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The Nervous System


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. – PowerPoint PPT presentation

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Title: The Nervous System

The Nervous System
The nervous system
  • A.     Two organ systems, the nervous system and
    the endocrine system, coordinate organ system
    activities in response to changing environmental

  • 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

  • 3.       The nervous system has three
    functions- 1- monitors the internal and external
    environments, 2- integrates sensory information
    and 3- coordinates voluntary and involuntary
  • 4. The functions of the two major anatomical
    subdivisions of the nervous system are shown in
    figure 8.1 pages 226.

  • 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
  • 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

  • 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.

  • 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.

  • 3. The neuroglia regulates the environmental
    around the neurons provide a support framework
    for neural tissue, and act as phagocytes.

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

  • 2. Neurons can have a variety of shapes the
    most common type of neuron in the CNS is shown on
    page 227.

  • 3. Neurons come in all shapes and sizes they
    have enough features in common that we can draw a

  • 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.

  • 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.

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.

  • 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

  • 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

E. Functional classification of Neurons
  • 1. Neurons are sorted into three functional
    groups sensory, motor and interneurons.

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.

  • 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

  • 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

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.

  •   3. The somatic motor neurons of the somatic
    nervous system and the visceral motor neurons of
    the autonomic nervous system. Example cardiac

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.

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.

  • 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.

  • 4. Ependymal cells- line fluid filled cavities
    within the CNS.

II. Neuron Functions
  • The nerve impulse
  • 1. The electrical activity in a nerve impulse
    is a flow of electrical charges along the cell
    membrane of a neuron.

  • 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.

  •  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

  • 6. At the ends of the axons are receptors.
    Receptors are special sensory neurons in sense
    organs that receive stimuli from the external
  • 7. The points of contact at which the impulses
    are passed from one cell to another are known as

  • 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.

III. The brain
  • Major divisions of the brain
  • 1.      The adult brain has six major regions
  •         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

  • 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.

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.

  • 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
  •   3. Each hemisphere of the cerebrum is divided
    into regions called lobes. These lobes are named
    for the skull bones.

  • 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.

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.

  • 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

  •  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.

  •  3. The epithalamus contains the pineal gland,
    an endocrine structure that secretes the hormone
    melatonin. Melantonin is important in regulating
    day-night cycles.

  •  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.

  • 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

 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.

  •   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

  • 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

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.

  • 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
II. Optic-sensory
III. Oculomotor Somatic Parasympathetic
 control of extrinsic eye muscles and pupil
IV. Trochlear Somatic control of muscles that
surround the eye.
V. Trigeminal Sensory Somatic control of
facial nerves and muscles for chewing. 
VI. Abducens  Somatic muscle control of lateral
movement of the eyeball.
VII. Facial Sensory, Somatic
Parasympatheticcontrol over facial expression
and taste. 
VIII. Vestibulocochlear or Auditory Nerve
Sensory nerve for hearing. 
IX. Glossopharyngeal Sensory, Somatic
Parasympathetic control over taste, the tongue
and the pharynx. 
X. Vagus Primary Parasympathetic nerve with some
Somatic and Sensory functions. Controls visceral
XI. Accessory  Somatic control over larynx,
pharynx and sternocleidomastoid muscles 
XII. Hypoglossal Somatic control over the
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.

  •   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.

  •   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.

  • 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

  • 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.

  • 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
  • 8. See page 293 for accommodation problems.

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

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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.

V. Clinical considerations
  • 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
  • 2. Seizure- a temporary cerebral disorder
    accompanied by abnormal, involuntary movements.
    This chronic condition is called epilepsy.

  • 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.

  • 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

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  • 7. Color blindness is a sex linked trait.

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