The Special Senses

Chapter 45

The Senses

• Senses do not only apply to telling us about the external environment (hearing, sight, taste, and smell)

• They tell us about the internal state of the body as well (stretching of muscles, blood pressure, and body position)

Sensory Receptors

• Two types of receptors– Exteroreceptors – sense stimuli that originate externally– Interoreceptors – sense stimuli that originate from

within the body• Three Categories

– Mechanoreceptors – stimulated by mechanical forces like pressure

– Chemoreceptors – detect chemicals or chemical changes

– Energy-detecting Receptors – react to electromagnetic and thermal energy like light

Conveying Sensory info

1. Stimulation – a stimulus impinges on a sensory neuron or receptor

2. Transduction – stimulus energy is transformed into potentials in the dendrites

of the sensory neuron3. Transmission – action potentials develop in the

axon of the sensory neuron and are conducted to the CNS

4. Interpretation – the signal to the brain is interpreted by a perception present within the brain

Gated Ion Channels

• Sensory cells respond to stimuli because they have stimulus gated ion channels– These channels open when exposed to the

proper stimulus– This in most cases depolarizes the receptor and

initiates the formation of an action potential which is conveyed to a sensory nerve pathway

– An action potential is only formed when the threshold of the sensory nerve is reached

Mechanoreceptors: Touch and Pressure

• Cutaneous receptors- in the skin– Sense heat, cold, pain, touch, and pressure– Pain Receptors (nociceptors) – alert body to

damage or potential damage• While there are specialized ones many other

receptors when hyper-stimulated can produce a pain perception in the brain.

• Most are free nerve endings located throughout the body and respond to different types of stimuli

Touch Receptors

• Meissner Corpuscle– Receptor sensitive to

fine touch, concentrated in hairless skin (fingertips, face)

– Shallow in the epidermis to sense slight pressure changes

Touch Receptors

• Pacinian Corpuscle– Deep pressure

receptors located in subcutaneous tissue

– Is only stimulated when internal nerve ending is displaced

Proprioceptors

• Muscle length and tension are measured by interoceptors called proprioceptors.

• This sensation allows the brain to be aware of body position without sight

• E.g. when your eyes are closed you still know where your arms and legs are, whether they are bent or not

Baroreceptors

• Blood pressure is measured in 2 areas of the body, the aortic arch and the carotid sinus (as the blood enters the brain)– The walls of vessels in these areas contain

baroreceptors which detect tension and stretching of the walls.

– These signals are perceived by the brain as blood pressure

– The CNS responds appropriately to an increase or decrease in the signals sent by these baroreceptors

Hearing

• Hair Cell– As stereocilia are

displaced toward the kinocilium, the nerve is stimulated

– As stereocilia are displaced away from the kinocilium, the nerve is inhibited

The Ear

• Organized to concentrate sound waves and channel them to tympanic membrane.

• Vibrations of the tympanic membrane cause movement of the middle ear bones: malleus (hammer), incus anvil, and stapes (stirrup)

• The stapes vibrates against the flexible membrane known as the oval window

• Because the oval window is much smaller in diameter than the tympanic membrane, vibrations against it produce more force heading into the inner ear

The Ear

The Cochlea

Cochlea

• The vibration of the oval window forms pressure waves in the fluid of the vestibular canal

• The wave travels all the way to the round window where the pressure can again be released to the middle ear.

• These pressure waves also cause the fluid in the cochlear duct to vibrate

• The bottom of the cochlear duct is called the basilar membrane

Cross section of the Cochlea

The Organ of Corti

• The basilar membrane is covered in sensory hair cells

• The stereo cilia of these cells is embedded in the gelatinous tectorial membrane.

• As the basilar membrane is vibrated by the pressure waves, the two membranes slide past one another causing the cilia to be displaced, this creates the action potential which is transmitted to the brain as sound.

Frequency Localization

• Different frequencies of sound will cause a maximal reaction at different points within the cochlea– High frequencies cause maximal reaction near

the beginning of the cochlea– Low frequencies cause their maximal reaction

near the end of the cochlea

• These differences allow us to hear different frequencies of sound (20-20,000 Hz)

Vestibular Apparatus

• Used to Maintain balance• The Utricle and Saccule

– Sense Horizontal and Vertical Acceleration– Each contain hair cells embedded in a gelatinous membrane

(Otolith membrane)• The otolith organ is oriented differently in each so they detect motion

in different directions• When the hairs are displaced the brain interprets a change in position.

• The Semicircular Canals– Oriented in 3 different plains– Contain a similar organ with hair cells that allow for sensing

angular motion

Taste

• Five Categories– Sweet – G-protein pathway– Sour – detected by H+ presence– Salty – detected by Na+ presence– Bitter – G-protein pathway– Umami – G-protein pathway

• Taste Buds are collections of chemoreceptors located on the sides of raised bumps called papillae

• Chemicals from food dissolve in saliva and enter the sensory cell through a taste pore

Smell

• Chemoreceptors in the upper nasal passage– Airborne chemicals dissolve in the mucous

surrounding the receptor cells– These dissolved chemicals cause potentials in

the receptors that are conveyed to the brain and interpreted as smells.

Vision

• Photoreceptors capture light energy and relay images to the brain through electical signals

The Eye• Sclera – the white of the eye• Cornea – transparent covering

that light enters the eye through

• Iris – colored portion of the eye, contraction limits amount of light that is allowed through the pupil (the opening)

• Lens – Transparent structure that focuses light onto the retina– Muscles stretch the lens to

focus light differently, this is how the eye focuses

– People who are near or far sighted do not focus light correctly onto the retina

Rods and Cones• Rods

– responsible for black/white vision in dim light– ~100 million per eye– Almost none in the fovea– Photopigment rhodopsin

• Cones– Responsible for high visual acuity (sharpness)– ~3 million per eye– Most located in the central fovea– Photopigment photopsin (3 different ones with slightly different

maximum absorption ranges)– Other species can have more or less cones that can allow them to

see a broader or more narrow range of the spectrum than humans can

Absorption Spectrum

The Retina

• 3 Layers of Cells– Photoreceptor layer (rods and cones)– Bipolar Cells – relay from photoreceptor to ganglion

cells– Ganglion Cells – transmit to optic nerve

• Light must pass through Ganglion and Bipolar cell layers to reach the photoreceptors

• Horizontal Cells – can inhibit nearby signal transduction increasing sharpness and sharpening an image (lateral inhibition)

The Confusing Part

1. Rods and Cones actually are active in the lack of light (they release an inhibitory neurotransmitter to the bipolar cells)

2. When light enters the eye, the rods and cones Stop releasing their neurotransmitter

3. At this point the bipolar cells can fire and release their neurotransmitters to ganglion cells which in turn create signals that are transmitted to the optic nerve.

The Fovea

• Used for high acuity vision (all cones)– Each cone has a 1 to 1 connection with a

bipolar cell and a ganglion cell– Outside the fovea many rods may synapse on a

single bipolar cell this makes this area more sensitive to dim light

Color Blindness

• Results from the lack of one or more type of cone

• Sex linked recessive trait– Therefore found mostly in males XbY– XbXb only possibility for females

Blind Spot• A spot where there is a lack of rods and cones on the

retina

• All have one where the optic nerve converges and heads out of the eye

X O