Special Senses and Functional Aspects of the Nervous System

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CHAPTER 10

• Special Senses and Functional Aspects of the
  Nervous System

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• 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)

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SENSORY FUNCTIONS

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

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SENSORY PATHWAYS

• 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
  receptor.
• From the receptor, the impulse is conducted along
  a sensory neuron to the CNS.

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RECEPTORS

• 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.

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• 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.

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Sensory adaptation

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

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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.

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• 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
  levels

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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.

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• 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
  processing.

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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
  cortex.

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

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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
  processed.

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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.

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• 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
  genitals.
• A second, less common set of touch receptors in
  the skin are Merkels discs, whose dendrites end
  in the skins epidermis.

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• 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.

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Temperature

• 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.

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Pain

• 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.

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• 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.

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• 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.

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

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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.

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• 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.

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• 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.

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SPECIAL SENSES

• The four special senses are
• SMELL
• TASTE
• SIGHT
• And HEARING

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• 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.

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SMELL

• 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.

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• 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.

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TASTE

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

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HEARING

• 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

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OUTER 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.

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MIDDLE EAR

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

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• 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.

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• 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.

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• 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
  malleus).

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• 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.

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• 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.

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• 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.

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Inner Ear

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

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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

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• 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.

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DYNAMIC EQUILIBRIUM

• 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.

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Integrative Functions

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

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Motor Areas

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

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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.

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Motor Functions

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