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Chapter 18: Senses


Chapter 18: Senses Sensory Receptors and Sensations Sensory receptors are specialized to detect certain types of stimuli. Each type of sensory receptor responds to a ... – PowerPoint PPT presentation

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Title: Chapter 18: Senses

Chapter 18 Senses
Sensory Receptors and Sensations
  • Sensory receptors are specialized to detect
    certain types of stimuli.
  • Each type of sensory receptor responds to a
    particular kind of stimulus.
  • Exteroceptors (hearing, sight receptors, for
    example) detect stimuli from outside the body.
  • Interoceptors receive stimuli from inside the
    body they are directly involved in homeostasis.

Types of Sensory Receptors
  • Chemoreceptors respond to chemical substances,
    such as changes in pH, or the senses of taste and
  • Pain receptors are chemoreceptors that respond to
    chemicals from damaged tissues.
  • Mechanoreceptors respond to mechanical forces.
  • The senses of hearing and balance both involve

  • Proprioceptors (mechanoreceptors) in tendons
    around joints make us aware of position
    pressoreceptors in arteries detect blood pressure
    changes, and stretch receptors in lungs detect
    degree of inflation.
  • Thermoreceptors respond to temperature changes
    there are both warm receptors and cold receptors.
  • Photoreceptors respond to light energy.
  • Special photoreceptors called rods result in
    black-and-white vision, while cones detect color.

How Sensation Occurs
  • Sensation occurs when nerve impulses reach the
    cerebral cortex.
  • Perception is an interpretation of the meaning of
  • The sensation that results depends on the part of
    the brain receiving the impulses.
  • Receptors may integrate signals before sending
    nerve impulses.
  • Sensory adaptation occurs when a stimulus
    continues but the receptor decreases its

Proprioceptors and Cutaneous Receptors
  • Proprioceptors
  • Proprioceptors help us know the position of our
    limbs in space.
  • Proprioceptors include muscle spindles that are
    stimulated when muscle fibers stretch a reflex
    is initiated and the muscle tightens in
    proportion to the degree of stretch.

  • These proprioceptors allow the muscles to
    maintain the proper length and tension, or muscle
  • The knee-jerk reflex involves muscle spindles.
  • Signals to the CNS from muscle spindles help
    maintain balance and posture.
  • Golgi tendon organs are proprioceptors with the
    opposite effect.

Muscle spindle
Cutaneous Receptors
  • The dermis of the skin contains sensory receptors
    for touch, pressure, pain, and temperature
    (warmth and cold).
  • Three types of cutaneous receptors are sensitive
    to fine touch
  • Meissner corpuscles are concentrated in finger
    tips, lips, tongue, nipples, and genital areas
  • Merkel discs are found where the epidermis meets
    the dermis and

  • 3) free nerve endings (root hair plexus) around
    hair follicles all detect touch.
  • Three different types of pressure receptors are
    Pacinian corpuscles, Ruffini endings, and Krause
    end bulbs.
  • Temperature receptors are simply free nerve
    endings in the epidermis some are responsive to
    cold and others are responsive to warmth,
    although there are no structural differences
    between them.

Sensory receptors in human skin
  • Pain Receptors
  • Nociceptors are pain receptors on internal organs
    and may be sensitive to temperature, pressure, or
  • Referred pain occurs when stimulation of internal
    pain receptors is felt as pain from the skin.
  • Referred pain most likely happens because of
    shared nerve pathways between the skin and
    internal organs.

Chemical Senses
  • Chemoreceptors in the carotid arteries and aorta
    respond to the pH of the blood and communicate
    with the medulla oblongata to change breathing
  • For example, when blood pH drops, these
    chemoreceptors signal the medulla respiratory
    center that triggers breathing rate to increase
    expiration of CO2 raises the pH of the blood to
  • Taste and smell are chemical senses.

Sense of Taste
  • The taste buds located in papillae on the tongue
    contain taste cells that communicate with sensory
    nerve fibers.
  • Microvilli on taste cells contain receptor
    proteins that match chemicals in food.
  • The brain determines the taste according to a
    weighted average of incoming impulses from
    taste buds sensitive to either sweet, sour,
    salty, or bitter tastes.

Taste buds
Sense of Smell
  • Olfactory cells (modified neurons) are located in
    epithelium in the roof of the nasal cavity.
  • After molecules bind to receptor proteins on the
    varied cilia of olfactory cells, nerve impulses
    lead to olfactory areas of the cerebral cortex.
  • The perceived odor is determined by the
    combination of olfactory cells stimulated.
  • The effects of smell and taste combine.

Olfactory cell location and anatomy
Sense of Vision
  • Vision is dependent on the eye and the visual
    areas of the cerebral cortex.
  • It is estimated that at least one-third of the
    cerebral cortex is involved in processing visual

Anatomy of the Eye
  • The eye has three layers.
  • The sclera is the outer layer seen as the white
    of the eye and includes the transparent bulge in
    the front of the eye called the cornea.
  • The choroid is the middle, darkly pigmented layer
    that absorbs stray light rays it also becomes
    the iris that regulates the size of the pupil.

  • Behind the iris, the choroid thickens and forms
    the ciliary body.
  • The ciliary body contains the ciliary muscle,
    which controls the shape of the lens for near and
    far vision.
  • The lens divides the eye into two compartments
    the anterior compartment (containing aqueous
    humor) and the posterior compartment (containing
    vitreous humor).

  • Rod cells and cone cells are located in the
    retina that forms the inner layer.
  • The retina lines the back half of the eye and has
    cone cells densely packed in one area called the
    fovea centralis.
  • Sensory fibers from the retina form the optic
    nerve leading to the brain.

Anatomy of the human eye
  • The cornea and the lens focus light rays on the
  • To see a close object, the ciliary muscles change
    the lens shape to provide visual accommodation.
  • After age 40, the lens is less able to
    accommodate and near vision is less acute.
  • Cataracts occur when the lens becomes opaque sun
    exposure might be a factor in developing

  • Both rod cells and cone cells have an outer
    segment with membranous disks containing embedded
  • Rods contain a deep purple pigment called
    rhodopsin that is composed of retinal (made from
    vitamin A) and the protein opsin.
  • Rods are numerous and provide peripheral vision,
    perception of motion, and vision in dim light at

  • When a rod absorbs light, rhodosin splits into
    opsin and retinal, leading to a cascade of
    reactions and the closing of rod membrane ion
  • Inhibitory neurotransmitters are no longer
    released from the rod.
  • Breakdown of rhodopsin in rods thus initiates
    nerve impulses.
  • Cones have three different pigments (red, green
    and blue) made from retinal and opsin opsin
    varies between the three.

Photoreceptors in the eye
Integration of Visual Signals in the Retina
  • The retina has three layers of neurons rods and
    cones are near the retina, bipolar cells are in
    the middle, and the innermost layer contains
    ganglion cells that carry impulses to the optic
  • The rod and cones synapse with the bipolar cells,
    which in turn synapse with ganglion cells that
    initiate nerve impulses.

  • As signals pass from one layer to the next,
    integration occurs because each layer contains
    fewer cells than the previous layer.
  • However each cone connects directly to one
    ganglion cell, while a hundred rods may synapse
    with only one ganglion cell.
  • It is likely that much processing occurs in the
    retina before impulses are sent to the brain.
  • There are no rods and cones where the optic nerve
    exits the retina this is the blind spot.

Structure and function of the retina
Integration of Visual Signals in the Brain
  • The visual pathway begins with the retina and
    passes through the thalamus before reaching the
    cerebral cortex.
  • The visual pathway and the visual cortex split
    the visual field apart, but the visual
    association areas rebuild it so we correctly
    perceive the entire visual field.

Optic chiasma
Abnormalities of the Eye
  • Color Blindness
  • The most common abnormality is a lack of red
    and/or green cones.
  • Distance Vision
  • Nearsighted individuals (elongated eyeball)
    cannot see distant objects this is corrected by
    a concave lens.
  • Farsighted individuals (shortened eyeball) see
    distant objects well but not up close this is
    corrected by a convex lens.
  • Astigmatism occurs with an uneven cornea or lens.

Common abnormalities of the eye
Sense of Hearing
  • The ear has two sensory functions hearing and
  • The sensory receptors for both senses are located
    in the inner ear, and both use a type of
    mechanoreceptor consisting of hair cells with
    stereocilia (long microvilli).

Anatomy of the Ear
  • The ear is divided into three parts.
  • The outer ear consists of the pinna and the
    auditory canal, which direct sound waves to the
    middle ear.
  • The middle ear begins at the tympanic membrane
    (eardrum) and contains the ossicles the malleus,
    incus, and stapes that amplify sound waves.

  • The malleus is attached to the tympanic membrane,
    and the stapes is attached to the oval window,
    which is covered by membrane.
  • The inner ear contains semicircular canals and
    vestibule involved in equilibrium, and the
    cochlea for hearing.

Anatomy of the human ear
Process of Hearing
  • Sound waves enter the auditory canal and vibrate
    the tympanic membrane.
  • If the vibrations are strong enough, the outer
    and middle portions (ossicles) of the ear convey
    and amplify the sound waves about 20 times and
    vibrate against the oval window.
  • These vibrations set up pressure waves within the
    fluid of the cochlea.

  • The cochlea contains the spiral organ consisting
    of hair cells on the basilar membrane whose
    stereocilia are embedded within the tectorial
  • Vibrations within the cochlea cause the
    sterocilia to vibrate against the tectorial
    membrane, thus generating nerve impulses.

  • Different regions are sensitive to different
    frequencies or pitch.
  • When the stereocilia of the hair cells bend,
    nerve impulses are generated in the cochlear
    nerve and are carried to the brain.

Mechanoreceptors for hearing
Sense of Equilibrium
  • Rotational Equilibrium
  • Rotational equilibrium depends on the stimulation
    of hair cells within the ampullae of the
    semicircular canals.
  • Continuous movement of fluid within the canals
    can cause motion sickness.
  • Vertigo is dizziness from a sensation of spinning.

  • Gravitational Equilibrium
  • Stimulation of hair cells within the utricle and
    the saccule, two sacs located in the vestibule,
    by the slippage of calcium carbonate granules or
    otoliths, provide impulses that tell the brain
    the direction of movement of the head.
  • The movement of the otoliths provides a sense of
    gravitational equilibrium.

Mechanoreceptors for equilibrium
Chapter Summary
  • Sensory receptors respond to specific
    environmental stimuli.
  • Sensation occurs in the brain when sensory
    receptors send nerve impulses to the brain.
  • Senses are divided into exteroceptors that detect
    stimuli from outside the body, and interoceptors
    that receive stimuli from inside the body.

  • Proprioceptors in muscles and joints help the
    body maintain balance and posture.
  • Cutaneous receptors in the skin respond to touch,
    pressure, pain, and temperature (both warmth and
  • In the mouth, the microvilli of taste cells have
    membrane protein receptors that respond to
    certain molecules.

  • Olfactory cells within the olfactory epithelium
    respond to molecules and result in a sense of
  • Photoreceptors for sight contain visual pigments,
    which respond to light rays.
  • Some integration occurs in the retina of the eye
    before nerve impulses are sent to the brain.

  • Sensory receptors for hearing are hair cells in
    the cochlea of the inner ear that respond to
    pressure waves.
  • Sensory receptors for balance are hair cells in
    the vestibule and semicircular canals of the
    inner ear that respond to the tilt of the head
    and to the movement of the body, respectively.