Autonomic Nervous System II

Autonomic Nervous System II

• Objectives:
  • Compare parasympathetic and sympathetic nervous system functions.
  • Identify neurotransmitters and receptor types involved in autonomic nervous system neurotransmission and target organs.

Functions of the Parasympathetic Nervous System (PNS)

• Dominates during normal physiological conditions.
• Primary function: "Rest and Digest".
• Note: The parasympathetic system is always active in the gastrointestinal (GIT) and urinary systems.

Functions of the Sympathetic Nervous System (SNS)

• Prepares the organism to handle environmental stresses.
• Primary function: "Fight or Flight".

Parasympathetic and Sympathetic Divisions

• Diagram (Page 5): Shows the pathways (cranial, thoracic, lumbar, sacral nerves) for each system, illustrating the effects on different organs (eg., pupils, heart, breathing, digestion, etc.). Sympathetic division increases activity during stress, while parasympathetic division works to return the body to a resting state.

Horner Syndrome

• Rare disorder caused by interruption of preganglionic/postganglionic sympathetic innervation to the face.
• Possible causes include injury to nerves, carotid artery injury, stroke, brain stem lesions, or tumors.
• Typically unilateral (one side).
• Hallmark triad:
  • Anhidrosis (reduced sweating).
  • Ptosis (drooping eyelid).
  • Miosis (constricted pupil).
• Additional symptoms may include: Enophthalmos (sunken eyeball), and vasodilation.

Chemical Transmission at Autonomic Junctions

• Cholinergic transmission: Primarily uses Acetylcholine (ACh).
• Adrenergic transmission: Primarily uses Epinephrine (E) / adrenaline, and Norepinephrine (NE) / noradrenaline.

Cholinergic Autonomic Neurons

• All preganglionic neurons release ACh.
• All parasympathetic postganglionic neurons release ACh.
• Some sympathetic postganglionic neurons release ACh (e.g., sweat glands, some blood vessels).

Adrenergic Neurons

• Most sympathetic postganglionic neurons release norepinephrine (NE).
• Exceptions: Sweat glands and some blood vessels.
• Adrenal glands.

ACh Synthesis and Degradation

• ACh is synthesized from acetyl CoA and choline.
• Stored in vesicles in nerve endings.
• After action, ACh is broken down by acetylcholinesterase (AChE).
• Choline is recycled.
• Some ACh diffuses away into the surrounding fluids, where pseudocholinesterase breaks it down.

NE Synthesis and Degradation

• NE is synthesized from tyrosine, dopamine, and norepinephrine.
• Stored in synaptic vesicles.
• After action, NE is broken down by monoamine oxidase (MAO) in nerve endings, and catechol-O-methyltransferase (COMT) in other tissues.

Autonomic Receptors

• Cholinergic:
  • Muscarinic
  • Nicotinic
• Adrenergic:
  • Alpha
  • Beta

Nicotinic Receptors

• Respond to ACh and nicotine.
• Do not respond to muscarine.
• Act through ligand-gated ion channels.
• Blocked by tubocurarine (neuromuscular blocker).
• Activation results in rapid increases in sodium (Na+) and calcium (Ca2+) conductance, depolarization, and excitation.
• Two subtypes:
  • Nm (neuromuscular end plate, skeletal muscle)
  • Nn (autonomic ganglia).

Muscarinic Receptors

• Respond to ACh, muscarine, and carbachol.
• Blocked by atropine.
• Coupled via G-proteins, activating second messengers (e.g., cAMP).
• Multiple subtypes (M1, M2, M3).

Adrenergic Receptors (Alpha and Beta)

• Alpha 1, Alpha 2 (located in various tissues).

• Beta 1, Beta 2, Beta 3 (located in various tissues).

• Different subtypes have different effects (e.g., on heart rate, blood vessels, etc.)

• Detailed receptor locations and effects of binding on various tissues (Page 21)