Download CNS consists of brain and spinal cord PNS consists of nerves 1

CNS consists of brain and spinal cord
PNS consists of nerves
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Input to nervous system
[Chapter 13]
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Cordlike organ of PNS
Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective
tissue
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Connective tissue coverings include
Endoneurium—loose connective tissue that encloses axons and their myelin
sheaths
Perineurium—coarse connective tissue that bundles fibers into fascicles
Epineurium—tough fibrous sheath around a nerve
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Peripheral nerves classified as cranial or spinal nerves
Classified according to direction transmit impulses
Types of fibers in mixed nerves:
Somatic afferent
Somatic efferent
Visceral afferent
Visceral efferent
Sensory (afferent) nerves – impulses only toward CNS
Motor (efferent) nerves – impulses only away from CNS
Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic
(visceral) fibers
Pure sensory (afferent) or motor (efferent) nerves are rare; most mixed
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Note that there are no neuronal cell bodies in peripheral nerves which are just
myelinated axons
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Contain neuron cell bodies associated with nerves in PNS
Ganglia associated with afferent nerve fibers contain cell bodies of sensory
neurons
Dorsal root ganglia (sensory, somatic) (Chapter 12)
Ganglia associated with efferent nerve fibers contain autonomic motor
neurons
Autonomic ganglia (motor, visceral) (Chapter 14
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31 pairs of mixed nerves named for point of issue from spinal cord
Supply all body parts but head and part of neck
8 cervical (C1–C8)
12 thoracic (T1–T12)
5 Lumbar (L1–L5)
5 Sacral (S1–S5)
1 Coccygeal (C0)
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Only 7 cervical vertebrae, yet 8 pairs cervical spinal nerves
7 exit vertebral canal superior to vertebrae for which named
1 exits canal inferior to C7
Other vertebrae exit inferior to vertebra for which named
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Mature neurons are amitotic but if
soma of damaged nerve is intact,
peripheral axon may regenerate
If peripheral axon damaged
Axon fragments (Wallerian
degeneration); spreads distally from
injury
Macrophages clean dead axon; myelin
sheath intact
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Axon filaments grow through
regeneration tube
Axon regenerates; new myelin sheath
forms
Greater distance between severed
ends-less chance of regeneration
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Macrophages clean dead axon; myelin
sheath intact
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Axon filaments grow through
regeneration tube
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Axon regenerates; new myelin sheath
forms
Greater distance between severed
ends-less chance of regeneration
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Most CNS fibers never regenerate
CNS oligodendrocytes bear growthinhibiting proteins that prevent CNS
fiber regeneration
Astrocytes at injury site form scar
tissue containing chondroitin sulfate
that blocks axonal regrowth
Treatment
Neutralizing growth inhibitors,
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blocking receptors for inhibitory
proteins, destroying chondroitin
sulfate promising
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Each spinal nerve connects to spinal cord via two roots
Ventral roots
Contain motor (efferent) fibers from ventral horn motor neurons
Fibers innervate skeletal muscles
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Dorsal roots
Contain sensory (afferent) fibers from sensory neurons in dorsal root ganglia
and conduct impulses from peripheral receptors
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Dorsal and ventral roots unite to form spinal nerves, which emerge from vertebral
column via intervertebral foramina
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Spinal roots longer as move
inferiorly in cord
Lumbar and sacral roots extend as
cauda equina
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Spinal nerves quite short (~1-2 cm)
Each branches into mixed rami
Dorsal ramus
Ventral ramus - larger
Meningeal branch – tiny, reenters vertebral canal, innervates meninges and
blood vessels
Rami communicantes (autonomic pathways) join ventral rami in thoracic
region
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All ventral rami except T2–T12 form
interlacing nerve networks called
nerve plexuses (cervical, brachial,
lumbar, and sacral)
Back innervated by dorsal rami via
several branches
Ventral rami of T2–T12 as intercostal
nerves supply muscles of ribs,
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anterolateral thorax, and abdominal
wall
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All ventral rami except T2–T12 form interlacing nerve networks called nerve plexuses
(cervical, brachial, lumbar, and sacral)
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Within plexus fibers criss-cross
Each branch contains fibers from several spinal nerves
Fibers from ventral ramus go to body periphery via several routes
Each limb muscle innervated by more than one spinal nerve
Damage to one does not  paralysis
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Formed by ventral rami of C1–C4
Most branches form cutaneous nerves
Innervate skin of neck, ear, back of head, and shoulders
Other branches innervate neck muscles
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Phrenic nerve
Major motor and sensory nerve of diaphragm (receives fibers from C3–C5)
Irritation  hiccups
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Formed by ventral rami of C5–C8 and T1 (and often C4 and/or T2)
Gives rise to nerves that innervate upper limb
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Major branches of this plexus:
Roots—five ventral rami (C5–T1), which form
Trunks—upper, middle, and lower, which form
Divisions—anterior and posterior, which form
Cords—lateral, medial, and posterior
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Brachial Plexus: Five Important Nerves
Axillary—innervates deltoid, teres minor, and skin and joint capsule of shoulder
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Musculocutaneous—innervates biceps brachii and brachialis, coracobrachialis, and
skin of lateral forearm
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Median—innervates skin, most flexors, forearm pronators, wrist and finger flexors,
thumb opposition muscles
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Ulnar—supplies flexor carpi ulnaris, part of flexor digitorum profundus, most intrinsic
hand muscles, skin of medial aspect of hand, wrist/finger flexion
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Radial—innervates essentially all extensor muscles, supinators, and posterior skin of
limb
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Arises from L1–L4
Innervates thigh, abdominal wall, and psoas muscle
Femoral nerve—innervates quadriceps and skin of anterior thigh and medial surface
of leg
Obturator nerve—passes through obturator foramen to innervate adductor muscles
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Cremaster muscle innervated by the genitofemoral nerve
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Arises from L4–S4
Serves the buttock, lower limb, pelvic structures, and perineum
Sciatic nerve
Longest and thickest nerve of body
Innervates hamstring muscles, adductor magnus, and most muscles in leg and
foot
Composed of two nerves: tibial and common fibular
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Ventral rami in thorax in simple segmental pattern
Form intercostal nerves that supply intercostal muscles, muscle and skin of
anterolateral thorax, most abdominal wall
Give off cutaneous branches to skin along course
Dorsal rami innervate posterior body trunk
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Dermatome - area of skin innervated by cutaneous branches of single spinal nerve
All spinal nerves except C1 participate in dermatomes
Extent of spinal cord injuries ascertained by affected dermatomes
Most dermatomes overlap, so destruction of a single spinal nerve will not cause
complete numbness
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Provides links from and to world outside body
All neural structures outside brain
Sensory receptors
Peripheral nerves and associated ganglia
Efferent motor endings
SENSORY RECEPTORS
Specialized to respond to changes in environment (stimuli)
Activation results in graded potentials that trigger nerve impulses
Sensation (awareness of stimulus) and perception (interpretation of meaning of
stimulus) occur in brain
Classification of receptors:
Based on
Type of stimulus they detect
Location in body
Structural complexity
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Classification by Stimulus Type
Mechanoreceptors—respond to touch, pressure, vibration, and stretch
Thermoreceptors—sensitive to changes in temperature
Photoreceptors—respond to light energy (e.g., retina)
Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood
chemistry)
Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive
pressure, inflammatory chemicals)
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Classification by Location
Exteroceptors
Respond to stimuli arising outside body
Receptors in skin for touch, pressure, pain, and temperature
Most special sense organs
Interoceptors (visceroceptors)
Respond to stimuli arising in internal viscera and blood vessels
Sensitive to chemical changes, tissue stretch, and temperature changes
Sometimes cause discomfort but usually unaware of their workings
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Interoceptors (visceroceptors)
Respond to stimuli arising in internal viscera and blood vessels
Sensitive to chemical changes, tissue stretch, and temperature changes
Sometimes cause discomfort but usually unaware of their workings
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Proprioceptors
Respond to stretch in skeletal muscles, tendons, joints, ligaments, and
connective tissue coverings of bones and muscles
Inform brain of one's movements
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Simple receptors for general senses
Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and
muscle sense
Modified dendritic endings of sensory neurons
Either nonencapsulated (free) or encapsulated
Nonencapsulated (free) nerve endings
Abundant in epithelia and connective tissues
Most nonmyelinated, small-diameter group C fibers; distal endings have
knoblike swellings
Respond mostly to temperature and pain; some to pressure-induced tissue
movement; itch
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Thermoreceptors
Cold receptors (10–40ºC); in superficial dermis
Heat receptors (32–48ºC); in deeper dermis
Outside those temperature ranges  nociceptors activated  pain
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Nociceptors
Player in detection – vanilloid receptor
Ion channel opened by heat, low pH, chemicals, e.g., capsaicin (red
peppers)
Respond to:
Pinching, chemicals from damaged tissue, capsaicin
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Light touch receptors
Tactile (Merkel) discs
Hair follicle receptors
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Encapsulated Dendritic Endings
All mechanoreceptors in connective
tissue capsule
Tactile (Meissner's) corpuscles—
discriminative touch
Lamellar (Pacinian) corpuscles—deep
pressure and vibration
Bulbous corpuscles (Ruffini endings)—
deep continuous pressure
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Encapsulated Dendritic Endings
Muscle spindles—muscle stretch
Tendon organs—stretch in tendons
Joint kinesthetic receptors—joint
position and motion
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Somatosensory system – part of sensory system serving body wall and limbs
Receives inputs from
Exteroceptors, proprioceptors, and interoceptors
Input relayed toward head, but processed along way
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Sensation - the awareness of changes in the internal and external environment
To produce a sensation
Receptors have specificity for stimulus energy
Stimulus must be applied in receptive field
Transduction occurs
Stimulus changed to graded potential
Generator potential or receptor potential
Graded potentials must reach threshold  Action Potential
In general sense receptors, graded potential called generator potential
Stimulus  Generator potential in afferent neuron  Action potential
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Receptors have specificity for stimulus energy
Stimulus must be applied in receptive field
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Transduction occurs
Stimulus changed to graded potential
Generator potential or receptor potential
Graded potentials proportional to stimulus
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Receptor and generator potentials are GRADED potentials
[effect on adjacent ionic concentrations noted as blue circles above]
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Note: inactivation gates of voltage-gated sodium channels are CLOSED until point 5
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Graded potential gets membrane potential to threshold
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graded potential lasts for many, many milliseconds
action potential lasts for less than 5 mS
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Persistent graded potential combines with falling and undershoot phases of action
potential
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In general sense receptors, graded potential called generator potential
Stimulus  Generator potential in afferent neuron  Action potential
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In special sense organs:
Stimulus

Graded potential in receptor cell called
receptor potential

Affects amount of neurotransmitter released

Neurotransmitters generate graded potentials in sensory neuron
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Adaptation is change in sensitivity in presence of constant stimulus
Receptor membranes become less responsive
Receptor potentials decline in frequency or stop
Phasic (fast-adapting) receptors signal beginning or end of stimulus
Examples - receptors for pressure, touch, and smell
Tonic receptors adapt slowly or not at all
Examples - nociceptors and most proprioceptors
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Phasic (fast-adapting) receptors signal beginning or end of stimulus
Examples - receptors for pressure, touch, and smell
Tonic receptors adapt slowly or not at all
Examples - nociceptors and most proprioceptors
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Twelve pairs of nerves associated
with brain
Two attach to forebrain; rest with brain
stem
Most mixed nerves; two pairs purely
sensory
Each numbered (I through XII) and
named from rostral to caudal
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Twelve pairs of nerves associated
with brain
Two attach to forebrain; rest with brain
stem
Most mixed nerves; two pairs purely
sensory
Each numbered (I through XII) and
named from rostral to cauxdal
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Make up your own mnemonic
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Cranial nerves with sensory
modalities:
Olfactory and optic nerves
Neuron cell bodies within special
sense organs
Other nerves with sensory
information (V, VII, VIII, IX, X)
Neuron cell bodies in cranial sensory
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ganglia
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PS = parasympathetic
[only I and II are pure sensory nerves]
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Sensory nerves of smell
Run from nasal mucosa to olfactory bulbs
Pass through cribriform plate of ethmoid bone
Fibers synapse in olfactory bulbs
Pathway terminates in primary olfactory cortex
Purely sensory (olfactory) function
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Arise from retinas; really a brain tract not a peripheral nerve
Pass through optic canals, converge and partially cross over at optic chiasma
Optic tracts continue to thalamus, where they synapse
Optic radiation fibers run to occipital (visual) cortex
Purely sensory (visual) function
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Fibers extend from ventral midbrain through superior orbital fissures to four of six
extrinsic eye muscles
Function in raising eyelid, directing eyeball, constricting iris (parasympathetic), and
controlling lens shape
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Fibers from dorsal midbrain enter orbits via superior orbital fissures to innervate
superior oblique muscle
Primarily motor nerve that directs eyeball
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Largest cranial nerves; fibers extend
from pons to face
Three divisions
Ophthalmic (V1) passes through
superior orbital fissure
Maxillary (V2) passes through foramen
rotundum
Mandibular (V3) passes through the
foramen ovale
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Convey sensory impulses from
various areas of face (V1) and (V2)
Supply motor fibers (V3) for
mastication
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Note location of the ganglion containing
the cell bodies of the sensory neurons of
Cranial nerve V
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Shown to emphasize the unipolar characteristics of Peripheral Nerve sensory cells in
Cranial Nerve.
From article about the very pretty stain.
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Just like Dorsal Root Ganglia
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Fibers from inferior pons enter orbits via superior orbital fissures
Primarily a motor, innervating lateral rectus muscle
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Fibers from pons travel through internal acoustic meatuses, and emerge through
stylomastoid foramina to lateral aspect of face
Chief motor nerves of face with 5 major branches
Motor functions include facial expression, parasympathetic impulses to lacrimal and
salivary glands
Sensory function (taste) from anterior two-thirds of tongue
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Afferent fibers from hearing receptors (cochlear division) and equilibrium receptors
(vestibular division) pass from inner ear through internal acoustic meatuses, and
enter brain stem at pons-medulla border
Mostly sensory function; small motor component for adjustment of sensitivity of
receptors
Formerly auditory nerve
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Fibers from medulla leave skull via jugular foramen and run to throat
Motor functions - innervate part of tongue and pharynx for swallowing, and provide
parasympathetic fibers to parotid salivary glands
Sensory functions - fibers conduct taste and general sensory impulses from pharynx
and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors
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Only cranial nerves that extend beyond head and neck region
Fibers from medulla exit skull via jugular foramen
Most motor fibers are parasympathetic fibers that help regulate activities of heart,
lungs, and abdominal viscera
Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors,
chemoreceptors, and taste buds of posterior tongue and pharynx
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Formed from ventral rootlets from C1–C5 region of spinal cord (not brain)
Rootlets pass into cranium via each foramen magnum
Accessory nerves exit skull via jugular foramina to innervate trapezius and
sternocleidomastoid muscles
Formerly spinal accessory nerve
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Fibers from medulla exit skull via hypoglossal canal
Innervate extrinsic and intrinsic muscles of tongue that contribute to swallowing and
speech
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