Download The Special Senses and Functional Aspects of the Nervous System

The Special Senses and Functional Aspects of the Nervous
System
General Sensory ReceptorsAll receptors share a common characteristic which is the capability
to excite, to generate nerve impulses.
 Mechanoreceptors- detect a mechanical or physical change in
the receptor of nearby cells
Example:
 Thermoreceptors- detect changes in temperature
Example:
 Nociceptors- detect pain usually resulting from chemical or
physical damage of cells
Example:
 Photoreceptors- sensitive to changes in light and present only
in retina.
Example:
 Chemoreceptors- detect chemicals dissolved in fluid which
create the senses of taste and smell.
Example:
4 Special Senses
 Smell- olfaction
 Taste- gustation
 Sight- vision
 Sound- audation
Smell - olfaction
Olfactory organs- chemoreceptors bunched into a small space
about 1/2 square inch, associated with other organs. These
chemoreceptors are embedded within the mucous membrane of the
nasal epithelium. At the free end of these cells are the olfactory
hairs, which extend beyond the epithelial layer and into the
mucous.
Olfactory pathway
Begins at the olfactory hairs of the chemoreceptors which react to
dissolved molecules in air
olfactory hairs create an action potential conducted along the
receptor cell
continues onto cranial cavity
transmitted to sensory neurons in the olfactory bulbs (swellings in
the olfactory nerves at base of brain)
Impulse runs along olfactory nerve to cerbral cortex
Interpretation occurs
Olfactory system is closely linked to the limbic system which is
the center for emotional expression. Why would this be
advantageous to humans?
Taste- gustation
Taste buds- special organs of taste scattered throughout the oral
cavity. Closely associated with smell. About 10,000 in any given
human.
Gustatory cells- group of taste receptors surrounded by supportive
epithelial cells which make up a taste bud. Free ends of these cells
have microvilli called taste hairs projecting through the taste pore.
4 Primary Tastes
 Sweet
 Sour
 Bitter
 Salty
All tastes are a combination of these
four tastes.
Gustatory pathway
Chemicals are dissolved by saliva
Chemicals combine with taste hairs to cause a change
If change is great enough, leads to an action potential
Impulse travels on fibers of face and nerves to medulla oblongata
Impulses then pass to the thalamus
Thalamus directs impulses to the gustatory center in the cerebral
cortex
Sight- vision
Eyes- organs of sight. Many photoreceptors lie within these.
Accessory Structures
 Eyelids and eyelashes- protect the anterior surface of the eyes
 conjunctiva- inner mucous membrane, last of four membranes
 Lacrimal apparatus- lacrimal gland (secretes tears), lacrimal sac
(collects tears), and nasolacrimal ducts (carry tears to nasal
cavity)
Structure of the Eye
Tunic
Fibrous Tunic
Structural
Parts
Sclera
thick outermost layer of the
eyeball
Cornea
Vascular Tunic
Choroid
Description
white fibrous tissue, “white of the eye”, protects
eye and gives shape, contains the abundance of
blood vessels, penetrated by the optic nerve
transparent part of eye, “window of the eye”,
light passes through
thin, dark membrane containing brown pigment
which absorbs light minimizing reflection.
Abundance of blood vessels
Ciliary body
thickest part of vascular tunic, smooth muscle
connecting to lens
Iris
colored part of eye, contraction of this controls
the amount of light entering the eye
the black opening which allows light in
Pupil
Lens
Nervous Tunic
Retina
located immediately behind pupil, can become
more or less convex to focus on objects, this
alteration is called accommodation
thin, fragile layer of neurons which detects light
and transports nerve impulses to optic nerve
Three Distinct Layers:
1.Photoreceptors- rod cells (sensitive to low light
levels) and cone cells (sensitive to color)
2. Bipolar neurons- middle layer, transmit signal
to inner layer
3. Ganglion cells- inner most layer, contains long
axons that converge to form optic nerve which
extends to the brain
area where the axons from the galion cells
converge to form the optic nerve, no
photoreceptors here so “blind spot”
Optic disc
Pathway of Light Through the Eye
Refraction- the bending of light rays as they hit the eye due to the
change in media from air to eye structures and fluids.
Accommodation- the ability of the lens to change shape in order to
move the visual focus. A normal or emmetropic eye can refract
light from an object onto the retina.
Myopia- when refraction is too great or the eyeball is too long, an
image is focused in front of the retina. Distant objects are blurred.
Hyperopia- image is focused behind the retina, distant objects are
clear. Results from a weak or lazy eye whose refraction is too
short.
Astigmatism- unequal curvature of the cornea or lens, causes
blurred vision for near and far objects.
Physiology of Vision
Image formed on retina
Image converted from light pattern to nerve impulses
Conversion initiated by cones and rods
Pigments break apart in the rods and cones when light energy is
absorbed which generates an action potential
Activated rods and cones increase neurotransmitter release which
generates a nerve impulse
Visual Pathway
Rods and cones send a stimulus to bipolar neurons then ganglion
cells
Ganglion cells generate an action potential if stimulus great enough
Action potential then travels along the axons of the ganglion cells
to the optic nerve
Impulses run along the optic nerve to the X-shaped optic chiasma
where they might split
Impulses from the left lateral half of eye and the right medial half
of eye go to the left side of brain to optic tract
Impulses from the right lateral half of eye and the left medial half
of eye go to the right side of brain to optic tract
Optic tract runs to the thalamus
From thalamus impulses enter visual pathways which lead to the
occipital lobes of the cerebral cortex called the visual cortex where
interpretation takes place
Hearing- audation
Ear- organ of hearing. Three parts to the ear:
 Outer ear
 Middle ear
 Inner ear
Outer earauricle- external appendage on each side of head, collects sound
waves in the air and directs them to the external auditory canal
external auditory canal- tube that extends into the temporal bone
and contains glands which excrete cerumen (ear wax)
Middle eartympanic cavity- lies between inner surface of the tympanic
membrane and the bone that form the outer wall of the inner ear.
Contains three auditory ossicles
auditory (eustachian) tube- connects the ear to the throat and
allows air to pass bwtn the throat and the tympanic cavity.
Equalizes pressure on both sides of the tympanic membrane.
tympanic membrane- thin barrier separating the outer and inner
ear. Mucous membrane covered with thin layer of skin on outer
surface. It receives sound waves from the external auditory canal
and vibrates in response. The vibrations are sent to the auditory
ossicles.
auditory ossicles malleus (hammer)
 incus (anvil)
 stapes (stirrup)
The three bones are interconnected forming a bridge form the
tympanic membrane to the inner ear. These bones amplify the
sound waves from the tympanic membrane to the malleus, to the
incus to the stirrup which transmits the biration to an opening in
the Tympanic cavity called the oval window, into the inner ear.
Inner ear- labrinth
bony labrinth- series of canals within the temporal bone. These
canals form three regions of the inner ear:
 Semicircular canals
 vestibule
 cochlea
Membranous labyrinth- an internal series of sacs and tubes within
the outer, bony walls.
perilymph- fluid between outer walls of bony labyrinth and the
membranous labyrinth.
Endolymph- fluid within the membranous labyrinth.
Semicircular canal- has three loops and functions in the sensation
of equilibrium
Vestibule- chamber between semicircular canal and the cochlea
functions also in sensation of equilibrium
Cochlea- region of inner ear resembling a snail
Sound pathway
Sound waves transmitted from external auditory canal to the
tympanic membrane
These vibrations then sent to malleus, to the incus, to the stapes
Stapes move back and forth to move oval window producing sound
waves in the perilymph of inner ear
Pressure waves in the perilymph cause pressure on the endolymph
in the membranous labyrinth
These pressure waves cause movement of hair cells which releases
neurotransmitters
The neurotransmitters stimulate sensory neurons in cochlear nerve
to generate an action potential
Functional Regions of the Cerebral Cortex
Cerebral cortex- final destination for most sensory impulses
entering the brain. The impulses are interpreted here and a motor
response is sent if necessary. These functions are separated into
regions:
 Sensory
 Association
 Motor
Sensory areas
Sensory Area
Description
Associated with...
General sensory area
located in the parietal lobe,
receives sensations mostly from
skin, helps pinpoint where
sensations are, sensitivity of
region is due to number of
sensory receptors and not size.
somesthetic association
area
Primary visual area
receives sensory impulses from
eyes, located in the occipital lobe,
interpret images and analyse what
is seen
visual association area
Primary auditory area
receives sensory impulses from
the ears and interprets sound,
located in temporal lobe
auditory association
area
Primary gustatory area
located in the parietal lobe and
interprets taste
X
primary olfactory area
located in temporal lobe,
interprets smell
X
Association Areas - receive impulses from each of the sensory
areas and makes connection between sensory and motor areas.
These areas recognize, analyze, and respond to sensory
information. Integrate Information!!! Association area functions
are learning and reasoning, memory storage and recall, language
abilities, and consciousness
Motor areas- located primarily in the frontal lobes. Receive
impulses for the initiation of motor response.
 primary motor area- consists of groups of motor neurons the
control specific muscles or groups of muscles. Maps of the
locations of neuronal groups are generally consistent among
humans
 premotor area- Receives impulses from various areas of brain,
such as primary motor area, cerebellum, basal ganglia.
Coordinates precise skeleltal muscle movement, usually learned
like typing, writing.
 visual motor areas- controls scanning movement
 motor speech area- controls speech muscles
Thought and memory
Thought- What is a thought and how is it produced?
A thought is a conscious understanding in the brain of image or
language or words. It is the result of billions of exchanges of
neurotransmitters across billions of synapses and the conductions
of millions of impulses through millions of neurons. The frontal
and temporal lobes appear to be most active when generating a
thought.
Memory- ability to recall past experiences. A neural event stored
within the cortex for retrieval at a later time. Enables learning.
Areas in the brain dealing with memory include areas of storing
and areas of where memory information is integrated. Memories
are interconnected and share information. Memories are stored in
several areas of the brain, i.e. visual memories in occipital lobe.
Recalling a memoryA sensory perception is formed in the cerebral cortex where
memory is stored
Connecting fibers carry this impulse the limbic system then to
diencephalon and prefrontal cortex
Connecting fibers then route the impulse from the prefrontal cortex
back to the sensory cortex
Here perception is formed
Emotions- limbic system- a functional region of the brain which
occupies parts of the cerebral cortex and basal ganglia of
cerebrum, the hypothalamus, the thalamus and the brain stem.
Responsible for feelings and emotions. Networks of nerve fibers
connect the higher and lower brain to receive and integrate
information form a variety of sources, i.e. smells. These
interconnections allow a complex response such as headaches,
muscle spasms, or warmth, security, etc. These responses are
possible because the limbic system is connected to the cerebral
cortex.