Page 568 - Fundamentals of anatomy physiology

slow-adapting receptors, while phasic receptors are Chapter 15 Sensory Pathways and the Somatic Nervous System 555
fast-adapting receptors. (Figure 15–3)
15-3 General sensory receptors are classified by the type of
stimulus that excites them p. 539
7. Three types of nociceptors in the body provide information
on pain as related to extremes of temperature, mechanical
damage, and dissolved chemicals. Myelinated Type A fibers
carry fast pain. Slower Type C fibers carry slow pain.
8. Thermoreceptors are found in the dermis. Mechanorecep-
tors are sensitive to distortion of their membranes, and in-
clude tactile receptors, baroreceptors, and proprioceptors.
There are six types of tactile receptors in the skin, and three
kinds of proprioceptors. Chemoreceptors include carotid
bodies and aortic bodies. (Figure 15–4)
15-4 Separate pathways carry somatic sensory and visceral
sensory information p. 544
9. Sensory neurons that deliver sensation to the CNS are referred
to as first-order neurons. These synapse on second-order
neurons in the brain stem or spinal cord. The next neuron in
this chain is a third-order neuron, found in the thalamus.
10. Three major somatic sensory pathways carry sensory informa-
tion from the skin and musculature of the body wall, head,
neck, and limbs: the spinothalamic pathway, the posterior column
pathway, and the spinocerebellar pathway. (Figure 15–5)
11. The spinothalamic pathway carries poorly localized
(“crude”) sensations of touch, pressure, pain, and temperature.
The axons involved decussate in the spinal cord and ascend
within the anterior and lateral spinothalamic tracts to
the ventral nuclei of the thalamus. Abnormalities along the
spinothalamic pathway can lead to phantom limb pain,
painful sensations that are perceived as real, and referred
pain, inaccurate localizations of the source of pain.
(Spotlight Figure 15–6, Figure 15–7; Table 15–1)
12. The posterior column pathway carries fine touch, pressure,
and proprioceptive sensations. The axons ascend within the
fasciculus gracilis and fasciculus cuneatus and relay infor-
mation to the thalamus by the medial lemniscus. Before the
axons enter the medial lemniscus, they cross over to the
opposite side of the brain stem. This crossing over is called
decussation. (Spotlight Figure 15–6; Table 15–1)
13. The spinocerebellar pathway, including the posterior and
anterior spinocerebellar tracts, carries sensations to the
cerebellum concerning the position of muscles, tendons, and
joints. (Spotlight Figure 15–6; Table 15–1)
14. Visceral sensory pathways carry information collected by in-
teroceptors. Sensory information from cranial nerves V, VII,
IX, and X is delivered to the solitary nucleus in the medulla
oblongata. Dorsal roots of spinal nerves T1–L2 carry visceral
Review Questions
LEVEL 1 Reviewing Facts and Terms
1. The larger the receptive field, the (a) larger the stimulus needed to
stimulate a sensory receptor, (b) fewer sensory receptors there are,
(c) harder it is to locate the exact point of stimulation, (d) larger the
area of the somatosensory cortex in the brain that deals with the area,
(e) closer together the receptor cells.
sensory information from organs between the diaphragm and
the pelvic cavity. Dorsal roots of spinal nerves S2–S4 carry
sensory information from more inferior structures.
15-5 The somatic nervous system is an efferent division that
controls skeletal muscles p. 549
15. Somatic motor (descending) pathways always involve an upper
motor neuron (whose cell body lies in a CNS processing cen-
ter) and a lower motor neuron (whose cell body is located in
a nucleus of the brain stem or spinal cord). (Figure 15–8)
16. The neurons of the primary motor cortex are pyramidal cells.
The corticospinal pathway provides voluntary skeletal muscle
control. The corticobulbar tracts end at the cranial nerve nu-
clei. The corticospinal tracts synapse on lower motor neurons
in the anterior gray horns of the spinal cord. The corticospinal
tracts are visible along the medulla oblongata as a pair of thick
bands, the pyramids, where most of the axons decussate to
enter the descending lateral corticospinal tracts. Those that
do not cross over enter the anterior corticospinal tracts. The
corticospinal pathway provides a rapid, direct mechanism for
controlling skeletal muscles. (Figure 15–9; Table 15–2)
17. The medial and lateral pathways include several other cen-
ters that issue motor commands as a result of processing per-
formed at a subconscious level. (Table 15–2)
15 18. The medial pathway primarily controls gross movements of
the neck, trunk, and proximal limbs. It includes the vestibulo-
spinal, tectospinal, and reticulospinal tracts. The vestibulospi-
nal tracts carry information related to maintaining balance
and posture. Commands carried by the tectospinal tracts
change the position of the head, neck, and upper limbs in
response to bright lights, sudden movements, or loud noises.
Motor commands carried by the reticulospinal tracts vary
according to the region stimulated. (Table 15–2)
19. The lateral pathway consists of the rubrospinal tracts, which
primarily control muscle tone and movements of the distal
muscles of the upper limbs. (Table 15–2)
20. The basal nuclei adjust the motor commands issued in other
processing centers and provide background patterns of move-
ment involved in voluntary motor activities.
21. The cerebellum monitors proprioceptive sensations, visual in-
formation, and vestibular (balance) sensations. The integrative
activities performed by neurons in the cortex and nuclei of the
cerebellum are essential for the precise control of movements.
22. Spinal and cranial reflexes provide rapid, involuntary, pre-
programmed responses that preserve homeostasis. Voluntary
responses are more complex and require more time to prepare
and execute.
23. During development, the spinal and cranial reflexes are first
to appear. Complex reflexes develop over years, as the CNS
matures and the brain grows in size and complexity.
See the blue Answers tab at the back of the book.
2. ___________ receptors are normally inactive, but become active for a
short time whenever there is a change in the modality that they monitor.
3. In sensory organ function what is adaptation?
4. The area of primary sensory cortex devoted to a body region is relative
to the (a) size of the body area, (b) distance of the body area from the
brain, (c) number of motor units in the area of the body, (d) number