Chapter 10 - Somatic and Special Senses

1
Chapter 10 - Somatic and Special
Senses Receptors and Sensations Changes within
or outside the body are picked up by sensory
receptors, which trigger nerve impulses. These
nerve impulses travel to the brain for
interpretation so that the person can experience
a certain feeling or sensation. Each type of
receptor is sensitive to a specific type of
change. There are five types 1. Chemoreceptors
- stimulated by changes in chemical
concentrations 2. Pain receptors - stimulated by
tissue damage 3. Thermoreceptors - stimulated by
changes is temperature 4. Mechanoreceptors -
stimulated by changes in pressure or
movement 5. Photoreceptors - stimulated by light
energy
2

• A sensation is a feeling that occurs when the
  brain interprets sensory impulses.
• All impulses that travel away from sensory
  receptors to the brain are alike, so the
  resulting sensation depends on the area of the
  brain that receives the impulse.
• At the time that a sensation forms, the cerebral
  cortex causes the feelings to seem to come from a
  certain body part (projection).
• This is why ears seem to hear and eyes seem to
  see.
• If a sensory receptor is continuously stimulated,
  it will usually undergo an adjustment called
  sensory adaptation.
• Impulses leaving the receptors at decreasing
  rates and may eventually stop sending signals
• once adaptation has occurred, impulses can only
  be triggered by a change in signal strength
• if a person enters a room with a very strong
  odor, the original intense smell gradually
  becomes less and less noticeable

3

• Somatic Senses
• Somatic senses are associated with receptors in
  the skin, muscles, joints, and viscera. This
  group includes touch, pressure, temperature, and
  pain.
• The senses of touch and pressure come from three
  types of receptors that sense mechanical forces
  that deform or displace tissues.
• 1. Sensory nerve fibers - common in epithelial
  tissues and associated with
• touch and pressure
• 2. Meissners corpuscles - small, oval masses of
  flattened connective tissue
• that has two or more sensory nerve fibers
  that branch into it and end
• within it as tiny knobs
• abundant in the hairless portions of the skin
  and respond to the motion of objects that barely
  contact the skin and interpret it as a sensation
  of light touch
• 3. Pacinian corpuscles - relatively large
  structures composed of connective
• tissue
• common in the deep subcutaneous tissues, muscle
  tendons, and joint ligaments respond to heavy
  and deep pressure

4

• Temperature sensation depends on two types of
  free nerve endings in the skin
• 1. Heat receptors that respond to warmer
  temperatures
• most responsive at temperatures above 25C
  (77F)
• unresponsive at temperatures above 45C (113F),
  which can stimulate pain receptors that produce a
  burning sensation
• 2. Cold receptors that respond to colder
  temperatures
• most sensitive to temperatures between 10C
  (50F) and 20C (68F)
• temperatures below 10C stimulate pain
  receptors, producing a freezing sensation
• Both heat and cold receptors rapidly adapt.
  Usually within a minute of continuous
  stimulation, the sensation of heat or cold begins
  to fade.

5

• Other receptors consist of free nerve endings and
  can sense pain. These are widely distributed
  throughout the skin and internal tissues with the
  exception of nervous tissue in the brain.
  (Therefore, the brain feels no pain.)
• Pain receptors are stimulated by tissue damage -
  the sensation is most often unpleasant and
  signals the person to take action to remove the
  stimulation.
• Pain receptors adapt poorly, if at all once a
  pain receptor is activated, it may send impulses
  to the CNS for some time (pain may persist).
• Pain sensations may be triggered by the build up
  of certain chemicals.
• Pain receptors are the only receptors in viscera
  that produce sensations.
• Widespread stimulation of visceral tissues may
  produce strong pain sensations.
• Visceral pain may sometimes seem as if it is
  coming from a part of the body other than the
  part being stimulated this is called referred
  pain.
• For example, pain originating in the heart may
  be referred to the left shoulder or left upper
  limb (heart attack).

6

• There are two types of fibers that move impulses
  away from pain receptors
• 1. Acute pain fibers - thin, myelinated nerve
  fibers
• very fast relay
• associated with sharp pain typically from the
  skin
• rarely continues after the pain stimulus stops
• 2. Chronic pain fibers - thin, unmyelinated nerve
  fibers
• much slower relay
• produce dull, aching pains that may be hard to
  pinpoint to a specific location
• may continue for some time after the pain
  stimulus is removed
• Both types of pain fibers are usually activated
  when an event stimulates pain receptors causing a
  dual sensation - a sharp, prickling sensation
  followed by a dull, aching one. The aching pain
  is usually more intense and may worsen over time.
• Awareness of pain occurs when the impulses reach
  the thalamus however, the intensity of the pain
  and its source is not determined until the
  impulses reach the cerebral cortex.

7

• Special Senses
• Smell, Taste, Sight, Hearing, Equilibrium
• __________________________________________________
  __________________________________________________
  __________________________________________________
  ___________________
• SMELL - associated with the complex sensory
  structures in the upper region of the nasal
  cavity
• Olfactory(smell) receptors are chemoreceptors
  that are stimulated by chemicals dissolved in
  liquids.
• The olfactory organs are yellowish brown masses
  that cover the upper parts of the nasal cavity,
  the superior nasal conchae, and a portion of the
  nasal septum.
• Olfactory Receptors
• bipolar neurons surrounded by epithelial cells
• cilia cover knobs at the ends of these neurons
  dendrites which project into the nasal cavity and
  act as the sensitive parts of the receptor
• detect chemicals that have partially dissolved
  into the watery fluids of the nasal cavity

8

• Olfactory Nerve Pathways
• receptor cells send nerve impulses along their
  axons
• these fibers connect with neurons located within
  olfactory bulbs
• here, impulses are analyzed and send more
  impulses along the olfactory tracts to the limbic
  system
• the area for interpretation (olfactory cortex)
  is located in the temporal lobes and at the base
  of the frontal lobes
• How Do We Smell?
• shapes of molecules fit into complementary areas
  on the membrane receptor sites of olfactory
  receptor cells
• when the molecules bind to its particular site,
  an impulse is sent
• since olfactory organs are located above the
  usual pathway of inhaled air, a person might have
  to sniff or force air up to the receptor areas
  in order to smell a faint odor
• olfactory receptors undergo sensory adaptation
  very quickly, but just because they have adapted
  to one scent does not mean that they wont detect
  others

9

• The loss of smell is called anosmia.
• Causes
• inflammation of the nasal cavity lining from a
  respiratory infection
• tobacco smoking
• use of certain drugs, such as cocaine
• Smell and taste function closely together and aid
  in food selection because we usually smell food
  at the same time we taste it.
• --------------------------------------------------
  -----------------------------------------
• Taste buds are the primary organs TASTE
• occur mostly on the surface of the tongue, while
  a few are scattered in the roof of the mouth and
  the walls of the throat
• each taste bud contains a group of gustatory
  (taste) cells which function as taste receptors
• each taste bud contains a small opening called a
  taste pore, which has small projections called
  taste hairs
• taste hairs are believed to be the sensitive
  parts of the receptor cells

10

• In order for a taste sensation to occur
• molecules must dissolve in the saliva that
  surrounds the taste buds
• molecules contact receptor surfaces on the taste
  hair and triggers an impulse
• the impulse is sent to the brain for
  interpretation
• Taste Sensations
• 1. Sweet
• 2. Sour
• 3. Salty
• 4. Bitter
• Each of the four primary taste sensations is
  concentrated in a certain region of the tongue.
• A flavor results from the stimulation of one of
  the primary sensations or a combination of two or
  more of them.

Some scientists now also recognize two other
taste sensations ALKALINE and METALLIC
11
Experiencing flavors involves 1. Taste -
concentrations of stimulating chemicals 2.
Sensations of odor 3. Texture (touch) 4.
Temperature 5. In some foods, such as chili
peppers and ginger, pain receptors may also
be stimulated Taste receptors can also undergo
sensory adaptation. However, by moving bits of
food over the surface of the tongue to stimulate
different receptors at different times, the taste
sensation remains. Taste Nerve Pathways 1.
Impulses from taste receptors travel along the
facial, glossopharyngeal, and vagus nerves
to the medulla oblongata. 2. From there, impulses
travel to the thalamus and on to the gustatory
cortex which is located in the parietal lobe of
the cerebrum.
12
The Basics of Hearing 1. Sound waves enter the
external auditory meatus. 2. The pressure from
the external waves causes the eardrum to
reproduce similar vibrations. 3. The vibrations
are amplified by the auditory ossicles (malleus,
incus, and stapes) as they pass. 4. The stapes
moves against the oval window to transmit the
sound vibrations to the perilymph in the scala
vestibuli. 5. The vibrations continue on through
the vestibular membrane and into the endolymph of
the cochlear duct. 6. Differences in the
frequencies stimulate different sets of hearing
receptor cells. 7. Once the receptor cells are
stimulated, its membrane becomes permeable to Ca
ions.
13

• 8. As Ca ions move into the cells, a
  neurotransmitter is released to stimulate nearby
  sensory neurons.
• 9. Impulses travel down the vestibulocochlear
  nerve to the auditory cortex located in the
  temporal lobe to be interpreted.
• Decibel (dB) - unit used to measure sound
  intensity
• whisper - 40 dB
• normal conversation - 60-70 dB
• heavy traffic - 80 dB
• rock concert - 120 dB produces discomfort
• plane take-off - 140 dB produces pain
• Frequent exposures to more than 90 dB can cause
  permanent hearing damage.

14

• Equilibrium
• 1. Static equilibrium - senses the position of
  the head and maintains posture
• and stability when the head and body are
  still
• 2. Dynamic equilibrium - detects sudden motion of
  the head and body and
• aids in maintaining balance
• Static Equilibrium
• within the vestibule are two chambers called the
  utricle and saccule
• each of these chambers has a tiny structure
  called a macula, which contains
  many hair cells that serve as
  sensory receptors
• when the head is upright, the hairs project into
  a gel-like material that contains structures
  called otoliths
• when the head changes position, the hair cells
  become stimulated by the shifting of the gel-like
  material and otoliths
• impulses sent along the vestibulocochlear nerve
  inform the brain of the heads new position
• the brain sends impulses to skeletal muscles to
  contract or relax in order to maintain balance

15

• Dynamic Equilibrium
• three semicircular canals detect motion of the
  head and aid in balancing the head and body
  during sudden movement
• the canals end in a swelling called an ampulla,
  which contains the organs of the semicircular
  canals called the crista ampullaris
• these organs contain hair cells similar to the
  maculae
• the hair cells extend into a gel-like structure
  called the cupula
• rapid movement of the head or body stimulates
  the hair cells
• the semicircular canals move with the head, but
  the fluid within the canals remains stationary
• this bends the cupula and causes the hair cells
  to become stimulated
• hair cells signal their associated nerve fibers
  and impulses travel to the brain, in particular
  the cerebellum

16

• Other structures that aid in maintaining
  equilibrium
• mechanoreceptors in the joints of the neck
• eyes detect changes in posture
• Motion sickness is a result of abnormal movements
  that disturb the organs of equilibrium. Symptoms
  include
• nausea
• vomiting
• dizziness
• headache

17

• MAJOR STRUCTURES OF THE EYE
• 1. Cornea - window of the eye
• helps focus entering light rays
• transparent because it contains few
• cells and no blood vessels
• 2. Sclera - white portion of the eye
• protects the eye
• attachment site for extrinsic muscles
• 3. Optic nerve - extends off the back of the eye
• responsible for transmitting visual information
  to the brain
• 4. Lens - structure capable of changing shape in
  order to focus by the
• process of accomodation
• held in place by suspensory ligaments
• when the s.l. are pulled tight, the lens has a
  thin shape and can focus on distant objects
• when the s.l. are relaxed, the lens has a
  thicker shape and can focus on closer objects
• located directly behind the iris and pupil

18

• 5. Iris - composed mostly of connective tissue
  and smooth muscle fibers
• color part of the eye (blue, green, brown, gray,
  hazel)
• located between the cornea and lens, and divides
  the anterior cavity into the anterior and
  posterior chambers
• 6. Pupil - circular opening in the center of the
  iris
• 7. Aqueous humor - watery fluid that fills the
  posterior and anterior
• chambers
• 8. Vitreous humor - jellylike fluid that fills
  the posterior cavity
• 9. Retina - contains the visual receptors
• a point called the fovea centralis is the area
  on the retina that produces the sharpest vision
• an area called the optic disk contains no
  receptor cells and therefore cannot detect visual
  information this is called your blind spot

19

• COMMON EYE DISORDERS
• 1. Cataracts - the lens or its capsule slowly
  becomes cloudy and opaque
• can cause blindness without treatment
• were once treated surgically and required a
  two-week recovery period
• now treated on an outpatient basis with laser
  therapy

20

• 2. Glaucoma - the rate of production of aqueous
  humor is greater than the
• rate of its removal
• as fluid accumulates, the pressure rises and can
  shut down the blood supply to the receptor cells
• if the receptor cells die, permanent blindness
  will result
• if diagnosed early, it can be treated with
  drugs, laser therapy, or surgery
• 3. Floaters - sometimes as a person ages, clumps
  of gel or crystal-like
• substances form in the vitreous humor
• they cast shadows on the retina
• cause the person to see small, moving flecks in
  the field of vision
• most apparent when looking at a plain
  background, such as the sky or wall
• 4. Retinoblastoma - inherited, highly malignant
  tumor in the retina
• may result in the loss of one or both eyes

21

• Light Refraction
• A person sees an object because it is either
  giving off light waves or light waves are
  reflecting from it.
• The light waves enter the eye and are focused on
  the retina. Focusing bends the light waves in a
  process called refraction.
• Refraction occurs when light waves pass from one
  medium to another.
• The shape of the lens causes the waves to
  converge on the retina.
• If the eye is normal, the waves will focus
  sharply on the retina.
• However, the image that forms on the retina will
  be upside down and reversed from left to right.
• The visual cortex of the brain then interprets
  the image in its proper position.

22
Common Vision Problems
23

• Visual Receptors
• Visual receptor cells are modified neurons that
  fall into two categories
• 1. Rods - extremely sensitive to light and can
  provide vision in very dim
• light
• do not detect color
• provide general outlines of objects
• 2. Cones - not as sensitive to light
• provides color vision
• produces sharper vision
• fovea centralis contains only tightly packed
  cones (no rods)
• Visual Pigments
• 1. Rods contain the pigment rhodopsin, or visual
  purple.
• In the presence of light, rhodopsin breaks down
  into a colorless pigment called opsin and a
  yellowish substance called retinal.
• In bright light, almost all rhodopsin in the
  rods decomposes and reduces rod sensitivity.
• In dim light, opsin and retinal regenerate
  rhodopsin faster than it is broken down.

24

• 2. There are three different types of cone cells,
  with each containing a
• different light-sensitive pigment. Each
  pigment detects a certain color
• wavelength. The color a person perceives
  depends on which cone or
• combination of cones is stimulated.
• Erythrolabe - sensitive to red light waves
• chlorolabe - sensitive to green light waves
• cyanolabe - sensitive to blue light waves
• If all three sets of cones are stimulated, the
  person senses the light as white.
• If none of the sets of cones are stimulated, the
  person senses black.