The Nervous System: Divisions and Cell Types

• The nervous system is the most complex system in the body.
• It is formed by a network of billions of nerve cells (neurons) which are assisted by many supporting cells called glial cells.
• Neurons have hundreds of interconnections with other neurons.
• Nerve tissue is an integrated communications network distributed throughout the body.

Divisions of the Nervous System

• Anatomically, the nervous system is divided into two divisions: The central nervous system (CNS) consisting of the brain and spinal cord and the peripheral nervous system (PNS) composed of the cranial, spinal, and peripheral nerves, and ganglia.
• PNS conducts impulses to and from the CNS to sensory and motor functions.
• Ganglia are small aggregates of nerve cells outside the CNS.

Cell Types

• Cells in both central and peripheral nerve tissue are of two kinds: neurons and glial cells.
• Neurons typically have long processes.
• Glial cells have short processes, and support and protect neurons and participate in many neural activities, neural nutrition, and defense of cells in the CNS.
• Neurons respond to environmental changes (stimuli) by altering the ionic gradient that exists across their plasma membranes.
• All cells maintain an electrical potential gradient, but neurons, muscle cells, and gland cells can rapidly change this potential in response to stimuli and are excitable/irritable.
• Neurons react to stimuli with a reversal of the ionic gradient (membrane depolarization).
• Depolarization spreads and propagates across the neuron's plasma membrane, which is called the action potential.
• The action potential/nerve impulse travels long distances along neuronal processes, transmitting signals to other neurons, muscles, and glands.
• By collecting, analyzing, and integrating information in such signals, the nervous system continuously stabilizes the intrinsic conditions of the body and maintains behavioral patterns.

Development of Nerve Tissue

• Occurs from the ectoderm (outermost of the three early embryonic layers) beginning in the third week of development.
• Signals from the notochord cause ectoderm on the mid-dorsal side of the embryo thickens to form the epithelial neural plate.
• The sides of this plate fold upward and grow toward each other medially, and within a few days fuse to form the neural tube.