Transcript For electrical signaling

LECTURE 2
Basic Electrical Properties of
Neurons
I.
Basic neural system
II. Membrane properties
III. Synapses
IV. Action potential generation
Two classes of cells
• Nerve cells (neurons，神经细胞或神经元)
− For electrical signaling
− The human brain has 1011 (1000 亿) neurons
• Glial cells (神经胶质细胞）
− Not for electrical signaling
− 10 to 50 times more glial cells than neurons
− Play an essential role in brain metabolism
− Support neurons
− Cover neurons with myelin
− Clean up debris
Neuron structure
• cell body
– contains nucleus （核）& organelles （细胞器）
• dendrite
– conducts signals to cell body
• axon
– long fibers
– specialized for electrical conduction
Structure of a typical neuron
Dendrite
Axon Terminal
Node of Ranvier
Cell Body
Axon
Nucleus
Schwann Cell
Myelin Sheath
Differences between neurons and other
cells
1. Neurons have specialized extensions called
dendrites and axons. Dendrites bring information to the cell
body and axons take information away from the cell body
2. Neurons communicate with each other through an
electrochemical process
3. Neurons contain some specialized structures (for example,
synapses) and chemicals (for example, neurotransmitters)
Classification of neurons by function
Sensory (or afferent) neurons (感觉神经元）
They send information from sensory receptors (e.g., in skin,
eyes, nose, tongue, ears) TOWARD the central nervous
system
Interneurons（中间神经元）
They send information between sensory neurons and motor
neurons. Most interneurons are located in the central nervous
system
Motor (or efferent) neurons（运动神经元）
They send information AWAY from the central nervous
system to muscles or glands
By the number of extensions extending
from the neuron's cell body
Pseudounipolar cells-伪单极神经元
One process extends centrally toward the spinal
cord, the other extends toward the skin or
muscle
( dorsal root ganglion cells)
Bipolar neurons - 双极神经元
Two processes extending from the cell body.
(retinal cells, olfactory epithelium cells)
Multipolar neurons - 多极神经元
Many processes but has only one axon. (spinal
motor neurons, pyramidal neurons, Purkinje
cells)
By neuron‘s
shape
105 synaptic inputs
thousands of
synaptic inputs
(103)
A cortical pyramidal cell
A Purkinje cell of the cerebellum
A stellate cell of the cerebral cortex
(magnified about 150 fold)
(Drawings from Cajal 1911)
I.
Basic neural system
II. Membrane properties
III. Synapses
IV. Action potential generation
The soma of a neuron: 4 - 100 μm in diameter
Nucleus
• 1 μm3: 1010 water molecules, 108 ions, 107 small molecules such
as amino acids and nucleotides, and 105 proteins
• In water, the molecules Na+, K+, Cl- and Ca2+ are in ionic form
A schematic diagram of a section of the
lipid bilayer
3 to 4 nm
~10 nm long
(Dayan and Abbott 2001)
Basic points: physiological specializations
• A wide variety of membrane-spanning ion channels
(Na+, K+, Ca2+, and Cl−)
• Ion channels control the flow of ions across the cell
membrane (voltage-gated, ligand-gated, and
others)
• This type of membrane is called semipermeable
I.
Basic neural system
II. Membrane properties
III. Synapses
IV. Action potential generation
Diagram of a synapse
(Kandel et al. 1991)
Signal transmission at synapse
Electrical synapses
At gap junctions, cells approach within about 3.5 nm of
each other, rather than the 20 to 40 nm distance that
separates cells at chemical synapses
Postsynaptic potential in electrical synapses is not caused by
the opening of ion channels by chemical transmitters, but by
direct electrical coupling between both neurons
Electrical synapses are therefore faster and more reliable than
chemical synapses. (Most of time, biderectional)
电突触主要存在于蚯蚓、虾、软体动物等无脊椎动物
I.
Basic neural system
II. Membrane properties
III. Synapses
IV. Action potential generation
A rough estimation of membrane
potential
qVT  kT
kT
VT 
q
kT: The thermal energy of an ion
K: Boltzmann constant
q: the charge of a single proton
VT: 24 ~27 mV
Membrane potentials: about -3 to +2 times VT
Intracellular Resistance
the intracellular resistivity
Membrane potentials measured at different places within a neuron
can take different values
For neurons with electrotonic
compactness, we have …
Membrane Capacitance and Resistance
membrane resistance
1.
membrane capacitance
membrane conductance: g = 1/ rm
2. membrane time constant: τm = RmCm = rmcm (10 and 100 ms)
(the basic time scale for changes in the membrane potential)
The restriction in measuring membrane
resistance
Why is the restriction to small currents and small voltage needed here?
Equilibrium potentials
-+
[inside]
+ +
+ +
+
+
+
+
[outside]
+
+
+
+
+
+
+
+
+ +
+
+
+
Concentration gradient
Voltage gradient
zqE
zE
[outside]  [inside] exp(
)  [inside] exp( )
kT
VT
+
+
Nernst equation
VT
[outside]
E  ln(
)
z
[inside]
Examples:
Sodium ions:
o = 60 mM, I = 440mM
Potassium ions: o = 400 mM, i=20 mM
E = 27·ln(440/60) = 54mV
E = 27·ln(20/400) = -80mV
Chloride ions: E = -65mV (near the resting potential of many neurons)
Calcium ions: E = 130mV
Equilibrium and reversal potentials
Equilibrium potentials
(The Nernst equation applies when the channels allow only one
type of ion to pass through them)
Some channels are not so selective, and in this case the potential
E is estimated by the Goldman equation
Reversal potentials takes a value intermediate between the
equilibrium potentials of the individual ion types that it conducts
The Membrane Current
• For one ion type with reversal potential E:
i  g (V  E )
• For several ions through different channels:
i   gi (V  Ei )
i
Leakage current
the resting potential
i  g L (V  EL )
Leakage conductance: a passive conductance
All of the time-independent contributions to the membrane current
can be lumped together into a single leakage term.
For example, the currents carried by ion pumps that maintain
the concentration gradients that make equilibrium potentials nonzero
Action potential
An action potential: a roughly 100 mV fluctuation in the electrical potential
across the cell membrane that lasts for about 1ms
Depolarization and hyperpolarization
Absolute refractory period: a few milliseconds just after an action potential
Relative refractory period: lasting up to tens of milliseconds after a spike
Subthreshold potential fluctuations
Subthreshold potential fluctuations are severely
attenuated over distances of 1 mm or less
Three simulated recordings from a
neuron
(Dayan and Abbott 2001)
作业及思考题
1. 胶质细胞主要类型和功能.
2. 如何估计膜电位数量级？
3. 说出钠、钾和氯离子通道平衡电位的数量级。
4. 如果用电极分别在锥体细胞的胞体内、胞外和距
胞体一定距离的轴突内记录动作电位串，结果有
何异同？