General Characteristics and functions: Composed of neurons and associated supportive cells called neuroglia.

• Neurons - The functional units of the nervous system. Neurons are highly specialized cells that generate and transmit electrical impulses (action potentials) permitting rapid communication between distant areas of the body. Neurons gather and process information from receptors in contact with the external environment, and generate appropriate response signals. Networks of neurons help integrate multiple sensory signals and are responsible for more complex behaviors such as abstract thinking, learning, memory, language, planning, etc. Neurons are permanent cells that do not regenerate when injured.

  
  

  

  
  The nervous system is divided into two main parts:

  • The central nervous system (CNS) - The brain and spinal cord. The brain is further divided into the cerebrum, diencephalon, basal ganglia, brainstem, and cerebellum.

  • The peripheral nervous system (PNS) - All nervous structures outside the CNS. Includes the cranial nerves which supply the head and neck, spinal nerves which supply the trunk and extremities, and the autonomic nervous system (ANS) which supplies the smooth muscle and glands of internal organs.

  Structural components of the neuron include:

  • A cell body (soma) containing the nucleus and most of the organelles responsible for maintaining the neuron's metabolic functions.

  • A long process called an axon which stretches from the cell body to a distance ranging from several millimeters up to one meter. Axons transmit nerve signals away from the nerve cell body towards other nerves, muscle, or glands - i.e., they conduct the output of the nervous system. Neurons usually have only one axon. Many, but not all, axons are sheathed in multiple layers of a lipid-rich material called myelin. This substance is derived from the cytoplasm of Schwann cells and oligodendrocytes (see below). Myelin functions as a kind of electrical "insulation" greatly facilitating nerve impulse transmission. The greater the amount of myelin around an axon, the faster the rate of nerve impulse transmission. Axons of the peripheral nervous system can regenerate following injury provided that the nerve cell body is not damaged.

  • Numerous short projections from the nerve cell body called dendrites. These structures transmit information towards the nerve cell body - i.e., they conduct input to the neuron. Dendrites are not myelinated.

  • Specialized cell junctions called chemical synapses that facilitate communication between neurons or between neurons and muscle. Communication at synapses is mediated by soluble chemical neurotransmitters secreted into the extracellular space by the terminal portion of an axon. Neurotransmitters depolarize the plasma membrane on target cells leading to propagation of the nerve impulse, or contraction of muscle.

  Within the CNS, nerve cell bodies are often clustered together into functional units called nuclei. Specific groups of nuclei tend to specialize in particular neurologic functions (sensory, motor, memory, speech, etc.). Nervous tissue containing nuclei has a pinkish-gray color and is called gray matter (e.g., the cerebral cortex). Clusters of nerve cell bodies in the peripheral nervous system are called  ganglia. Ganglia serve as neuronal relay centers in the peripheral nervous system.

  Axons, the wiring of the nervous system, are commonly bundled into  tracts and pathways each conducting specific types of neural information (motor, pain, touch, etc.) within the CNS. Because of their myelination, axonal tracts have a yellow-white color (white matter). Bundles of axons in the peripheral nervous system are called nerves.

  

• Neuroglia - Cells that provide metabolic support and immune protection for neurons. Neuroglia outnumber neurons by about 10:1 in the CNS. Neuroglia do not generate or conduct nerve impulses. However, unlike neurons, glial cells can regenerate if injured.

  • Astrocytes - Provide for the energy and other metabolic needs of neurons as well as giving nervous tissue structural support. When neurons of the brain or the spinal cord are injured and destroyed, they are replaced with scar tissue made up of astrocytes (a process called gliosis).

  • Microglia - Phagocytic cells, similar to macrophages, that perform a housekeeping function by removing dead cellular material and bacteria from the CNS.

  • Ependymal Cells - Cells that line the cerebral spinal fluid (CSF) containing cavities of the brain - the ventricles. CSF is secreted by a specialized subset of ependymal cells in the walls of the ventricles of the brain called the choroid plexus.

  • Oligodendrocytes - Cells responsible for myelination of axons within the CNS. Glial cells that myelinate the axons of peripheral nerves are called Schwann cells. These cells wrap their cytoplasm in a spiral fashion around short segments of axons. Because the myelin sheath is formed from numerous Schwann cells (or oligodendrocytes) arranged sequentially along the axon, there are gaps between adjacent myelinating cells producing myelin-free areas of axon called Nodes of Ranvier. These play an important role in nerve impulse conduction.

• Connective tissue in the nervous system

  • Peripheral nervous system - Peripheral nerve axons and their supporting Schwann cells are held together by connective tissue forming nerve fibers. The connective tissue of nerves has three distinct components - endoneurium, perineurium, and epineurium. Endoneurium is a thin layer of connective tissue associated with individual axons. Perineurium surrounds small groups of axons (each wrapped in endoneurium) called fascicles that conduct similar types of information or arise from a specific anatomic region of the body. The epineurium is a layer of dense connective tissue that surrounds and binds together many nerve fascicles into a common bundle.

  • Central nervous system - The brain and spinal cord are enclosed within three sequential connective tissue membranes called meninges. The innermost layer resting directly on the surface of the brain and spinal cord is called the pia mater. The outermost layer is a relatively thick sheet of dense connective tissue called the dura mater. The delicate arachnoid mater is attached to the inner surface of the dura and extends to the pia mater on the surface of the brain and spinal cord. The loose organization of the arachnoid produces a space containing cerebrospinal fluid called the subarachnoid space. Cerebral vessels course through the subarachnoid space.

• The Blood-Brain Barrier - An extensive arrangement of tight junctions between endothelial cells lining the blood vessels of the brain prevents the passage of most molecules from the circulation into the CNS. Astrocytes surrounding these blood vessels also contribute to the function of the blood-brain barrier. This system essentially isolates the CNS from the rest of the body and insures that the unique extracellular environment required for normal neuron functioning is maintained.

Germ Cell Origin: Neurons and glial cells are derived from ectoderm.