Small electrical EEG signals need to be amplified and noise-reduced to be detectable. Noise reduction techniques include mathematical averaging and signal processing techniques like filtering.
Neuronal activity is electrical and generates potential differences around each neuron. Potential changes generate electrical fields. These electrical field are what EEG electrodes pick up (Fig. 1).
Fig. 1. Electrical field generated by a neuron (top) and the correspodning electromagnetic field (bottom). The former is detected with EEG, the latter with MEG. Source: Picton (2002)
The intervening tissues (dura mater and the skull) are not very conductive and indeed cause those potentials to be weakened. Worse, still, the electrical fields diffuse on their way to the scalp, meaning that adjacent patches of neurons cannot reliably be distinguished at the scalp. In other words, the spatial resolution of the EEG is very poor. The small amplitude of the potentials simply needs signal amplification to make them measurable. However, as @user49102 points out nicely, background noise is amplified with it. Hence, the magic hat of an electrophysiologist may be needed to extract the signal on the scalp from the noise by using noise reduction techniques such as signal averaging and filtering. A notable example where signal averaging is used is the event-related potential, or ERP. A stimulus (e.g. a flash of light) is repeatedly presented and the time-locked EEG is repeatedly measured and averaged offline. The random noise is reduced by the averaging process and neat ERP responses can be extracted without much filtering needed. A specialized example is the visual evoked potential, or VEP (Fig. 2):
VEP. Source Creel (2015)
Your question whether scalp cells (skin cells) have their own electrical responses - Well yes and no. The skin cells do not generate action potentials and feature only steady membrane potentials. Hence, if they are picked up by the electrodes it will only generate a stable offset, i.e., a baseline shift. Nothing to worry about. Neurons feature active potential differences through their action potentials, and that is what is picked up - voltage differences.
The low spatial resolution and weak signal means that the activity of single neurons cannot be measured. Instead, the gross signal from many thousands of neurons is collectively measured. In turn, when those neurons fire randomly, they will cancel each other's responses out and little signal is obtained. The most robust responses are obtained when the brain generates synchronized oscillatory potentials, for example during slow-wave sleep (Roth, 2009).
While EEG has poor spatial resolution of EEG, it excels in the temporal domain. Responses of a few milliseconds can be measured reliably (Fig. 2). As a comparison, fMRI has a temporal resolution in the order of seconds.
- Creel, Visual Evoked Potentials, In: Webvision
The Organization of the Retina and Visual System (2015)
- Picton, Int J Bioelectromagnetism (2002); 4(2): 225 - 8
- Roth, J Clin Sleep Med (2009); 5(2 Suppl): S4–S5