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 10 11 (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 Nucleus Axon 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)

10 5 synaptic inputs By neuron‘s shape thousands of synaptic inputs (10 3 )

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 μm 3 : 10 10 water molecules, 10 8 ions, 10 7 small molecules such as amino acids and nucleotides, and 10 5 proteins • In water, the molecules Na + , K + , Cl and Ca 2+ 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

qV T



kT V T



kT q

kT: The thermal energy of an ion K: Boltzmann constant q: the charge of a single proton

V

T : 24 ~27 mV Membrane potentials: about -3 to +2 times V T

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 membrane capacitance 1. membrane conductance: g = 1/ r m 2.

membrane time constant:

τ m = R

m

C

m = r m

c

m (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

[ outside ]  [ inside ] exp(

zqE

)

kT

 [ inside

zE

] exp(

V T

)

Nernst equation

E



V T z

ln( [ outside [ inside ] ] )

Examples: Sodium ions:

o = 60 mM, I = 440mM E = 27 ·

ln

(440/60) = 54mV

Potassium ions:

o = 400 mM, i=20 mM 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



i



g i

(

V



E i

)

Leakage current

the resting potential

i



g L

(

V



E L

) 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？