Lab4 sensory

SPECIAL SENSES

Last revised: 04/12/2023

The Senses

Sensory receptors transduce different forms of energy in the “real world” into nerve impulses.

Different sensory perceptions (sound, light, pressure) arise from differences in neural pathways. If the optic nerve delivers an impulse, the brain interprets it as

light.

Functional Categories of Sensory Receptors

Receptors can be classified according to the type of signal they transduce:

Chemoreceptors – sense chemicals in the environment Taste, smell, or blood

Photoreceptors – sense light Thermoreceptors – respond to cold or heat Mechanoreceptors – stimulated by mechanical deformation of the

receptor. Touch Hearing

Nociceptors – sense pain; damaged tissue release chemicals that excite sensory endings

Nociceptors

Pain receptors that depolarize when tissues are damaged. Stimuli can include heat, cold, pressure, or chemicals Glutamate and substance P are the main neurotransmitters. May be activated by chemicals released by damaged

tissues, such as ATP. Perception of pain can be enchanced by emotions and

expectations Pain reduction depends on endogenous opioids.

Nociceptors can be myelinated or unmyelinated Sudden, sharp pain is transmitted by myelinated

neurons. Dull, persistent pain in transmitted by unmyelinated

neurons.

Tonic and Phasic Receptors

Receptors can be categorized based on how they respond to a stimulus.

Phasic: respond with a burst of activity when a stimulus is first applied but quickly decrease their firing rate—adapt to the stimulus—if the stimulus is maintained. (fast-adapting)

Alerts us to changes in the environment Allow sensory adaptation Smell, touch, temperature

Tonic: maintain a high firing rate as long as the stimulus is applied. (slow-adapting)

Cutaneous Receptors Pain, cold, and heat receptors are naked

dendrites Cold receptors – located close to epidermis Warm receptors – located deeper in the dermis. Hot receptors – pain experienced by a hot stimulus is

sensed by a special nociceptor called a capsaicin receptor.

Touch and pressure receptors have special structures around their dendrites.

Meissner’s corpuscles Encapsulated dendrites in connective tissue Changes in texture and slow vibration

Pacinian corpuscles Encapsulated dendrites by concentric lamellae of

connective tissue structures Deep pressure and fast vibrations

Ruffini endings Sustained pressure Enlarged dendritic endings with open, elongated

capsule Merkel’s discs

Expanded dendritic endings Sustained touch and pressure Slow adapting

Two-Point Threshold Test

Measures the density of touch receptors The minimum distance at which two points of contact

can be felt.

High density of receptive fields = shorter minimum distance

Low density of receptive fields = longer minimum distance

HEARING AND EQUILIBRIUMThe Ears

Vestibular Apparatus

Provides a sense of equilibrium Located in the inner ear Consists of:

Otolith organs Linear acceleration

Utricle (horizontal) Saccule (vertical)

Semicircular canals Rotational acceleration

Both structures in the vestibular apparatus are: Filled with endolymph Contain sensory hair cells which

are activated by bending.

Sensory Hair Cells

Hair cells

Sensory nerve fiber Supporting cells

Otoliths

(a) Head upright (b) Head bent forward

Maculaof utricle

Hairs ofhair cells bend

Gelatinousmaterial sags

Gravitationalforce

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Hair cell

Supporting cells

Sensory (afferent) nerve fibers

Hairs

Cupula Cristaampullaris

(a) Head in still position

(b) Head rotating


(c)

Crista ampullaris

Semicircular canal

Endolymph

Ampulla


Semicircular Canals

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Clinical Applications

Nystagmus Involuntary oscillations of the eyes

when spinning is suddenly stopped.

Eyes continue to move in the direction of the spin, then jerk rapidly back to the midline.

When a person begins spinning, the cupula bends in the opposite direction.

If the movement suddenly stops, inertia of endolymph causes it to continue moving in the direction of the spin.

This is a normal phenomenon that helps maintain balance during spinning, however, nystagmus can also be a symptom of certain diseases, like Meniere’s disease.

Vertigo Loss of equilibrium with the illusion

of spinning May be caused by anything that alters

the firing rate of one of the vestibulocochlear nervers.

May be due to spinning or pathologically induced by by viral infections.

Tx: Antivert® (meclizine) Anticholinergic action Blocks conduction in the middle ear

vestibular-cerebellar pathways.

Anatomy of the Ear

Structures of the Middle Ear

Cavity between the tympanic membrane and the cochlea

Contains three bones called ossicles: Malleus Incus Stapes Vibrations are transmitted

and amplified along the bones.

The stapes is attached to the oval window, which transfers the vibrations into the inner ear.

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• Group of hearing receptor cells, called hair cells.

• On upper surface of basilar membrane

• Different frequencies of vibration move different parts of basilar membrane

• Particular sound frequencies cause hairs of receptor cells to bend

• Nerve impulse generated

Spiral organ (organ of Corti)

Hair cells

Basilar membrane

(a)

(b)

Scala vestibuli(contains perilymph)

Cochlear duct(contains endolymph)

Scala tympani(contains perilymph)

Branch ofcochlearnerve

Tectorialmembrane

Basilarmembrane

Supportingcells

Nervefibers

Branch ofcochlear nerve

Vestibular membrane


Organ of Corti

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Figure 10-20 Sound transmission through the ear

Sound waves strikethe tympanicmembrane andbecome vibrations.

The sound waveenergy is transferredto the three bonesof the middle ear,which vibrate.

The stapes is attached tothe membrane of the ovalwindow. Vibrations of theoval window create fluidwaves within the cochlea.

The fluid waves push on theflexible membranes of thecochlear duct. Hair cells bendand ion channels open,creating an electrical signal thatalters neurotransmitter release.

Energy from the wavestransfers across thecochlear duct into thetympanic duct and isdissipated back intothe middle ear at theround window.

Neurotransmitter releaseonto sensory neuronscreates action potentialsthat travel through thecochlear nerve tothe brain.

Ear canal Malleus

Incus

Stapes

Cochlear nerve

Vestibular duct(perilymph)

Roundwindow

Tympanicmembrane

Cochlear duct(endolymph)

Tympanic duct(perilymph)

Ovalwindow

Clinical Applications

Conduction deafness Sound waves are not conducted

from the outer to inner ear. May be due to a buildup of

earwax, too much fluid in the middle ear, damage to eardrum, or overgrowth of bone in the middle ear.

Impairs hearing of all sound frequencies.

Can be helped by hearing aids.

Sensorineural deafness Nerve impulses are not conducted

from the cochlea to the auditory cortex. May be due to damaged hair cells. May only impair hearing of a

particular sound frequency. May be helped by cochlear

implants.

VISIONThe Eyes

Image is inverted on retina due to refraction of light.

Degree of refraction depends on: Refractive index (RI) of

media RI of air = 1.00 RI of cornea = 1.38

Curvature of the interface between the two media.

Functional Anatomy of the Eye

Functional Anatomy of the Eye

Rods: Provide black and white vision under low light intensities

Cones: Provide sharp color vision when light intensity is great Humans have trichromatic

vision due to the presence of three different types of cones: Blue, Green, and Red.

Photoreceptors

Visual Acuity

Sharpness of vision Depends upon resolving power

Ability of the visual system to resolve two closely spaced dots

Visual Abnormalities Myopia (nearsightedness)

Hyperopia (farsightedness)

Astigmatism uneven cornea or lens

Presbyopia hardening of the lens impedes accommodation