44 terms

Chpt 13 handout/ Homeostatic imbalance/ T and F

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1. Define peripheral nervous system
Portion of the nervous system consisting of nerves (cranial and spinal) and ganglia (collection of nerve cell bodies outside of CNS ) that lie outside of the brain and spinal cord
List PNS components.
1. Sensory (afferent) division
2. Motor (efferent) division
3. Somatic nervous system (Branched from motor) "Voluntary"
4. Autonomic nervous system (ANS) (branched from motor division) "Involuntary"
5. Sympathetic Division (branched from ANS)
6. Parasympathetic Division (branched from ANS)
General Sensory Receptors are classified into
1. Stimulus type
2. Location
3. Structure complexity
Classify general sensory receptors by structure (In PNS)
Structure:There are 2 types of receptor structures (simple and complex)

Simple Structure:
1. Unencapsulated dendritic endings
2. Encapsulated dendritic endings

Simple receptors are general senses, and may be unencapsulated or encapsulated dendritic endings.

1. Unencapsulated dendritic endings are free, or naked, nerve endings, and detect temperature, pain, itch, or light touch.
2. Encapsulated dendritic endings consist of a dendrite enclosed in a connective tissue capsule and detect discriminatory touch, initial, continuous, and deep pressure, and stretch of muscles, tendons, and joint capsules.

Simple receptors are modified dendritic endings of sensory neurons.

Complex receptors are actually sense organs, localized collections of cells (usually of many types) associated with the special senses (vision, hearing, equilibrium, smell, and taste).
Classify general sensory receptors by stimulus detected (In PNS)
Stimulus:
1. Mechanoreceptors
2. Thermoreceptors
3. Photoreceptors
4. Chemoreceptors
5. Nociceptors



1. Mechanoreceptors respond to mechanical force such as
touch, pressure (including blood pressure), vibration, and
stretch.
2. Thermoreceptors are sensitive to temperature changes
3. Photoreceptors, such as those of the retina of the eye, respond
to light energy.
4. Chemoreceptors respond to chemicals in solution (molecules
smelled or tasted, or changes in blood or interstitial
fluid chemistry).
5. Nociceptors (nose-septorz; noci harm) respond to
potentially damaging stimuli that result in pain. For example,
searing heat, extreme cold, excessive pressure, and
inflammatory chemicals are all interpreted as painful.
These signals stimulate subtypes of thermoreceptors,
mechanoreceptors, and chemoreceptors.
Classify general sensory receptors by
body location. (In PNS)
Body location :
1. Exteroceptors
2. Interoceptors
3. Proprioceptors



1. Exteroceptors (ekster-o-septorz) are sensitive to stimuli
arising outside the body (extero outside), so most exteroceptors
are near or at the body surface. They include
touch, pressure, pain, and temperature receptors in the
skin and most receptors of the special senses (vision, hearing,
equilibrium, taste, smell).
2. Interoceptors (inter-o-septorz), also called visceroceptors,
respond to stimuli within the body (interoinside), such
as from the internal viscera and blood vessels. They monitor
a variety of stimuli, including chemical changes, tissue
stretch, and temperature. Sometimes their activity causes
us to feel pain, discomfort, hunger, or thirst. However, we
are usually unaware of their workings.
3. Proprioceptors (propre-o-septorz), like interoceptors,
respond to internal stimuli; however, their location is
much more restricted. Proprioceptors occur in skeletal
muscles, tendons, joints, and ligaments and in connective
tissue coverings of bones and muscles. (Some authorities
include the equilibrium receptors of the inner ear in this
class.) Proprioceptors constantly advise the brain of our
body movements (propria one's own) by monitoring
how much the organs containing these receptors are stretched.
sensation (awareness of
changes in the internal and external environments)
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Outline the events that lead to sensation and perception
The somatosensory system, the part of the sensory system serving the body wall and limbs, involves the receptor level, the circuit level, and the perceptual level (pp. 488-491; Fig. 13.2).
1. Processing at the receptor level involves a stimulus that must excite a receptor in order for sensation to occur.
2. Processing at the circuit level is involved with delivery of impulses to the appropriate region of the cerebral cortex for stimulus localization and perception.
3. Processing at the perceptual level involves interpretation of sensory input in the cerebral cortex.
4. The perception of pain protects the body from damage, and is stimulated by extremes of pressure and temperature, as well as chemicals released from damaged tissues.
Describe receptor and generator potentials and sensory adaptation.
The stimulus must be applied within a sensory receptor's receptive field, the particular area monitored by the receptor. Typically, the smaller the receptive field, the greater the abil- ity of the brain to accurately localize the stimulus site.
■ The stimulus energy must be converted into the energy of a graded potential called a receptor potential, a process called transduction. This receptor potential may be a depolarizing or hyperpolarizing graded potential similar to the EPSPs or IPSPs generated at postsynaptic membranes in response to neurotransmitter binding, as described in Chapter 11 (pp. 411- 412). Membrane depolarizations that summate and directly lead to generation of action potentials in an afferent fiber are called generator potentials.

Information about the stimulus—its strength, duration, and pattern—is encoded in the frequency of nerve impulses (the greater the frequency, the stronger the stimulus). Many but not all sensory receptors exhibit adaptation, a change in sensitivity and nerve impulse generation) in the presence of a constant stimulus.
perception (conscious interpretation of those stimuli).
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5. Describe the main aspects of sensory perception
The main aspects of sensory perception include the following:
■ Perceptual detection is the ability to detect that a stimulus has occurred. This is the simplest level of perception. As a general rule, inputs from several receptors must be summed for perceptual detection to occur.
■ Magnitude estimation is the ability to detect how intense the stimulus is. Because of frequency coding, perception in- creases as stimulus intensity increases
■ Spatial discrimination allows us to identify the site or pattern of stimulation. A common tool for studying this quality in the laboratory is the two-point discrimination test. The test determines how close together two points on the skin can be and still be perceived as two points rather than as one. This test provides a crude map of the density of tactile receptors in the various regions of the skin. The distance between perceived points varies from less than 5 mm on highly sensitive body areas (tip of the tongue) to more than 50 mm on less sensitive areas (back).
■ Feature abstraction is the mechanism by which a neuron or circuit is tuned to one feature in preference to others. Sensation usually involves an interplay of several stimulus properties or features. For example, one touch tells us that velvet is warm, compressible, and smooth but not completely continuous, each a feature that contributes to our perception of "velvet." Feature abstraction enables us to identify more complex aspects of a sensation.
■ Quality discrimination is the ability to differentiate the submodalities of a particular sensation. Each sensory modality has several qualities, or submodalities. For example, the sub-modalities of taste include sweet and bitter.
■ Pattern recognition is the ability to take in the scene around us and recognize a familiar pattern, an unfamiliar one, or one that has special significance for us. For example, a figure made of dots may be recognized as a familiar face, and when we listen to music, we hear the melody, not just a string of notes.
sensory fiber a nerve fiber that carries impulses toward the central nervous system
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Nerve-A threadlike process of a neuron, especially the prolonged axon that conducts nerve impulses
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tactile sensation
the sensation produced by pressure receptors in the skin; "she likes the touch of silk on her skin"; "the surface had a greasy feeling
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6.Define ganglion
Collection of nerve cell bodies outside the CNS
6. Indicate the general body location of ganglia.
In PNS:



1. Ganglia associated with (sensory) afferent nerve fibers contain cell bodies of sensory neurons. (These are the dorsal root ganglia

2. Ganglia associated with (motor) efferent nerve fibers mostly contain cell bodies of autonomic motor neurons.
7. Describe the general structure of a nerve.
A nerve is a cordlike organ consisting of parallel bundles of peripheral axons enclosed by connective tissue wrappings
8. Follow the process of nerve regeneration.
Short version:
1. The axon becomes fragmented at the injury site. (the injury site begin to disintegrate because they cannot receive nutrients from the cell body this process, Wallerian degeneration,)
2. Macrophages clean out the dead axon distal to the injury. (Schwann cells proliferate in response to mitosis-stimulating chemicals released by the macrophages)
3. Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells.
4. The axon regenerates and a new myelin sheath forms.

Longer version:
Almost immediately after a peripheral axon has been severed or crushed, the separated ends seal themselves off and then swell as substances being transported along the axon begin to accu- mulate in the sealed ends. Within a few hours, the axon and its myelin sheath distal to the injury site begin to disintegrate because they cannot receive nutrients from the cell body (Fig- ure 13.4, ). This process, Wallerian degeneration, spreads distally from the injury site, completely fragmenting the axon. Generally, the entire axon distal to the injury is degraded by phagocytes within a week, but the neurilemma remains intact within the endoneurium (Figure 13.4, ). After the debris has been disposed of, surviving Schwann cells proliferate in response to mitosis-stimulating chemicals released by the macrophages, and migrate into the injury site. Once there, they release growth factors and begin to express cell adhesion mole- cules (CAMs) that encourage axonal growth. Additionally, they form a regeneration tube, a system of cellular cords that guide the regenerating axon "sprouts" across the gap and to their original contacts (Figure 13.4, and ). The same Schwann cells protect, support, and remyelinate the regenerating axons.
Changes also occur in the neuronal cell body after the axon has been destroyed. Within two days, its chromatophilic substance breaks apart, and then the cell body swells as protein synthesis revs up to support regeneration of its axon.
Axons regenerate at the approximate rate of 1.5 mm a day. The greater the distance between the severed endings, the less the chance of recovery because adjacent tissues block growth by pro- truding into the gaps, and axonal sprouts fail to find the regener- ation tube.
9. Name the 12 pairs of cranial nerves;
12 cranial nerves:
1. Olfactory
2. Optic
3. Oculomotor
4. Trochlear
5. Trigeminal
6. Abducens
7. Facial
8. Acoustics
9. Glossopharyngeal
10. Vagus
11. Hypoglossal
12. Accessory Nerves.
9. Indicate the body region and structures innervated by the 12 pairs of cranial nerves.
I. Olfactory. These are the tiny sensory nerves (filaments) of smell, which run from the nasal mucosa to synapse with the olfactory bulbs.
Olfactory nerve fibers arise from olfactory receptor cells located in olfactory epithelium of nasal cavity and pass through cribriform plate of ethmoid bone to synapse in olfactory bulb.

II. Optic. Because this sensory nerve of vision develops as an outgrowth of the brain, it is really a brain tract.

Origin and course: Fibers arise from retina of eye to form optic nerve, which passes through optic canal of orbit. The optic nerves converge to form the optic chiasma where fibers partially cross over, continue on as optic tracts, enter thalamus, and synapse there. Thalamic fibers run (as the optic radi- ation) to occipital (visual) cortex, where visual interpretation occurs

III. Oculomotor. The name oculomotor means "eye mover." This nerve supplies four of the six extrinsic muscles that move the eyeball in the orbit.
Origin and course: Fibers extend from ventral midbrain (near its junction with pons) and pass through bony orbit, via superior orbital fissure, to eye.

IV. Trochlear.
The name trochlear means"pulley"and it innervates an extrinsic eye muscle that loops through a pulley-shaped ligament in the orbit.

Origin and course: Fibers emerge from dorsal midbrain and course ventrally around midbrain to enter orbit through superior orbital fissure along with oculomotor nerves.

V. Trigeminal. Three (tri) branches spring from this, the largest of the cranial nerves. It supplies sensory fibers to the face and motor fibers to the chewing muscles.
Largest of cranial nerves; fibers extend from pons to face, and form three divisions (trigemina threefold): ophthalmic, maxillary, and mandibular divisions. As major general sensory nerves of face, transmit afferent impulses from touch, temperature, and pain receptors. Cell bodies of sensory neurons of all three divisions are located in large trigeminal ganglion.


VI. Abducens. This nerve controls the extrinsic eye muscle that abducts the eyeball (turns it laterally).
Origin and course: Fibers leave inferior pons and enter orbit via superior orbital fissure to run to eye.


VII. Facial. A large nerve that innervates muscles of facial expression (among other things).
Origin and course: Fibers issue from pons, just lateral to abducens nerves (see Figure 13.5), enter temporal bone via internal acoustic meatus, and run within bone (and through inner ear cavity) before emerging through stylomastoid foramen; nerve then courses to lateral aspect of face.


VIII. Vestibulocochlear. This sensory nerve for hearing and balance was formerly called the auditory nerve.
Origin and course: Fibers arise from hearing and equilibrium apparatus located within inner ear of temporal bone and pass through internal acoustic meatus to enter brain stem at pons-medulla border. Afferent fibers from hearing receptors in cochlea form the cochlear division; those from equilibrium receptors in semicircular canals and vestibule form the vestibular division (ves- tibular nerve); the two divisions merge to form vestibulocochlear nerve

IX. Glossopharyngeal. The name glossopharyngeal means "tongue and pharynx," and reveals the structures that this nerve helps to innervate.
Origin and course: Fibers emerge from medulla and leave skull via jugular foramen to run to throat.
Function: Mixed nerves that innervate part of tongue and pharynx. Provide somatic motor fibers to, and carry proprioceptor fibers from, a superior pharyngeal muscle called the stylopharyngeus, which ele- vates the pharynx in swallowing. Provide parasympathetic motor fibers to parotid salivary glands (some of the nerve cell bodies of these parasympathetic motor neurons are located in otic ganglion).

X. Vagus.
This nerve's name means "wanderer" or "vagabond," and it is the only cranial nerve to extend beyond the head and neck to the thorax and abdomen.
Origin and course: The only cranial nerves to extend beyond head and neck region. Fibers emerge from medulla, pass through skull via jugular foramen, and descend through neck region into thorax and abdomen.

XI. Accessory. Considered an accessory part of the vagus nerve, this nerve was formerly called the spinal accessory nerve.
Origin and course: Unique in that they are formed from ventral rootlets that emerge from the spinal cord, not the brain stem. These rootlets arise from superior region (C1-C5) of spinal cord, pass upward along spinal cord, and enter the skull as the accessory nerves via fora- men magnum. The accessory nerves exit from skull through jugular foramen together with the vagus nerves, and supply two large neck muscles. Until recently, it was thought that the accessory nerves also received a contribution from cranial rootlets, but it has now been de- termined that in almost all people, these cranial rootlets are instead part of the vagus nerves. This raises an interesting question: Should the accessory nerves still be considered cranial nerves? Some anato- mists say "yes" because they pass through the cranium. Others say "no" because they don't arise from the brain. Stay tuned!

XII. Hypoglossal. The name hypoglossal means under the tongue. This nerve runs inferior to the tongue and innervates the tongue muscles.
Origin and course: As their name implies (hypo below; glossal tongue), hypoglossal nerves mainly serve the tongue. Fibers arise by a series of roots from medulla and exit from skull via hypoglossal canal to travel to tongue.
10. Describe the formation of a spinal nerve and the general distribution of its rami.
Thirty-one pairs of spinal nerves, each containing thousands of nerve fibers, arise from the spinal cord and supply all parts of the body except the head and some areas of the neck. All are mixed nerves.
Each spinal nerve connects to the spinal cord by a dorsal root and a ventral root.

1. Rami lie distal to and are lateral branches of the spinal nerves that carry both motor and sensory fibers.

2. The back is innervated by the dorsal rami with each ramus innervating the muscle in line with the point of origin from the spinal column.

3. Only in the thorax are the ventral rami arranged in a simple segmental pattern corresponding to that of the dorsal rami.

A spinal nerve is quite short (only 1-2 cm). Almost immedi- ately after emerging from its foramen, it divides into a small dorsal ramus, a larger ventral ramus (ramus; "branch"), and a tiny meningeal branch (me ̆-ninje-al) that reenters the verte- bral canal to innervate the meninges and blood vessels within. Each ramus, like the spinal nerve itself, is mixed. Finally, joined to the base of the ventral rami of the thoracic spinal nerves are special rami called rami communicantes, which contain auto- nomic (visceral) nerve fibers.
The spinal nerve rami and their main branches supply the entire somatic region of the body (skeletal muscles and skin) from the neck down. The dorsal rami supply the posterior body trunk. The thicker ventral rami supply the rest of the trunk and the limbs.
11. Define plexus
A network of converging and diverging nerve fibers, blood vessels or lymphatics.
11. Name the major plexuses and describe the distribution and function of the peripheral nerves arising from each plexus.
Major Plexuses: (nerves)
1. The cervical plexus is formed by the ventral rami of the first four cervical nerves.ts branches are summa- rized in Table 13.3. Most branches are cutaneous nerves that supply only the skin. They transmit sensory impulses from the skin of the neck, the ear area, the back of the head, and the shoulder. Other branches innervate muscles of the anterior neck.

2. The brachial plexus is situated partly in the neck and partly in the axilla and gives rise to virtually all the nerves that innervate the upper limb. It can be palpated (felt) in a living person just superior to the clavicle at the lateral border of the sternocleidomastoid muscle.

3. The sacral and lumbar plexuses overlap and because many fibers of the lumbar plexus contribute to the sacral plexus via the lumbosacral trunk, the two plexuses are often referred to as the lumbosacral plexus.

Lumbar Plexus The lumbar plexus arises from the spinal nerves L1-L4 and lies within the psoas major muscle (Figure 13.10). Its proximal branches innervate parts of the abdominal wall muscles and the psoas muscle, but its major branches descend to in- nervate the anterior and medial thigh.

Sacral Plexus The sacral plexus arises from spinal nerves L4-S4 and lies immediately caudal to the lumbar plexus (Figure 13.11). Some fibers of the lumbar plexus contribute to the sacral plexus via the lumbosacral trunk, as mentioned earlier. The sacral plexus has about a dozen named branches. About half of these serve the buttock and lower limb; the others innervate pelvic structures and the perineum. The most important branches are described here. Table 13.6 summarizes all but the smallest ones.
12. Compare and contrast the motor endings of somatic and autonomic nerve fibers.
1. Somatic-The terminals of the somatic motor fibers that innervate voluntary muscles form elaborate neuromuscular junctions with their effector cells and they release the neurotransmitter acetylcholine (p. 512).
2. Autonomic- The junctions between autonomic motor endings and the visceral effectors involve varicosities and release either acetylcholine or epinephrine as their neurotransmitter
Levels of motor control hierarchacy
1. Segmented level
2. Projection level
3. Precommand level
13. Outline the three levels of the motor hierarchy.
Levels of Motor Control
1. The segmental level is the lowest level on the motor control hierarchy and consists of the spinal cord circuits.
2. The projection level has direct control of the spinal cord.
3. The precommand level is made up of the cerebellum and the basal nuclei and is the highest level of the motor system hierarchy.
14. Compare the roles of the cerebellum and basal nuclei in controlling motor activity.
1. Cerebullum- The key center for "online" sensorimotor integration and control is the cerebellum. Remember the cerebellum is a target of ascending proprioceptor, tactile, equilibrium, and visual inputs—feedback that it needs for rapid correction of "errors" in motor activity. It also receives information via branches from descending pyramidal tracts, and from various brain stem nuclei. The cerebellum lacks direct connections to the spinal cord; it acts on motor pathways through the projection areas of the brain stem and on the motor cortex via the thalamus to fine-tune motor activity.

2. The basal nuclei receive inputs from all cortical areas and send their output back mainly to premotor and prefrontal cor- tical areas via the thalamus. Compared to the cerebellum, the basal nuclei appear to be involved in more complex aspects of motor control. Under resting conditions, the basal nuclei inhibit various motor centers of the brain, but when the motor centers are released from inhibition, coordinated motions can begin.
Cells in both the basal nuclei and the cerebellum are involved in this unconscious planning and discharge in advance of willed movements.
15. Name the components of a reflex arc and distinguish between autonomic and somatic reflexes.
Components of Reflex arc:
1. Receptor: Site of the stimulus action.

2. Sensory neuron: Transmits afferent impulses to the CNS.

3. Integration center: In simple reflex arcs, the integration center may be a single synapse between a sensory neuron and a motor neuron (monosynaptic reflex). More complex reflex arcs involve multiple synapses with chains of interneu- rons (polysynaptic reflex). The integration center for the reflexes we will describe in this chapter is within the CNS.

4. Motor neuron: Conducts efferent impulses from the integration center to an effector organ.

5. Effector: Muscle fiber or gland cell that responds to the ef- ferent impulses (by contracting or secreting).

Autonomic and somatic reflexes:

Reflexes are classified functionally as somatic reflexes if they activate skeletal muscle,

or as Autonomic (visceral) reflexes if they activate visceral effectors (smooth or cardiac muscle or glands).
16. Compare and contrast stretch, flexor, crossed-extensor, and Golgi tendon reflexes.
Reflexes:

1.Stretch- In the stretch reflex the muscle spindle is stretched and excited by either an external stretch or an internal stretch.
2. Golgi tendon- The Golgi tendon reflex produces muscle relaxation and lengthening in response to contraction.
3. Flexor- The flexor, or withdrawal, reflex is initiated by a painful stimulus and causes automatic withdrawal of the threatened body part from the stimulus.
4. Cross extensor- The crossed-extensor reflex is a complex spinal reflex consisting of an ipsilateral withdrawal reflex and a contralateral extensor reflex.
17. Describe the developmental relationship between the segmented arrangement of peripheral nerves, skeletal muscles, and skin dermatomes.
Most skeletal muscles derive from paired blocks of mesoderm (somites) distributed segmentally down the posteromedial aspect of the embryo. The spinal nerves branch from the devel- oping spinal cord and adjacent neural crest and exit between the forming vertebrae, and each nerve becomes associated with the adjacent muscle mass. The spinal nerves supply both sensory and motor fibers to the developing muscles and help direct their maturation. Cranial nerves innervate muscles of the head in a comparable manner.
The distribution of cutaneous nerves to the skin follows a similar pattern. Most of the scalp and facial skin is innervated by the trigeminal nerves. Spinal nerves supply cutaneous branches to specific (adjacent) dermatomes that eventually become dermal segments. As a result, the distribution and growth of the spinal nerves correlate with the segmented body plan, which is established by the fourth week of embryonic development.
Growth of the limbs and unequal growth of other body areas result in an adult pattern of dermatomes with unequal sizes and shapes and varying degrees of overlap. Because embryonic mus- cle cells migrate extensively, much of the early segmental pat- tern is lost. Understanding the general pattern of sensory nerve distribution is critical for physicians. For example, in areas of substantial dermatome overlap, two or three spinal nerves must be blocked to perform local surgery.
18. List the changes that occur in the peripheral nervous system with aging.
Sensory receptors atrophy to some degree with age, and there is some lessening of muscle tone in the face and neck. Reflexes occur a bit more slowly during old age. This deterioration seems to reflect a general loss of neurons, fewer synapses per neuron, and a slowdown in central processing rather than any major changes in the peripheral nerve fibers. In fact, the peripheral nerves remain viable and normally functional throughout life unless subjected to traumatic injury or ischemia. The most common symptom of ischemia is a tingling sensation or numb- ness in the affected region.
Homeostatic Imbalances
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HOMEOSTATIC IMBALANCE
Normally a steady state is maintained that correlates injury and pain. Long-lasting or very intense pain inputs, such as limb am- putation, can disrupt this system, leading to hyperalgesia (pain amplification), chronic pain, and phantom limb pain. Intense or long-duration pain causes activation of NMDA receptors, the same receptors that strengthen neural connections during certain kinds of learning. Essentially, the spinal cord learns hyperalgesia. In light of this, it is crucial that pain is effectively managed early to prevent the establishment of chronic pain. Phantom limb pain (pain perceived in tissue that is no longer present) is a curious ex- ample of hyperalgesia. Until recently, surgical limb amputations were conducted under general anesthesia only and the spinal cord still experienced the pain of amputation. Blocking neurotrans- mission in the spinal cord by additionally using epidural anes- thetics greatly reduces the incidence of phantom limb pain. ■
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HOMEOSTATIC IMBALANCE
Irritation of the phrenic nerve causes spasms of the diaphragm, or hiccups. If both phrenic nerves are severed, or if the C3-C5 region of the spinal cord is crushed or destroyed, the diaphragm is paralyzed and respiratory arrest occurs. Victims are kept alive by mechanical respirators that force air into their lungs and do their breathing for them. ■
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HOMEOSTATIC IMBALANCE
Median nerve injury makes it difficult to use the pincer grasp (opposed thumb and index finger) to pick up small objects. Because this nerve runs down the midline of the forearm and wrist, it is a frequent casualty of wrist-slashing suicide attempts. In carpal tunnel syndrome (see p. 231), the median nerve is compressed. ■
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HOMEOSTATIC IMBALANCE
Where it takes a superficial course, the ulnar nerve is very vulner- able to injury. Striking the "funny bone"—the spot where this nerve rests against the medial epicondyle—causes tingling of the little finger. Severe or chronic damage can lead to sensory loss, paralysis, and muscle atrophy. Affected individuals have trouble making a fist and gripping objects. As the little and ring fingers become hyperextended at the knuckles and flexed at the distal interphalangeal joints, the hand contorts into a clawhand. ■
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HOMEOSTATIC IMBALANCE
Injuries to the brachial plexus are common; when severe, they cause weakness or paralysis of the entire upper limb. Such injuries may occur when the upper limb is pulled hard and the plexus is stretched (as when a football tackler yanks the arm of the running back), and by blows to the top of the shoulder that force the humerus inferiorly (as when a cyclist is pitched headfirst off a motorcycle and his shoulder grinds into the pavement)
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HOMEOSTATIC IMBALANCE
Trauma to the radial nerve results in wrist drop, an inability to extend the hand at the wrist. Improper use of a crutch or "Saturday night paralysis," in which an intoxicated person falls asleep with an arm draped over the back of a chair or sofa edge, causes radial nerve compression and ischemia (deprivation of blood supply). ■
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HOMEOSTATIC IMBALANCE
Compression of the spinal roots of the lumbar plexus, as by a herniated disc, results in gait problems because the femoral nerve serves the prime movers of both hip flexion and knee extension. Other symptoms are pain or anesthesia of the anterior thigh and of the medial thigh if the obturator nerve is impaired.
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HOMEOSTATIC IMBALANCE
Injury to the proximal part of the sciatic nerve, as might follow a fall, disc herniation, or improper administration of an injection into the buttock, results in a number of lower limb impairments, depending on the precise nerve roots injured. Sciatica (si-at ̆ı- kah), characterized by stabbing pain radiating over the course of the sciatic nerve, is common. When the nerve is transected, the leg is nearly useless. The leg cannot be flexed (because the hamstrings are paralyzed), and all foot and ankle movement is lost. The foot drops into plantar flexion (it dangles), a condition called footdrop. Recovery from sciatic nerve injury is usually slow and incomplete.
If the lesion occurs below the knee, thigh muscles are spared. When the tibial nerve is injured, the paralyzed calf muscles can- not plantar flex the foot and a shuffling gait develops. The com- mon fibular nerve is susceptible to injury largely because of its superficial location at the head and neck of the fibula. Even a tight leg cast, or remaining too long in a side-lying position on a firm mattress, can compress this nerve and cause footdrop.
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HOMEOSTATIC IMBALANCE
Stretch reflexes tend to be either hypoactive or absent in cases of peripheral nerve damage or ventral horn injury involving the tested area. These reflexes are absent in those with chronic dia- betes mellitus or neurosyphilis and during coma. However, they are hyperactive when lesions of the corticospinal tract reduce the inhibitory effect of the brain on the spinal cord (as in stroke patients). ■
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Hyperalgesia (pain amplification), chronic pain, and phantom limb pain.
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Irritation of the phrenic nerve causes spasms of the diaphragm, or hiccups
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Median nerve injury makes it difficult to use the pincer grasp (opposed thumb and index finger) to pick up small objects.
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