The Nervous System
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I.
The nervous system
A. Two organ systems, the nervous
system and the endocrine system,
coordinate organ system activities in
response to changing environmental
conditions.
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1. The nervous system responds
relatively fast but briefly to stimuli,
whereas endocrine responses develop
slower but last longer.
2. The nervous system is the most
complex organ system.
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3.
The nervous system has three
functions- 1- monitors the internal and
external environments, 2- integrates sensory
information and 3- coordinates voluntary and
involuntary responses.
4. The functions of the two major anatomical
subdivisions of the nervous system are shown
in figure 8.1 pages 226.
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5. The central nervous system (CNS):
consisting of the brain and spinal cord,
integrates and coordinates sensory data and
motor commands. The CNS is also the site
for intelligence memory and emotions.
6. All communication between the CNS and
the rest of the body occurs over the
peripheral nervous system (PNS). The PNS
includes all the neural tissue OUTSIDE the
CNS.
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7. Its afferent division brings sensory
information TO the CNS.
8. The efferent division carries motor
commands AWAY from the CNS to the
muscles and glands.
9. The PNS is divided into the somatic
nervous system (SNS) and the autonomic
nervous system (ANS)
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10. The SNS provides control over skeletal
muscle contractions.
11. The ANS provides automatic involuntary
regulations of the smooth, cardiac muscles
and glandular secretions.
12. The ANS includes a sympathetic and a
parasympathetic division, which commonly
have opposite effects.
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13. Example: sympathetic division
accelerates the heart rate and
parasympathetic slows it down.
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B. Cellular organization in
Neural tissue
1. Neural tissue consists of two kinds
of cells, neurons and neuroglia.
2. Neurons are the basic unit of the
nervous system. All neural functions
involve the communication of neurons
with one another and with other cells.
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3. The neuroglia regulates the
environmental around the neurons;
provide a support framework for neural
tissue, and act as phagocytes.
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C. The general structure of the
neurons
1. The “model” neuron as cell body,
several branching dendrites which
receive incoming signals, an elongate
axon which carries outgoing signals
toward the one or more synaptic
terminals.
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2. Neurons can
have a variety of
shapes the most
common type of
neuron in the CNS is
shown on page 227.
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3. Neurons come in all shapes and
sizes; they have enough features in
common that we can draw a neuron.
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a. The largest part of the neuron is the
cell body. The cell body contains the
nucleus and much of the cytoplasm.
b. Spreading out from the cell body are
short-branched extensions called dendrites.
Dendrites carry impulses from the
environment or from other neurons toward
the cell body.
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c. The long fiber that carries
impulses away from the cell body is
called the axon.
d. The axon ends in a series of
small swellings called axon
terminates.
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D. Structural classification of
neurons
1. A multipolar
neuron has multiple
processes extending
away from the cell
body. These are
very common in the
CNS.
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2. A unipolar neuron,
the dendrites and axon
are continuous, and the
cell body lies off to one
side. In a unipolar
neuron, the action
potential begins at the
base of the dendrites
and the rest of the
process is considered
an axon
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3. Bipolar neurons
have two processes,
one dendrites and one
axon, with the cell body
between them. Bipolar
neurons are rare but
occur in special sense
organs such as the eye
and the ear.
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E. Functional classification of
Neurons
1. Neurons are sorted into three
functional groups: sensory, motor and
interneurons.
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F. Sensory neurons of the afferent
division convey information from both
external and internal environments to
other neurons
inside the CNS.
1. Receptors may be grouped into three
categories based on the information
they carry.
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2. The somatic sensory receptors carry two
types of information; one about the outside
world and the other about our position within
it.
3. The external receptors provide information
about the external environment in the form of
touch, temperature and pressure sensations
and more complex senses of sight, smell,
hearing and touch.
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4. The proprioceptors monitor the position
and movement of skeletal muscles and joints.
5. The visceral receptors or internal
receptors monitor activities of digestive,
respiratory, cardiovascular, urinary and
reproductive systems and provide sensation
for taste deep pressure and pain.
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G. Motor Neurons
1. The half million motor neurons of the
efferent division carry instructions from the
CNS to other tissues, organs or organ
systems.
2. The peripheral targets are called effectors
because they change their activities in
response to the commands issued by the
motor neurons.
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3. The somatic motor neurons of the
somatic nervous system and the
visceral motor neurons of the
autonomic nervous system. Example:
cardiac tissue.
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H. Interneurons
1. The 20 billion interneurons are
located entirely within the brain and the
spinal cord.
2. Interneurons are responsible for the
connection between sensory and motor
activity.
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I. Neuroglia-are both found in both CNS
and PNS, but the CNS has the greatest
diversity of glial cells. There are four
types of glial cells in the CNS.
1. Astrocytes- largest and the most
numerous neuroglia. They secrete
chemicals vital to the maintenance of
the blood-brain barrier, which isolates
the CNS from the general circulation.
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2. Oligodendrocytes- has cytoplasmic
extensions that wrap around axons
creating a sheath called myelin. Myelin
speeds up an impulse.
3. Microglia- are the smallest and
rarest. They are phagocytic cells that
eat cellular waste and pathogens.
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4. Ependymal cells- line fluid filled
cavities within the CNS.
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II. Neuron Functions
A. The nerve impulse
1. The electrical activity in a nerve
impulse is a flow of electrical charges
along the cell membrane of a neuron.
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2. Nerve cell has an electrical potential
across its cell membrane because of the
difference in the number of positively and
negatively charged ions on each side of the
cell. The potential is 70 millivolts.
3. Myelin improves the rate of impulses
along an axon. Myelin is composed of 80
percent lipid and 20 percent protein forms an
insulated sheath around the axon.
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4. Most important feature is there are small
nodes or gaps in thy myelin allowing the
impulse to jump from note to node instead of
moving along the membrane. Jumping
greatly increases the speed of the impulse.
5. The minimum level of a stimulus that is
required to activate a neuron is called a
threshold. Any stimulus that is weaker than
the threshold will produce no impulse. Any
stimulus that is stronger than the threshold
will have an impulse.
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6. At the ends of the axons are receptors.
Receptors are special sensory neurons in
sense organs that receive stimuli from the
external environment.
7. The points of contact at which the
impulses are passed from one cell to another
are known as synapses.
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8. The axon terminals at a synapse
contain tiny vesicles, or sacs. These
tiny vesicles are filled with chemicals
known as neurotransmitters. A
neurotransmitter is a substance that is
used by one neuron to signal another.
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III. The brain
A. Major divisions of the brain
1.
The adult brain has six major regions:
·
·
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·
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Cerebrum
Diencephalon
Midbrain
Pons
Medulla oblongata
Cerebellum
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2. The brain is the main switching unit
of the CNS. The spinal cord provides
the link between the brain and the rest
of the body.
3. The brain is wrapped in three layers
of connective tissue known as
meninges
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4. The inner layer covers and is bound to the
surface of the brain is called pia matter.
5. The outermost layer is called dura
mater.
6. Between the pia mater and the dura mater
is cobweb like layer called arachnoid. This
area is filled with cerebrospinal fluid. The
fluid protects the brain from injury. Acts like
a shock absorber.
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B. The cerebrum is the largest
prominent part of the human brain. It is
responsible for all the voluntary activities
in the body. It is the site of intelligence. .
Hearing and judgment.
1. Is divided into two hemispheres, the
right and left. It is separated by a deep
groove. The hemispheres are
connected in a region known as the
corpus callosum.
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2. The cerebrum has numerous folds.
These folds and grooves increase the
surface area of the cerebrum. It allows
more to fit in a small area.
3. Each hemisphere of the cerebrum is
divided into regions called lobes.
These lobes are named for the skull
bones.
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4. Each half of the cerebrum deals with
the opposite side of the body.
5. It is thought that right hemisphere is
associated with creativity and artistic
ability, whereas the left hemisphere is
associated with analytical and
mathematical ability.
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6. The cerebral cortex is the outer
surface and it consists of gray matter.
The inner surface is called the cerebral
medulla. This consists of white matter.
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C. The cerebellum is the second
largest part of the brain is located at the
back of the skull.
1. Its function is balance and
coordination.
2. Major part of learning how to
perform physical activities seems to be
related to the cerebellum. Ex.
Shooting a basketball.
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D. The diencephalon provides switching
and relay centers that integrate
conscious and unconscious sensory
information and motor commands. It
consists of the epithalamus, thalamus
and hypothalamus.
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1. The thalamus serves as a switching
station for sensory input.
2. The hypothalamus controls center for
hunger, thirst, fatigue, and anger and
body temperature.
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3. The epithalamus contains the pineal
gland, an endocrine structure that
secretes the hormone melatonin.
Melantonin is important in regulating
day-night cycles.
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E. Just above the medulla are the pons.
Pons means bridge and this is provides
a link between the cerebral cortex and
the cerebellum.
F. Above the pons is the midbrain, the
smallest division of the brainstem. This
area involves hearing and vision.
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G. The medulla oblongata connects
the brain with the spinal cord. It relays
sensory information to the thalamus
and other brain stem cells. It also
regulates autonomic functions like heart
rate, blood pressure, respiration and
digestion
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H. Functions of the Brain are performed
in the Grey matter of the cerebral cortex.
1. Electroencephalogram (EEG) gives a
general idea of the activity of the brain.
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2. When the activity of the cerebral cortex
falls to the lowest possible level the person
becomes unconscious-sleep. Sleep is a state
of unconsciousness in which a person can be
awakened by normal sensory stimulation.
3. Rapid Eye movement (REM) is deep sleep
this is when all systems are the slowest.
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4. Memory is also a function of the
brain. There are two types of memory.
Short term and long term. Short term
is not permanent-you can easily forget
it. Long-term memories are more
permanent.
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I. Spinal Cord is a link between the
brain and the peripheral nervous
system. It carries impulses to and from
the brain, the spinal cord regulates
reflexes. Examples sneezing and
blinking.
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IV. The Peripheral Nervous
system
A. The PNS is the link between the neurons
and the CNS and the rest of the body.
1. The PNS transmits impulses from
sense organs (ears, taste buds) to the CNS.
The motor division transmits impulses from
the CNS to the muscles or glands. The motor
division is further divided into the somatic
nervous system and the autonomic
nervous system.
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2. There are 12 pairs of cranial nerves
that connect to the brain.
3. See page 255- cranial nerves.
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I. Olfactory-Smell
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II. Optic-sensory
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III. Oculomotor: Somatic & Parasympathetic:
control of extrinsic eye muscles and pupil dilation
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IV. Trochlear: Somatic
control of muscles that surround the eye.
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V. Trigeminal: Sensory & Somatic
control of facial nerves and muscles for chewing.
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VI. Abducens: Somatic
muscle control of lateral movement of the
eyeball.
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VII. Facial: Sensory, Somatic &
Parasympathetic
control over facial expression and taste.
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VIII. Vestibulocochlear or Auditory Nerve:
Sensory
nerve for hearing.
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IX. Glossopharyngeal: Sensory, Somatic &
Parasympathetic
control over taste, the tongue and the pharynx.
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X. Vagus Primary Parasympathetic
nerve with some Somatic and Sensory
functions. Controls visceral organs.
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XI. Accessory: Somatic
control over larynx, pharynx and
sternocleidomastoid muscles
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XII. Hypoglossal: Somatic
control over the tongue.
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B.
The Somatic nervous system regulates
activities that are under conscious
control.
1. If you step on a tac, you will pull
your foot up. The information is taken
to your muscle before it is taken to your
brain. This is called a reflex arc.
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C. The autonomic nervous system
regulates activities that are automatic,
involuntary. The autonomic nervous
system is further subdivided into two
parts, sympathetic nervous system
and the parasympathetic nervous
system.
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1. Most organs controlled by the
autonomic nervous system are under
the control of the sympathetic and
parasympathetic neurons. Ex. Heart
rate is speeded up by the sympathetic
nervous and para slows it down.
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D. The senses each of the five senses (sight,
hearing, smell, taste, and touch) have a
specific sense organ associated with it.
1. Vision the eye-is composed with three
layers. The outer layer consists of the sclera
and the cornea; the middle layer contains
the chorioid, ciliary body and iris. The
inner layer consists of the retina.
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2. The sclera (white) helps maintain
the shape of the eye and also provides
a means of attachment for the muscles
that move they eye.
3. In the front of the eye is the cornea,
the iris the part of the eye through
which light enters.
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4. Inside the cornea is a small chamber
filled with a fluid known as aqueous
humor.
5. The pupil appears as the small black
disk in the center of the eye. Tiny
muscles in the iris regulate the size of
the pupil.
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6. Behind the iris is the lens. This allows
bending which enables the normal eye to
focus on close and distant objects.
7. Rods and cones are photoreceptors.
Rods will see black and white and cones will
see color.
8. See page 293 for accommodation
problems.
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E. Ear
1. Sound vibrations strike the eardrum and
the little bones in the ear, malleus,
hammer, anvil and stapes vibrate.
2. The ears contain structures for detecting
movements and allow us to maintain our
balance. Located within the inner ear are
semicircular canals filled with fluid. The
movement of fluid will detect movement.
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F. Smell-because the sense of smell is a
chemical sense; the cells that are responsible
fore smell are called chemoreceptors.
These contain cilia that react to chemicals in
the air. Little is known about smell.
G. Taste-it is also chemical with
chemoreceptors. The sense organs are
taste buds. Taste is detected by sweet, salty,
sour, and bitter.
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H. Touch and Related senses-all
regions of the skin are sensitive to
touch. There are two different types of
sensory receptor heat and cold and
pain.
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V. Clinical considerations
A. CNS and PNS
1. Meningitis- is the inflammation of the
meningeal membranes following a bacterial
or a viral infection. Meningitis is dangerous
because it can disrupt the normal
circulatory and CSF fluid.
2. Seizure- a temporary cerebral disorder
accompanied by abnormal, involuntary
movements. This chronic condition is called
epilepsy.
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3. The term cerebral palsy refers to a
number of disorders affecting voluntary motor
performance, such as movement and posture
that appear during infancy or childhood and
persist throughout life.
4. Cataract- when the lens loses its
transparency. As aging proceeds, the lens
becomes less elastic, takes on a yellowish
hue and eventually begins to lose
transparency.
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5. Visual acuity- clarity of vision is
rated on the basis of the sight of a
“normal” person.
6. Night blindness- The dim light
proves insufficient to activate the rods.
This can be treated with vitamin A.
Carrots contain vitamin A.
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7. Color blindness is a sex linked trait.
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THE END
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