regenerate axons peripherally, it is not possible in the CNS. This is because the PNS nerves have a covering called a neurilemma that creates a path for the nerve axon to regenerate. Neurons can be divided into being afferent neurons, efferent neurons, or interneurons. Interneurons connect two neurons together and can go to or from the CNS. Afferent neurons are sensory neurons that carry peripheral signals to the CNS. They have long dendrites and shorter axons. Efferent neurons send signals from the CNS to the periphery—often to muscles or glands. They tend to have short dendrites and very long axons. The peripheral nervous system can be afferent (sensory) or efferent (motor).
DIGESTIVE SYSTEMS The digestive system of animals has a major digestive tract and several accessory organs. It takes larger molecules through ingestion, and processes them to make smaller molecules that can be absorbed in the GI tract. The major functions of the digestive system are to ingest food, break it down mechanically, break it down chemically, and absorb the small, absorbable molecules. The final act of the digestive tract is the elimination of waste products that are not digestible. The digestive tract, while diverse throughout the system, is basically a continuous tube from mouth to anus. It starts with ingestion through some type of mouth. In many cases, there are teeth that start the process of mechanical digestion (or breakdown). In mammals, there is a small amount of chemical digestion (via saliva) in the mouth as well. The stomach also participates in both the mechanical and chemical digestion, breaking up proteins and churning food into chyme. Most chemical digestion takes place through the action of hydrolysis in the upper small intestine using pancreatic enzymes and brush border enzymes along the intestinal wall to break down proteins, carbohydrates, and fats into absorbable nutrients. The swallowing action of the mouth is called deglutition. This is voluntary. Outside of that, there are involuntary muscles that undergo peristalsis in order to pass food through the GI tract. Besides peristaltic movements, there are segmental movements in the small intestine that move chyme back and forth so it can be maximally absorbed. It 209
