Chapter 7:
The Nervous System
I. Functions of the Nervous System
A. Sensory Input- gathering
information to monitor changes
occurring inside and outside the
body
B. Integration- to process and
interpret sensory input and decide
if action is needed
C. Motor Output- A response to
integrated stimuli, the response
activates muscles or glands
II. Organization of the Nervous System
A. Structural Classification- Two
Subdivisions:
1. Central Nervous System (CNS)
- occupies the dorsal cavity,
integration and commanding
centers
a. includes the brain and
spinal cord
2. Peripheral Nervous System
(PNS) - contains mainly the
nerves that extend from the
brain and spinal cord
a. includes cranial and spinal
nerves
B. Functional Classification- concerned only with the PNS
and has two subdivisions
1. Sensory or Afferent division- Nerve fibers that
carry information to the central nervous system
2. Motor or Efferent division- Nerve fibers that carry
impulses away from the central nervous system.
The Two subdivisions are the somatic and
autonomic nervous system.
a. Somatic nervous system = voluntary
b. Autonomic nervous system = involuntary
further divided into the sympathetic
and parasympathetic divisions.
i. Sympathetic division- the “fightor-flight” system. Its activity is
evident when we are frightened,
in emergency, or threatening
situations.
ii. Parasympathetic divisionsometimes called the “resting
and digesting” system.
It is
most active when the body is at
rest and not threatened.
Organization of the Nervous System
Sensory neurons
Motor neurons
A. Supporting Cells
1. Astrocytes- abundant
star-shaped cells, brace
neurons, form the
boundary between
capillaries and neurons,
and control the
chemical environment of
the brain
2. Microglia- spider-like
phagocytes that dispose
of debris
-mutant-type SOD1 genes activate microglial
cells; in turn, the activated microglial cells release
proteins that produce nitric oxide or others
(cytokines) that cause inflammation, thus causing
damage to motor neurons and accelerating the
progress of ALS.
III. Nervous Tissue: Structure and Function
A. Supporting Cells
3. Ependymal
cells- line
cavities of brain
and spinal cord,
help circulate
cerebrospinal
fluid
4.Oligodendrocytesproduce myelin
sheath around
nerve fibers in CNS
5. Satellite cellsprotect neuron cell
bodies
6. Schwann cellsform myelin sheath
in PNS
B. Neurons- excitable little cells that make
use of their potential!
1. Cells specialized to transmit messages
2. Major regions of neurons:
a. Cell body- nucleus and metabolic
center
b. Processes- fibers that extend from the
body
1) dendrites- convey messages
towards the cell body
2) axons- conduct messages away
from the cell body, axons usually
end with terminals that contain
vesicles (small sacs) that
contain neurotransmitters
c. Synapse- the space between two
neurons
3. Nerve Fiber Coverings:
a. Schwann Cells- produce myelin sheathswrapped membranes that enclose the axon
b. Nodes of Ranvier- gaps in myelin sheaths
along the axon- allows the impulse to travel
quickly down the neuron
B. Neurons- excitable little cells that make
use of their potential!
Reflex Arc Animation
4. Functional Classification of
neurons- grouped according to
the direction the impulse is
traveling relative to the CNS
a. Sensory or Afferent neuronscarry impulses from sensory
receptors. Examplescutaneous nerve receptors
b. Motor or Efferent Neuronscarry impulses from the CNS
c. Interneuron (association
neurons) - connect sensory
and motor neurons
B. Neurons- excitable little cells that make
use of their potential! 5. Structural neuron classification:
(determined by the number of
processes extending from the cell
body)
a. Unipolar-have a short process
leaving the cell body;found in
sensory neurons of PNS
b. Bipolar-one axon and one
dendrite; found in sense organs
(eye, ear)
c. Multipolar- many extensions
from the cell body; most
common type
B. Neurons- excitable little cells that make
use of their potential!
C. Functional Properties of Neurons
1. Irritability- neurons have the
ability to respond to a stimulus
2. Conductivity- the ability to transmit
an impulse
3. The plasma membrane at rest is
polarized, this is called the
Resting potential (-70 mV); this
means fewer positive ions are
inside the cell (K+) than outside
(Na+). As long as the inside
remains more negative than the
outside the neuron remains
inactive.
Resting Membrane Potential Animation
D. Action Potentials or Nerve Impulses- an electrical charge
traveling down a neuron
1.
2.
A nerve impulse begins with a stimulus- usually this is
a neurotransmitter released by other neurons, pain
receptors, light excites receptors in the eye, etc.
Once the neuron is stimulated the “sodium gates” of
the neuron open and sodium ions begin flowing
across the cell membrane. This is called
Depolarization: a decrease in membrane potential
(inside less negative) increases the chances of an
impulse (action potential).
3.
If the action potential starts, it continues down the
entire neuron. Action potentials are an “all or
nothing” response.
4.
Potassium ions rush out of the neuron after sodium
ions rush in, which repolarizes the membrane
(returns to resting). A neuron cannot conduct another
impulse until repolarization occurs.
Action Potential Animation
D. Action Potentials or Nerve Impulses- an
electrical charge traveling down a neuron
5.
The propagation of a nerve impulse occurs
more rapidly in cells that contain a myelin
sheath. This helps the impulse jump from
node to node along the length of a neuron.
This is called saltatory conduction.
Action Potential Animation
E.
Conductivity- How does the impulse travel to
the next neuron?
1.
2.
3.
Once the impulse reaches the
axon endings, vesicles
containing neurotransmitters
fuse with the plasma
membrane, which ruptures
releasing the neurotransmitter
(a chemical messenger).
Some neurotransmitters fit
receptor sites, some return
through reuptake sites and are
reused, and some are
destroyed by enzymes.
If enough neurotransmitter is
released the action potential
will continue in the next
neuron.
E.
Conductivity- How does the impulse travel to the next neuron?
F. Reflexes are a predictable,
rapid, involuntary response to a
stimulus. These responses do
not carry the impulse to the
brain, they take a shorter path to
allow for quicker response.
1. Reflex Arcs- the direct route
from a sensory neuron, to
interneuron, to an effector.
2. Autonomic reflexes include
regulation of: smooth muscle,
blood pressure, digestive
system, and glands.
3. Somatic reflexes involve the
activation of skeletal muscles.
1.
All reflex arcs have five key
elements:
a. sensory receptor- reacts
to a stimulus
b. effector organ- gland or
muscle stimulated
c.
afferent neurons- carry
the message to the
integration center
d. efferent neurons- carry
the message away from
the integration center
e.
integration center- the
connection between the
afferent and efferent
pathways
IV. Developmental Aspects of the Nervous
System
A. The nervous system begins to form in the first month
of embryonic development. Therefore maternal and
environmental factors may impair brain development.
Mothers that smoke impair the bodies ability to carry
oxygen sufficiently which increased the chances of
oxygen deprivation to the babies brain cells that are
forming. Other severe congenital brain disorders
include cerebral palsy which is thought to be caused
by a temporary lack of oxygen during delivery.
Furthermore, premature babies have trouble
regulating their temperature because the
hypothalamus is one of the last brain areas to mature
prenatally.
B. The development of motor control indicates the
Cerebral refers to the affected area of the brain,
the cerebrum, and palsy refers to disorder of
movement. Cerebral palsy is caused by damage to
the motor control centers of the young developing
brain and can occur during pregnancy, during
childbirth, or after birth up to about age three.
progressive myelination and maturation of a child’s
nervous system.
IV. Developmental Aspects of the Nervous System
C. Neurons continue to die throughout life and
cannot be replaced. However, most healthy
aged people maintain optimal intellectual
functioning. Cardiovascular disease is the
major cause of declining mental function with
age.