For multicellular organisms such as humans it is important that there is effective communication between spatially separated cells, tissues and organs. For example if you are the Spanish matador in the image on the right confronted by a charging bull, it is essential that the presence of the bull (detected by your eyes) is communicated quickly to your legs so that evasive action can be taken.
Failure of these communication systems can have quite dramatic consequences. 
Two major forms of communication exist within the human body:
Chemical Communication.
Some tissues synthesise chemical messengers (known as hormones) and secrete these into the bloodstream. These hormones usually travel through the systemic circulation and bind to receptors associated with their target tissues.
Not surprisingly communication mediated by hormones is relatively slow. Hormonal communication forms the basis of the endocrine system and is dealt with a little later in semester.
Electrical Communication.
Other tissues utilise electrical signals that travel along cells and can be readily transferred from cell to cell. These electrical signals travel very quickly and consequently constitute a much more rapid form of communication.
Tissues that utilise electrical signals for communication are know as excitable tissues (reflecting their electrical excitability) and are the topic of this lesson.
The major excitable tissues are neurones and all three varieties of muscle (skeletal, cardiac and smooth). Interestingly however a number of other cells types have been shown to exhibit electrically excitable properties that underpin their physiology. For example female gametes (oocytes) use their electrical properties to prevent fertilisation by more than one sperm during conception. You will learn about this process next semester,
Excitable tissues are traditionally considered one of the toughest topics in human physiology. For the most part the principles of electricity are not popular with biologists (who may indeed have turned to the life sciences because of a dislike of the physical sciences). Interestingly however this divide between the biological and physical scientist hasn't always been so wide. In fact, towards the end of the 18th Century excitable tissues (in the form of amphibian nerves and muscles) were instrumental in unraveling some of the basic physical principles of electricity. 
Despite their complexity, the study of excitable tissues is intrinsically interesting because electrical activity in the nervous system determines things like consciousness, memory and personality. Therefore, in many respects, excitable tissues really define what we, as humans, ‘are’.
Fortunately the physics required to understand excitable tissues are fairly rudimentary and grasping a few fundamental concepts is all that you need to comprehend communication in these types of tissues. In fact, virtually all of the basic principles required to understand the physiology of excitable tissues have already been covered in the previous lesson on membrane transport. Before continuing with his lesson, make sure that you understand the following concepts:
- What is meant by the term potential difference, what are its units and how you would measure it?
- What is a membrane potential and how would you measure it?
- The difference between anions and cations and the terms monovalent and divalent?
- How the membrane potential can be modified by movement of ions across the membrane.
- If a membrane is selectively permeable to one ion, the magnitude of the membrane potential is directly proportional to the concentration gradient of the ion across the membrane.
- What is meant by the term equilibrium potential.
- What is the Nernst Equation and how it can be used.
If any of these concepts are unfamiliar to you then you may like to revise the relevant sections of the membrane transport lesson. To test your understanding of these concepts you may like try the following review questions.
In order to comprehend excitable tissues you really only need to understand three different forms of the membrane potential. In order to make this as straightforward as possible we will initially focus how these membrane potentials look and ignore (for the moment) what causes them. We will return to the physiological-basis of these membrane potentials a little later in the lesson.