Define PNS and list major components
The Peripheral Nervous System (PNS) is composed of all neural structures outside the brain:
- Sensory receptors
- Peripheral nerves and associated ganglia
- Motor endings
Describe sensory receptors
Specialized to respond to changes in their environment (stimuli)
Activation results in graded potentials that trigger nerve impulses
Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain
Name three ways receptors are classified
Based on:
- Stimulus type
- Location
- Structural complexity
List and describe classifications of sensory receptors based on stimulus type
Mechanoreceptors�respond to touch, pressure, vibration, stretch, and itch
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 c
List and describe classifications of sensory receptors based on location
Exteroceptors: respond to stimuli arising outside the body; receptors in the skin for touch, pressure, pain, and temperature (general sensation), as well as most special sense organs (taste, smell, sight, hearing, equilibrium)
Interoceptors (visceroceptor
List and describe classifications of sensory receptors based on structure
1. Complex receptors (special sense organs):sense vision, hearing, equilibrium, smell, and taste (Chapter 15)
2. Simple receptors for general senses (dendritic nerve endings): for tactile sensations (touch, pressure, stretch, vibration), temperature, pain
List and describe types of sensory receptors with unencapsulated dendritic endings
See chart, p. 487
1. Thermoreceptors:
- Cold receptors (10-40�C); in superficial dermis
- Heat receptors (32-48�C); in deeper dermis
2. Nociceptors, responding to:
- Pinching
- Chemicals from damaged tissue
- Temperatures outside the range of thermorecept
List and describe types of sensory receptors with encapsulated dendritic endings
1. Meissner's (tactile) corpuscles�discriminative touch (found in hairless skin: nipples, fingertips, soles of feet, eyelids, external genitalia )
2. Pacinian (lamellated) corpuscles�deep pressure and vibration (found in dermis, hypodermis, tendons, joint
Define Sensation
Sensation: the awareness of changes in the internal and external environment
Define Perception
Perception: the conscious interpretation of those stimuli
Describe sensory integration
Input comes from exteroceptors, proprioceptors, and interoceptors
Input is relayed toward the head, but is processed along the way
What are the three levels of neural integration in sensory systems?
Receptor level�the sensor receptors
Circuit level�ascending pathways
Perceptual level�neuronal circuits in the cerebral cortex
Describe the structure of a nerve
Cordlike organ of the PNS
Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue
List and describe three types of connective tissue enclosing a nerve
Endoneurium�loose connective tissue that encloses single axons and their myelin sheaths
Perineurium�coarse connective tissue that bundles fibers into fascicles
Epineurium�tough fibrous sheath around a nerve (enclosing fascicles + blood vessels, lymph)
How are most nerves classified?
Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers
Pure sensory (afferent) or motor (efferent) nerves are rare
Peripheral nerves classified as cranial or spinal nerves
What are the two types of fibers in mixed nerves?
1. Somatic afferent and somatic efferent (skin, muscles, joints)
2. Visceral afferent and visceral efferent (internal organs)
Describe Ganglia
Contain neuron cell bodies associated with nerves
Dorsal root ganglia (sensory, somatic) (Chapter 12)
Autonomic ganglia (motor, visceral) (Chapter 14)
Describe Regeneration of Nerve Fibers
Mature neurons are amitotic (if cell body is damaged, regneration is unlikely)
If the soma of a damaged PNS nerve is intact, axon will regenerate; this involves coordinated activity among:
- Macrophages�remove debris
- Schwann cells�form regeneration tube
Wallerian degeneration
Changes that occur in the axon distal to the site of axonal damage; the axon swells up and becomes irregular; eventually, the axon and the terminals are broken down into fragments that are phagocytosed by adjacent macrophages and Schwann cells.
How quickly can a PNS axon regrow?
About 1.5 mm of growth per day
Describe cranial nerves
Twelve pairs of nerves associated with the brain
Most are mixed in function; two pairs are purely sensory
Each nerve is identified by a number (I through XII) and a name
"On occasion, our trusty truck acts funny�very good vehicle anyhow"
OR: "Oooh ooh ooh
List the twelve cranial nerves
I - Olfactory: smelling
II - Optic: seeing
III - Oculomotor: eye movement
IV - Trochlear: eye movement
V - Trigeminal: face, taste
VI - Abducens: eye movement
VII - Facial: facial sensations
VIII - Vestibulocochlear: hearing & balance
IX - Glossopharyngea
Describe cranial nerve I: The Olfactory Nerves
Arise from the olfactory receptor cells of nasal cavity
Pass through the cribriform plate of the ethmoid bone
Fibers synapse in the olfactory bulbs
Pathway terminates in the primary olfactory cortex (temporal lobe)
Purely sensory (olfactory) function
Chem
Describe cranial nerve II: The Optic Nerves
Arise from the retinas
Pass through the optic canals, converge and partially cross over at the optic chiasma
Optic tracts continue to the thalamus, where they synapse
Optic radiation fibers run to the occipital (visual) cortex
Purely sensory (visual) func
Describe cranial nerve III: The Oculomotor Nerves
Fibers extend from the ventral midbrain through the superior orbital fissures to the extrinsic eye muscles
Functions in raising the eyelid, directing the eyeball, constricting the iris (parasympathetic), and controlling lens shape
Describe cranial nerve 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 motor nerve that directs the eyeball
Describe cranial nerve V: The Trigeminal Nerves
Largest cranial nerves; fibers extend from pons to face
Three divisions
Ophthalmic (V1) passes through the superior orbital fissure
Maxillary (V2) passes through the foramen rotundum
Mandibular (V3) passes through the foramen ovale
Convey SENSORY impulses
Describe cranial nerve VI: The Abducens Nerves
Fibers from the inferior pons enter the orbits via the superior orbital fissures
Primarily a motor, innervating the lateral rectus muscle
Describe cranial nerve VII: The Facial Nerves
Fibers from the pons travel through the internal acoustic meatuses, and emerge through the stylomastoid foramina to the lateral aspect of the face
Chief MOTOR nerves of the face with 5 major branches
MOTOR functions include facial expression, parasympathe
Describe cranial nerve VIII: The Vestibulocochlear Nerves
Afferent fibers from the hearing receptors (cochlear division) and equilibrium receptors (vestibular division) pass from the inner ear through the internal acoustic meatuses, and enter the brain stem at the pons-medulla border
MOSTLY sensory function; sma
Describe cranial nerve IX: The Glossopharyngeal Nerves
Fibers from the medulla leave the skull via the jugular foramen and run to the throat
MOTOR functions: innervate part of the tongue and pharynx for swallowing, and provide parasympathetic fibers to the parotid salivary glands
SENSORY functions: fibers con
Describe cranial nerve X: The Vagus Nerves
The only cranial nerves that extend beyond the head and neck region
Fibers from the medulla exit the skull via the jugular foramen
Most MOTOR fibers are parasympathetic fibers that help regulate the activities of the heart, lungs, and abdominal viscera
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Describe cranial nerve XI: The Accessory Nerves
Formed from ventral rootlets from the C1-C5 region of the spinal cord (not the brain)
Rootlets pass into the cranium via each foramen magnum
Accessory nerves exit the skull via the jugular foramina to innervate the trapezius and sternocleidomastoid muscle
Describe cranial nerve XII: The Hypoglossal Nerves
Fibers from the medulla exit the skull via the hypoglossal canal
Innervate extrinsic and intrinsic muscles of the tongue that contribute to swallowing and speech
Describe Spinal Nerves
These are only somatic, and every one is mixed: both sensory and motor
The 31 pairs are named according to their point of issue from the spinal cord
8 cervical (C1-C8)
12 thoracic (T1-T12)
5 Lumbar (L1-L5)
5 Sacral (S1-S5)
1 Coccygeal (C0)
Describe Roots of Spinal Nerves
Each spinal nerve connects to the spinal cord via two roots
1. Ventral roots
- Contain motor (efferent) fibers from the ventral horn motor neurons
- Fibers innervate skeletal muscles
2. Dorsal roots
- Contain sensory (afferent) fibers from sensory neurons
What do dorsal and ventral roots combine to create?
Dorsal and ventral roots unite to form spinal nerves (1-2 cm long before branching into rami), which then emerge from the vertebral column via the intervertebral foramina
Describe the rami of spinal nerves
Each spinal nerve branches into mixed rami:
- Dorsal ramus
- Larger ventral ramus
- Meningeal branch
- Rami communicantes (autonomic pathways) join to the ventral rami in the thoracic region
Which ventral rami form nerve networks and what are they called?
All ventral rami except T2-T12 form interlacing nerve networks called plexuses (cervical, brachial, lumbar, and sacral)
What do the ventral rami of T2 - T12 do?
Ventral rami of T2-T12 as intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall
Describe the Cervical Plexus
Formed mostly by ventral rami of C1-C4 (and a bit of C5)
Innervates skin and muscles of the neck, ear, back of head, and some of the shoulders
Includes Phrenic nerve: major motor and sensory nerve of the diaphragm (receives fibers from C3-C5)
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Describe the 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 (arms)
Major branches of this plexus:
Roots�five ventral rami (C5-T1)
Trunks�upper, middle, and lower
Divisions�anterior and posterior
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List and describe the five nerves of the brachial plexus (including the areas they innervate)
1. Axillary�innervates the deltoid, teres minor, and skin and joint capsule of the shoulder
2. Musculocutaneous�innervates the biceps brachii and brachialis and skin of lateral forearm
3. Median�innervates the skin, most flexors and pronators in the forea
Describe the Lumbar Plexus
Arises from L1-L4 (mostly cauda equina)
Innervates the thigh, abdominal wall, and psoas muscle
Includes two important nerves:
Femoral nerve�innervates quadriceps and skin of anterior thigh and medial surface of leg (lower)
Obturator nerve�passes through o
Describe the Sacral Plexus
Arises from L4-S4
Serves the buttock, lower limb, pelvic structures, and perineum
Includes the Sciatic nerve:
- Longest and thickest nerve of the body (about 1" wide)
- Innervates the hamstring muscles, adductor magnus, and most muscles in the leg an
Define Dermatome
The area of skin innervated by the cutaneous branches of a single spinal nerve (most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness)
Which is the only spinal nerve not participating in a dermatome?
All spinal nerves except C1 participate in dermatomes