Nervous System Organization

• The nervous system's primary function is communication throughout the body acting as a messenger system
• The process the nervous system achieves this is through sensory input, interperetation and motor response

Invertebrate Nervous Organization

• Hydras possess a nerve net composed of neurons that are in contact with one another
• These neurons are also in contact with contractile epitheliomuscular cells
• Planarians have a ladder-like nervous system and exhibit cephalization
• Epitheliomuscular cells are skin/muscle cells for movement and more
• Cephalization - a concentration of ganglia (cluster of neurons) and sensory receptors in the head
• Annelids, Arthropods, and Mollusks have complex and true nervous systems

Evolution of Nervous System

• Hydras feature a nerve net
• Planarians feature cerebral ganglia, transverse nerves, lateral nerve cords, and eyespots
• Earthworms feature a brain and a ventral nerve cord with ganglia

Vertebrate Nervous Organization

• Humans have a Central Nervous System (CNS) containing the brain and spinal cord
• Peripheral Nervous System (PNS) includes everything outside the CNS
• Nervous tissue is made of neuroglia (glial cells) and neurons
• Neuroglia are cells that nourish and support the neurons which transmit the signals

Neurons

• Cell body contains the nucleus and other organelles
• Dendrites receive signals from sensory receptors
• Axon, a long fiber, conducts nerve impulses
• 1 to thousands of receptors from different sources
• A long axon may be called a nerve fiber
• Axons are covered by a myelin sheath, a fatty material which helps speed up the sending of the message
• Schwann cells create the myelin sheath in the PNS
• Oligodendrocytes are neuroglia in the CNS
• Nodes of Ranvier (uncovered) work with the myelin sheath to speed up transmission
• Nodes are where the electrical signal transmits and the sheath causes the electrical signal to jump to the next node instead of transmitting down the entire length
• Axon terminal releases a neurotransmitter to the next cell

Neurons 2

• The CNS has two specific types of nerve tissue called white matter and gray matter
• White matter contains long myelinated axons
• Gray matter contains short non-myelinated axons
• Glial cells are for physical support and form myelin
• Neurons send and receive impulses
• The difference in color is due to the myelin
• The reason it is tan, is because the tissue is no longer living. It was cut and preserved on a microscope slide.

Types of Neurons

• Sensory neurons have sensory receptors at one end that detect changes in the environment
• Sensory neurons transmit electrical impulses containing the information to the CNS
• Sensory neurons may respond to light, sound, heat, touch, pain, odors, and taste
• Interneurons convey nerve impulses between various parts of the CNS, and comprise 90% of all neurons
• Motor neurons accept nerve impulses from the CNS and transmit them to muscles or glands
• Motor neurons stimulate muscle contractions or gland secretions

How Neurons Work

• Reflexes are under the control of the spinal cord
• Peripheral nervous system, central nervous system (brain and spinal cord)