1. Unit Nine: The Nervous System: A
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

2. 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

3. 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)

4. 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

5. 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
Meissner’s 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)

6. Detection and Transmission of Tactile Sensations
Tactile Receptors (cont.)
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”

7. Detection and Transmission of Tactile Sensations
Tactile Receptors (cont.)
Fig Iggo dome receptor containing multiple layers of
Merkel’s discs connected to a single large
myelinated nerve fiber

8. 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

9. Detection and Transmission of Tactile Sensations
Transmission of Tactile Signals in Peripheral Nerve Fibers
Detection of Vibration
Detection of Tickle and Itch by Mechanoreceptors

10. 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

11. 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

12. Sensory Pathways for Transmitting Somatic Signals into the CNS
Anatomy of the Dorsal Column
Fig. 47.2

13. Sensory Pathways for Transmitting Somatic Signals into the CNS
Anatomy of the Dorsal Column
Fig. 47.3
Fig. 47.4

14. Sensory Pathways for Transmitting Somatic Signals into the CNS
Somatosensory Cortex
Fig Structurally distince areas, called Brodmann’s areas of the
human cerebral cortex

15. 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

16. Sensory Pathways for Transmitting Somatic Signals into the CNS
Somatosensory Areas I and II
Fig Two somatosensory cortical areas; I and II

17. Sensory Pathways for Transmitting Somatic Signals into the CNS
Spatial Orientation of Signals from Different Parts of
the Body in Area I
Fig Sensory homunculus

18. 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

19. 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

20. 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

21. 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

22. 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
Receives information from somatosensory area I, ventro-
basal nuclei of the thalamus, other areas of the thalamus,
visual cortex, and the auditory cortex

23. 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 Transmission of a pinpoint stimulus
signal to the cerebral cortex

24. Sensory Pathways for Transmitting Somatic Signals into the CNS
Two-Point Discrimination
Fig Transmission of signals to the
cortex from two adjacent
pinpoint stimuli

25. 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

26. 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

27. 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.

28. 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
minimal rotation

29. Fig. 47.12 Typical responses of five different thalamic neurons when the
knee joint is moved through its range of motion

30. Transmission of Less Critical Sensory Signals in the 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

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

32. 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

33. Transmission of Less Critical Sensory Signals in the Anterolateral Pathway
Segmental Fields of Stimulation—Dermatomes
See Fig in the text