An agent that is smaller than a virus causes scrapie. It is either a protein called a prion or it is another substance closely associated with the prion protein. Scientists still have a lot to learn about the scrapie agent and exactly how it works.
Tissue valves are made with tissues from porcine (pig) heart valves or bovine (cow) cardiac tissue because they function like human heart valves. Once the tissue is removed from the animal, it is chemically treated to preserve the tissue and prevent immulogic reactions once it is placed in a patient.
Nervous tissue is composed of nerves containing neurons, specialized cells. Muscle tissue contains myocytes, which have completely different structure than neurons.Also, muscle is organized in many fibers covered by connective tissue; these fibers build up stronger fibers etc.ervous tissue is made up of different cells working together to  perform a given function -- in this case, to carry messages from  one part of the body to another. Differen â¦ t nervous tissues working  together make up nervous system organs, such as nerves, the spinal  cord, or the brain.