Download Peripheral Nervous System (PNS)

Crafton Hills College
Human Anatomy and Physiology 150
Peripheral Nervous System (PNS)
Chapter 13
A. General Characteristics & Descriptions
1. All neural structures outside the brain & spinal cord
2. Includes: sensory receptors, peripheral nerves, associated ganglia, & motor endings
3. Provides links to and from the external environment
B. Sensory Receptors
1. Characteristics
a. Structures specialized to respond to stimuli
b. Activation of sensory receptors results in depolarizations that trigger impulses to the CNS
c. Awareness of these stimuli (sensation / perception) occurs in the brain
2. Classification by Stimulus Type
a. Mechanoreceptors: Touch, pressure, vibration, stretch, & itch
b. Thermoreceptors: Changes in temperature
c. Photoreceptors: Light energy (e.g., retina)
d. Chemoreceptors: Chemicals (e.g., smell, taste, changes in blood chemistry)
e. Nociceptors: Pain-causing stimuli
3. Classification by Location
a. Exteroceptors
i. Respond to stimuli arising outside the body
ii. Near body's surface
iii. Sensitive to touch, pressure, pain, and temperature
iv. Include the special sense organs
b. Interoceptors
i. Respond to stimuli arising within the body
ii. Found in internal viscera and blood vessels
iii. Sensitive to chemical changes, stretch, temperature changes
c. Proprioceptors
i. Respond to degree of stretch of the organs they occupy
ii. Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of
bones and muscles
iii. Constantly "advise" the brain of one's movements
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4. Classification by Structural Complexity
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Classified as either simple or complex
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Complex receptors are special sense organs
a. Simple
i. Unencapsulated
1) Free dendritic nerve endings
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Respond chiefly to temperature and pain
2) Example:
a) Merkel (tactile) discs
b) Hair follicle receptors
ii. Encapsulated
1) Meissner's corpuscles (tactile corpuscles)
2) Pacinian corpuscles (lamellated corpuscles)
3) Muscle Spindles
4) Joint kinesthetic receptors
5. Processing
a. Steps
i. Receptor's stimulation converted to graded potential
ii. Graded potential meeting/exceeding threshold initiates A.P.
b. Levels
i. Circuit
1) Chains of three neurons (first-, second-, and third-order) conduct sensory impulses to
brain
a) First-order neurons
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Soma in dorsal root (CNS: cranial ganglia)
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Conducts impulses from skin to spinal cord or brain stem
b) Second-order neurons
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Soma in dorsal horn of spinal cord (CNS:medullary nuclei )
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Transmit impulses to thalamus or cerebellum
c) Third-order neurons
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In Thalamus: Send A.P.s to cerebral somatosensory cortex
ii. Somatosensory cortex receives impulse
1) Sensory association areas interpret input
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Interprets shape, texture, magnitude of signal, spatial discrimination
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c. Adaptation
i. Occurs when sensory receptors are subjected to an unchanging stimulus
1) Receptor membranes become less responsive
2) Receptor potentials decline in frequency or stop
3) Effect strong in pressure, touch, & smell receptors
C. Structure of a Nerve
1. Nerve - cordlike organ of the PNS consisting of peripheral axons enclosed by connective tissue
2. Connective tissue coverings include:
a. Endoneurium - Loose connective tissue that surrounds axons
b. Perineurium - Coarse connective tissue that bundles fibers into fascicles
c. Epineurium - Tough fibrous sheath around a nerve
3. Divisions
a. Sensory (afferent) - carry impulse to the CNS
b. Motor (efferent) - carry impulses from CNS
c. Mixed nerves (4 types) - carry somatic & autonomic (visceral) impulses
i. Somatic afferent and somatic efferent
ii. Visceral afferent and visceral efferent
D. Cranial Nerves
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General Characteristics
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12 pairs of cranial nerves come from brain
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Functions are sensory, motor, or both
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Each nerve identified by number (I - XII) and name
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Four cranial nerves carry parasympathetic fibers that serve muscles and glands
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CRANIAL NERVES
1. Olfactory (I)
a. From nasal tissue, through cribriform plate, olfactory bulb & end in primary olfactory cortex
b. Carries afferent impulses for the sense of smell
2. Optic (II)
a. Goes from retina (eye), as optic nerves, through optic canals, optic chiasm (converge), to
thalamus where they synapse, optic radiation fibers go to visual cortex
b. Carries afferent impulses for vision
3. Oculomotor (III)
a. Goes from ventral midbrain, through superior orbital fissure, to extrinsic eye muscles
b. Functions: Raising eyelid, directing eyeball, constricting iris, and shaping lens
4. Trochlear (IV)
a. From dorsal midbrain, in orbits via superior orbtl fissures; innervates superior oblique muscle
b. Motor nerve (primarily) aids in directing the eyeball
5. Trigeminal (V)
a. 3 divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3)
b. From face to pons via superior orbital fissure (V1), foramen rotundum (V2), foramen ovale (V3)
c. Sensory impulses from face (V1) and (V2), Motor fibers (V3) for mastication
6. Abdcuens(VI)
a. Fibers leave the inferior pons and enter the orbit via the superior orbital fissure
b. Primarily a motor nerve innervating the lateral rectus muscle
7. Facial (VII)
a. From pons, through internal acoustic meatus, stylomastoid foramen to side of face
b. Mixed nerve with five major branches
c. Motor functions: Facial expression, autonomic innervation of lacrimal & salivary glands
d. Sensory function: Taste (anterior 2/3 of tongue)
8. Vestibulocochlear (VIII)
a. Goes from inner ear, through internal acoustic meatus, into brainstem at pons-medulla border
b. Two divisions: Cochlear (hearing) & vestibular (balance)
c. Functions: Sensory only! (equilibrium & hearing)
9. Glossopharyngeal (IX)
a. Goes from medulla, through jugular foramen, to throat
b. A mixed nerve with motor & sensory functions
i. Motor: Innervate some tongue & pharynx, parotid salivary gland
ii. Sensory: Taste & general sensory from tongue & pharynx
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10. Vagus (X)
a. Only cranial nerve beyond head & neck
b. Mixed nerved that goes from medulla via the jugular foramen
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Parasympathetic to heart, lungs, visceral organs & Sensory function: Taste
11. Accessory (XI)
a. Goes from Medulla / spinal border upward into cranium via foramen magnum & leaves
cranium via the jugular foramen
b. Function: Motor nerve (primarily)
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Innervates: Trapezius & sternocleidomastoid, (moves head / neck)
12. Hypoglossal (XII)
a. From medulla, exit skull via hypoglossal canal
b. Innervates Extrinsic & Intrinsic muscles of tongue (aids swallowing & speech)
E. Reflexes
1. Definition
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Rapid, predictable motor response to a stimulus
2. General Characteristics
a. Inborn (intrinsic) or learned (acquired)
b. Can involve only peripheral nerves & spinal cord
c. Can involve higher brain centers as well
3. Reflex Arc
a. 5 Components
i. Receptor - Site of stimulus
ii. Sensory neuron - Transmits an afferent impulse to CNS
iii. Integration center - Region within the CNS
iv. Motor neuron - Conducts efferent impulses from integration center to an effector
v. Effector - Muscle fiber or gland that responds to the efferent impulse
4. Type of Reflex
a. Stretch Reflex
i. Stretching the muscle (tapping) activates muscle spindle
ii. Muscle spindle excites motor neurons causing the stretched muscle to contract
iii. Afferent impulses from the spindle result in inhibition of the antagonist
iv. Example: patellar reflex
1) Tapping the patellar tendon stretches the quadriceps and starts the reflex action
2) Quadriceps contract while the simultaneously inhibited antagonistic hamstrings relax
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F. Spinal Nerves
1. 31 pairs of mixed nerves arise from spinal cord; Supplies all of body except the head
2. Named according to their point of issue
a. 8 cervical (C1-C8)
b. 12 thoracic (T1-T12)
c. 5 Lumbar (L1-L5)
d. 5 Sacral (S1-S5)
e. 1 Coccygeal (C0)
3. Roots
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Each spinal nerve connects to the spinal cord via two (branched) medial roots
a. Ventral roots: From the anterior horn - contain motor (efferent) fibers
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Rami supplies thorax and abdominal area
b. Dorsal roots: From sensory neurons (dorsal root ganglion) & contain sensory (afferent) fibers
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Rami innervates back via several branches
4. Nerve Plexus
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Plexuses (distal extensions of roots) are found in cervical, brachial, lumbar, & sacral regions
a. Cervical Plexus
i. Nerves of neck, ear, back of head, and shoulders
ii. Phrenic nerve (most important) is major motor & sensory nerve of diaphragm
b. Brachial Plexus
i. Formed by primarily by C5-C8
ii. Innervate the upper limb
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E.g. Deltoid & teres minor, Biceps brachii & brachialis, flexors & extensors
c. Lumbar Plexus
i. From L1-L4 & innervates thigh, abdominal wall, and psoas muscle
ii. Major nerves: Femoral & Obturator
d. Sacral Plexus
i. From L4-S4 & serves buttock, lower limb, pelvic structures
ii. Major nerve: Sciatic (longest, thickest nerve of body)
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Composed of two nerves: Tibial and Common fibular (peroneal) nerves
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