A&P 13: The Peripheral Nervous System & Reflex Activity Flashcards
Peripheral Nervous System (PNS)
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
Sensory receptors
specialized to respond to changes in their environment
Stimuli
changes in the environment that cause a response in receptors
Mechanoreceptors
respond to mechanical force (touch, pressure, including BP, vibration, and stretch)
Thermoreceptors
respond to temperature changes
Photoreceptors
respond to light (such as those of the retina of the eye)
Chemoreceptors
respond to chemicals in solution (molecules smelled or tasted) or changed in blood or interstitial fluid chemistry
Nociceptors
respond to potentially damaging stimuli that result in pain; stimulate subtypes of thermoreceptors, mechanoreceptors, and chemoreceptors
Exteroceptors
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
Interoceptors
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)
Proprioceptors
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
General senses
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
Special senses
receptors for vision, hearing, equilibrium, smell, and taste
Sense organs
receptors for special senses are housed in these complex organs
Nonencapsulated (free) nerve endings
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
Tactile (Merkel) discs
lie in the deepest layer of the epidermis; function as light touch receptors
Hair follicle receptors
free nerve endings that wrap basket-like around hair follicles; light touch receptors that detect bending of hairs
Encapsulated nerve endings
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
Tactile corpuscles (Meissner’s corpuscles)
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
Lamellar corpuscles
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
Bulbous corpuscles
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
Muscle spindles
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
Tendon organs
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
Joint kinesthetic receptors
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)
Sensation
awareness of changes in the internal and external environments
Perception
conscious interpretation of stimuli
Somatosensory system
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)
Transduction
process in which the stimulus energy must be converted into the energy of a graded potential
Generator potential
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
Receptor potential
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
Adaptation
many, but not all, sensory receptors exhibit this change in sensitivity (& nerve impulse generation) in the presence of a constant stimulus
Phasic receptors
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
Tonic receptors
receptors providing a sustained response with little or no adaptation; nociceptors & most propioceptors are this type because of the protective importance of their info
Projection
phenomenon in which the exact point in the cortex that is activated always refers to the same “where,” regardless of how it is activated
Perceptual detection
ability to detect a stimulus has occurred; simplest level of perception
Magnitude estimation
ability to detect how intense the stimulus is; perceived intensity increases as stimulus intensity increases because of frequency coding
Spatial discrimination
allows us to identify the site or pattern of stimulation
Two-point discrimination test
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
Feature abstraction
mechanism by which a neuron or circuit is tuned to one feature, or property, of a stimulus in preference to others
Quality discrimination
ability to differentiate the submodalities of a particular sensation
Qualities
each sensory modality has several of these submodalities; ex taste is a sensory modality with submodalities including sweet and bitter
Pattern recognition
ability to take in the scene around us and recognize a familiar pattern, an unfamiliar one, or one that has special significance for us
Hyperalgesia
pain amplification
Phantom limb pain
pain perceived in tissue that is no longer present (such as after an amputation); an example of hyperalgesia
Referred pain
phenomenon in which stimuli arising in 1 part of the body are perceived as coming from another part
Nerve
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
Endoneurium
each axon is surrounded by this delicate layer of loose connective tissue that also encloses the fiber’s associated Schwann cells
Perineurium
a coarser connective tissue wrapping that binds groups of fibers into bundles (fascicles)
Fascicles
bundles of fibers
Epineurium
a tough, fibrous sheath which encloses all the fascicles to form the nerve
Mixed nerves
contain both sensory and motor fibers and transmit impulses both to and from the central nervous system; most nerves fall into this category
Sensory (afferent) nerves
carry impulses only toward the CNS
Motor (efferent) nerves
carry impulses only away from the CNS
Ganglia
collections of neuron cell bodies associated with nerves in the PNS
Wallerian degeneration
a process of disintegration of an axon that occurs when it is crushed or severed and cannot receive nutrients from the cell body
Cranial nerves
12 pairs of these are associated with the brain; 1st 2 attach to the forebrain; the rest are associated with the brain stem
Olfactory
cranial nerve I; tiny sensory nerves/filaments of smell; run from the nasal mucosa to synapse with the olfactory bulbs
Optic
CN II; sensory nerve of vision; develops as an outgrowth of the brain so really a brain tract
Oculomotor
CN III; “eye mover”; supplies 4 of the 6 extrinsic muscles that move the eyeball in the orbit
Trochlear
CN IV; “pulley”; innervates an extrinsic eye muscle that loops through a pulley-shaped ligament in the orbit
Trigeminal
CN V; 3 branches springs from this, the largest cranial nerve; supplies sensory fibers from the face and motor fibers to the chewing muscles
Abducens
CN VI; controls the extrinsic eye muscle that ABDUCTS the eyeball (turns it laterally)
Facial
CN VII; a large nerve that innervates muscles of FACIAL expression
Vestibulocochlear
CN VIII; mostly sensory nerve for hearing and balance; formerly called the auditory nerve
Glossopharyngeal
CN IX; “tongue & pharynx,” structures that this nerve helps innervate
Vagus
CN X; “wanderer” or “vagabond;” only cranial nerve to extend beyond the head and neck to the thorax and abdomen
Accessory
CN XI; considered an accessory part of the vagus nerve; formerly called the spinal accessory nerve
Hypoglossal
CN XII; “under the tongue;” runs inferior to the tongue and innervates the tongue muscles
Cranial sensory ganglia
in sensory neurons contributing to cranial nerves (V, VII, IX, & X), the cell bodies are located in these clusters just outside the brain
Spinal nerves
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
Rootlets
each dorsal and ventral root forms from a series of these that attach along the length of the corresponding spinal cord segment
Ventral roots
contain motor (efferent) fibers that arise from ventral horn motor neurons and extend to an innervate the skeletal muscles
Dorsal roots
contain sensory (afferent) fibers that arise from sensory neurons in the dorsal root ganglia and conduct impulses from peripheral receptors to the spinal cord
Dorsal ramus
small branch of a spinal nerve almost immediately after emerging from its foramen
Ventral ramus
larger branch of a spinal nerve
Meningeal branch
tiny branch of a spinal nerve that reenters the vertebral canal to innervate the meninges and blood vessels within
Rami communicantes
special rami which contain autonomic (visceral) nerve fibers; attach to the base of the ventral rami of the thoracic spinal nerves
Nerve plexuses
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
Cervical plexus
buried deep in the neck under the sternocleidomastoid muscle, the ventral rami of the 1st 4 cervical nerves form this looping plexus
Cutaneous nerves
most of the branches of the cervical plexus are these nerves that supply only the skin
Phrenic nerve
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
Brachial plexus
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
Roots of the brachial plexus (5)
ventral rami C5-T1; lie deep to the sternocleiodmastoid
Upper, middle, & lower trunks
at the lateral border of the sternocleiodmastoid
Anterior & posterior divisions
upper/middle/lower trunks divide almost immediately into these divisions
Lateral, medial, and posterior cords
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)
Axillary nerve
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
Musculocutaneous nerve
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
Median nerve
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
Ulnar nerve
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
Radial nerve
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
Lumbosacral trunk
many fibers of the lumbar plexus contribute to the sacral plexus via this
Lumbosacral plexus
lumbar plexus + sacral plexus; serves mainly the lower limb; also sends some branches to the abdomen, pelvis, and buttock
Lumbar plexus
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
Femoral nerve
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
Obturator nerve
enters the medial thigh via the obturator foramen and innervates the adductor muscles
Sacral plexus
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
Sciatic nerve
the largest branch of the sacral plexus; the thickest and longest nerve in the body; supplies the entire lower limb except the anteromedial thigh
Tibial nerve
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
Sural nerve
in the vicinity of the knee, the tibial nerve gives off this nerve, which serves the skin of the posterolateral leg
Medial/lateral plantar nerves
at the ankle, the tibial nerve divides into these 2 nerves, which serve most of the foot
Common fibular nerve
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)
Superior/inferior gluteal nerves
next largest sacral plexus branches; together they innervate the buttock and tensor fasciae latae muscles
Pudendal nerve
“shameful;” innervates the muscles and skin of the perineum and helps stimulate erection and control urination
Intercostal nerves
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
Subcostal nerve
lies inferior to the 12th rib, which makes it this type of nerve
Dermatome
“skin segment;” area of skin innervated by cutaneous branches of a single spinal nerve; every spinal nerve except C1 innervates these
Hilton’s Law
any nerve serving a muscle that produces movement at a joint also innervates the joint and the skin over the joint
Motor endings
the PNS elements that activate effectors by releasing neurotransmittesr
Neuromuscular junctions
terminals of somatic motor fibers that innervate voluntary muscles form elaborate ___ ___ with their effector cells
Varicosities
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
Segmental level
lowest level of the motor hierarchy; consists of reflexes and spinal cord circuits that control automatic movements
Central pattern generators
circuits that control locomotion and other specific and oft-repeated motor activities
Projection level
spinal cord is under the direct control of this level of motor control; consists of neurons acting through the direct and indirect motor pathways
Precommand areas
systems which control the outputs of the cortex and brain stem motor centers and stand at the highest level of the motor hierarchy
Cerebellum
key center for “online” sensorimotor integration and control; target of ascending proprioceptor, tactile, equilibrium, and visual inputs
Basal nuclei
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
Monosynaptic reflex
a single synapse between a sensory neuron and a motor neuron
Polysynaptic reflex
more complex reflex arc involving multiple synapses with chains of interneurons
Somatic reflexes
classification of reflexes that activate skeletal muscles
Autonomic (visceral) reflexes
classification of reflexes that activate visceral effectors (smooth or cardiac muscles or glands)
Spinal reflexes
somatic reflexes that are mediated by the spinal cord
Intrafusal muscle fibers
each muscle spindle consists of 3-10 of these modified skeletal muscle fibers enclosed in a connective tissue capsule
Extrafusal muscle fibers
effector fibers of the muscles
Anulospiral endings
AKA primary sensory endings; endings of large axons that wrap around the spindle center; stimulated by both the rate and degree of stretch
Flower spray endings
AKA secondary sensory endings; formed by smaller axons that supply the spindle ends; stimulated only by degree of stretch
Gamma efferent fibers
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
Alpha efferent fibers
fibers of the large alpha motor neurons
Alpha motor neurons
stimulate the extrafusal muscle fibers to contract
Alpha-gamma coactivation
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
Stretch reflex
reflex that makes sure the muscle stays at a set length
Patellar/knee-jerk reflex
stretch reflex that helps keep your knees from buckling when you are standing upright
Reciprocal inhibition
branches of the afferent fibers synapse with interneurons that inhibit motor neurons controlling antagonistic muscles (parallel processing); resultant inhibition is this
Monosynaptic & ipsilateral
involving a single synapse and motor activity on the same side of the body
Tendon reflexes
muscles relax and lengthen in response to tension
Reciprocal activation
phenomenon in which, simultaneously, motor neurons in spinal cord circuits supplying the contracting muscles are activated; contracting muscle relaxes as its antagonist is activated
Flexor (withdrawal) reflex
a painful stimulus initiates this reflex; causes automatic withdrawal of the threatened body part from the stimulus
Crossed-extensor reflex
often accompanies the flexor reflex in weight-bearing limbs and is particularly important in maintaining balance
Superficial reflexes
elicited by gentle cutaneous stimulation; depend on functional upper motor pathways and on cord-level reflex arcs; best known are plantar and abdominal reflexes
Plantar reflex
tests the integrity of the spinal cord from L4 to S2; indirectly determines if the corticospinal tracts are functioning properly; normal response = toes curl
Babinski’s sign
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
Abdominal reflexes
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
