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 1 – External Surface

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

shaped

Figure 7.13

Type 1 – 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

Figure 7.14

Vertebrate Tactile Receptors

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

(e.g., Pacinian corpuscle)

Figure 7.15

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

Figure 7.16a

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

Figure 7.23

Cristae Detect Angular Acceleration

Figure 7.24

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

Figure 7.25

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

Figure 7.26a

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

Figure 7.26b

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

Figure 7.27

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

Figure 7.29

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

Figure 7.33

Flat-sheet Eyes

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

adults

Figure 7.33a

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

Figure 7.33c

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

Figure 7.35

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

Figure 7.35

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

Figure 7.36a

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”)

Figure 7.37a

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

Figure 7.37a

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

Figure 7.39

The Brain Processes the Visual Signal

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

Figure 7.41

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)

Figure 7.42

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