Download General classification of peripheral nervous system

‫إجابة إمتحان مادة الفيزياء الحيوية الفرقة الثالثة لطلبة‬
)‫( تكنولوجية حيوية‬
1122 / 6 / 12 ‫ الثالثاء‬: ‫تاريخ االمتحان‬
‫ سميرة محمد سالم‬.‫ د‬:‫استاذ المادة‬
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Q(1) -The structure and function of center nervous system (CNS):
The brain and spinal cord comprise the CNS. About 9 neurons are
contained in the human CNS. They are present in the young child and do
not undergo mitosis even to replace those destroyed by accident or
disease. The neurons grow in size during maturation of an individual but
they do not increase in number.
a) Brain:
The brain is the most complicated organ in the body, and a complex,
fully-formed brain is one of the derived characteristics of vertebrates.
Fig.(1) shown six of major components in CNS. The forebrain
(prosencephalon) and midbrain (mesencephalon) are collectively called
the cerebrum. The cerebellum, Pons, medulla, and cord complete the
most prominent divisions of the CNS.
Fig.(1) as in page 79 in textbook.
The cerebrum is the largest part of the brain, and fills the upper
portion of the skull. The cerebral surface is called the cortex and is a
highly convoluted or folded surface. The cerebrum is divided into two
hemispheres, right and left, each half of the cerebrum hemisphere is
divided into lobes. The lobes are geographical regions of the cerebrum
and are shown in fig. (2).
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Fig.(2) The geographical regions of the cerebrum.
b) Spinal cord and spinal nerves
In the simplest reflex arc, messages from receptor organs are
transferred within the spinal cord directly from afferent fibers to efferent
fibers, which then send appropriate messages to efferent organs.
The function of the spinalcord:
is
to receive incoming impulses, integrate and coordinate them, transmit
them to wherever they should go within the central nervous system, and
send responses to the peripheral nervous system as appropriate.
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Fig.(3). A cross section of spinal card. The outer region is white mater; the central
area is gray mater, from which extended the dorsal and ventral roots that fuse to form
spinal nerves.
The general structure of the spinal cord: is best exemplified by a
cross-section of the spinal cord of an amniotes (Fig. 3):
• The grey matter lies on the interior of the cord while white matter lies
on the exterior
• The grey matter resembles the letter H, with the upper arms called the
dorsal columns or horns, and the lower arms called the ventral columns or
horns
• The grey commissural makes up the cross arm of the H and transmits
fibers from one side of the cord to the other
• The external white matter is divided into right and left sides by the
dorsomedian sulks and the ventromedian fissure
• The dorsal horn of the cord receives terminations of primary sensory
neurons
• The ventral horn contains the dendrites and cell bodies of motor
neurons.
Q(2) -The peripheral nervous system is (PNS):
The PNS is to connect the CNS to the limbs and organs. Unlike the
central nervous system, the PNS is not protected by bone or by the bloodbrain barrier, leaving it exposed to toxins and mechanical injuries.
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The peripheral nervous system is divided into: the somatic nervous
system and the autonomic nervous system.
General classification of peripheral nervous system:
a-By direction
There are two types of directions of the neurons:
1. Sensory neurons are afferent (i.e. relaying impulses TO the central
nervous system) as in fig.(4a).
2. Motor neurons are efferent (i.e. relaying impulses FROM the
central nervous system) as in fig.(4b). However, there are relay
neurons in the CNS.
( a)
(b)
Fig.(4a&b). Shows the motions of neurons are, a) afferent (sensory neuron) & b)
efferent (motor neuron).
b- By function
The peripheral nervous system is functionally as well as structurally
divided into the somatic nervous system and autonomic nervous system.
6.3.1. The somatic nervous system :
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is responsible for coordinating the body movements, and also for
receiving external stimuli. It is the system that regulates activities that are
under conscious control.
a)The sensory-somatic system consists of :


12 pairs of cranial nerves and
31 pairs of spinal nerves.
The cranial nerves are complex groups of nerves that are connected
to the CNS above the spinal cord.
 cranial nerves:
There are twelve pairs of cranial nerves that are attached to the CNS
at the base of the brain and at the brain stem. They serve specific
peripheral structures. The examples of cranial nerves are:
Nerves
Type
Function
I
Olfactory
sensory
Transmits information related to the sense of smell.
sensory
Arises from retina of eye and transmits information related to
vision.
II
Optic
(Contain 38% of all the axons connecting to the brain.)
III
motor*
Oculomotor
IV
Trochlear
V
Trigeminal
motor*
mixed
Arises in muscles that attach to the eyelid and eyeball and
move it in various directions.
Is a single muscle attached to the eyeball muscles
Nerve has a motor and sensory root. the former is
distributed to muscles for chewing; Sensory serve as in facial
and mouth sensation
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It should be consider that, there are thirty-one pairs of spinal nerves
that join the CNS by dorsal and ventral roots.
 The Spinal Nerves:
All of the spinal nerves are "mixed"; that is, they contain both sensory
and motor neurons.
All our conscious awareness of the external environment and all our
motor activity to cope with it operate through the sensory-somatic
division of the PNS.
b) The Autonomic Nervous System
The autonomic nervous system consists of sensory neurons and motor
neurons that run between the central nervous system (especially the
hypothalamus and medulla oblongata) and various internal organs such
as the(heart, lungs, viscera and glands).
The contraction of both smooth muscle and cardiac muscle is
controlled by motor neurons of the autonomic system.
The actions of the autonomic nervous system are largely involuntary
(in contrast to those of the sensory-somatic system). It also differs from
the sensory-somatic system is using two groups of motor neurons to
stimulate the effectors instead of one.
The first, the preganglionic neurons, arise in the CNS and run to a
ganglion in the body. Here they synapse with postganglionic neurons,
which run to the effector organ (cardiac muscle, smooth muscle, or a
gland).All the above discussion refer that the nerves response to
transfer the signals from muscles to CNS or reveres depended on the type
of neuron and stimulus for it.
1- The neuritis acting as the core conductors.
The complexities of the whole living cell will disregard and consider
it as body of cytoplasm with a membrane around it. One of the crucial
assumptions of cable theory is to go one step further and to also remove a
good deal of the morphological complexity of a neuron. Essentially, the
idea is the following: if we have a long, thin neuritis (also called "neural
process" which means just a part of a dendrite or axon), the voltage will
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vary much more along the long axis of the neural process than
perpendicular to it. So, we might just as well neglect this small variation
and only consider the variation along the long axis. This has the
important advantage that we can simplify our model from 3 dimensions
to one! This is the single-most important assumption of cable theory.
Let us consider the currents and voltages in a neural process. By
our simplification, the only spatial dimension that currents and voltages
depend on is the long axis of the nitrite, to which we assign the spatial
coordinate, x . Let us subdivide the process into little pieces of length
,
small enough so that the voltage is approximately constant everywhere
within each such piece. The cell membrane is an insulator and both the
inside and the outside of the cell are reasonably good conductors,
therefore the membrane can be considered a capacitor. Let the
capacitance per unit length be . The membrane is, however, not a
perfect insulator, therefore currents can flow across it or, in other words,
it has a finite conductance. Let the conductance per unit length be and
let be the leakage (or resting) potential of the cell. Then, the capacitive
and leakage currents across the membrane of our piece of neurite of
length
are, respectively,
(1)
Let us assume for simplicity that the specific resistance per unit length
for currents flowing along the neurite (not across the membrane) is
constant along the neuritis. The resistance for a piece of length ∆x.
Note that if we look at two points along the conductor, the resistance
between them increases proportionally to their distance. In contrast, the
transmembrane conductance (and capacity) of the membrane between the
two points are decreases with their distance. In other words, the core
conductance is in series while the transmembrane conductance is in
parallel.
The current between the locations is proportional to the voltage
difference and the inverse resistance of the conductor, thus
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( 2)
Likewise, the current between
and
is
(3)
Kirchhoff's current law applies:
and together with equations 1, 2 and 3, we obtain (4)
After dividing by
and taking the limit
spatial derivative on the LHS and we obtain
, we find the second
( 5)
Conventionally, we divide by the leakage conductance and the coefficient
of the first temporal derivative on the LHS becomes
.
decays exponentially towards the equilibrium value , with a
characteristic time . Likewise, the coefficient of the second spatial
derivative on the LHS after division by
becomes
and it
has the units of
. The length is the characteristic length over which
solutions of eq.5 decay in space. We can now write eq.5 in the convenient
form:
(6)
This is a Partial Differential Equation called the Telegrapher's
Equation.
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2-
The microstructure of muscle and what it's role of in
contraction of muscle:
Skeletal muscle is made up of thousands of cylindrical muscle fibers
often running all the way from origin to insertion. The fibers are bound
together by connective tissue through which run blood vessels and
nerves.
Each muscle fiber as in fig.( 5) contains:




an array of myofibrils that are stacked lengthwise and run the entire
length of the fiber;
mitochondria;
an extensive smooth endoplasmic reticulum (SER);
many nuclei.
The multiple nuclei arise from the fact that each muscle fiber develops
from the fusion of many cells (called myoblasts).
The number of fibers is probably fixed early in life. This is regulated
by myostatin, a cytokine that is synthesized in muscle cells (and
circulates as a hormone later in life). Myostatin suppresses skeletal
muscle
development.
Fig.(5).Shown the compassion of skeletal muscle which is contained the many of
muscle fibers.
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The muscle fiber is not a single cell, its parts are often given special
names such as :
plasma membrane, endoplasmic reticulum, mitochondria and
cytoplasm. although this tends to obscure the essential similarity in
structure and function of these structures and those found in other cells.
The nuclei and mitochondria are located just beneath the plasma
membrane. The endoplasmic reticulum extends between the myofibrils.
Fig. ( 6 ). Shown the striated muscle fiber which is containing many nuclei and
mitochondria are located just beneath the plasma membrane and the endoplasmic
reticulum extends between the myofibrils.
Seen from the side under the microscope, skeletal muscle fibers show a
pattern of cross banding, which gives rise to the other name: striated
muscle as in fig.( 6 ). The striated appearance of the muscle fiber is
created by a pattern of alternating, dark A bands and light I bands.
 The A bands are bisected by the H zone running through the center
of which is the M line.
 The I bands are bisected by the Z disk.
Each myofibril is made up of arrays of parallel filaments as in fig.( 7 ).
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

The thick filaments have a diameter of about 15 nm. They are
composed of the protein myosin.
The thin filaments have a diameter of about 5 nm. They are
composed chiefly of the protein actin .
Fig.( 7 ). Shown the structure of myofiber which is contain alternating
pattern, dark A bands and light I bands.
Shortening of the sarcomeres in a myofibril produces the shortening of
the myofibril and, in turn, of the muscle fiber of which it is a part.
The "contracting filament hypothesis" proposed that the filaments
themselves contract. Electron microscope observations, however, did not
support this hypothesis. Neither the thick nor thin bands changed in
length when the muscle contracted. Only the degree of overlap between
thick and thin filaments changed. Huxley alternatively proponed the
Sliding Filament Model, suggesting muscle contraction results as the
crossbridges linking the actin and myosin molecules pull the filaments
over one another.
In a contracted muscle the degree of overlap between thick and thin
filaments should be high. Consequently, Huxley hypothesized the force
generated by a muscle should depend on the degree of overlap.
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3- The heart conduction system:
The sinoatrial node (SAN), located within the wall of the right atrium
(RA), normally generates electrical impulses that are carried by special
conducting tissue to the atrioventricular node (AVN).
Upon reaching the AVN, located between the atria and ventricles, the
electrical impulse is relayed down conducting tissue (Bundle of HIS) that
branches into pathways that supply the right and left ventricles. These
paths are called the right bundle branch (RBB ) and left bundle branch
(LBB ) respectively. The left bundle branch further divides into two sub
branches (called fascicles).
Electrical impulses generated in the SAN cause the right and left atria
to contract first. Depolarization (heart muscle contraction caused by
electrical stimulation) occurs nearly simultaneously in the right and left
ventricles 1-2 tenths of a second after atrial depolarization. The entire
sequence of depolarization, from beginning to end (for one heart beat),
takes 2-3 tenths of a second.
All heart cells, muscle and conducting tissue, are capable of
generating electrical impulses that can trigger the heart to beat. Under
normal circumstances all parts of the heart conducting system can
conduct over 140-200 signals (and corresponding heart beats) per minute.
The SAN is known as the "heart's pacemaker" because electrical
impulses are normally generated here. At rest the SAN usually produces
60-70 signals a minute. It is the SAN that increases its' rate due to stimuli
such as exercise, stimulant drugs, or fever. Should the SAN fail to
produce impulses the AVN can take over. The resting rate of the AVN is
slower, generating 40-60 beats a minute. The AVN and remaining parts
of the conducting system are less capable of increasing heart rate due to
stimuli previously mentioned than the SAN.
The Bundle of HIS can generate 30-40 signals a minute. Ventricular
muscle cells may generate 20-30 signals a minute.
Heart rates below 35-40 beats a minute for a prolonged period usually
cause problems due to not enough blood flow to vital organs.Problems
with signal conduction, due to disease or abnormalities of the conducting
system, can occur anyplace along the heart's conduction pathway.
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Abnormally conducted signals , resulting in alterations of the heart's
normal beating, are called arrhythmias or dysrrythmia.
The intrinsic conduction system is a group of specialized cardiac cells
that pass an electrical signal throughout the heart. This ensures that heart
muscle tissue depolarizes and contracts in a sequential manner (from atria
to ventricles) resulting in a coordinated heart beat. The intrinsic
conduction systme is composed of the SA (sinoatrial) node, the AV
(atrioventrical) node, the bundle of His, right and left bundle branches,
and the Purkinje fibers. Referring to the figure below, you can see that
these components spread the depolarization waves from the top (atria) of
the heart down through the ventricles. These signals from the spinal cord
and brain are carried via nerves (the sympathetic and parasympathetic
nerves, respectively), and release neurotransmitters onto the heart which
speed or slow down the heart, respectively. The sympathetic nervous
system also increases the strength of the contraction.
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