Creating a membrane section and manipulating its attributes

Membrane sections are represented by Section objects. They are instantiated with no arguments (although in the future it would be nice to be able to set section properties in the constructor):

>>> soma = Section()
>>> type(soma)
<type 'nrn.Section'>

As in hoc, a section's length, axial resistance and number of segments may be accessed and changed using dot notation:

>>> soma.L
100.0
>>> soma.nseg
1
>>> soma.Ra
35.399999999999999
>>> soma.nseg = 3
>>> soma.nseg
3

Each segment of the section may be accessed in turn by iterating over the section:

>>> for seg in soma:
...   print seg.x, seg.diam
...
0.166666666667 500.0
0.5 500.0
0.833333333333 500.0

or an individual segment may be accessed by calling the Section object with the x-location (0-1) as an argument:

>>> central_seg = soma(0.5)
>>> type(central_seg)
<type 'nrn.Segment'>
>>> central_seg.diam
500.0

Note the difference in syntax with respect to hoc:

oc> soma.v(0.5)
    -65
>>> soma(0.5).v
-65.0

The currently-accessed section

The concept of the currently-accessed section is less important in Python than in hoc, but it exists nonetheless. When we created the soma section above, it became the currently accessed section by default. The function cas() in the nrn module returns the currently accessed section as a Section object:

>>> soma.name()
'PySec_402d1040'
>>> cas().name()
'PySec_402d1040'

If we now create a new section, soma is still the currently-accessed section:

>>> dend = Section()
>>> soma == cas()
True
>>> dend == cas()
False

To make dend the currently-accessed section, use its push() method:

>>> dend.push()
>>> dend == cas()
True

We can now perform operations on dend using hoc calls, e.g.:

>>> dend.L
100.0
>>> h('L = 200')
1
>>> dend.L
200.0

(The 'h' HocObject comes from the neuron module. For more information on HocObjects, see accessing hoc from Python).

To return to the previously-access section, use the hoc pop_section() function:

>>> h.pop_section()
1.0
>>> soma == cas()
True

Connecting sections together

To connect two sections, call the connect() method of the child Section object with the parent section as the argument:

>>> dend.connect(soma)

By default, the '0' end of the child is connected to the '1' end of the parent. Which point on the parent to connect to and which end of the child to connect can be controlled with additional, optional arguments:

>>> axon = Section()
>>> axon.connect(soma, 0.1, 1)
>>> for sec in dend, axon:
...   sec.push()
...   h.psection()
...   h.pop_section()
...
<nrn.Section object at 0x402d1050>
PySec_402d1050 { nseg=1  L=200  Ra=35.4
        PySec_402d1040 connect PySec_402d1050 (0), 1
        /* First segment only */
        insert morphology { diam=500}
        insert capacitance { cm=1}
}
<nrn.Section object at 0x402d1060>
PySec_402d1060 { nseg=1  L=100  Ra=35.4
        PySec_402d1040 connect PySec_402d1060 (1), 0.1
        /* First segment only */
        insert morphology { diam=500}
        insert capacitance { cm=1}
}

It is often not necessary to explicitly push the section onto the stack, as most functions take an optional sec keyword argument:

>>> h.psection(sec=soma)
PySec_402d1040 { nseg=3  L=100  Ra=35.4
    /*location 0 attached to cell 0*/
    /* First segment only */
    insert capacitance { cm=1}
    insert morphology { diam=500}
}

Inserting membrane mechanisms

>>> soma.insert('pas')

Accessing range variables can be done in two ways: using a more object-oriented notation:

>>> soma(0.5).pas.g
0.001
>>> soma(0.5).pas.e
-70.0

or with a more hoc-compatible syntax using underscores:

>>> soma(0.5).g_pas
0.001
>>> soma(0.5).e_pas
-70.0

Contrast with the hoc syntax:

oc> soma.g_pas(0.5)
         0.001
oc> soma.e_pas(0.5)
         -70

To set values for all the segments in a section, iterate over them:

>>> for seg in soma:
...     seg.pas.g = 0.01*seg.x

Or, to set the same value for all segments:

>>> soma(0.5).e_pas = -64.0

For the most fine-scale control, the mechanisms can be addressed as Python objects:

for seg in soma:
...   for mech in seg:
...     if mech.name() == 'pas':
...       print seg.x, mech.g, mech.e
...
0.166666666667 0.00166666666667 -64.0
0.5 0.005 -64.0
0.833333333333 0.00833333333333 -64.0

Creating and inserting point processes

All hoc classes are accessible in Python through the h object. Of these, point processes such as IClamps must be associated with a membrane section, so we must either push the section onto the stack using, e.g. soma.push(), or, which is more convenient, pass the section as the sec keyword argument:

>>> stim = h.IClamp(0.5, sec=soma)
>>> type(stim)
<type 'hoc.HocObject'>
>>> stim.amp
0.0
>>> stim.dur
0.0
>>> stim.del
  File "<stdin>", line 1
    stim.del
           ^
SyntaxError: invalid syntax

What happened there? del is a reserved word in Python, which sometimes conflicts with names in hoc. For this reason, the IClamp delay attribute, which is called del in hoc, has been renamed to delay in Python:

>>> stim.delay = 50.0

Note, however, that the original name can still be accessed using the Python getattr() and setattr() functions:

>>> getattr(stim, 'del')
50.0
>>> setattr(stim, 'del', 100.0)
>>> stim.delay
100.0