Download Keshara Senanayake Page # 1 -an individual nerve cells is called

Keshara Senanayake
Page # 1
-an individual nerve cells is called the "neuron"
-Each neuron performs (4) functions
1) receive information from the internal or external environment or from other neurons
2) integrate the information it receives and produce an appropriate output signal
3) conduct the signal to its terminal endings
4) transmit the signal to other nerve cells or to glands/muscles
-(4) structural regions in a neuron. 1) dendrites 2) cell body 3) axon 4) synaptic terminals
1) dendrites -> branched terminals that extend outward from a nerve cell body -> responds to signals from
other neurons or from external environment --> they receive information
>in neurons of the brain and spinal cord, dendrites respond to the chemical neurotransmitters released by
other neurons
>these dendrites have protein receptors in their membrane that bind to specific neurotransmitters -->
produces electrical signals
Dendrites of sensory neurons --> special membrane adaptations to produce electrical signals in response to
specific stimuli
>electrical signals --> dendrite --> converge to neurons cell body (integration center) [takes sum of signals
if sum > threshold] --> produces action potential
>cell body has many organelle that make complex molecules and coordinate metabolic activities of the cell
-in a neuron, a long thin fiber called an "axon" --> extends outward from cell body --> conducts electricity
>neurons are the longest cells in the body
-axons are distributing lines --> carry action potentials from cell body to the synaptic terminals, located at
the far end of each axon
>axons are bundles to form "nerves)
-plasma membrane of axon conduct action potentials undiminished from the cell body to their synaptic
terminals
>axons are wrapped with insulated called myelin --> allows electrical signal to be conducted more rapidly
>formed from non-neural cells that wrap around the axon
>transmission of the signal to other cells occurs at the synaptic terminals --> swelling at terminal branches
>synaptic terminals contain neurotransmitters --> chemical releases in response to action potential reaching
the terminal. -->synaptic terminal of one neuron communicated with a gland, a muscle cell, or dendrites of
cell body of a 2nd neuron
>the output of the first becomes the input of the 2nd cell --> the site at which synaptic terminals
communicate with other cells is called the synapse
-unstimulated inactive neurons maintain a constant electrical difference "potential" across their plasma
membranes
>this potential is called "resting potential" --> from -40 t -90 mV
>if neuron is stimulated the (-) inside can be made either more or less negative.
>if potential is made much less (-) that it reaches threshold --> action potential triggered
>during action potential --> potential rapidly rises to about +50 mV inside cell --> lasts few milli seconds
before the cell of restored
>(+) charge of the action potential flows rapidly down the axon to the synaptic terminal --> signal is
communicated to another cell at the synapse
--> once action potential reaches the synaptic terminal or a neuron --> signal must be transmitted to another
neuron, muscle cell, or a glad --> this occurs at synapses
-> signals transmitted at synapses are called postsynaptic potentials
>synapse terminals -> action potential encounters a synapse, where parts of two neurons are specialized to
communicate to one another
>tiny gap separates the synaptic terminal of the first neuron (presynaptic neuron) from the second neuron
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(postsynaptic neuron).
>both dendrites and the cell bodies of neurons are covered with synapses
>synapses include the synaptic terminal of the presynaptic neuron, the gap, and the specialized membrane
of the post synaptic neuron (contains receptors for neurotransmitters)
-when action potential reaches synaptic terminal --> inside of terminal becomes (+) --> charge causes
storage vesicles in the synaptic terminal to release neurotransmitters into the gap between the cells
>the neurotransmitter molecules rapidly diffuse across the gap and briefly bind to receptors in the
membrane of the postsynaptic neuron before diffusing away and being taken back into the presynaptic
neuron
-exicitory or inhibitory postsynaptic potentials are produced at synapses and summate in the cell body
>receptors proteins in the postsynaptic membrane bind to neurotransmitters
>causes specific types of ion channels in postsynaptic membrane to open --> ion flow across plasma
membrane along concentration gradients -> flow of ions in postsynaptic neuron causes small (brief)
changes in electrical charge called the postsynaptic potential (PSP)
>type of PSP depends on type of channel opened and movement of ions through them
>EPSP (excitory) make neurons less (-) and more likely to produce action potential
-EPSP open Na+ channels, allowing more Na+ ions to flow into neuron --> closer to threshold
>IPSP (inhibitory) make neuron more (-) and less likely to produce action potential
-ISP open K+ channels allowing K+ to flow out
>post synaptic potentials are small, rapid signals --> travel far enough to reach cell body
>they determine is an action potential will be produced
>dendrites and cell body of a single neuron receive IPSP and EPSP but synaptic terminals of 1000's of
presynaptic neurons. Postsynaptic body does a summation. If IPSP and EPSP added together > than
threshold --> action potential produced
--- "A closer look" at ions and electrical signal
-The resting potential is based on a balance between chemical and electrical gradients, maintained by active
transport and permeability of the membrane
>ions of the cytoplasm is mainly (+) charged K+ and large (-) organic molecules [proteins] which cannot
leave the cell
>outside the cell, extracellular fluid has Na+ and (-) Cl-. Concentration differences maintained via
specialized membrane protein called a Na+/K+ pump --> pumps K+ into and Na+ out of cell
-unstimulated neuron --> K+ can only cross membrane by traveling through specific membrane proteins
(potassium channels). Na+ channels are closed. K+ goes down concentration gradient and diffuses out
>the neuron has a (-) charge at rest become the inside the negative organic molecules remain
-neuron signals are carried long distanced by action potentials. Action potentials occur if the PSP brings
potential in neuron to threshold.
>at threshold Na+ channels are opened allowing Na+ to flow in --> soon after they open they close and K+
channels are opened allowing K+ to flow out of the cell and restore negative resting potentials
>an action potential is fasting moving, and doesn't diminish in size along the axon
>(+) charge carried into the axon by Na+ causes Na+ channels farther along the axon to open. More Na+
can flow and more channel farther down the axon are opened, and the action potential continues along the
axon
-as wave of (+) charge passes a given point, the resting potential is restored as K+ flows out
-action potentials are "all or none" reach threshold = only way for action potential
>synaptic terminal contains many neurotransmitter-filled vesicles ---> action potential enters synaptic
terminal --> vesicles release their neurotransmitter into the space between the neurons --> neurotransmitter
diffuses rapidly across the gap, binds to postsynaptic receptors and causes ion channels to open --> ions
flow through these open channels causing postsynaptic potential in the postsynaptic cell
>individual neurons use action potential let animals do complex things
-Nervous system has to be able to do (4) things
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1) determine the type of stimulus 2) signals the intensity of a stimulus 3) integrate information from many
sources 4) initiate the direct response
>nervous system is able to identify the type of stimulus
>the nervous system monitors which neurons produce the action potential --> brain interprets action
potentials that occur in the axons of your optic nerves as sensation of light --> action potentials in olfactory
nerves as orders ect
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>drugs activate the "reward circuitry" of the brain
>synapses in the brain that use the neurotransmitters dopamine, serotonin, or norepinphrine contribute to
out energy level and our overall sense of well-being
>normally presynaptic neuron after releasing a neurotransmitter immediately starts to pumping it back in -> limiting effects
-drugs like cocaine work by blocking this pump mechanism
>as a result the neurotransmitter remain in their synapses much long and reach high levels --> fells
euphoria and energetic until the brain attempts to reduce the impact of the drug --> the postsynaptic neuron
decreases its number of receptors for these neurotransmitters --> fewer receptors = higher levels of
neurotransmitters are needed to feel normal --> when drug is withdrawn the postsynaptic neurons are
inadequately stimulated and user feels a crash --> needs more and more
___________________________________________________________
>all action potentials are the same magnitude and duration
>intensity of coded in two ways 1) intensity can be signaled by the frequency of action potentials in a
single neuron --> more intense stimulus the faster the neuron produces action potential (or fires)
2) stronger stimuli tend to excite more neurons, where weaker stimulate fewer
>brain is bombarded by sensory stimuli from inside and outside of the body
>brain must filter all these inputs --> determine which ones are important
-nervous system uses convergence, many neurons funnel their signals to fewer neurons
>many sensory neurons many converge onto a small number of brain cells
-these brain cells add up postsynaptic potentials that result from the synaptic activity of the sensory neurons
(depending on their relative strengths --> and produce appropriate output
-actions directed by the brain many involve many parts of the body.
>these actions require divergence, the flow of electrical signals from a relatively small # of decision
making cells onto many different neurons that control the activity of muscles or glands
-neural pathways direct behavior
>neuron to muscle composed of four elements
1) sensory neurons --> respond to stimulus (internal or external)
2) associative neurons --> receive signals from many sources (including sensory neurons, hormones,
neurons that store memories, and many others) --> they active motor neurons
3) motor neurons --> receive instruction from associative neurons --> activate muscles
4) effectors --> muscles or glands that perform the response directed by the nervous system
-simplest type of behavior in animals is the reflex
>largely involuntary movement
-occurs without conscious portion of the brain
--> many occur entirely within the spinal cord and peripheral neurons
>by integrating postsynaptic potentials from several sources, the associative neurons can decide what to do
and can stimulate motor neurons to direct the appropriate activity in muscles and glands
>in animal kingdom mainly (2) nervous systems 1) diffuse nervous system (in hydra) and centralized
nervous system (humans)
-radially symmetrical = no "front end" = hydra
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-most animals are bilaterally symmetrical with a definite head and tail ends
-"cephalization" --> "head' "brain" increase in complexity
-vertebrate nervous system can be divided into two parts: central and peripheral nervous systems
>central nervous system --> brain and spinal cord. It extends down the dorsal torso.
>peripheral nervous system --> consists of nerves that connect the CNS to the rest of the body
>PNS consists of peripheral nerves --> link brain and spinal cod to the rest of the body (including muscles,
sensory organs, reparatory, excretory, and circulatory systems)
>within peripheral nerves are axons of sensory neurons that bring sensory information to the CNS from all
parts of the body
>peripheral nerves also contains axons of motor neurons that carry signals from the central nervous system
to organs/muscles
-motor potion of the PNS (PNS is sensory portion and motor portion) can be divided into two parts -->
somatic and autonomic nervous system
>motor neurons of the somatic nervous system form synapses on skeletal muscles and control voluntary
movement
>cell bodies of somatic motor neurons are located in the gray matter of the spinal cord and their axons go
directly to the muscles they control
>motor neurons of the autonomic nervous system control involuntary responses
>form synapses on heart, smooth muscle, and glands
>autonomic nervous system is controlled primarily by the hypothalamus of the brain
>has two divisions: sympathetic division and the parasympathetic division
>two divisions of the autonomic nervous system generally make synaptic contacts with the same organ but
usually produce opposite effects
-Sympathetic visions releases neurotransmitter norepinphrine --> onto target organs --> during fight/flight
-Parasympathetic releases acetylcholine onto its target organs, dominates during maintenance of activities
>two major difference between these two.
-in Sympathetic division, the synapse occurs in sympathetic ganglia near the spinal cord
-in parasympathetic division, the synapse occurs in smaller ganglia located on or very near each target
organ
-2nd their nerves originate at different levels of the CNS
-parasympathetic nerves --> Pons and medulla at the base of the brain (also the lowest region of the spinal
cord)
-all sympathetic nerves emerge from the middle regions of the spinal cord
-spinal cord and brain make up CNS
>this portion of the nervous system receives and processes sensory information
>CNS consists of primarily associative neurons
>brain and spinal cord protected in (3) ways
>1) skull -> surrounds brain and vertebral column --> protects the spinal cord
2) beneath the bones lies a triple layer of connective tissues called the meninges
>between layers of meninges the cerebrospinal fluid (clear liquid like blood plasma) cushions brain and
spinal cord as it nourishes the cells of the CNS --> 3) delicate cells of the brain are also protected from
potentially damaging chemicals that reach the bloodstream because brain capillaries are far less permeable
(blood-brain barrier)
-spinal cord is a neural cable that extends from the base of the brain to the lower back
>protected by bones of the vertebrae column
>between vertebrae, nerves carry axons of sensory neurons and motor neurons arise from the dorsal and
ventral portions of the spinal cord, respectively --> merge to form PNS of the spinal cord (part of the PNS)
>in the center of the spinal cord is the butterfly shaped area of GRAY MATTER --> consists of cell bodies
of several types of neurons that control voluntary muscles and the automatic nervous system + neurons that
communicate with the brain and other parts of the spinal cord
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>gray matter is surrounded by white matter --> containing myelin-coated axons of neurons that extend up
or down the spinal cord
>these axons carry sensory signals from internal organs/muscles/skin to the brain
>axons also extend downward from the brain, carrying signals that direct the motor portions of the PNS;
the motor neurons of the spinal cord also control the muscles involves in conscious voluntary activities -->
that are directly activated by motor portions of the brain
>spinal cord contains neural pathways for certain simple behaviors such as reflexes
>the cell bodies of the sensory neurons from the skin are located in the dorsal root ganglia
>clusters of neurons on spinal nerves just outside the spinal cord
>both associative neurons and motor neurons are found in gray matter in the center of the spinal cord
>associative neurons not only form synapses on motor neurons --> also have axons that extend up to the
brain
>signals carried along these axons alert the brain to the painful event --> brain sends impulses down axons
in white matter to cells in gray matter
>these signals can modify spinal reflexes --> you can suppress pain withdrawal with enough motivation
-all the neurons and interconnections needed for basic movements of walking and running are contained in
the spinal word
>brain's role is semi-automatic behavior that initiate, guide, and modify the activity of spinal motor neurons
>for balance --> brain uses sensory input from muscles --> command motor neurons to adjust way muscles
move
white matter = contains myelinated axons
central canal = contains cerebrospinal fluid
gray matter = contains cell bodies of motor and associative neurons
dorsal root = contains axons of sensory neurons
dorsal root ganglion = contains cell bodies of sensory neurons
ventral root = contains axons of motor neurons
-vertebrate brain has 3 main parts
1) hindbrain 2) midbrain 3) forebrain
>hindbrain is comprised of the medulla, the Pons, and the cerebellum
>medulla is a lot like a extension of the spinal cord
>the medulla also has neuron cell bodies at its center surrounded by a layer of myelin-covered axons; it
controls several automatic functions such as breathing, heart rate, blood pressure, and swallowing
>certain neurons called "Pons" --> located above medulla --> influence sleep and wakefulness between
sleep stages
>cerebellum -> coordinating movements of the body --> receives information from command center in the
conscious areas of the brain that controls movement --> also from position sensors in muscles and joins
>by comparing info from two sources they cerebellum guides smooth motions --> cerebellum is also
involved in learning and memory storage for behaviors
-midbrain contains an auditory relay center and a center that controls reflex movements of the eyes, as well
as portions of the reticular formation
>the neurons of this is an important relay center that extends all the way from the central core of the
medulla to lower regions of the forebrain
>it receives input from all sense, every part of body, and many areas of the body
>plays a role in sleep/wakefulness
--> it filters sensory inputs before they reach the conscious regions of the brain, although the selectivity of
the filtering varies
>forebrain is also called the cerebrum --> includes the thalamus, limbic system, and the cerebral cortex
>in mammals the cerebral cortex is enlarged.
>the thalamus is a complex relay center that channels sensory information from all parts of the brain to the
limbic system and cerebral cortex
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>signals from the cerebellum and limbic system back to the cerebral cortex are also channeled here
>limbic system is a diverse group of structures
-->located between arc between the thalamus and cerebral cortex
--> work together to produce our most basic and primitive emotions, drives, and behaviors
--> including fear, rage, calm, and sexual responses
>portion of limbic system is also important for formation of memories
>limbic system includes the hypothalamus, the amygdala and the hippocampus
-the hypothalamus --> contains many clusters of neurons
>some neurosecretory cells that release hormones into the blood; others control the release of a variety of
hormones from the pituitary gland
>other regions direct activities of the autonomic nervous system --> the hypothalamus through its hormone
production and neural connections acts as a major coordinating center, maintaining homeostasis in a variety
of ways
>amygdala --> produce pleasure, fear, sexual arousal when stimulated
>hippocampus --> variety of emotions
>also plays an important role in the formation of long term memory
>in humans the largest part of the brain is the cerebral cortex (outer layer of the forebrain)
>cerebral cortex and its underlying parts of the forebrain are divided into
--> two halves called cerebral hemispheres and communicate via the corpus collosum
-> cortex is folded into convolutions which increases its area
>in cortex cell bodies of neurons predominate--> giving this layer a gray color --> "gray matter" --> these
neurons receive sensory information, process it, create memories for future use --> ect
>cerebral cortex is divided into 4 anatomical regions
1) frontal 2) parietal 3) occipital 4) temporal
>cortex contains primary sensory areas --> regions where signals originating in sensory organs such as eyes
and ears are received and converted into subjective impressions
>nearby associative areas interpret sounds as speech or music
>associative areas also link the stimuli with memories stored in the cortex and generate commands to
produce speech
>primary sensory areas in the parietal lobe interpret touch that originates all over the body
>an adjacent area to the frontal lobe, primary motor areas command movements in corresponding areas of
the body by stimulating motor neurons that from synapses with muscles allowing you to do things
>like the primary sensory area the primary motor area which seems to be responsible for directing the
motor area to produce more-complex movements.
>behind the bones of the forebrain lies another association area of the frontal lobe, which is important in
complex reasoning function --> decision making/predicting consequences
Frontal lobe = primary motor area/premotor area/higher intellectual function/speech motor area
Parietal lobe = primary sensory area/sensory association area
Occipital lobe = visual association area/primary visual area
Temporal lobe = memory, primary auditory area, auditory association, language comprehension
Left hemisphere: controls right side of the body, input from right visual field, right ear, and left nostril,
center for language/mathematics
Right hemisphere: controls left side of the body, input from left visual field, left ear, and right nostril,
centers for spatial perception, music, creativity
- for uncontrollable epilepsy they severe the corpus collosum
- brain functions as a coordinated unit
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-at rest your brain is responsible for 20-25% of your total energy consumption
-working memory is followed by long term memory
-working memory (short term) is electrical in nature
>involving in the repeated activity of a particular neural circuit in the brain
-as long as the circuit is active the memory stays
>in other cases working memory involves temporary biochemical changes within neurons of a circuit -->
resulting in stronger synaptic connections between them
-long term memory is structural --> requires formation of new, long lasting synaptic connections between
specific neurons
-working memory can be converted into long term memory --> seems to involve the hippocampus and then
transfer them to the cerebral cortex for permanent storage
>temporal lobes of the cerebral hemispheres are important in the retrieval (recall) of long term memories
MRI =magnetic resonance imaging
PET = positron emission resonance imaging)
-Receptor is a structure that changes when it is acted on by a stimulus from its surrounding causing a signal
to be produced
- a receptor may be a membrane proteins that changes configuration when it binds to a specific hormone or
neurotransmitter
-sensory receptor may be an entire specialized cell that produces an electrical response to a particular
stimuli --> it translates sensory stimuli in the "language" of the nervous system
--> all sensory receptors produce electrical signals --> each receptor type is specialized to produce its
signal only in response to a particular type of environmental stimulus
>some receptors are called free nerve endings --> consist of branched dendrites or sensory neurons -->
other receptors have specialized structures that help them respond to specific stimulus
>many sensory receptors are clustered into sensory organs (eye, ear, tongue, skin, ect)
-their electrical energy (after being processed by the brain) gives rise to the subjective perceptions of
light/sound/touch/taste
-the stimulation of a sensory receptor causes an electrical signal called a receptor potential
>amplitude of this varies with the intensity of the stimulus
>stronger stimuli = large receptor potential
>sensory receptors of different types influence postsynaptic neurons differently
>in some sensory receptor neurons, a receptor potential will bring cell above threshold and cause action
potentials
>in some very small sensory receptors, receptor potentials directly cause neurotransmitters to be released
onto postsynaptic neurons which --> produces action potential
>a large (+) receptor potential will cause a higher frequency of action potentials
____________
>Sound is produced by any vibrating object
>the ear of the human has (3) parts
1) outer 2) middle 3) inner
>outer ear consists of the external ear and the auditory canal
>external ear (with its fleshy folds) modifies sound waves in a way brain uses to determine location of the
sound sources
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>air-filled auditory canal conducts the sound waves to the middle ear, consisting of the tympanic
membrane (eardrum), and three tiny bones called the hammer, anvil, and the stirrup; and the auditory tube
>auditory tube --> connects middle ear to the pharynx and equalizes the air pressure between the middle ear
and the atmosphere
>this tube may become swollen shut if you have a cold
>within the middle ear sound vibrates the tympanic membrane --> vibrates the hammer, anvil, and stirrup -> these bones transmit vibrations to the inner ear
>fluid filled hollow bones of the inner ear form the spiral shaped cochlea as well as the vestibular system
that detect head movement and the pull of gravity
>stirrup bone transmits vibrations to the fluid within the cochlea by vibrating a membrane in the cochlea
called the oval window --> the round window is a second membrane below the oval window; it allows the
fluid within the cochlea to shift back and forth as the stirrup bone vibrates the oval window
>in the cochlea, consists of three fluid filled compartments
>central compartment houses the receptors and the supporting structures that activate them in response to
sound vibrations. The floor of the central chamber consists of the basilar membrane
>on top sits the mechanoreceptors = hair cells --> have small cell bodies that are like stiff cilia
>some hairs are embedded into the tectorial membrane that protrudes into the central canal
-the oval window passes vibrations from small bones of the middle ear to the fluid in the cochlea -->
vibrates the basilar membrane, causing it to move up and down. This movement bend the hairs of the hair
cells, opening channels in the hair cell membrane that allows the flow of ions, producing receptor potentials
>receptor potentials cause the hair cells to release neurotransmitter onto neurons whose axons form
auditory nerve --> causes action potential within the auditory nerve --> transmitted to auditory processing
centers within the brain
-cochlea allows us to perceive loudness (magnitude) and pitch (frequency)
>weak sounds cause small vibrations (bend hairs slightly) --> results in low frequency of action potentials
in axons of the auditory nerve
-loud sounds damage hair cells
-structure of basilar membrane allows the perception of pitch
>basilar is stuff and narrow at the end near the oval window but more flexible and wider near the tip of the
cochlea
>progressive change in structure causes each successive portion of the membrane to resonate or vibrate in
synchrony
>brain interprets signals from receptors near oval window as high pitched sounds --> signals from receptors
located progressively closer to the tip of cochlea --> increasingly lower in pitch
Thermoreceptors - free nerve endings (sensory cell type) heat/cold (stimulus) skin (location)
Mechanoreceptors :
hair (sensory cell type) vibration, motion, gravity (stimulus) inner ear (location)
specialized nerve endings and free nerve endings in the skin (sensory cell type) vibration, motion, gravity
(stimulus) skin (location
specialized nerve endings (sensory cell type) stretch (stimulus) muscles, tendons (location)
Photoreceptors:
rod, cone (sensory cell type) light (stimulus) retina of eye (location)
Chemoreceptors:
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Olfactory receptor (sensory cell type) odor (stimulus) nasal cavity (location)
Taste receptor (sensory cell type) taste (stimulus) tongue (location)
Pain receptor (sensory cell type) chemicals released in tissue injury (stimulus)widespread in body (location)
________
-all forms of vision use photoreceptors --> these sensory cells contain receptor molecules called
photopigments which absorb light and chemically changed in the process
>this chemical change alters ion channels in the receptor cell membrane, producing receptor potential
-arthropods (spiders/insects) have compound eyes which consist of mosaic of many individual light
sensitive subunits called ommatidia
>cornea - transparent covering over the front of the eyeball that collects light waves and begins to focus
them
>behind the cornea, light passes through a chamber filled watery fluid called aqueous humor which
provides nourishment for both the lens and cornea
>amount of light that enters the eye is adjusted by the iris (pigmented muscular tissue)
>iris regulates the size of the pupil, a circular opening in the center of the iris
>light passing through the pupil encounters the lens (a structure that is like a flattened sphere composed of
transparent protein fibers)
>lens is suspended behind the pupil by muscles that regulate shape and allow fine focusing of the image
>behind the lens if a much larger chamber filled with vitreous humor (allows light to pass freely while
maintaining shape of the eyeball)
-after passing through the vitreous humor light reaches the retina --> multilayered sheet of photoreceptors
>there light is converted into electrical nerve impulse that are transmitted to the brain
>behind the retina is the choroids --> its rich blood supply helps nourish the cells of the retina --> its dark
pigment absorbs stray light whose reflection inside the eyeball would interfere with the clear vision
>the sclera surrounds the outer potion of the eyeball --> it is a tough connective tissue layer that is visible
as the white of the eye and is continuous with the cornea
-visual image is focused most sharply on a small area of the retina called the fovea
>although focusing begins at the cornea, whose rounded contour bends light rays, the lens is responsible for
final, sharp focusing.
>shape of the lens is adjusted by its encircling muscle
>eyeball too long = nearsighted ; too short = far sighted
>corrective lens needed --> for farsighted "converges to retina" ; for nearsighted "diverges to retina"
-photoreceptors in eye --> rods and cones
>the retinal layer nearest the vitreous humor consists of ganglion cells, whose axons make up the optic
nerve
>ganglion axons must pass back through the retina to reach the brain at a location called the blind spot
>the receptor potential from the photoreceptors is processed by other retinal neurons
>the much modified signal is finally converted into action potentials which are carried to the brain by the
optic nerve
>photoreception in both rods and cones begins with the absorption of light by the photo pigment molecules
that embedded in the plasma membranes of the photoreceptors.
>light hitting the photo pigment molecules causes a change in the receptor membrane's permeability to ions
---> producing a receptor potential in the photoreceptor cell
>although cones are located throughout the retina, they are concentrated in the fovea (no rods in fovea)
>where lens focuses images most sharply
>fovea is a depression near the center of the retina
>layers of signal-processing neurons are pushed aside while retaining their synaptic connections
>allows light to reach cones of the fovea
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>humans have 3 verities of cones --> deals with color
>rods dominate in the peripheral portions of the retina -> rods have longer outer segments and more
photopigments than cones --> more sensitive to light --> deals with DIM LIGHT --> do not distinguish
color
>forward facing eyes are called "binocular vision" allows depth perception, the accurate judgment of the
size and distance of an object from the eyes
-the ability of smell arises from olfactory receptors
-receptors for olfaction (smell) are nerve cells located in mucus covered epithelial tissues in the upper
portion of each nasal cavity
>human olfactory epithelium is small compared to other mammals
-olfactory receptors have hair like dendrites that protrude into the nasal cavity and are embedded in a layer
of mucus
>odorous molecules in the air diffuse into the mucus layer and bind with receptors on the dendrites
>1000+ receptor proteins in the olfactory dendrites
>each receptor protein is specialized to bind a particular type of molecule and stimulate the olfactory
receptor to send a message to the brain
>human tongue has 10,000 taste buds, structures embedded in small bumps (papillae) that covers the
tongue's surface. Each taste bud consists of a cluster of 60-80 taste receptors cells surrounded by supporting
cells in a small pit. The cells in the pit communicate with the mouth through a taste pore. Microvilli of taste
receptors protrude through the pore. Dissolved chemicals enter the pore and bind to the receptor molecules
on the Microvilli --> producing receptor potential
>5 major types of taste receptors: sweet, sour, salty, and bitter. 5th type, umani responds to glutamate, an
amino acid that serves as a neurotransmitter
>We can perceive a great variety of tastes because of two mechanisms. A particular substance may
stimulated two or more receptors types to different degrees. The substance being tasted usually releases
molecules into the air inside the mouth. These odors diffuse to the olfactory receptors, which contribute an
odor component to the basic flavor.
>pain is a specialized chemical sense
-pain is caused by tissue damage
-when cells/capillaries are damaged their contents flow into the extracellular fluid
>cell contents include K+ which stimulate pain receptors --> damaged cells release enzymes that convert
certain blood proteins into chemicals (bradykinin) --> another stimulus that activated pain receptors