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Somatic Senses
Chapter 21
CHAPTER SUMMARY
This chapter begins with a general introduction to sensations: the definition of sensations, characteristics of sensations,
classification of sensations, and the classification of sensory receptors are described. Detailed explanations are provided
for the somatic sensations, namely tactile sensations (including touch, pressure, vibration, itch and tickle), thermal
sensations, and pain sensations, as well as the proprioceptive sensations (including the characteristics of muscle spindles,
tendon organs, and joint kinesthetic receptors). The somatic sensory pathways, the somatic motor pathways, and
integration of sensory input and output are all thoroughly described. A glossary of key medical terms associated with
general senses and sensory and motor pathways is provided. This chapter concludes with a thorough study outline, an
excellent self-quiz, critical thinking questions, and answers to questions that accompany chapter figures.
STUDENT OBJECTIVES
1. Define a sensation and describe the conditions necessary for a sensation to occur.
2. Describe the different ways to classify sensory receptors.
3. List the locations and functions of the receptors for tactile sensations (touch, pressure, vibration, itch and tickle),
thermal sensations (warmth and cold), and pain sensations.
4. Identify the receptors for proprioception and describe their functions.
5. Discuss the neuronal components and functions of the posterior column-medial lemniscus, the anterolateral, and the
spinocerebellar pathways.
6. Compare the locations and functions of the direct and indirect motor pathways.
7. Explain how the basal ganglia and cerebellum contribute to motor responses.
8. Explain how sensory input and motor output are integrated.
LECTURE OUTLINE
A. Overview of Sensations (p. 652)
1. Sensation is the conscious or subconscious awareness of external or internal stimuli.
2. For a sensation to arise, four events typically occur:
i. a stimulus capable of activating specific sensory neurons must occur
ii. a sensory receptor or sense organ must respond to the stimulus and transduce (convert) it into a nerve
impulse
iii. nerve impulses are conducted to the brain
iv. a region of the brain must receive and integrate the nerve impulses, producing a sensation
3. Sensory receptors vary in complexity; some are free nerve endings, some are encapsulated nerve endings, and
others are specialized, separate cells that synapse with sensory neurons.
4. Conscious sensations or perceptions are integrated in the cerebral cortex; the distinct quality that makes one
sensation (e.g., touch) different from another sensation (e.g., pain) is its modality.
5. A characteristic of many sensory receptors is adaptation, i.e., a decrease in perception of a
sensation during a prolonged stimulus; receptors vary in how quickly they adapt.
6. Sensations can be grouped into two classes:
i. general senses, which include both somatic senses (which include touch, pressure, vibration, warm,
cold, pain, and proprioceptive sensations) and visceral senses
ii. special senses, which include smell, taste, vision, hearing, and equilibrium (balance).
7. Sensory receptors can be classified according to several characteristics (see Table 21.1):
i. by location:
- exteroceptors are located at or near the surface of the body
- interoceptors are located in blood vessels and viscera
- proprioceptors are located in muscles, tendons, joints, and the internal ear
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ii.
by type of stimulus they detect:
- mechanoreceptors detect mechanical pressure or stretching
- thermoreceptors detect changes in temperature
- nociceptors detect pain, usually as a result of damage to tissues
- photoreceptors detect light in the eye
- chemoreceptors detect chemicals
- osmoreceptors detect the osmotic pressure of body fluids
iii. by their degree of complexity:
- simple receptors are associated with the general senses
- complex receptors are associated with the special senses
B. Somatic Sensations (see Table 21.2) (p. 653)
1. Somatic sensations arise from stimulation of sensory receptors embedded in:
i. skin or subcutaneous layer
ii. mucous membranes of the mouth, vagina, and anus
iii. muscles, tendons, and joints
iv. internal ear
2. Some parts of the body are densely populated with receptors (e.g., tip of tongue, lips, fingertips) and other parts
of the body have few receptors (e.g., back of neck).
3. Somatic sensations that arise from stimulation of the skin surface are called cutaneous sensations.
4. Tactile sensations include touch, pressure and vibration (all of which are detected by a variety of encapsulated
receptors) as well as itch and tickle (which are detected by free nerve endings).
i. Touch sensations arise from stimulation of tactile receptors located in the skin and subcutaneous
layer; the receptors for both crude touch and fine touch include:
a. corpuscles of touch or Meissner’s corpuscles for fine touch
b. hair root plexuses that detect hair movement
c. type I cutaneous mechanoreceptors or Merkel disks for fine touch
d. type II cutaneous mechanoreceptors or Ruffini corpuscles that detect stretching that occurs
during digit or limb movements
ii. Pressure sensations are detected by tactile receptors in numerous tissues that are widely distributed in
the body; the receptors for pressure include:
a. corpuscles of touch
b. type I cutaneous mechanoreceptors
c. lamellated or Pacinian corpuscles
iii. Vibration sensations are detected by corpuscles of touch and lamellated corpuscles.
iv. Itch sensation arises from stimulation by certain body chemicals, often as a result of a
local inflammatory response.
v. Tickle sensation arises from stimulation of free nerve endings and lamellated corpuscles.
5. Thermal sensations are detected by thermoreceptors which are free nerve endings:
i. cold receptors are located in the stratum basale of the epidermis and are activated by temperatures
ranging between 10 and 40C
ii. warm receptors are located in the dermis and are activated by temperatures ranging between 32 and
48C
iii. temperatures below 10C and above 48C stimulate pain receptors
6. Pain sensations provide information about tissue-damaging stimuli and therefore enable us to protect ourselves
against further damage;
i. pain receptors or nociceptors are free nerve endings and are located in almost every tissue except the
brain; they adapt only slightly or not at all
ii. there are two major types of pain, fast pain and slow pain; pain can also be classified as:
a. somatic pain, which includes superficial somatic pain and deep somatic pain
b. visceral pain
iii. referred pain is a phenomenon in which visceral pain is felt in the skin overlying the stimulated organ
or in a surface area far from a stimulated organ
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iv. patients who have had a limb amputated may experience sensations such as pain, itching, pressure,
tingling, or itch as though the limb were still present; this phenomenon is called phantom limb
sensation
7. Proprioceptive Sensations:
i. The proprioceptive sensations and kinesthesia provide an awareness of positions and movements of
parts of the body.
ii. Proprioceptors adapt slowly and only slightly; they include:
a. muscle spindles located within skeletal muscles
b. tendon organs located within tendons
c. joint kinesthetic receptors located within synovial joint capsules
d. hair cells of the inner ear (provide information for maintaining balance)
iii. Proprioceptive nerve impulses pass along ascending tracts in the spinal cord to the thalamus and from
there to the somatosensory area of the cerebral cortex; proprioceptive impulses also pass along
spinocerebellar tracts to the cerebellum.
iv. Muscle spindles:
a. a muscle spindle consists of 3 to 10 intrafusal muscle fibers that are enclosed in a spindleshaped connective tissue capsule; surrounding a muscle spindle are extrafusal (i.e., regular)
muscle fibers
b. the central area of an intrafusal fiber is innervated by two types of afferent fibers which detect
stretching of the central area
c. both ends of an intrafusal fiber contain actin and myosin filaments and contract when
stimulated by gamma motor neurons; the extrafusal muscle fibers are innervated by alpha
motor neurons
d. muscle spindles monitor changes in the length of skeletal muscles in order to prevent
overstretching of muscles; this information is relayed to the cerebral cortex to provide
perception of limb position, and is also relayed to the cerebellum to aid in the coordination of
muscle contraction
v. Tendon organs (or Golgi tendon organs):
a. tendon organs are located at the junction of a tendon with a muscle
b. a tendon organ consists of a thin capsule of connective tissue that encloses a few tendon
fascicles (bundles of collagen fibers)
c. a tendon organ is innervated by sensory fibers
d. tendon organs detect tension applied to a tendon
e. tendon organs help protect tendons and their associated muscles from damage due to excessive
tension; they also monitor the force of contraction of associated muscles
vi. Joint kinesthetic receptors:
a. there are several types of joint kinesthetic receptors located within and around the articular
capsules of synovial joints:
- free nerve endings and type II cutaneous mechanoreceptors in joint capsules detect
pressure
- lamellated corpuscles outside articular capsules detect acceleration and deceleration
of joint movement
- receptors (similar to tendon organs) in articular ligaments adjust adjacent muscles
when excessive strain is placed on a joint
C. Somatic Sensory Pathways (p. 658)
1. Somatic sensory pathways relay information from somatic receptors to the primary somatosensory areas in the
cerebral cortex and to the cerebellum; the pathways to the cerebral cortex consist of thousands of sets of three
neuron:
i. first-order neurons carry signals from somatic receptors into the brain stem or spinal cord via cranial
nerves or spinal nerves
ii. second-order neurons carry signals from the spinal cord and brain stem to the thalamus; axons of
second-order neurons decussate to the opposite side before ascending to the thalamus
iii. third-order neurons project from the thalamus to the primary somatosensory areas where conscious
perception of sensations results
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2. There are two general pathways by which somatic sensory signals entering the spinal cord ascend to the
cerebral cortex (see Table 21.3):
i. posterior column-medial lemniscus pathways:
a. transmit nerve impulses for conscious proprioception and most tactile sensations
b. axons of the first-order neurons form the posterior (dorsal) columns (gracile fasciculus and
cuneate fasciculus) in each side of the spinal cord
c. axons of the second-order neurons decussate in the medulla and enter the medial lemniscus
which is a projection tract that extends from the medulla to the thalamus
d. third-order neurons project from the thalamus to the primary somatosensory area
e. impulses conducted along this pathway give rise to the following sensations:
- fine touch
- stereognosis
- proprioception and kinesthesia
- weight discrimination
- vibratory sensations
ii. anterolateral (spinothalamic) pathways:
a. transmit nerve impulses for pain and temperature as well as for tickle, itch, crude touch, and
pressure
b. axons of the first-order neurons enter the spinal cord and synapse with second-order neurons in
the posterior gray horns
c. axons of second-order neurons decussate and ascend to the brain stem in either:
- lateral spinothalamic tract which conveys impulses for pain and temperature
- anterior spinothalamic tract which conveys impulses for tickle, itch, crude touch,
pressure and vibrations
d. axons of second-order neurons enter the thalamus and synapse with third-order neurons which
project to the primary somatosensory area
3. The primary somatosensory area in each parietal lobe has been mapped out:
i. some parts of the body are represented by large areas while other parts of the body are represented by
small areas in the primary somatosensory cortex
ii. the relative sizes of the areas are proportional to the number of receptors in each respective part of the
body
4. Proprioceptive information reaches the cerebellum via two pathways:
i. posterior spinocerebellar tract:
ii. anterior spinocerebellar tract:
D. Somatic Motor Pathways (p. 661)
1. Both direct and indirect pathways (see Table 21.4) extend from the cerebral cortex to skeletal muscles.
2. Motor cortex:
i. the primary motor area (located in the precentral gyrus) of the cerebral cortex is the major control
region for initiation of voluntary movements
ii. the adjacent premotor area and the primary somatosensory area also contribute fibers to the
descending motor pathways
iii. the degree of representation of different muscles in the primary motor area is proportional to the
number of motor units present in a particular muscle
3. Direct motor pathways:
i. voluntary motor impulses are transmitted from the motor cortex to somatic efferent neurons that
innervate skeletal muscles via the direct or pyramidal pathways
ii. these pathways consists of the following neurons:
a. local circuit neurons receive input from somatic sensory receptors and from higher centers in
the brain
b. upper motor neurons (UMNs) whose axons descend into the medulla, where about 90% of the
axons decussate (the remaining 10% decussate at lower levels), and terminate in nuclei of
cranial nerves or in the anterior gray horns of the spinal cord
c. basal ganglia neurons assist movement by providing input via the thalamus to UMNs
d. cerebellar neurons also assist movement by providing input via the thalamus to UMNs
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e.
axons of most UMNs synapse with interneurons which in turn synapse with lower motor
neurons (LMNs) whose axons innervate skeletal muscles; since each LMN receives and
integrates excitatory and inhibitory input from many presynaptic neurons (e.g., UMNs and
interneurons), LMNs are also called the final common pathway
iii. the direct pathways convey impulses that result in precise, voluntary movements via three pairs of
tracts containing axons of UMNs:
a. lateral corticospinal tracts:
- about 90% of the axons of UMNs decussate in the medulla to form the lateral
corticospinal tracts in the right and left lateral white columns of the spinal cord
- axons of LMNs exit all levels of the spinal cord to innervate skeletal muscles in the
limbs, hands, and feet
b. anterior corticospinal tracts:
- about 10% of the axons of UMNs do not cross over in the medulla but descend on
the same side to form the anterior corticospinal tracts in the right and left anterior
white columns
- some axons of these UMNs decussate at several spinal cord levels and then synapse
with interneurons or LMNs in the anterior gray horns
- axons of these LMNs exit the cervical and upper thoracic segments of the spinal cord
to innervate muscles that control muscles of the neck and part of the trunk, thus
coordinating movements of the axial skeleton
c. corticobulbar tracts:
- some axons of UMNs extend to the midbrain where they form the corticobulbar
tracts in right and left cerebral peduncles
- theaxons terminate in nuclei of nine pairs of cranial nerves; LMNs of these cranial
nerves convey nerve impulses that control precise, voluntary movements of the eyes,
tongue, neck, chewing, facial expression, and speech
4. Indirect motor pathways:
i. Indirect (extrapyramidal) pathways include all somatic motor tracts other than the corticospinal and
corticobulbar tracts.
ii. Indirect (extrapyramidal) motor output from the brain arises from various nuclei in the brain stem and
travels along five major pairs of spinal cord tracts (and terminate on interneurons and LMNs): (see
Table 21.4)
a. rubrospinal tract begins in the red nucleus and transmits nerve impulses to the opposite side
of the body to regulate precise, discrete movements, especially of the distal limbs
b. tectospinal tract begins in the superior colliculus and transmits nerve impulses to the opposite
side of the body to neck muscles that control movements of the head in response to visual
stimuli
c. vestibulospinal tract begins in the vestibular nucleus and transmits nerve impulses to the same
side of the body to regulate muscle tone in response to movements of the head; it therefore
plays a major role in balance
d. lateral reticulospinal tract begins in the reticular formation of the medulla; it facilitates (on
the same side of the body) flexor reflexes, inhibits extensor reflexes, and decreases muscle tone
in muscles of the axial skeleton and proximal limbs
e. medial reticulospinal tract begins in the pons; it facilitates (on the same side of the body)
extensor reflexes, inhibits flexor reflexes, and increases muscle tone in muscles of the axial
skeleton and proximal limbs
5. Roles of the Basal Ganglia:
i.
Since basal ganglia have many connections with other parts of the brain, they are able to help
program habitual or automatic movement sequences and to set an appropriate level of muscle tone
ii.
Basal ganglia are also involved in all aspects of cortical function, including
sensory, limbic, cognitive, and linguistic functions.
6. Roles of the Cerebellum:
i. The cerebellum is active in:
a. learning and performing rapid, coordinated, highly skilled movements
b. maintaining proper posture and equilibrium
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ii. Cerebellar functions involve four activities:
a. monitoring intentions for movements
b. monitoring actual movement
c. comparing command signals (intentions for movements) with sensory information (actual
performance)
d. sending out corrective signals
E. Integration of Sensory Input and Motor Output (p. 666)
1. Sensory input informs the CNS about changes in the external and internal environment.
2. Incoming sensory information is integrated with other sensory information within many regions of the CNS,
i.e., spinal cord, brain stem, cerebellum, basal ganglia, and cerebral cortex.
3. As a result, any motor response to make a muscle contract or a gland secrete can be modified and responded to
in any of these regions.
4. Motor portions of the cerebral cortex play a major role for initiating and controlling precise muscle movements.
5. The basal ganglia largely integrate semivoluntary, automatic movements (e.g., walking).
6. The cerebellum assists the motor cortex and basal ganglia by making body movements smooth and coordinated
and by contributing significantly to maintaining normal posture and balance.
F. Key Medical Terms Associated with Somatic Senses (p. 667)
1. Students should familiarize themselves with the glossary of key medical terms.