Cranial Nerves and Peripheral Nervous System

Cranial Nerves

12 pairs of cranial nerves are associated with brain; two attach to forebrain, rest with brain stem.

Olfactory Nerves (I)

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.

Optic Nerves (II)

Arise from retinas; really a brain tract, pass through optic canals, converge, partially cross over at optic chiasma, optic tracts continue to thalamus where they synapse, optic radiation fibers run to occipital (visual) cortex.

Oculomotor Nerves (III)

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.

Trochlear Nerves (IV)

Fibers from dorsal midbrain enter orbits via superior orbital fissures to innervate superior oblique muscle; primarily motor nerve that directs eyeball.

Trigeminal Nerves (V)

Largest cranial nerves; fibers extend from pons to face with three divisions: Ophthalmic (V1) passes through superior orbital fissure, Maxillary (V2) passes through foramen rotundum, Mandibular (V3) passes through foramen ovale; convey sensory impulses from various areas of face (V1 and V2) and supply motor fibers (V3) for mastication.

Abducens Nerves (VI)

Fibers from inferior pons enter orbits via superior orbital fissures; primarily a motor nerve, innervating lateral rectus muscle.

VII: Facial nerves

Chief motor nerves of face with five major branches; motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands; sensory function (taste) from anterior two-thirds of tongue.

VIII: Vestibulocochlear nerves

Mostly sensory function; small motor component for adjustment of sensitivity of receptors; 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.

IX: Glossopharyngeal nerves

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.

X: Vagus nerves

Only cranial nerves that extend beyond head and neck region; 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.

XI: Accessory nerves

Formed from ventral rootlets from C1 to 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.

XII: Hypoglossal nerves

Fibers from medulla exit skull via hypoglossal canal; innervate extrinsic and intrinsic muscles of tongue that contribute to swallowing and speech.

Composition of Cranial Nerves (1 of 2)

Neuron cell bodies located within special sense organs; other nerves with sensory information (V, VII, IX, and X) have neuron cell bodies located in cranial sensory ganglia.

Composition of Cranial Nerves (2 of 2)

Some mixed nerves contain both somatic and autonomic fibers; most motor neuron cell bodies in ventral gray matter of brain stem; some autonomic motor neurons in ganglia.

Mnemonic for cranial nerves functions

To remember primary functions of cranial nerves as sensory, motor, both: 'Some say marry money, but my brother believes (it's) bad business (to) marry money.'

Peripheral Nervous System (PNS)

PNS provides links from and to world outside our body.

Sensory Receptors

Part 1 of the PNS that detects changes in the environment.

Transmission Lines

Part 2 of the PNS that includes nerves and their structure and repair.

Motor Endings

Part 3 of the PNS that relates to motor activity.

Reflex Activity

Part 4 of the PNS that involves reflex actions.

Sensory receptors

Specialized to respond to changes in environment (stimuli); activation results in graded potentials that trigger nerve impulses.

Mechanoreceptors

Respond to touch, pressure, vibration, and stretch.

Thermoreceptors

Sensitive to changes in temperature.

Photoreceptors

Respond to light energy (example: retina).

Chemoreceptors

Respond to chemicals (examples: smell, taste, changes in blood chemistry).

Nociceptors

Sensitive to pain-causing stimuli (examples: extreme heat or cold, excessive pressure, inflammatory chemicals).

Exteroceptors

Respond to stimuli arising outside body; receptors in skin for touch, pressure, pain, and temperature.

Interoceptors (visceroceptors)

Respond to stimuli arising in internal viscera and blood vessels; sensitive to chemical changes, tissue stretch, and temperature changes.

Proprioceptors

Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles.

Simple receptors of the general senses

Modified dendritic endings of sensory neurons found throughout body that monitor most types of general sensory information.

General senses

Include tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense.

Nonencapsulated (free) nerve endings

Abundant in epithelia and connective tissues; respond mostly to temperature, pain, or light touch.

Cold receptors

Activated by temperatures from 10 to 40ºC located in superficial dermis.

Heat receptors

Activated from 32 to 48ºC located in deeper dermis.

Nociceptors (pain receptors)

Triggered by extreme temperature changes, pinch, or release of chemicals from damaged tissue.

Vanilloid receptor

Protein in nerve membrane that acts as an ion channel opened by heat, low pH, and chemicals (example: capsaicin in red peppers).

Itch receptors

Located in dermis and can be triggered by chemicals such as histamine.

Tactile (Merkel) discs

Function as light touch receptors located in deeper layers of epidermis.

Hair follicle receptors

Free nerve endings that wrap around hair follicles and act as light touch receptors that detect bending of hairs.

Encapsulated dendritic endings

Almost all are mechanoreceptors whose terminal endings are encased in connective tissue capsule.

Tactile (Meissner's) corpuscles

Small receptors involved in discriminative touch, found just below skin, mostly in sensitive and hairless areas (fingertips).

Lamellar (Pacinian) corpuscles

Large receptors that respond to deep pressure and vibration when first applied (then turn off), located in deep dermis.

Bulbous corpuscles (Ruffini endings)

Respond to deep and continuous pressure, located in dermis.

Muscle spindles

Spindle-shaped proprioceptors that respond to muscle stretch.

Tendon organ

Proprioceptors located in tendons that detect stretch.

Joint kinesthetic receptors

Proprioceptors that monitor joint position and motion.

Sensation

The awareness of changes in the internal and external environment.

Perception

The conscious interpretation of those stimuli.

Somatosensory system

Part of sensory system serving body wall and limbs.

Exteroceptors

Receptors that receive external stimuli.

Proprioceptors

Receptors that provide information about body position and movement.

Interoceptors

Receptors that receive stimuli from within the body.

Receptor level

Sensory receptors in the levels of neural integration in sensory systems.

Circuit level

Processing in ascending pathways in the levels of neural integration in sensory systems.

Perceptual level

Processing in cortical sensory areas in the levels of neural integration in sensory systems.

Generator potential

Graded potential generated in general receptors when stimulus energy is converted.

Receptor potential

Graded potential generated in special sense receptors when stimulus energy is converted.

Adaptation

Change in sensitivity in presence of constant stimulus where receptor membranes become less responsive.

Phasic receptors

Fast-adapting receptors that send signals at the beginning or end of a stimulus.

Tonic receptors

Receptors that adapt slowly or not at all.

First-order sensory neurons

Neurons that conduct impulses from receptor level to spinal reflexes or second-order neurons in CNS.

Second-order sensory neurons

Neurons that transmit impulses to third-order sensory neurons.

Third-order sensory neurons

Neurons that conduct impulses from thalamus to the somatosensory cortex.

Perceptual detection

Ability to detect a stimulus, requiring summation of impulses.

Magnitude estimation

Intensity of a stimulus coded in frequency of impulses.

Spatial discrimination

Identifying site or pattern of stimulus, studied by two-point discrimination test.

Feature abstraction

Identification of more complex aspects and several stimulus properties.

Quality discrimination

Ability to identify submodalities of a sensation, such as sweet or sour tastes.

Pattern recognition

Recognition of familiar or significant patterns in stimuli, such as melody in music.

Pain tolerance

Varies among individuals; 'sensitive to pain' indicates low pain tolerance, not low pain threshold.

Visceral pain

Pain resulting from stimulation of visceral organ receptors, felt as vague aching or burning.

Referred pain

Pain from one body region perceived as coming from a different region due to shared nerve pathways.

Endogenous opioids

Neurotransmitters that block some pain impulses, examples include endorphins.

Stimuli for pain

Include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykinin.

Pain impulse transmission

Impulses travel on fibers that release neurotransmitters glutamate and substance P.

Hyperalgesia

Pain amplification that can result from long-lasting or intense pain.

Chronic pain

Long-lasting pain that can occur after events such as limb amputation.

Phantom limb pain

Pain felt in a limb that has been amputated.

NMDA receptors

Receptors activated by long-lasting or intense pain that allow the spinal cord to learn hyperalgesia.

Epidural anesthesia

Anesthesia used during surgery to reduce phantom pain.

Nerve

A cordlike organ of the peripheral nervous system (PNS) consisting of a bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue.

Endoneurium

Loose connective tissue that encloses axons and their myelin sheaths (Schwann cells).

Perineurium

Coarse connective tissue that bundles fibers into fascicles.

Epineurium

Tough fibrous sheath around all fascicles to form the nerve.

Mixed nerves

Nerves that contain both sensory and motor fibers, allowing impulses to travel both to and from the CNS.

Sensory (afferent) nerves

Nerves that transmit impulses only toward the CNS.

Motor (efferent) nerves

Nerves that transmit impulses only away from the CNS.

Somatic afferent fibers

Sensory fibers that carry impulses from muscles to the brain.

Somatic efferent fibers

Motor fibers that carry impulses from the brain to muscles.

Visceral afferent fibers

Sensory fibers that carry impulses from organs to the brain.

Visceral efferent fibers

Motor fibers that carry impulses from the brain to organs.

Ganglia

Structures that contain neuron cell bodies associated with nerves in the PNS.

Dorsal root ganglia

Ganglia associated with afferent nerve fibers that contain cell bodies of sensory neurons.

Autonomic ganglia

Ganglia associated with efferent nerve fibers that contain autonomic motor neurons.

Amitotic neurons

Mature neurons that do not undergo mitosis.

Wallerian degeneration

The process where axon fragments and myelin sheaths distal to injury degenerate.

Macrophages

Cells that clean dead axon debris during nerve regeneration.

Schwann cells

Cells that are stimulated to divide during the regeneration of peripheral nerves.