Nervous Syste

CH 10 Nervous System: Nervous Tissue and Brain

Divisions of the Nervous System

Two parts of the Nervous System

Central Nervous System (CNS)- includes brain and spinal cord

Location: Dorsal Cavity

Location of Brain: Cranial Cavity; the spinal cord is enclosed in spinal cavity

Peripheral Nervous System (PNS)

Location: Located outside of the CNS

Consists of the nerves that connect the CNS with the rest of the body

*Nervous System three general functions: sensory function, integrative function, and a motor

Sensory Function

Sensory Nerves gather information from inside the body and from outside environment
Transmitted to the brain

Integrative Function

Sensory information brought to the CNS is interpreted

Motor Function

Convey information from the CNS to muscles and glands of the body
Carry out the plans made by the CNS
Motor nerve converts the plan into action

*Nervous tissue composed of two types of cells: The neuroglia and the neurons

Neuroglia (Glial Cells)- most abundant of the nerve cells

Location: In the CNS

Glial cells support, protect, insulate for the delicate neurons

Two common glial cells: Astrocytes and Ependymal Cells

Astrocytes (star-shaped)- most abundant of the glial cells; support neurons, covers surface of the brain and help form protective barrier called blood-brain barrier
Secrete nerve growth promote neuron growth and enhance synaptic develop
Ependymal Cell- line the inside cavities of the brain and assist formation of CNS

Neuron (second type of cell within the NS)

Enables the nervous system; vast communication network

Sensory Neurons: carry information to the CNS

Motor Neurons: carry information away from the CNS

Interneurons: found only in the CNS. Forms connection between sensory and motor neurons. Regulate motor activity

Parts of Neuron (Dendrites, Cell Body, and Axon)

Dendrites- treelike
Receive signals from other neurons; transmit signals to cell body
Cell Body
Contains the nucleus and is essential for life of the cell
Axon
Long extension
Transmits signals away from the cell body

The Axon: A Special Structure

Myelin Sheath
Protects and insulates the axon
Unmyelinated Neurons: neurons not incased in myelin
Schwann: surrounding the axon of neuron in PNS
Neurilemma: nuclei and cytoplasm Schwann cells lie outside myelin sheath

White Matter Vs. Gray Matter

Clusters of cell bodies located in the CNS referred to as nuclei
Small clusters of cell bodies in PNS called ganglia

The Nerve Impulse: What It Is

The nerve impulse (action potential)- electrical signals; conveys information along a neuron

*Two phases of action potential: Depolarization and Repolarization

Resting Membrane Potential- refers to the electrical charge
The inside resting neuro is more negative
Resting cell said to be polarized
The cell is quiet, or resting
Depolarization- The cell charges from negative to positive
Repolarization- very quickly
The inside of the cell again become negative
Returns to resting state or repolizes

Fig 20.6

Negative (-) 90 millivolt reading inside the unstimulated neuron) resting membrane potential
Threshold potential- degree of depolarization that must be attain for neuron to fully depolarize to +20mv

The Nerve Impulse: What Causes It

Chief intracellular cation (resting state)- potassium (K+)
Chief extracellular cation- sodium (Na+)

Depolarization

It is the rapid inward diffusion of the Na+ that causes depolarization

Repolarization (membrane permeability does two things)

It stops additional diffusion of Na+ into the cell
It allows K+ to rapidly diffuse out of the cell

The Nerve Impulse: What Causes It To Move

A nerve impulse (action potential) must move the length of neuron, from the cell body to the axon terminal

The Nerve Impulse: What Causes It To Move Quickly

Myelination increases that movement of the nerve impulse along axonal membrane
“Jumping” from node to node is saltatory conduction

Synapse Across Neurons- helps information move chemically from neuron to the next

Parts of a Synapse

Synaptic cleft- a space

Space exists because the axon terminal of neuron A (presynaptic neuron) does not physically touch the dendrite of neuron B (postsynaptic neuron)

Receptors- places on the membrane which the neurotransmitters bind
Neurotransmitters- contain thousands of tiny vesicles that store chemical substances
Inactivators- substances that terminate the activity of the neurotransmitters when completed task

Events at the Synapse

The nerve impulse travels along neuron A to its axon terminal
The vesicles open and release neurotransmitter into synaptic cleft
The neurotransmitter diffuses across the synaptic cleft and binds to the receptor site
Electrical information travels toward the cell body and axon of neuron B

Brain: Parts and Function

Brain

Location: Cranial cavity

Structure: Pinkish gray, delicate structure with soft consistency

“Boss it all” weighs only 3lbs
Brain weighs only 2% total body weight
Requires 20% of body’s oxygen supply
Primary energy source of the brain is glucose

Unchangeable brain gradually gave way to a different view, called neuroplasticity

The brain is divided into four areas

The cerebrum - The diencephalon
The brain stem - The cerebellum

Cerebrum

Cerebrum

Largest part of the brain
Divided right and left hemispheres
Cerebral hemispheres joined together by white matter form large fiber tract called corpus callosum

*Corpus Callosum allows right and left sides brain to communicate with each other

Cerebral Hemisphere four lobes: Four lobes named for the overlying cranial bones

Frontal, Temporal, Parietal, Occipital

Gray on the outside, White on the inside

Cerebrum contains gray and white matter
Thin layer of gray matter called cerebral cortex (forms the outermost portion of the cerebrum)
Cerebral cortex- composed cell bodies and interneurons; gray
Gray matter of cerebral cortex allows us to perform higher mental task such as learning, reasoning, language, and memory

Bulk of the cerebrum composed of white matter

Location: Directly below the cortex

White matter is composed primarily of myelinated axons

Form connections between parts of the brain and spinal cord

* Scattered throughout the white matter are the patches gray matter called nuclei

Marking of Cerebrum

Surface of cerebrum- folded into elevation that resemble speed bumps on a road

Elevations called- convolutions or gyri

Extensive folding- increases the amount of cerebral cortex/thinking tissue

Gyri

Separated by groove called sulci (deep sulsus called fissure)

Central Sulcus

Separates the frontal lobe from the parietal lobes
Separates precentral and postcentral gyri

Precentral Gyrus

Location: Frontal lobe; front of the central sulcus, primary motor area

Postcentral Gyrus

Location: Parietal lobe; behind of central sulcus, somatosensory area

* Lateral sulcus separates the temporal lobe from the frontal and parietal lobes

* Longitudinal fissure separates left and right cerebral hemispheres

Frontal Lobe

Frontal Lobe

Location: Front of the cranium under frontal bone

Voluntary motor activity, emotional and behavioral expression etc called executive functions

Homunculus

Amount of brain tissue; particular body part
Each body part controls the cerebral cortex of the precentral gyrus

Voluntary motor activity frontal lobe (key role in motor speech)

- Motor speech called Broca’s area (frontal eye field)

Parietal Lobe

Location: Posterior to the central sulcus

Receives general sensory information from the body

Primary Somatosensory Area (parietal lobe)- allows experience temp, pain,light touch

Receives sensations from the body
Receives information from the skin and muscles

Parietal Lobe

Reading, speech, and taste

Temporal Lobe

Temporal Lobe

Location: Inferior to the lateral fissure; above the ear

Contains primary auditory cortex- receives sensory information from the ears and allows you to hear
Receives information from the nose; olfactory area (sense of smell)

Wernicke’s Area (broad region)

Location: Parietal and temporal lobes

Concerned translation thought to words

Occipital Lobe

Occipital Lobe

Location: Posterior cerebrum, underlying occipital bone

Visual reflexes and vision-related (reading)
Sensory fibers from the eye

Functions Involving Many Cerebral

Speech Area

Location: temporal, parietal, and occipital lobes

Most people speech area located in left hemisphere

* Frontal lobe directs muscles: larynx, tongue, cheeks, and lips to speak

Association Areas

Association Areas- large areas of cerebral cortex

Concerned: analyzing, interpreting, and integrating information

Auditory Association Area- identify and give meaning to the sound

Auditory Cortex- hears the noise and interprets noise

Patches of Gray

Basal Nuclei- help regulate body movement and facial expression

Neurotransmitter dopamine responsible for activity of basal nuclei

Diencephalon

Diencephalon- second main area of the brain

Location: Beneath the cerebrum; above brain stem

Thalamus- serves as a relay station for sensory fibers (lower brain and spinal cord)

Hypothalamus- second structure in diencephalon

Helps regulate the function of autonomic (involuntary) nerves
Exerts effect on heart rate, blood pressure, and respiration

* Located under hypothalamus is pituitary gland- affects almost every hormone in the body

Brain Stem

Cyyonnects: spinal cord with higher brain structures
Composed of the midbrain, pons, and medulla oblongata

Midbrain

Extends from the lower diencephalon to the pons
Contains nuclei functions; vision and hearing

Pons (bridge)

Extends from the midbrain to the medulla oblongata
Information traveling to and from several brain structures
Regulation of breathing rate and rhythm

Medulla Oblongata

Connects: spinal cord with the pons
Sensory and motor information
Sensitive to certain drugs- morphine

Vomiting Center

Indirect stimulation of vomiting center- from chemoreceptor trigger zone

Location: floor of the fourth ventricle (CTZ)

Cerebellum (fourth major area)

Protrudes from under the occipital love
Connected to the brain stem
Connections allow cerebellum to receive, integrate, and deliver information
Voluntary muscle activity and maintaining equilibrium and posture

Structures Across Divisions of the Brain

- limbic system, the reticular system, and memory areas

Limbic System: The Emotional Brain

- functions in emotional states and behavior

Reticular Formation (special mass of gray matter)

extending entire brain and diencephalon
Primary sensory function- to alert the cerebral cortex
Primary motor function- regulate muscle tone

Reticular Activating System: Wakefulness, Sleep, and Coma

Sleep-wake cycle
Cerebral cortex from RAS keeps us awake

Memory Areas

Three categories: immediate memory, short-term memory, long-term memory

Protecting the Central Nervous System

Tissue of CNS is very delicate

Four structures: bone, meninges, cerebrospinal fluid, and blood-brain barrier

Bone: First Layer of Protection

CNS protected by bone
Encased in the cranium, and spinal cord is encased in vertebral column

Meninges: Second Layer of Protection

Three layers of connective tissue surround the brain and spinal cord

Subarachnoid Space- between arachnoid layer and pia mater

Cerebrospinal Fluid- circulates within space and forms cushion → brain and spinal cord

Meninges from a brain PAD
Infection can spread to the brain

P- pia mater

A- arachnoid matter

D- dura matter

Cerebrospinal Fluid: Third Layer of Protection

CSF is formed from the blood within the brain
Composed of water, glucose, protein, and several ions, especially Na+ and Cl-

Choroid Plexus- grapelike collection of blood vessels and ependymal cells

where CSF formed
Four ventricles are two lateral ventricles and third/fourth ventricles

Blood-Brain Barrier: Fourth Layer of Protection

Act as a barrier of potentially harmful chemicals into CNS
Astrocytes and capillary cells select the substances allowed to enter CNS to the blood

CH 11 Nervous System: Spinal Cord and Peripheral Nerves

Sensory pathway: information traveling from the periphery to the brain

Motor pathway: information coming from the brain and going to the periphery

Reflex center: spinal cord acts as major reflex center

What the Spinal Cord is

Location and Size

Spinal cord: continuation of the brain

Tubelike structure
Located within spinal cavity
Spinal cord is about 17 inches; ends at L1

Gray on the Inside, White on the Outside

Gray Matter

Located in the center
Shaped-like butterfly
Composed primarily cell bodies, interneurons, and synapses

* Two projections of gray matter: dorsal (posterior) horn and ventral (anterior) horn

CSF circulates throughout subarachnoid space surrounding brain and spinal cord

White Matter

Composed of myelinated and unmyelinated axons
Neuronal axons grouped sensory and motor tracts

What the Spinal Cord does

Sensory tract carry information from the periphery, up spinal cord; toward brain (ascending tracts)
Spinothalamic- carries sensory information for touch, pressure, and pain from the spinal cord to the thalamus (brain)
Motor tracts carry information from the brain, down spinal cord; toward periphery (descending tracts)

* Major descending tracts: the pyramidal and extrapyramidal tracts

Decussation

Tracts decussate or cross over from one side to another

- Spinal cord severed neck region, trunk, and all four extremities condition called: quadriplegia

- Spinal cord injury is lower, involving only the lumbar region of the spinal cord: paraplegia

What Reflexes Are

Reflex- involuntary response to stimulus
Spinal cord plays crucial role in reflex activity

The Reflex Arc

Nerve pathway involved in reflex

Sensory receptor- stimulates sensory receptors in the anterior thigh muscles (quadriceps femoris)
Afferent or sensory neuron- nerve impulse carried from the receptors along a sensory neuron to the spinal cord
Efferent or motor neuron- nerve impulse carry by a motor nerve to the muscles of thigh
Effector organ- response to the motor nerve impulse, the muscles and extend the leg at the knee

The Withdrawal Reflex

Helps protect you from injury

Organ Reflexes

Help regulate organ function

Peripheral Nervous System

Consists of the nerve and ganglia
Located outside CNS

Nerves (single nerve cell)

Wrapped in connective tissue
Located outside the CNS

Sensory nerves- composed only sensory neurons

Motor nerves- composed only motor neurons

Mixed nerves- both sensory and motor neurons

* Peripheral nervous system classified in two ways:

structurally (by the anatomy) or functionally (according to what they do)

Structural Classification of the Peripheral Nervous System

Divides the nerves into cranial and spinal nerves

Names and Numbers of Cranial Nerves

Cranial Nerves

Exit the brain
Supply head, neck, and shoulder areas
Ten of 12 cranial nerves originate brain stem

Functions of Cranial Nerves

Sensory information special senses: smell, vision, taste, hearing, and balance
Sensory information general senses: touch, pressure, pain, temp, and proprioception
Motor information results om contraction of skeletal muscles
Motor information results in secretion of glands/contraction of cardiac and smooth muscle

*Location of each Cranial Nerve

CN I, olfactory nerve
- Carries information from temporal and parietal lobes of cerebrum
- Concerned with sense of smell
CN II, optic nerve
- Carries visual information from the eye to visual cortex
CN III, oculomotor nerve
- Causes contraction most of extrinsic eye muscles
- Located close to hard tentorium with cranium
CN IV, trochlear nerve
- Nervates one of extrinsic muscles of eyeball (helping eyeball to move)
CN V, trigeminal nerve
- Carry information from the face, scalp, eye, and teeth to the brain
Cranial VI, abducens nerve
- Like the trochlear, controls eye movement by innervating the extrinsic eye muscles
Cranial VII, facial nerve
- Performs mostly motor functions
Cranial VIII, vestibulocochlear nerve
- Carries information from hearing and balance from inner ear
- Responsible for equilibrium and hearing
Cranial IX, glossopharyngeal nerve
- Taste sensation from the posterior tongue to the brain
- Motor fibers stimulate secretion of salivary glands in mouth
- Associated with gag reflex
Cranial X, vagus nerve
- Innervates the organs in thoracic and abdominal cavities
Cranial XI, accessory nerve
- Sternocleidomastoid and trapezius muscles
- Controlling head and shoulder region movement
Cranial XII, hypoglossal nerve
- Controls movement of tongue

Spinal Nerves

Attached to the spinal cord

Spinal Cord by two roots: the dorsal root and ventral root

Ventral root is composed of motor fibers

Names and Numbers of Spinal Cord (31 pairs are grouped)

8 pairs cervical nerves
12 pairs thoracic nerves
5 pairs lumbar nerves
6 pairs sacral nerves
1 pair coccygeal nerves
Cauda equina: horse-like tail
Foramina: tiny holes in vertebrae

Spinal Nerve Plexuses

Three major nerve plexuses: cervical plexus, brachial plexus, and lumbosacral plexus
Cervical plexus- fibers from the cervical plexus supply muscle and skin of neck
Phrenic nerve- stimulates the contraction of diaphragm, major breathing muscle

* Brachial plexus- emerge from the brachial plexus supply the muscles and skin of shoulder, arm, forearm, wrist, and hand

* Lumbosacral plexus- gives rise to nerves, supply muscles and skin of lower extremities

What a Dermatome is

Sensory thing
Spinal nerve innervates certain area of skin → dermatome

Functional Classification of the Peripheral Nervous System

Afferent nerves- bring sensory information from different parts of the body
Efferent nerves- bring information from the CNS to skeletal muscles throughout the body
Autonomic nervous system- supply the organs, smooth muscle, and glands

CH 12 Autonomic Nervous System

Autonomic = automatic!

Autonomic (Visceral) Reflexes

What They Do

Knee-jerk is mediated by somatic nerves
Visceral reflexes control: heart rate, BP, body temp, and pupillary responses

Pathway

Visceral reflex

Activation of receptor, transmission of sensory information to CNS

Division of the Autonomic Nervous System

ANS part of PNS
Glands, smooth muscles within the organs tubes, and heart

* Two divisions of the ANS: Sympathetic and Parasympathetic Nervous System

* Divisions of ANS called dual innervation

Sympathetic Nervous System: Fight or Flight

Sympathetic Nervous System (fight-or-flight)

- Causes you to prepared either to confront or away from threatening situation

- Activates during period of stress

Parasympathetic Nervous System: Feed and Bree

- Most active during quiet, nonstressful conditions (calming effect on the body)

- Regulation of digestion and reproductive function

- Resting and digesting

Autonomic Terminology and Pharmacology

Sympathomimetic

- Increases heart rate, force of cardiac contraction, and BP

Parasympathomimetic

- Decreases heart rate and increases digestive activity

Autonomic Tone and Vasomotor Tone

- Parasympathetics and Sympathetics are active at the same time

- Background autonomic activity → autonomic tone

- Parasympathetic heart rate: 72 beats/min

- Symohathetically induced of blood vessels constrict → vasomotor tone or sympathetic tone

Numbers of Ganglia

Cell body of neuron 1 is located in CNS (brain or spinal cord)
Axon of neuron 1 → preganglionic fiber
Axon neuron 2 leaves the ganglion and target organ → postganglionic fiber

*Postganglionic fiber: key to understanding autonomic function

Neurons of the Sympathetic Nervous System

Thoracolumbar column- neurons of SNS leave spinal cord and lumbar levels

Synapse within ganglia located close to the spinal cord
Sympathetic ganglia form chain called paravertebral ganglia or sympathetic chain ganglia
Paravertebral ganglia provide site where each preganglionic fiber synapses

Neuros of the Parasympathetic Nervous System

Parasympathetic nervous system (craniosacral flow)

- Ganglia of PNS located close to or within target organs

Running with Cranial Nerves

Parasympathetic fibers travel from the brain stem- four cranial nerves

- Oculomotor (CN III) -Glossopharyngeal (CN IX)

- Facial (CN VII) - Vagus (CN X)

Oculomotor Never (CN III)

Carries parasympathetic fibers to two intrinsic eye muscles:

constrictor muscle of the eye- causes pupillary constriction

ciliary muscle- controls the shape of the eye lens

Facial Nerve (CN VII)

Facial nerve carries parasympathetic fibers to the tear glands, salivary glands, nasal glands

Glossopharyngeal Nerve (CN IX)

Carries parasympathetic fibers to the salivary glands in the mouth

Vagus Nerve (CN X)

Carries over 80% of parasympathetic fibers
Travels from the brain stem to organs within thoracic and abdominal cavities

Naming Fibers and Neurotransmitters

*Two major neurotransmitters of ANS are acetylcholine (ACh) and norepinephrine (NE)

Fiber that secretes NE called adrenergic fiber
Fiber that secretes ACh called cholinergic fiber

Neurotransmitters: Termination of Activity

ACh secreted by cholinergic fiber and diffuses to its receptor
NE secreted by adrenergic fibers
Termination of NE called reuptake
Degraded by another enzyme found surrounding tissue → catechol-O-methyltransferase

Receptors of the Autonomic Nervous System

Cholinergic receptors activated by ACh

* Two types of Cholinergic receptors: muscarinic receptors and nicotinic receptors

Muscarinic receptors

Located on effector or target organs (cardiac muscle, smooth muscle, glands)
Nicotinic neutral receptors located within ganglia ANS

Adrenergic Receptors

Two types of adrenergic receptors

alpha- (a) adrenergic and beta- (B) adrenergic receptors

alpha1-, alpha2-, beta-1, and beta-2 adrenergic receptors
Adrenergic receptors located on target organs of sympathetic nerves

CH 13 Sensory System

The sensory system acts as a warning system

Receptors and Sensation

Cells that Detect Stimuli

Sensory neurons transmit information to CNS

Five types of sensory receptors

Chemoreceptors- stimulated by changes chemicals; hydrogen(H+), calcium, and food
Pain receptors (nociceptors)- stimulated by tissue damage or distention
Thermoreceptors- stimulated by changes in temperature
Mechanoreceptors- stimulated by changes in pressure or movements body fluid
Protoreceptors- stimulated by light

* Others may taste shape/words, others see musical sounds- neurologic intertwining senses called synesthesia

Experiencing a Sensation

Four Components

Stimulus- light stimulus for the sense of sight. Absence of light, you cannot see
Receptor- light waves stimulate the photoreceptors in eye, producing nerve impulse
Sensory Nerve- nerve impulse conducted by a sensory nerve to the lobe of the brain
Special area of the brain- interpreted as sight in the occipital love of the brain

Two Characteristics of Sensation

Two characteristics of sensation: projection and Adaption

Projection describes process of the brain, after receiving sensation back to its source
Sensory adaption illustrated by the sense of smell

Two groups of senses: general and special

General senses- receptors widely distributed throughout the body
Special senses- localized within particular organ in the head

* Special senses include- taste, smell, sight, hearing, and balance

The General Senses

Pain, touch, pressure, temp, and prorioception
Receptors general senses found in the skin, muscles, joints, and viscera

Pain (protective function)

Receptors of pain called nociceptors, consist of free nerve ending; stimulated by tissue damage
Pain receptors distributed throughout the skin and other internal tissues
If blood supply internal organ is diminished- condition called ischemia
When pain feels coming from where it originates called referred pain
Body travel up the spinal cord in a sensory tract called spinothalamic tract

Touch and Pressure

Receptors for touch and pressure are mechanoreceptors; respond to move, press or deform tissue
Touch receptors also called tactile receptors found in the skin; allows us to feel
Receptors for heavy pressure located in the skin, subcutaneous tissue, and deep tissue
Touch first sensory to develop in fetus (growth and development)

Temperature

Thermoreceptors found on free nerve endings and scattered throughout the body
Cold receptors: between 50F and 76F
Heat receptors: between 76F and 112F

Proprioception

Sense of orientation or position
Allows you to locate body part without looking at it
Receptors for proprioception called proprioceptors- located in muscles, tendons, joints
Proprioceptors also found in the inner ear where they function balance or equilibrium
Cerebelum- coordinating skeletal activity, receives sensory information from receptors

The Special Senses

Smell, taste, sight, hearing, and balance
These receptors located in the head

Sense of smell: The Nose

Smell (olfaction) associated with sensory structures
Located in the upper nose
Olfactory receptors located in the high nose, where the air circulation is poor

Sense of taste: The Tongue

Sense of taste often called social sense
Taste receptors located on the tongue (chemoreceptors)
Small number of taste buds located on palate, tonsils, and throat
Tip of the tongue mostly sensitive to sweet and salty substances
Sour sensations found on the side of the tongue
Bitter substances tasted on the back of the tongu
* When taste receptors stimulated, taste impulses travel along cranial nerves (facial, glosopharyngeal, and vagus nerves)

Sense of sight: The Eye

Study of the eye: ophthalmology

Visual Accessory Organs

Eyebrows, eyelids, conjunctiva, eyelashes, lacrimal apparatus, and extrinsic eye muscles

Eyebrows

Eyebrows, patches of hair located above the eyes, perform a protective role

Eyelids (palpebrae)

Protects the eyes
Corners are called medial (inner) canthus and lateral (outer) canthus

Eyelids four layers

skin, skeletal muscle, connective tissue, and inner lining called conjunctiva

Conjunctiva

Think mucous membrane likes the inner surface of the eyelids
Very vascular; has many blood vessels (pink eye)

Lacrimal Apparatus

Secretion, distribution, and drainage of tears
Blinking stimulates lacrimation and helps spread tears over the eye surface

Exrinsic Eye Muscles

Visual accessory organ

The Eyeball

Eyeball has spherical shape approximately ¾ inch to 1 inch
Most eyeball sits bony orbital cavity of the skull surrounded layer of orbital fat
Eyeball is recessed and well-protected

Eyeball is composed three layers: the sclera, the choroid and retina

Layers of the Eyeball

Outermost layer is called sclera
Tough fibrous connective tissue that covers of the eyeball
Helps contain the contents of the eye
Sclera extends toward the front of the eye
Anterior sclera covered by conjunctiva is called white of the eye

Choroid

Middle layer of the eye

Provides the retina with rich supply of blood
Dark pigments located in the choroid absorb any excess light to prevent glare

Retina

Innermost layer of the eyeball
Lines the posterior two-thirds of the eyeball

Two types photoreceptors: rods and cones

Rods- scattered throughout the retina, abundant along its periphery
Cones- most abundant in the central portion of the retina (provides color vision)

* Bulging optic disc seen on ophthalmic examination called choked disc or papilledema

Cavities and Fluids

Two cavities in the eyeball: the posterior and anterior cavities

Posterior cavity located between the lens and the retina
Anterior cavity located between the lens and the cornea

Aqueous humor performs two functions:

Maintains the shape of the anterior portion of the eye
Provides nourishment for the cornea

Tiny canals located at the junction f the sclera and the cornea
Outlet canals are called venous sinuses or the canals of Schlemm

Muscles of the Eye

Two groups of muscles associated- extrinsic and intrinsic eye muscles

Extrinsic eye muscles move the eyeball in its bony orbit

Intrinsic eye muscles- move structures within the eyeball

Intrinsic Eye Muscles

Smooth muscles
Located in the eyeball

Muscles of the iris

the radial muscle and circular muscle
Second muscle located in the iris
Parasympathetic nerve stimulates causes pupillary constriction, or miosis

Ciliary Muscle

Third intrinsic eye muscle
Located the ciliary muscles
Ciliary muscles attach to the suspensory ligaments

Pathway of Light

Light waves must enter the eye and focus on retina
Each structure of this pathway must retain its ability to allow for the passage of light

Refraction and Accomodation

Ability of the lens to change its shape to focus on a close object
Emmetropia refers to the ability of the eye to refract life

Stimulation of Photoreceptors

Must stimulate the photoreceptors (rods and cones)

Night Vision

Rods widely scattered throughout the retina (more abundant in periphery)
Dilated pupil allows the light ray to scatter along periphery of the retina

Color Vision

Cones are the photoreceptors for color vision
Most abundant in central portion of retina

Informing the Brain: The Visual Pathway

Nerve impulses travel along fibers optic nerves and optic tracts to occipital lobe of brain
Left and right eyes crisscross called chiasm- located directly in front of pituitary gland

Structure of the Ear

Ear divided into three parts: external, middle, and inner

External ear

- composed of the auricle and external auditory canal (cartilage, layer of fitting skin)

- passageway allowing sound to waves to enter the ear, hollowed temporal bone

1 inch long and ½ wide

Middle Ear

- Small air-filled chamber located in between tympanic membrane, end bony wall

Structure

tympanic membrane, tiny bones, several small bones, and eustachian tube
Passageway connecting to pharynx or throat

Inner Ear

Consist of an intricate system of tubes passageways
Membranous labyrinth filled thick fluid called endolymph
Cells within the organ of Corti contain tiny hair

Sense of Balance: The Ear

Inner ear three parts: vestibule, semicircular canals, and cochlea

Cochlea- hearing
Vestibule and semicircular canals- equilibrium or balance
Receptors for balance located within semicircular and vestibule

Two Types of equilibrium or balance: static and dynamic equilibrium

Static senses position of head and maintains posture (no sudden movement)
Detects sudden movements of the head and maintain posture while moving/rotating

Static Equilibrium

Ability to maintain steady position of the head and body in gravity

Dynamic Equilibrium

Three semicircular canals are organs of dynamic equilibrium respond sudden movement
located at right angles to each other
Semi circular canals is ampulla: base of each ampulla is small elevation → crista
Cerebellum analyzes the input from CN VIII and sends information to skeletal muscles, maintain balance and posture