THE NEURON
 1. The CELLS that Carry Messages Throughout the
Nervous System are called NEURONS. (Figure 50-8)
 2. The Neuron is the Basic Functional Unit of the
Nervous System.
 3. Whatever their specific function, all neurons have
the same physical parts: The Cell Body, Dendrites and
One Axon.
 4. Messages take the form of ELECTRICAL SIGNALS,
and are known as IMPULSES. A Neuron carries
impulses in only ONE direction.
 5. Neurons can be classified into THREE
TYPES:
 A. SENSORY (RECEPTOR) NEURONS
(AFFERENT) - Carry impulses from the SENSE
ORGANS (RECEPTORS) to the Brain and Spinal
Cord. Receptors detect external or internal
changes and send the information to the Central
Nervous System in the form of impulses by way of
the Afferent Neurons.
 B. MOTOR NEURONS (EFFERENT) - Carry impulses
from the Brain and Spinal Cord to MUSCLES or
GLANDS. Muscles and Glands are Two Types of
Effectors. In response to impulses, Muscles Contract
and Glands Secrete.
 C. INTERNEURONS - Connect Sensory and Motor
neurons and carry impulses between them. They are
found entirely within the Central Nervous System.
THE ANATOMY OF A NEURON
 6. A Neuron consists of THREE MAIN PARTS:
(Figure 50-8)
 A. CELL BODY - The largest part, contains the
nucleus and much of the cytoplasm (area between the
nucleus and the cell membrane), most of the
metabolic activity of the cell, including the generation
of ATP (Adenine Triphosphate Compound that Stores
Energy) and synthesis of protein.
 B. DENDRITES - Short branch extensions spreading
out from the cell body. Dendrites Receive STIMULUS
(Action Potentials) and carry IMPULSES from the
ENVIRONMENT or from other NEURONS AND
CARRY THEM TOWARD THE CELL BODY.
 C. AXON - A Long Fiber that CARRIES IMPULSES
AWAY FROM THE CELL BODY. Each neuron has
only ONE AXON. The Axon Ends in a series of small
swellings called AXON TERMINALS.
 7. Neurons may have Dozens or even Hundreds of
DENDRITES but usually ONLY ONE AXON.
 8. The Axons of most Neurons are covered with a Lipid
Layer known as the MYELIN SHEATH.
 9. The Myelin Sheath both Insulates and Speeds Up
transmission of Action Potentials through the Axon.
 10. In the Peripheral Nervous System, Myelin is
produced by SCHWANN CELLS, which surround the
Axon.
 11. GAPS (NODES) in the Myelin Sheath along the
length of the Axon are known as the NODES OF
RANVIER.
TRANSMISSION OF NERVE IMPULSES
 RESTING POTENTIAL
 A Nerve Cell has ELECTRICAL POTENTIAL across its
cell membrane because of a difference in the number of
Positively and Negatively Charged IONS on each side of
the Cell Membrane. ( + outside, - inside )
 As a result of its Resting Potential, the Neuron is said to
be POLARIZED.
 A Neuron maintains this polarization until it is
stimulated.
 The ability of a neuron to respond to a stimulus and
convert it into a nerve impulse is known as
EXCITABILITY.
Nerve Impulse Conduction
 At the beginning of an impulse, the Sodium Gates
OPEN, allowing positively charged Na+ ions to flow
INSIDE the Cell Membrane.
 The INSIDE of the membrane temporarily becomes
MORE POSITIVE than the OUTSIDE. This is
Depolarization .
 As the impulse passes, the Potassium Gates OPEN,
allowing positively charged K+ ions to FLOW
OUT. Repolarization: the inside of the axon resumes
a negative charge.
 11. The DEPOLARIZATION and
REPOLARIZATION of a Neuron Membrane is
called an ACTION POTENTIAL. Action
Potential is another name for a Nerve Impulse.
PROPAGATION
 An impulse is self-propagating. Once started it
continues, and moves only in one direction.
MYELIN SHEATH
 Myelin Sheaths greatly increase the speed of impulse
along an axon.
 Myelin is composed of 80% lipid and 20% protein.
 Myelin gives these axons a white appearance.
 There are SMALL NODES or GAPS called the Nodes
of Ranvier between adjacent myelin sheath cells
along the axon. (Figure 50-8)
 As an impulse moves down a myelinated (covered
with myelin) axon, the impulse JUMPS form Node to
Node instead of moving along the membrane.
 This jumping from Node to Node greatly increase the
speed of the impulse.
 Some myelinated axons conduct impulses as rapid as
200 meters per second.
 The formation of myelin around axons can be thought
of as a crucial event in evolution of vertebrates.
THE THRESHOLD
 The MINIMUM LEVEL of a STIMULUS that is
REQUIRED to Activate a neuron is called the
THRESHOLD.
 Any Stimulus WEAKER than the Threshold will
produce NO impulse.
 Any Stimulus STRONGER than the Threshold WILL
produce an impulse.
 A nerve impulse follows the ALL-OR-NONE Principle.
THE SYNAPTIC CLEFT OR SYNAPSE
 The Axon ends with many small swellings called
AXON TERMINALS.
 At these Terminals the neuron may make contact
with the DENDRITES of another neuron, with a
RECEPTOR, or with an EFFECTOR.
 RECEPTORS are special SENSORY NEURONS
in SENSE ORGANS that RECEIVE Stimuli from
the EXTERNAL ENVIRONMENT.
 EFFECTORS are MUSCLES
or GLANDS that bring
about a COORDINATE
RESPONSE.
 The point of contact at which
impulses are passed from one
cell to another are known as
THE SYNAPTIC CLEFT OR
SYNAPSE.
 Neurons that transmit impulses to other neurons
DO NOT actually touch one another. The Small
Gap or Space between the axon of one neuron and
the dendrites or cell body on the next neuron is
called the Synapse. One importance of the
presence of Synapses is that they ensures one-way
transmission of impulses in a living person. A
nerve impulse CANNOT go backward across a
Synapse.
 The Axon Terminals at a Synapse contain tiny vesicles,
or sacs.
 These tiny vesicles are filled with CHEMICALS known
as NEUROTRANSMITTERS. (Acetylcholine)
 9. A
NEUROTRANSMITTER is
a chemical substance that
is used by one neuron to
signal another. The
impulse is changed from
and Electrical Impulse to a
Chemical Impulse
(Electrochemical
Impulses).
 When an impulse reaches the Axon Terminal, dozen of
vesicles fuse with the cell membrane and discharge the
Neurotransmitter into the Synaptic Cleft (GAP).
 The molecules of the neurotransmitter diffuse across
the gap and attach themselves to SPECIAL
RECEPTORS on the membrane of the neuron
receiving the impulse.
 When the neurotransmitter becomes attached to the
cell membrane of the adjacent nerve cell, it changes
the permeability of that membrane.
 As a result, Na+ ions diffuse through the membrane
into the cell.
 14. If enough neurotransmitter is released by the axon
terminal, so many Na+ ions diffuse into the neuron
that the neuron becomes DEPOLARIZED.
 15. DEPOLARIZED = Inside the membrane becomes
more positive than outside.
 16. This causes a THRESHOLD to be REACHED and an
impulse (ACTION POTENTIAL) begins in the second
cell.
 17. After the neurotransmitter relays it message it is
rapidly REMOVED or DESTROYED, thus halting its
effect.
 18. The molecules of the neurotransmitter may be broken
down by ENZYMES, taken up again by the axon terminal
and recycled, or they may simply diffuse away.
 Synapses are the slowest part of the nervous
system. The advantage to having many neurons, with
gaps between them, is that we can control and receive
information from different parts of the body at
different times. They also ensure One-Way
Transmission of impulses in a living person.
The Brain - Meninges
 The brain and spinal cord are surrounded by
membranes called meninges that lie between the bone
and the soft tissues.
dura mater
 Outermost meninx is made up of tough, white dense
connective tissue, contains many blood vessels
 In some areas, the dura mater forms partitions
between lobes of the brain, and in others, it forms
dural sinuses.
 The sheath around the spinal cord is separated from
the vertebrae by an epidural space. (where the epidural
is placed when having a child)
Dura mater intact
arachnoid mater (spider web appearance)
 The middle meninx, the arachnoid mater, is thin and
lacks blood vessels.
 It does not follow the convolutions of the brain.
 Between the arachnoid and pia maters is a
subarachnoid space containing cerebrospinal fluid.
Brain, whole; Right hemisphere has arachnoid
mater intact {CNS}
pia mater
 The innermost pia mater is thin and contains many
blood vessels and nerves.
 It is attached to the surface of the brain and spinal
cord and follows their contours.
Brain--Cerebrum

Conscious thought, intellectual functions, memory
storage & processing, regulation of motor patterns
Lobes (Cerebrum)
 Frontal- voluntary control of




skeletal muscles
Parietal- perception of touch,
pressure, vibration, pain, temp,
taste
Occipital- perception of visual
stimuli
Temporal- perception of
olfactory and auditory stimuli
All lobes have association areas
where sensory data is integrated
& processes
 Cerebrum is divided into 2 hemispheres-separated by
a deep longitudinal fissure and connected by a thick
band of axons called the corpus callosum.
 Left controls right side
 Right controls left side
Corpus callosum
Cerebellum
Inferior & posterior to


cerebrum
Coordinates
adjustments that
maintain balance and
equilibrium
Fine tunes voluntary &
involuntary movements
to ensure smooth
movements.
Thalamus


Atop brain stem
Relays and processes
sensory info
Hypothalamus


Inferior to thalamus
Controls emotions,
autonomic functions,
hormone production
Midbrain



Processing of visual
and auditory data
Generation of
involuntary motor
responses
Maintenance of
consciousness
Pons
 Relays sensory info to
cerebellum & thalamus
 Involuntary motor center
Medulla Oblongata


Relays sensory info to
thalamus
Autonomic centers for
regulation of
cardiovascular,
respiratory, & digestive
activities
Spinal Cord
 The spinal cord begins at the base of the brain and
extends as a slender cord to the level of
the intervertebral
disk between the first and second lumbar vertebrae.
 The spinal cord consists of 31 segments, each of which
gives rise to a pair of spinal nerves.
 A cervical enlargement gives rise to nerves leading to
the upper limbs, and a lumbar enlargement gives rise
to those innervating the lower limbs.
1. Cervical enlargement
2. Lumbar enlargement
3. Ventral rami of spinal
nerves
 Two deep longitudinal
grooves (anterior median
fissure and posterior
median sulcus) divide
the cord into
right and left halves.
 White matter, made up
of bundles of myelinated
nerve fibers (nerve
tracts), surrounds a
butterfly-shaped core of
gray matter housing
interneurons.
 A central canal contains
cerebrospinal fluid.
 The spinal cord has two major functions: to transmit
impulses to and from the brain, and to house spinal
reflexes.
 Tracts carrying sensory
information to the brain
are called ascending
tracts; descending tracts
carry motor
information from the
brain.
 The names that identify nerve tracts identify the
origin and termination of the fibers in the tract.
 Many spinal reflexes also pass through the spinal
cord.
Spinal Nerves
 1. Thirty-one pairs of mixed nerves make up the spinal
nerves.
 2. Spinal nerves are
grouped according to the
level from which they
arise and are numbered
in sequence, beginning
with those in the cervical
region.
 3. Each spinal nerve
arises from two roots: a
dorsal, or sensory, root,
and a ventral, or motor,
root.
 4. The main branches of
some spinal nerves form
plexuses.
Cervical Plexuses
 a. The cervical plexuses
lie on either side of the
neck and supply muscles
and skin of the neck.
Brachial Plexuses
 a. The brachial plexuses
arise from lower cervical
and upper thoracic
nerves and lead to the
upper limbs.
Lumbosacral Plexuses
 a. The lumbosacral
plexuses arise from the
lower spinal cord and
lead to the lower
abdomen, external
genitalia, buttocks, and
legs.
Observe the Brain….when they get in...
Cranial Nerves
 1. Twelve pairs of cranial nerves arise from the underside
of the brain, most of which are mixed nerves.
Cranial nerves
 2. The 12 pairs are designated by











number and name and include
the:
olfactory,
optic,
oculomotor,
trochlear,
trigenimal,
abducens,
facial,
vestibulocochlear,
glossopharyngeal,
vagus,
accessory, and
hypoglossal nerves.
HEARING AND BALANCE
 1. The EAR is really TWO
Sense Organs in
ONE. It not only detects
Sound Waves, it also
senses the Position of
the HEAD, whether it is
STILL, MOVING IN A
STRAIGHT LINE, OR
ROTATING.
 2. Sound is nothing more than Vibrations in the Air
around us.
 3. Deep LOW-PITCHED Sounds result from slow
vibrations.
 HIGH-PITCHED Sounds are caused from faster
Vibrations.
 4. In addition to Pitch, sounds differ by their Loudness
or Volume.
 5. The Sense Organ that can distinguish BOTH PITCH
AND LOUDNESS of SOUNDS are the EARS.
 6. The External Ear consists of the visible fleshy part
helps to COLLECT Sounds and FUNNEL them into the
AUDITORY CANAL. The Auditory Canal connects the
External Ear with the TYMPANIC MEMBRANE, also
called the Eardrum.
 7. The Auditory Canal contains small Hairs and WAX
Producing GLANDS that PREVENT Foreign objects
from entering the ear.
 8. The Auditory Canal extends into the bone of the
head, but stops at the EARDRUM OR TYMPANIC
MEMBRANE.
 9. The Eardrum is the beginning of the MIDDLE EAR.
 10. Sound Vibrations
STRIKE the EARDRUM
and are Transmitted
through THREE TINY
BONES: THE
MALLEUS (HAMMER),
INCUS (ANVIL), AND
STAPES (STIRRUP).
 11. The Stirrup transfers
the Vibrations to a thin
membrane covering an
opening called the OVAL
WINDOW.
 12. This Membrane
transmits the vibrations to
the COCHLEA, which
begins the INNER EAR.
 13. The COCHLEA is
SNAIL SHAPED,
consisting of Three
FLUID FILLED
Chambers that are
separated by
membranes.
 14. The Middle Chamber
contains the ORGAN OF
CORTI, which is the organ
of Hearing.
 15. When the Fluid
Vibrates, tiny Hair Cells
lining the Cochlea are
PUSHED back and forth,
providing Stimulation that
is turned into NERVE
IMPULSES.
 16. These Nerve Impulses are
carried to the Brain by the
AUDITORY OR ACOUSTIC
NERVE.
 17. The EARS also contain
structures for DETECTING
STIMULI that make us aware
of our MOVEMENTS and
allow us to maintain our
BALANCE.
 18. Located within the
INNER EAR just above the
Cochlea are three tiny canals
that lie at right angles to
each other.
 19. They are called the
SEMICIRCULAR CANALS
BECAUSE THEY EACH
MAKE HALF A CIRCLE.
 20. The Semicircular Canals and the TWO Tiny Sacs
located behind them help us to SENSE BALANCE OR
EQUILIBRIUM.
 21. Both the Canals and the Sacs are filled with Fluid
and Lined with Hair Cells (Mechanoreceptors).
 22. There are also Tiny
Grains of Calcium
Carbonate and Protein
called OTOLITHS,
Otoliths roll back and
forth in response to
gravity, acceleration, and
deceleration.
 23. The Movement of Fluid and Otoliths bend the hair
on the Hair Cells, and in turn sends the impulses to
the Brain that enable it to determine BODY MOTION
AND POSITION.
VISION
 1. The Sense Organ we use to sense Light is the EYES.
(Figure 50-13)
 2. The EYE is composed
of THREE LAYERS:
 A. The OUTER Layer
consists of the SCLERA
AND CORNEA.
 B. The MIDDLE Layer
contains the
CHOROID, CILIARY
BODY, AND IRIS.
 C. The INNER Layer
consists of the
RETINA.
 3. The SCLERA (WHITE OF THE EYE) consist of
tough white connective tissue. The Sclera helps
MAINTAIN the SHAPE OF EYE, and also provides a
means of ATTACHMENT for the MUSCLES THAT
MOVE THE EYE.
 4. IN THE FRONT OF
THE EYE, THE SCLERA
FORMS A
TRANSPARENT LAYER
CALLED THE CORNEA.
 5. The CORNEA is the
part of the eye through
which LIGHT ENTERS.
 6. Just inside the Cornea is a small chamber filled with
FLUID known as the AQUEOUS HUMOR.
 7. At the BACK of this chamber, the PIGMENTED
CHOROID, WHICH CONTAINS THE BLOOD
VESSELS OF THE EYE, BECOMES a disk-like
structure called the IRIS.
 8. The IRIS (A DIAPHRAGM) is the portion of the eye
that gives your eye its COLOR. The Iris controls the
amount of Light entering the eye by altering the
Diameter of the Pupil.
 9. In the MIDDLE of the Iris is a small opening called
the PUPIL, through which LIGHT ENTERS THE EYE.
 11. In DIM LIGHT the Pupil OPENS to INCREASE the
amount of Light, In BRIGHT LIGHT the Pupil
CLOSES to DECREASE the amount of Light entering
the Eye.
 12. Behind the Iris is the LENS. Light is Focused by
the Lens, which changes shape when pulled by
muscles around its edges.
 13. The CELLS that form the Lens contain a special
PROTEIN called CRYSTALIN. CRYSTALIN is almost
transparent and allows light to pass through.
 14. Small Muscles
attached to the Lens
cause it to bend, this
enables the eye to
FOCUS on close and
distant objects.
 15. Behind the Lens is a Large Chamber called the
VITREAL CHAMBER filled with a transparent Jellylike Fluid called VITREOUS HUMOR.
 16. Special Light Sensitive
RECEPTOR CELLS, or
PHOTORECEPTORS, are
arranged in a layer in the
RETINA, at the BACK of
the EYE.
 17. The
PHOTORECEPTORS
CONVERT LIGHT
ENERGY INTO IMPULSES
THAT ARE CARRIED TO
THE CNS.
 18. THERE ARE TWO
TYPES OF
PHOTORECEPTORS:
RODS AND
CONES. We have about
125 million RODS and 7
million CONES on a
single Retina.
 19. Photoreceptors contain a PIGMENT called
RHODOPSIN, that can respond to most wavelengths
of light.
 20. RODS are extremely sensitive to ALL COLORS of
LIGHT, but DO NOT DISTINGUISH DIFFERENT
COLORS.
 21. CONES are less sensitive than RODS, but they DO
RESPOND DIFFERENTLY TO LIGHT OF
DIFFERENT COLORS, PRODUCING COLOR
VISION.
 22. Humans have three kinds of cones. Each type of
cone contains a pigment that absorbs different
wavelengths of light. When the signals from these
three kinds of cone are integrated, a person is able to
see all the colors in the visible spectrum.
 23. In DIM Light, when only RODS are activated, you
may see objects clearly, but not their colors.
 24. As the amount of Light INCREASE, the CONES are
stimulated and the colors become clear.
 25. The Impulses leave the
Eye by way of the OPTIC
NERVE, and CARRIED to
the part of BRAIN Known
as the OPTIC LOBE OR
OCCIPITAL LOBE. Here
the Brain Interprets the
visual images and provides
information about the
external world.
SMELL

1. The Sense of Smell is a CHEMICAL SENSE, the Cells
responsible for smell are Specialized Chemoreceptors
called OLFACTORY RECEPTORS.

2. These Cells are located in the Upper Part of the Nasal
Cavity.
 3. Chemoreceptors contain Cilia that extend into the
air passageways of the nose and react to Chemicals in
the Air. Chemicals that come into contact with the
Chemoreceptors Stimulate them, causing Impulses to
be sent to the Brain by the OLFACTORY NERVE
TASTE
 1. The Sense of Taste is a Chemical Sense.
 2. The Cells that are Stimulated by the Chemicals are
called CHEMORECEPTORS
CHEMORECEPTORS.
 3. The Sense Organ that detects Taste are the TASTE
BUDS. NOT THE TONGUE!
 4. Most of the 10,000 Taste Buds are embedded
between bumps called PAPILLAE on the tongue, but
can also be found on the roof of the mouth, on the
lips, and in the throat.
 5. Humans can detect FOUR main kinds of
TASTE: SWEET, SALTY, SOUR, AND BITTER.
 6. Each Taste Bud shows a
particular Sensitivity to one of
these taste. The Sense of Taste
is converted to Nerve Impulses
in the Taste Buds and
transmitted to the Brain by
TWO Nerves - The FACIAL
NERVE AND
GLOSSOPHARYNGEAL
NERVES.
 7. Many of the Sensation associated with taste are
actually SMELL Sensations, You depend on both
Senses to detect Flavors in Food.
 8. That is why when you have a Cold and Your Smell
Receptors are blocked, food seems to have little or no
taste.
Eye Dissection..
 Reliability of Vision Lab
 Smell, Hearing, Equilibrium, taste, labs……
 Study Guide
 Test…