Sensory Receptors

Sensory Receptors

• Range from simple neurons to complex sense organs

• Types: chemoreceptors, mechanoreceptors, photoreceptors, electroreceptors, magnetoreceptors, thermoreceptors

• All transduce incoming stimuli into changes in membrane potential

Sensory Receptors

Classification of Sensory Receptors

• Based on stimulus location• Telereceptors – detect distant stimuli, e.g.,

vision and hearing• Exteroceptors – detect stimuli on the outside

of the body, e.g., pressure and temperature• Interoceptors – detect stimuli inside the body,

e.g., blood pressure and blood oxygen

Classification of Sensory Receptors

Based on type of stimuli the receptors can detect (stimulus modality)• Chemoreceptors – chemicals, e.g., smell and taste• Mechanoreceptors – pressure and movement, e.g.,

touch, hearing, balance, blood pressure• Photoreceptors – light, e.g., vision; detect photons• Electroreceptors – electrical fields• Magnetoreceptors – magnetic fields• Thermoreceptors - temperature

Receptors and stimulus

• Location: Can distinguish the location of the stimulus (touch, light or odour)

• Duration: Determine length of stimulus by responding to the stimulus for the duration of the stimulus.

• Intensity: Increase in action potential frequency or increase in neurotransmitter release.

Sensitivity to Multiple Modalities

• Adequate stimulus – preferred or most sensitive stimulus modality

• Many receptors can also be excited by other stimuli, if sufficiently large, e.g., pressure on eyelid perceive bright light

• Polymodal receptors – naturally sensitive to more than one stimulus modality, e.g., ampullae of Lorenzini in sharks

• Nociceptors – sensitive to strong stimuli, e.g., pain; many are polymodal receptors

Stimulus Encoding

• All stimuli are ultimately converted into action potentials in the primary afferent neurons

• How can organisms differentiate among stimuli or detect the strength of the signal?

• Sensory receptors must encode four types of information• Stimulus modality• Stimulus location• Stimulus intensity• Stimulus duration

Dynamic Range

• Action potentials code stimulus intensity through changes in frequency, e.g., strong stimuli high frequency

• Dynamic range – range of intensities for which receptors can encode stimuli

• Threshold detection – weakest stimulus that produces a response in a receptor 50% of the time

• Saturation – top of the dynamic range; all available proteins have been stimulated

Range Fractionation

Relationships between stimulus intensity and AP frequency• Linear across large range of

intensities: large change in stimulus causes a small change in AP frequency large dynamic range, poor sensory discrimination

• Linear across small range of intensities: small change in stimulus causes a large change in AP frequency small dynamic range, high sensory discrimination

Range fractionation – groups of receptors work together to increase dynamic range without decreasing sensory discrimination

Figure 7.4b-c

Tonic and Phasic Receptors

Two classes of receptors that encode stimulus duration• Phasic – produce APs only at the beginning or end of the

stimulus encode changes in stimulus, but not stimulus duration

• Tonic – produce APs as long as the stimulus continues• Receptor adaptation – AP frequency decreases if stimulus

intensity is maintained at the same level

Tonic and Phasic Receptors, Cont.

Pain

• Pain and itching are mediated by Nocireceptors• Itch comes form Nocireceptors in the skin. Higher

pathways for itch are not well understood• Pain is s subjective perception

Chemoreception

• Most cells can sense incoming chemical signals• Animals have many types of chemoreceptors• Multicellular organisms typically use taste and smell• Olfaction – sense of smell

• Detection of chemicals carried in air• Gustation – sense of taste

• Detection of chemicals emitted from ingested food• Distinct due to structural criteria• Performed by different sense organs• Use different signal transduction mechanisms• Are processed in different integrating centers

The Olfactory System

Evolved independently in vertebrates and insects

Vertebrate olfactory system

• Can distinguish thousands of odorants

• Located in the roof of the nasal cavity

• Mucus layer to moisten olfactory epithelium

• Odorant binding proteins – allow lipophilic odorants to dissolve in mucus

• Receptor cells are bipolar neurons and are covered in cilia

• Odorant receptor proteins are located in the cilia

Odorant Receptors are G Proteins

• Each olfactory neuron expresses only one odorant receptor protein

• Each odorant receptor can recognize more than one odorant

Pheromones

Vomeronasal organ – detects pheromones

Structurally and molecularly distinct from the primary olfactory epithelium

• Location• Base of nasal cavity near the

septum in mammals• Palate in reptiles

• Transduction• Activates a phospholipase C-

based signal transduction system; adenylate cyclase-cAMP in other olfactory receptors

Taste Buds in Vertebrates

Group of taste receptor cells

Located on tongue, soft palate, larynx, and esophagus; external surface of the body in some fish

Taste Buds in Vertebrates

• 50 to 150 taste cells

• Epithelial cells that have apical and basal sides and joined by tight junctions

• Life span of 10-14 days

• Basal stem cells divide to regenerate taste cells

• Microvilli on its apical surface that project into the mucus of the tongue

• Taste receptor proteins are found in the microvilli

• Chemicals are soluble and diffuse to the bind to their receptors

• Different cells in the same bud can detect NaCl, sucrose, H+ and quinine (bitter)

• Taste cell forms a chemical synapse with a sensory neuron that projects to the brain from the tongue

Taste buds and peripheral innervation

A generic taste cell.

• Apical surface: both channels and G-protein-coupled receptors that are activated by chemical stimuli

• Basolateral surface: voltage-gated Na+, K+, and Ca2+ channels, as well as all the machinery for synaptic transmission mediated by serotonin

• The increase in intracellular Ca2+ is either by the activation of voltage-gated Ca2+ channels or via the release from intracellular stores causes synaptic vesicles to fuse and release their transmitter onto receptors on primary sensory neurons

• Each cell contains the standard complement of neuronal proteins including Na+/K+ ATPase at the basal level, voltage-gated Na+ and Ca2+ channels, leak K+ channel

A generic taste cell…cont.

• The response to the chemical is mediated by the expression of receptors for that chemical in the microvilli

• The response is a depolarization of the cell sometimes enough to generate an action potential

• The signaling of the cell to the sensory neuron depends on a sufficient depolarization to open the voltage-gated Ca2+ channels necessary for vesicle fusion and neurotransmitter release.

Transduction mechanisms- again

G-Protein-Coupled Receptors

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G-protein and adenylate cyclase

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The inositol-phospholipid signaling pathway

Salt taste

• The Na+ enters into the cell through the passive amiloride-sensitive Na+ channel

• These proteins are found in frog skin and kidney• Amiloride will block Na+ salt taste reception• Entry of Na+ into the cell of course causes the

cell to depolarize• Need a large concentration of Na+ to trigger a

sufficient depolarization to signal to the post-synaptic sensory neuron

Salt taste

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Sour taste

Taste response produced by acids, excess protons (H+). These positive ions enter the cell through a H+, cation specific ion channel and in turn depolarize the cell to threshold for an action potential.

Sour taste

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Sweet taste • There are specific membrane receptors for different sweeteners and sugars

• These receptors are not ligand gated ion channels but rather are metabotropic receptors

• These receptors belong to the family of seven transmembrane domain proteins that are linked to signaling cascades through G proteins. In mammals a combination of the T1R2/T1R3 receptors have a response to sugars and sweeteners

• These receptors stimulate a G protein (Gp) which in this case activates phosopholipase C (PLC)

• PLC breaks down PIP2 (phosphatidylinositol 4,5-bisphosphate) into IP3 (inositol triphosphosphate) and DAG

• IP3 will bind to and activate a ion channel (TRP channel called TRPM5) which allows Ca2+ to influx into the cell

• This pathway leads to a depolarization and threshold is reached to trigger an action potential 

Sweet taste • In other animals sugars also appear to bind to receptors that stimulate G

proteins (Gs) that activate adenylate cyclase

• This results in an increase in cAMP in the cell that activates a protein kinase (PKA) which in turn phosphorylates a K+ channel to close the channel

• Once the K+ channel is close the cell will depolarize

• Both these signaling cascades are used in multiple biological systems

• In the nervous system neurotransmitter binding to specific metabotropic receptors can trigger these cascades

• Photoreceptor and olfactory neurons also use parts of these cascades for their sensory transduction

Sweet taste

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Bitter taste Different cells have different mechanisms of bitter taste transduction

1. In mammals the bitter receptor is a metabotropic receptor called T2R. There are about 30 different subtypes in mammals

These signal through a G protein called gustducin to PLC and thus generate IP3

Like sweet receptors the IP3 activates a TRPM5 channel to open and allow Ca2+ to influx into the cell.

2. Some bitter chemicals such as quinine bind to and block specific K+ channels and thus result in depolarization of the cell

Bitter taste

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Amino acid taste cellsIn some animals (catfish) there are a high number of amino acid taste cells

There appears to be multiple ways that animals respond to amino aicds

1. In fish and other amphibians, amino acids such as L-arginine and L-proline bind to specific receptors which are ligand gated ion channels

2. In mammals there are taste cells that respond to L-glutamate. In these cells L-glutamate activates a metabotropic receptor glutamate receptor linked to a G protein. Glutamate binds to many different metabotropic receptors and in taste cells it is the mGluR4 that is responsible for the taste transduction

3. In mammals there are also two metabotropic receptors T1R1/T1R3 that combine to respond to the standard 20 amino acids. This combination signals through G protein activation of PLC and the generation of IP3 and the activation of the TRPM5 channel.

Sensory Receptors – Part II

Based on type of stimuli the receptors can detect (stimulus modality)• Chemoreceptors – chemicals, e.g., smell and taste• Mechanoreceptors – pressure and movement, e.g.,

touch, hearing, balance, blood pressure• Photoreceptors – light, e.g., vision; detect photons• Electroreceptors – electrical fields• Magnetoreceptors – magnetic fields• Thermoreceptors - temperature

Mechanoreceptors

• Transform mechanical stimuli into electrical signals• All organisms and cells can sense and respond to mechanical

stimuli• Two main types

• ENaC – epithelial sodium channels• TRP – transient receptor potential

Touch and Pressure

Three classes• Baroreceptors – interoceptors that detect pressure

changes• Tactile receptors – exteroceptors that detect touch,

pressure, and vibration on the body surface• Proprioceptors – monitor the position of the body

Insects

Two types of mechanoreceptors

Type I – External Surface

Two common types of sensilla• Trichoid – hairlike• Campaniform – bell-

shaped

Type II – Internal Surface

• Scolopidia – bipolar neuron and complex accessory cell (scolopale)

• Can be isolated or grouped to form chordotonal organs

• Most function in proprioception

• Can be modified into tympanal organs for sound detection

Vertebrate Tactile Receptors

• Widely dispersed• Function as isolated sensory cells• Free nerves endings or enclosed in accessory structures

(e.g., Pacinian corpuscle)

Proprioceptors

Monitor the position of the body• Three major groups

• Muscle spindles – located on the surface of the muscle and monitor muscle length

• Golgi tendon organs – located at the junction between skeletal muscles and tendons and monitor tendon tension

• Joint capsule receptors – located in the capsules that enclose joints and detect pressure, tension, and movement in the joint

Equilibrium and Hearing

• Utilize mechanoreceptors• Equilibrium or balance – detecting position of the body

relative to gravity• Hearing – detecting and interpreting sound waves • Vertebrates: ear is responsible for both equilibrium and

hearing• Invertebrates: organs for equilibrium are different from

organs of hearing (e.g., tympanal organs)

Statocysts

• Organ of equilibrium in invertebrates• Hollow, fluid filled cavities lined with mechanosensory

neurons• Contain statoliths – dense particles of calcium carbonate

Hair cells

• Mechanoreceptor cells used for hearing and balance in vertebrates

• Modified epithelial cells• Have extensive

extracellular structures and cilia that extend from the apical end

Signal Transduction in Hair Cells

Can detect movement and direction

Fish

• Use hair cells in ears for hearing and for detecting body position and orientation

• Have neuromasts that detect water movement

• Neuromast – hair cell and accessory cupula

• Lateral line system – array of neuromasts within pits or tubes running along the side of the body

Vertebrate Ears

Function in both equilibrium and hearing

Equilibrium

• Vestibular apparatus detects movements• Vestibular apparatus – three semi-circular canals with

enlarged region at one end (ampulla) and two sacklike swellings (utricle and saccule)

• All regions contain hair cells

Vestibular Apparatus

• Utricle and saccule contain mineralized otoliths suspended in a macula covering >100,000 hair cells

• Ampullae lack otoliths and contain cristae (hair cells located in a cupula)

Maculae Detect Linear Acceleration and Tilting

Cristae Detect Angular Acceleration

Sound Detection

• Inner ear detects sound• In fish, incoming sound waves cause otoliths to move

which bend cilia of hair cells• Some fish use the swim bladder to amplify sounds

Terrestrial Vertebrates

• Hearing involves the inner, middle, and outer ears

• Problem: sound transfers poorly between air and the fluid-filled inner ear

• Solution: amply sound• Pinna acts as a funnel to

collect more sound• Middle ear bones increase

the amplitude of vibrations from the tympanic membrane to the oval window

Mammalian Inner Ear

• Specialized for sound detection• Cochlea is coiled in mammals• Perilymph – fills vestibular and tympanic ducts and is similar to

extracellular fluids• Endolymph – fills cochlea duct and is high in K+ and low in Na+

• Organ of Corti contains hair cells and sits on basilar membrane• Two types of hair cells

• Inner hair cells detect sound• Outer hair cells amplify sounds

Sound Transduction

Steps • Incoming sound• Oval window vibrates • Waves in perilymph of vestibular duct• Basilar membrane vibrates • Stereocilia on the inner hair cells bend • Depolarization• Release of neurotransmitter (glutamate)• Excite sensory neuron

Round window serves as a pressure valve

Sound Encoding

Basilar membrane is stiff and narrow at the proximal end and flexible and wide at distal end

Frequency• High stiff end vibrates• Low flexible end vibrates

Amplification

Loudness• Loud sounds movement of basilar membrane

depolarization of inner hair cells AP frequency

Outer hair cells• Change shape in response to sound instead of

releasing neurotransmitter• Change in shape causes basilar membrane to move

more and causes a larger stimulus to the inner hair cells

• Amplifies sound

Sound Location

• Brain uses information on time lags and differences in sound intensity

• Sound to right ear first sound located to the right• Sound louder in right ear sound located to the right

Photoreception

• Ability to detect a small proportion of the electromagnetic spectrum from ultraviolet to near infrared

• Concentration on this range or wavelengths supports idea that animals evolved in water

Photoreceptors

Organs range from single light-sensitive cells to complex, image forming eyes

Two major types• Ciliary photoreceptors – have single, highly folded

cilium; folds form disks that contain photopigments • Rhabdomeric photoreceptors – apical surface is

covered with multiple outfoldings called microvillar projections

Photopigments - molecules that absorb energy from photons

Vertebrate Photoreceptors

All are ciliary photoreceptors

Two types• Rods• Cones

Characteristics of Rods and Cones

Nocturnal animals have relatively more rods

Photopigments

Photopigments have two covalently bonded parts• Chromophore – pigment that is a derivative of vitamin

A, e.g., retinal• Opsin – G-protein-coupled receptors

Steps in photoreception• Chromophore absorbs energy from photon• Chromophore changes shape• Photoreceptor protein changes shape• Signal transduction cascade• Change in membrane potential

Bleaching – process where activated retinal no longer bonds to opsin, thereby activating opsin

Phototransduction

Transduction cascades differ in rhabdomeric and ciliary photoreceptors

The Eye

• Eyespots are single cells or regions of a cell that contain photosensitive pigment, e.g., protist Euglena

• Eyes are complex organs

Flat-sheet Eyes

• Provide some sense of light direction and intensity• Most often seen in larval forms or as accessory eyes in

adults

Cup-shaped Eyes

• Retinal sheet is folded to form a narrow aperture• Better discrimination of light direction and intensity• Seen in the Nautilus

Vesicular Eyes

• Use a lens in the aperture to improve clarity and intensity• Lens refracts light and focuses it onto a single point on

the retina• Present in most vertebrates

Convex Eye

•Photoreceptors radiate outward forming a convex retina•Present in annelids, molluscs, and arthropods (eeeeeeeeeek)

Compound Eyes

Most complex convex eyes found in arthropods

Composed of ommatidia

Form images in two ways• Apposition compound eyes – ommatidium operate

independently; afferent neurons make interconnection to generate an image

• Superposition compound eyes – ommatidium work together to form an image on the retina

The Vertebrate Eye

Forms bright, focused images

Parts• Sclera – white of the

eye• Cornea – transparent

layer• Choroid – pigmented

layer• Tapetum – layer in

the choroid of nocturnal animals that reflects light

The Vertebrate Eye, Cont.

Parts• Iris – two layers of pigmented

smooth muscle• Pupil – opening in iris• Lens – focuses image• Ciliary body – muscles for

changing lens shape• Aqueous humor – fluid in the

anterior chamber• Vitreous humor – gelatinous

mass in the posterior chamber

Image Formation

• Refraction – bending light rays

• Both the cornea and the lens act as converting lens to focus light on the retina

• In terrestrial vertebrates, most of the refraction occurs between the air and the cornea

Image Accommodation

•Accommodation - incoming light rays must converge on the retina to produce a clear image

•Focal point – point at which light waves converge•Focal distance – distance from a lens to its focal point•Distant object: light rays are parallel when entering the lens•Close object: light rays are not parallel when entering the lens and must be refracted more

•Light rays are focused on the retina by changing the shape of the lens

The Retina

• Arranged into several layers• Rods and cones are are at

the back and their tips face backwards

• Axons of ganglion cells join together to form the optic nerve

• Optic nerve exits the retina at the optic disk (“blind spot”)

The Fovea

• Small depression in the center of the retina where overlying bipolar and ganglion cells are pushed to the side

• Contains only cones• Provides the sharpest

images

Signal Processing in the Retina

Rods and cones form different images

Rods• Principle of convergence – as many as 100 rods

synapse with a single bipolar cell many bipolar cells synapse with a ganglion cell

• Large visual field • Fuzzy image

Cones• One cone synapses with one bipolar cell which

connects to one ganglion cell• Small visual field• High resolution image

Signal Processing in the Retina, Cont.

Complex “on” and “off” regions of the receptive fields of ganglion cells improve their ability to detect contrasts between light and dark

The Brain Processes the Visual Signal

• Optic nerves optic chiasm optic tract lateral geniculate nucleus visual cortex

Color Vision

Detecting different wavelengths of light

Requires multiple types of photoreceptors with different maximal sensitivities• Humans: three (trichromatic)• Most mammals: two (dichromatic)• Some bird, reptiles and fish: three, four, or five (pentachromatic)

Thermoreception

Central thermoreceptors – located in the hypothalamus and monitor internal temperature

Peripheral thermoreceptors – monitor environmental temperature• Warm-sensitive• Cold-sensitive• Thermal nociceptors – detect painfully hot stimuli

ThermoTRPs – TRP ion channel thermoreceptor proteins

Specialized Thermoreception

• Specialized organs for detecting heat radiating objects at a distance

• Pit organs – pit found between the eye and the nostril of pit vipers

• Can detect 0.003°C changes (0.5°C for humans)

Magnetoreception

• Ability to detect magnetic fields

e.g., migratory birds, homing salmon• Neurons in the olfactory epithelium of rainbow trout contain particles that resemble magnetite