The Autonomic Nervous System

The autonomic nervous system (ANS) operates below levels of consciousness. It monitors and controls all basic life processes. The ANS is a motor system with 3 target tissues; smooth muscles, glands (exocrine and endocrine), and cardiac muscles.

The autonomic nervous system is organized into pre-ganglionic and post-ganglionic neurons. Synapses between the pre and post-ganglionic neurons are made in the autonomic ganglia. Pre-ganglionic neurons of both divisions, sympathetic and parasympathetic, have their cell bodies in the central nervous system, while post-ganglionic neurons have their cell bodies in the autonomic ganglia and synapse on various organs. The sympathetic ganglia are located in the sympathetic trunk along the vertebral column, while the parasympathetic ganglia are located in or near the target organs.

The ANS contains higher autonomic centers, including the hypothalamus, which monitors blood and hormones, as well as coordinates signals to other centers. The hypothalamus contains the temperature regulation center, as well as the thirst and food intake regulation centers. The ANS is divided into the sympathetic and parasympathetic nervous systems. The sympathetic division is for immediate survival, instant reactions, and instant change. The parasympathetic works to keep a homeostatic flow within the body and focuses on long term survival. When the sympathetic system is activated, it increases heart rate, force of heart beat, blood pressure, adrenaline, respiration, and decreases renal and digestive functions. The parasympathetic system decreases heart rate, respiratory rate, blood pressure, and increases digestive and renal functions.

The sympathetic system is made up of two types of ganglia. There are sympathetic trunk (chain) ganglia and collateral ganglia (in the abdominal cavity). The signal comes from a pre-ganglionic neuron to the ganglion. From this ganglion, the signal can synapse with 20 or more post-ganglionic neurons. This is known as amplification. The post-ganglionic neurotransmitter output is nor-epinephrine. There is also adrenaline releases by a gland on the kidney known as the adrenal medulla.

The parasympathetic nervous system focuses on rest and regeneration. The main functions are salivation, lacrination, urination, defecation, and digestion. It tends to be slow acting; organ by organ. This system has much less amplification than the sympathetic system.

The main neurotransmitters of the autonomic nervous system are acetylcholine and norepinephrine. Acetylcholine is released by the pre-ganglionic neurons in both the sympathetic and parasympathetic divisions. Norepinephrine is released by the post-ganglionic neurons of the sympathetic division, while acetylcholine is released by the post ganglionic neurons of the parasympathetic division.