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.

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

Structure

The nervous system is divided into two parts:

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

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:

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 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.

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

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 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.

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.

| 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