Unit Nine: The Nervous System: A. General Principles and Sensory Physiology

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Unit Nine The Nervous System A. General
Principles and Sensory Physiology

• Chapter 47 Somatic Sensations. I. General
  Organization, the Tactile and Position Senses

Guyton and Hall, Textbook of Medical Physiology,
12th edition
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Classification of Somatic Senses

• Mechanoreceptic Somatic Senses- include both
  tactile
• and position sensations stimulated by mechanical
• displacement
• Thermoreceptive Senses- detect heat and cold
• Pain Sense- activated by factors that damage
  tissues

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Other Classifications of Somatic Senses

• Exteroreceptive Sensations- from the surface
  of the body
• Proprioceptive Sensations- relating to the
  physical state
• of the body (position, tendons, muscles,
  equilibrium)
• Visceral Sensations- sensations from the
  internal organs
• Deep Sensations- come from the deep tissues
  (fascia,
• muscles, and bone)

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Detection and Transmission of Tactile Sensations

• Interrelaitons Among the Tactile Sensations of
  Touch,
• Pressure, and Vibration- three principle
  differences
• Touch sensation generally results from
  stimulation of
• tactile receptors in the skin or s.c. tissues
• Pressure sensation generally results from
  deformation
• of deeper tissues
• Vibration sensation results from rapidly
  repetitive
• sensory signals

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Detection and Transmission of Tactile Sensations

• Tactile Receptors
• Free nerve endings- found everywhere in the skin
  and in
• many other tissues can detect touch and
  pressure
• Meissners Corpuscles- touch receptor with great
  sensitivity
• elongated, encapsulated nerve ending of a large
  myelin-
• ated nerve fiber present in the non-hairy areas
  of the skin
• (i.e. the fingertips)

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Detection and Transmission of Tactile Sensations

• Tactile Receptors (cont.)
• Merkels discs- expanded tip tactile receptor
  transmit an
• initially strong but partially adapting signal
  and then a
• continuing weaker signal that adapts slowly
  found in the
• hairy parts of the skin often grouped together
  in a Iggo
• dome receptor

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Detection and Transmission of Tactile Sensations

• Tactile Receptors (cont.)

Fig. 47.1 Iggo dome receptor containing multiple
layers of Merkels discs
connected to a single large
myelinated nerve fiber
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Detection and Transmission of Tactile Sensations

• Tactile Receptors (cont.)
• d. Hair end organ- touch receptor around each
  hair
• movement and initial contact with the body
• e. Ruffinis endings- multibranched
  encapsulated, adapt
• slowly prolonged touch and pressure
  sensations
• found in joint capsules

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Detection and Transmission of Tactile Sensations

• Transmission of Tactile Signals in Peripheral
  Nerve Fibers
• Detection of Vibration
• Detection of Tickle and Itch by
  Mechanoreceptors

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Dorsal Column- Medial Lemniscal System
• Touch sensations requiring high degree of
  localization
• Touch sensations requiring transmission of fine
• gradations of intensity
• Phasic sensations, such as vibratory sensations
• Sensations that signal movement against the skin
• Position sensations from the joints
• Pressure sensations related to fine degrees of
• judgment of pressure intensity

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Anterolateral System
• Pain
• Thermal sensations, both warm and cold
• Crude touch and pressure
• Tickle and itch sensations
• Sexual sensations

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Anatomy of the Dorsal Column

Fig. 47.2
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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Anatomy of the Dorsal Column

Fig. 47.3
Fig. 47.4
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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Somatosensory Cortex

Fig. 47.5 Structurally distince areas, called
Brodmanns areas of the human
cerebral cortex
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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Somatosensory Cortex
• Sensory signals from all modalities terminate
  just
• posterior to the central fissure
• Anterior half of the parietal lobe-reception and
• interpretation of somatosensory signals
• Posterior half of t he parietal lobe-provides
  still
• higher levels of interpretation
• Visual signals terminate in the occipital lobe
• Auditory signals terminate in the temporal lobe
• Anterior to the central fissure is the motor
  cortex

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Somatosensory Areas I and II

Fig. 47.6 Two somatosensory cortical areas I
and II
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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Spatial Orientation of Signals from Different
  Parts of
• the Body in Area I

Fig. 47.7 Sensory homunculus
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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Layers of the Somatosensory Cortex and Their
  Function-
• contains six layers of neurons (1 is next to
  the brain
• surface)

Fig. 47.8
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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Layers of the Somatosensory Cortex and Their
  Function
• Incoming sensory signal excites layer IV first
  signal
• spreads toward the surface and also deeper
  layers
• Layers I and II receive diffuse nonspecific input
  signals
• Neurons in II and III send axons to related
  portions of
• the cerebral cortex and to the opposite
  hemisphere via
• the corpus callosum
• Neurons in V and VI send axons to deeper parts of
  the
• nervous system

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Sensory Cortex is Organized in Vertical
  Columns
• Each column detects a different sensory spot on
  the
• body with a specific sensory modality
• Functions of Somatosensory Area I-bilateral
  excision
• cause the following types of sensory judgement
• Person is unable to localize discretely the
  different
• sensations in different parts of the body can
• localize the sensations crudely

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Functions of Somatosensory Area I
• Person is unable to judge critical degrees of
  pressure
• against the body
• Person is unable to judge the weights of objects
• Person is unable to judge shapes or forms of
  objects
• Person is unable to judge texture of materials

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Somatosensory Association Areas
• a. Brodmanns Areas 5 and 7- play an important
  role in
• deciphering deeper meanings of the sensory
  information
• Receives information from somatosensory area I,
  ventro-
• basal nuclei of the thalamus, other areas of the
  thalamus,
• visual cortex, and the auditory cortex

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Overall Characteristics of Signal Transmission
  and
• Analysis in the Dorsal Column- (lower part of
  Fig. 47.9)

Fig. 47.9 Transmission of a pinpoint stimulus
signal to the cerebral cortex
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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Two-Point Discrimination

Fig. 47.10 Transmission of signals to the
cortex from two adjacent
pinpoint stimuli
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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Effect of Lateral Inhibition- increases the
  degree of
• contrast in the perceived spatial pattern
• Virtually every sensory pathway, when excited,
  gives
• rise simultaneously to lateral inhibitory
  signals
• Importance of lateral inhibition is that it
  blocks the
• lateral spread of excitatory signals and
  therefore,
• increases the degree of contrast in the sensory
  pattern
• perceived in the cerebral cortex
• In the dorsal column lateral inhibition signals
  occur at
• each synaptic level

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Transmission of Rapidly Changing and Repetitive
• Sensations- dorsal column can recognize
  changing
• stimuli that occur in as little as 1/400 of a
  second
• Vibratory Sensation- rapidly repetitive and can
  be
• detected up to 700 cycles/second

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Position Senses (Proprioceptive Senses)- two
  subtypes
• (1) static position sense, and (2) rate of
  movement
• sense (kinesthesia or dynamic proprioception)
• Knowledge of position depends on knowing the
  degrees
• of angulation of all joints in all planes and
  their rates of
• change
• Multiple different types of receptors are used
• Deep receptors
• Corpuscles
• Muscle spindles, etc.

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Sensory Pathways for Transmitting Somatic Signals
into the CNS

• Processing of Position Sense Information- thalmic
• neurons responding to joint rotation are of two
• types
• Those maximally stimulated when the joint is at
• full rotation
• Those maximally stimulated when the joint is at
• minimal rotation

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Fig. 47.12 Typical responses of five different
thalamic neurons when the knee
joint is moved through its range of motion
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Transmission of Less Critical Sensory Signals in
the Anterolateral Pathway

• Anterolateral Pathway
• Transmits sensory signals that do not require
  highly
• discrete localization or discrimination of fine
• gradations of intensity
• Pain
• Heat and cold
• Crude tactile
• Tickle and itch
• Sexual sensations

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Transmission of Less Critical Sensory Signals in
the Anterolateral Pathway

• Anatomy of the Anterolateral Pathway

Fig. 47.13
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Transmission of Less Critical Sensory Signals in
the Anterolateral Pathway

• Characteristics of Transmission
• Velocity of transmission is 1/3 of that of the
  dorsal column
• Degree of spatial localization of signals is poor
• Gradations of intensities are less accurate
• Ability to transmit rapidly changing or
  repetitive
• signals is poor

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Transmission of Less Critical Sensory Signals in
the Anterolateral Pathway

• Segmental Fields of StimulationDermatomes
• See Fig. 47.14 in the text