2. 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.
3. 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
4. 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.
5. 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)
6. 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
7. 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
9. 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.
13. Clinical Applications
Nystagmus Vertigo
Involuntary oscillations of the eyes Loss of equilibrium with the
when spinning is suddenly illusion of spinning
stopped. May be caused by anything that alters
the firing rate of one of the
Eyes continue to move in the vestibulocochlear nervers.
direction of the spin, then jerk May be due to spinning or
rapidly back to the midline. pathologically induced by by viral
When a person begins spinning, the cupula infections.
bends in the opposite direction.
If the movement suddenly stops, inertia of
Tx: Antivert® (meclizine)
endolymph causes it to continue moving in Anticholinergic action
the direction of the spin. Blocks conduction in the middle ear
This is a normal phenomenon that helps
vestibular-cerebellar pathways.
maintain balance during spinning, however,
nystagmus can also be a symptom of certain
diseases, like Meniere’s disease.
15. 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.
18. Clinical Applications
Conduction deafness Sensorineural deafness
Sound waves are not conducted Nerve impulses are not conducted
from the outer to inner ear. from the cochlea to the auditory
May be due to a buildup of earwax, cortex.
too much fluid in the middle ear, May be due to damaged hair cells.
damage to eardrum, or May only impair hearing of a
overgrowth of bone in the middle particular sound frequency.
ear.
May be helped by cochlear
Impairs hearing of all sound implants.
frequencies.
Can be helped by hearing aids.
20. Functional Anatomy of the Eye
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.
22. Photoreceptors
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.
23. 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