Nervous and Endocrine Systems

The endocrine system comprises glands, which secrete hormones (chemical messengers) into the bloodstream. The hormones then travel within the bloodstream to their target tissue, where they can exert their effect. Endocrine glands are ductless organs of many different types. Regardless of their type, they all function to secrete hormones into the body's fluid to help with systemic regulation and communication. There are nine endocrine glands.

• The hypothalamus is a structure in the brain that links the nervous and endocrine systems, responding to signals from other parts of the brain and hormone feedback mechanisms. The hypothalamus produces hormones that regulate the pituitary gland.
• The pituitary gland is located near the bottom of the brain and is often called the master gland because its hormones control several other endocrine glands. It consists of two lobes, each producing different hormones. One such hormone is growth hormone (GH), which aids in development and growth.
• The pineal gland is a small endocrine gland located in the brain that plays a role in the production of melatonin, a hormone that aids in the regulation of sleep patterns.
• The thyroid gland is an endocrine gland located in the front of the neck that produces thyroid hormones that help regulate metabolic processes, heat production, and the development and function of other organ systems. The thyroid hormones contain iodine.
• A parathyroid gland is one of four small endocrine glands located near or within the thyroid gland that release hormones that help maintain calcium and phosphorus levels in the body. These hormones are important for bone development.
• An adrenal gland is one of two endocrine glands located on the top of each kidney consisting of the medulla and the cortex that release hormones that help regulate critical functions in the body, such as blood pressure, the immune system, and the metabolism. The inner portion, called the adrenal medulla, releases hormones such as epinephrine that increase heart rate and increase blood flow to the muscles. The adrenal cortex, found on the outer portion of the gland, helps control blood sugar levels, responds to stressful stimuli, and helps the kidneys regulate blood pressure.
• The pancreas is found near the stomach and small intestine. It secretes digestive enzymes that help with digestion. It also releases hormones to help maintain healthy glucose levels in the body. If blood sugar levels are too high, the pancreas secretes insulin. If blood sugar is too low, the pancreas causes the release of stored glucagon to increase blood sugar.
• There are reproductive glands in both males and females. The ovary aids in production of estrogen and progesterone hormones in females. Both hormones are important to maintain sexual traits and aid in pregnancy. The testes produce testosterone that helps with male development and the maintenance of male sexual traits.

Multicellular organisms have complex nervous systems that function to receive and process internal or external information and to control body responses. These systems must be able to receive and interpret sensory signals that come from external stimuli. Once received, these signals must be transmitted through the body so that the organism can respond. All body activities are performed through a coordinated action of the nervous system with other body systems. The nervous system is divided into two broad categories: central nervous system (CNS) and peripheral nervous system (PNS). The central nervous system consists of the brain and spinal cord. The peripheral nervous system consists of nerves that travel from the CNS to the rest of the body.

The brain is the control center, regulating internal responses to stimuli, controlling organism behavior, and maintaining homeostasis. It processes external information received from sensory organs, as well as internal information from hormone feedback. The spinal cord links the brain to the rest of the body through nerves of the PNS. This is regarded as a neural highway where neural signals mediate the flow of information to and from the brain. The PNS is further divided into two systems: the somatic nervous system and autonomic nervous system. The somatic nervous system provides signals to and from the skeletal muscles. Most of the voluntary responses to signals in the environment are controlled by the somatic nervous system. For example, the process of catching a ball involves a variety of sensory signals and voluntary muscular responses.

The autonomic nervous system plays a role in involuntary processes such as heartbeats and digestion. It also plays a role in homeostasis through the components of the sympathetic nervous system, the system that stimulates the body for a fight-or-flight response during an environmental stimuli response. The autonomic nervous system stimulates routine functions like the regulation of digestive enzymes or saliva production through the components of the parasympathetic nervous system. The parasympathetic nervous system slows down the heart rate after the fight-or-flight response elicited from the sympathetic nervous system.

The nervous system includes two cell types: neurons and glial cells. Cells that make up the nerves and brain are called neurons. Glial cells support and regulate neurons. The structure of a neuron includes the cell body, dendrite, axon, and axon terminal. The cell body houses the nucleus of the neuron. Dendrites are used to receive signals, while the axon is the main conducting unit of a neuron. The axon is the site where electrical signals travel along the neuron. These signals can travel long distances, depending on the length of the axon. An insulating membrane called myelin sheath is wrapped around the axons of most nerve cells of vertebrates. This membrane is produced by glial cells. Each separation point between myelin sheaths is called a node of Ranvier. Myelin aids in electrical signaling by increasing the rate of nerve impulse transmission. At the end of a nerve cell (or neuron) is an axon terminal. Synapses are the junctions between adjacent neurons. At these synapses neurons are excited, inhibited, or modulated. Most synapses are chemical synapses, in which a chemical messenger, called a neurotransmitter, is involved in the movement of the nerve signal to the next neuron. In chemical synapses, there is a small space between the adjacent neurons. Some specific types of synapses, usually those involved in escape responses, are electrical synapses. These synapses have a protein channel connecting adjacent neurons through which ions can travel to propagate the neural signal.