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Chapter 9-11: NERVOUS SYSTEM and SENSES
Master controlling communicating system of the body
 Responsible for every thought, action and emotion

Cells respond quickly and almost instantaneously
1. Monitors changes (stimulus)
o Gathered information is called sensory input
2. Processes and interprets, then decides what should be done
o A process called integration
3. Causes a response by activating effector organs (motor output)
Two divisions of our Nervous System
1. CNS (central nervous system) – brain and spinal cord

Integrating and command center
2. PNS (peripheral nervous system) – outside CNS

Nerves that extend from the brain and spinal cord (cranial and spinal nerves both to
and from)
Two functional subdivisions
 Sensory (afferent) division – fibers that send the info (impulses) to the CNS
o Somatic – skin, muscles, joints
o Visceral – organs inside ventral cavity

Motor (efferent) division – transmits impulses from CNS to effector organs
Two main parts
 SNS (Somatic nervous system) – somatic motor nerves
- conduct impulses to skeletal muscles (voluntary)

ANS (autonomic nervous system) – consists of visceral motor nerve fibers
- regulates smooth muscle, cardiac muscles & glands (involuntary action)
Two functional subdivisions
 Sympathetic – “fight or flight” system ( prepares for action)

Parasympathetic – “resting and digestion” system (↑ digestion and
absorption)
*Both are always in counterbalance of each other, not necessarily equal
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Two types of cells - supporting cells (surround or wrap) and neurons (excitable nerves)
I. Neurons (nerve cell)
 basic structural and functional component of nervous system
o conduct messages in the form of nerve impulses

Dendrites, Cell Body, Axon
o Myelin sheath (fatty covering) (fig 9-4)

excitability (irritability) – highly responsive to stimuli

amitotic – once neurons link to communicate they lose the ability to divide

Increased metabolic rate – high need for O2 and glucose

Longevity – will live a lifetime with proper nutrition
Synapse:
- mediates the information transfer from one neuron to another or an effector cell
1. presynaptic neuron vs. postsynaptic neuron
-
typical synapse has: axon terminals & receptor regions (ie. dendrites; sarcolemma)
-
specialized for the release and reception of chemical transmitters
-
Neurotransmitters
o Excitatory or inhibitory
o Ach (Acetylcholine) excitatory
o Norepinepherine and Epinepharine (adrenaline) excitatory and inhibitory (ANS)
o Dopamine (inhibitory) secreted in the brain to control movement and increase
motivation
Functional classification:
1. Sensory (afferent) neurons – afferent division of the PNS (~10 million neurons)
a. extend between a receptor and the CNS
b. somatic vs. visceral
c. Categorized as Exteroceptors, Proprioceptors, or Interoceptors
2. Motor (efferent) neurons – (~500,000 neurons)
a. cell bodies are located in the CNS
b. Somatic (SNS) vs. Visceral (ANS)
c. Organs controlled by these fibers are called effectors
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3. Interneurons (association) - (~20 billion neurons)
a. Located in the brain and spinal cord
b. shuttle information between sensory and motor neurons
c. also involved in memory, planning and learning
Nerurons travel in groups of similar function

Tracts – CNS (ascend or decend)

Nerves – PNS (afferent or efferent)

Covered by connective tisseue – Endoneurium (nerve), Perineurium (fascicle),
Epineurium (entire nerve)
II. Supporting cells also called neuroglia (or “glial cells)

cells, such as Astrocytes, that guide the development of neurons, protect neurons,
regulate the surrounding extracellular fluid

In the PNS, Schwann cells increase impulse travel and aid in the repair of axons
NEUROPHYSIOLOGY
Resting membrane potential (Polarized State) 
more K+ and Proteins- inside the neuron and more Na+ and Cl- outside the neuron

electrochemical gradient for Na+ to enter neuron
Membrane potentials as Signals – stimulus can change resting potential by:
1. changing the permeability of the membrane to a particular solute
2. altering solute concentrations on either side of the membrane

This creates an action potential or electrical change that can be transmitted along the
neurolemma (plasma membrane of a neuron)

A myelin sheath on an axon will pass this charge on quicker – jumps from node to node
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Spinal Cord Gross Anatomy
 18” long (45 cm), .55” wide (14 mm), runs down vertebral foramen to L1 – L2 level

Grey matter : surrounds central canal; highly organized
o
Dominated by cell bodies of neurons, neuroglia and unmyelinated axons

o
Projections are called horns: groups of cell bodies that have a common function


Dorsal and Ventral horns
Contain the cell bodies of neurons that pass outside the CNS and/or travel
up or down the spinal cord
White matter
o
Contains a large number of myelinated axons (some myelinated)
o
Divided into three columns which are made up of tracts (or fascicles) – bundles of
axons in the CNS with the same function and structure

Ascending tracts carry sensory info to the brain

Descending tracts convey motor commands to the spinal cord from the
brain
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
Inferior to lumbar enlargement the spinal cord tapers (conus medullaris)
o Filum terminale (slender strand of fibrous tissues) provides longitudinal support
of the

Dorsal roots – axons of sensory

Cell bodies of these neurons form dorsal root ganglia

Ventral roots – axons of motor neurons that extend into the periphery

Distal to the dorsal root ganglion, each sensory and motor roots are bound into Spinal
nerves: 31 pairs spinal nerves
- Supply all parts of the body except the head and some areas of the neck
- All are mixed nerves (both sensory and motor)
- Named for their point of issue from the spinal cord

Dermatomes – specific region of the skin in which a nerve is associated
Reflex Arc – most are complex some are very simple (give example)
 A complete pathway through the nervous system in response to a stimulus
o Includes a Receptor – specialized extensions (dendrite) detect stimulus
o Sensory neuron – sends impulse through dorsal horn of spinal cord
o CNS interpretation – impulses are coordinated and distributed
o Motor neuron – impulse sent out of CNS through the ventral horn
o Effector – A muscle or gland that carries out the response
Nerve roots in cervical, lumbar, and sacral areas form plexuses (networks): give rise to nerves
supplying skin, muscles, and joints of upper and lower extremities.
Nerve Plexuses:
Cervical Plexus: supplies head, neck, shoulder, & chest

Phrenic nerve innervates the entire diaphragm
Brachial Plexus: supplies axilla, skin & muscles of upper extremity

Ulnar nerve innervates posteromedial surface of forearm (funny bone)
Lumbosacral Plexus: supplies lower trunk, pelvis, and legs
 sciatic nerve – largest nerve of this plexus (~1in)
 passes beneath gluteus maximus and extends down back of thigh
o Sciatica – damage to nerve caused by ruptured disc or tight muscles
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Disorders of the Spinal Cord and Spinal Nerves
M.S. – Multiple Sclerosis
 Myelin degeneration within the white matter of CNS
o Caused by an autoimmune disorder combined with environmental factors
 Most common of the chronic CNS diseases in the U.S.
 Progression varies; marked by relapses/remissions; no cure
Poliomyelitis
 Viral disease of the nervous system that affect mostly children
 Caused by contaminated water
 Virus multiplies within motor neurons especially of the respiratory system
 All but eliminated through toddler vaccinations
Peripheral Neuropathies (or neuritis) (palsies) – loss of sensory or motor function
 Various causes (intoxication, infection, diabetes)
 Location of dermatome loss indicates location of damage (symptom)
 Paresthesia – excessive stimulation “pins and needles”
Paralysis – loss of synapses between motor neurons (box 9-1)
 Caused by injury (most common – many involve alcohol and drugs), tumors, or infection
 -plegia is loss of (monoplegia, diplegia, paraplegia, hemiplegic, quadriplegia)
Trauma – damage to neurons may or may not regenerate
 Spinal contusion – white matter of the spinal cord degenerates at the site of injury
 Spinal laceration – partial tearing from fragments
 Spinal compression – cord is squeezed from a smaller vertebral canal or foramina
 Spinal transection – complete severing ( quadriplegia vs. paraplegia)
Shingles (herpes zoster) – viral infection within dorsal root ganglia
 Causes a painful rash at the area of distribution of the affected sensory nerves usually
from the thoracic nerves
 Caused by a reactivation of a prior chickenpox virus
 Creates fever, pain, contagious fluid filled vesicles along a dermatome for 2-4wks; may
affect nerve function for years
AUTONOMIC NERVOUS SYSTEM (summary table 9-3 and fig 9-16)


Coordinates CV, respiratory, digestive, urinary, and reproductive functions.
ANS provides stability of internal environment: homeostasis
o Adjusts internal water, electrolyte, nutrient, and dissolved gas concentrations –
without interference from the conscious mind.

ANS vs. SNS
Both are efferent divisions that carry motor commands
Differences between Somatic and Autonomic Nervous Systems:
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1. Pathways include 2 consecutive motor neurons (relay b/w 2 neurons)
 presynaptic (preganglionic) - cell body in CNS – axon extends to visceral ganglion

postsynaptic (postganglionic) - cell body in autonomic ganglion – outside of the
CNS contains 100s to 1000s ganglionic neurons – axon extends to peripheral
target organs
2. Different effectors – cardiac muscle, smooth muscle, glands and adipose tissue
Consists of 2 subdivisions:
I. Sympathetic (thoracolumbar)
- cell bodies located in lateral horn of gray matter between T1 and L2
- dominant under stress (fight or flight response)
-
Increased alertness, feeling of energy and euphoria, increased activity in CV and
Resp. center, elevation of muscle tone, and mobilization of energy reserves
Preganglionic axons from these neurons:

exit from spinal cord in ventral root of spinal nerves then enter the sympathetic
chain ganglia

Axons are relatively short compared to the postganglionic axons
Postganglionic Neurons of the Sympathetic Division:

cell bodies located in ganglia (chain ganglia or collateral ganglia) or in adrenal
medulla

axons of postganglionic neurons are relatively long and terminate with varicosities
o most release epinepharine (E) or norepinephrine (NE) at their effectors
II. Parasympathetic (craniosacral)
- cell bodies of these neurons are located in lateral horns of gray matter in S 1-S4 and in
brainstem (mesencephalon, pons, medulla oblongata)
- Functions center on relaxation, food processing, and energy absorption
- dominant under "status quo"
-
Constriction of pupils, secretion of digestive glands, hormones that promote
nutrient absorption, glandular activity associated w/sexual arousal, increase in
smooth muscle activity, stimulation/coordination of defecation and urination,
constriction of respiratory passageways, reduction of HR
Preganglionic axons from these neurons

are associated with cranial nerves (III, VII, IX & X)

synapse w/ postganglionic neuron in terminal ganglia right next to or within
effector

are relatively long compared to postganglionic axons
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Postganglionic Neurons of the Parasympathetic Division:

cell bodies of these neurons are at synapses next to or within the effector

axons of postganglionic neurons
o are relatively short compared to preganglionic neuron’s axons
o secrete acetylcholine at effectors (cholinergic)
Most vital organs receive instructions from both the sympathetic and parasympathetic divisions
– commonly having opposing effects (Box 9-3)
Spinal Meninges
 layers of specialized membranes that surround and protect the brain and spinal cord
 provides physical stability and shock absorption in the vertebral canal
Three layers
1. Dura Mater – tough fibrous outer covering
 Superiorly fuses with the periosteum of the occipital bone and become the cranial
dura mater; inferiorly fuses with coccygeal ligament

Creates pockets within the cranium for venous channels (dural sinuses)

Anesthetics are injected into this space (epidural block)
Epidural hemorrhage – blood forced between the dura mater and cranium as a result of
damage to meningeal vessels (arteries vs. veins)
Subdural hemorrhage – blood below the dura mater that may extend into the arachnoid.
Results from damage to small vein or dural sinus. Symptoms progress slower because of the
lower BP in these vessels
2. Arachnoid – middle layer
 A delicate network of collagen and elastic fibers makes up the trabeculae

Space between trabeculae and pia mater (deeper layer) contains
o Cerebral Spinal Fluid (CSF) – clear liquid that carries nutrients to nervous
tissue
o Spinal Tap – (lumbar puncture)
3. Pia Mater – innermost layer
 Meshwork of elastic and collagen fibers firmly attached to underlying neural tissue

Supports the blood vessels that serve the brain and spinal cord
Meningitis – inflammation of the meninges



Bacterial infection through the ear nose or throat (meningococcus)
Survival includes early treatment of antibiotics
Disrupts CSF and circulation, damages neuroglia and neurons
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The Brain & Cranial Nerves
 The brain weighs ~1.4kg (3 lb)
 Males brains are 10% larger than a females
o No correlation exists between size and intelligence
Major landmarks: (table 10-1)
 Cerebrum is divided into large paired cerebral hemispheres
o Area of higher mental functions – conscious thought, intellect, memory and movement

The surface of the hemispheres are folded and covered by neural cortex
o superficial layer of gray matter – cerebral cortex

This cerebral cortex forms a series of elevated ridges, or gyri – increase surface area

Gyri are separated by shallow depressions (sulci) and deeper grooves (fissures)

Connected by white matter called the corpus callosum

Central Sulcus separates areas of the primary motor cortex (precentral gyrus) and the
control center for somatic senses of touch, temp, pain, and taste
o (fig 9-10) primary motor area – control of skeletal muscle

Visual Area (Primary Visual Cortex): extreme posterior tip and medial aspect of occipital
lobe receives input from retina (reading)
o General interpretive area receives info from all sensory association areas and
puts them together (left hemisphere)
o Broca's Area: (speech center) generally exists only in the left hemisphere

motor and panning speech area
o Prefrontal Cortex: anterior area of the frontal lobe coordinates information from
all areas of the cortex. Used in planning and intellectual decision making

Hemispheres of the cerebellum lie below the cerebrum

Cerebellum also covered with neural cortex – cerebellar cortex
o Adjusts movements by comparing incoming signals with previous experiences

Once the cerebrum is removed, diencephalon can be seen – links cerebrum with spinal cord
o Made up of the right and left thalamus – contain sensory relay and processing centers

Thalamus is the final relay point for ascending tracts before they go on to the primary
sensory cortex
o left and right thalamus are separated by the third ventricle

The hypothalamus, the floor of the diencephalon, is involved with emotion, autonomic
function, and hormone production
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o the infundibulum extends inferiorly to connect the hypothalamus to the
pituitary gland
o main visceral (autonomic) control center
o controls HR, pupil size, BP, produces hormones such as ADH and oxytocin,
regulates thirst and hunger, sex drive, monitors body temp, and controls
Circadian cycles (sleep-wake cycles)

The infundibulum connects the hypothalamus with the pituitary gland which
o Integrates the nervous system with the endocrine system

Brain stem includes the mesencephalon, pons, and medulla oblongata
o Mesencephalon contains nuclei that process visual and auditory info and control
the reflexes associated – hearing and looking at a loud noise
o Pons connects the cerebellum to the brain stem – contains nuclei that control
visceral and somatic motor control
o Medulla oblongata connects the spinal cord to the brain (stem)

Relays sensory info to the hypothalamus and rest of brain and regulate
autonomic functions such as HR, BP, and digestion
Limbic System






complex group of fiber tracts that function to establish emotional states, link conscious
to the unconscious, and facilitate memory storage and retrieval
found between the border of the cerebral hemispheres (around the corpus callosum)
and outlying areas of the diencephalons
responsible for emotional responses; “fight or flight”, pleasure, pain, anger, sorrow
hippocampus – nucleus deep to the limbic lobe responsible for long-term memory
storage and retrieval
link between emotional stress and ill health through over stimulation in these areas
limbic system explains why you want to do complex tasks – this is your motivation
center
Ventricles - Expanded hollow chambers




continuous with each other and the central canal of the spinal chord
- filled with cerebrospinal fluid (CSF) and lined with ependymal cells
- Supports the brain – weighs 3 lb in the air but only a few ounces in CSF
- Transports nutrients, Chemical messengers, and Waste products – ependymal layer is
freely permeable which allows constant communication with interstitial fluid
- CNS function produces changes in the CSF – Spinal tap (or lumbar puncture)
lateral ventricles: contained in each cerebral hemisphere
third ventricle: exists within the diencephalon and communicates directly with the lateral
ventricles through the interventricular foramen
fourth ventricle: extends from the posterior surface of the pons down to the superior
portion of the medulla oblongata, connected to the third ventricle via the cerebral aqueduct
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Hydrocephalus – fluid accumulation within the cranium
 Accumulation created pressure on the bran and therefore decreased function

Results from trauma or more often masses or tumors that block circulation

In infants, the cranium expands because bone plates have not fused
Cranial nerves: (pgs 217-219)
 12 pairs of cranial nerves are located on the ventrolateral surface of the brain stem
Olfactory
Optic
Oculomotor
Trochlear
Trigeminal

Tic douloureux -
Abducens
Facial

Bell’s Palsy -
Vestibulocochlear
Glossopharyngeal
Vagus
Accessory
Hypoglossal
Aging and Disorders of the Nervous System
Structural and functional changes begin after 30 yrs of age but aren’t realized until later years
1. Reduction in brain size and weight: primarily from the cerebral cortex
2. Reduction in the number of neurons
3. Decrease in the blood flow to the brain
4. Changes in the structure of synapse in the brain
5. Changes in the structure of CNS neurons
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Brain Disorders
CVA (cerebrovascular accident) – results from a blood clot or cerebral hemorrhage that
reduces blood flow to a certain part of the brain.

Occurs mainly in individuals over 40 with associated poor lifestyle habits

Aphasia – affliction of speech and language areas of the cerebral cortex
Cerebral Palsy – affects voluntary motor movement (motor pathways)

Appear during or after childbirth (alcohol, drugs, infection)

Permanent brain damage results
Concussions – trauma related
Encephalitis: inflammation of the brain
Alzheimer’s disease – degenerative disease resulting in the progressive decline of higher
cerebral functions

Early stages are usually senile dementia
Parkinson’s disease is a brain disorder that causes a general increase in muscle tone

Spasticity vs. tremors
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Sensory Receptors: specialized structures designed to respond to changes in their
environment (outside or inside)
A. Special Senses

(vision, hearing, equilibrium, smell, taste) receptors located in sense organs
B. General Senses (simple in structure)

Tactile sensations (touch, pressure, vibration), Temperature regulation, Pain

Majority of receptors have modified dendritic endings
1. Sensation – any stimulus received and interpreted by the CNS
2. Perception – conscious awareness of a sensation
Adaptation – reduction in the sensory response to a constant stimulus
Classified according to Stimulus Type Detected
a. Nociceptors – nerve endings sensitive to pain (extreme temp., damage)
b. Thermoreceptors
c. Mechanoreceptors
i. Tactile Receptors (touch, pressure, vibration)
ii. Baroreceptors – stretch receptors that monitor pressure
d. Proprioceptors – monitor positions of joints
e. Chemoreceptors
i. Carotid or Aortic bodies – monitor pH, CO2 and O2 levels in blood
SPECIAL SENSES
Interpretation occurs in cerebral cortex
Vision, olfactory, auditory (equilibrium), and gustatory are considered the special senses
Perceptions of sensory inputs are overlapping, therefore our “experiences” are a blending of
stimulus effects
Chemical senses: Smell and Taste
OLFACTION (SMELL)
Receptors: olfactory epithelium located in the roof of the nasal cavity
Odorants – small organic molecules that stimulate olfactory receptors

olfactory epithelium contains millions of olfactory receptor cells – but still cant compare to
dogs and even fish
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



receptor cells are highly modified cells that have a knob that projects past the epithelium
– extensions of olfactory cilia extend within the surrounding mucus
we can detect over 2000 chemical smells based on a combination of about 50 primary
smells
Information (scent) passed via Olfactory tracts to either:
I.
to olfactory cortex (medial surface of temporal lobe) and part of frontal lobe
above orbits
II.
to hypothalamus & limbic system where emotional aspects of smell are
analyzed and/or responded to
Receptor cells adapt rapidly and therefore need to be turned over – one of the few areas
where neuronal replacement exists within adults
GUSTATION (TASTE)
Receptors: gustatory hairs (cilia) of taste cells




Taste cells located in taste buds of tongue (most), pharynx, larynx, & soft/hard palate – an
adult has ~3000 taste buds
Most all taste buds located in lingual papillae (epithelial projections on the tongue)
Taste bud cells are replaced every 7 to 10 days by basal cells
For a chemical to be tasted it must dissolve in saliva, diffuse into the taste pore, contact
gustatory hairs (chemoreceptors)
o Different gustatory cells have different thresholds for activation
4 basic tastes (sweet, sour, salty, biter) - not spatially distributed





-sweet: elicited by organic substances including sugar, alcohol, and some AA’s
-sour: produced by acids with a high H ion concentration
-salty: produced by inorganic salts (metal ions) such as sodium chloride
-bitter: elicited by alkaloids (caffeine, nicotine, morphine)
Two additional tastes have been discovered
 Umami and Water receptors
 Pleasant tastes of broth and cheese, and yes we can taste water
Taste cells coiled by sensory dendrites



(VII) facial: monitors taste buds on the anterior 2/3 of tongue
(IX) Glossopharyngeal: taste buds on the posterior 1/3 of tongue and the pharynx
(X) Vagus: cheeks, larynx, lower pharynx, soft/hard palate
- triggers reflexes involved with digestion (saliva and gastric juices)
Sensitivity to tastes differs among individuals and it declines as we age
Taste is intimately associated with olfaction – if olfactory receptors are blocked, then our
perception of taste diminishes
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VISION
70% of all sensory receptors of the body are in the eyes
A. Accessory Structures: (eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye
muscles)
1. eyebrows: help protect the eye – sun, dust, etc
2. eyelids (palpebrae): lubricate and clean off surface of the eye
-
-
eyelashes protect from foreign material
o associated with a tarsal gland that secretes a lipid-rich product – keep from
sticking
a blockage in any of these ducts may produce a localized swelling - sty
3. Conjunctiva: epithelium covering the inner surface of the eyelids and outer surface of
the eye
a. Extends over the eye to the cornea where the epithelium thins
Conjunctivitis – (pinkeye) results from irritation of the conjunctiva surface
o Caused by streptococcus bacteria
4. Lacrimal apparatus: lacrimal gland and the ducts that drain excess secretions into the
nasal cavity
b. Lacrimal secretions: fluid containing mucus, antibodies, and lysozome (attacks
bacteria)
c. Blinking sweeps this fluid across the eye to clean and maintain lubrication
5. Extrinsic eye muscles – inferior, medial, superior, & lateral rectus, and superior &
inferior oblique
B. Structure of the Eye:

The eye sits in the orbit surrounded by the extrinsic muscles, the lacrimal gland, orbital fat,
and the cranial nerves and blood vessels that supply the eye and area around it

Sclera: glistening white dense fibrous connective tissue; “white of the eyes”
o Thickest in the posterior and thinnest in the anterior; extrinsic muscles insert on the
sclerea

Cornea: regular arrangement of its collagen fibers makes it transparent, forming a clear
window that allows light to enter the eye
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o well supplied w/ nerve endings; but has no blood vessels
o vulnerable to damage – if scratched, replacement is common

Iris: visible colored part of eye
o houses pupil (opening in eye for light to enter) & acts to vary pupil size
o papillary dilator and constrictor muscles
o iris color determined by the density of melanocytes and density of pigmented
epithelium

Ciliary Body: encircles lens posterior to the iris
o predominately made up of ciliary muscles (smooth) - important in lens focusing
o pulls on suspensory ligaments which keep the lens centered

Choroid: provides nutrients to all eye tunics - contains an extensive capillary network
Retina: (innermost layer)

absorbs light and prevents it from bouncing back producing visual “echoes”

contains photoreceptors – cells that detect light (rods and cones)
1. Rods - highly sensitive to light; allow us to see in poor lighting
o Mostly span the periphery of the retina
o Improper # or Vitamin A deficiency leads to night-blindness
2. Cones – require more intense light; provide us with color vision
o Highly concentrated in the fovea (central area) – center of site where
image is projected – visual axis
o Low # in any three cause certain color blindness

Rods and cones synapse with bipolar cells which in turn synapse with ganglion cells Optic nerve is formed from the axons of ganglion cells

Trauma may lead to retinal tearing or detachment – surgery required
Optic disc (blind spot): site where optic nerve exits the eye and central artery and vein enter
the eye

There are no photoreceptors this area – continual involuntary eye movements allow the
brain to fill on the missing spot
Internal Chambers & Fluids
A. Anterior Cavity (anterior to lens)
• Anterior segment filled with aqueous humor
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-supplies nutrients and removes wastes
-creates pressure that stabilizes the shape of the eye and position of the retina
- intraocular pressure – 12 to 21 mm Hg
*Glaucoma – for various reasons, aqueous humor cannot enter the canal of Shlemm.
Intraocular pressure builds and leads to disruption of signals (blindness)
B. Posterior cavity (behind lens & is the largest)
• filled with vitreous humor (body) – viscous, gelatinous liquid
•transmits light
•helps stabilizes the shape of the eye and supports the retina
•also contributes to intraocular pressure
C. Lens: transparent, flexible structure that can change shape
•spherical when relaxed
•avascular and held in place by the suspensory ligaments
*Cataract – lens loses its transparency
Senile cataract – yellowing of the lens, natural consequence of aging
Vision:
Stimulus: electromagnetic energy wave lengths - Rays of light - ROYGBIV
Accommodation of the lens – the greater the convex curve, the more the light rays bend
toward each other. It can change focusing power by becoming moderately curved or greatly
curved
o
o
o
Emmetropic eye – sufficiently refracts light rays to focus an image on the retina
o normal vision
Myopia (nearsightedness) or hyperopia (farsightedness) occur here
o Affected by the depth of the eyeball or resting shape of the lens
o Corrected by photorefractive keraectomy (PRK)
Presbyopia – age related hyperopia
Remember: visual images are focused upside down and undergo a mirror reversal on the
retina - brain "turns" it right side up
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Light impulses are converted to nerve impulses on the surface of the retina on the quarter
billion photoreceptors found in the neural retina (rods and cones) – name refers to the shape of
the outer segment of the photoreceptor
HEARING and EQUILIBRIUM
Stimulus: sound waves
3 major areas of ear: external, middle, & inner
External Ear: funnels sound waves of air into the auditory canal
 pinna = flap of elastic cartilage sends waves down the external auditory canal
o waxy secretions (cerumen) and tiny hairs protect substances from entering

tympanic membrane (eardrum) boundary between the external auditory canal and the
middle ear – thin membranous sheet
o sound waves make it vibrate, which then transfers the energy to the bones of the
middle ear.
Middle Ear (tympanic cavity): air filled mucosa lined cavity
 auditory (Eustachian) tube: links middle ear with nasopharynx: equalizes pressure
 otitis media – middle ear infection

Houses smallest bones of the body: ossicles
 malleus (hammer), incus (anvil), stapes (stirrup) – smallest synovial joints in the
body

malleus secured to the eardrum and stapes fits into the oval window (opening to inner ear)
 transmits vibrations from ear drum to oval window
Inner (internal) Ear: senses of hearing and equilibrium exist here
 2 major divisions:
 Membranous labyrinth: tubes and sacs filled with endolymph
o Fluid with electrolytes

Bony labyrinth: shell of dense bone filled w/ perilymph (similar to CSF) which
surrounds membranous labyrinth
o channels through the temporal bone subdivided into:
 vestibule, cochlea, semicircular canals
o Vestibule- central egg shaped cavity
 lateral wall contains the oval window

contain receptor regions that respond to the pull of gravity and position of
head (calcium carbonate crystals - statoconia)
o Semicircular canals: posterior to vestibule
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
3 canals that are oriented in all three planes of space; involved in
equilibrium (stimulated by rotation of the head)
Equilibrium: refers to a state of balance between opposing forces.
There are two kinds of equilibrium (Static and Dynamic).
o Static Equilibrium: refers to the balance and posture of the body (mainly the head) when
the body is not moving.
o Dynamic Equilibrium: refers to the balance and posture of the body (mainly the head) in
response to rotational movements. Collectively, the receptor organs for equilibrium are
in the internal ear and are referred to as the vestibular apparatus. It includes the
saccule, utricle, (membranous sacs in the vestibule) and semi-circular ducts.
o Cochlea: spiral bony chamber (snail shaped) – contains receptors for hearing
 Organ of Corti (organ of hearing) – consists of supporting cells and hair
cells, which are receptors for auditory sensations

hair cells are in contact with the cochlear branch of the vestibulocochlear
(VII) and covered by the tectorial membrane
Sound waves
- frequency of vibration is its pitch (high frequency Vs low frequency)
o intensity (energy) of sound wave is measured in decibels (40-50 dBs are
comfortable
o wave energy produces pressure
Deafness due to loud sounds, blockage, or drug interaction
Physiology of hearing
1. Sound waves that reach the ear are directed by the auricle into the external auditory
canal.
2. When the waves strike the eardrum, the eardrum vibrates. The distance the eardrum
moves is relative to the intensity and frequency of the sound waves. It vibrates slowly in
response to low-frequency sounds and rapidly in response to high-frequency sounds.
3. As the stapes moves back and forth, it pushes the oval window in and out. The
movement of the oval window sets up waves in the perilymph of the cochlea.
4. As the oval window bulges inward, it pushes on the perilymph of the scala vestibuli.
Pressure waves are transmitted from the scala vestibuli to the scala tympani and
eventually to the round window, causing it to bulge outward into the middle ear.
5. As the pressure waves push on the walls of the scala vestibuli and scala tympani, they
also push the vestibular membrane back and forth. As a result, the pressures of the
endolymph inside the cochlear duct changes.
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6. The pressure changes in the endolymph move the basilar membrane slightly. When
the basilar membrane vibrates, the hair cells of the spiral organ move against the
tectorial membrane. The bending of the hairs ultimately leads to the generation of nerve
impulses.
Sensory Disorders
Tactile sensations determined by the two-point discrimination test
o Anesthesia vs. Hypesthesia
Pain Management
Acute vs. Chronic Pain
Local Anesthesia vs. General Anesthesia
Analgesics create Analgesia without affecting touch or pressure
o Non-Narcotics: aspirin and acetaminophen
o Narcotics: morphine and codeine
Secondary Disorders of the Senses
Retinal occlusion results if there is a blockage of the blood supply to the retina
o May result in
Macular degeneration is an age related disorder associated with the uncontrolled growth and
proliferation of blood vessels within the choroids.
o Unknown cause; no cure
Diabetic retinopathy (result of chronic diabetes mellitus) – capillaries within the eye leak fluid
and hemorrhage into the retina
o Leading cause of blindness
Vertigo – illusion of movement
o Different than being “dizzy”
o An event that sends endolymph into motion; eyes and inner ear not on the same page
Otitis Media (infection of middle ear) and Otitis externa (infection of external ear)
Deafness
Conductive vs. Nerve deafness
o Bone conduction tests are used to discriminate between nerve and conduction deafness