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Function

The endocrine system is made up of glands that produce and secrete hormones, chemical substances produced in the body that regulate the activity of cells or organs. These hormones regulate the body's growth, metabolism (the physical and chemical processes of the body), and sexual development and function. The hormones are released into the bloodstream and may affect one or several organs throughout the body.

Hormones are chemical messengers created by the body. They transfer information from one set of cells to another to coordinate the functions of different parts of the body.

The major glands of the endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, and the reproductive organs (ovaries and testes). The pancreas is also a part of this system; it has a role in hormone production as well as in digestion.

The endocrine system is regulated by feedback in much the same way that a thermostat regulates the temperature in a room. For the hormones that are regulated by the pituitary gland, a signal is sent from the hypothalamus to the pituitary gland in the form of a "releasing hormone," which stimulates the pituitary to secrete a "stimulating hormone" into the circulation. The stimulating hormone then signals the target gland to secrete its hormone. As the level of this hormone rises in the circulation, the hypothalamus and the pituitary gland shut down secretion of the releasing hormone and the stimulating hormone, which in turn slows the secretion by the target gland. This system results in stable blood concentrations of the hormones that are regulated by the pituitary gland.

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Hypothalamus
The hypothalamus is located in the lower central part of the brain. This part of the brain is important in regulation of satiety, metabolism, and body temperature. In addition, it secretes hormones that stimulate or suppress the release of hormones in the pituitary gland. Many of these hormones are releasing hormones, which are secreted into an artery (the hypophyseal portal system) that carries them directly to the pituitary gland. In the pituitary gland, these releasing hormones signal secretion of stimulating hormones. The hypothalamus also secretes a hormone called somatostatin, which causes the pituitary gland to stop the release of growth hormone.

Pituitary Gland
The pituitary gland is located at the base of the brain beneath the hypothalamus and is no larger than a pea. It is often considered the most important part of the endocrine system because it produces hormones that control many functions of other endocrine glands. When the pituitary gland does not produce one or more of its hormones or not enough of them, it is called hypopituitarism.

The pituitary gland is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe produces the following hormones, which are regulated by the hypothalamus:

Growth hormone: Stimulates growth of bone and tissue (Growth hormone deficiency results in growth failure. Growth hormone deficiency in adults results in problems in maintaining proper amounts of body fat and muscle and bone mass. It is also involved in emotional well-being.)
Thyroid-stimulating hormone (TSH): Stimulates the thyroid gland to produce thyroid hormones (A lack of thyroid hormones either because of a defect in the pituitary or the thyroid itself is called hypothyroidism.)
Adrenocorticotropin hormone (ACTH): Stimulates the adrenal gland to produce several related steroid hormones
Luteinizing hormone (LH) and follicle-stimulating hormone (FSH): Hormones that control sexual function and production of the sex steroids, estrogen and progesterone in females or testosterone in males
Prolactin: Hormone that stimulates milk production in females
The posterior lobe produces the following hormones, which are not regulated by the hypothalamus:

Antidiuretic hormone (vasopressin): Controls water loss by the kidneys
Oxytocin: Contracts the uterus during childbirth and stimulates milk production
The hormones secreted by the posterior pituitary are actually produced in the brain and carried to the pituitary gland through nerves. They are stored in the pituitary gland.

Thyroid Gland
The thyroid gland is located in the lower front part of the neck. It produces thyroid hormones that regulate the body's metabolism. It also plays a role in bone growth and development of the brain and nervous system in children. The pituitary gland controls the release of thyroid hormones. Thyroid hormones also help maintain normal blood pressure, heart rate, digestion, muscle tone, and reproductive functions.

Parathyroid Glands
The parathyroid glands are two pairs of small glands embedded in the surface of the thyroid gland, one pair on each side. They release parathyroid hormone, which plays a role in regulating calcium levels in the blood and bone metabolism.
Adrenal Glands
The two adrenal glands are triangular-shaped glands located on top of each kidney. The adrenal glands are made up of two parts. The outer part is called the adrenal cortex, and the inner part is called the adrenal medulla. The outer part produces hormones called corticosteroids, which regulate the body's metabolism, the balance of salt and water in the body, the immune system, and sexual function. The inner part, or adrenal medulla, produces hormones called catecholamines (for example, adrenaline). These hormones help the body cope with physical and emotional stress by increasing the heart rate and blood pressure.
Pineal Body
The pineal body, or pineal gland, is located in the middle of the brain. It secretes a hormone called melatonin, which may help regulate the wake-sleep cycle of the body.

Reproductive Glands
The reproductive glands are the main source of sex hormones. In males, the testes, located in the scrotum, secrete hormones called androgens; the most important of which is testosterone. These hormones affect many male characteristics (for example, sexual development, growth of facial hair and pubic hair) as well as sperm production. In females, the ovaries, located on both sides of the uterus, produce estrogen and progesterone as well as eggs. These hormones control the development of female characteristics (for example, breast growth), and they are also involved in reproductive functions (for example, menstruation, pregnancy).

Pancreas
The pancreas is an elongated organ located toward the back of the abdomen behind the stomach. The pancreas has digestive and hormonal functions. One part of the pancreas, the exocrine pancreas, secretes digestive enzymes. The other part of the pancreas, the endocrine pancreas, secretes hormones called insulin and glucagon. These hormones regulate the level of glucose (sugar) in the blood.

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Organisation
The nervous system consists of two parts, shown in Figure 1:

The central nervous system (CNS) consists of the brain and spinal cord.

The peripheral nervous system (PNS) consists of nerves outside the CNS.

Nerves of the PNS are classified in three ways. First, PNS nerves are classified by how they are connected to the CNS. Cranial nerves originate from or terminate in the brain, while spinal nerves originate from or terminate at the spinal cord.

Second, nerves of the PNS are classified by the direction of nerve propagation. Sensory ( afferent) neurons transmit impulses from skin and other sensory organs or from various places within the body to the CNS. Motor ( efferent) neurons transmit impulses from the CNS to effectors (muscles or glands).

Third, motor neurons are further classified according to the effectors they target. The somatic nervous system (SNS) directs the contraction of skeletal muscles. The autonomic nervous system (ANS) controls the activities of organs, glands, and various involuntary muscles, such as cardiac and smooth muscles.

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The Action Potential
The resting potential tells us about what happens when a neurone is at rest.  An action potential occurs when a neurone sends information down an axon.  This involves an explosion of electrical activity, where the nerve and muscle cells resting membrane potential changes.

In nerve and muscle cells the membranes are electrically excitable, which means they can change their membrane potential, and this is the basis of the nerve impulse. The sodium and potassium channels in these cells are voltage-gated, which means that they can open and close depending on the voltage across the membrane.

The normal membrane potential inside the axon of nerve cells is –70mV, and since this potential can change in nerve cells it is called the resting potential. When a stimulus is applied a brief reversal of the membrane potential, lasting about a millisecond, occurs. This brief reversal is called the action potential:

An action potential has 2 main phases called depolarisation and repolarisation:
An action potential has 2 main phases called depolarisation and repolarisation:

| At rest, the inside of the neuron is slightly negative due to a higher concentration of positively charged sodium ions outside the neuron.
| When stimulated past threshold (about –30mV in humans), sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon.  This is called depolarisation
| The region of positive charge causes nearby voltage gated sodium channels to close. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon, so the charge across the membrane is brought back to its resting potential.  This is called repolarisation.
| This process continues as a chain-reaction along the axon.  The influx of sodium depolarises the axon, and the outflow of potassium repolarises the axon.
| The sodium/potassium pump restores the resting concentrations of sodium and potassium ions

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Peripheral system

The peripheral system connects the central nervous system to the rest of the body. The main divisions of the Peripheral Nervous System are:

The autonomic nervous system — which controls the automatic functions of the body: the heart, smooth muscle (organs) and glands. It is divided into the “fight-or-flight” system and the “resting and digesting" system.

The somatic nervous system — which allows us to consciously or voluntarily control our skeletal muscles. The somatic system contains 12 cranial nerves and 31 spinal nerves.

Nerves — which are made up of special cells called neurons. Neurons are comprised of a dendrite, a cell body and an axon. Impulses travel to the dendrite into the cell body and then onto the axon. A special sheath called myelin, which increases the conductivity of the neuron, covers some nerves.

As messages travel from one neuron to the next they move across a synapse. At each synapse there is a chemical called a neurotransmitter. At various parts of the body specific neurotransmitters facilitate communication, for example dopamine (motor function), serotonin (mood) and endorphins (painkillers). Sensory neurons carry messages from a receptor to the brain. The brain then interprets the message. Motor neurons then send the message to an affector in muscles and glands.

Receptor (sensory organ) sends a signal to the sensory neuron which sends a signal to the brain/spinal chord which sends a signal to the motor neuron which sends a signal to the affector (muscle/gland).

The neurone

The basic unit of the nervous system, is a specialised cell called the neurone. These nerve cells make up a massive network of specialised cells that transmit messages, very rapidly, from one part of the body to another. Information is transmitted via electrical impulses.

The neurone is comprised of a nerve cell and its adjoining processes called an axon and dendrites. Every nerve cell has one or more processes attached to it. Electrical impulses enter the neurone via the dendrites and leave via the axon. The space between the axon of one cell and the dendrites of another is called a synapse. Specialised chemicals called neurotransmitters help conduct impulses through the synapse onto the next cell.

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Central nervous system

These structures are protected by bone and cushioned from injury by the cerebrospinal fluid (CSF).

Brain

The brain is a mass of soft nerve tissue, which is encapsulated within the skull. It is made up of grey matter, mainly nerve cell bodies, and white matter which are the cell processes. The grey matter is found at the periphery of the brain and in the centre of the spinal cord. White matter is found deep within the brain, at the periphery of the spinal cord and as the peripheral nerves.

The brain is divided into:

Cerebrum - the largest part of the brain. It is the centre for thought and intelligence. It is divided into right and left hemispheres. The right controls movement and activities on the left side of the body. The left controls the right side of the body. Within the cerebrum are areas for speech, hearing, smell, sight, memory, learning and motor and sensory areas.

Cerebral cortex - the outside of the cerebrum. Its function is learning, reasoning, language and memory.

Cerebellum - lies below the cerebrum at the back of the skull. Its functions are to control voluntary muscles, balance and muscle tone.

Medulla - controls heart rate, breathing, swallowing, coughing and vomiting. Together with the pons and the midbrain, the medulla forms the brainstem that connects the cerebrum to the spinal chord

Lobes of the brain

It is important to have an understanding of how the brain functions and which parts control our functioning and behaviour. For example, when a casualty suffers from a stroke, the part of the brain that is affected controls function. If it is the frontal lobe, speech, thought and movement may be affected

The Spinal cord

The spinal cord is about 45 cms long, extending from the medulla down to the second lumbar vertebrae. It acts as a message pathway between the brain and the rest of the body. Nerves conveying impulses from the brain, otherwise known as efferent or motor nerves, travel through the spinal cord down to the various organs of the body. When the impulses reach the appropriate level they leave the cord to travel to the' target organ.

Sensory or afferent nerve impulses also use the spinal cord to travel from various parts of the body up to the brain.

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Function

The nervous system has three general functions: a sensory function, an interpretative function and a motor function.

1. Sensory nerves gather information from inside the body and the outside environment. The nerves then carry the information to central nervous system (CNS).

2. Sensory information brought to the CNS is processed and interpreted.

3. Motor nerves convey information from the CNS to the muscles and the glands of the body.

Structure

The nervous system is divided into two parts:

1. the central nervous system consisting of the brain and spinal cord. These structures are protected by bone and cushioned from injury by the cerebrospinal fluid (CSF)

2. the peripheral system which connects the central nervous system to the rest of the body.