BIOL 307 Chp 11

Transducer/Transduction

A receptor or sensor that converts stimulus into intracellular signal; Changes a signal from one form to another form, usually from a sensory input to a chemical signal.

Proprioception

Awareness of where limbs are in relation to space and body; Mediated by muscle and joint receptors.

Chemoreceptor

Respond to chemical signals that bind to receptor (smell and taste).

Mechanoreceptor

Respond to mechanical energy (pressure, vibration, gravity, stretch, acceleration, and sound).

Thermoreceptor

Responds to temperature.

Photoreceptor

Responds to light (usually in the retina for vision).

Receptor Potential

Electrical signal occurring in sensory receptor when it is activated.

Receptive Field

The bodily areas (somatic sensory and visual neuron) where a stimulus can be detected to active a neuron.

Somatosensory Cortex

Location in parietal lobe (brain) where all information about touch, temperature, body position, itch and pain is sent.

Nociceptor

Neurons with free nerve endings; Pain receptor.

Sensory Adaptation

Lack of response to stimulus after periods of constant stimulus.

Olfaction

Sense of smell; ability to detect and interpret odors.

Gustation

Ability to taste; sensory perception of flavors from chemicals in food and drinks.

Hair Cell

Highly specialized non-neural receptors found in the ear for sound transduction and equilibrium; Release chemical signals upon activation, which initiate action potential in the associated sensory neurons.

Pupil

Opening through which light passes into the eye; black part of the eye.

Iris

Colored part of the eye around the pupil.

Lens

The transparent disk that focuses light for eye; 1/3rd of light bending occurs here.

Cornea

Clear, outermost layer of the eye covering the pupil and iris, and focusing light on the retina; 2/3rd of the total refraction occurs here.

Retina

Layer inside the eye containing photoreceptors (light-sensitive).

Optic Nerve

Bundle of optic fibers carrying information from the retina to the brain.

Accommodation

Process of adjusting lens curvature to keep things in focus.

Myopia

Near sightedness, cannot see far.

Hyperopia

Far sightedness, cannot see close.

What is the function of all sensory receptors?

Transduction of an environmental signal/energy into an electrical signal.

Distinguish between the special senses and the somatic senses, and give examples of each.

Special Sense:

• Use specialized sense organs in the head to detect stimuli.

• Ex: Vision, hearing, taste, smell

Somatic Sense:

• Use receptors to have a conscious perception of sensation from skin, muscles, and joint.

• Ex: Touch

Classify a particular sensory receptor as a mechanoreceptor, chemoreceptor, etc. (e.g. into its “class”).

Mechanoreceptor:

• Responds to various forms of mechanical energy.

• Stimuli can be pressure, vibration, gravity, acceleration, or sound.

• Ex: Touch, hearing

Chemoreceptor:

• Responds to chemical ligands that bind to receptors.

• Stimuli can be oxygen, pH, various organic molecules (glucose).

• Ex: Smell, taste

Photoreceptor:

• Responds to light.

• Stimuli can be photons of light.

• Ex: Vision

Thermoreceptor:

• Responds to temperature.

• Stimuli can be varying degrees of heat.

Describe the steps of transduction generally in a mechanoreceptor, including applying your knowledge of how neurons work to these cells.

• Conversion of stimulus energy into information that can be processed by the nervous system.

  • Occurs by opening or closing of ion channels in many receptors.

• Some transduction includes signal transduction and second messenger systems which initiate change in membrane potential.

• Each sensory receptor has adequate stimulus.

  • Specific form of energy to which receptor is most receptive.

  • Ex: Thermoreceptors most receptive to temperature changes, mechanoreceptors most receptive to stimuli which deform the cell membrane.

Give examples of chemoreceptors and mechanoreceptors.

Chemoreceptors:

• Olfactory, airway, hypothalamic, gastrointestinal, central (medulla), peripheral (carotid and aortic bodies).

Mechanoreceptors:

• Cutaneous (touch), proprioception (position/movement), auditory and vestibular (hair cells of ear).

Recognize that for a given sensory receptor cell, increased stimulus intensity is always “coded” as increased action potential frequency, no matter what the sense.

Increased intensity means increased action potential frequency, NOT increased action potential magnitude.

Distinguish between tonic and phasic receptors, explain the mechanism underlying the difference and give examples of each.

Tonic:

• Maintains fire as long as the stimulus is present.

• Stimuli of tonic receptors are parameters which must be continuously monitored by body.

• Ex: Pressure sensitive, if someone is touching you.

Phasic:

• Fires when stimulus is first presented, but stops is the stimulus strength remains constant.

• Receptive to changes in parameters.

• Allow brain to filter out extraneous sensory information, and focus on new, different, or essential sensory information.

• Ex: Sense of smell, losing the smell of your perfume throughout the day.

For each of the major senses, describe the general pathway by which information travels from sensory receptors to the cerebral cortex (e.g., for somatosensation you would say “touch sensation synapses first in the spinal cord or medulla, then crosses over to the other side of the CNS and synapses in the thalamus, then goes to the somatosensory cortex”. Broadly speaking all senses have similar pathways and a high level of detail is not needed, so this is not too complex, but you will need to spend time on it; the idea is to see the overall anatomical pattern, but the senses are not all the same).

Sound:

• Vestibulocochlear nerve -> medulla -> thalamus -> auditory complex.

Sight:

• Rod -> bipolar cell -> ganglion cell -> optic nerve -> optic chiasm -> optic tract -> thalamus -> visual cortex.

Smell:

• Olfactory neuron -> cranial nerve I -> olfactory bulb -> olfactory tract -> olfactory cortex -> cerebral cortex.

Nociception, Temperature, Coarse Touch:

• Neurons synapse onto secondary neurons after entering the spinal cord -> secondary neurons cross the midline in the spinal cord -> ascend to the brain.

Fine Touch, Vibration, Proprioception:

• Neurons have long axons that project up the spinal cord all the way to the medulla -> secondary neurons cross midline in the medulla -> results sensations on one side of the body to be processed in the opposite side of the brain.

Explain how we can identify many more chemicals (smells) than we have olfactory receptor types.

• Axons of cells with same receptors converge on few secondary neurons in olfactory bulb.

• Brain uses info from hundreds of olfactory neurons in different combinations to create perceptions of many different smells.

Describe or identify several major differences between the human senses of olfaction and gustation.

Olfaction:

• Receptors are located in the epithelium of nasal cavity.

• Receptors are neurons from cranial nerve I and project to olfactory bulb.

• Odorants must dissolve in mucus and then binds to receptors, which activates the G-protein completes which increases intracellular cAMP, which leads to ion channels opening, which depolarize the cell.

• Olfaction does not travel through the thalamus.

Gustation:

• Epithelial taste receptors are grouped into taste buds.

• Receptors are non-neuronal epithelial cells.

• Use various methods of signal transduction.

• Travels through the thalamus.

Describe how people feel touch, hear, taste, smell, and see (e.g., “We hear using auditory receptors, which are modified mechanoreceptors with cilia on them that bend when a sound ‘hits’ them, opening ion channels”).

Touch:

• Mechanoreceptor

• Stimulated by physical deformation.

• Mechanically gated ion channels open -> depolarization.

Hear:

• Hair cell (mechanoreceptor)

• Stimulated by sound vibrations.

• Stereocilia bend -> channels open -> depolarization.

Taste:

• Taste receptor cell (chemoreceptor).

• Stimulated by chemicals.

• Channels/GPCRs activated -> depolarization -> neurotransmitter.

Smell:

• Olfactory receptor neuron (chemoreceptor).

• Stimulated by odor molecules.

• GPCR pathway -> channels open -> depolarization -> AP.

See:

• Rods and cones (photoreceptor).

• Stimulated by light.

• Signaling cascade -> channels close -> hyperpolarization.

Name the five things we can taste.

• Sweet

• Sour/Acidic

• Salty

• Bitter

• Umami

What processes the senses signals?

A dedicated region of the cerebral cortex for each sense.

Describe how sound is conducted to the cochlea and then how the sound stimulates auditory receptors and results in action potentials.

Outer ear funnels soundwaves -> sound vibrates on eardrum -> vibrations pass to ossicles in middle ear to amplify sound -> fluid waves bend basilar membrane causing hair cells to move -> hair cells bend against tectorial membrane, opening mechanically gated ion channels -> K+ influx depolarizes -> neurotransmitter release onto auditory nerve fibers -> action potentials travel via auditory nerve to brainstem -> go to thalamus -> go to auditory cortex.

Explain how the mammalian cochlea provides information about both amplitude and frequency of sounds to the brain.

Amplitude:

• Louder sounds bend hair cells more, which release more neurotransmitters, leading to more frequent action potentials being sent to the brain from the cochlea.

Frequency:

• Different frequencies are detected in different parts of the basilar membrane, so action potentials are sent from different sensory neurons.

  • Higher frequency sounds are in the base of the cochlea, lower frequency sounds are in the apex of the cochlea.

Explain what it is that changes in the eye to allow you to focus on close objects.

Close Objects:

• Lens bends and becomes more round/curved (thicker) for more refraction.

  • Controlled by the ciliary muscles.

• Pupil constricts.

• Eyes rotate medially (convergence).

Far Objects:

• Pupil is relaxed.

• Lens flattens.

• Far focused objects come into focus.

Distinguish between the function of rods and cones.

Rods:

• Function well in low light, and used in night vision.

• Sensitive to light, but not color.

• Less detail and more blurry (low acuity).

• Concentrated around periphery of eye.

Cones:

• Responsible for high-acuity vision and color vision during daytime.

• Detect green, red, and blue.

• Functions the best in bright lights.

Explain the steps of phototransduction (Sometimes on tests, students write the pathway of visual information to the cerebral cortex in “answer” to this goal, rather than the steps of phototransduction. Make sure you know what “transduction” means, remember that it occurs entirely within one cell (one cell in the retina, in this case), and see Fig 11.31).

Dark:

• At rest.

• Na 2+/Ca 2+ channels are open.

• Photoreceptors depolarize and release glutamate.

Light:

• Retinal activates by changing shape -> activates transducin (which is a G-protein) -> PDE enzyme activates, which breaks down cGMP -> less cGMP -> Na 2+/Ca 2+ channels close -> K+ leaks out -> neurotransmitter release decreases.