AUTONOMIC NERVOUS SYSTEM

• The Autonomic Nervous System (ANS) is controlled by centers in the spinal cord, brain stem, and hypothalamus. It responds to signals from internal organs and influences organ activities through reflexes.
• The ANS has two parts: the sympathetic nervous system and the parasympathetic nervous system, each with specific functions.
• For the autonomic, neurons are lightly myelinated or non-myelinated, and there will be a two-neuron chain from the central nervous system to effector organs.

  PREGANGLIONIC NEURON

  POSTGANGLIONIC NEURON
• THE NEURONS release either acetylcholine or norepinephrine, resulting in either a stimulatory or inhibitory response, in contrast with somatic which is always stimulatory.
• • The parasympathetic division is for “rest and digest actions”.
  • It directs the digestion of food and the expulsion of waste, along with other maintenance-related actions that occur best in a state of relaxation.
• • The sympathetic division, on the other hand, springs into action in emergency situations, enacting the fight-or-flight instincts, raising the heart rate, constricting blood vessels, and releasing more glucose from the liver, all actions that help an organism evade danger.
• 1. Sympathetic fibers originate in between, in the thoracic and lumbar regions of the spinal cord.
• 1. The Sympathetic Nervous System also has some interesting roles such as dilation of pupils, inhibits salivation, relaxes bronchi, heartbeat acceleration, and many more.
• 1. Parasympathetic fibers originate in the brain and sacral region of the spinal cord, which are the opposite ends of the central nervous system.
• 1. Parasympathetic has long preganglionic and short postganglionic fibers, with the ganglia residing in the effector organs.
• Sympathetic Division of the ANS has short preganglionic and long postganglionic fibers.
• 1. Parasympathetic System is responsible for the constriction of pupils and bronchi, slowing of heartbeat, contraction of bladder and many more.
• The nervous system has two main branches: sympathetic and parasympathetic. These branches release different chemicals, called neurotransmitters, to communicate with target organs.
• Cholinergic: Nerve fibers that release acetylcholine.
• Adrenergic: Nerve fibers that release norepinephrine (also known as noradrenaline).
• Preganglionic Neurons: In both sympathetic and parasympathetic systems, the initial neurons are cholinergic.
• Postganglionic Neurons:

  Parasympathetic: Almost all postganglionic neurons are cholinergic.Sympathetic: Most are adrenergic, but a few are cholinergic (e.g., those going to sweat glands).
• Neurotransmitters:

  Acetylcholine: Mainly associated with parasympathetic effects.
• Neurotransmitters:

  Norepinephrine: Mainly associated with sympathetic effects.
• cholinergic fibers release acetylcholine, which is parasympathetic in nature. Adrenergic fibers release norepinephrine, which is associated with sympathetic effects.
• Nerve Endings and Varicosities:

  • Nerve endings of autonomic nerves, especially in the parasympathetic system, touch or pass by target cells.
  • Some have bulging parts called varicosities where neurotransmitter vesicles are made and stored.
• Action Potential and Secretion:

  
  

  When an action potential (nerve signal) reaches the nerve endings, it causes depolarization.

  • This depolarization allows calcium ions to enter varicosities.
• Calcium ions trigger the release of neurotransmitter vesicles from varicosities to the exterior.
• Secretion Mechanism:

  This process is how acetylcholine or norepinephrine is secreted and acts on target cells.
• acetylcholine is made, released, and then broken down for reuse, ensuring a continuous cycle of nerve signal transmission.
• When released, acetylcholine stays in the tissue for a few seconds to transmit nerve signals.
• acetylcholine is made and stored in nerve endings (varicosities) of cholinergic fibers.
• • Norepinephrine is made in the nerve endings of adrenergic fibers and completed inside vesicles.
• In the adrenal medulla, about 80% of norepinephrine is transformed into epinephrine.
• After release, norepinephrine is removed in three ways:

  • Reuptake into nerve endings (50-80%).
  • Diffusion into body fluids and then into the blood (most of the rest).
  • Destruction by enzymes like monoamine oxidase and catechol-O-methyltransferase.
• Norepinephrine in tissues lasts only a few seconds.
• • Norepinephrine in the blood remains active for 10 to 30 seconds before declining over 1 to several minutes.
• In essence, norepinephrine is made, released, and then removed by reuptake, diffusion, and enzyme destruction, with its activity lasting for a brief period depending on the location.
• Receptors are on the cell membrane, attached to a protein that goes through the membrane.
• • The altered protein either excites or inhibits the cell.
• Functional Changes:

  • This is often achieved by changing cell membrane permeability to ions or by activating/inactivating enzymesinside the cell.
• 1. Receptor proteins on the cell membrane can change shape. - This change can open or close ion channels in the protein.
• 1. Opening sodium or calcium channels lets these ions into the cell, usually exciting it.
• 1. Opening potassium channels lets potassium ions out, typically inhibiting the cell.
• 1. Influx of ions (like sodium or calcium) can depolarize the cell membrane, exciting the cell.