nervous system dendrites,axons, Autonomic Nervous System

• Individual nerve cells consist of a cell body, where the nucleus resides, and other cell parts.

  Dendrites bring signals into the cell body.

  
  

  Axons carry signals away from the nucleus to neighboring cells.

  
  

  Schwann cells in the peripheral nervous system and oligodendrocytes in the CNS form a myelin sheath that surrounds and protects axons. Without myelin sheathing, signal conduction is not well coordinated between cells and becomes sporadic. Sporadic nerve conduction makes coordinated muscle movement difficult.

  
  
• • Autonomic nerves regulate functions of the body that are not controlled consciously. They are responsible for response to stress stimuli, “fight or flight” response; regulated by sympathetic nerves. Norepinephrine is released by the sympathetic nerves as part of the “flight or fight” response.

    The adrenal medulla releases epinephrine into circulation; often referred to as adrenaline; adrenergic – nerve cells in which epinephrine, and norepinephrine act as a neurotransmitter.

    Parasympathetic nerves regulate restful body functions; called cholinergic because of their type of receptor, and their primary neurotransmitter acetylcholine (ACh).

    Adrenergic receptors include alpha and beta receptors; activated by norepinephrine and epinephrine in the sympathetic part of the autonomic system. Alpha receptors are found in blood vessels; constrict blood vessels, raising blood pressure when stimulated; when blocked by drugs, blood pressure lowers; used to treat hypertension. 

    Beta receptors are divided in two:
  • • Beta-one receptors are found mostly within the heart; increase heart rate and contraction force when stimulated.
    • Beta-two receptors are found in smooth muscle or arteries and bronchioles in the lungs and other tissues; cause blood vessels and bronchioles to dilate when stimulated.
  • When blocked by drugs beta blockers heart rate slows and demand for oxygen in the heart decreases, lowering blood pressure. Adrenergic agonist agents are used to exert effects of stimulating alpha and beta receptors in urgent situations.
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