The Human Nervous System

• The central nervous system (CNS) • The peripheral nervous system (PNS)

The CNS is composed of the brain and the spinal cord

The PNS is composed of all the nerves that carry messages to and from the CNS, as well as clusters of neurons called ganglia that are mostly located very near to the CNS. Ganglia contain the cell bodies of sensory neurons.

A nerve cell specialised to carry electrochemical signals. It carries electrochemical impulses from one region of the nervous system to another.

• Sensory neurons • Interneurons (association neurons) • Motor neurons

They carry impulses from sense organs towards the central nervous system.

Part of the sensory neuron is outside the CNS and part is inside.

Interneurons carry impulses from one neuron to another, completely within the CNS.

Interneurons are located entirely within the CNS.

They carry impulses from an interneuron to an effector.

An organ or tissue that carries out an action in response to a signal from the nervous system.

Dendrites; cell body; nucleus; axon; myelin sheath (Schwann cells); and axon terminals.

The lengths of the dendrites and axons.

Sensory neurons have very long dendrites and axons, with a cell body at a specific location in between.

Motor neurons have very short dendrites and long axons.

In general, interneurons have very short dendrites and axons.

Dendrites are nerve endings that receive impulses from other neurons or from sensory cells. They transmit the impulse towards the cell body.

It is responsible for the upkeep of the cell and for producing neurotransmitter-chemicals responsible for transmitting the impulse on to another neuron or effector.

A chemical substance released by a neuron to transmit a nerve impulse to another neuron or effector.

The axon is responsible for carrying an electrochemical impulse away from the cell body and onwards towards the axon terminals where the impulse will be transmitted on to another neuron or effector.

• Myelin sheath is a substance made from lipids that wraps around axons and dendrites. • It is made by a special type of cell called a Schwann cell. The Schwann cell wraps its cell membrane around the axon or dendrite a number of times to create the myelin sheath.

Myelin sheath protects the axon and dendrites and also has the effect of speeding up electrochemical impulses and maintaining their strength all the way to the end of the axon or dendrite. Without it, impulses are not transmitted properly.

Axon terminals are present at the end of the axon and contain synaptic vesicles, which in turn contain neurotransmitter chemicals.

Neurotransmitter chemicals.