chapter 47: somatic sensations. i. general organization, the tactile and position senses guyton and...

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

Classification of Somatic Senses

• Mechanoreceptic Somatic Senses- include both tactileand position sensations stimulated by mechanicaldisplacement

• 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 stateof 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 deformationof deeper tissues

c. Vibration sensation results from rapidly repetitive sensory signals


• Tactile Receptors

a. Free nerve endings- found everywhere in the skin and inmany other tissues; can detect touch and pressure

b. 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)


• Tactile Receptors (cont.)

c. Merkel’s discs- expanded tip tactile receptor; transmit aninitially strong but partially adapting signal and then acontinuing weaker signal that adapts slowly; found in thehairy parts of the skin; often grouped together in a “Iggodome receptor”



Fig. 47.1 Iggo dome receptor containing multiple layers of Merkel’s discs connected to a single large myelinated nerve fiber



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


• 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 localizationb. Touch sensations requiring transmission of fine

gradations of intensityc. Phasic sensations, such as vibratory sensationsd. Sensations that signal movement against the skine. Position sensations from the jointsf. Pressure sensations related to fine degrees of

judgment of pressure intensity


• Anterolateral System

a. Painb. Thermal sensations, both warm and coldc. Crude touch and pressured. Tickle and itch sensationse. Sexual sensations


• Anatomy of the Dorsal Column

Fig. 47.2


• Anatomy of the Dorsal Column

Fig. 47.3 Fig. 47.4


• Somatosensory Cortex

Fig. 47.5 Structurally distince areas, called Brodmann’s areas of the human cerebral cortex


• 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 stillhigher levels of interpretation

d. Visual signals terminate in the occipital lobee. Auditory signals terminate in the temporal lobe f. Anterior to the central fissure is the motor cortex


• Somatosensory Areas I and II

Fig. 47.6 Two somatosensory cortical areas; I and II


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

Fig. 47.7 Sensory homunculus


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

Fig. 47.8


• 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 signalsc. Neurons in II and III send axons to related portions of

the cerebral cortex and to the opposite hemisphere viathe corpus callosum

d. Neurons in V and VI send axons to deeper parts of the nervous system


• Sensory Cortex is Organized in Vertical Columns

a. Each column detects a different sensory spot on thebody with a specific sensory modality

• Functions of Somatosensory Area I-bilateral excisioncause 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


• Functions of Somatosensory Area I

b. Person is unable to judge critical degrees of pressureagainst 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


• Somatosensory Association Areas

a. Brodmann’s Areas 5 and 7- play an important role indeciphering deeper meanings of the sensory information

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


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

Fig. 47.9 Transmission of a pinpoint stimulus signal to the cerebral cortex


• Two-Point Discrimination

Fig. 47.10 Transmission of signals to the cortex from two adjacent pinpoint stimuli


• Effect of Lateral Inhibition- increases the degree ofcontrast in the perceived spatial pattern

a. Virtually every sensory pathway, when excited, givesrise 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 patternperceived in the cerebral cortex

c. In the dorsal column lateral inhibition signals occur ateach synaptic level


• Transmission of Rapidly Changing and RepetitiveSensations- dorsal column can recognize changingstimuli that occur in as little as 1/400 of a second

• Vibratory Sensation- rapidly repetitive and can bedetected up to 700 cycles/second


• Position Senses (Proprioceptive Senses)- two subtypes:(1) static position sense, and (2) rate of movementsense (kinesthesia or dynamic proprioception)

a. Knowledge of position depends on knowing the degreesof angulation of all joints in all planes and their rates ofchange

b. Multiple different types of receptors are used:

1. Deep receptors2. Corpuscles3. Muscle spindles, etc.


• Processing of Position Sense Information- thalmicneurons responding to joint rotation are of twotypes:

a. Those maximally stimulated when the joint is atfull rotation

b. Those maximally stimulated when the joint is atminimal 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 highlydiscrete localization or discrimination of finegradations of intensity

1. Pain2. Heat and cold3. Crude tactile4. Tickle and itch5. Sexual sensations


• Anatomy of the Anterolateral Pathway

Fig. 47.13


• Characteristics of Transmission

a. Velocity of transmission is 1/3 of that of the dorsal columnb. Degree of spatial localization of signals is poorc. Gradations of intensities are less accurated. Ability to transmit rapidly changing or repetitive

signals is poor


• Segmental Fields of Stimulation—Dermatomes

• See Fig. 47.14 in the text

chapter 47: somatic sensations. i. general organization, the tactile and position senses guyton and...

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