Download nerve part 1

‫بسم هللا الرحمن الرحيم‬
‫﴿و ما أوتيتم من العلم إال قليال﴾‬
‫صدق هللا العظيم‬
‫االسراء اية ‪58‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Medical Physiology
Member of American Society of Physiology
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Organization of Human Body
• The human body is made up of different systems
e.g. digestive system, whose functions are
coordinated and integrated together.
• Each system consists of many organs that made
up of many tissues of complementary functions.
• Each tissue consists of millions of similar cells.
• The cell is the basic unit of structure and function
in the body
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Nervous System
• Nervous system is divided into;
• i) Central NS→consists of the brain and the spinal
cord.
• ii) Peripheral NS→ consists of;
• 12 pairs of cranial nerves arising from the brain.
• 31 pairs of spinal nerves arising from the spinal cord.
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Functions of NS
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Functions of NS
• 1-Sensory functions:
• It receives the sensory stimuli coming from skin,
viscera and special sense organs (eye, nose, and
ear).
• 2-Motor functions:
• It controls the activity of skeletal ms, smooth ms
and cardiac ms.
• 3-Higher mental functions:
• As learning and memory.
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Types of cells in NS
• NS is composed of 2 types of cells:
Neurons (Nerve cells)
Neuroglial cells
Def. They are the structural They support and
and functional units of protect the neurons
the NS
No. one trillion neurons
10 to 50 times No. of
neurons
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Types of cells in NS
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Neuron (Nerve Cell)
• 1) Def.,
• It is the structural and functional unit of the nervous
system.
• 2) No.
• One trillion neurons
• 3) Classification of neurons:
• a) Histological classification:
• Neurons are either multipolar, bipolar and unipolar
neurons
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Neuron (Nerve Cell)
• b) Functional (physiological)
classification:
• i) Sensory (afferent) neurons:
• They conduct impulses from
receptors to the CNS.
• ii) Motor (efferent) neurons:
• They conduct impulses from the CNS
to the effector organs.
• iii) Interneurons:
• They are small neurons.
• They serve the integrative functions
of the NS.
• They are about 99% of all neurons.
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Structure of Neuron
• 4) Structure of neuron:
• It is formed of cell body and cell processes
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Cell Body or Soma
• The cell body is the
enlarged part of the
neuron, which contains
the nucleus.
• It controls the metabolic
processes and provides
nutrition for the whole
neuron.
• It contains; mtiochondria,
microtubules and
microfilaments etc
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Cell Processes
• There are 2 cell processes;
Axon (Axis
Dendrites
cylinder)
Definition Single longer
Multiple short
process
processes
Functions Conduction of -↑ surface area of
impulses.
the cell body
- Collection of
impulses and
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Cell Processes
• There are 2 cell processes;
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Axonal Sheaths
• There are two sheaths that cover the axons:
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Axonal Transport
• Transport of materials in the axons can be done by 2 mechansims;
Slow axonal transport
Speed
1 mm/day.
Mechanism done
by
Fast axonal transport
400 mm/day.
protoplasmic done
streaming
through
the
microtubules
Transports new cytoplasm and proteins Materials
(nutrients
and
metabolites)
Direction
from the cell body toward the be either away from the cell
axon
direction
terminals
i.e.one body or towards it i.e. 2
directions
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Nerve Fibers
• The nerve (or nerve trunk) is composed of a larger number of nerve
fibers.
• Each nerve fiber is an axon covered by a myelin sheath and a Schwann
sheath.
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Properties of nerve fibers
• The nerve fibers have 2 properties,
1. Excitability is the ability of the nerve fibers to
respond to stimuli, and convert these stimuli into
nerve impulses
2. Conductivity is the ability of the nerve fibers to
conduct nerve impulses from one site to another
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Stimulus
• 1) Def.
• It is a change in the surrounding environment
(either external or internal environment)
• 2) Types of stimuli:
• a) Electrical stimuli:
• b) Chemical stimuli
• c) Physical stimuli
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Factors that determine the
effectiveness of the stimulus
• a) Strength of the stimulus
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Factors that determine the
effectiveness of the stimulus
• b) Duration
• -The stimulus to be effective must act for a
certain length of time known as the excitation
time. Within limit, the stronger the stimulus, the
shorter is excitation time
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Strength and Duration curve
(Excitability curve)
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Factors that determine the
effectiveness of the stimulus
• c) Rate of rise of intensity
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Nature of Excitability
• Excitability is a, bioelectric phenomenon in which
the nerve fibers respond to stimuli by rapid
changes in their resting membrane potential and
conduct action potentials along the nerve fibers to
their terminals
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Resting membrane potential
(RMP)
• 1-Def.
• It is the potential difference between the outside
and inside of nerve fiber during rest.
• 2-Value:
• It is about 70 mV (in nerve fiber) and 90 (in
skeletal ms)
• It is -70 mV because the inner surface of the cell
membrane is -vely charged relative to the
interstitial fluid.
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Resting membrane potential
(RMP)
• 3-Measurement of the resting membrane
potential
• By two microelectrodes with very fine tips less
than 1 um connected with a special voltmeter.
• One electrode is introduced inside the nerve fiber
and the other electrode is placed on its outer
surface.
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Resting membrane potential
(RMP)
• 3-Measurement of the resting membrane
potential
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4-Causes of the resting
membrane potential
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Causes of RMP
• A) Distribution of ions inside and outside the
nerve fiber
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Causes of RMP
• B) Selective permeability of the cell membrane
• The cell membrane is made up of double layers of
lipids, with specialized proteins penetrating the
double layers.
• These proteins form channels which regulate the
movements of ions across the membrane.
• Certain ions (Na, K, Cl and Ca) can cross the
membrane only through these protein pores
channels
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AS Biology, Cell membranes and
Transport
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Causes of RMP
• B) Selective permeability of the cell membrane
• These channels are 3 types
1. Passive ion channels
2. Chemically activated ion channels
3. Voltage activated ion channels
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Causes of RMP
•
•
•
•
•
B) Selective permeability of the cell membrane
Diffusion of ions through the cell membrane
a) K diffusion: (the main cause of RMP(
b) Na diffusion
c) Cl diffusion
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Causes of RMP
• Diffusion of ions through cell membrane
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Causes of RMP
• C) Sodium and potassium pump: (Na- K Pump):
• i) Structure:
• The pump is formed of a large protein molecule
(act as a carrier) in the cell membrane, which has:
a. ATPase property (ability to split ATP).
b. Na binding site→ at the inner surface
c. K binding site→ at the outer surface of the cell
membrane.
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Causes of RMP
• C) Sodium and potassium pump: (Na- K Pump):
• ii) Operation:
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Causes of RMP
• C) Sodium and potassium pump: (Na- K Pump):
• iii) Function of Na-K pump:
• It helps to maintain a high concentration of Na ion
outside and a high concentration of K ion inside the
nerve cells
• Any condition decreases the metabolic activity of the
cell e.g. by cooling →inhibits Na-K pump →Na+ ions
will accumulate inside the cell and neutralize the -ve
charges of protein ions and K ions that held on the
outer surface escape away, and RMP becomes
progressively lost.
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THANKS
Action Potential
• A) Def.
• It is the electrical changes, which occur in the RMP
as a result of stimulation of N.F. by an effective
stimulus
• B) Initiation or phases of action potential
• i) Depolarization
• ii) Repolarization
• iii) Redistribution of ions inside and outside
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i) Depolarization
• The stimulus ↑es the permeability of the cell membrane
(several hundred folds) to Na ions, which diffuse inside
causing gradual change in the membrane potential from
the resting potential (-70m.v) to the threshold potential or
the firing level (-55m.v).
• At -55m.v, m gates of the voltage activated sodium
channels open and Na ions flow into the cell (Na influx).
• As a result of sudden Na influx, the membrane potential
quickly reaches zero potential and then overshoots to
about +35 mv, so there is a momentary reversal in polarity
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i) Depolarization
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ii) Repolarization
• At the threshold potential (-55 mv), the voltage sensitive K
channels also open, but begin to act after a slight delay
time.
• Repolarization results from closure of h gates of voltage
gated Na channels and opening of the n gates of voltage
gated K channels, allowing K ions to diffuse out of the cell
(K efflux or outflux).
• Diffusion of K ions outside returns the inside of the
membrane to its original -ve potential l(-70m.v) i.e.
restore the RMP i.e. repolarization
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ii) Repolarization
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iii) Redistribution of ions inside and
outside
• After depolarization and repolarization, the ionic
composition inside and outside the cell membrane is
slightly disturbed.
• Redistribution of Na and K ions to the normal resting
condition is established by the Na-K pump which actively
transports Na out and K into the cell
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Propagation or Conduction of
Action Potential
• The action potential must be propagated in order
to transfer information from one place in the
nervous system to another site
• There 2 types of propagation of action potential;
a. Saltatory conduction in myelinated nerve fibers
b. Continuous conduction in unmyelinated nerve
fibers
• conduction velocity is ↑ by fiber size &
myelination.
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Continuous Conduction
• occurs in unmyelinated nerve fibers
• relatively slow→ 0.5-2.0 m/sec
• Mechanism
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Saltatory Conduction
• occurs in myelinated nerve fibers
• fast (may reach up to 120 m/sec)
• Mechanism
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Saltatory Conduction
• Significance
• a- It ↑es the velocity of conduction
• b- It ↓es the energy needed for the Na-K pump →
restricted to the nodes of Ranvier.
• Myelinated fibers use about 1% of the energy used by
unmyelinated fibers.
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Recording of the action potential
• i) Aparatus
• The cathode ray oscilloscope (CRO) consists cathode ray
tube which fires a beam of electrons directed to a special
screen
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Recording of the action potential
• ii) Methods
• 2 methods for recording of
action potential
• A) Biphasic action
potential:
• a) Method:
• By two micro electrodes
placed on the outer surface
of the nerve fibers→
connected with the CRO.
• b) Record:
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Recording of the action potential
•
•
•
•
ii) Methods
B) Monophasic action potential
a) Method: recorded by one of the following methods:
i) If the nerve is damaged in between the two electrodes
used in recording the biphasic action potential.
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Recording of the action potential
•
•
•
•
ii) Methods
B) Monophasic action potential
a) Method:
ii) If one micro electrode (recording electrode) is
introduced inside the nerve fiber and the other electrode
(reference electrode) is placed in the extracellular fluid far
away from the excited region.
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Monophasic Record of Action Potential
• Monophasic action
potential shows the
following:
• ◊ Latent period:
• A) Spike potential
• B) After potentials
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Monophasic Record of Action Potential
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Effects of stimulation of the nerve fibers
• A) Stimulation by ineffective (subminimal) stimuli
• Ineffective stimuli produce a localized area of
depolarization which does not reach the firing level
(-55 mv) known as "the local excitatory state".
• B) Stimulation by effective stimuli produce a
propagated action potential or a nerve impulse.
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Local excitatory state
• Local excitatory state is characterized by;
• 1) It is localized to the site of stimulation and
nearby area.
• 2) During conduction, it decreases gradually with
distance till it disappears (conducted with
decrement)
• 3) Its duration is very short (less than 1 m. sec).
• 4) Can be graded: its magnitude is proportional
with the strength of the subminimal stimulus i.e. it
dose not obey the all or non law.Also, it has no
threshold.
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Local excitatory state
• Local excitatory state is characterized by;
• 5) It can be summated to the local excitatory state
of other subminimal stimuli.
• Two types of summation can occur:
• i) Spatial summation: By applying several
subminimal stimuli at the same time at different
sites near each other.
• ii) Temporal summation: By applying several
subminimal stimuli at the same site within a very
short time (less than the duration of the local
excitatory state)
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Local excitatory state
• Local excitatory state is characterized by;
• 6) It is associated with increased excitability
because the nerve fibers can respond to
subminimal stimuli applied at the same time.
• 7) It has no refractory period.
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Properties of the nerve impulse
• I. The nerve impulse is a wave:
• It is characterized by
Type A fibers
Type B fibers
Type C fibers
Meylination
thick myelinated
thin myelinated
thin unmyelinated
Diameters
2-20 um
1-3 um
0.5-1 um
Speed
10-120met/sec
3-15 met/sec
0.5-2 meter/sec
Examples
i)
somatic
sensory -preganglionic
fibers
cutaneous
carrying sympath
and
i)postganglionic
and
autonomic fibers
deep parasympathetic fibers. ii)
Somatic sensory
sensation
fibers
carrying
pain
ii) Somatic motor fibers
sensation (delayed pain).
to skeletal ms.
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Properties of the nerve impulse
• I. The nerve impulse is a wave:
• It is characterized by
Type A fibers
Type B fibers
Type C fibers
Nerve
- high magnitude -moderate
-low magnitude
impulse
–short duration
magnitude
-prolonged
-0.5-1 m. sec
-moderate
duration
duration
-2.0 m. sec
-1.2 m. sec
Sensitive
prolonged
deep O2 lack
local anaesthetics
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Action potential in mixed nerve fibers
• The AP in the mixed nerve is
called the compound action
potential
• The mixed nerve is formed of
bundles of nerve fibers of
different types
• The action potentials
produced in these fibers are
formed of multiple peaks
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Properties of the nerve impulse
• II. The nerve impulse obeys all or non law
• The nerve impulse either occurs maximally or it does not
occur at all, provided that all other conditions remain
constant. So;
• -All subminimal stimuli do not produce response
• -Minimal (threshold) stimulus produces a maximal
response.
• -Further increase in the intensity of the stimuli
(superminimal, maximal, supermaximal) do not
produce any further increase in the response
• All or non law is applied in the single nerve fiber and not
applied in the nerve trunk.
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Amplitude
Local excitatory state
Nerve impulse
(Graded potential)
(Action potential)
Proportional with the intensity of Not
proportional
with
the
the stimulus i.e. not obey all or non intensity of the stimulus i.e.
law.
Summation
obey all or non law.
Can be summated→ temporal Can't be summated
and spatial
Threshold
Has no threshold.
Has a threshold.
Refractory period Not followed by a refractory It is followed by a refractory
period.
Conduction
period.
Localized to site of stimulation or Conducted by saltatory or
nearby area.
continuous conduction.
Passive conduction.
Active conduction.
Conducted with decrement.
Conducted without decrement.
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THANKS