Clinical Anatomy 28:692–693 (2015)
Letter to the Editor A Proposed New Function of the Cerebrospinal Fluid To the Editor, Clinical Anatomy: Cerebrospinal ﬂuid (CSF) has been said to have several potential functions including:
1. Maintains a constant environment for neurons and glia 2. Removes potentially harmful brain metabolites 3. Acts as a cushion for the brain within the skull 4. Provides buoyancy to the brain 5. Serves as a conduit for polypeptide hormones 6. Acts as the lymphatic system of the brain 7. Its pH affects ventilation and cerebral blood ﬂow CSF compartments communicate with each other and the subarachnoid space is bounded externally by the arachnoid membrane and internally by the pia mater, which extends along blood vessels that penetrate the surface of the brain. As an electrophysiologist specialized in extracellular, intracellular patch clamp (both current and voltage clamps) recordings and stimulation and after performing almost 2,000 deep brain stimulation procedures, I propose a new function of the CSF–shielding the brain from external electricity. This newly proposed function is based on a comparison experiment that demonstrated that an “Artiﬁcial CSF Faraday cage” was as effective as a copper Faraday cage in shielding an electronic device (cell phone) from electricity (Fig. 1). A Faraday cage is a container covered with conductive material, which blocks external static and non-static electric ﬁelds by channeling electricity along and around the conductive material with no current ﬂowing through the structure. This effectively provides for constant voltage on all sides of the container. An analogy to such a system is a computer: a metal frame serves as common grounding connecting all computer components and is covered with a metal case. Several issues support such a role of the CSF. First, the CSF is contained in a closed space (Faraday cage) between two nonconductive tissues, dura and pia mater, which house this conductive saline solution. Second, common grounding of referent potential 0 for all brain interfacing with CSF (given the importance of cortical folding between gyrus and sulcus, such folding creates huge surface area, which has maximum contact with CSF). The evolution of such a “natural Faraday cage” minimizes the interference of any external electrical disturbance. Such a system may also be the reason that lightning strikes result in cardiac arrest but not direct brain damage (personal communication with Li Feng MD, PhD, JD). In fact, Faraday cages are used to protect electronic equipment from lightning
2015 Wiley Periodicals, Inc.
Fig. 1. This is an example of a CSF Faraday cage. A cellphone sealed in a plastic bag and placed in the center of the container had a diminished signal reception.
strikes and electrostatic discharges. A Faraday cage operates because an external static electrical ﬁeld causes the electric charges within the cage’s conducting material to be distributed such that they cancel the ﬁeld’s effect in the cage’s interior. Faraday cages cannot block static or slowly varying magnetic ﬁelds, such as the Earth’s magnetic ﬁeld. To a large degree, though, they shield the interior from external electromagnetic radiation if the conductor is thick enough and any holes are signiﬁcantly smaller than the wavelength of the radiation. Additionally, the pia (Yamazaki and Kitamura, 2003) and dura mater have low conductivity but the CSF, skull, and scalp greatly attenuate brain signals (Yamada, 1995). The surface recorded electrical activity is greatly attenuated and *Correspondence to: C. Chris Kao, Department of Neurological Surgery, Vanderbilt University Medical Center, Nashville, Tennessee. E-mail: [email protected]
Received 27 May 2015; Accepted 27 May 2015 Published online 28 June 2015 in Wiley Online (wileyonlinelibrary.com). DOI: 10.1002/ca.22578
Letter to the Editor distorted by the time it reaches to the scalp. This is due to the inhomogeneous tissues (CSF, dura, skull, scalp) that intervene between the recording electrodes and cortical surface. Therefore, CSF might prevent electrical signals from both entering and leaving the brain. I have recorded millions of neurons using a microelectrode to do intraoperative brain mapping for the DBS cases. If the electrode entered the 3rd ventricle from a nucleus, all the active neuronal activities sudden disappeared because the tip of the electrode was “grounded” by the CSF. Similarly, when doing in vitro recording, artiﬁcial CSF immersed brain slices or cultured neurons would diminish when a conductive metal tipped shunt was used with the artiﬁcial CSF and this is why we use a non-conductive glass pipette electrode (Kao and Coulter, 1997).
C. Chris Kao* Department of Neurological Surgery, Vanderbilt University Medical Center, Nashville, Tennessee
REFERENCES Kao CQ, Coulter DA. 1997. Physiology and pharmacology of corticothalamic stimulation-evoked responses in rat somatosensory thalamic neurons in vitro. J Neurophysiol 77:2661–2676. Yamada T. 1995. The objective and perspective of recording electrical activity from the central nervous system. Rinsho Shinkeigaku 35:1323–1331. Yamazaki J, Kitamura K. 2003. Intercellular electrical coupling in vascular cells present in rat intact cerebral arterioles. J Vasc Res 40:11–27.