Anatomy

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Chapter 15

• Anatomy Physiology
• Fifth Edition
• Seeley/Stephens/Tate
• (c) The McGraw-Hill Companies, Inc.

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The Senses

• Information gathered by the sensory receptors go
  up the afferent neurons and are processed at the
  cerebral cortex.
• Sensory receptors work in response to the
  stimulation from the environment as well as the
  change observed within the body, where regulation
  by negative feed back is needed.
• Each specific sensory receptor is designed to
  pick up corresponding stimuli, such as heat,
  pressure, light, sound, et and transmit to the
  primary sensory cortex of its own.
• However, if the stimulus is applied to the
  receptor other than the form of its primary
  function, the receptor may still respond and give
  a signal to the sensory cortex. You will see a
  flash of light when your eye is hit.

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• Furthermore, if the input is given to a simple
  nerve ending, the neuron may pick up any stimuli
  without discrimination.
• The sensory receptors are known to adapt to the
  level of input. Adopting to hot bath, noise,
  taste, smell, etc
• On the contrary, the sensitivity of sensory
  receptors may be increased by the command from
  the higher order. For example, if you are
  commanded to listen carefully you do indeed
  listen carefully.

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• Classification the senses may be divided into 3
  major types, which use 5 different types of
  receptors. (Table 15.1)
• Three types of senses are
• Somatic sense response of the body to the
  environment, such as touch, pressure,
  temperature, proprioception and pain.
• Visceral sense sensation within the body, such
  as pain pressure.
• Special sense sensation felt by specialized
  organs, such as smell, taste, sight, sound and
  balance.

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• In order to detect these stimuli, at least 5
  receptors have been identified.
• Mechanoreceptors
• Chemoreceptors
• Photoreceptors
• Thermoreceptors
• Nociceptors
• As has been mentioned, these receptors may
  respond to other stimuli than originally
  identified.

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Sensation

• A stimulus may excite a receptor and generate an
  action potential.
• But to be recognized as stimulation, the action
  potential must travel through afferent neurons to
  the cerebral cortex to be sensed as a stimulus.
• The action potential may travel to the parts of
  the brain such as the cerebellum and remain as
  unconscious information.
• Adaptation or accommodation to stimuli may be
  processed by CNS.
• Another modification to stimuli may be found in
  proprioceptors. There are two types of
  proprioceptors tonic and phasic.

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• Tonic receptors generate action potentials as
  long as stimuli are applied and accommodate
  (adapt) slowly, locate position of your body
  parts.
• Phasic receptors accommodate rapidly and sense
  only when there is a change moving body parts.

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• Types of Afferent nerve Endings somatic and
  visceral sensation may arise from at least 8
  types of sensory nerve endings (Table 15.2 and
  Fig. 15.1)
• Free nerve endings distributed throughout all
  parts of the body and responsible for a number of
  sensations , such pain, temperature, itch and
  movement.
• Thermoreceptors are essentially free nerve
  endings scattered immediately beneath the skin,
  skeletal muscles, the liver, and the
  hypothalamus.
• There are several times more cold receptors than
  hot receptors and their structures are
  indistinguishable.
• The pathways of pain receptors and those of the
  thermoreceptors are about the same, ii.e. the
  reticular formation, the thalamus, the primary
  sensory cortex.
• Thermoreceptors are sensitive to the temperature
  change and can quickly adapt.

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• Pain receptors pain receptors are found in the
  skin, joint capsules, periosteal of bone, the
  walls of blood vessels.
• Found a little in deep tissues or visceral
  organs.
• Pains are transmitted with types of axons fast
  pain with myelinated axons and slow pain with
  non-mylinated axons.
• Fast pain ca be localized and respond quick
  somatic reflexes or specific sensory cortexes.
• Slow pain, such as caused by burning, identifies
  only general area of pain.
• Pain from visceral organs are often confused with
  the surface pain coming from the same spinal
  nerves referred pain.
• Thus, the cause of upper chest or left arm pain
  may be from the heart.
• The awareness of pain may be reduced by
  inhibition of pain center in the thalamus,
  reticular formation, lower brain stem and spinal
  cord.

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• Merkels (or tactile) disks flattened axonal
  ending associated with epithelial cells.
• At the basal layers of the epidermis.
• Touch, pressure, and vibration.
• Fine touch and pressure receptors for exact
  location, shape, size, texture and movement.
• Hair follicle receptor at the root of hair
  follicles and respond to the motion of hair.
• Pacinis or Laminated Corpuscles single
  dendrite is found at the center of this laminated
  receptor.
• Located in the dermis or hypodermis. (finger,
  breasts, external genitalia.)
• Senses cutaneous pressure and vibration.
• The corpuscles associated with the joints help
  relay proprioceptive information.

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• Meissners or Tactile Corpuscles in the dermal
  papillae.
• Two point discrimination touch.
• Different distribution densities leading to the
  different level of position resolution.
• Ruffinis End Organs in the dermis of the skin
  of fingers.
• Respond to pressure and stretch.
• Golgi Tendon Organ proprioceptive nerve ending
  at the tendon.
• Respond to stretch of the tendon.
• The afferent signal from the Golgi organ inhibits
  the motor neuron of the associated muscle and
  makes it to relax. Thus, prevents damage to the
  muscle by excessive contraction.

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• Muscle Spindles relatively short 3-10 muscles
  fibers which are striated only on the ends of the
  fibers, ii.e only the ends can contract.
• Sensory afferent neurons wrap around the
  non-striated central region. Efferent gamma motor
  neurons attach the striated ends.
• When the muscle is stretched, stimulates the
  muscle spindle.
• The afferent neuron synapses with the alpha motor
  neuron.
• Muscle contracts stretch reflex.

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Olfaction

• The structure of olfactory recess and bulb are
  shown in Fig. 15.3.
• Olfactory receptors are found in the olfactory
  epithelium of olfactory organ together with the
  basal (these are stem cells) and supporting
  cells.
• Exception to the rule that neuronal cells do not
  replicate, the basal cells regenerate olfactory
  cells every two months.
• The olfactory cells dendrites ends are embedded
  in the mucus layer excreted from olfactory glands
  and the axons extend into olfactory bulbs, where
  the cranial nerve N I are found.
• The olfactory receptors are highly modified
  neurons and humans have about 10-20 millions per
  5cm², while a dog can have the receptor surface
  more than 72 times of this.
• The average person can distinguish about 4,000
  different odors. Though it is not known how.

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• Seven primary classes of odors have been
  proposed
• Comphoraceous
• Musky
• Floral
• Peppermint
• Ethereal
• Pungent
• Putrid
• The olfactory receptor cells are highly sensitive
  and respond to the dissolved chemicals in the
  mucus and change its permeability of the membrane
  leading to a formation of an action potential.
• The information is often transmitted as the
  frequency of action potentials.
• The axons from the olfactory bulbs directly reach
  the olfactory cortex, but effector neurons reach
  hypothalamus and the limbic system raising
  emotional behavioral responses. Smell good and
  smell bad. (adaptation, effect of perfume)

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Taste

• Gustatory (taste) receptors are located within
  the taste buds, which are in turn protectively
  placed in the specialized papillae to avoid
  mechanical stress. There are four types of
  papillae circumvallate, fungiform, foliate, and
  filiform which have no taste buds.
• At the ending of gustatory cell, microvilli are
  found.
• The microvilli, responding to the chemical that
  causes depolarization to initiate the action
  potential
• There are four primary taste sweet, salt, sour,
  and bitter.
• Surprisingly, the chemical structures of
  artificial sweeteners are not the same as
  sucrose.
• Peppery and burning sensation are with the
  general sensory receptors.

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• The afferent nerves synapses in the medulla,
  again in the thalamus, and to the primary
  sensory cortex. Good taste and bad taste.
• Perception of taste is extensively related to
  other senses.
• Integration with the olfactory receptor is
  especially evident. For example, bad smell could
  ruin your appetite.

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Visual Physiology

• How subjects are recognized? Scattering,
  absorption and reflection.
• Projection to retina.
• Visual recognition starts with recognition of
  light with photoreceptors, rods and cones.
• In all the available instruments, human eyes are
  the most capable of working under a very wide
  intensity of light, ii.e. from the dim light
  under the moon to the bright daylight in the
  beach. Minimum light intensity to be recognized
  is about 40 photons.
• Our eyes are also sensitive to visible wave
  length of the light ranging from red, orange,
  yellow, green, blue, indigo and violet.

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• Light (Fig. 15.16)
• Visible light and UV/IR
• color
• Light refraction and reflection
• Lens focused imaged
• Reflection by the surface of an object
• Inner structure of an eye (fig.15.13)
• The Retina
• Rods and Cones their names, rods and cones,
  come from the appearance of their outer segments
  (15.18/19)

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• Rods are very sensitive to the light. But do not
  distinguish colors. They are found more at the
  outer edge in the retina. In the outer segment of
  the rods there are light absorbing pigment called
  rhodopsin.
• Cones are less sensitive to light, but they
  distinguish color. Because there are at least
  three types of cones with specific colors of
  iodopsins red, green and blue cones, which will
  be decribed later.
• Cones are concentrated near the opposite end of
  the lens, fovea, at a higher density to provide
  high resolution.
• Failure for any these cones to develop ends in
  color blindness. The most common color blindness
  is missing of the red cones, thus the subject
  cannot distinguish red light from green light (
  red-green color blindness).
• Color blindness is caused by recessive sex linked
  genes.

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• Photoreceptor Function in the outer portion of
  each photoreceptor, many layers of membranous
  discs are packed within the membrane sack.
• Photoreceptor pigments are located in these
  disks.
• When the photoreceptor is at rest, gated sodium
  channels are open in the outer segment causing
  depolarization at the synaptic end of the cell.
• Thus, in the dark, the neurotransmitters are
  continuously released from the synaptic region of
  the photoreceptor leading to a high frequency of
  spiked signals
• Activated rhodopsin closes the Na gated channel
  and the cell becomes hyperpolarized to stop
  sending the spiked signals.
• When the receptor is stimulated with light, the
  Na channels are closed and the rate of release
  of synaptic vesicles decreases, thus the cell
  becomes hyperpolarized.

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• Visual Pigments
• a visual pigment, rhodopsin, consists of enzymes
  opsin and retinal from Vitamin A.
• Retinal, with a slight modification, is the same
  for both rods and cones.
• Three slightly different opsins bind with retinal
  to form three iodopsins found in cones. Red,
  green and blue.
• Recognition of Light
• There are two forms of retinal cis (bent) and
  trans (stretched) forms.
• In the dark, the retinal takes the cis form and
  the rhodopsin is inactivated.
• This keeps open the gated Na channels of the
  outer segment of the cell and keeps the cell
  depolarized.

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• The cell continues to send spiked signals.
• When the cell is illuminated, the retinal becomes
  isomerized to the trans-retinal and activated
  rhodopsin.
• activated rhodopsin closes the Na channels and
  the cell becomes hyperpolarized to stop sending
  the spiked signals.
• These biochemical processes create an optical
  image on the retinal surface and the image will
  be transmitted through the optic nerves to the
  cerebral cortex.
• Shortly after the photo excitation, the
  trans-retinal will dissociate from the rhodopsin
  (bleaching). The opsin will be recycled and the
  trans-retinal will be put back to cis-retinal
  using the other enzymes and ATP.
• Dark adaptation
• Fading image in the retina.

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• Visual Pathway recall the anatomy of retina
• There is a significant complexity of neuronal
  pathways at the level of the retina.
• Eventually, the ganglionic cells extend their
  axons together as optic nerves, cross over 50 of
  them at the optic chiasm, and reach lateral
  geniculate nuclei at the thalamus before finally
  arriving at large area of visual cortex. (Fig.
  15.22)
• Visual information is spread over the wide areas
  of the cerebral cortex via lateral geniculate
  laterals from the optic tracts.
• Visual inputs to the pineal gland establish a
  daily pattern of activity, thus circadian rhythm.

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• Equilibrium and hearing (review)
• Anatomy of the Ear in the lab
• Auditory Function
• Sound is generated by vibrating matter such as
  drums, tables etc
• It travels through matter such as air,
  water,wood, metals etc, but does not travel
  through a vacuum.
• It has pitch and volume.
• Humans can hear within a certain range of pitch
  (20 20,000 cps) of louder than 0db, but less
  than 125 db with pain.
• While other animals have different hearing
  ranges. Example a dog whistle can be heard by
  dogs, but not by humans.

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• Sound traveled through the air is
• Collected by external ear and reaches tympanic
  membrane (15.23)
• The vibration of tympanic membrane is transmitted
  to auditory ossicles (three tiny bones) in the
  middle ear consisting of malleus, incus and
  staples, while tensor tympani and stapedius
  muscles provide sound attenuation reflex to
  protect the structure. (15.30)
• The vibrating foot of staples transmits the sound
  to oval window. When overall window is pushed,
  the perilymph will be pushed through the scala
  vestibuli and scala tympani, while basilar
  membrane is pushed down. These events will end up
  pushing out the round window.

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• The vibration frequency of the perilymph will be
  enhanced at a specific resonating region along
  the basilar membrane the higher frequencies are
  closer to the oval window and the lower
  frequencies are at the further end of the
  cochlea.
• The position oriented movement of basilar
  membrane moves hair cells causing them to rub
  against the tectorial membrane. (15.26)
• Hair cells attached to the basilar membrane are
  exposed to endolymph. There is about a 80 mV
  difference between endolymph and perilymph across
  the vestitublar membrane ( endocochlear
  potential). The difference in potential may be
  the cause of the forming action potential on the
  hair cells.
• A large number of cochlear nerves are attached to
  the entire length of the cochlea hair cells to
  detect sounds of different frequencies.
• Neuronal Pathways for Hearing are shown in Fig.
  15.32. Both hearing and balance senses are
  transmitted by the vestibulochlear (VIII) nerve
  to cochlear nucleus and eventually reach the
  auditory cortex

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• Balance
• The organs of balance consist of two major parts
• Static labyrinth consisting of the utricle and
  saccule. Defines the position of the head
  relative to gravity.
• In the utricle and saccule, there are two oval
  shaped maculae with receptor cells.
• These receptor cells support gelatinous matter on
  which is composed of high density mineral
  crystals of otolith.
• Any motion against otolith is felt with the
  receptors as motion against the gravity or linear
  acceleration.

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• Kinetic labyrinth consisting of three
  semicircular canals. Defines the movements of the
  head. (15.35). The three semicircular ducts
  anterior, posterior and lateral create three
  dimension perception towards movement of the body
• The root of each duct, ampulla, a cupula with
  receptor where receptor hair cells are found.
• The motion of gelatinous liquid in the
  semicircular ducts is felt by theses receptors
  cells to provide rotational sensation

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The End.