HUMAN ANATOMY & PHYSIOLOGY
Copyright W
iley, 2020
548
CHAPTER 16
In the previous four chapters we described the organization of
the nervous system. In this chapter, we explore the levels and
components of sensation. We also examine the pathways that
convey somatic sensory nerve impulses from the body to the
ain
and the pathways that ca
y impulses from the
ain to skeletal
muscles to produce movements. As sensory impulses reach the CNS,
they become part of a large pool of sensory input. However, not every
nerve impulse transmitted to the CNS elicits a response. Rather,
each piece of incoming information is combined with other a
iving
and previously stored information in a process called integration.
Integration occurs at many places along pathways in the CNS,
such as the spinal cord,
ainstem, cerebellum, basal nuclei, and
cere
al cortex. You will also learn how the motor responses that
govern muscle contraction are modified at several of these levels.
To conclude this chapter, we introduce three complex integrative
functions of the
ain: (1) wakefulness and sleep, (2) learning and
memory, and (3) language.
Q Did you ever wonder how drugs such as aspirin and
ibuprofen relieve pain?
Sensory, Motor, and
Integrative Systems
Sensory, Motor, and Integrative Systems
and Homeostasis
The sensory and motor pathways of the body provide routes for input into the
ain and spinal
cord and for output to targeted organs for responses such as muscle contraction.
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iley, 2020
16.1 Sensation 549
In this chapter we discuss the somatic senses and visceral pain.
The special senses are the focus of Chapter 17. Visceral senses were
discussed in Chapter 15 and will be further described in association
with individual organs in later chapters.
The Process of Sensation
The process of sensation begins in a sensory receptor, which can be
either a specialized cell or the dendrites of a sensory neuron. A given
sensory receptor responds vigorously to one particular kind of stimu-
lus, a change in the environment that can activate certain sensory
eceptors. A sensory receptor responds only weakly or not at all to other
stimuli. This characteristic of sensory receptors is known as selectivity.
For a sensation to arise, the following four events typically occur:
1. Stimulation of the sensory receptor. An appropriate stimulus
must occur within the sensory receptor’s receptive field, that is, the
ody region where stimulation activates the receptor and produces
a response.
2. Transduction of the stimulus. A sensory receptor converts the
energy in the stimulus into a graded potential, a process known
as transduction. Recall that graded potentials vary in amplitude
(size), depending on the strength of the stimulus that causes them,
and are not propagated. (See Section 12.3 to review the diff er-
ences between action potentials and graded potentials.) Each type
of sensory receptor exhibits selectivity: It can transduce (convert)
only one kind of stimulus. For example, odorant molecules in the
air stimulate olfactory (smell) receptors in the nose, which trans-
duce the molecules’ chemical energy into electrical energy in the
form of a graded potential.
3. Generation of nerve impulses. When a graded potential in a sen-
sory neuron reaches threshold, it triggers one or more nerve im-
pulses, which then propagate toward the CNS. Sensory neurons
that conduct impulses from the PNS into the CNS are called first-
order neurons (see Section 16.3).
4. Integration of sensory input. A particular region of the CNS
receives and integrates (processes) the sensory nerve impulses.
Conscious sensations or perceptions are integrated in the cere
al
cortex. You seem to see with your eyes, hear with your ears, and feel
pain in an injured part of your body because sensory impulses from
each part of the body a
ive in a specific region of the cere
al cor-
tex, which interprets the sensation as coming from the stimulated
sensory receptors.
Sensory Receptors
Types of Sensory Receptors Several structural and
functional characteristics of sensory receptors can be used to group
them into diff erent classes. These include (1) microscopic structure,
(2) location of the receptors and the origin of stimuli that activate
them, and (3) type of stimulus detected.
MICROSCOPIC STRUCTURE On a microscopic level, sensory recep-
tors may be one of the following: (1) free nerve endings of first-order
16.1 Sensation
OBJECTIVES
• Define sensation, and discuss the components of sensation.
• Describe the diff erent ways to classify sensory receptors.
In its
oadest definition, sensation is the conscious or subconscious
awareness of changes in the external or internal environment. The
nature of the sensation and the type of reaction generated vary
according to the ultimate destination of nerve impulses (action
potentials) that convey sensory information to the CNS. Sensory
impulses that reach the spinal cord may serve as input for spinal
eflexes, such as the stretch reflex you learned about in Chapter 13.
Sensory impulses that reach the lower
ainstem elicit more complex
eflexes, such as changes in heart rate or
eathing rate. When sen-
sory impulses reach the cere
al cortex, we become consciously
aware of the sensory stimuli and can precisely locate and identify spe-
cific sensations such as touch, pain, hearing, or taste. As you learned
in Chapter 14, perception is the conscious interpretation of sensa-
tions and is primarily a function of the cere
al cortex. We have no
perception of some sensory information because it never reaches the
cere
al cortex. For example, certain sensory receptors constantly
monitor the pressure of blood in blood vessels. Because the nerve
impulses conveying blood pressure information propagate to the car-
diovascular center in the medulla oblongata rather than to the cere-
al cortex, blood pressure is not consciously perceived.
Sensory Modalities
Each unique type of sensation—such as touch, pain, vision, or hear-
ing—is called a sensory modality (mō-DAL-i-tē). A given sensory neu-
on ca
ies information for only one sensory modality. Neurons
elaying impulses for touch to the somatosensory area of the cere
al
cortex do not transmit impulses for pain. Likewise, nerve impulses
from the eyes are perceived as sight, and those from the ears are per-
ceived as sounds.
The diff erent sensory modalities can be grouped into two classes:
general senses and special senses.
1. The general senses refer to both somatic senses and visceral
senses. Somatic senses (somat- = of the body) include tactile sen-
sations (touch, pressure, vi
ation, itch, and tickle), thermal sensa-
tions (warm and cold), pain sensations, and proprioceptive
sensations. Proprioceptive sensations allow perception of both the
static (nonmoving) positions of limbs and body parts (joint and
muscle position sense) and movements of the limbs and head.
Visceral senses provide information about conditions within
internal organs, for example, pressure, stretch, chemicals, nausea,
hunger, and temperature.
2. The special senses include the sensory modalities of smell, taste,
vision, hearing, and equili
ium or balance.
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550 CHAPTER 16 Sensory, Motor, and Integrative Systems
in the inner ear, gustatory receptors in taste buds (Figure 16.1c) and
photoreceptors in the retina of the eye for vision. The olfactory recep-
tors for the sense of smell are not separate cells; instead, they are
located in olfactory cilia, which are hair like structures that project
from the dendrite of an olfactory receptor cell (a type of neuron). You
will learn more about the receptors for the special senses in Chapter 17.
A sensory receptor responds to a stimulus by generating a graded
potential known as a receptor potential (Figure 16.1a–c). In sensory
eceptors that are free nerve endings or encapsulated nerve endings,
if the receptor potential is large enough to reach threshold, it triggers
one or more nerve impulses in the axon of the sensory neuron (Figure
16.1a, b). The nerve impulses then propagate along the axon into
the CNS. In sensory receptors that are separate cells, the receptor
potential triggers release of neurotransmitter through exocytosis of
sensory neurons, (2) encapsulated nerve endings of first-order sen-
sory neurons, or (3) separate cells that synapse with first-order sen-
sory neurons. Free nerve endings are bare (not encapsulated)
dendrites; they lack any structural specializations that can be seen
under a light microscope (Figure 16.1a). Receptors for pain, tempera-
ture, tickle, itch, and some touch sensations are free nerve endings.
Receptors for other somatic and visceral sensations, such as pressure,
vi
ation, and some touch sensations, are encapsulated nerve end-
ings. Their dendrites are enclosed in a connective tissue capsule that
has a distinctive microscopic structure—for example, lamellated cor-
puscles (Figure 16.1b). The diff erent types of capsules enhance the
sensitivity or specificity of the receptor. Sensory receptors for some
special senses are specialized, separate cells that synapse with
sensory neurons. These include hair cells for hearing and equili
ium
FIGURE 16.1 Types of sensory receptors and their relationship to first-order sensory neurons.
(a) Free nerve endings: in this case, a cold-sensitive receptor. These endings are bare dendrites of first-
order neurons with no apparent structural specialization. (b) An encapsulated nerve ending: in this case,
a vi
ation-sensitive receptor. Encapsulated nerve endings are dendrites of first-order neurons. (c) A
separate receptor cell—here, a gustatory (taste) receptor—and its synapse with a first-order neuron.
Sensory receptors respond to stimuli by generating receptor potentials.
(a) First-order sensory
neuron with free nerve
endings
(b) First-order sensory
neuron with
encapsulated nerve
endings
(c) Sensory receptor
synapses with first-order
sensory neuron
Free nerve endings
(dendrites)
Triggers
Triggers Triggers
Cold
stimulus
Axon
Axon
Axon
Propagate
into CNS
Propagate
into CNS
Recepto
potential
Vi
ation
stimulus
Encapsulated
nerve ending
Nerve
impulses
Nerve
impulses
Recepto
potential
Suga
molecule
Gustatory (taste)
eceptor Synaptic vesicle
Dendrite
Release of
neurotransmitte
from sensory
ecepto
Neurotransmitte
Dendrite
Q Which senses are served by receptors that are separate cells?
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Copyright W
iley, 2020
16.1 Sensation 551
you may already have guessed, this causes the frequency of nerve
impulses in the sensory neuron to decrease. Because of adaptation,
the perception of a sensation may fade or disappear even though the
stimulus persists. For example, when you first step into a hot shower,
the water may feel very hot, but soon the sensation decreases to one of
comfortable warmth even though the stimulus (the high temperature
of the water) does not change.
Receptors vary in how quickly they adapt. Rapidly adapting re-
ceptors adapt very quickly. They are specialized for signaling changes
in a stimulus. Receptors associated with vi
ation, touch, and smell
are rapidly adapting. Slowly adapting receptors, by contrast, adapt
synaptic vesicles (Figure 16.1c). The neurotransmitter molecules lib-
erated from the synaptic vesicles diff use across the synaptic cleft and
produce a postsynaptic potential (PSP), a type of graded potential, in
the sensory neuron. If threshold is reached, the PSP will trigger one or
more nerve impulses, which propagate along the axon into the CNS.
The amplitude of a receptor potential varies with the intensity of
the stimulus, with an intense stimulus producing a large potential and
a weak stimulus eliciting a small one. Similarly, large receptor poten-
tials trigger nerve impulses at high frequencies in the first-order
neuron, in contrast to small receptor potentials, which trigger nerve
impulses at lower frequencies.
LOCATION OF RECEPTORS AND ORIGIN OF ACTIVATING STIMULI
Another way to group sensory receptors is based on the location of
the receptors and the origin of the stimuli that activate them.
• Exteroceptors (EKS-ter-ō-sep′-tors) are located at or near the
external surface of the body; they are sensitive to stimuli origi-
nating outside the body and provide information about the external
environment. The sensations of hearing, vision, smell, taste,
touch, pressure, vi
ation, temperature, and pain are conveyed by
exteroceptors.
• Interoceptors (IN-ter-ō-sep′-tors) or visceroceptors are located in
lood vessels, visceral organs, muscles, and the nervous system and
monitor conditions in the internal environment. The nerve impulses
produced by interoceptors usually are not consciously perceived;
occasionally, however, activation of interoceptors by strong stimuli
may be felt as pain or pressure.
• Proprioceptors (PRŌ-prē-ō-sep′-tors) are located in muscles, ten-
dons, joints, and the inner ear. They provide information about body
position, muscle length and tension, and the position and move-
ment of your joints.
TYPE OF STIMULUS DETECTED A third way to group sensory recep-