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Special Senses and Functional Aspects of the Nervous System


... Sensory Pathways Conduction pathways that carry impulses from complex receptors, such as those found within special sensory organs like the eye and ear, ... – PowerPoint PPT presentation

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Title: Special Senses and Functional Aspects of the Nervous System

  • Special Senses and Functional Aspects of the
    Nervous System

  • Overall function of the nervous system is the
    maintenance of homeostasis.
  • This is achieved by three ways
  • Monitoring the body and the external environment
    for changing conditions (sensory function)
  • Processing this sensory info to coordinate the
    proper response (integrative function)
  • Stimulating the response (motor function)

  • Sensations- a sensation is a state of awareness
    of the external or internal conditions of the

  • All sensory pathways begin with a stimulus, or
    change in the environment that is great enough to
    initiate a nerve impulse.
  • The stimulus is converted to a nerve impulse by a
  • From the receptor, the impulse is conducted along
    a sensory neuron to the CNS.

  • Sensory receptors are variable in structure.
  • Ex.a receptor may be the simple dendrites of a
    sensory neuron that end bare in the skin, or it
    may be a complex organ such as the eye of ear.
  • However, all receptors share in common the
    capability to excite.

  • They are capable of generating an action
    potential(nerve impulse).
  • Receptors are sensitive to a particular
    environmental change but insensitive to all
    others. Exthe receptors in the eye have a low
    threshold to light and will initiate a nerve
    impulse in response to it, but have a very high
    threshold to slight changes in temp. and will not
    respond to it.
  • Thus, receptors are stimulus-specific.

Sensory adaptation
  • In many receptors, the threshold level for a
    particular stimulus may rise after continuous
    stimulation. This phenomenon is called sensory
  • In these receptors the impulses are generated at
    decreasing rates until they stop completely and
    sensation ends.

Types of receptors
  • Receptors are classified according to their types
    of sensitivity.
  • Mechanoreceptors detect a mechanical or physical
    change in the receptor or nearby cells. They are
    sensitive to touch, pressure, muscle tension,
    hearing, equilibrium, and blood pressure.

  • Thermoreceptors detect temperature changes.
  • Nociceptors detect pain that usually results from
    chemical or physical damage to nearby cells.
  • Photoreceptors are sensitive to changes in the
    amount of light and are present only in the
    retina of the eye.
  • Chemoreceptors detect chemicals dissolved in
    fluid, providing the senses of smell and taste.
    They also provide the detection of O2 and CO2

General Sensory Pathways
  • Conduction pathways that carry impulses from a
    simple receptorsuch as those found in the skin,
    visceral organs, and musclesto the brain are
    known as general sensory pathways.

  • From the receptor, the impulse is conducted along
    three sensory neurons before it reaches its
    destination in the brain.
  • 1. First-order neuron---A single sensory neuron
    connects the receptor to the spinal cord.
  • 2. It synapses with a second-order neuron that
    conducts the impulse upward to the thalamus.
  • 3. From the thalamus, a third-order neuron
    conducts the impulse to the cerebral cortex for

Special Sensory Pathways
  • Conduction pathways that carry impulses from
    complex receptors, such as those found within
    special sensory organs like the eye and ear, are
    called special sensory pathways.
  • Basically, a special sensory pathway includes at
    least three sensory neurons that connect a
    receptor with a specific region of the cerebral

  • Since the special senses are located in the head
    (eye, ear, tongue, and nasal epithelium), the
    impulses travel along cranial nerve routes to the
  • Impulses entering the brain pass through the
    thalamus before continuing to the cerebral cortex.

General Senses
  • The general senses are sensations detected by
    simple receptors.
  • They are touch and pressure, temperature, pain,
    and body position.
  • Impulses travel from these receptors along
    general sensory pathways to the cerebral cortex,
    where the sensations are interpreted and

Touch and Pressure
  • The sensations of touch and pressure are often
    called cutaneous sensations, because many of
    their receptors are located within the skin.
  • They are detected by mechanoreceptors, which are
    widely distributed.

  • The sense of touch is primarily detected by
    Meisseners corpuscles.
  • Meissners corpuscles are abundant in the skin of
    the fingers, palms, soles, lips, and external
  • A second, less common set of touch receptors in
    the skin are Merkels discs, whose dendrites end
    in the skins epidermis.

  • The sense of pressure requires a greater
    mechanical disturbance than touch and is
    longer-lasting and felt over a larger area.
  • Its receptors are located within the deep regions
    of the skin, around joints and tendons, and in
    certain visceral organs, and they respond to
    heavy pressure.
  • These receptors are known as Pacinian corpuscles.

  • Although it is poorly understood, it is thought
    to be detected by the free nerve endings in the
    skin that are stimulated by temperature changes.
  • As extreme temps are reached (lt10C and gt than
    45C), pain receptors are also triggered,
    producing a burning sensation.

  • The sensation of pain is a necessary evil, for
    it provides an important protective function.
  • Pain is detected by branching dendrites of
    sensory neurons that end freely throughout the
    skin, muscles, and most visceral organs.
  • It is caused by chemicals that are released
    during an event involving the damage or
    destruction of cells.

  • A restricted blood flow to a body part may also
    stimulate pain receptors.
  • Pain may be caused by receptors other than the
    specialized pain receptors. It is caused by
    excessive stimulation of any type of receptor.
    Ex. photoreceptors (bright light)
  • Pain that is generated by pain receptors located
    in the walls of visceral organs is known as
    visceral pain.

  • Visceral pain is hard to trace because it travels
    along pathways that are shared by other neurons
    leading to the brain. (cerebral cortex).
  • This is known as referred pain.
  • The conduction pathway of an impulse traveling
    from the source of pain extends along sensory
    neurons to the cerebral cortex, where recognition
    of the intensity of pain occurs.

  • It is possible to reduce the intensity of pain by
    interfering with the transmission of impulses
    along this pathway.

Body Positions
  • You are made aware of the position of your body
    by the activities of muscles, tendons, joints,
    and equilibrium centers in the inner ear.
  • The receptors that detect these activities are
    called proprioceptors.
  • They provide info on the degree of muscle
    contraction, tension in the tendons, position of
    a joint, and the position of the head relative to
    the ground.

  • They allow you to control your body movements
    without using your sense of vision, hearing,
    touch, or any other senses.
  • The primary receptors of body position are muscle
    spindles in skeletal muscle and tendon organs
    between a tendon and a skeletal muscle.

  • Muscle spindles are stimulated by muscle stretch
    and can monitor the actual length of a skeletal
    muscle during its contraction cycle.
  • Tendon organs are stimulated by tension in the
    surrounding tendon.
  • Other proprioceptors are located in and around
    joints and within the inner ear.

  • The four special senses are

  • These specialized, complex structures are true
    organs since they are composed of two or more
    tissue types.
  • They are, therefore, properly termed special
    sensory organs.

  • The sense of smell is also known as olfaction.
  • Detected by thousands of chemoreceptors located
    in the upper wall of the nasal cavities.
  • Because olfactory organs are located high in the
    nasal cavities out of the usual pathway of
    inhaled air, they are in a poor position to
    collect scents.
  • The olfactory organs provide many individual
    sensations of smell, perhaps 50 or more.

  • The sense of smell declines with age.
  • Olfactory pathways are closely linked to the
    limbic system of the brain, which is the center
    for emotional expression.
  • Olfactory sensations are long-lasting and form a
    very important part of our memories and emotions.
  • Ex.a particular smell sensation sometimes has
    enough impact to sweep us back to a memory of an
    experience thought to be long-forgotten.

  • The sense of taste is celled gustation.
  • Four primary types of taste sensations
  • 1. Sweet
  • 2. Sour
  • 3. Bitter
  • 4. Salty

  • The sense of hearing, or audation, is a very
    specialized sense.
  • It is combined with the sense of equilibrium.
  • The ear consists of three portions
  • 1. The outer ear
  • 2. The middle ear
  • 3. The inner ear

  • The outer ear consists of the external appendage
    on each side of the head, called the auricle, and
    the tube that extends into the temporal bone,
    known as the external auditory canal.
  • The canal occupies the space in the bone formed
    by the external auditory meatus and contains
    glands in its lining of the skin that secrete a
    waxy substance called cerumen (ear wax).
  • The canal channels the sound waves to the
    eardrum, which is the entrance to the middle ear.

  • The middle ear contains an air-filled space
    within the temporal bone, called the tympanic
  • It also contains the eardrum, or tympanic
    membrane, and three small bones called the
    auditory ossicles.

  • The tympanic cavity lies between the inner
    surface of the tympanic membrane and the bone
    that forms the outer wall of the inner ear.
  • It contains the three auditory ossicles (malleus,
    or hammer, the incus, or anvil, and the stapes,
    or stirrup.

  • It communicates with the throat by way of a
    narrow tube called the auditory tube often called
    the eustachian tube.
  • This tube allows air to pass between the throat
    and tympanic cavity.
  • Its function is to equalize air pressure on both
    sides of the tympanic membrane.

  • The tympanic membrane is a thin, semitransparent
    barrier separating the outer and middle ears.
  • It is slightly cone-shaped, with its apex
    pointing into the tympanic cavity, where it
    connects to one of the auditory ossicles (the

  • The tympanic membrane receives sound waves from
    the external auditory canal.
  • It vibrates in response to these sound waves.
  • Its vibrations are then transmitted to the
    auditroy ossicles.
  • The auditory ossicles are the smallest bones of
    the body.

  • When the tympanic membrane vibrates in response
    to sound waves, the vibration is transmitted
  • Firstto the malleus, which responds by vibrating
    in unison.
  • Secondfrom the malleus the vibration is passed
    to the incus.
  • Thirdthen the stapes.

  • The stapes transmits the vibration to an opening
    in the wall of the tympanic cavity, known as the
    oval window.
  • The oval window opens into the cochlea of the
    inner ear.
  • Within the inner ear, the vibrations from the
    stapes cause fluid to move, stimulating the
    receptors for hearing.

Inner Ear
  • The inner ear is also known as the labyrinth.
  • It consists of a winding, complicated series of
    passageways or canals.

Sense of Equilibrium
  • Equilibrium is detected by receptor cells in the
    inner ear.
  • The sensation of equilibrium is of two types
  • Static equilibrium
  • Dynamic equilibrium

  • Static Equilibrium---refers to the sensation of
    body positions.
  • The membranous labyrinth of the vestibule
    contains two sacs called the utricle and saccule.
  • Otoliths are carbonate crystals.
  • The position of the head is monitored by the
    movement of otoliths in the macula.

  • Dynamic Equilibrium---is the sensation of rapid
    movements, mostly of the head.
  • The receptors that sense this movement are
    located within the fluid-filled semicircular
    canals of the inner ear.
  • The ampulla contains the sensory organs of
    dynamic equilibrium, called the cristae.

Integrative Functions
  • The ability of the nervous system to process and
    interpret sensory information before sending out
    a motor response is called its integrative
  • It is performed primarily by the cerebral cortex
    of the brain.
  • It includes activities such as thought, memory,
    and emotions.

Motor Areas
  • The Motor Areas of the cerebral cortex are
    located primarily in the frontal lobes in front
    of the central sulcus.

Emotions The Limbic System
  • The Lymbic System is a region of the brain that
    occupies parts of the cerebral cortex and basal
    ganglia of the cerebrum, the hypothalamus, the
    thalamus, and the brain stem.
  • It is the center of the brain that is responsible
    for our emotions, or feelings about ourselves and
    the world.

Motor Functions
  • The region that is dedicated to the generation
    and control of motor impulses is called the motor
  • The motor cortex is the primary site of origin
    for motor impulses requiring thought, memory,
    skillful body coordination, and precise muscular
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