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The Nervous System
• The nervous system is unique in the vast
complexity of thought processes and control
actions it can perform.
• It receives each minute literally millions of
bits of information from the different
sensory nerves and sensory organs and then
integrates all these to determine responses
to be made by the body.
Chapter objectives
1. Principles of neurophysiology
The function of neurons
Synaptic transmission
2. The functions of nervous system
Sensory function
Regulation of posture and movement
Regulation of visceral function
Advanced function
Section 1
The Functions of Neuron and
Neuroglia
(31 pairs)
Neuron
The basic structural
and functional
unit of the
nervous system is
the individual
nerve cell, called
as neuron.
There are about 100
billion neurons in
the CNS.
Neuron
The mostly function
of neurons operates
by generating and
transmitting
electrical signals
that move from one
part of the cell to
another part of the
same cell or to
neighboring cells.
Neuron
The elementary functions of neuron
(1) Receive the excitations or inhibitions induced by
internal or external stimulations.
(2) Analyze and integrate the information from every
organs.
(3) Generate or carry the demands regulating the
activities of the effectors.
(4) Some neurons have neuroendocrine function.
Basic Neuron types
(1) Bipolar----one process projecting from either end
of an elongated cell.
(2) Unipolar---nerve cell possessing only a single
process.
(3) Multipolar---numerous dendrites projecting from
cell body.
Structure of neuron
Cell body
dendrite
Receiving part
AP
Initial segment
Conduction
of AP
Axon collateral
axon
Transmitter
release
terminal
Fig. Diagrammatic representation of a neuron.
Function of Neuron
Processing of information.
Dendrite: receive the
nervous impulse.
Soma: intergrate the
message
Axon: carries the impulse
away from the cell body.
Action potential propagation in
an unmyelinated axon
※
Propagation of the action
potential
Conduction Velocity
• 1. One way of increasing the speed of
conduction is by increasing the size of the axon.
This reduces the internal electrical resistance
and increases the passive depolarization.
• 2. The advantage of myelination is an increase
in the speed of conduction, without a large
increase in metabolic cost.
• 3. Another way of increasing the speed of
conduction is by increasing the temperature.
• Classification of nerve fibers：
• 1.The letter system:
• Based on electrophysiological properties, mainly
the conduction velocity and characteristics of
Ap, this electrophysiological classification
divides fibers into three groups: A, B and C.
2.The number system:
• According to diameters and origins of fibers,
they are divided into four groups:Ⅰ, Ⅱ, Ⅲ and
Ⅳ.
efferent nerves
afferent nerves
Type
Ia
Origin
Type
Muscle spindle,
Aα
annulospinal ending
Ib
Golgi tendom organ
Aβ
II
Muscle spindle, flowerspray Aγ
ending,touch, pressure
III
Pain and cold receptor
Aδ
some touch receptors
IV
Pain,temperature, and
C
other receptors.
注：痛觉传入纤维习惯用Aδ类纤维和C类纤维
Types of nerve fibers
Fiber
type
Aα (I)
Aβ (II)
Function
motor α – fibers
spindle afferents (Ia)
tendon organs (Ib)
touch and pressure
Fiber Diameter,
μm
Conduction
velocity, m/s
13-22
70-120
8-13
30-70
Aγ
motor to muscle
spindles
4-8
15-30
Aδ(III)
pain, pressure,
temperature
1-4
12-30
B
preganglionic
1-3
3-15
C (IV)
pain, touch, heat
0.4-1.2
0.6-2.0
Axoplasmic transport
 Anterograde anxoplasmic transport:
soma → terminals
- Rapid transport : 410mm/d,
 organelles with membrane,
 neurotransmitters( neuropeptide),
 mitochondria and enzymes
- Slow transport: 1-12(0.5-10)mm/d.
microtubule and microfilament,
 Retrograde axoplasmic transport :
soma ← terminals
205mm/d.
NGF, virus and toxin,etc. by endocytosis.
• Anterograde
axoplasmic transport:
– Fast transport: occurs at
400mm/d,
such
as
mitochondria, secretory
vesicles.
Mechanism :kinesin
– Slow transport: occurs
at 0.5-10mm/d, such as
cytoskeleton proteins.
Mechanism :microbule
, microfilament
Fig. Axopasmic transport
• Retrograde axoplasmic transport:
• nerve growth factor,
• viruses. （Rabies）
• Mechanism :dynein
Fig. The method of horseradish peroxidase
Nerve retrograde tract-tracing
• HRP is injected to
the brain, then take
the brain to conduct
histochemical
reaction after two
weeks. HRP can be
used to the Nerve
retrograde tracttracing.
Fig. The method of horseradish peroxidase
HRP
• The enzyme horseradish peroxidase, found in horseradish, is
used extensively in molecular biology and in antibody
amplification and detection, among other things. For example,
"In recent years the technique of marking neurons with the
enzyme horseradish peroxidase (HRP) has become a major
tool. In its brief history, this method has probably been used by
more neurobiologists than have used the Golgi stain since its
discovery in 1870." Horseradish peroxidase is also highly used
in techniques such as Western blotting and ELISAs.
• HRP is widely used as an enzymatic label in immunoassays.
Usually, the enzyme is coupled to antibodies, lectins or haptens.
Coupling to antibodies etc. may be performed through the
carbohydrate side chains of the HRP.
Trophic action between nerve and tissue
1. Neurotrophic action on tissue:
muscle atrophy after nerve injury
glycogen synthesis ↓
protein decomposition ↑
poliomyelitis
poliomyelitis
poliomyelitis
• An acute infectious disease of humans, particularly children,
caused by any of three serotypes of human poliovirus
(POLIOVIRUS). Usually the infection is limited to the
gastrointestinal tract and nasopharynx, and is often
asymptomatic. The central nervous system, primarily the
spinal cord, may be affected, leading to rapidly progressive
paralysis, coarse FASCICULATION and hyporeflexia.
Motor neurons are primarily affected. Encephalitis may also
occur. The virus replicates in the nervous system, and may
cause significant neuronal loss, most notably in the spinal
cord. A rare related condition, nonpoliovirus poliomyelitis,
may result from infections with nonpoliovirus enteroviruses.
(From Adams et al., Principles of Neurology, 6th ed, pp7645)
Trophic action between nerve and tissue
2. Trophic action on nerve
Neurotrophin
•
•
•
•
•
•
•
•
•
•
•
•
Nerve growth factor(NGF)
Brain-derived neurotrophin factor(BDNF)
Neurotrophin 3
Neurotrophin 4/5
Neurotrophin 6
Ciliary neurotrophin factor(CNTF)
Glial cell-derived neurotrophin factor(GDNF)
Leukemia inhibitory factor(LIF)
Insulin-like growth factorⅠ(IGF-Ⅰ)
Transforming growth factor(TGF)
Fibroblast growth factor(TGF)
Platelet-derived growth factor(PDGF)
Trophic action between nerve and tissue
3. Receptors for neurotrophic factors
Neuroglia
 About 1.0×1012~ 5.0×1012 neuroglia cells ,
10~50 fold of neurons
 Dendrites and axons can
not be distinguished clearly
 No synapse formed
and no AP produced
The types of glia
 CNS - astrocyte
oligodendrocyte
microglia
ependymal cell
Choroidal epithelium
 PNS - Schwann cell
satellite cell
Neuroglial cells
CNS
Ependymal
Cell
Microglia
Oligodendrocyte
Astrocyte
Functions of glial cells
 Astrocytes (Astroglia)
- Support the neurons
- Clean up brain "debris"( damaged material)
and fill in the damaged area
- Transport nutrients to neurons
- regulate the external chemical environment of
neurons by removing excess ions, and
recycling neurotransmitters.
1.
2.
Transport nutrients to neurons,
They regulate the external chemical environment of neurons
by removing excess ions, notably potassium, and recycling
neurotransmitters released during synaptic transmission
• Astrocytes
– Regulate extracellular brain fluid composition
– Promote tight junctions to form blood-brain barrier
Ependymal Cells
– Line brain ventricles and spinal cord central canal
– Help form choroid plexuses that secrete
cerebrospinal fluid (CSF)
 Oligodendrocytes and Schwann cells
- myelinate axons
(1) insulate the axons
(2) facilitate the conduction of electrical impulses.
 Microglia
- act as the immune cells of the CNS
- remove most of the waste and cellular debris
from the CNS
- derivation,action in brain injury, action in
other diseases.
Section 2
General interactions
between neurons
• There are 10 11 neurons in central nervous system.
• Each neuron make around 1000 synapses.
• There are approximately 1014 synapses in the CNS.
Classifications of Synapses
– Chemical synapses (Classical Synapse)
neurotransmitter
▪Directed synapses (Typical synapses)
▪Non- directed synapses (Varicosity)
– Electrical synapses (Gap Junction)
local current
※
Synapse
• Synapse:
Synapse
is
the
junction
between
neuron and another
neuron or in some
cases a muscle or
gland cell where
information
is
transmitted from one
neuron to another
cell.
Synapse
Typical Synapses (chemical synapses)
Synapse
The small gap or space between the axon terminals of
one neuron and the dendrites or cell body of the
next neuron is called the Synapse .
Structure of Synapse
• Membrane of
presynaptic neuron
• Synaptic cleft
• Membrane of
postsynaptic neuron
Synapse
• Chemical synapse:
Most synapses use
chemical transmitter called
chemical synapse.
• Synaptic cleft is about
20-40nm.The pre- and
post-synaptic membranes
are often specialized.
• pre-synaptic knob contains
large members of vesicles
called synaptic vesicles.
gap junction
serial synapse
mixed synapses
reciprocal synapses
※ Synaptic transmission
Ap depolarizes the presynaptic membrane
Ca2+ channel open
Ca2+ influx
Vesicle move
release of transmitter
binding to the receptors
permeability of some
ions increase
postsynaptic potential
Synaptic transmission
Process of Typical Synaptic Transmission
1. An arriving action potential
depolarizes the presynaptic membrane.
2. Calcium ions enter the cytoplasma of
the synaptic knob.
3. Neurotransmitters release.
4. Neurotransmitters diffuse to and bind
to the receptors on postsynaptic
membrane.
5. Receptors on the postsynaptic
membrane are activated, producing a
postsynaptic potential.
6. Neurotransmitters are broken down.
Inactivation of Neurotransmitters
1. Be reuptaken by presynaptic membrane
or by Glial cells (Serotonin,NE)
2. Diffusion (Neuropeptide)
3. Enzymatic degradation (ACh )
Synaptic transmission
Electrical Activities of Postsynaptic Neurons
(Postsynaptic Potential)
Postsynaptic Potentials
•When a neuron responds to
the neurotransmitter
postsynaptically, it allows
ions to move across its
membrane.
Excitatory
postsynaptic potential
•The movement of ions
changes the membrane
potential of the postsynaptic
neuron.
•It is called the
Inhibitory postsynaptic potential
“postsynaptic potential”.
※
• The EPSP is produced by depolarization of the
postsynaptic membrane. During this potential, the
excitability of the neuron to other stimuli is
increased, and this potential is called the EPSP.
• The IPSP is produced by hyperpolarization of the
postsynaptic membrane. During this potential, the
excitability of the neuron to other stimuli is
decreased, and this potential is called the IPSP.
※
Types of postsynaptic potentials
EPSP:
 excitatory
postsynaptic
potential
 can help lead to the
production of an
action potential
 causes a
depolarization
IPSP:
 inhibitory
postsynaptic
potential
 can help to prevent
the production of an
action potential
 causes a
hyperpolarization
※
Postsynaptic potential
• EPSP (excitatory postsynaptic potential):
– Depolarization.
– Brings cell closer to threshold for an AP.
– Often Na+ and K+ channels.
• IPSP (inhibitory postsynaptic potential):
– Hyperpolarization.
– Takes cell further away from threshold for
an AP.
– Often Cl- channels.
Excitatory Post-synaptic Potential (EPSP)
AP depolarizes the presynaptic membrane
→ Ca2+ channel open,
Ca2+ influx
→release of transmitter
(excitatory)
→binding to the
receptors
→Na+, K+ permeability
increase
→ Na+ influx much more
readily than K+ efflux →
EPSPs→action potential
※
Inhibitory Post-synaptic Potential (IPSP)
AP depolarizes the presynaptic membrane
→ Ca2+ influx
→ release of transmitter
(inhibitory)
→binding to the receptors
→Cl-, K+ permeability
increase
→ Cl- influx, K+ efflux
→ IPSPs
→ reduces the probability
of producing excitation
※
Postsynaptic Potentials
EPSP and IPSP
Dorsal root ganglion,
DRG
Recording
Extensor motor neuron
Stim electrode
Inhibitory interneuron
Flexor motor neuron
Neuron Classification
By function (connections)
Sensory
Motor
Interneuron
Some synapses form on the dendrites, cell
body, or the axon hillock.
•At any given time, any
number of these
presynaptic neurons
(probably hundreds)
may be firing and thus
influencing the
postsynaptic neuron's
level of activity.
grand postsynaptic potential
• The total potential in the postsynaptic
neuron, the grand postsynaptic
potential (GPSP), is a composite of all
EPSPs and IPSPs occurring at
approximately the same time.
Summation
Summation:
EPSPs + IPSPs =
tells the axon of that cell whether to AP or not to AP!
• Temporal summation
– Many EPSPs or IPSPs arriving in a
short period of time (might all be coming from
the same presynaptic neuron)
• Spatial summation
– Many EPSPs and IPSPs arriving from many
different neurons (but at about the same time)
Summation of EPSP and IPSP
• Temporal summation
– Many EPSPs or IPSPs arriving in a
short period of time (might all be coming from the same
presynaptic neuron)
• Spatial summation
– Many EPSPs and IPSPs arriving from many different
neurons (but at about the same time)
Non-directed synapse
• Varicosity:
• -sympathetic
nerve
endings in smooth
and cardiac muscle.
• -The multiple
branches are beaded
with enlargements
that are not covered
by Schwann cells and
contain synaptic
vesicles;
• -high concentration
NE in the vesicles
Non-synaptic chemical transmission
• -No recognizable
postsynaptic specializations;
• -Transmitter is apparently
released at each varicosity, at
many locations along each
axon;
• -One neuron innervate many
effector cells.
• no 1:1 relationship
Non-synaptic chemical transmission
• Varicosity and effector
is not closely together
（>20μm），need 1 s
• The effects of
transmitters depend
on the type of
postsynaptic receptors
Electrical Synapses
•Represents low-resistance
pathways
•No synaptic cleft or
vesicles cell membranes in
direct contact
•Communication not
polarized- electric current
can flow between cells in
either direction
•Characteristics：
fast, bi-direction
The gap junction
Electrical Synapses
• significance：synchronous discharges of
neurons. an important role in the precise
synchronization of the activity of many cells
Electrical Synapse
Chemical Synapse
Purves, 2001
Summary
•
•
•
•
Terms:
1. Synapse
2. Excitatory postsynaptic potential (EPSP)
3. Inhibitory postsynaptic potential (IPSP)
Summary
• Questions:
1.The process of typical synaptic transmission.
2. The characters of AP conduction
(Properties of conduction in nerve fiber).
3. The process of excitatory postsynaptic
potential (EPSP)?
4. The process of inhibitory postsynaptic
potential (IPSP)?