Explain the need for neuronal communication over chemical signalling or passive electrical signalling
Neurons are required to send messages at high speeds over long distances. For example, reflex reactions occur in the order of fractions of seconds, and some neurons carry messages over 1m in distance.According to the Stoke-Einstein law, time taken for diffusion of chemicals such as hormones is proportional to the square of distance. The Stoke-Einstein law is given by t=x2/D, where t=time, x=distance and D is the diffusion coefficient. The diffusion coefficient factors in additional variables such as protein size and medium viscosity. This means that diffusion becomes a rate-limiting factor for transporting chemicals such as hormones over long distances.Additionally, the space constant (lambda) describes the decay of voltage as a signal travels passively over a distance. The space constant relates voltage at time t (Vt) to starting voltage (Vo) via the equation V=Vo e-x/lambda. This means that the voltage will decay by around 37% when passing across a distance of lambda, giving an exponential decay of voltage over distance. The myelin sheath of neurons has a high resistance and low capacitance, preventing the conduction of electrical signals across the sheath. Instead, the current jumps between the Nodes of Ranvier, where there is no myelin. This effectively increases the space constant. The importance of myelin sheaths is indicated by diseases such as Multiple Sclerosis, where episodic autoimmune destruction of myelin in the central nervous system causes severe neurological defects.
