(Sensory receptor)(Sensory receptor)

Sensory receptor

• There are several general classes of receptors that are characterized by the type of energy to which they are sensitive.

• 1. Mechanoreceptors respond to mechanical stimuli, such as pressure or stretch, and are responsible for many types of sensory information, including touch, blood pressure, and muscle tention.

Sensory receptor

• 2. Thermoreceptors detect both sensations of cold and warmth.

• 3. Photoreceptors respond to particular light wavelengths.

• 4. Chemoreceptors respond to the binding of particular chemicals to the receptive membrane. This type of receptor provides the senses of smell and taste and detects blood pH and oxygen concentration.

Sensory receptor

• 5. Nociceptors are specialized nerve endings that respond to a number of different painful stimuli, such as heat or tissue damage.

General sensory receptor structure

• Free nerve endings: dendrites interspersed among other cells/tissues (pain, temperature, touch)

General sensory receptor structure

• Encapsulated nerve endings: dendrites with special supporting structures (mechanoreceptors and proprioceptors)

Which receptor?

Classification of Receptors1. Location

1) Externoceptors

Located on the body surface or specialized to detect external stimuli.

Pressure, pain, temp, touch, etc.

2) Visceroceptors

Located within internal organs, detect internal stimuli.

Blood pressure, pain, fullness.

1. Location

3) Proprioceptors

Found in the joints and muscles

Also in the vestibular structures and the semicircular canals of the inner ear.

Limb and body position and movement.

Classification of Receptors

2 Modalities

1) Mechanoceptive

2) Thermoceptive

3) Nociceptive (pain)

4) Photoreceptors

5) Chemoceptive

1. Adequate stimulus

• In addition, within the general energy type that serves as a receptor’s adequate stimulus, a particular receptor responds best (i.e., at lowest threshold) to only a very narrow range of stimulus energies.

• For example, different individual receptors in the eye respond best to light (the adequate stimulus) in different wavelengths.

Generator Potential

Receptor/Generator Potential

Stretch Receptors:

Weak stretch causes low impulse frequency on neuron leaving receptor.

Strong stretch causes high impulse frequency on neuron leaving receptor. Time Membrane

potential

Frequency Code

Sensory Adaptation is one form of Integration

Touch receptors quickly adapt. The frequency of action potentials diminishes or stops if the stimulus is unchanging.

Pain receptors Baroreceptors.

adapt slowly or not at all.

“On-off ” responses

Visual optics – depends on refraction of light by the cornea and

lens of the eye.• Coarse Fixed FocusingCoarse Fixed Focusing

» Cornea ShapeCornea Shape• AccommodationAccommodation- - adjust configuration ofadjust configuration of

» Lens ShapeLens Shape» Pupil SizePupil Size

– the overall effect is to provide a convergent optical system.

– Diverging rays of light be brought back to a focus in the plane of the retina.

Lens- transparent biconvex structure, flexible• Attached by suspensory ligaments to ciliary

body• focuses image onto retina• changes lens thickness to allow light to be

properly focused onto retina

Accommodation

Focusing

Muscles working

Lens more spherical

Focus near

Muscles relaxed

Lens less spherical

Focus far

Accommodation

• Near accommodation

• Object close to the eyes→ optic cortex → parasympathetic N→ ciliary muscle contraction → suspensory ligaments relaxed → the lens become thick → refractory power increase →formation sharp image

Pupil

• Pupil – central opening of the iris

–Regulates the amount of light entering the eye during:

•Close vision and bright light – pupils constrict

•Distant vision and dim light – pupils dilate

spherical aberration spherical aberration

chromatic aberration chromatic aberration

The retinal imaging is more clear. The retinal imaging is more clear.

bright light normal light dim light

pupillary light reflexpupillary light reflex

Convergence of eyeballs

Viewing near object causes reflexly both eyes to move inward to focus on a near object, this process is called convergence reflex.

Presbyopia Since the lens must be elastic to assume a more

spherical shape during accommodation for near vision, the increasing stiffness of the lens that occurs with aging makes accommodation for near vision increasingly difficult.

This condition, known as prsbyopia, is a normal part of the aging process and is the reason that people around 45 years of age may have to begin wearing reading glasses or bifocals for close work.

The retina is the light sensitive portion of the eye.

From outside to inside:

(1)Pigment layer

(2)Photoreceptor cell layer

(3)Bipolar cell layer

(4)Ganglion cell layer

Structure of Retina

•Receptor cells: rods and cones, sensitive to light

•bipolar cells: carry signals from receptors to ganglion cells

•Ganglion cells: axons of ganglion cells form the optic

nerve

The photoreceptor cells are two types, rods and cones

The photosensitive element is rhodopsin, which is composed of opsin and 11-cis retinal (an aldehyde of vitamin A), are located in the membrane of the outer segment of rods and cones.

Wiring: each cone has its own bipolar and ganglion cell while several rods share one bipolar and ganglion cell

The Retina: Ganglion Cells and the Optic Disc

• Ganglion cell axons:– Run along the inner surface of the retina– Leave the eye as the optic nerve

• The optic disc:– Is the site where the optic nerve leaves the eye– Lacks photoreceptors

(the blind spot)

The rod system consists of rods and subsequent bipolar cells and ganglion cells, which correlate with the rods.

The rod system consists of rods and subsequent bipolar cells and ganglion cells, which correlate with the rods.

Rods see in low light but lack detail.

Rods see in low light but lack detail.

The cone system is composed of cones and subsequent bipolar cells and ganglion cells.

The cone system is composed of cones and subsequent bipolar cells and ganglion cells.

Cones see detail but require bright light

Cones see detail but require bright light

Retinal – Light Sensitive Pigment11-cis-Retinal - All-trans-Retinal

Light

Dark

When light energy is absorbed by rhodopsin, the rhodopsin begin within trillionths of a second to decompose.

This decomposition converts 11-cis retinal (bent shape form) into all-trans retinal (straight chain form)

Photoisomerization of rhodopsin

In order to maintain the ability to detect light, the rods must reconvert the all-trans retinal into 11-cis retinal.

This process requires metabolic energy and is catalyzed by the retinal isomerase.

This process only occurs under the dark environment.

Under the dim light, the 11-cis and 11-trans keep dynamic balance

Light

Dark

rhodopsin 11-cis retinal + opsin

all-trans retinal+ opsin

isomerase

11-cisretinal

All-transretinal

opsin opsin

All-trans retinol is one form of vitamin A

Vitamin A is present both in the cytoplasm of the rods and in the pigment layer of the retina.

Therefore, vitamin A is always available to form new retinal when needed.

Night blindness.

Rhodopsin Cycle

Rod Cell Hyperpolarization

Theories of Color Vision

• Trichromatic theory– Occurs at the receptor level– Primary Colors: sets of 3 colors that can be mixed

to produce any other color– Short-wave (blue)

Medium-wave (green)

Long-wave (red)– Ratio of activated cones = color differentiation–99%red,42%green,0%blue =Orange–83%red,83%green,0%blue =Yellow

Color Blindness

• Sex-linked conditions: Genes on X chromosome, so more common in men.– Protanopia, missing red photopigment– Deuteranopia, missing green photopigment

• Non-sex-linked condition– Tritanopia, missing blue photopigment or

blue cones

–monochromats: people who are totally colorblind, more severe

•It is often assessed using the visual acuity chart, with its rows of different sized letters.

•It is often assessed using the visual acuity chart, with its rows of different sized letters.

Concept Dark Adaptation

From brightly lighted surrounding to a dimly lighted environment, the retinas slowly become more sensitive to light.

Light Adaptation:When one passes suddenly from a dim to a brightly lighted environment, the light seems intensely and even uncomfortably bright until the eyes adapt to the increased illumination and the visual threshold rises.

Light Adaptation:When one passes suddenly from a dim to a brightly lighted environment, the light seems intensely and even uncomfortably bright until the eyes adapt to the increased illumination and the visual threshold rises.

Cones adaptation: early, rapid but small in amplitude

Rods adaptation: late, slowly and great in amplitude

Dark AdaptationDark Adaptation

Dark Adaptation

• dark adaptation: increased sensitivity of rods and cones in darkness

—e.g., entering a darkened room

• Cones: adapt for 10 minutes bet never become very sensitive

• Rods: continue adapting for 30 minutes and become much more sensitive

Light Adaptation

• Exposure to very intense light initial

glare large scale bleaching of pigment Decrease of sensitivity of retina Retinal neurons switch from rod to

cone system for reception

Visual fields• The visual field may be defined as the region

of space within which a visual stimulus can be detected without moving the eye. There is also a blind spot on the temporal side of the field for each eye. Images from this area fall on the optic disc on the nasal side of the retina, where there are no photoreceptors.

white blue red green﹥ ﹥ ﹥

Sound

• Sound energy is transmitted through a gaseous, liguid, or solid medium by setting up a vibration of the medium’s molecules, air being the most common medium.

• When there are no molecules, as in a vacuum, there can be no sound.

Basilar Membrane oscillations at different sound frequencies

Effect of Sound Waves on Cochlear Structures

Hair cell transduction channels

Summary

1. Dark Adaptation

2. Light Adaptation