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We now know that the cell membrane contains ion channels that allow specific ions to flow through the lipid bilayer and we treated the neuron cells as small round objects. But Neuron cells are not small round objects. They contain three major areas
- The Soma (cell body)
- The Axon
This section covers the dendrites
The Dendrites are the inputs to the neuron cell. They have extensive arborization and in the case of the Purkinje cells connect to hundreds of thousands of other cells. Normally they are connected to about 10,000 cells. Each connection contributes to the operation of the neuron.
If you have not done so already download and install the SETI Net Neuron Simulator here.
One of the most famous images in neurobiology.
Two Purkinje neurons hand drawn by Santiago Ramon y Cajal
The cell Soma, which is the largest part of the neuron, typically has a diameter of 100 uM but the diameter of the dendrite may be down to 1 uM or less. The dendrites extend 100's of microns but some can extend several meters as in the giraffe brain.
Branching - Dendrites tend to bifurcate repeatedly and create (often several) large and complicated trees. Cerebellar Purkinje cells typically bear one very complicated tree with approximately 400 tips. Cells therefrom cat spinal cord usually have 8-12 trees each with approximately 30 terminal tips.
Diameters - Dendrites are thin tubes of nerve membrane. Near the soma they start with a diameter of few um and fall below 1 um as they successively branch.
Length - Dendritic trees may range from very short (100-200 um) to quite long (1-2 mm) and the total dendritic length may reach 1 cm and more.
Area and Volume - The majority of the brain volume and area is occupied by dendrites. The surface area of a single dendritic tree is in the range of 2,000 to 750,000 Cum.
A detailed multi-compartmental model of a cerebellar Purkinje cell , created with GENESIS simulator. This shows an input spike coming into the dendritic tree and spreading out as it moves down to the soma (the orange ball). The output of the soma is through a single axon - not shown
A neuron cell and its dendrite tree simplified for analysis as compartments:
These dendrite compartments can be thought of Leaky Electrical Cables with capacitance.
This picture is a view of the electrical properties of a uniform section of passive dendrite having length l and diameter d.
The ion channels are as described 1.0 The Cell. The current in and out (I axial) is defined by axial resistance to the current and the length of the compartment.
The conducting cytoplasm inside the neuron, the insulating neural membrane, and the liquid (similar to salt water) surrounding the neuron form a cable with a capacitance Cm.
The inner conductor, the cytoplasm, is a poorer conductor than the copper wire used in an undersea cable, and it has an resistance along the length of the cable Ra, the "axial resistance". The membrane in not a perfect insulator due to the ion-conducting channels that pass through it.
The "passive channels" do not vary in conductance, and the "active channels" that have conductances varying with voltage, calcium concentration, or synaptic input. The passive channels account for the membrane resistance Rm and the associated leakage current Ileak. The active channels are represented by the various variable conductances.
Compartment Electrical Equivalent
The various components of the cell compartment can be thought of as an electrical circuit and can be analyzed as such.
Simplest cable equation is the steady state where you can ignore the current flowing into the membrane capacitance Cm. The membrane voltage is given by (membrane current times the membrane resistance).
The quantities Rm, Ra, Cm, Vm, in the diagram and equation are given in ohms, farads, or volts, and will depend on the size of the compartment.
Specific units - In order to specify parameters that are independent of the compartment dimensions, specific units are used. For a cylindrical compartment, the membrane resistance is inversely proportional to the area of the cylinder, so we define a specific membrane resistance RM, which has units of ohms·m². The membrane capacitance is proportional to the area, so it is expressed in terms of a specific membrane capacitance CM, with units of farads/m².
Compartments are connected to each other through their axial resistances Ra. The axial resistance of a cylindrical compartment is proportional to its length and inversely proportional to its cross-sectional area. Therefore, we define the specific axial resistance RA to have units of ohms·m.
For a piece of dendrite or a compartment of length l and diameter d we then have:
These results are useful for determining the length of the model compartment. Sections of dendrite that have a continuous variation of voltage along the length are replaced by a "lumped parameter model" with discrete jumps in membrane potential. By using very many short compartments, the compartmental model can approach the result of the continuous cable equation.
Determining values of CM, RM, and RA
This is done by assuming that the values are the same in all compartments, as they are intrinsic properties of the neural membrane and cytoplasm. CM depends on the intrinsic properties of the thickness and dielectric constant of the membrane, and is usually close to 0.01F/m2 (n this simulator the equivalent value of 1uF/m2 is used).
The membrane time constant for a short uniform section is given by:
This Dendrite simulator calculates the RC Time Constant of a 1 cm2 patch of cell membrane by applying it to the following formula
The time constant is modeled by a passive membrane resistance, Rm, a membrane capacitance, Cm, and a current source. The membrane capacitance equals 1uF/m2 is, or simply 1 uF.
- Vm is the compartment membrane voltage at time t
- Vs is the height of the voltage step
- Vo is the voltage on the capacitor (t-1)
- RC is the calculated time constant of the compartment
Note that the Time Constant (RC) is independent of the dimensions, because Rm is inversely proportional to the surface area, and Cm is proportional to the surface area. RC is important to determining the time that it takes for ionic currents to produce changes in the membrane potential.
The exponential decay of the membrane potential as the current traverses this section of leaky cable is:
Where lambda is the point where the current will have been reduced by 63%
If you were to increase the leakage resistance Rm the slope would become more shallow. Conversely if the axial resistance Raxial were to increase you would get a sharper, faster drop in potential.
Excited and Depressed Dendrites
Dendritic connections from other neurons may be attached to any one of thousands of places along the dendritic tree, and each connection point consists of an ion channel that restricts the flow of ions through the membrane. The configuration of each connected Ion channel determines if the junction will stimulate or retard the downstream soma from firing. If the ion channel stimulates the soma, it is called Excitatory Post Synaptic Potential (EPSP). If the connection channel depresses the downstream soma from firing it is called an Inhibitory Post Synaptic Potential (IPSP)
The type of connection (EPSP or IPSP) is set up on the simulator when defining the input to the dendrite compartment. An EPSP connection is shown by a Black connection line. To define a synaptic connection as IPSP click the right column next to the connection you wish to designate. The word 'yes' will appear and the connecting line will change to red. A second click will remove the 'yes' and return the connection to EPSP.
The best way to get a feel for the interaction of the various parameters of the dendrite is to run a simulation. Download and install the SETI network Simulator if you have not already (as described in section 3. Neuron Simulator).