Nervous System 3
Peripheral Nervous System (PNS)
• All neural structures outside the brain
• ___________ receptors
• Peripheral ___________ and associated ___________
• Motor endings
Sensory Receptors
• Specialized to respond to changes in their ___________
(stimuli)
• Activation results in graded potentials that trigger nerve
impulses
• Sensation (awareness of ___________) and perception
(___________ of the meaning of the stimulus) occur in the
brain
Classification of Receptors
• Based on:
• Stimulus type
• ___________
• Structural complexity
Classification by Stimulus Type
• ___________ —respond to touch, pressure, vibration, stretch, and itch
• ___________ —sensitive to changes in temperature
• ___________ —respond to light energy (e.g., retina)
• ___________ —respond to chemicals (e.g., smell, taste, changes in
blood chemistry)
• ___________ —sensitive to pain-causing stimuli (e.g. extreme heat or
cold, excessive pressure, inflammatory chemicals)
Classification by Location
1.Exteroceptors
• Respond to stimuli arising ___________ the body
• Receptors in the skin for ___________, pressure, ___________,
and temperature
• Most special sense organs
Classification by Location
2.Interoceptors (___________)
• Respond to stimuli arising in internal viscera and blood vessels
• Sensitive to chemical changes, tissue stretch, and temperature
changes
Classification by Location
3.Proprioceptors
• Respond to stretch in skeletal ___________, tendons,
___________, ligaments, and connective tissue coverings of
bones and muscles
• Inform the brain of one’s movements
Classification by Structural Complexity
1.Complex receptors (special sense organs)
• ___________, hearing, ___________, smell, and ___________
(Chapter 15)
2.Simple receptors for general senses:
• Tactile sensations (touch, ___________, stretch, vibration),
temperature, pain, and muscle sense
• Unencapsulated (___________) or encapsulated dendritic
endings
Unencapsulated Dendritic Endings
• Thermoreceptors
• ___________ receptors (10–40ºC); in superficial dermis
• ___________ receptors (32–48ºC); in deeper dermis
Unencapsulated Dendritic Endings
• Nociceptors
• Respond to:
• Pinching
• ___________ from damaged tissue
• Temperatures outside the range of ___________
• Capsaicin
Unencapsulated Dendritic Endings
• Light touch receptors
• ___________ (Merkel) discs
• ___________ ___________ receptors
Encapsulated Dendritic Endings
• All are mechanoreceptors
•
•
•
•
•
•
___________ (tactile) corpuscles—discriminative touch
Pacinian (___________) corpuscles—deep pressure and vibration
___________ endings—deep continuous pressure
Muscle spindles—muscle stretch
___________ tendon organs—stretch in tendons
Joint ___________ receptors—stretch in articular capsules
From Sensation to Perception
• Survival depends upon sensation and perception
• ___________: the awareness of changes in the internal and
external environment
• ___________: the conscious interpretation of those stimuli
Sensory Integration
• Input comes from ___________, proprioceptors, and
interoceptors
• Input is relayed toward the ___________, but is processed
along the way
Sensory Integration
• Levels of neural integration in sensory systems:
1. ___________ level—the sensor receptors
2. Circuit level—ascending pathways
3. ___________ level—neuronal circuits in the cerebral cortex
Processing at the Receptor Level
• Receptors have specificity for stimulus energy
• Stimulus must be applied in a ___________ field
• ___________ occurs
• Stimulus energy is converted into a graded potential called a
receptor potential
Processing at the Receptor Level
• In general sense receptors, the receptor potential and
generator potential are the same thing
stimulus

receptor/generator potential in ___________ neuron

action potential at first node of ___________
Processing at the Receptor Level
• In special sense organs:
stimulus

___________ potential in receptor cell

___________ of neurotransmitter

generator potential in first-order sensory neuron

___________ potentials (if threshold is reached)
Adaptation of Sensory Receptors
• Adaptation is a ___________ in sensitivity in the presence of a
constant stimulus
• ___________ membranes become less responsive
• Receptor potentials ___________ in frequency or stop
Adaptation of Sensory Receptors
• ___________ (fast-adapting) receptors signal the beginning or
end of a stimulus
• Examples: receptors for pressure, touch, and smell
• ___________ receptors adapt slowly or not at all
• Examples: nociceptors and most proprioceptors
Processing at the Circuit Level
• Pathways of ___________ neurons conduct sensory impulses upward
to the appropriate brain regions
• First-order neurons
• Conduct impulses from the ___________ level to the second-order
neurons in the CNS
• ___________ -order neurons
• Transmit impulses to the thalamus or cerebellum
• Third-order neurons
• Conduct impulses from the thalamus to the somatosensory cortex
(perceptual level)
Processing at the Perceptual Level
• Identification of the ___________ depends on the specific location of
the target neurons in the sensory cortex
• Aspects of sensory perception:
• ___________ detection—ability to detect a stimulus (requires summation
of impulses)
• ___________ estimation—intensity is coded in the frequency of impulses
• ___________ discrimination—identifying the site or pattern of the stimulus
(studied by the two-point discrimination test)
Main Aspects of Sensory Perception
• ___________ abstraction—identification of more complex
aspects and several stimulus properties
• Quality ___________ —the ability to identify ___________ of a
sensation (e.g., sweet or sour tastes)
• ___________ recognition—recognition of familiar or significant
patterns in stimuli (e.g., the melody in a piece of music)
Perception of Pain
• Warns of ___________ or ___________ tissue damage
• Stimuli include ___________ pressure and temperature,
histamine, K+, ATP, acids, and ___________
• Impulses travel on fibers that release neurotransmitters
glutamate and substance P
• Some ___________ impulses are blocked by inhibitory
endogenous opioids
Structure of a Nerve
• ___________ organ of the PNS
• Bundle of ___________ and ___________ peripheral axons
enclosed by connective tissue
Structure of a Nerve
• Connective tissue coverings include:
• ___________ —loose connective tissue that encloses axons and
their myelin sheaths
• ___________ —coarse connective tissue that bundles fibers into
fascicles
• ___________ —tough fibrous sheath around a nerve
Classification of Nerves
• Most nerves are mixtures of afferent and efferent fibers and somatic and
autonomic (___________) fibers
• Pure sensory (___________) or motor (___________) nerves are
rare
• Types of fibers in mixed nerves:
• Somatic afferent and somatic efferent
• ___________ afferent and visceral efferent
• Peripheral nerves classified as cranial or spinal nerves
Ganglia
• Contain neuron cell bodies associated with nerves
• Dorsal root ganglia (___________, somatic) (Chapter 12)
• Autonomic ganglia (motor, ___________) (Chapter 14)
Regeneration of Nerve Fibers
• Mature neurons are ___________
• If the soma of a damaged nerve is intact, axon will regenerate
• Involves coordinated activity among:
• ___________ —remove debris
• ___________ cells—form regeneration tube and secrete growth factors
• ___________ —regenerate damaged part
• CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS
fiber regeneration
Cranial Nerves
• ___________ pairs of nerves associated with the brain
• Most are mixed in function; two pairs are purely ___________
• Each nerve is identified by a number (I through XII) and a name
“On occasion, our trusty truck acts funny—very good vehicle
anyhow”
I: The Olfactory Nerves
• Arise from the ___________ receptor cells of nasal cavity
• Pass through the ___________ plate of the ethmoid bone
• Fibers ___________ in the olfactory bulbs
• Pathway terminates in the primary olfactory cortex
• Purely sensory (___________) function
II: The Optic Nerves
• Arise from the ___________
• Pass through the optic canals, converge and partially cross
over at the optic ___________
• Optic ___________ continue to the thalamus, where they
synapse
• Optic ___________ fibers run to the occipital (visual) cortex
• Purely sensory (___________) function
III: The Oculomotor Nerves
• Fibers extend from the ventral midbrain through the superior
___________ fissures to the extrinsic eye muscles
• Functions in raising the eyelid, directing the eyeball,
constricting the iris (___________), and controlling lens shape
IV: The Trochlear Nerves
• Fibers from the dorsal midbrain enter the orbits via the superior
orbital fissures to innervate the superior oblique muscle
• Primarily a ___________ nerve that directs the eyeball
V: The Trigeminal Nerves
• Largest ___________ nerves; fibers extend from pons to face
• Three divisions
• ___________ (V1) passes through the superior orbital fissure
• ___________ (V2) passes through the foramen rotundum
• ___________ (V3) passes through the foramen ovale
• Convey sensory impulses from various areas of the face (V1) and (V2),
and supplies ___________ fibers (V3) for mastication
VI: The Abducens Nerves
• Fibers from the inferior ___________ enter the orbits via the
superior orbital fissures
• Primarily a ___________, innervating the lateral rectus muscle
VII: The Facial Nerves
• Fibers from the pons travel through the internal acoustic ___________,
and emerge through the ___________ foramina to the lateral aspect of
the face
• Chief motor nerves of the face with 5 major branches
• Motor functions include facial expression, parasympathetic impulses to
lacrimal and salivary glands
• Sensory function (___________) from the anterior two-thirds of the
tongue
VIII: The Vestibulocochlear Nerves
• Afferent fibers from the ___________ receptors (cochlear
division) and ___________ receptors (vestibular division) pass
from the inner ear through the internal acoustic meatuses, and
enter the brain stem at the pons-medulla border
• Mostly ___________ function; small motor component for
adjustment of sensitivity of receptors
IX: The Glossopharyngeal Nerves
• ___________ from the ___________ leave the skull via the
jugular foramen and run to the throat
• Motor functions: innervate part of the tongue and pharynx for
___________, and provide ___________ fibers to the parotid
salivary glands
• Sensory functions: fibers conduct taste and general sensory
impulses from the pharynx and posterior tongue, and impulses
from carotid chemoreceptors and baroreceptors
X: The Vagus Nerves
• The only ___________ nerves that extend beyond the head and neck
region
• Fibers from the medulla exit the skull via the jugular foramen
• Most motor fibers are ___________ fibers that help regulate the
activities of the ___________, ___________, and ___________
viscera
• Sensory fibers carry impulses from thoracic and abdominal viscera,
___________, ___________, and taste buds of posterior tongue and
pharynx
XI: The Accessory Nerves
• Formed from ___________ rootlets from the C1–C5 region of
the spinal cord (not the brain)
• Rootlets pass into the ___________ via each ___________
magnum
• Accessory nerves exit the skull via the jugular foramina to
innervate the ___________ and ___________ muscles
XII: The Hypoglossal Nerves
• Fibers from the ___________ exit the skull via the hypoglossal
canal
• Innervate extrinsic and ___________ muscles of the tongue
that contribute to swallowing and speech
Spinal Nerves
• 31 pairs of mixed nerves named according to their point of
issue from the spinal cord
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•
•
•
•
8 ___________ (C1–C8)
12 ___________ (T1–T12)
5 ___________ (L1–L5)
5 ___________ (S1–S5)
1 ___________ (C0)
Spinal Nerves: Roots
• Each spinal nerve connects to the spinal cord via ___________
roots
• Ventral roots
• Contain motor (___________) fibers from the ventral horn motor
neurons
• Fibers ___________ skeletal muscles)
Spinal Nerves: Roots
• Dorsal roots
• Contain sensory (___________) fibers from sensory neurons in
the dorsal root ganglia
• Conduct impulses from peripheral receptors
• ___________ and ___________ roots unite to form spinal
nerves, which then emerge from the vertebral column via the
intervertebral foramina
Spinal Nerves: Rami
• Each spinal nerve branches into mixed rami
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•
•
•
___________ ramus
Larger ___________ ramus
___________ branch
Rami communicantes (___________ pathways) join to the ventral
rami in the thoracic region
Spinal Nerves: Rami
• All ___________ ___________ except T2–T12 form interlacing
nerve networks called plexuses (cervical, ___________,
lumbar, and sacral)
• The back is innervated by dorsal rami via several branches
• Ventral rami of T2–T12 as ___________ nerves supply muscles
of the ribs, anterolateral ___________, and ___________ wall
Cervical Plexus
• Formed by ventral rami of C1–C4
• Innervates skin and muscles of the ___________,
___________, back of head, and shoulders
• ___________ nerve
• Major motor and sensory nerve of the diaphragm (receives fibers
from C3–C5)
Brachial Plexus
• Formed by ventral rami of C5–C8 and T1 (and often C4 and T2)
• It gives rise to the nerves that innervate the upper limb
• Major branches of this plexus:
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•
•
•
___________ —five ventral rami (C5–T1)
___________ —upper, middle, and lower
___________ —anterior and posterior
___________ —lateral, medial, and posterior
Brachial Plexus: Nerves
• ___________ —innervates the deltoid, teres minor, and skin and joint
•
•
•
•
capsule of the shoulder
___________ —innervates the biceps brachii and brachialis and skin
of lateral forearm
___________ —innervates the skin, most flexors and pronators in the
forearm, and some intrinsic muscles of the hand
___________ —supplies the flexor carpi ulnaris, part of the flexor
digitorum profundus, most intrinsic muscles of the hand, and skin of
medial aspect of hand
Radial—innervates essentially all extensor muscles, supinators, and
posterior skin of limb
Lumbar Plexus
• Arises from L1–L4
• Innervates the ___________, ___________ wall, and psoas
muscle
• ___________ nerve—innervates quadriceps and skin of
anterior thigh and medial surface of leg
• ___________ nerve—passes through obturator foramen to
innervate adductor muscles
Sacral Plexus
• Arises from L4–S4
• Serves the ___________, lower limb, ___________ structures, and
perineum
• ___________ nerve
• ___________ and ___________ nerve of the body
• Innervates the hamstring muscles, adductor magnus, and most muscles in
the leg and foot
• Composed of two nerves: tibial and common fibular
Innervation of Skin
• ___________: the area of skin innervated by the cutaneous
branches of a single spinal nerve
• All spinal nerves except C1 participate in dermatomes
• Most dermatomes ___________, so destruction of a single
spinal nerve will not cause complete numbness
Innervation of Joints
• ___________ law: Any nerve serving a muscle that produces
movement at a joint also innervates the joint and the skin over
the joint
Motor Endings
• ___________ elements that activate effectors by releasing
neurotransmitters
Review of Innervation of Skeletal Muscle
• Takes place at a neuromusclular junction
• ___________ (ACh) is the neurotransmitter
• ACh binds to receptors, resulting in:
• Movement of Na+ and K+ across the membrane
• ___________ of the muscle cell
• An end ___________ potential, which triggers an action potential
Review of Innervation of Visceral Muscle and Glands
• Autonomic motor endings and visceral effectors are simpler
than somatic junctions
• Branches form synapses en passant via varicosities
• ___________ and ___________ act indirectly via second
messengers
• ___________ motor responses are slower than somatic
responses
Levels of Motor Control
• ___________ level
• ___________ level
• ___________ level
Segmental Level
• The lowest level of the ___________ hierarchy
• Central pattern generators (___________): segmental circuits
that activate networks of ventral horn neurons to stimulate
specific groups of muscles
• Controls ___________ and ___________, oft-repeated motor
activity
Projection Level
• Consists of:
• ___________ motor neurons that direct the direct (pyramidal)
system to produce voluntary skeletal muscle movements
• ___________ ___________ motor areas that oversee the indirect
(extrapyramidal) system to control reflex and CPG-controlled
motor actions
• Projection motor pathways keep ___________ command levels
informed of what is happening
Precommand Level
• Neurons in the cerebellum and basal nuclei
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•
•
•
•
___________ motor activity
Precisely ___________ or ___________ movements
Coordinate movements with ___________
Block ___________ movements
Monitor muscle tone
• Perform ___________ planning and discharge in advance of
willed movements
Precommand Level
• Cerebellum
• Acts on ___________ pathways through projection areas of the
brain stem
• Acts on the motor cortex via the ___________
• Basal nuclei
• Inhibit various motor centers under ___________ conditions
Reflexes
• Inborn (___________) reflex: a rapid, involuntary, predictable
motor response to a stimulus
• Learned (___________) reflexes result from practice or
repetition,
• Example: driving skills
Reflex Arc
• Components of a reflex arc (neural path)
1. ___________ —site of stimulus action
2. ___________ neuron—transmits afferent impulses to the CNS
3. Integration center—either monosynaptic or polysynaptic region within the
CNS
4. ___________ neuron—conducts efferent impulses from the integration
center to an effector organ
5. ___________ —muscle fiber or gland cell that responds to the efferent
impulses by contracting or secreting
Spinal Reflexes
• Spinal somatic reflexes
• ___________ center is in the spinal cord
• Effectors are skeletal muscle
• Testing of ___________ reflexes is important clinically to
assess the condition of the nervous system
Stretch and Golgi Tendon Reflexes
• For skeletal muscle activity to be ___________ coordinated,
___________ input is necessary
• Muscle spindles inform the nervous system of the length of the
muscle
• ___________ tendon organs inform the brain as to the amount of
tension in the muscle and tendons
Muscle Spindles
• Composed of 3–10 short ___________ muscle fibers in a
connective tissue capsule
• Intrafusal fibers
• ___________ in their central regions (lack myofilaments)
• Wrapped with two types of afferent endings: primary sensory
endings of type Ia fibers and secondary sensory endings of type
II fibers
Muscle Spindles
• ___________ end regions are innervated by gamma ()
efferent fibers that maintain spindle sensitivity
• Note: ___________ fibers (contractile muscle fibers) are
innervated by alpha () efferent fibers
Muscle Spindles
• Excited in ___________ ways:
1. External ___________ of muscle and muscle spindle
2. Internal stretch of muscle spindle:
• Activating the  motor neurons stimulates the ends to
___________, thereby stretching the spindle
• ___________ causes an increased rate of impulses in Ia fibers
Muscle Spindles
• ___________ the muscle reduces tension on the muscle
spindle
• Sensitivity would be lost unless the muscle spindle is shortened
by impulses in the  motor neurons
• – ___________ maintains the tension and sensitivity of the
spindle during muscle contraction
Stretch Reflexes
• Maintain muscle tone in large ___________ muscles
• Cause muscle contraction in response to increased muscle
length (___________)
Stretch Reflexes
• How a stretch ___________ works:
• Stretch activates the muscle spindle
• IIa sensory neurons synapse directly with  motor neurons in the
spinal cord
•  ___________ neurons cause the stretched muscle to contract
• All stretch reflexes are ___________ and ___________
Stretch Reflexes
• ___________ inhibition also occurs—IIa fibers synapse with
interneurons that inhibit the  motor neurons of antagonistic
muscles
• Example: In the patellar reflex, the stretched muscle
(___________) contracts and the antagonists (hamstrings)
relax
Golgi Tendon Reflexes
• ___________ reflexes
• Help to prevent damage due to excessive stretch
• Important for smooth onset and ___________ of muscle
contraction
Golgi Tendon Reflexes
• Produce muscle relaxation (___________) in response to tension
• Contraction or passive stretch activates Golgi tendon organs
• ___________ impulses are transmitted to spinal cord
• Contracting muscle relaxes and the antagonist contracts (reciprocal
activation)
• Information transmitted ___________ to the cerebellum is used to adjust
muscle tension
Flexor and Crossed-Extensor Reflexes
• Flexor (___________) reflex
• Initiated by a painful stimulus
• Causes automatic withdrawal of the threatened body part
• Ipsilateral and ___________
Flexor and Crossed-Extensor Reflexes
• Crossed ___________ reflex
• Occurs with flexor reflexes in weight-bearing limbs to maintain
balance
• Consists of an ___________ flexor reflex and a ___________
extensor reflex
• The stimulated side is withdrawn (flexed)
• The contralateral side is extended
Superficial Reflexes
• Elicited by gentle ___________ stimulation
• Depend on upper motor pathways and cord-level reflex arcs
Superficial Reflexes
• Plantar reflex
• ___________: stroking lateral aspect of the sole of the foot
• ___________: downward flexion of the toes
• Tests for function of corticospinal tracts
Superficial Reflexes
• ___________ sign
•
•
•
•
Stimulus: as above
Response: ___________ of hallux and fanning of toes
Present in infants due to incomplete ___________
In adults, indicates corticospinal or motor cortex damage
Superficial Reflexes
• Abdominal reflexes
• Cause contraction of abdominal muscles and movement of the
___________ in response to stroking of the skin
• Vary in intensity from one person to another
• Absent when ___________ tract lesions are present
Developmental Aspects of the PNS
• ___________ nerves branch from the developing spinal cord
and neural crest cells
• Supply both motor and sensory fibers to developing muscles to
help direct their maturation
• Cranial nerves innervate muscles of the head
Developmental Aspects of the PNS
• Distribution and growth of spinal nerves ___________ with the
segmented body plan
• Sensory receptors atrophy with age and muscle tone lessens
due to loss of ___________, decreased numbers of synapses
per neuron, and slower central processing
• Peripheral nerves remain viable throughout life unless
subjected to trauma