best Nerve system physiology ppt

1. School of medicine
Department of Medicine
Physiology of Nerve tissues, Membrane Potentials and synaptic transmission
Zelalembanjaw9@gmail.com
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2. Outlines
1. Nerve tissue-Neurons and neuroglial cells
2. Electrical conduction via nerves
3. Synapse and neurotransmitters
4. Cerebral blood flow & cerebrospinal fluid

3. Nerve tissue Physiology

4. Nervous System
Nervous system is divided into:
1. Central nervous system (CNS)
 Brain
 Spinal cord
2. Peripheral nervous system (PNS)
 Cranial nerves
 Spinal nerves
2 principal cells in the nervous system:
I. Neurons
 Functional, signal conducting cells
 specialized for Sensory function, Generation of
thought, Storage of memory, Integrates idea,
Coordinates muscular activities
II. Neuroglia
 Supporting cells
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5. I. Neuroglia
 Neuroglial cells are:
 Supporting cells
 Non-excitable cell
 5 to 50 times more numerous.
 Are smaller in size than neurons
 Can multiply after maturation
 Potential causes of glioma (brain
tumour)
 Neuroglia provides:
 Support, nourish, and protect the
neurons & maintain homeostasis in
the interstitial fluid that bathes them.
 6 types of neuroglia
I. Astrocytes
II. Microglia
III.Oligodendrocytes
IV.ependymal cells
V. Schwann cells and
VI.satellite cells
 The first four neuroglias are found only
in CNS while the later two are found
only in PNS.

6. I. Neuroglia…
1. Microglia
 Specialized immune cells that act as the macrophages
of the CNS.
 Why is it important for the CNS to have its own
army of immune cells?
2. Astrocytes
 Star-shaped, abundant
 Provide nourishment to the Neurons and involved
in the formation of the blood brain barrier
 Inter connect nerve fibers(axons) and blood vessels
3. Oligodendrocytes
 Produce the myelin sheath which provides the
electrical insulation for certain neurons in the CNS
4. Ependymal cells
 line the ventricles of the brain and the central
canal of the spinal cord
Secrete cerebrospinal fluid (CSF) and assist
in its circulation.
5. Schwann cells (Neurolemnocyte)
 Form myelin sheaths around the larger nerve
fibers in the PNS
 Vital to neuronal regeneration (participate in
regeneration of PNS axons.)
6. Satellite cells
Small cells that line the exterior surface of the
PNS and regulate the external chemical
environment.

7. I. Neuroglia…

8. 8
II. Neurons
 Neurons are functional & structural units of the nervous system.
 A neuron has 3 distinct parts. These are:
 Cell body
 Dendrites and
 Axon
 Specialized to conduct information from one part of the body to another
 There are different types of neurons but most have certain structural and
functional characteristics in common:
– Cell body (soma)
– One or more specialized, slender processes (axons/dendrites)
– An input region (dendrites/soma)
– A conducting component (axon)
– A secretory (output) region (axon terminal)

9. II. Neurons…
• Dendrites & Cell body
 Input Zone
• Nucleus
 Biosynthetic center
• Axon hillock
 Trigger Zone
• Axon
 Conducting Zone

10. a. Soma
 Contains nucleus plus most normal organelles.
 Biosynthetic center of the neuron.
 Contains a very active & developed rough
endoplasmic reticulum which is responsible for the
synthesis of Neurotransmitters.
 The neuronal RER is referred to as the Nissl body.
 Contain multiple mitochondria.
 Acts as a receptive service for interaction with other
neurons.
 Most somata are found in the bony environs of the
CNS.
 Clusters of somata in the CNS are known as nuclei.
 Clusters of somata in the PNS are known as ganglia.

11. Neuronal Processes
 2 types of processes that differ in structure and
function:
i. Dendrites &
ii. Axons
 Tracts : Bundles of processes in the CNS
 Nerves : Bundles of processes in the PNS
i. Dendrites
Are thin, branched processes whose main function is to
receive incoming signals.
They effectively increase the surface area of a neuron to
increase its ability to communicate with other neurons.
 Small, mushroom-shaped dendritic spines further increase
the surface area
Convey info towards the soma through the use of graded
potentials – which are some what similar to action potentials.

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ii. Axons
 Myelinated /unmyelinated
 Most neurons have a single axon (long up to 1m)
process designed to convey info away from the cell body.
 Originates from a special region of the cell body called
the axon hillock.
 Transmit APs from the soma toward the end of the axon
where they cause NT release.
 Often branch sparsely, forming collaterals.
 Each collateral may split into telodendria which end in
a synaptic knob, which contains synaptic vesicles –
membranous bags of NTs.
Neuronal Processes…

13. Synapse
The site where two neurons or a neuron and
an effector cell can communicate is termed a
synapse
The tips of most axon terminals swell into
synaptic end bulbs
These bulb-shaped structures contain
synaptic vesicles, tiny sacs that store
chemicals called Neurotransmitters
The neurotransmitter molecules released
from synaptic vesicles are the means of
communication at a synapse.

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Neuronal Processes…
Axolemma
 Axon plasma membrane
 Surrounded by a myelin sheath, a wrapping of lipid which:
– Protects the axon & electrically isolates it
– Increases the rate of AP transmission
Q: What myelin sheath is made up of in PNS& CNS are?
 This wrapping is never complete. Interspersed along
the axon are gaps where there is no myelin – these are
nodes of Ranvier.
 In the PNS, the exterior of the Schwann cell
surrounding an axon is the neurolemma

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Types of Nerve Fibers
1. Group A (α, β, γ, δ)
 Axons of the somatic sensory neurons and motor neurons serving the skin,
skeletal muscles, and joints.
 Large diameters and thick myelin sheaths
– How does this influence their AP conduction?
2. Group B
 Type B are lightly myelinated & of intermediate diameter.
 Belong to autonomic preganglionic neurons
3. Group C
 Type C are unmyelinated and have the smallest diameter.
 Autonomic nervous system fibers serving the visceral organs, visceral
sensory fibers.
 Small somatic sensory fibers are Type B and Type C fibers.

16. Membrane potential

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Membrane potential
Resting Membrane potential
 All cells have a voltage difference across their plasma membrane.
This is called membrane potential.
 The membrane potential (VM) at rest is called resting membrane
potential (RMP).
 At rest, there are electropositivity out and electronegativity inside in
cell membrane of the neuron.
 Cell’s ability to fire an action potential is due to the cell’s ability to
maintain the cellular resting potential at approximately –70 mV.

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What are the causes of the RMP?
1. An outward diffusion of K+ through K+ leak channels. The ECF is
very high in Na+ while the ICF is very high in K+. As a result, K+ is
constantly leaking out of the cell.
2. The Na+/K+ pump is constantly pumping 3 Na+ ions out and 2 K+
ions in for every ATP used. Thus more positive charge is leaving
than entering.
3. There are protein anions (i.e., negatively charged proteins) within
the ICF that cannot travel through the PM.
 What this adds up to is the fact that the inside of the cell is negative
with respect to the outside. The interior has less positive charge
than the exterior.

19. Basic Electrophysiological Terms
 Polarization
- a state in which membrane is polarized at rest, negative
inside and positive outside.
 Depolarization:
- the membrane potential becomes less negative than the
resting potential (-70 mV).(the reverse of polarization
due to the rapid opening of Na+ channel )
 Repolarization:
- when the membrane recover (returns) to the resting
potential after depolarization.(due to the slower opening
of K+channels and the closing of Na+ channels )
 Hyperpolarization:
- membrane potential become more negative than the
resting potential (-70 mV).(due to the out flow of K+
may be so great)
• Excitability: the ability to respond to a
stimulus and convert it into an action
potential.
• Excitable cells: cells that generate action
potential during excitation.
• Stimulus : any change in the environment
(internal or external environmental condition
of the cell).
• Threshold stimulus: any stimulus strong
enough to initiate an action potential (nerve
impulse).
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 Changes in membrane potential are due to changes in ion movement across the
membrane.
 Movement is through ion channels.
I. Leak channels: non-gated, always open
II. Gated channels: opened and closed in response to specific triggering events.
Of three types:
1. Voltage-gated channels:
- respond to changes in membrane potential
2. Chemically gated channels (ligand gated):
- respond to binding of a specific chemical messenger
3. Mechanically gated channels:
- respond to stretching or other mechanical deformation.
Role of Ion Channels

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Action Potentials
 An immediate change of the RMP in to
depolarization is called action potential (nerve
impulse).
 Action potential is a rapid, conductive, and
reversible change of the membrane potential after
the cell is stimulated.
 If VM reaches threshold, Na+ channels open and
Na+ influx , depolarizing the cell and causing the
VM to increase.
 Eventually, the Na+ channel will have inactivated
and the K+ channels will be open. Now, K+
effluxes and repolarization occurs.
 K+ channels are slow to open and slow to
close. This causes the VM is became
Hyperpolarization.

22. Refractory Periods
 The period of time during which the neurone
can not generate another nerve action
potential is called the refractory periods.
 Until repolarization occurs , the neuron can
not conduct another nerve impulse.
 Period of decreased excitability following an
action potential
1. Absolute refractory period
 During the time interval between the opening
of the Na+ channel activation gate and the
opening of the inactivation gate, a Na+
channel CANNOT be stimulated.
2. Relative refractory period
 Follows absolute refractory period
 Additional action potential possible with
stronger stimulus . the stronger the stimulus
→ the more likely to cause an additional AP.
 Some Na channels still inactivated
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23. Refractory Periods
Absolute refractory period
 Axon membrane is incapable of
producing another AP.
Relative refractory period
 VG ion channel shape alters at the
molecular level.
 VG K+ channels are open.
 Axon membrane can produce
another action potential, but
requires stronger stimulus.
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Conduction of Action Potential
 If an AP is generated at the axon hillock, it will travel all the way
down to the synaptic knob.
 The manner in which it travels depends on whether the neuron is
myelinated or unmyelinated.
i. Unmyelinated neurons undergo the Sweeping /continuous
conduction/ of an AP whereas
ii. Myelinated neurons undergo jumping /saltatory conduction/ of
an AP.

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1. Continuous (Sweeping) Conduction
 Occurs in unmyelinated axons.
 The whole length of the
membrane is depolarized (AP
occurs on whole length of the
axon membrane)
 Velocity of conduction is slow
 Consumes large amount of
energy

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2. Saltatory (Jumping) Conduction
 Occurs in myelinated axons.
 AP occurs at the axon of nodes of Ranvier.
 Velocity of conduction is faster
 Consumes few amount of energy
Orthodromic Vs antidromic Conduction
• Orthodromic conduction
 Soma Axon terminal
• Antidromic conduction
 Axon terminal Soma

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Saltatory Conduction
Nodes of Ranvier
• Only region where current flow across the membrane exist.
• Region of concentrated voltage-gated Na channels.

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Factors Affecting Rates of AP Conduction
1. presence or absence of myelination
 Myelinated axons faster rate of AP conduction than unmyelinated axons
2. Diameter of fiber (size of nerve fiber)
 An axon with a large diameter conduct an AP faster than axon with a
small diameter .
3. Age
 Slower in babies and elderly but maximum b/n the age of 5-15 years .
4. Temperature
 When warmed, nerve fibers conduct impulse at highest speed; when cooled
at lower speed.

29. Synaptic Transmission

30. Synapses
 The region where there is a transfer
of message from a neuron to the
next.
 The junction between two cells in
which one must be a neuron.
There are 3 types of synapses
1. Neuro-neuronal junction
 junction b/n two neuron.
 Presynaptic neuron and
postsynaptic neuron
2. Neuro-muscular junction
 junction occurred between neuron &
muscle.
3. Neuro-glandualr junction
 junction occurred between neuron & gland
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31. Types of neuroneuronal junctions
3 types of neuroneuronal
junctions
1.1 Axo-dendritic junctions
1.2 Axo-somatic junctions
1.3 Axo-axonic junctions
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32. Synaptic Transmission
Two modes of transmission
1. Electrical transmission
 via gap junctions
2. Chemical transmission
 Via neurotransmitters.
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Chemical Synapses
 Chemical messenger (neurotransmitter) is released from a
neuron into the synaptic cleft.
 Neurotransmitter in the synaptic cleft binds to a receptor on the
target cell.
 Acts slower than electrical synapses because the
neurotransmitter must diffuse across the synaptic cleft to bind
the receptor.
 Advantages over electrical synapses
 One-way direction of communication
 Allows for different kinds of signaling events.

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Components of Axo-Somatic synapse
1. Presynaptic terminal
 contains neurotransmitter
(NT)
1. Synaptic cleft
 contains ECF and Enzymes
2. Postsynaptic neuron
 contains receptor for the
action of NT

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Mechanism of Chemical Synaptic Transmission
• An AP reaches the presynaptic axon terminal of the presynaptic
cell and causes V-gated Ca2+ channels to open.
• Ca2+ rushes in, binds to regulatory proteins & initiates NT release
by exocytosis.
• NTs diffuse across the synaptic cleft and then bind to specific
receptors on the postsynaptic membrane and initiate postsynaptic
potentials.
• NT-Receptor interaction results in either EPSP or IPSP.

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Mechanism of Chemical Synaptic Transmission…
• When the NT-R combination
triggers the opening of ligand
gated Na-channels, this leads to
membrane depolarization, EPSP.
e.g. Ach on Nicotinic receptor
• When the NT-R combination
triggers the opening of ligand
gated K or Cl-channels, this leads
to membrane hyperpolarization,
IPSP.
e.g. GABA on GABAb receptor

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Excitatory Vs Inhibitory Synapses
• Excitatory
 more likely to have action potential
 depolarization
• Inhibitory
 less likely to have action potential
 hyperpolarization
 membrane stabilization

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Excitatory Synapses
 Depolarizes postsynaptic cell
 Causes depolarization
because of the stronger force
of Na+ to flow into the cell.
 Depolarization= EPSP
(excitatory postsynaptic
potential)

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 Neurotransmitter binds to receptor,
channels for either K or Cl open
 hyperpolarizes the cell
 If K +channels open
 K + moves outIPSP (inhibitory
postsynaptic potential)
 If Cl channels open, either
 Cl moves in  IPSP
 Cl stabilizes membrane potential
Inhibitory Synapses

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Neurotransmitter Removal
 NTs are removed from the
synaptic cleft via:
 Enzymatic degradation
 Diffusion
 Reuptake

41. Properties of synaptic transmission
1. Unidirectional conduction
2. Synaptic delay (0.5 -1.0m/s)
3. Fatigue -↓in response of
postsynaptic neurons after repetitive
stimulation by the pre-synaptic
neurons
4. Synaptic potentiation
(facilitation):– persistence of out put
signals after the stoppage of in put
signal
5. PH: Alkalosois ↑ Synaptic transmission,
Acidosis ↓ Synaptic transmission
6. Hypoxia ↓ Synaptic transmission
7. Synapse is a target for the action of several
groups of drugs
 Caffeine, theophylline, theobromine
↑Synaptic transmission
 Strychinine ↑ Synaptic transmission
 Hypnotics, anesthtics, tranquilizers ↓
Synaptic transmission
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Classes of neurotransmitters
Classes Neurotransmitters Receptors Distribution, role
I Acetylcholine (Ach) Nicotinic receptors
Muscarinic receptors
Excitatory in
CNS, PNS
II Amines Adrenaline, nor-adrenaline
Dopamine
Histamine
α and β adrenoreceptors
Dopaminergic Rs: A, B
Histaminergic Rs: H1 & 2
Excitatory
Inhibitory in BG
Excitatory
III Amino
Acids
GABA (γ-amino butyric acid)
Glycine
Aspartate
Glutamate
GABA-A and B receptors
Glycine receptors
NMDA receptors
NMDA receptors
Inhibitory in BG
Inhibitory
Excitatory
Excitatory
IV Polypep Hypothalamic hormones,
pituitary hormones, ANG-II
Hormone receptors Stimulatory or
inhibitory
V Nitric oxide Memory

43. Reading Assignment
1. Cerebral blood flow & cerebrospinal fluid

44. Thank you!