Nervous System

The Nervous System
The Nervous System consists of two parts: The central nervous system (CNS) is made up of the brain and the spinal cord. They act as the control centre receiving messages from all parts of the body, examining the information received and then sending out messages to tell parts of the body what to do.
 * The Central Nervous System
 * The Peripheral Nervous System

The peripheral nervous system (PNS) is made up of nerves and sensory receptors. They continuously inform the CNS of changing conditions, and transmit the decision made by the CNS back to effector organs.

Types of Neurons
There are three types of neurons and they are classified by the direction that they send information.
 * Sensory (or afferent) neurons: send information from sensory receptors (e.g., in skin, eyes, nose, tongue, ears) TOWARD the central nervous system.
 * Motor (or efferent) neurons: send information AWAY from the central nervous system to muscles or glands.
 * Interneurons: send information between sensory neurons and motor neurons. Most interneurons are located in the central nervous system.\

Examples of Neurotransmitters
Acetylcholine: movement and breathing [Curare blocks the reception of acetylcholine]

Noradrenalin: alertness [amphetamines, cocaine and ecstasy increase the production of Noradrenalin]

Dopamine: emotions

Sending an Impulse
When an impulse reaches the axon terminals of causes chemical compounds called neurotransmitters to be released into the synapse, or gap, between them and dendrites of the next neuron, or between them and the effector organ, muscle or gland on which they end.

While the impulse travels, it changes from an electrical message (within the neuron) to a chemical message across synapses, then back to an electrical message again in the next neuron.

Electrical impulses -> Chemical Neurotransmitters -> Chemical Impulses -> Chemical

Many substances including alcohol, some painkillers, drugs such as heroin and cannabis, and some medicines prescribed to treat depression interfere with the way in which messages pass from one nerve cell to another.

The Synapse
Synapses are similar to a switchboard and they allow messages to be directed to the correct places. This message is carried out by chemical compounds known as neurotransmitters.

Information from one neuron flows to another neuron across a synapse. The synapse contains a small gap separating neurons. The synapse consists of:
 * 1) a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles
 * 2) a postsynaptic ending that contains receptor sites for neurotransmitters
 * 3) a synaptic cleft or space between the presynaptic and postsynaptic endings.

Things to Remember
Neurons are grouped together in bundles called nerves.

Dendrites make contact with other cells and receive messages from them.

Axon carries information away from the cell.

Myelin insulates axon and enables messages to pass more quickly.

Reflex Arc
Sometimes actions need to be carried out automatically and without thinking. These usually require only a few neurons and are very fast. These actions are done by a pathway known as a reflex arc.

Examples are blinking, sweating, shivering, coughing and knee-jerking.

Stimulus - Response Pathway
Stimulus -> sensory receptors -> sensory receptors -> central nervous system [interneurons] -> motor neurons -> effector (muscle or gland) -> action or response

The Functions of a Brain
The human brain is the center of our nervous system and is a complex organ that allows us to think, move, feel, see, hear, taste, and smell. It controls our body, receives information, analyzes information, and stores information (our memories).

The brain produces electrical signals, which, together with chemical reactions, let the parts of the body communicate. Nerves send these signals throughout the body.

The Size of a Human Brain
The average human brain weighs about 3 pounds (1300-1400 g).

At birth, the human brain weighs less than a pound (0.78-0.88 pounds or 350-400 g). As a child grows, the number of cell remains relatively stable, but the cells grow in size and the number of connections increases. The human brain reaches its full size at about 6 years of age.

Composition of the Brain
The brain consists of gray matter (40%) and white matter (60%) contained within the skull. Brain cells include neurons and glial cells.

The brain has three main parts: the cerebrum, the cerebellum, and the brain stem (medulla).

Differences between grey and white matter

 * Grey matter is made up of nerve cell bodies and white matter is made of fibres.
 * Unlike the white matter, the neurons of grey matter does have and extended axon
 * Grey matter occupies 40% of the brain and the white matter fills the rest.
 * Grey matter has a grey colour because of the grey nuclei that comprises the cells. Myelin is responsible for the white appearance of the white matter
 * Processing is concluded in the grey matter while white matter allows communication to and from grey matter areas and between the grey matter and the other parts of the body.
 * Grey matter has no myelin sheath while the white matter is myelinated.