and spinal cord

b.     Substance P (neurokinin-1) is a peptide neurotransmitter released by nociceptors at the dorsal horn of spinal cord

c.     Opioids (e.g. morphine) reduce pain by:

          i.     presynaptic: opioids reduce the synaptic release of Substance P and glutamate by inhibiting presynaptic Ca²⁺ influx

          ii.     postsynaptic: opioids antagonize the depolarizing effects of Substance P & glutamate by increasing in K+ conductance

d.     Endorphins are > 20 peptide hormones secreted by the anterior pituitary

e.     Functions:

            i.     Pain inhibition: In the PNS and CNS, endorphins bind to opioid receptors at both pre- and post- synaptic nerve terminals and inhibit the release of substance P

            ii.     Reward and pleasure: In the CNS, endorphins bind opioid receptors and inhibit the release of GABA, resulting in excess production of dopamine

           iii.     Exercise was shown to increase β-endorphin in peripheral blood in humans

f.      Endorphins bind to opioid receptors at both pre- and post-synaptic nerve terminals and inhibit the release of substance P

g.     Endorphins bind to opioid receptors and inhibit the release of GABA, resulting in excess release of dopamine

Why do touch and pain have different speeds?

a.     Speed is lower with less myelin

b.     Mechanoreceptors of skin have thicker myelin diameter than pain/temperature/itch

What is the gate control model of pain modulation?

a.     Peripheral non-noxious stimulation will inhibit presynaptically the pain-conducting pulses at the spinal cord target cells and will prevent pain sensation from being transmitted to higher centers

b.     this is why rubbing can lessen pain and scratching can reduce itch

What is the difference between the ascending pathway of fine touch and the pathway of pain, temperature, and coarse touch? [END LECTURE 20]

a.     Coarse touch, pain, temperature: opposite side of pathway from source/sensation (crosses at spinal cord)

b.     Fine touch: same side of pathway as source/sensation (crosses in brainstem)

c.     Left side spinal cord damage

       i.     Below cut, left side will lose [fine] touch sensation (same side)

       ii.     Below cut, right side will lose pain, temperature, coarse touch (opposite side)

Know these structures and their functions: tympanic membrane, three middle ear bones, oval window, round window, hair cells, basilar membrane, and tectorial membrane.

a.     Middle ear:

        i.     Tympanic membrane (ear drum): thin diaphragm, ~9mm in diameter

       ii.     Three bones: malleus, incus, stapes

      iii.     Oval window: bony covering of the cochlea through which vibration enters cochlea

      iv.     Round window: membrane covered opening on cochlea through which vibration dissipates

b.     Ripples in the cochlear fluid cause the vibration of the tectorial membrane across the hair cells, altering ion movements into those cells, and increasing neurotransmitter release

What cells and cellular processes transduce the mechanical vibration of sound into an electric signal in the auditory nerve?

a.     Vibrating objects create percussion waves in the air that cause the tympanic membrane to vibrate

b.     The three bones of the middle ear transmit the vibrations of moving air to the oval window on the cochlea

c.     These vibrations create pressure waves in the fluid in the cochlea that travel through the vestibular canal

d.     Pressure waves in the canal cause the tectorial membrane to vibrate, bending cilia on hair cells

e.     This bending of cilia depolarizes the membranes of mechanoreceptors, release of neurotransmitter, and sends action potentials to the brain via the auditory nerve

What ion flow depolarizes the hair cells?

a.     Extracellular fluid (endolymph) has high K⁺ concentration, depolarization is caused by inward K⁺ current

b.     Mechanically gated TRPA1 channel open allowing (K+) ion influx to depolarize

What is the auditory pathway from auditory nerves to the primary auditory cortex?

Sound waves into left cochlea > cochlear branch of left vestibulocochlear nerve (VIII) > cochlear nuclei (at medulla, same side and crosses to opposite side after medulla) > to cerebellum and up to L/R thalamus > L/R auditory cortex

Where are the two places we see the separation of sound frequencies?

a.     Cochlea and temporal lobe

            i.     Higher frequencies travel shallower in cochlea

           ii.     Lower frequencies travel deeper in cochlea

What does the McGurk effect show about how we process sensory information?

a.     When visual and auditory sensations conflict, visual sensation prevails

           i.     What we see is what we hear

What are the two major components of the vestibular system and their corresponding functions?

a.     Semicircular canals and otolith organs

b.     Interconnected fluid-filled chambers to detect body movement, position, and balance

c.     The utricle and saccule contain granules called otoliths that allow us to perceive position relative to gravity or linear movement

d.     Three semicircular canals contain fluid (endolymph, high K+/low Na+) and can detect angular movement in any direction through hair cells

             i.     Mechanical force activates depolarization in hair cells?

e.     Equilibrium pathways project primarily to the cerebellum (also to cerebral cortex)

Cerebellum responsible for balance and movement

Know the structures of the eye (cornea, lens, ciliary muscle, retina, fovea, optic nerve, optic disk). [SLIDE 20/FIGURE 10.25]

a.     Cornea

b.     Lens: bends light to focus it on the retina

c.     Ciliary muscle: contraction alters curvature of the lens

           i.     Contracted = lower tension/more rounded lens

          ii.     Relaxed = higher tension/more flattened lens

d.     Retina: layer that contains photoreceptors

           i.     light penetration past the ganglion (neuron), bipolar, amacrine, and horizontal cells occurs prior to transduction by the rods and cones

e.     Fovea: region of sharpest vision (highest concentration of colors?)

f.      Optic nerve

g.     Optic disc: (blind spot) region where optic nerve and blood vessels leave the eye

How does the ciliary muscle control the accommodation?

a.     The contraction state of the ciliary muscles (a sphincter muscle) determines the amount of tension that the zonular fibers exert on the lens

               i.     sphincter contracted = lower tension and more rounded lens

              ii.     sphincter relaxed = higher tension and more flattened lens

What is the principle of vision correction?

a.     Hyperopia/far-sightedness: Lens too flat/not rounded enough, focus behind retina, fix is concave lens

b.     Myopia/near-sightedness: Shape too rounded, focus in front of retina, fix is concave lens (more common)

What are the cell types in the retina?

light penetration past the ganglion (neuron), bipolar, amacrine, and horizontal cells occurs prior to transduction by the rods and cones

What is the difference between rods and cones?

a.     light penetration past the ganglion (neuron), bipolar, amacrine, and horizontal cells occurs prior to transduction by the rods and cones

b.     Fovea is low density/no rods, lots of cones

         i.     Look next to object in dark room to see it clearly

c.     Cones = color, rods = (low) light

What is the adaptation in sensory receptors?

a.     a decrease in receptor sensitivity even given constant stimulus strength

b.     General Principle: this applies to somatic senses, smell, taste, hearing and vision (except for some pain)

c.     Phasic receptors: rapidly adapt to a constant stimulus and turn off - odor, touch, and temperature, quick adaptation (also photoreceptors)

             i.     Not related to danger, action potentials stop in the nerve quickly

d.     Tonic receptors: slowly adapting receptors that respond for the duration of the stimulus – pain, slower adaptation/long term

What is the biological basis of afterimage illusion?

a.     1. three different types of photoreceptors

b.     2. when staring at an image, some receptors are being used and go into adaptation.

c.     3. White light has all colors, but the receptors in adaptation will not respond, only the receptors not previously used will respond.

d.     4. The “opposite” color will show temporarily before new adaptation is formed.

e.     An afterimage or ghost image or image burn-in is an optical illusion that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased.

f.      When the eyes are then diverted to a blank space, the adapted photoreceptors send out a weak signal and those colors remain muted. However, the surrounding cones that were not being excited by that color are still "fresh” and send out a strong signal. The signal is exactly the same as if looking at the opposite color, which is how the brain interprets it.

What molecular and cellular events happen after a photoreceptor is exposed to light?

a.     Three pieces of the puzzle:

          i.     GTP cycle (GTP – cGMP – GMP)

         ii.     2. cGMP opens CNG channel

         iii.     3. Light – rhodopsin – transducin (a G protein) – PDE

b.     In darkness, rhodopsin is inactive, cGMP is high, and CNG and K+ channels are open > light bleaches rhodopsin. Opsin decreases cGMP, closes CNG channels, and hyperpolarizes the cell > in the recovery phase, retinal recombines with opsin

           i.     Membrane potential in the dark = -40 mV

          ii.     Membrane hyperpolarizes to -70 mV              

c.     Transduction of visual information to the nervous system begins when light induces the conversion of retinal conformation (from cis- to trans-).

d.     trans-retinal activates rhodopsin, which activates a G protein, eventually leading to hydrolysis of cyclic GMP.

e.     When cyclic GMP breaks down, Na⁺ channels close.

f.      Light hyperpolarizes the photoreceptor.

g.     The signal transduction pathway usually shuts off again as enzymes convert retinal back to the cis form.

  Why would vitamin A deficiency cause night blindness?

Vitamin A deficiency causes night blindness because it is required for the synthesis of the retinal portion of rhodopsin

In which type of cells are action potentials first started in the optical nerve?

a.     Photoreceptors > bipolar cells > ganglion cells (neuron) > brain (via optic nerve)

b.     Ganglion cells

How are the two sides of the visual field projected to the visual cortex? (in particular, locations of the optic nerve, optic chiasm, optic tract, and how the visual field is divided at these locations) [SLIDE 56/57 diagram]

a.     Visual cortex > optic tract > optic chiasm > optic nerve > eye

b.     Right visual field through both eyes projected to left brain

c.     Left visual field through both eyes projected to right brain

d.     Optic nerve equivalent to eye (right = right)

e.     Optic tract in brain (opposite)

f.      Optic chiasm = peripheral

g.     Outer half does not cross; inner half does cross

h.     Binocular zone is where left and right visual fields overlap

What are the two common symptoms of pituitary adenomas?

a.     Impaired vision (blurry or double vision), headaches

What are the receptors for sensing light and regulating the circadian rhythm?

a.     Melanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light

b.     SCN (suprachiasmatic nucleus), located in the hypothalamus, is the control center of circadian rhythm

c. Ganglion cells in retina > SCN > pineal gland for melatonin/other pathways

What is the olfactory pathway from the nose to the brain?

Olfactory neurons in the olfactory epithelium > cranial nerve I > olfactory bulb > olfactory tract > olfactory cortex > cerebral cortex or limbic system

How do odorant receptors transduce smells? Are they neurons?

a.     Olfactory transduction involves the interaction of odorant molecules in nasal mucus with receptors on the ciliated endings of olfactory neurons

b.     Olfactory receptors are neurons

c.     Stimulated odorant receptors activate a G protein-mediated pathway that increases cAMP, which in turn opens nonselective cation channels and depolarizes the cell

What is the difference between phasic and tonic receptors?

a.     Phasic receptors: rapidly adapt to a constant stimulus and turn off - odor, touch, and temperature, quick adaptation (also photoreceptors)

               i.     Not related to danger, action potentials stop in the nerve quickly

a.     Tonic receptors: slowly adapting receptors that respond for the duration of the stimulus – pain, slower adaptation/long term

What are the five tastes?

a.     Sweetness: sugars, honey, - carbohydrate

b.     Sourness:  high H⁺ concentration

c.     Saltiness: high Na⁺ concentration

d.     Bitterness: K^+, Mg²⁺, Quinine (a lot of poisonous substances)

e.     Umami: “delicious”, MSG

f.      Fat (as well as water – see next slide) is proposed to be the sixth taste

g.     Spiciness is not a taste, but a pain sensation

How do taste receptors transduce tastes? Are they neurons?

a.     Sense of taste depends on sense of smell

b.     Taste receptors are NOT neurons.

a.     Signal transduction causes release of neurotransmitters (5-HT) or ATP

b.     Ligands activate the taste cells > various intracellular pathways are activated > Ca2+ signal in the cytoplasm triggers exocytosis or ATP formation > neurotransmitter or ATP is released > primary sensory neuron fires and action potentials are sent to the brain