Peripheral Nervous System and Special Senses Flashcards
1
Q
PNS Structural Classification
A
o Nerves extending from the brain and spinal cord
Spinal nerves carry impulses to and from the spinal cord
Cranial nerves carry impulses to and from the brain
o Functions
Serve as communication lines among sensory organs, the brain and spinal cord, and glands or muscles
2
Q
PNS Functional Classification
A
Motor (efferent) division (continued)
Two subdivisions
Somatic nervous system = voluntary
• Consciously (voluntarily) controls skeletal muscles
Autonomic nervous system = involuntary
• Automatically controls smooth and cardiac muscles and glands
• Further divided into the sympathetic and parasympathetic nervous systems
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Q
PNS Glial Cells
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Schwann cells
Form myelin sheath around nerve fibers in the PNS
Satellite cells
Protect and cushion neuron cell bodies
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Q
Nervous Tissue: Schwann Cell, Nodes of Ranvier, Oligodendocytes
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Myelin sheaths
Schwann cells
Wrap axons in a jelly roll–like fashion (PNS) to form the myelin sheath
Neurilemma
• Part of the Schwann cell external to the myelin sheath
• Important in nerve fibre regeneration
Nodes of Ranvier
• Gaps in myelin sheath along the axon
• Speed up propagation of nerve impulses
Oligodendrocytes
Produce myelin sheaths around axons of the CNS
Lack a neurilemma
5
Q
Structure of a Nerve
A
- Nerves are bundles of neurons found outside the CNS
- Endoneurium is a CT sheath that surrounds each fiber (surrounds neurolemma and node of Ranvier
- Perineurium wraps groups of fibers bound into a fascicle
- Epineurium binds groups of fascicles (cord like strength)
6
Q
Nerve Types
A
Mixed nerves
o Contain both sensory and motor fibers
o All spinal nerves
Sensory (afferent) nerves
o Carry impulses toward the CNS
Motor (efferent) nerves
o Carry impulses away from the CNS`
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Q
Cranial Nerves
A
- 12 pairs of nerves serve mostly the head and neck from the left and right side of the brain
- Only the pair of vagus nerves extends to thoracic and abdominal cavities
- Most are mixed nerves, but three are sensory only o Optic (sight) o Olfactory (sight) o Vestibulocochlear (hearing and balance)
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Q
Distribution of Cranial Nerves
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- Olfactory sensory impulses for smell
- Optic sensory impulses for sight
- Oculomotor, Trochlear and Abducens sensory/motor impulses for eye movements
- Trigeminal sensory/motor impulses for jaw motion and facial sensation
- Facial sensory/motor impulses for facial sensation and taste
- Vestibulocochlear sensory impulses relating to hearing and balance
- Glossopharyngeal sensory/motor impulses for taste and movement of throat muscles
- Vagus Sensory/Motor relating to many autonomic functions
- Accessory nerve impulses related to moving of neck muscles
- Hypoglossal motor/sensory for tongue movement and sensation
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Q
Spinal Nerves
A
- 31 pairs from right and left side of spinal cord
- Formed by the combination of the ventral and dorsal roots of the spinal cord
- Named for the region of the spinal cord from which they arise
- Spinal nerves divide soon after leaving the spinal cord into a dorsal ramus and a ventral ramus
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Q
Rami
A
All rami are mixed nerves
o Ramus
Branch of a spinal nerve; contains both motor and sensory fibers
o Dorsal rami
Serve the skin and muscles of the posterior trunk
o Ventral rami (T1–T12)
Form the intercostal nerves that supply muscles and skin of the ribs and trunk
o Ventral rami (except T1–T12)
Form a complex of networks (plexus) for the anterior
11
Q
Plexus
A
o Networks of nerves serving motor and sensory needs of the limbs
o Form from ventral rami of spinal nerves in the cervical, lumbar, and sacral regions
o Four plexuses
Cervical
Brachial
• 5 separate nerves branch out
• Transmit information to and from the skin and muscles of the shoulder, upper arm, forearm and hand
Lumbar
• 3 main nerves branch out
• Transmit information to and from the anterior and medial thigh and lower leg and thigh and hip
Sacral
• 3 main nerves branch out
• Transmit impulses to and from the skin of the posterior thigh and lower leg, gluteal muscles, hamstring muscles and those that control the foot
12
Q
Autonomic Nervous System
A
Motor subdivision of the PNS
o Consists only of motor nerves
o Controls the body automatically (and is also known as the involuntary nervous system)
o Regulates cardiac and smooth muscles and glands
13
Q
ANS vs SNS
A
Somatic nervous system
o Motor neuron cell bodies originate inside the CNS
o Axons extends to skeletal muscles that are served
Autonomic nervous system
o Chain of two motor neurons
Preganglionic neuron is in the brain or spinal cord
• Those found in spinal cord begin at the lateral horn
Postganglionic neuron extends to the organ
14
Q
Anatomy of the Parasympathetic Division
A
- Parasympathetic division is also known as the craniosacral division
- Preganglionic neurons originate in:
o Cranial nerves III, VII, IX, and X
o S2 through S4 regions of the spinal cord - Preganglionic neurons synapse with terminal ganglia.
o From there, postganglionic axons extend to organs that are served
15
Q
Anatomy of the Sympathetic Division
A
- Sympathetic division is also known as the thoracolumbar division
- Preganglionic neurons originate from T1 through L2
o Axons pass through a ramus communicans to enter a sympathetic trunk ganglion
o Sympathetic trunk, or chain, lies near the spinal cord - After synapsing at the ganglion, the axon may synapse with a second neuron at the same or different level
- Or the preganglionic neuron may pass through the ganglion without synapsing and form part of the splanchnic nerves
o Splanchnic nerves travel to the collateral ganglion
o Collateral ganglia serve the abdominal and pelvic organs
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Q
Anatomic Functioning
A
- Body organs served by the autonomic nervous system receive fibers from both divisions
o Exceptions: blood vessels, structures of the skin, some glands, and the adrenal medulla
o These exceptions receive only sympathetic fibers - When body divisions serve the same organ, they cause antagonistic effects due to different neurotransmitters
o Parasympathetic (cholinergic) fibers release acetylcholine
o Sympathetic postganglionic (adrenergic) fibers release norepinephrine
o Preganglionic axons of both divisions release acetycholine
17
Q
Sympathetic - Fight or Flight
A
o Response to unusual stimulus when emotionally or physically stressed or threatened o Takes over to increase activities o Remember as the “E” division Exercise Excitement Emergency Embarrassment
18
Q
Parasympathetic - Rest and Digest
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o “Rest-and-digest” system o Conserves energy o Maintains daily necessary body functions o Remember as the “D” division Digestion Defecation Diuresis
19
Q
Special Senses
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- Special senses include: o Smell o Taste o Sight o Hearing o Equilibrium
- Special sense receptors
o Large, complex sensory organs
o Localized clusters of receptors
20
Q
The Eye and Vision
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- 70 percent of all sensory receptors are in the eyes
- Each eye has over 1 million nerve fibers carrying information to the brain
21
Q
Anatomy of the Eye - Accessory Structures
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- Accessory structures include the: o Extrinsic eye muscles o Eyelids o Conjunctiva o Lacrimal apparatus
22
Q
External and Accessory Structures - Explained
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- Eyelids
o Meet at the medial and lateral commissure (canthus) - Eyelashes
o Tarsal glands produce an oily secretion that lubricates the eye
o Ciliary glands are located between the eyelashes - Conjunctiva
o Membrane that lines the eyelids and eyeball
o Connects with the transparent cornea
o Secretes mucus to lubricate the eye and keep it moist - Lacrimal apparatus = lacrimal gland + ducts
o Lacrimal gland—produces lacrimal fluid (tears); situated on lateral end of each eye
o Tears drain across the eye into the lacrimal canaliculi, then the lacrimal sac, and into the nasolacrimal duct, which empties into the nasal cavity
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Q
Tears
A
- Tears contain: o Dilute salt solution o Mucus o Antibodies o Lysozyme (enzyme that destroys bacteria)
- Function of tears
o Cleanse, protect, moisten, lubricate the eye
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Q
Extrinsic Eye Muscles
A
- Six muscles attach to the outer surface of the eye
- Produce gross eye movements
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Internal Structures: The Eyeball
- Three layers, or tunics, form the wall of the eyeball
o Fibrous layer: outside layer
o Vascular layer: middle layer
o Sensory layer: inside layer
- Humors are fluids that fill the interior of the eyeball
- Lens divides the eye into two chambers
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Fibrous Layer: Eyeball
Fibrous layer = sclera + cornea
o Sclera
White connective tissue layer
Seen anteriorly as the “white of the eye”
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o Cornea
Transparent, central anterior portion
Allows for light to pass through
Repairs itself easily
The only human tissue that can be transplanted without fear of rejection (no blood supply)
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Vascular Layer: Eyeball
- Choroid is a blood-rich nutritive layer that contains a pigment (prevents light from scattering)
- Choroid is modified anteriorly into two smooth muscle structures
o Ciliary body
o Iris
Regulates amount of light entering eye
Pigmented layer that gives eye colour
Pupil Rounded opening in the iris
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Sensory Layer: Eyeball
Sensory layer
o Retina contains two layers
Outer pigmented layer absorbs light and prevents it from scattering
Inner neural layer contains receptor cells (photoreceptors)
• Rods – dim light
• Cones – bright light and colour vision
o Electrical signals pass from photoreceptors via a two neuron chain
Bipolar neurons
Ganglion cells
o Signals leave the retina toward the brain through the optic nerve
o Optic disc (blind spot) is where the optic nerve leaves the eyeball
Cannot see images focused on the optic disc
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Rods
Most are found toward the edges of the retina
Allow vision in dim light and peripheral vision
All perception is in gray tones
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Cones
Allow for detailed color vision
Densest in the center of the retina
Fovea centralis (area of sharpest vision)–lateral to blind spot
• Area of the retina with only cones
• Visual acuity (sharpest vision) is here
No photoreceptor cells are at the optic disc, or blind spot
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Cone Sensitivity
Three types of cones
| Each cone type is sensitive to different wavelengths of visible light
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Optic Disc
Blind spot on the retina
Junction between optic nerve and eye
Cannot transmit optic information
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Internal Structures: Lens
- Flexible, biconvex crystal-like structure
- Held in place by a suspensory ligament attached to the ciliary body
Lens divides the eye into two chambers
o Anterior (aqueous) segment
Anterior to the lens
Contains aqueous humor, a clear, watery fluid
Maintains intracochlear pressure and provides nutrients for lens and cornea
o Posterior (vitreous) segment
Posterior to the lens
Contains vitreous humor, a gel-like substance
Prevents eye from collapsing by maintaining intracochlear pressure
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Physiology of Vision - Visual Pathway to Brain
Visual fields and visual pathways to the brain
o Optic nerve
Bundle of axons that exit the back of the eye carrying impulses from the retina
o Optic chiasma
Location where the optic nerves cross
Fibers from the medial side of each eye cross over to the opposite side of the brain
o Optic tracts
Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye
Synapse with neurons in the thalamus
o Optic radiation
Axons from the thalamus run to the occipital lobe
Synapse with cortical cells, and vision interpretation (seeing) occurs
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Summary of the pathway of impulses from the retina to the point of visual interpretation
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1. Optic nerve
2. Optic chiasma
3. Optic tract
4. Thalamus
5. Optic radiation
6. Optic cortex in occipital lobe of brain
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Visual Fields
o Each eye “sees” a slightly different view
o Field of view overlaps for each eye
- Binocular vision results and provides:
o Depth perception (three-dimensional vision)
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Emmetropia
(harmonious vision)
| o Eye focuses images correctly on the retina
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Myopia (Nearsightedness)
o Distant objects appear blurry
o Light from those objects fails to reach the retina and are focused in front of it
o Results from an eyeball that is too long
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Hyperopia (farsightedness)
o Near objects are blurry, whereas distant objects are clear
o Distant objects are focused behind the retina
o Results from an eyeball that is too short or from a “lazy lens”
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Astigmatism
o Images are blurry
o Results from light focusing as lines, not points, on the retina because of unequal curvatures of the cornea or lens
o Corrective lenses can partially or completely correct astigmatism
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The Ear: Hearing and Balance
- Ear houses two senses
o Hearing
o Equilibrium (balance)
- Receptors are mechanoreceptors
- Different organs house receptors for each sense
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Anatomy of the Ear: External (outer) Ear
Auricle (pinna)
External acoustic meatus (auditory canal)
• Narrow chamber in the temporal bone
• Lined with skin and ceruminous (earwax) glands
o Cleans and protects the ear
• Ends at the tympanic membrane (eardrum)
o Separates outer ear from inner ear
External ear is involved only in collecting sound waves
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Anatomy of the Ear: Middle Ear
Air-filled, mucosa-lined cavity within the temporal bone
Involved only in the sense of hearing
Located between tympanic membrane and oval window and round window
Tympanic Cavity
• Three bones (ossicles) span the cavity
o Malleus (hammer)
o Incus (anvil)
o Stapes (stirrup)
o Connect tympanic membrane to the membrane of oval window
o Transmit and amplify soundwaves
• Function
o Transmit vibrations from tympanic membrane to the fluids of the inner ear
o Vibrations travel from the malleus → incus → stapes → oval window of inner ear
Pharyngotympanic tube (auditory tube)
• Links middle ear cavity with the throat
• Equalizes pressure in the middle ear cavity so the eardrum can vibrate
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Internal (inner) ear
Includes sense organs for hearing and balance
Bony labyrinth (osseous labyrinth) consists of:
• Cochlea
• Vestibule
• Semicircular canals
Bony labyrinth is filled with perilymph
• Membranous labyrinth is suspended in perilymph and contains endolymph
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Equilibrium
- Equilibrium receptors of the inner ear are called the vestibular apparatus
- Vestibular apparatus has two functional parts
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Static Equilibrium
Maculae—receptors in the vestibule
• Report on the position of the head
• Help us keep our head erect
• Send information via the vestibular nerve (division of cranial nerve VIII) to the cerebellum of the brain
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Anatomy of the Maculae
* Hair cells are embedded in the otolithic membrane
* Otoliths (tiny stones) float in a gel around hair cells
* Movements cause otoliths to roll and bend hair cells
* Impulse transmitted via vestibulocochlear nerve
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Dynamic Equilibrium
Crista ampullaris
• Responds to angular or rotational movements of the head
• Located in the ampulla of each semicircular canal
• Tuft of hair cells covered with cupula (gelatinous cap)
• If the head moves, the cupula drags against the endolymph
• Hair cells are stimulated, and the impulse travels the vestibular nerve to the cerebellum
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Hearing: Spiral Organs of Corti
o Located within the cochlear duct
o Receptors = hair cells on the basilar membrane extend into endolymph of cochlear duct
o Gel-like tectorial membrane is capable of bending hair cells
o Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe
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Pathway of Vibrations from Sound Waves
Pathway of Vibrations from Sound Waves
o Move by the ossicles from the eardrum to the oval window
o Sound is amplified by the ossicles
o Pressure waves cause vibrations in the basilar membrane in the spiral organ of Corti
o Hair cells of the tectorial membrane are bent when the basilar membrane vibrates against it
o An action potential starts in the cochlear nerve (cranial nerve VIII), and the impulse travels to the temporal lobe
- High-pitched sounds disturb the short, stiff fibers of the basilar membrane
o Receptor cells close to the oval window are stimulated
- Low-pitched sounds disturb the long, floppy fibers of the basilar membrane
o Specific hair cells further along the cochlea are affected
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Hearing Deficits
- Deafness refers to any loss of hearing
o Results from injury, loud sounds, excessive earwax, scarring of the tympanic membrane
- Tinnitus refers to ringing in the ear
o Results from damage to hair cells in the inner ear
o Ringing caused by random movement of hair cells
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Chemical Senses: Smell and Taste
Chemoreceptors
o Stimulated by chemicals in solution
o Taste has five types of receptors
o Smell can differentiate a wider range of chemicals
- Both senses complement each other and respond to many of the same stimuli
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Olfactory Receptors and Sense of Smell
- Olfactory receptors are in roof of nasal cavity
o Olfactory receptor cells (neurons) with long cilia known as olfactory hairs detect chemicals
o Chemicals must be dissolved in mucus for detection by chemoreceptors called olfactory receptors
- Impulses are transmitted via the olfactory filaments to the olfactory nerve (cranial nerve I)
- Smells are interpreted in the olfactory cortex
- When suffering a cold , olfactory receptors are partially blocked
- Nerve pathway between nose and brain travel through the limbic system
o Part of brain responsible for emotion
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Taste Buds and Sense of Taste
- Taste buds house the receptor organs
- Locations of taste buds
o Most are on the tongue
o Soft palate
o Superior part of the pharynx
o Cheeks
- The tongue is covered with projections called papillae that contain taste buds
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Gustatory Cells
Gustatory cells are the taste receptors
o Possess gustatory hairs (long microvilli)
o Gustatory hairs protrude through a taste pore
o Hairs are stimulated by chemicals dissolved in saliva
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Areas in which Taste Buds are found
Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas
o Facial nerve (cranial nerve VII)
o Glossopharyngeal nerve (cranial nerve IX)
o Vagus nerve (cranial nerve X)
- Taste buds are replaced frequently by basal cells (every 7 to 10 days)
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Five Basic Taste Sensations
- Sweet receptors respond to sugars, saccharine, some amino acids
- Sour receptors respond to H+ ions or acids
- Bitter receptors respond to alkaloids
- Salty receptors respond to metal ions
- Umami receptors respond to the amino acid glutamate or the beefy taste of meat
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Disorders of the Nose and Tongue
- Rhinitis is inflammation of the mucous membranes in the nasal passage
o Caused by infections, allergies, strong chemical odours and certain drugs
- Septum problems
- Infections, injuries and abnormal tissue growth affect the tongue
o Inadequate oral hygiene can lead to growth of hairs on the tongue
