Chapter 47: Somatic Sensations. I.
General Organization, the Tactile and
Position Senses
Guyton and Hall, Textbook of Medical Physiology, 12th edition
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
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)
Detection and Transmission of Tactile Sensations
• Interrelaitons Among the Tactile Sensations of Touch,
Pressure, and Vibration- three principle differences
a. Touch sensation generally results from stimulation of
tactile receptors in the skin or s.c. tissues
b. Pressure sensation generally results from deformation
of deeper tissues
c. Vibration sensation results from rapidly repetitive
sensory signals
Detection and Transmission of Tactile Sensations
• Tactile Receptors
a. Free nerve endings- found everywhere in the skin and in
many other tissues; can detect touch and pressure
b. Meissner’s Corpuscles- touch receptor with great sensitivity;
elongated, encapsulated nerve ending of a large myelinated nerve fiber; present in the non-hairy areas of the skin
(i.e. the fingertips)
Detection and Transmission of Tactile Sensations
• Tactile Receptors (cont.)
c. Merkel’s 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”
Detection and Transmission of Tactile Sensations
• Tactile Receptors (cont.)
Fig. 47.1 Iggo dome receptor containing multiple layers of
Merkel’s discs connected to a single large
myelinated nerve fiber
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. Ruffini’s endings- multibranched encapsulated, adapt
slowly; prolonged touch and pressure sensations;
found in joint capsules
Detection and Transmission of Tactile Sensations
• Transmission of Tactile Signals in Peripheral Nerve Fibers
• Detection of Vibration
• Detection of Tickle and Itch by Mechanoreceptors
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Dorsal Column- Medial Lemniscal System
a. Touch sensations requiring high degree of localization
b. Touch sensations requiring transmission of fine
gradations of intensity
c. Phasic sensations, such as vibratory sensations
d. Sensations that signal movement against the skin
e. Position sensations from the joints
f. Pressure sensations related to fine degrees of
judgment of pressure intensity
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Anterolateral System
a.
b.
c.
d.
e.
Pain
Thermal sensations, both warm and cold
Crude touch and pressure
Tickle and itch sensations
Sexual sensations
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Anatomy of the Dorsal Column
Fig. 47.2
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Anatomy of the Dorsal Column
Fig. 47.3
Fig. 47.4
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Somatosensory Cortex
Fig. 47.5 Structurally distince areas, called Brodmann’s areas of the
human cerebral cortex
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Somatosensory Cortex
a. Sensory signals from all modalities terminate just
posterior to the central fissure
b. Anterior half of the parietal lobe-reception and
interpretation of somatosensory signals
c. Posterior half of t he parietal lobe-provides still
higher levels of interpretation
d. Visual signals terminate in the occipital lobe
e. Auditory signals terminate in the temporal lobe
f. Anterior to the central fissure is the motor cortex
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Somatosensory Areas I and II
Fig. 47.6 Two somatosensory cortical areas; I and II
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
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Layers of the Somatosensory Cortex and Their Functioncontains six layers of neurons (#1 is next to the brain
surface)
Fig. 47.8
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Layers of the Somatosensory Cortex and Their Function
a. Incoming sensory signal excites layer IV first; signal
spreads toward the surface and also deeper layers
b. Layers I and II receive diffuse nonspecific input signals
c. Neurons in II and III send axons to related portions of
the cerebral cortex and to the opposite hemisphere via
the corpus callosum
d. Neurons in V and VI send axons to deeper parts of the
nervous system
Sensory Pathways for Transmitting Somatic Signals into the CNS
• Sensory Cortex is Organized in Vertical Columns
a. 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:
a. Person is unable to localize discretely the different
sensations in different parts of the body; can
localize the sensations crudely
Sensory Pathways for Transmitting Somatic Signals into the CNS
•
Functions of Somatosensory Area I
b. Person is unable to judge critical degrees of pressure
against the body
c. Person is unable to judge the weights of objects
d. Person is unable to judge shapes or forms of objects
e. Person is unable to judge texture of materials
Sensory Pathways for Transmitting Somatic Signals into the CNS
•
Somatosensory Association Areas
a. Brodmann’s Areas 5 and 7- play an important role in
deciphering deeper meanings of the sensory information
b. Receives information from somatosensory area I, ventrobasal nuclei of the thalamus, other areas of the thalamus,
visual cortex, and the auditory cortex
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
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
Sensory Pathways for Transmitting Somatic Signals into the CNS
•
Effect of Lateral Inhibition- increases the degree of
contrast in the perceived spatial pattern
a. Virtually every sensory pathway, when excited, gives
rise simultaneously to lateral inhibitory signals
b. 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
c. In the dorsal column lateral inhibition signals occur at
each synaptic level
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
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)
a. Knowledge of position depends on knowing the degrees
of angulation of all joints in all planes and their rates of
change
b. Multiple different types of receptors are used:
1. Deep receptors
2. Corpuscles
3. Muscle spindles, etc.
Sensory Pathways for Transmitting Somatic Signals into the CNS
•
Processing of Position Sense Information- thalmic
neurons responding to joint rotation are of two
types:
a. Those maximally stimulated when the joint is at
full rotation
b. Those maximally stimulated when the joint is at
minimal rotation
Fig. 47.12 Typical responses of five different thalamic neurons when the
knee joint is moved through its range of motion
Transmission of Less Critical Sensory Signals in the Anterolateral
Pathway
• Anterolateral Pathway
a. Transmits sensory signals that do not require highly
discrete localization or discrimination of fine
gradations of intensity
1.
2.
3.
4.
5.
Pain
Heat and cold
Crude tactile
Tickle and itch
Sexual sensations
Transmission of Less Critical Sensory Signals in the Anterolateral
Pathway
• Anatomy of the Anterolateral Pathway
Fig. 47.13
Transmission of Less Critical Sensory Signals in the Anterolateral
Pathway
• Characteristics of Transmission
a.
b.
c.
d.
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
Transmission of Less Critical Sensory Signals in the Anterolateral
Pathway
• Segmental Fields of Stimulation—Dermatomes
•See Fig. 47.14 in the text