Nervous System


     The nervous and endocrine systems are involved with the coordination of all the activities of the body’s cells. The nervous system controls various activities by the rapid communication achieved by its highly specialized nerve cells or neurons.
     The functions of the various neurons that make up the nervous system can be divided into three basic functions:
  1. Sensory input
  Nerve cells collect information about the external environment as well as the internal conditions of the body.
  2. Integration
  Nerve cells receive the sensory input, analyze it, and make decisions about the appropriate response to make.
  3. Motor output
  Nerve cells carry out the instructions that result from integration by impulses sent to various effector organs that include muscles and glands.

Organization of the Nervous System
     The nervous system, as a whole, can be divided into:
  Central nervous system (CNS) which includes all the nervous tissue contained within the brain and spinal cord; and
  Peripheral nervous system (PNS) which includes all the nerves that enter or exit from the brain and spinal cord or that exit external to the CNS. The PNS can be further subdivided on a functional basis into:
  Sensory or afferent neurons
  These neurons provide sensory input and are subcategorized according to the type of information they provide:
  Special sensory – sends information about vision, hearing, equilibrium, smell and taste.
  Somatic sensory – sends information about touch, temperature and tissue damage from receptors in the skin, muscles and joints.
  Visceral sensory - conveys information about conditions of various internal (visceral) organs.
  Motor or efferent neurons
  These neurons carry commands to effector organs and are subcategorized according to the effector organs they control:
  Somatic (voluntary) motor – control skeletal muscles.
  Autonomic (involuntary) motor – regulate cardiac and smooth muscle and glands.

Neuron Anatomy
     Neurons, or nerve cells, are highly specialized cells that rapidly transmit messages from one cell to another. All nerve cells have:
  Cell body
  The cell body is the part of the nerve cell that contains the nucleus and most other organelles. It is the metabolic center of the cell. Rough endoplasmic reticulum, the organelle responsible for protein synthesis, is abundant in the cell body. When a special stain is used, the large amount of RER in the cell body becomes visible as Nissl substance.
  Processes
  Processes extend from the cell body and can vary greatly in length and complexity. There are two basic types of processes:
  Dendrites
  Dendrites are processes that convey impulses to the cell body. A neuron may have hundreds of dendrites that have complicated branching patterns.
  Axon
  A neuron has only one axon. The axon conveys impulses away from the cell body and can be incredibly long (up to 3 to 4 feet). The axon originates on the cell body on a cone-like projection called the axon hillock. Toward their ends, axons branch to form axon terminals that form synapses with effector organs and other neurons.

Supporting Cells
     Because nerve cells are so specialized, they require other cells to protect and support them physically and metabolically.
  CNS
  The supporting cells of the CNS are called neuroglia. These cells can also be referred to as glia or glial cells. There are four types of glial cells in the CNS:
  1. Astrocytes
     Astrocytes are the most abundant glial cell in the CNS. These cells have many processes that extend from the central part of the cell in a manner that resembles rays radiating from a star (hence astrocyte = star + cell). The ends of the processes come into contact with the neurons and the blood capillaries.
     Astrocytes regulate the exchanges between neurons and the blood. Astrocytes also maintain the extracellular environment of the neurons by absorbing excess ions and released neurotransmitters.
  2. Microglia
     These cells are phagocytes that dispose of cellular debris and engulf pathogens such as bacteria.
   3. Ependymal cells
     The ependymal cells line the cavities of the brain and spinal cord. These cells regulate exchange between the cerebrospinal fluid (CSF) that is produced at specific locations within the cavities of the brain and the nervous tissue of the CNS.
  4. Oligodendrocytes
     Oligodendrocytes have process that wrap around the axons of neurons of the CNS to form myelin sheaths.  
    
  PNS
  There are two kinds of supportive cells in the PNS:
  1. Schwann cells
     Schwann cells surround and support the axons of the neurons of the PNS. These cells are also responsible for the formation of the myelin sheath. The myelin sheath results from supportive cells (Schwann cells or oligodendrocytes) wrapping around the axon of a neuron in a jelly-roll fashion so that the axon comes to be surrounded by many layers of plasma membrane. The myelin sheath dramatically speeds the nerve impulse along the axon.
  2. Satellite cells
     Satellite cells are similar to Schwann cells except that they are found around the cells bodies of neurons in the PNS.
    

Synapses
     Synapses are the functional connections between neurons and the cells they communicate with. In the vast majority of synapses, the cell membrane of the cell sending the message comes very close to, but does not touch, the cell receiving the message. The space between the cell membranes is called the synaptic cleft. Chemicals called neurotransmitters are released by the cell sending the message, cross the synaptic cleft, and come into contact with the cell receiving the message.