6. DIVISION
• The nervous system is the most important control
system in the human body. The other system involved
in organ control is the endocrine system, but whereas
this uses hormones as messengers, the nervous system
uses electrical stimuli which travel a great deal faster.
Nerves provide the wiring through which electrical
impulses are received from and sent to virtually all
parts of the body.
• The nervous system consists of the central nervous
system (CNS) and the peripheral nervous system (PNS).
8. THE CENTRAL NERVOUS SYSTEM
• The central nervous system consists of the brain and
the spinal cord. It is in the brain that the higher
senses, both cognitive and emotional, are found. It is
also responsible for producing sensations and
controlling movements. The brain acts as a
computer, integrating all incoming
information, selecting an appropriate response, then
instructing the involved body parts to take appropriate
action. Thus the nervous system forms a vital
link, allowing communication and coordination of
interaction between the various tissues in the body as
well as with the outside world.
9. THE PERIPHERAL NERVOUS SYSTEM
• The peripheral nervous system consists of all the
nervous tissue outside the central nervous system: the
peripheral nerves that inervate muscles and organs.
• Nervous tissue consists of an intricate , interconnected
network of specialized cells called neurons, which are
enclosed within a supportive tissue that has the same
function in the nervous system that connective tissue
fulfils everywhere. The characteristic support cells of
the brain are known generally as glia and include
astrocytes and oligodendrocytes.
11. NEURONS
• The functional and structural unit of the nervous
system is the neuron. Although neurons vary
widely in form and size they all share the same
basic structure, consisting of:
1) The cell body, or soma (contains the nucleus
surrounded by cytplasm)
2) The dendrites (numerous multi-branched
extensions or processes that make contact with
other neurons – convey impulses to the cell
body).
13. -
3) The axon (Usually a long single process which acts as
the neuron`s transmitter, conducting impulses away
from the cell body). Near its end, an axon splits into
numerous branches, which are called axon terminals or
terminal fibrils. The tips of these terminals are dilated
into tiny bulbs known as the synaptic knobs. These
knobs house numerous vesicles (sacs) filled with
chemicals, known as neurotransmitters, that are used
for communication between a neuron and another cell.
15. -
There are three types of neurons:
a) Unipolar (a single axon divides into two
branches)
b) Bipolar (two axons originate from
differentt points of the neuron cell body)
c) Multipolar (an axon and many dendrites
originate from the cell body)
18. THE NERVE IMPULSE
(how it is generated and how it travels through a neuron)
• A nerve impulse – an electrical charge – is the
signal that passes from one neuron to the next
and finally to an end organ.
Resting Membrane Potential:
The cell membrane of a neuron at rest has a
negative electrical potential of about -70mV. That
means that the electrical charges found inside the
cell and the charges found outside the cell differ
by 70mV, and that the inside is more negative
relative to the outside.
20. -
• This potential difference is known as the resting
membrane potential, or RMP. It is caused by a
separation of charges across the membrane.
When the charges along the membrane differ, the
membrane is polarized. This polarization is due to
a high concentration of potassium ions (K+) on
the inside and a high concentration of sodium
ions (Na+) on the outside because the sodium-
potassium pump actively moves sodium out of
the cell and potassium into it.
21. -
Depolarization and Hyperpolarization
If the middle of the cell becoms less negative relative to
the ouside, the potential difference across the
membrane will decrease. The membrane is now
depolarized. This depolarization occurs anytime the
change difference becomes less than the RMP of -
70mV, moving closer to zero. The opposite can also
occur. If the charge difference across the membrane
increases, then the membrane becomes more
polarized. This is known as hyperpolarization.
22. -
• Graded potentials
Graded potentials are localized changes in the
membrane potential. They can be either
depolarizations or hyperporizations. Graded
potentials are triggered by a change in the
neuron`s local environment. A graded potential
is usually just a local event, and the
depolarization does not spread very far along the
neuron. To travel the full distance, an impulse
must generate an action potential.
23. -
Action Potentials
An action potential is a rapid and substantial
depolarization of the neuron`s membrane. It
usually lasts only about 1 ms. Typically, the
membrane potential changes from RMP of -70
mV to a value of 30 mV, then rapidly returns
to its resting value.
25. -
Threshold and the All-Or-None Principle
All action potentials begin as graded potentials. When
enough stimulation occurs to cause a depolarization of
at least 15 to 20 mV, an action potential results. That
means if the membrane depolarizes from the RMP of -
70 mV to a value of -50 to -55 mV, the cell will
experience an action potential. The minimum
depolarization required to produce an action potential
is called the threshold.
Repolarization is when the neuron returns to its normal
resting state
27. -
Propagation of the Action Potential
The Myelin Sheath
The axons of most neurons are myelinated, covered with a
sheath formed by myelin, a fatty substance that insulates
the cell membrane. In the peripheral nervous system, this
sheath is formed by Schwann cells.
The sheath is not continuous. It exibits gaps between
adjacent Schwann cells, leaving the axon uninsulated at
those points. These gaps are referred to as nodes of
Ranvier. The action potential appears to jump from one
node to the next as it traverses the myelinated fiber. This is
referred to as saltatory conduction, a much faster rate of
conduction than in unmyelinated fibers
30. The Synapse
• Once the action potential is fired, the nerve impulse travels
the full length of the axon, ultimately reaching the axon
terminals. Neurons communicate with each other across
synapses. A synapse is the site of impulse transmission
from one neuron to another.
The neuron sending the impulse across the synapse is called
the presynaptic neuron, so axon terminals are presyaptic
terminals. Similarly, the neuron receiving the impulse on
the opposite sideof the synapse is called the postsynaptic
neuron, and it has the postsynaptic receptors The axon
terminals and postsynaptic receptors are not physically in
contact with each other. A narrow gap, the synaptic
cleft, separates them.
32. -
• A nerve impulse can be transmitted across a synapse only
in one direction: from the axon terminals of the presynaptic
neuron to the postsynaptic receptors usually on the
dendrites of the postsynaptic neuron.
• The presynaptic terminals of the axon contain a large
number of sac-like structures, called synaptic vesicles.
These sacs contain neurotransmitter chemicals. When the
impulse reaches the presynaptic terminals, the synaptic
vesicles respond by dumping their chemicals into the
synaptic cleft. The neurotransmitters then diffuse across
the synaptic cleft to the postsynaptic neurons receptors.
The postsynaptic receptors bind the neurotransmitter once
it diffuses across the synaptic cleft.
33. -
• When this binding occurs, the impulse has been
transmitted successfully to the next neuron and can be
transmitted onward. More than 40 neurotransmitters
have been identified. Acetylcholine and
norepinephrine are the two major neurotransmitters.
Neurotransmitters can have either excitatory or
inhibitory effects, or both.
• Once the neurotransmitter binds to the postsynaptic
receptor, the nerve impulse has been successfully
transmitted. The neurotransmitter is then either
destroyed by enzymes or actively transported back into
the presynaptic terminals for reuse when the next
impulse arrives.
35. THE NERVES
• The nerves are responsible for conveying nerve stimuli in
the peripheral nervous system. They form bundles and
some are long, extending from the spinal cord to the tip of
a finger or toe.
• There are two types of nerves, defined according to the
function: the somatic nerves, which are involved in
volunatary functions and are the type that stimulate the
muscles to produce movement; and the autonomic
nerves, which control involuntary functions such as the
functioning of the different organs.
• One of the most important nerves of the autonomic
nervous system is the vagus nerve, which controls many
vital functions such as heart rate, digestion, and breathing.
36. Structures of the Nerves
• Nerves are structures of different thickness and length. The
cell bodies of the neuronal axons that form the nerves are
situated in the central nervous system or in the collections
of cell bodies (ganglia) that lie next to the spinal cord.
• Each nerve is formed by one or more bundles of nerve
fibers. Each individual nerve fiber is the axon of a neuron
which is covered by the cytoplasm of a supporting cell
known as a Schwann cell. Large-diameter fibers are covered
by several concentric layers of Schwann cells, which form a
sheath of myelin.
• Each bundle of nerve fibers is surrounded by a layer of
connective tissue called the perineurium; if the nerve
contains many bundles, these are surrounded by another
layer known as the epineurium.
39. THE CENTRAL NERVOUS SYSTEM
The central nervous system is made up of the
brain and spinal cord. The brain functions to
receive nerve impulses from the spinal cord
and cranial nerves. The spinal cord contains
the nerves that carry messages between the
brain and the body
40. The Brain
The brain is composed of four major parts:
1. The Cerebrum
2. The Diencephalon
3. The Cerebellum
4. The Brain Stem
47. The Cerebrum
• The cerebrum is composed of the right and left
cerebral hemispheres. These are connected to each
other by fiber bundles referred to as the corpus
callosum alowing the two hemispheres to
communicate with each other. The cerebral cortex
forms the outer portion of the cerebral hemispheres
and has been referred to as the site of the mind and
intellect. It is also called the gray matter, which simply
reflects its distinctive color resulting from lack of
myelin on the cell bodies located in this area. The
cerebral cortex is our conscious brain. It allows us to
think, to be aware of sensory stimuli, and to voluntarily
control our movements.
51. -
• The cerebrum consists of five lobes – four
outer lobes and the central insula. The four
outer lobes are named after the bones that lie
directly above them.
• Numerous folds, or convolutions, called gyri
are found in the cerebrum surface. These are
separated by furrows called fissures or sulci.
• The remainder of the cerebrum is composed
primarily of white matter (myelinated axons)
53. The Diencephalon
• This region of the brain is mostly composed of the
thalamus and hypothalamus. The thalamus is an
important sensory integration center. All sensory input
(except smell) enters the thalamus and is relayed to the
appropriate area of the cortex. The thalamus regulates
what sensory input reaches our conscious brain, and
thus is very important for motor control.
• The hypothalamus, directly below the thalamus, is
responsible for maintaining homeostasis by regulating
almost all processes that affect the body`s internal
environment.
55. The Cerebellum
• The cerebelum is located behind the brain
stem. It is connected to numerous parts of the
brain and has a crucial role in controlling
movement. When the cerebrum initiates
muscular movement, the cerebellum
coordinates and refines the movement. The
cerebellum also maintains the equilibrium and
balance of the body
57. The Brain Stem
• The brain stem composed of the middle brain
(mesencephalon), the pons, and the medulla
oblongata, is the stalk of the brain, connecting
the brain and the spinal cord. All sensory and
motor nerves pass through the brain stem as they
relay information between the brain and the
spinal cord. This is site of origin for 10 of the 12
pairs of cranial nerves. The brain stem also
contains the major autonomic regulatory centers
that exert control over respiratory and
cardiovascular systems.
58.
59. THE SPINAL CORD
• The lowest part of the brain stem, the medulla
oblongata, is continuous below with the spinal
cord. The spinal cord is composed of tracts of
nerve fibers that allow two-way conduction of
nerve impulses. The sensory (afferent) fibers
carry neural signals from sensory receptors, such
as those in the muscles and joints, to the upper
levels of the CNS. Motor (efferent) fibers from the
brain and upper spinal cord travel down to the
organs.
61. Meninges
• Both the brain and the spinal cord are protected
against injury by bones. The brain is enclosed
within the skull and the spinal cord is enclosed
within the vertebral column. In addition, both the
brain and the spinal cord receive limited
protection from a set of three coverings called
meninges. The outermost coat, the dura mater, is
tough and fibrous. Immediately beneath the dura
mater is a cavity called the subdural space. It is
filled with serous fluid. The next layer of the
meninges is the arachnoid.
63. -
• A subarachnoid space filled with cerebrospinal
fluid, provides additional protection for the brain
and spinal cord by acting as a shock absorber.
Finally, the innermost layer, the pia
mater, contains numerous blood vessels and
lymphatics, which provide nourishment for the
underlying tissues. Cerebral fluid circulates
around the spinal cord and the brain and through
spaces called ventricles. These ventricles are
located within the inner portion of the brain.
65. THE PERIPHERAL NERVOUS SYSTEM
• The peripheral nervous system contains 43
pairs of nerves: 12 pairs of cranial nerves that
connect with tthe brain and 31 pairs of spinal
nerves that connect with the spinal cord.
Spinal nerves directly supply the spinal
muscles. Functionally, the peripheral nervous
system has two major divisioons: the sensory
division and the motor division.
67. The Sensory Division
• The sensory division of the peripheral nervous
system carries sensory information toward the
central nervous system. Sensory neurons
originate in such areas as: blood and lymph
vessels, internal organs, organs of special
sense (taste, touch, smell, hearing, vision), the
skin and muscles and tendons.
69. The Motor Division
• The central nervous system transmits
information out to various parts of the body
through the motor, or efferent, division of the
peripheral nervous system. Once the CNS has
processed the information it receives from the
sensory division, it decides how the body
should respond to that input. From the brain
and spinal cord, intricate networks of neurons
go out to all parts of the body providing
detailed instructions to the target areas.
70. THE AUTONOMIC NERVOUS SYSTEM
• The autonomic nervous system, often considered
part of the motor division of the PNS, controls
our body`s involuntary internal functions such as:
heart rate, blood pressure, respiration...
• The autonomic nervous system has two major
divisions: the sympathetic nervous system and
the parasympathetic nervous system. These
originate from different sections of the spinal
cord and from the base of the brain. The effects
of the two systems are often antagonistic, but
both systems always function together.
72. The Sympathetic Nervous System
• The sympathetic nervous system is our fight-or-flight
system – it prepares the body to face a crisis. When we
are excited, our sympathetic nervous system produces
a massive discharge through the body, preparing us for
action: increased heart rate, vasodilation, increased
blood pressure, bronchodilation, increased metabolic
rate, release of glucose from the liver...
• These basic alternations in body functions facilitate our
motor response, demonstrating the importance of the
autonomic nervous system in preparing us for acute
stress or physical activity
74. The Parasympathetic Nervous System
• The parasympathetic nervous system is our
body`s housekeeping system. It has a major
role in carrying out such processes as
digestion, urination, glandular secretion, and
conservation of energy. The system is more
active when we are calm and at rest. Its
effects tend to oppose those of the
sympathetic system. It causes: decreased
heart rate, constriction of coronary
vessels, and bronchoconstriction.
