Somatic Sensations: I. General Organization, the Tactile and Position Senses

Question 1

The somatic senses can be classified into three physiological types:

Answer 1

(1) the mechanoreceptive somatic senses, which
include both tactile and position sensations that are stimulated by mechanical displacement of some tissue of the
body; (2) the thermoreceptive senses, which detect heat
and cold; and (3) the pain sense, which is activated by
factors that damage the tissues.

Question 2

diffrences bewteen touch pressure and vibration sensation

Answer 2

1) touch sensation generally results from stimulation of tactile receptors in the skin or in tissues immediately beneath the skin; (2) pressure sensation generally
results from deformation of deeper tissues; and (3) vibration sensation results from rapidly repetitive sensory
signals

Question 3

e free nerve endings,

Answer 3

which are found everywhere in the skin and in many other tissues, can detect
touch and pressure. For instance, even light contact with
the cornea of the eye, which contains no other type of
nerve ending besides free nerve endings, can nevertheless
elicit touch and pressure sensations.

Question 4

Meissner’s corpuscle

Answer 4

ated encapsulated nerve ending of a large (type Aβ) myelinated sensory nerve fiber. Inside the capsulation
are many branching terminal nerve filaments. These corpuscles are present in the nonhairy parts of the skin
and are particularly abundant in the fingertips, lips, and other areas of the skin where one’s ability to discern
spatial locations of touch sensations is highly developed. Meissner corpuscles adapt in a fraction of a second after
they are stimulated, which means that they are particularly sensitive to movement of objects over the surface of
the skin, as well as to low-frequency vibration.

Question 5

Merkel’s discs,

Answer 5

These receptors differ from
Meissner’s corpuscles in that they transmit an initially
strong but partially adapting signal and then a continuing weaker signal that adapts only slowly. Therefore, they are
responsible for giving steady-state signals that allow one
to determine continuous touch of objects against the skin

Question 6

hair end-organ,

Answer 6

fourth, slight movement of any hair on the body stimulates a nerve fiber entwining its base. Thus, each hair
and its basal nerve fiber, called the hair end-organ, are
also touch receptors

Question 7

Ruffini’s endings,

Answer 7

These endings adapt very slowly and,
therefore, are important for signaling continuous states of
deformation of the tissues, such as heavy prolonged touch
and pressure signals. They are also found in joint capsules
and help to signal the degree of joint rotation

Question 8

Pacinian corpuscles,

Answer 8

They are
stimulated only by rapid local compression of the tissues
because they adapt in a few hundredths of a second Therefore, they are particularly important for detecting
tissue vibration or other rapid changes in the mechanical
state of the tissues.

Question 9

ithcing and tickle is transmitted by

Answer 9

type c that transmits slow pain signals

Question 10

Almost all sensory information from the somatic segments of the body enters the spinal cord through the

Answer 10

dorsal root of spinal nerves

Question 11

rom the entry
point into the cord and then to the brain, the sensory
signals are carried through one of two alternative sensory
pathways

Answer 11

he dorsal column–medial lemniscal system
or (2) the anterolateral system. These two systems come
back together partially at the level of the thalamus.

Question 12

dorsal column- medial lemniscal system

Answer 12

The dorsal column–medial lemniscal system, as its name implies, carries signals upward to the medulla of the brain mainly in the dorsal columns of the cord. Then,
after the signals synapse and cross to the opposite side in the medulla, they continue upward through the brain
stem to the thalamus by way of the medial lemniscus

Question 13

anterolateral system

Answer 13

signals in the anterolateral system, immediately after entering the spinal cord from the dorsal
spinal nerve roots, synapse in the dorsal horns of the
spinal gray matter, then cross to the opposite side of the
cord and ascend through the anterior and lateral white
columns of the cord. They terminate at all levels of the
lower brain stem and in the thalamus

Question 14

difference bewteen dorsal column system and anterolateral system

Answer 14

dorsal column system has large myelinated fibers and it is faster
they also have spatial orientation and transmits discrete types of mechanoreceptive sensations
while the anterolatera trasnmits abroad spectrum of sensations like pain cold warmth they are slow and does ave spatial orintation

Question 15

medial branch

Answer 15

The medial branch turns medially first and then upward in the dorsal column, proceeding by way of
the dorsal column pathway all the way to the brain.

Question 16

lateral branch

Answer 16

The lateral branch enters the dorsal horn of the cord
gray matter, then divides many times to provide terminals
that synapse with local neurons in the intermediate and
anterior portions of the cord gray matter

Question 17

Dorsal Column–Medial Lemniscal Pathway

Answer 17

that nerve fibers entering the dorsal columns pass uninterrupted up to the dorsal medulla, where they synapse in the dorsal column nuclei (the cuneate and gracile nuclei). From there, second-order neurons decussate immediately to the opposite side of the brain stem and continue upward through the medial lemnisci to the thalamus. In this pathway through the brain stem, each medial lemniscus is joined by additional fibers from the sensory nuclei of the trigeminal nerve; these fibers subserve the same sensory functions for the head that the dorsal column fibers subserve for the body. In the thalamus, the medial lemniscal fibers terminate in the thalamic sensory relay area, called the ventrobasal complex. From the ventrobasal complex, third-order nerve fibers project, as shown in , mainly to the postcentral gyrus of the cerebral cortex, which is called somatic
sensory area I these fibers also project to a smaller area in the lateral parietal cortex called somatic sensory area II)

Question 18

brodmann area

Answer 18

cerebral cortex,
showing that it is divided into about 50 distinct areas called Brodmann’s areas based on histological structural
differences.

Question 19

sensory signals of all modalities sensation goes to what part of brain

Answer 19

posterior to central fissure

Question 20

anterior part of parietal lobe is concerned with

Answer 20

intepretation and reception of all somatosensory signals

Question 21

visual and auditory signals terminate

Answer 21

in occipital and temporal lobe

Question 22

motor cortex is where

Answer 22

anterior part of central fissure and posterior part of frontal lobe is called as motor cortex

Question 23

somatosensensory area 1 and 2 are

Answer 23

anterior parietal lobe

Question 24

somatosensensory 1

Answer 24

high degree of localization and most important

Question 25

what functions are lost if there somatosensory area is cutted out

Answer 25

unable to judge the discre localization of touch
unable to judge critical pressure
weight of objects
shape and forms of objects called ASTEREOGENESIS
person is unable to judge the texture of things

Question 26

somatosensory association area

Answer 26

Brodmann’s areas 5 and 7 of the cerebral cortex, located
in the parietal cortex behind somatosensory area I play important roles in deciphering deeper
meanings of the sensory information in the somatosensory areas. Therefore, these areas are called somatosensory association areas.
Electrical stimulation in a somatosensory association
area can occasionally cause an awake person to experience a complex body sensation, sometimes even the
“feeling” of an object such as a knife or a ball. Therefore,
it seems clear that the somatosensory association area
combines information arriving from multiple points in
the primary somatosensory area to decipher its meaning.

Question 27

somatosensory association area receive tracts from

Answer 27

(1) somatosensory area I, (2) the ventrobasal nuclei of the
thalamus, (3) other areas of the thalamus, (4) the visual
cortex, and (5) the auditory cortex.

Question 28

amorphorsynthesis

Answer 28

Likewise, when feeling objects, the
person tends to recognize only one side of the object and
forgets that the other side even exists. This complex
sensory deficit is called amorphosynthesis
When the somatosensory
association area is removed on one side of the brain, the
person loses the ability to recognize complex objects and complex forms felt on the opposite side of the body. In
addition, he or she loses most of the sense of form of his
or her own body or body parts on the opposite side

Question 29

lateral inhibition

Answer 29

these
inhibitory signals spread to the sides of the excitatory
signal and inhibit adjacent neurons. For instance, consider an excited neuron in a dorsal column nucleus. Aside
from the central excitatory signal, short lateral pathways
transmit inhibitory signals to the surrounding neurons—
that is, these signals pass through additional interneurons
that secrete an inhibitory transmitter.
The importance of lateral inhibition is that it blocks
lateral spread of the excitatory signals and, therefore,
increases the degree of contrast in the sensory pattern
perceived in the cerebral cortex.

Question 30

the spinal cord anterolateral fibers originiate

Answer 30

in the dorsal horn laminae 1 4 5 and 6

Question 31

anatomy of anterolateral pathway

Answer 31

The spinal cord anterolateral fibers originate mainly in
dorsal horn laminae I, IV, V, and VI .These laminae are where many of the dorsal root sensory nerve fibers terminate after entering the cord. the anterolateral fibers cross immediately in the anterior commissure of the cord to the opposite anterior and lateral white columns, where they turn upward toward the brain by way of the anterior spinothalamic and lateral spinothalamic tracts. The upper terminus of the two spinothalamic tracts is mainly twofold: (1) throughout the reticular nuclei of the brain stem and (2) in two different nuclear complexes of the thalamus, the ventrobasal complex and the intralaminar nuclei. In general, the tactile signals are transmitted mainly into the ventrobasal complex, terminating in some of the same thalamic nuclei where the dorsal column tactile signals terminate. From here, the signals are transmitted to the somatosensory cortex along with the signals from the dorsal columns. Conversely, only a small fraction of the pain signals project directly to the ventrobasal complex of the thalamus. Instead, most pain signals terminate in the reticular nuclei of the brain stem and from there are relayed to the intralaminar nuclei of the thalamus where the pain signalsare further processed,

Question 32

different betwene anterolateral and dosrsal column

Answer 32

(1) the velocities of transmission are only one third to one
half those in the dorsal column–medial lemniscal system,
ranging between 8 and 40 m/sec; (2) the degree of spatial
localization of signals is poor; (3) the gradations of intensities are also far less accurate, with most of the sensations being recognized in 10 to 20 gradations of strength, rather
than as many as 100 gradations for the dorsal column
system; and (4) the ability to transmit rapidly changing or
rapidly repetitive signals is poor