Anatomy & Physiology

Anatomy & Physiology Study Guide bolus , or small oval mass of gastrointestinal contents, along the length of the digestive tract. During a peristaltic movement, the circular muscles contract behind the bolus while circular muscles ahead of the bolus relax. 23.4 Control of Digestive Functions The actions of the digestive system are controlled by neural, hormonal, and local mechanisms. Neur l Mechanisms The movement of materials along the digestive tract, as well as many secretory functions, is controlled primarily by neural mechanisms. The motor neurons that control smooth muscle contraction and glandular secretion are located in the myenteric plexus. These neurons are usually considered parasympathetic because some of them are innervated by parasympathetic preganglionic fibers. However, the plexus also contains sensory neurons, motor neurons, and interneurons responsible for local reflexes whose internal workings operate entirely outside the control of the central nervous system. These reflexes are also called myenteric reflexes , and the term enteric nervous system is often used to refer to the neural network that coordinates the myenteric reflexes along the digestive tract. The digestive tract contains sensory receptors that also pass along the information to the CNS which can trigger long reflexes. These long reflexes involve interneurons and motor neurons in the CNS and provide a higher level of control over digestive and glandular activities. Generally controlling large- scale peristaltic waves that move materials from one region of the digestive tract to another. Hormonal Mechanisms The digestive tract produces at least 18 hormones that affect almost every aspect of digestive function and can enhance or inhibit the sensitivity of the smooth muscle cells to neural commands. The hormones (gastrin, secretin, cholecystokinin, and others), reach their target organs after their distribution in the bloodstream. Local Mechanisms Prostaglandins, histamine, and other chemicals released into interstitial fluid may affect adjacent cells within a small segment of the digestive tract. These local messengers are essential in coordinating a response to changing conditions (such as variations in the local pH or specific chemical or physical stimuli) that affect only a portion of the tract. 23.5 The Oral Cavity The oral cavity contains the tongue, salivary glands, and teeth, each with specific functions. The mouth opens into the oral cavity or buccal cavity . The duties of the oral cavity include sensory analysis of material before swallowing; mechanical processing through the actions of the teeth, tongue, and palatal surfaces; lubrication by mixing with mucus and salivary gland secretions; and limited digestion of carbohydrates and lipids. Achieve Page 316 of 368 ©2018