A&P 13: The Peripheral Nervous System & Reflex Activity

Question 1

Peripheral Nervous System (PNS)

Answer 1

provides links from and to the world outside our bodies; includes all neural structures outside the brain and spinal cord (sensory receptors, peripheral nerves & associated ganglia, and efferent motor ending

Question 2

Sensory receptors

Answer 2

specialized to respond to changes in their environment

Question 3

Stimuli

Answer 3

changes in the environment that cause a response in receptors

Question 4

Mechanoreceptors

Answer 4

respond to mechanical force (touch, pressure, including BP, vibration, and stretch)

Question 5

Thermoreceptors

Answer 5

respond to temperature changes

Question 6

Photoreceptors

Answer 6

respond to light (such as those of the retina of the eye)

Question 7

Chemoreceptors

Answer 7

respond to chemicals in solution (molecules smelled or tasted) or changed in blood or interstitial fluid chemistry

Question 8

Nociceptors

Answer 8

respond to potentially damaging stimuli that result in pain; stimulate subtypes of thermoreceptors, mechanoreceptors, and chemoreceptors

Question 9

Exteroceptors

Answer 9

sensitive to stimuli arising outside the body; most are near or at the body surface; include touch, pressure, pain, and temperature receptors in the skin and most receptors of the special senses

Question 10

Interoceptors

Answer 10

AKA visceroceptors; respond to stimuli within the body, such as from the internal viscera and blood vessels; monitor a variety of stimuli, including chemical changes, tissue stretch, and temperature; sometimes their activity causes us to feel pain, discomfort, hunger, or thirst (we are usually unaware of their workings)

Question 11

Proprioceptors

Answer 11

respond to internal stimuli; location is more restricted than interoceptors; occur in skeletal muscles, tendons, joints, ligaments, and in connective tissue coverings of bones and muscles; constantly advise the brain of our body movements by monitoring how much the organs containing those receptors are stretched

Question 12

General senses

Answer 12

overwhelming majority of sensory receptors belong to this class; simply = the modified dendritic endings of sensory neurons; found throughout the body and monitor most types of general sensory info

Question 13

Special senses

Answer 13

receptors for vision, hearing, equilibrium, smell, and taste

Question 14

Sense organs

Answer 14

receptors for special senses are housed in these complex organs

Question 15

Nonencapsulated (free) nerve endings

Answer 15

present nearly everywhere in the body; particularly abundant in epithelia and connective tissues; most are nonmyelinated, small-diameter, group C fibers; their distal endings (sensory terminals) usually have knoblike swellings; respond chiefly to temperature and painful stimuli; some respond to tissue movements caused by pressure as well

Question 16

Tactile (Merkel) discs

Answer 16

lie in the deepest layer of the epidermis; function as light touch receptors

Question 17

Hair follicle receptors

Answer 17

free nerve endings that wrap basket-like around hair follicles; light touch receptors that detect bending of hairs

Question 18

Encapsulated nerve endings

Answer 18

all consist of 1 or more fiber terminals of sensory neurons enclosed in a connective tissue capsule; virtually all are mechanoreceptors, but they vary greatly in shape, size, and distribution in the body; include tactile corpuscles, lamellar corpuscles, bulbous corpuscles, muscle spindles, tendon organs, and joint kinesthetic receptors

Question 19

Tactile corpuscles (Meissner’s corpuscles)

Answer 19

small receptors in which a few spiraling sensory terminals are surrounded by Schwann cells and then by a thin egg-shaped connective tissue capsule; found just beneath the epidermis in the dermal papillae; especially numerous in sensitive and hairless skin areas; receptors for discriminative touch & play the same role in sensing light touch in hairless skin that hair follicle receptors do in hairy skin

Question 20

Lamellar corpuscles

Answer 20

Pacinian corpuscles; scattered deep in the dermis & in subcutaneous tissue underlying the skin; mechanoreceptors stimulated by deep pressure; respond only when the pressure is 1st applied and thus are best suited to monitoring vibration (“on/off” pressure stimulus); largest corpuscular receptors; resembles a cut onion; its single dendrite is surrounded by a capsule containing up to 60 layers of collagen fibers and flattened supporting cells

Question 21

Bulbous corpuscles

Answer 21

Ruffini endings; lie in the dermis, subcutaneous tissue, and joint capsules; contain a spray of receptor endings enclosed by a flattened capsule; bear a striking resemblance to tendon organs (which monitor tendon stretch) and probably play a similar role in other dense connective tissues where they respond to deep and continuous pressure

Question 22

Muscle spindles

Answer 22

fusiform (spindle-shaped) propioceptors found throughout the perimysium of a skeletal muscle; each consists of a bundle of modified skeletal fibers (intrafusal fibers), enclosed in a connective tissue capsule; detect muscle stretch and initiate a reflex that resists the stretch

Question 23

Tendon organs

Answer 23

proprioceptors located in tendons, close to the skeletal muscle insertion; consist of small bundles of tendon (collagen) fibers enclosed in a layered capsule, with sensory terminals coiling between and around the fibers

Question 24

Joint kinesthetic receptors

Answer 24

proprioceptors that monitor stretch in the articular capsules that enclose synovial joints; contains at least 4 receptor types - lamellar corpuscles, bulbous corpuscles, free nerve endings, and receptors resembling tendon organs; together these receptors provide info on joint position and motion (a sensation of which we are highly conscious)

Question 25

Sensation

Answer 25

awareness of changes in the internal and external environments

Question 26

Perception

Answer 26

conscious interpretation of stimuli

Question 27

Somatosensory system

Answer 27

part of the sensory system serving the body wall and limbs; receives inputs from exteroceptors, propioceptors, and interoceptors; transmits info about several different sensory modalities (types of sensation)

Question 28

Transduction

Answer 28

process in which the stimulus energy must be converted into the energy of a graded potential

Question 29

Generator potential

Answer 29

when the receptor region is part of a sensory neuron (as with free dendrites or the encapsulated receptors of most general sense receptors), the graded potential is called this because it generates action potentials in a sensory neuron

Question 30

Receptor potential

Answer 30

when the receptor is a separate cell (as in most special senses), the graded potential is called this because it occurs in a separate receptor cell

Question 31

Adaptation

Answer 31

many, but not all, sensory receptors exhibit this change in sensitivity (& nerve impulse generation) in the presence of a constant stimulus

Question 32

Phasic receptors

Answer 32

fast adapting receptors, often giving bursts of impulses at the beginning and the end of the stimulus; report changes in the internal or external environment; examples = lamellar and tactile corpuscles

Question 33

Tonic receptors

Answer 33

receptors providing a sustained response with little or no adaptation; nociceptors & most propioceptors are this type because of the protective importance of their info

Question 34

Projection

Answer 34

phenomenon in which the exact point in the cortex that is activated always refers to the same “where,” regardless of how it is activated

Question 35

Perceptual detection

Answer 35

ability to detect a stimulus has occurred; simplest level of perception

Question 36

Magnitude estimation

Answer 36

ability to detect how intense the stimulus is; perceived intensity increases as stimulus intensity increases because of frequency coding

Question 37

Spatial discrimination

Answer 37

allows us to identify the site or pattern of stimulation

Question 38

Two-point discrimination test

Answer 38

a common tool for studying spatial discrimination in the lab; determines how close together 2 points on the skin can be and still be perceived as 2 points rather than as 1; provides a crude map of the density of tactile receptors in the various regions of the skin

Question 39

Feature abstraction

Answer 39

mechanism by which a neuron or circuit is tuned to one feature, or property, of a stimulus in preference to others

Question 40

Quality discrimination

Answer 40

ability to differentiate the submodalities of a particular sensation

Question 41

Qualities

Answer 41

each sensory modality has several of these submodalities; ex taste is a sensory modality with submodalities including sweet and bitter

Question 42

Pattern recognition

Answer 42

ability to take in the scene around us and recognize a familiar pattern, an unfamiliar one, or one that has special significance for us

Question 43

Hyperalgesia

Answer 43

pain amplification

Question 44

Phantom limb pain

Answer 44

pain perceived in tissue that is no longer present (such as after an amputation); an example of hyperalgesia

Question 45

Referred pain

Answer 45

phenomenon in which stimuli arising in 1 part of the body are perceived as coming from another part

Question 46

Nerve

Answer 46

cordlike organ that is part of the PNS; vary in size, but every one consists of parallel bundles of peripheral axons (some myelinated, some not) enclosed by successive wrappings of connective tissue

Question 47

Endoneurium

Answer 47

each axon is surrounded by this delicate layer of loose connective tissue that also encloses the fiber’s associated Schwann cells

Question 48

Perineurium

Answer 48

a coarser connective tissue wrapping that binds groups of fibers into bundles (fascicles)

Question 49

Fascicles

Answer 49

bundles of fibers

Question 50

Epineurium

Answer 50

a tough, fibrous sheath which encloses all the fascicles to form the nerve

Question 51

Mixed nerves

Answer 51

contain both sensory and motor fibers and transmit impulses both to and from the central nervous system; most nerves fall into this category

Question 52

Sensory (afferent) nerves

Answer 52

carry impulses only toward the CNS

Question 53

Motor (efferent) nerves

Answer 53

carry impulses only away from the CNS

Question 54

Ganglia

Answer 54

collections of neuron cell bodies associated with nerves in the PNS

Question 55

Wallerian degeneration

Answer 55

a process of disintegration of an axon that occurs when it is crushed or severed and cannot receive nutrients from the cell body

Question 56

Cranial nerves

Answer 56

12 pairs of these are associated with the brain; 1st 2 attach to the forebrain; the rest are associated with the brain stem

Question 57

Olfactory

Answer 57

cranial nerve I; tiny sensory nerves/filaments of smell; run from the nasal mucosa to synapse with the olfactory bulbs

Question 58

Optic

Answer 58

CN II; sensory nerve of vision; develops as an outgrowth of the brain so really a brain tract

Question 59

Oculomotor

Answer 59

CN III; “eye mover”; supplies 4 of the 6 extrinsic muscles that move the eyeball in the orbit

Question 60

Trochlear

Answer 60

CN IV; “pulley”; innervates an extrinsic eye muscle that loops through a pulley-shaped ligament in the orbit

Question 61

Trigeminal

Answer 61

CN V; 3 branches springs from this, the largest cranial nerve; supplies sensory fibers from the face and motor fibers to the chewing muscles

Question 62

Abducens

Answer 62

CN VI; controls the extrinsic eye muscle that ABDUCTS the eyeball (turns it laterally)

Question 63

Facial

Answer 63

CN VII; a large nerve that innervates muscles of FACIAL expression

Question 64

Vestibulocochlear

Answer 64

CN VIII; mostly sensory nerve for hearing and balance; formerly called the auditory nerve

Question 65

Glossopharyngeal

Answer 65

CN IX; “tongue & pharynx,” structures that this nerve helps innervate

Question 66

Vagus

Answer 66

CN X; “wanderer” or “vagabond;” only cranial nerve to extend beyond the head and neck to the thorax and abdomen

Question 67

Accessory

Answer 67

CN XI; considered an accessory part of the vagus nerve; formerly called the spinal accessory nerve

Question 68

Hypoglossal

Answer 68

CN XII; “under the tongue;” runs inferior to the tongue and innervates the tongue muscles

Question 69

Cranial sensory ganglia

Answer 69

in sensory neurons contributing to cranial nerves (V, VII, IX, & X), the cell bodies are located in these clusters just outside the brain

Question 70

Spinal nerves

Answer 70

31 pairs, 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

Question 71

Rootlets

Answer 71

each dorsal and ventral root forms from a series of these that attach along the length of the corresponding spinal cord segment

Question 72

Ventral roots

Answer 72

contain motor (efferent) fibers that arise from ventral horn motor neurons and extend to an innervate the skeletal muscles

Question 73

Dorsal roots

Answer 73

contain sensory (afferent) fibers that arise from sensory neurons in the dorsal root ganglia and conduct impulses from peripheral receptors to the spinal cord

Question 74

Dorsal ramus

Answer 74

small branch of a spinal nerve almost immediately after emerging from its foramen

Question 75

Ventral ramus

Answer 75

larger branch of a spinal nerve

Question 76

Meningeal branch

Answer 76

tiny branch of a spinal nerve that reenters the vertebral canal to innervate the meninges and blood vessels within

Question 77

Rami communicantes

Answer 77

special rami which contain autonomic (visceral) nerve fibers; attach to the base of the ventral rami of the thoracic spinal nerves

Question 78

Nerve plexuses

Answer 78

all ventral rami branch and join one another lateral to the vertebral column, forming complicated interlacing nerve networks called this; occur in the cervical, brachial, lumbar, and sacral regions and primarily serve the limbs; ONLY VENTRAL RAMI form these

Question 79

Cervical plexus

Answer 79

buried deep in the neck under the sternocleidomastoid muscle, the ventral rami of the 1st 4 cervical nerves form this looping plexus

Question 80

Cutaneous nerves

Answer 80

most of the branches of the cervical plexus are these nerves that supply only the skin

Question 81

Phrenic nerve

Answer 81

single most important nerve from the cervical plexus; receives fibers from C3, C4, & C5; runs inferiorly through the thorax and supplies both motor and sensory fibers to the diaphragm

Question 82

Brachial plexus

Answer 82

large, important plexus situated partly in the neck and partly in the axilla; gives rise to virtually all the nerves the innervate the upper limb; can be palpated in a living person just superior to the clavicle at the lateral border of the sternocleidomastoid

Question 83

Roots of the brachial plexus (5)

Answer 83

ventral rami C5-T1; lie deep to the sternocleiodmastoid

Question 84

Upper, middle, & lower trunks

Answer 84

at the lateral border of the sternocleiodmastoid

Question 85

Anterior & posterior divisions

Answer 85

upper/middle/lower trunks divide almost immediately into these divisions

Question 86

Lateral, medial, and posterior cords

Answer 86

the anterior/posterior divisions (indicate which fibers serve the front/back of the limb) pass deep to the clavicle and enter the axilla; there they give rise to these 3 large fiber bundles (named for their relationship to the axillary artery, which runs through the axilla)

Question 87

Axillary nerve

Answer 87

branches off the posterior cord and runs posterior to the surgical neck of the humerus; innervates the deltoid and teres minor and the skin & joint capsule of the shoulder

Question 88

Musculocutaneous nerve

Answer 88

major end branch of the lateral cord; courses inferiorly in the anterior arm, supplying motor fibers to the biceps brachii, brachialis, and coracobrachialis muscles; distal to the elbow, it provides cutaneous sensation in the lateral forearm

Question 89

Median nerve

Answer 89

descends through the arm to the anterior forearm, where it gives off branches to the skin and to most flexor muscles; on reaching the hand, it innervates 5 intrinsic muscles of the lateral palm; activates muscles that pronate the forearm, flex the wrist and fingers, and oppose the thumb

Question 90

Ulnar nerve

Answer 90

branches off the medial cord of the brachial plexus; descends along the medial aspect of the arm toward the elbow, swings behind the medial epicondyle, and then follows the ulna along the medial forearm; supplies the flexor carpi ulnaris and the medial part of the flexor digitorum profundus (flexors not supplied by the median nerve); continues into the hand, where it innervates most intrinsic hand muscles and the skin of the medial aspect of the hand; causes the wrist and fingers to flex, and (with the median nerve) adducts and abducts the medial fingers

Question 91

Radial nerve

Answer 91

largest branch of the brachial plexus; continuation of the posterior cord; wraps around the humerus (in the radial groove) then runs anteriorly around the lateral epicondyle at the elbow; divides into a superficial branch that follows the lateral edge of the radius to the hand and a deep branch that runs posteriorly; supplies the posterior skin of the limb along its entire course; its motor branches innervate essentially all the extensor muscles of the upper limb; muscles controlled by this nerve extend the elbow, supinate the forearm, extend the wrist/fingers, and abduct the thumb

Question 92

Lumbosacral trunk

Answer 92

many fibers of the lumbar plexus contribute to the sacral plexus via this

Question 93

Lumbosacral plexus

Answer 93

lumbar plexus + sacral plexus; serves mainly the lower limb; also sends some branches to the abdomen, pelvis, and buttock

Question 94

Lumbar plexus

Answer 94

arises from spinal nerves L1-L4 & lies within the psoas major muscle; its proximal branches innervate parts of the abdominal wall muscles and the psoas muscle, but its major branches descend to innervate the anterior and medial thigh

Question 95

Femoral nerve

Answer 95

largest terminal nerve of the lumbosacral plexus; runs deep to the inguinal ligament to enter the thigh and then divides into several large branches; the motor branches innervate anterior thigh muscles (quads = principle thigh flexors and knee extensors); cutaneous branches serve the skin of the anterior thigh and the medial surface of the leg from knee to foot

Question 96

Obturator nerve

Answer 96

enters the medial thigh via the obturator foramen and innervates the adductor muscles

Question 97

Sacral plexus

Answer 97

arises from spinal nerves L4-S4; lies immediately caudal to the lumbar plexus; has a dozen named branches; about half serve the buttock and lower limb; the others innervate pelvic structures and the perineum

Question 98

Sciatic nerve

Answer 98

the largest branch of the sacral plexus; the thickest and longest nerve in the body; supplies the entire lower limb except the anteromedial thigh

Question 99

Tibial nerve

Answer 99

nerve that continues through the popliteal fossa and supplies the posterior compartment muscles of the leg and the skin of the posterior calf and sole of the foot

Question 100

Sural nerve

Answer 100

in the vicinity of the knee, the tibial nerve gives off this nerve, which serves the skin of the posterolateral leg

Question 101

Medial/lateral plantar nerves

Answer 101

at the ankle, the tibial nerve divides into these 2 nerves, which serve most of the foot

Question 102

Common fibular nerve

Answer 102

AKA common peroneal nerve; descends from its point of origin, wraps around the neck of the fibula, and then divides into superficial and deep branches; the branches innervate the knee joint, skin of the anterior and lateral leg and dorsum of the foot, and muscles of the anterolateral leg (extensors that dorsiflex the foot)

Question 103

Superior/inferior gluteal nerves

Answer 103

next largest sacral plexus branches; together they innervate the buttock and tensor fasciae latae muscles

Question 104

Pudendal nerve

Answer 104

“shameful;” innervates the muscles and skin of the perineum and helps stimulate erection and control urination

Question 105

Intercostal nerves

Answer 105

ventral rami of T1-T12 mostly course anteriorly, deep to each rib as these nerves, which supply the intercostal muscles, the muscle and skin of the anterolateral thorax, and most of the abdominal wall; give off cutaneous branches to the skin

Question 106

Subcostal nerve

Answer 106

lies inferior to the 12th rib, which makes it this type of nerve

Question 107

Dermatome

Answer 107

“skin segment;” area of skin innervated by cutaneous branches of a single spinal nerve; every spinal nerve except C1 innervates these

Question 108

Hilton’s Law

Answer 108

any nerve serving a muscle that produces movement at a joint also innervates the joint and the skin over the joint

Question 109

Motor endings

Answer 109

the PNS elements that activate effectors by releasing neurotransmittesr

Question 110

Neuromuscular junctions

Answer 110

terminals of somatic motor fibers that innervate voluntary muscles form elaborate ___ ___ with their effector cells

Question 111

Varicosities

Answer 111

an axon serving smooth muscle or a gland (NOT cardiac muscle) has a series of these knoblike swellings containing mitochondria and synaptic vesicles (make it look like a string of beads

Question 112

Segmental level

Answer 112

lowest level of the motor hierarchy; consists of reflexes and spinal cord circuits that control automatic movements

Question 113

Central pattern generators

Answer 113

circuits that control locomotion and other specific and oft-repeated motor activities

Question 114

Projection level

Answer 114

spinal cord is under the direct control of this level of motor control; consists of neurons acting through the direct and indirect motor pathways

Question 115

Precommand areas

Answer 115

systems which control the outputs of the cortex and brain stem motor centers and stand at the highest level of the motor hierarchy

Question 116

Cerebellum

Answer 116

key center for “online” sensorimotor integration and control; target of ascending proprioceptor, tactile, equilibrium, and visual inputs

Question 117

Basal nuclei

Answer 117

receives inputs from all cortical areas and send their output back mainly to premotor and prefrontal cortical areas via the thalamus; appear to be involved in more complex aspects of motor control; under resting conditions, they inhibit various motor centers of the brain

Question 118

Monosynaptic reflex

Answer 118

a single synapse between a sensory neuron and a motor neuron

Question 119

Polysynaptic reflex

Answer 119

more complex reflex arc involving multiple synapses with chains of interneurons

Question 120

Somatic reflexes

Answer 120

classification of reflexes that activate skeletal muscles

Question 121

Autonomic (visceral) reflexes

Answer 121

classification of reflexes that activate visceral effectors (smooth or cardiac muscles or glands)

Question 122

Spinal reflexes

Answer 122

somatic reflexes that are mediated by the spinal cord

Question 123

Intrafusal muscle fibers

Answer 123

each muscle spindle consists of 3-10 of these modified skeletal muscle fibers enclosed in a connective tissue capsule

Question 124

Extrafusal muscle fibers

Answer 124

effector fibers of the muscles

Question 125

Anulospiral endings

Answer 125

AKA primary sensory endings; endings of large axons that wrap around the spindle center; stimulated by both the rate and degree of stretch

Question 126

Flower spray endings

Answer 126

AKA secondary sensory endings; formed by smaller axons that supply the spindle ends; stimulated only by degree of stretch

Question 127

Gamma efferent fibers

Answer 127

the intrafusal muscle fibers have contractile regions at their ends, which are the only areas containing actin and myosin myofilaments; arise from small motor neurons in the ventral horn of the spinal cord; maintain spindle sensitivity

Question 128

Alpha efferent fibers

Answer 128

fibers of the large alpha motor neurons

Question 129

Alpha motor neurons

Answer 129

stimulate the extrafusal muscle fibers to contract

Question 130

Alpha-gamma coactivation

Answer 130

descending fibers of motor pathways synapse with both alpha and gamma motor neurons, and motor impulses are simultaneously sent to the large extrafusal fibers and to muscle spindle intrafusal fibers; stimulating the intrafusal fibers maintains the spindle’s tension (and sensitivity) during muscle contraction, so that the brain continues to be notified of changes in the muscle length

Question 131

Stretch reflex

Answer 131

reflex that makes sure the muscle stays at a set length

Question 132

Patellar/knee-jerk reflex

Answer 132

stretch reflex that helps keep your knees from buckling when you are standing upright

Question 133

Reciprocal inhibition

Answer 133

branches of the afferent fibers synapse with interneurons that inhibit motor neurons controlling antagonistic muscles (parallel processing); resultant inhibition is this

Question 134

Monosynaptic & ipsilateral

Answer 134

involving a single synapse and motor activity on the same side of the body

Question 135

Tendon reflexes

Answer 135

muscles relax and lengthen in response to tension

Question 136

Reciprocal activation

Answer 136

phenomenon in which, simultaneously, motor neurons in spinal cord circuits supplying the contracting muscles are activated; contracting muscle relaxes as its antagonist is activated

Question 137

Flexor (withdrawal) reflex

Answer 137

a painful stimulus initiates this reflex; causes automatic withdrawal of the threatened body part from the stimulus

Question 138

Crossed-extensor reflex

Answer 138

often accompanies the flexor reflex in weight-bearing limbs and is particularly important in maintaining balance

Question 139

Superficial reflexes

Answer 139

elicited by gentle cutaneous stimulation; depend on functional upper motor pathways and on cord-level reflex arcs; best known are plantar and abdominal reflexes

Question 140

Plantar reflex

Answer 140

tests the integrity of the spinal cord from L4 to S2; indirectly determines if the corticospinal tracts are functioning properly; normal response = toes curl

Question 141

Babinski’s sign

Answer 141

if the primary motor cortex or corticospinal tract is damaged, the plantar reflex is replaced by this abnormal reflex, in which the great toe dorsiflexes and the smaller toes fan laterally

Question 142

Abdominal reflexes

Answer 142

check the integrity of the spinal cord and ventral rami from T8 to T12; vary in intensity from one person to another; absence indicates lesions in the corticospinal tract