I
just bought a nice book on Broca’s area called “Broca’s Region” (Eds.
Grodzinsky and Amunts 2006). The book grew out of a workshop on Broca’s Region
Workshop that took place in Juelich, Germany in 2004. I don’t know whether many
people agree with Grodzinsky’s particular view of Broca’s area, but the book is
nice regardless, especially since we can buy it used for several bucks now and
it includes historical papers by Paul Broca and Norman Geschwind, to name a
few. Unfortunately for me, its biological section focuses on mapping /
localization of the Broca’s area and less on cortical microcircuits. It does
show a multivariate analysis of cytoarchitectonic asymmetry, in that the
Broca’s area (both BA44 and BA45) shows marked difference from BA6 (premotor
and supplementary motor area) in the left hemisphere compared to the right
hemisphere (Amunts and Zilles 2006).
Going
back to Tardif’s 2007 paper, it mentions the asymmetry regarding the extent of
horizontal connections in the Broca’s area. Bilaterally speaking (before going
into asymmetry), supragranular tracer injection found horizontal extension of several
mm found in in layers II, III, lesser in IV-VI. Layer IV injection found horizontal extension up
to 3.7 mm in I-IV, lesser in V, VI. Infragranular injection resulted in smaller
extension confined within infragranular layers. In terms of asymmetry, more
interlaminar difference in the horizontal extension was found in the left
hemisphere.
Before
concluding today’s blog, I’d like to take stock of what I know about generic (i.e.
not specific to Broca’s) intrinsic cortical microstructure before moving on to
interconnections between regions. I’m taking time here because it’s important
for me to internalize this microcircuit. So here’s the skeletal circuit (Harris et al. 2013), now including main
inhibitory neurons (Lee et al. 2010, Ma et al. 2010, Markman et al. 2004, Rudy
et al. 2001, 2011). Basket cells reside in layers II through VI, providing wide-area inhibition (axon branching up to 300-700 microns). The Martinotti cells also reside in layers II through VI but more in deep layers, and their axons extend to layer I and spread over a wide area. The function of bipolar cells is less clear, but they disinhibit other inhibitory cells, among other things.
Amunts
K, Zilles K. A multimodal analysis of structure and function in Broca’s region.
Grodzinsky & Amunts, ibid. 2006 Mar 24.
Grodzinsky
Y, Amunts K, editors. Broca’s region. Oxford University Press; 2006 Mar 24.
Harris
KD, Mrsic-Flogel TD. Cortical connectivity and sensory coding. Nature. 2013 Nov
7;503(7474):51-8.
Lee
S, Hjerling-Leffler J, Zagha E, Fishell G, Rudy B. The largest group of
superficial neocortical GABAergic interneurons expresses ionotropic serotonin
receptors. The Journal of Neuroscience. 2010 Dec 15;30(50):16796-808.
Ma
WP, Liu BH, Li YT, Huang ZJ, Zhang LI, Tao HW. Visual representations by
cortical somatostatin inhibitory neurons—selective but with weak and delayed responses.
The Journal of Neuroscience. 2010 Oct 27;30(43):14371-9.
Markram
H, Toledo-Rodriguez M, Wang Y, Gupta A, Silberberg G, Wu C. Interneurons of the
neocortical inhibitory system. Nature Reviews Neuroscience. 2004 Oct
1;5(10):793-807.
Rudy
B, Fishell G, Lee S, Hjerling‐Leffler J. Three groups of interneurons account
for nearly 100% of neocortical GABAergic neurons. Developmental neurobiology.
2011 Jan 1;71(1):45-61.
Rudy
B, McBain CJ. Kv3 channels: voltage-gated K+ channels designed for high-frequency
repetitive firing. Trends in neurosciences. 2001 Sep 1;24(9):517-26.
