The nervous system of the leech is said to be very similar to the human nervous system. As the pictures in Figure 1 and 2 show, the nervous systems of a leech and human have some similarities, the most obvious is the central nervous tissue and ladder-like nerve network.
These similarities are of enormous benefit to researchers in their quest for the answers to human problems. The medicinal leech Hirudo medicinalis can completely repair its central nervous system (CNS) after injury. This invertebrate model offers unique opportunities to study the molecular and cellular basis of the CNS repair process.
In humans, the development of the CNS starts with the formation of the neural plate about the 18/19th day post fertilization (Figure 4) – the time when the embryo begins to acquire the shape of a leech (Figure 3).
2. Intermediate zone of neural crest
3. Neural groove
4. Neural crest
5. Neural fold
6. Dorsal root ganglion
7. Neural tube
8. Surface ectoderm
|1. Neural plate
2. Primitive node
3. Primitive streak
4. Cut edge of amnion
|1. Primitive streak
2. Cut edge of amnion
3. Neural fold
4. Neural groove
6. Primitive node
|1. Pericardial bulge
2. Anterior neuropore
4. Posterior neuropore
− Figure 4. The central nervous system appears in the middle of the 3rd week of the development as a thickened area of the embryonic ectoderm. Source: http://www.chronolab.com/atlas/embryo/nervous.htm
The nervous system develops from embryonic tissue called the ectoderm. The first sign of the developing nervous system is the neural plate that can be seen at about the 18th day of development. Over the next few days, a “trench” is formed in the neural plate – this creates a neural groove. By the 21st day of development, a neural tube is formed when the edges of the neural groove meet. At 23 days post-fertilization the shape of the embryo’s entire body is tubular, which gives the embryo its leech-like appearance. The rostral (front) part of the neural tubes goes on to develop into the brain and the rest of the neural tube develops into the spinal cord. Neural crest cells become the peripheral nervous system. 
1. Biopharm Leeches: http://www.biopharm-leeches.com/links.html (accessed 30 June 2013).
2. Human-Leech Nervous system: http://commons.wikimedia.org/wiki/File:Human-leech-nervous-system-comparison.png (accessed 30 June 2013).
3. The medicinal leech, once used in medicine to draw blood from patients, may provide new clues to neural regeneration. Purdue researcher Christie Sahley and her research assistant, Orie Shafer (background) are using the leeches to analyze the role nitric oxide synthase may play in neural regeneration. Medicinal leech may offer clues to neural regeneration, Purdue News, 1996. (Accessed 26 July 2013).
4. Gonzalo Moscoso: Structural Development of the CNS, Chapter 2: Early Embryonic Development of the Brain. (accessed 30 June 2013).
5. Neuroscience For Kids: http://faculty.washington.edu/chudler/dev.html (accessed 30 June 2013).