chapter 13 Flashcards
(247 cards)
1
Q
The first layer of protection for the central nervous system is
A
the hard bony skull and vertebral column.
2
Q
The second protective layer of the CNS
A
is the meninges, three membranes that lie between the bony encasement and the nervous tissue in both the brain and spinal cord
3
Q
The final protective layer of the CNS is
A
cerebrospinal fluid
4
Q
The spinal cord is located within
A
the vertebral canal of the vertebral column
5
Q
meninges (me-NIN-jēz; singular is meninx [MĒ-ninks])
A
three distinct protective, connective tissue coverings that encircle the spinal cord and brain.
6
Q
From superficial to deep the menegies are .
A
the (1) dura mater, (2) arachnoid mater, and (3) pia mater
7
Q
epidural space (ep′-i-DOO-ral),
A
a space between the dura mater and the wall of the vertebral canal
8
Q
There is a cushion layer of fat in the
A
epidural space
9
Q
Dura mater (DOO-ra MĀ-ter = tough mother).
A
The most superficial of the three spinal meninges is a thick strong layer composed of dense irregular connective tissue.
10
Q
Arachnoid mater (a-RAK-noyd MĀ-ter; arachn- = spider; - oid= similar to).
A
This layer, the middle of the meningeal membranes, is a thin, avascular covering comprised of cells and thin, loosely arranged collagen and elastic fibers
11
Q
Between the dura mater and the arachnoid mater is a thin _________ which contains interstitial fluid.
A
subdural space
12
Q
Pia mater (PĒ-a MĀ-ter; pia = delicate).
A
This innermost meninx is a thin transparent connective tissue layer that adheres to the surface of the spinal cord and brain.
13
Q
Within the pia mater are
A
many blood vessels that supply oxygen and nutrients to the spinal cord.
14
Q
denticulate ligaments (den-TIK-ū-lāt = small tooth),
A
Triangular-shaped membranous extensions of the pia mater suspend the spinal cord in the middle of its dural sheath
15
Q
Extending along the entire length of the spinal cord, the denticulate ligaments
A
protect the spinal cord against sudden displacement that could result in shock.
16
Q
Between the arachnoid mater and pia mater is a space,
A
the subarachnoid space, which also contains shock-absorbing cerebrospinal fluid.
17
Q
Why does the spinal cord not extend to the end of the spine
A
Elongation of the spinal cord stops around age 4 or 5, but growth of the vertebral column continues. Thus, the spinal cord does not extend the entire length of the adult vertebral column.
18
Q
the cervical enlargement,
A
extends from the fourth cervical vertebra (C4) to the first thoracic vertebra (T1). Nerves to and from the upper limbs arise from the cervical enlargement.
19
Q
the lumbosacral enlargement,
A
extends from the ninth to the twelfth thoracic vertebra. Nerves to and from the lower limbs arise from the lumbar enlargement.
20
Q
Inferior to the lumbar enlargement, the spinal cord terminates as a tapering, conical structure called the __________which ends at the level of the _______________
A
conus medullaris
intervertebral disc between the first and second lumbar vertebrae (L1–L2) in adults.
21
Q
Arising from the conus medullaris is the _______________which is ______________
A
filum terminale (FĪ-lum ter-mi-NAL-ē = terminal filament),
an extension of the pia mater that extends inferiorly, fuses with the arachnoid mater and dura mater, and anchors the spinal cord to the coccyx.
22
Q
In a spinal tap (lumbar puncture),
A
a local anesthetic is given, and a long hollow needle is inserted into the subarachnoid space to withdraw cerebrospinal fluid (CSF)
23
Q
Spinal nerves are
A
the paths of communication between the spinal cord and specific regions of the body.
24
Q
How many pairs of nerves are at each segment of the spine
A
There are 8 pairs of cervical nerves (represented in Figure 13.2 as C1–C8), 12 pairs of thoracic nerves (T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5), and 1 pair of coccygeal nerves (Co1).
25
Two bundles of axons, called roots,
connect each spinal nerve to a segment of the cord by even smaller bundles of axons called rootlets
26
The posterior root and rootlets contain
only sensory axons, which conduct nerve impulses from sensory receptors in the skin, muscles, and internal organs into the central nervous system.
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spinal (dorsal root) ganglion
A swelling in the posterior root which contains the cell bodies of sensory neurons.
28
The anterior root and rootlets contain
axons of motor neurons, which conduct nerve impulses from the CNS to effectors (muscles and glands).
29
cauda equina
The roots of these lower spinal nerves angle inferiorly alongside the filum terminale in the vertebral canal like wisps of hair.
30
As spinal nerves branch from the spinal cord,
they pass laterally to exit the vertebral canal through the intervertebral foramina between adjacent vertebrae.
31
The spinal cord extends
from the medulla oblongata of the brain to the superior border of the second lumbar vertebra.
32
A transverse section of the spinal cord
reveals regions of white matter that surround an inner core of gray matter
33
The white matter of the spinal cord consists primarily of
bundles of myelinated axons of neurons.
34
The anterior median fissure
a wide groove on the anterior (ventral) side of the spinal cord
35
The posterior median sulcus is
a narrow furrow on the posterior (dorsal) side of the spinal cord
36
The gray matter of the spinal cord is shaped like the letter H or a butterfly; it consists of
dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia.
37
The gray commissure (KOM-mi-shur)
forms the crossbar of the H of the gray matter
38
the central canal
extends the entire length of the spinal cord and is filled with cerebrospinal fluid. At its superior end, the central canal is continuous with the fourth ventricle (a space that contains cerebrospinal fluid) in the medulla oblongata of the brain.
39
Anterior to the gray commissure is the anterior white commissure, which
connects the white matter of the right and left sides of the spinal cord.
40
In the gray matter of the spinal cord and brain, clusters of neuronal cell bodies form functional groups called
nuclei.
41
The posterior gray horns contain
axons of incoming sensory neurons as well as cell bodies and axons of interneurons
42
The anterior gray horns contain
somatic motor nuclei, which are clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles.
43
The lateral gray horns contain
autonomic motor nuclei, which are clusters of cell bodies of autonomic motor neurons that regulate the activity of cardiac muscle, smooth muscle, and glands.
44
The anterior and posterior gray horns divide the white matter on each side into three broad areas called
funiculi
45
tracts
distinct bundles of axons having a funiculus origin or destination and carrying similar information. These bundles, which may extend long distances up or down the spinal cord
46
Sensory (ascending) tracts consist of
axons that conduct nerve impulses toward the brain
47
Tracts consisting of axons that carry nerve impulses from the brain are called
motor (descending) tracts.
48
How does the does the spinal cord allow sensory input and motor output to be processed by the spinal cord
1. Sensory receptors detect a sensory stimulus.
2. Sensory neurons convey this sensory input in the form of nerve impulses along their axons, which extend from sensory receptors into the spinal nerve and then into the posterior root. From the posterior root, axons of sensory neurons may proceed along three possible paths (see steps 3, 4, and 5).
3. Axons of sensory neurons may enter the posterior gray horn and then extend into the white matter of the spinal cord and ascend to the brain as part of a sensory tract.
4. Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons whose axons extend into the white matter of the spinal cord and then ascend to the brain as part of a sensory tract.
5, Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons that in turn synapse with somatic motor neurons that are involved in spinal reflex pathways. Spinal cord reflexes are described in more detail later in this chapter.
49
How does the spinal cord allow motor outouts to be processed by the spinal cord
1.Motor output from the spinal cord to skeletal muscles involves somatic motor neurons of the anterior gray horn. Many somatic motor neurons are regulated by the brain. Axons from higher brain centers form motor tracts that descend from the brain into the white matter of the spinal cord. There they synapse with somatic motor neurons either directly or indirectly by first synapsing with interneurons that in turn synapse with somatic motor neurons.
2. When activated, somatic motor neurons convey motor output in the form of nerve impulses along their axons, which sequentially pass through the anterior gray horn and anterior root to enter the spinal nerve. From the spinal nerve, axons of somatic motor neurons extend to skeletal muscles of the body.
3. Motor output from the spinal cord to cardiac muscle, smooth muscle, and glands involves autonomic motor neurons of the lateral gray horn. When activated, autonomic motor neurons convey motor output in the form of nerve impulses along their axons, which sequentially pass through the lateral gray horn, anterior gray horn, and anterior root to enter the spinal nerve.
4. From the spinal nerve, axons of autonomic motor neurons from the spinal cord synapse with another group of autonomic motor neurons located in the peripheral nervous system (PNS). The axons of this second group of autonomic motor neurons in turn synapse with cardiac muscle, smooth muscle, and glands. You will learn more about autonomic motor neurons when the autonomic nervous system is described in Chapter 15.
50
Sensory input is conveyed
from sensory receptors to the posterior gray horns of the spinal cord
51
motor output is conveyed
from the anterior and lateral gray horns of the spinal cord to effectors (muscles and glands).
52
the amount of white matter decreases
from cervical to sacral segments of the spinal cord.
53
why does the amount of white matter decreases from cervical to sacral segments of the spinal cord.
(1) As the spinal cord ascends from sacral to cervical segments, more ascending axons are added to spinal cord white matter to form more sensory tracts. (2) As the spinal cord descends from cervical to sacral segments, the motor tracts decrease in thickness as more descending axons leave the motor tracts to synapse with neurons in the gray matter of the spinal cord.
54
Describe the characteristics of the cervical segment of the spinal cord
Relatively large diameter, relatively large amounts of white matter, oval; in upper cervical segments (C1–C4), posterior gray horn is large but anterior gray horn is relatively small; in lower cervical segments (C5 and below), posterior gray horns are enlarged and anterior gray horns are well developed.
55
Describe the characteristics of the thoracic segment of the spinal cord.
Small diameter due to relatively small amounts of gray matter; except for first thoracic segment, anterior and posterior gray horns are relatively small; small lateral gray horn is present.
56
Describe the characteristics of the lumbar segment of the spinal cord.
Nearly circular; very large anterior and posterior gray horns; small lateral gray horn is present in upper segments; relatively less white matter than cervical segments.
57
Describe the characteristics of the sacral segment of the spinal cord.
Relatively small, but relatively large amounts of gray matter; relatively small amounts of white matter; anterior and posterior gray horns are large and thick.
58
Describe the characteristics of the coccygeal segment of the spinal cord.
Resembles lower sacral spinal segments, but much smaller.
59
Spinal nerves
are associated with the spinal cord and, like all nerves of the peripheral nervous system (PNS), are parallel bundles of axons and their associated neuroglial cells wrapped in several layers of connective tissue.
60
The 31 pairs of spinal nerves are named and numbered according to
the region and level of the vertebral column from which they emerge
61
The first cervical pair of spinal nerves emerges from the spinal cord
between the occipital bone and the atlas
62
Spinal nerves C1–C7 exit the vertebral canal
above their corresponding vertebrae.
63
Spinal nerve C8 exits the vertebral canal
between vertebrae C7 and T1
64
Spinal nerves T1–L5 exit the vertebral canal
below their corresponding vertebrae.
65
From the spinal cord, the roots of the sacral spinal nerves (S1–S5) and the coccygeal spinal nerves.
enter the sacral canal, the part of the vertebral canal in the sacrum
66
spinal nerves S1–S4 exit the sacral canal via
the four pairs of anterior and posterior sacral foramina, and spinal nerves S5 and Co1 exit the sacral canal via the sacral hiatus.
67
The posterior and anterior roots unite to form a spinal nerve at the
intervertebral foramen.
68
Because the posterior root contains sensory axons and the anterior root contains motor axons, a spinal nerve is classified as a
mixed nerve
69
Each spinal nerve and cranial nerve consists of
many individual axons and contains layers of protective connective tissue coverings
70
Individual axons within a nerve, whether myelinated or unmyelinated, are wrapped in
endoneurium (en′-doˉ -NOO-rē-um; endo- = within or inner; -neurium = nerve), the innermost layer.
71
Groups of axons with their endoneurium are held together in bundles called
nerve fascicles, each of which is wrapped in perineurium (per′-i-NOO-rē-um; peri- = around), the middle layer.
72
The outermost covering over the entire nerve is the
epineurium (ep′-i-NOO-rē-um; epi- = over). It consists of fibroblasts and thick collagen fibers.
73
Three layers of connective tissue wrappings protect axons:
Endoneurium surrounds individual axons, perineurium surrounds bundles of axons (nerve fascicles), and epineurium surrounds an entire nerve.
74
A short distance after passing through its intervertebral foramen,
a spinal nerve divides into several branches. These branches are known as rami
75
The posterior (dorsal) ramus (RĀ-mus; singular form) serves
the deep muscles and skin of the posterior surface of the trunk.
76
The anterior (ventral) ramus serves
the muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk.
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meningeal branch (me-NIN-jē′-al).
This branch reenters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges
78
The branches of a spinal nerve are
the posterior ramus, the anterior ramus, the meningeal branch, and the communicating rami.
79
Axons from the anterior rami of spinal nerves, except for thoracic nerves T2–T12,
do not go directly to the body structures they supply.
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plexus
networks on both the left and right sides of the body that join with various numbers of axons from anterior rami of adjacent nerves.
81
The principal plexuses are the
cervical plexus, brachial plexus, lumbar plexus, and sacral plexus. A smaller coccygeal plexus is also present
82
The anterior rami of spinal nerves T2–T12
do not enter into the formation of plexuses and are known as intercostal nerves or thoracic nerves.
83
After leaving its intervertebral foramen, the anterior ramus of nerve T2 innervates the
intercostal muscles of the second intercostal space and supplies the skin of the axilla and posteromedial aspect of the arm.
84
Nerves T3–T6 .
extend along the costal grooves of the ribs and then to the intercostal muscles and skin of the anterior and lateral chest wall
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Nerves T7–T12 supply
the intercostal muscles and abdominal muscles, along with the overlying skin.
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dermatome
The area of the skin that provides sensory input to the CNS via one pair of spinal nerves or the trigeminal (V) nerve
87
Knowing which spinal cord segments supply each dermatome makes it possible to
locate damaged regions of the spinal cord
88
The cervical plexus (SER-vi-kul) is formed by
the roots (anterior rami) of the first four cervical nerves (C1–C4), with contributions from C5 (Figure 13.8). There is one on each side of the neck alongside the first four cervical vertebrae.
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The cervical plexus supplies
the skin and muscles of the head, neck, and superior part of the shoulders and chest. The phrenic nerves arise from the cervical plexuses and supply motor fibers to the diaphragm. Branches of the cervical plexus also run parallel to two cranial nerves, the accessory (XI) nerve and hypoglossal (XII) nerve.
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the Lesser occipital nerve
originates from C2 and is a sensory branch that serves the Skin of scalp posterior and superior to ear.
91
Great auricular nerve
originates from C2-C3 and is a sensory branch that serves the Skin anterior, inferior, and over ear, and over parotid glands.
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Transverse cervical branch
a senxory branch that Originates from C2-C3 and serves the Skin over anterior and lateral aspect of neck.
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Supraclavicular branch
A sensory branch originating from C3-C4 that serves the Skin over superior portion of chest and shoulder.
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Ansa cervicalis
Motor branch that Divides into superior and inferior roots.
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Superior Root
Motor branch that originates at C1 and controls the Infrahyoid and geniohyoid muscles of neck.
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Inferior Root
Motor branch that originates at C2-C3 and controls the Infrahyoid muscles of the neck
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Phrenic
Motor branch that originates at C3-C5 and controls the diaphragm
98
Segmental branches
Motor branches that originate at C1-C5 and control Prevertebral (deep) muscles of neck, levator scapulae, and scalenus medius muscles.
99
The cervical plexus supplies
the skin and muscles of the head, neck, superior portion of the shoulders and chest, and diaphragm.
100
The roots (anterior rami) of spinal nerves C5–C8 and T1 form
the brachial plexus
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the brachial plexus
extends inferiorly and laterally on either side of the last four cervical and first thoracic vertebrae (Figure 13.9a). It passes above the first rib posterior to the clavicle and then enters the axilla.
102
As with the cervical and other plexuses, the roots of the brachial plexus are the
anterior rami of the spinal nerves.
103
The roots of several spinal nerves from the brachial plexus unite to form
trunks in the inferior part of the neck.
104
Posterior to the clavicles, the trunks of the brachial plexus diverge into
divisions, called the anterior and posterior divisions.
105
In the axillae, the divisions of the brachial plexus unite to form
cords called the lateral, medial, and posterior cords.
106
The branches of the brachial plexus form
the principal nerves of the brachial plexus.
107
Five large terminal branches arise from the brachial plexus:
(1) The axillary nerve supplies the deltoid and teres minor muscles. (2) The musculocutaneous nerve supplies the anterior muscles of the arm. (3) The radial nerve supplies the muscles on the posterior aspect of the arm and forearm. (4) The median nerve supplies most of the muscles of the anterior forearm and some of the muscles of the hand. (5) The ulnar nerve supplies the anteromedial muscles of the forearm and most of the muscles of the hand.
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The Dorsal scapular Nerve
Originates at C5 and supplies the Levator scapulae, rhomboid major, and rhomboid minor muscles.
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The long thoracic nerve
Originates at C5-C7 and supplies Serratus anterior muscle.
110
Nerve to subclavius
Originates at C5-C6 and supplies the subclavius muscle
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The suprascapular nerve
Originates at C5-C6 and supplies Supraspinatus and infraspinatus muscles.
112
Musculocutaneous
Originates at C5-C7 and supplies Coracobrachialis, biceps brachii, and brachialis muscles.
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The lateral pectoral nerve
Originates at C5-C7 and supplies the pectoralis major muscle
114
The superior subscapular nerve
Originates at C5-C6 and supplies the Subscapularis muscle.
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the Thoracodorsal nerve
Originates at C6-C8 and supplies the Latissimus dorsi muscle.
116
the Inferior subscapular nerve
Originates at C5-C6 and supplies Subscapularis and teres major muscles.
117
The axillary nerve
Originates at C5-C6 and supplies the Deltoid and teres minor muscles; skin over deltoid and superior posterior aspect of arm.
118
The median nerve
Originates at C5-T1 and supplies Flexors of forearm, except flexor carpi ulnaris; ulnar half of flexor digitorum profundus, and some muscles of hand (lateral palm); skin of lateral two-thirds of palm and lateral 3½ digits.
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The radial nerve
Originates at C5-T1 and supplies Triceps brachii, anconeus, and extensor muscles of forearm; skin of posterior arm and forearm, lateral two-thirds of dorsum of hand, and lateral 3½ digits.
120
The medial pectoral nerve
Originates at C8-T1 and supplies Pectoralis major and pectoralis minor muscles.
121
Medial cutaneous nerve of arm
Originates at C8-T1 and supplies Skin of medial and posterior aspects of distal third of arm.
122
Medial cutaneous nerve of forearm
Originates at C8-T1 and supplies Skin of medial and posterior aspects of forearm.
123
The Ulnar nerve
Originates at C8-T1 and supplies Flexor carpi ulnaris, ulnar half of flexor digitorum profundus, and most muscles of hand; skin of medial side of hand, and medial 1½ digits.
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Erb-Duchenne palsy or waiter’s tip position
The presentation of this injury is characterized by an upper limb in which the shoulder is adducted, the arm is medially rotated, the elbow is extended, the forearm is pronated, and the wrist is flexed
125
Injury to the radial (and axillary) nerve can be caused by
improperly administered intramuscular injections into the deltoid muscle. The radial nerve may also be injured when a cast is applied too tightly around the mid- humerus.
126
Radial nerve injury is indicated by
wrist drop, the inability to extend the wrist and fingers
127
Injury to the median nerve may result in
median nerve palsy
128
median nerve palsy is indicated by
by numbness, tingling, and pain in the palm and lateral 3½. digits.
129
Injury to the ulnar nerve may result in
ulnar nerve palsy
130
ulnar nerve palsy
is indicated by an inability to abduct or adduct the digits, atrophy of the interosseous muscles of the hand, hyperextension of the metacarpophalangeal joints, and flexion of the interphalangeal joints, a condition called clawhand (Figure 13.9c). There is also loss of sensation over the medial 1½ digits.
131
Injury to the long thoracic nerve results in
paralysis of the serratus anterior muscle.The medial border of the scapula protrudes, giving it the appearance of a wing. this condition is called winged scapula
132
Compression of the brachial plexus on one or more of its nerves is sometimes known as
thoracic outlet syndrome.
133
What are the symptoms of thoracic outlet syndrome
The patient may experience pain, numbness, weakness, or tingling in the upper limb, across the upper thoracic area, and over the scapula on the affected side. The symptoms of thoracic outlet syndrome are exaggerated during physical or emotional stress because the added stress increases the contraction of the involved muscles.
134
The roots (anterior rami) of spinal nerves L1–L4 form the
lumbar plexus
135
in the lumbar plexus
there is minimal intermingling of fibers
136
the Iliohypogastric nerve
Originates at L1 and supplies Muscles of anterolateral abdominal wall; skin of inferior abdomen and buttocks.
137
The Ilioinguinal nerve
Originates at L1 and supplies Muscles of anterolateral abdominal wall; skin of superior and medial aspect of thigh, root of penis and scrotum in male, and labia majora and mons pubis in female.
138
The Genitofemoral nerve
Originates at L1-L2 and supplies Cremaster muscle; skin over middle anterior surface of thigh, scrotum in male, and labia majora in female.
139
The Lateral femoral cutaneous nerve
Originates at L2-L3 and supplies Skin over lateral, anterior, and posterior aspects of thigh.
140
The femoral nerve
Originates at L2-L4 and supplies Largest nerve arising from lumbar plexus; distributed to flexor muscles of hip joint and extensor muscles of knee joint, skin over anterior and medial aspect of thigh and medial side of leg and foot.
141
The obturator nerve
Originates at L2-L4 and supplies Adductor muscles of hip joint; skin over medial aspect of thigh.
142
The lumbar plexus supplies
the anterolateral abdominal wall, external genitals, and part of the lower limbs.
143
Injuries to the femoral nerve are indicated by
an inability to extend the leg and by loss of sensation in the skin over the anteromedial aspect of the thigh.
144
Injuries to the obturator nerve result in
paralysis of the adductor muscles of the thigh and loss of sensation over the medial aspect of the thigh. It may result from pressure on the nerve by the fetal head during pregnancy.
145
The roots (anterior rami) of spinal nerves L4–L5 and S1–S4 form the
sacral plexus
146
The sacral plexus supplies
the buttocks, perineum, and lower limbs.
147
The largest nerve in the body—the sciatic nerve—arises from
sacral plexus.
148
The roots (anterior rami) of spinal nerves S4–S5 and the coccygeal nerves form a small
Coccygeal plexus
149
ancoccygeal nerves
arises from the coccygeal plexus and supplys the skin in the coccygeal region
150
Injury to the sciatic nerve results in
sciatica (sī-AT-i-ka), pain that may extend from the buttock down the posterior and lateral aspect of the leg and the lateral aspect of the foot.
151
The sciatic nerve is actually
two nerves—tibial and common fibular—bound together by a common sheath of connective tissue.
152
Damage to the common fibular nerve causes
the foot to be plantar flexed, a condition called footdrop, and inverted, a condition called equinovarus (e-KWĪ-nō-va-rus).
153
Injury to the tibial portion of the sciatic nerve results in
dorsiflexion of the foot plus eversion, a condition called calcaneovalgus
154
Superior gluteal nerve
originates at L4–L5 and S1 and suplies Gluteus minimus, gluteus medius, and tensor fasciae latae muscles.
155
Inferior gluteal nerve
originates at L5-S2 and suplies Gluteus maximus muscle.
156
Nerve to piriformis
originates at S1-S2 and suplies Piriformis muscle.
157
nerve to quadratus femoris (quod-RĀ-tus FEM-or-is) and inferior gemellus (jem-EL-us)
originates at S1-S2 and suplies Quadratus femoris and inferior gemellus muscles.
158
Nerve to obturator internus (OB-too-rā′-tor in-TER-nus) and superior gemellus
originates at L5-S2 and suplies Obturator internus and superior gemellus muscles.
159
Perforating cutaneous nerve
originates at S2-S3 and suplies Skin over inferior medial aspect of buttocks.
160
Posterior femoral cutaneous nerve
originates at S1-S3 and suplies Skin over anal region, inferior lateral aspect of buttocks, superior posterior aspect of thigh, superior part of calf, scrotum in male, and labia majora in female.
161
Pudendal nerve
originates at S2-S4 and suplies Muscles of perineum; skin of penis and scrotum in male and clitoris, labia majora, labia minora, and vagina in female.
162
Sciatic nerve
originates at L4-S3 and is Actually two nerves—tibial and common fibular—bound together by common sheath of connective tissue; splits into its two divisions, usually at the knee. (See below for distributions.) As sciatic nerve descends through thigh, it sends branches to hamstring muscles and adductor magnus.
163
Tibial nerve
originates at L4-S3 and suplies Gastrocnemius, plantaris, soleus, popliteus, tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles. Branches of tibial nerve in foot are medial plantar nerve and lateral plantar nerve
164
Medial plantar nerve
suplies Abductor hallucis, flexor digitorum brevis, and flexor hallucis brevis muscles; skin over medial two-thirds of plantar surface of foot
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Lateral plantar nerve
suplies Remaining muscles of foot not supplied by medial plantar nerve; skin over lateral third of plantar surface of foot.
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common fibular nerve
originates at L4-s2 and Divides into superficial fibular and deep fibular branch.
167
Superficial fibular nerve
suplies Fibularis longus and fibularis brevis muscles; skin over distal third of anterior aspect of leg and dorsum of foot.
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Deep fibular nerve
suplies Tibialis anterior, extensor hallucis longus, fibularis tertius, and extensor digitorum longus and extensor digitorum brevis muscles; skin on adjacent sides of great and second toes.
169
The sacral plexus supplies
the buttocks, perineum, and lower limbs.
170
The spinal cord has two principal functions in maintaining homeostasis:
nerve impulse propagation and integration of information.
171
The white matter tracts in the spinal cord are highways for
both sensory and motor nerve impulse propagation.
172
The gray matter of the spinal cord
receives and integrates incoming and outgoing information.
173
Usually the name of the sensory or motor tracts takes on this order
First word: Directional term that indicates the position of the tract on the white matter
Second word: compound word the first part of the word indicates the origin of the tract (for example cortico stands for cerebral cortex) for sensory tracts the first part of the second word is almost always spino because sensory tracts always originate in the spinal cord The second part of the word describeswhere the tract ends
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The name of a tract often indicates
its location in the white matter and where it begins and ends.
175
Nerve impulses from sensory receptors propagate up the spinal cord to the brain along two main routes on each side:
the spinothalamic tract and the posterior funiculi.
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The spinothalamic tract (spī′-nō-tha-LAM-ik) conveys nerve impulses for sensing
pain, temperature, itch, and tickle.
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The posterior funiculi convey nerve impulses for
touch, pressure, vibration, and conscious proprioception (the awareness of the positions and movements of muscles, tendons, and joints).
178
The cerebral cortex, the outer part of the brain, plays a major role in
controlling precise voluntary muscular movements.
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Motor output to skeletal muscles travels down the spinal cord in two types of descending pathways:
direct and indirect
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The direct motor pathways, also called pyramidal pathways, include
the lateral corticospinal (kor′-ti-kō-SPĪ-nal), anterior corticospinal, and corticobulbar tracts (kor′-ti-kō-BUL-bar).They convey nerve impulses that originate in the cerebral cortex and are destined to cause voluntary movements of skeletal muscles.
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Indirect motor pathways, also called extrapyramidal pathways, include the
rubrospinal (ROO-brō-spī-nal), tectospinal (TEK-tō-spī-nal), vestibulospinal (ves-TIB-ū-lō-spī-nal), lateral reticulospinal (re-TIK-ū-lō-spī-nal), and medial reticulospinal tracts. These tracts convey nerve impulses from the brainstem to cause automatic movements and help coordinate body movements with visual stimuli.
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The second way the spinal cord promotes homeostasis is by
serving as an integrating center for some reflexes.
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A reflex is a
fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus.
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When integration takes place in the spinal cord gray matter, the reflex is a
spinal reflex
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If integration occurs in the brainstem rather than the spinal cord, the reflex is called a
cranial reflex.
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somatic reflexes involve
contraction of skeletal muscles.
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autonomic reflexes
involve responses of smooth muscle, cardiac muscle, and glands.
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The pathway followed by nerve impulses that produce a reflex is a
reflex arc (reflex circuit).
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What are the five functional components of a reflex arc
1. sensory receptor
2. Sensory neuron
3. Integrating center
4. Motor neuron
5 effector
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What is the role of a sensory receptor in a reflex arc
The distal end of a sensory neuron (dendrite) or an associated sensory structure serves as a sensory receptor. It responds to a specific stimulus—a change in the internal or external environment—by producing a graded potential called a generator (or receptor) potential (described in Section 16.1). If a generator potential reaches the threshold level of depolarization, it will trigger one or more nerve impulses in the sensory neuron.
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what is the role of the sensory neuron in a reflex arc
The nerve impulses propagate from the sensory receptor along the axon of the sensory neuron to the axon terminals, which are located in the gray matter of the spinal cord or brainstem. From here, relay neurons send nerve impulses to the area of the brain that allows conscious awareness that the reflex has occurred.
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What is the role of the integrating center in a reflex arc
One or more regions of gray matter within the CNS acts as an integrating center. In the simplest type of reflex, the integrating center is a single synapse between a sensory neuron and a motor neuron. A reflex pathway having only one synapse in the CNS is termed a monosynaptic reflex arc (mon′-ō-si-NAP-tik; mono- = one). More often, the integrating center consists of one or more interneurons, which may relay impulses to other interneurons as well as to a motor neuron. A polysynaptic reflex arc (poly- = many) involves more than two types of neurons and more than one CNS synapse.
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what is the role of the motor neuron in a reflex arc
Impulses triggered by the integrating center propagate out of the CNS along a motor neuron to the part of the body that will respond.
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What is the role of the effector in a reflex arc
The part of the body that responds to the motor nerve impulse, such as a muscle or gland, is the effector. Its action is called a reflex. If the effector is skeletal muscle, the reflex is a somatic reflex. If the effector is smooth muscle, cardiac muscle, or a gland, the reflex is an autonomic (visceral) reflex.
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A stretch reflex operates as follows
1. Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindles (shown in more detail in Figure 16.4). The spindles monitor changes in the length of the muscle.
2. In response to being stretched, a muscle spindle generates one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve and into the spinal cord.
3. In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with, and thereby activates, a motor neuron in the anterior gray horn.
4. If the excitation is strong enough, one or more nerve impulses arises in the motor neuron and propagates, along its axon, which extends from the spinal cord into the anterior root and through peripheral nerves to the stimulated muscle. The axon terminals of the motor neuron form neuromuscular junctions (NMJs) with skeletal muscle fibers of the stretched muscle.
5. Acetylcholine released by nerve impulses at the NMJs triggers one or more muscle nerve impulses in the stretched muscle (effector), and the muscle contracts. Thus, muscle stretch is followed by muscle contraction, which relieves the stretching.
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in an ipsilateral reflex
sensory nerve impulses enter the spinal cord on the same side from which motor nerve impulses leave it.
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all monosynaptic reflexes are
ipsilateral
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muscle tone
the small degree of contraction present while the muscle is at rest.
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Although the stretch reflex pathway itself is monosynaptic (just two neurons and one synapse),
a polysynaptic reflex arc to the antagonistic muscles operates at the same time.
200
how does a polysynaptic reflex occur
This arc involves three neurons and two synapses. An axon collateral (branch) from the muscle spindle sensory neuron also synapses with an inhibitory interneuron in the integrating center. In turn, the interneuron synapses with and inhibits a motor neuron that normally excites the antagonistic muscles
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The following atatement describes which type of innervation? when the stretched muscle contracts during a stretch reflex, antagonistic muscles that oppose the contraction relax
reciprocal innervation
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Reciprocal innervation prevents
conflict between opposing muscles and is vital in coordinating body movements.
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The stretch reflex operates as a
feedback mechanism to control muscle length by causing muscle contraction.
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the tendon reflex operates as a feedback mechanism to control muscle tension
by causing muscle relaxation before muscle force becomes so great that tendons might be torn.
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Like the stretch reflex, the tendon reflex is
ipsilateral
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The sensory receptors for a tendon reflex are called
tendon (Golgi tendon) organs (shown in more detail in Figure 16.4), which lie within a tendon near its junction with a muscle.
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In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to .
changes in muscle tension that are caused by passive stretch or muscular contraction
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A tendon reflex operates as follows
1. As the tension applied to a tendon increases, the tendon organ (sensory receptor) is stimulated (depolarized to threshold).
2. Nerve impulses arise and propagate into the spinal cord along a sensory neuron.
3. Within the spinal cord (integrating center), the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron.
4. The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses.
5. The muscle relaxes and relieves excess tension.
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The tendon reflex causes
relaxation of the muscle attached to the stimulated tendon organ.
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The flexor reflex causes
withdrawal of a part of the body in response to a painful stimulus.
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the flexor reflex or withdrawal reflex, operates as follows
1. Stepping on a tack stimulates the dendrites (sensory receptor) of a pain-sensitive neuron.
2. This sensory neuron then generates nerve impulses, which propagate into the spinal cord.
3. Within the spinal cord (integrating center), the sensory neuron activates interneurons that extend to several spinal cord segments.
4. The interneurons activate motor neurons in several spinal cord segments. As a result, the motor neurons generate nerve impulses, which propagate toward the axon terminals.
5. Acetylcholine released by the motor neurons causes the flexor muscles in the thigh (effectors) to contract, producing withdrawal of the leg. This reflex is protective because contraction of flexor muscles moves a limb away from the source of a possibly damaging stimulus.
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The flexor reflex, like the stretch reflex, is
ipsilateral—the incoming and outgoing impulses propagate into and out of the same side of the spinal cord.
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nerve impulses from one sensory neuron ascend and descend in the spinal cord and activate interneurons in several segments of the spinal cord, this type of reflex is called an
intersegmental reflex arc
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Through intersegmental reflex arcs,
a single sensory neuron can activate several motor neurons, thereby stimulating more than one effector
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How does a crossed extensor reflex work
1. Stepping on a tack stimulates the sensory receptor of a pain-sensitive neuron in the right foot.
2. This sensory neuron then generates nerve impulses, which propagate into the spinal cord.
3. Within the spinal cord (integrating center), the sensory neuron activates several interneurons that synapse with motor neurons on the left side of the spinal cord in several spinal cord segments. Thus, incoming pain signals cross to the opposite side through interneurons at that level, and at several levels above and below the point of entry into the spinal cord.
4. The interneurons excite motor neurons in several spinal cord segments that innervate extensor muscles. The motor neurons in turn generate more nerve impulses, which propagate toward the axon terminals.
5. Acetylcholine released by the motor neurons causes extensor muscles in the thigh (effectors) of the unstimulated left limb to contract, producing extension of the left leg. In this way, weight can be placed on the foot that must now support the entire body. A comparable reflex occurs with painful stimulation of the left lower limb or either upper limb.
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Unlike the flexor reflex, which is an ipsilateral reflex, the crossed extensor reflex involves a
contralateral reflex arc
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how does a contralateral reflex arc work
Sensory impulses enter one side of the spinal cord and motor impulses exit on the opposite side. Thus, a crossed extensor reflex synchronizes the extension of the contralateral limb with the withdrawal (flexion) of the stimulated limb.
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the patellar reflex can be blocked by
damage to the sensory or motor nerves supplying the muscle or to the integrating centers in the second, third, or fourth lumbar segments of the spinal cord.
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Absence of the Achilles reflex indicates damage to
the nerves supplying the posterior leg muscles or to neurons in the lumbosacral region of the spinal cord. This reflex may also disappear in people with chronic diabetes, neurosyphilis, alcoholism, and subarachnoid hemorrhages.
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An exaggerated Achilles reflex indicates
cervical cord compression or a lesion of the motor tracts of the first or second sacral segments of the cord.
221
A positive Babinski sign after age 1½ is abnormal and indicates
an interruption of the corticospinal tract as the result of a lesion of the tract, usually in the upper portion.
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Abdominal reflex.
This reflex involves contraction of the muscles that compress the abdominal wall in response to stroking the side of the abdomen.
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abscence of the abdominal reflex is associated with
lesions of the corticospinal tracts. It may also be absent because of lesions of the peripheral nerves, lesions of integrating centers in the thoracic part of the cord, or multiple sclerosis
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Most autonomic reflexes are not practical diagnostic tools because it is difficult to stimulate visceral effectors, which are deep inside the body. An exception is the
pupillary light reflex,
225
absence of a normal pupillary light reflex may indicate
brain damage or injury.
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A crossed extensor reflex causes
contraction of muscles that extend joints in the limb opposite a painful stimulus.
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Monoplegia (mon′-ō-PLĒ-jē-a; mono- = one; -plegia = blow or strike) is
paralysis of one limb only.
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Diplegia (di- = two) is
paralysis of both upper limbs or both lower limbs.
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Paraplegia (para- = beyond) is
paralysis of both lower limbs.
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Hemiplegia (hemi- = half) is
paralysis of the upper limb, trunk, and lower limb on one side of the body,
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quadriplegia (quad- = four) is
paralysis of all four limbs.
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Complete transection (tran-SEK-shun; trans- = across; -section = a cut) of the spinal cord means that
the cord is severed from one side to the other, thus cutting all sensory and motor tracts.
233
Hemisection is
a partial transection of the spinal cord on either the right or the left side.
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After hemisection, three main symptoms, known together as Brown-Séquard syndrome (sē-KAR), occur below the level of the injury:
(1) Damage of the posterior funiculus (sensory tracts) causes loss of proprioception and fine touch sensations on the ipsilateral (same) side as the injury. (2) Damage of the lateral corticospinal tract (motor tract) causes ipsilateral paralysis. (3) Damage of the spinothalamic tracts (sensory tracts) causes loss of pain and temperature sensations on the contralateral (opposite) side.
235
Spinal shock is an immediate response to
spinal cord injury characterized by temporary areflexia (a′-re-FLEK-sē-a), loss of reflex function.
236
Signs of acute spinal shock include
slow heart rate, low blood pressure, flaccid paralysis of skeletal muscles, loss of somatic sensations, and urinary bladder dysfunction. Spinal shock may begin within 1 hour after injury and may last from several minutes to several months, after which reflex activity gradually returns.
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Spinal cord compression may result from
fractured vertebrae, herniated intervertebral discs, tumors, osteoporosis, or infections.
238
symptoms of spinal cord compression include
pain, weakness or paralysis, and either decreased or complete loss of sensation below the level of the injury.
239
Shingles is
an acute infection of the peripheral nervous system caused by herpes zoster (HER-pēz ZOS-ter), the virus that also causes chickenpox
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post-polio syndrome.
This neurological disorder is characterized by progressive muscle weakness, extreme fatigue, loss of function, and pain, especially in muscles and joints.
241
Epidural block (ep′-i-DOO-ral)
Injection of an anesthetic drug into the epidural space, the space between the dura mater and the vertebral column, in order to cause a temporary loss of sensation. Such injections in the lower lumbar region are used to control pain during childbirth.
242
Meningitis (men-in-JĪ-tis; -itis = inflammation)
Inflammation of the meninges due to an infection, usually caused by a bacterium or virus. Symptoms include fever, headache, stiff neck, vomiting, confusion, lethargy, and drowsiness. Bacterial meningitis is much more serious and is treated with antibiotics. Viral meningitis has no specific treatment. Bacterial meningitis may be fatal if not treated promptly; viral meningitis usually resolves on its own in 1–2 weeks. A vaccine is available to help protect against some types of bacterial meningitis.
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Myelitis (mī-e-LĪ-tis; myel- = spinal cord)
Inflammation of the spinal cord.
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Nerve block
Loss of sensation in a region due to injection of a local anesthetic; an example is local dental anesthesia.
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Neuralgia (noo-RAL-jē-a; neur- = nerve; -algia = pain)
Attacks of pain along the entire course or a branch of a sensory nerve.
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Neuritis (neur- = nerve; -itis = inflammation)
Inflammation of one or several nerves that may result from irritation to the nerve produced by direct blows, bone fractures, contusions, or penetrating injuries. Additional causes include infections, vitamin deficiency (usually thiamine), and poisons such as carbon monoxide, carbon tetrachloride, heavy metals, and some drugs.
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Paresthesia (par-es-THĒ-zē-a; par- = departure from normal; -esthesia = sensation)
An abnormal sensation such as burning, pricking, tickling, or tingling resulting from a disorder of a sensory nerve.
