Autonomic Nervous System (ANS) and NeuroMuscular Junction (NMJ)

Autonomic Nervous System (ANS) and NeuroMuscular Junction (NMJ)

• The autonomic nervous system (ANS) is a subdivision of the peripheral nervous system (PNS)
• The ANS is further divided into two branches:
  • Parasympathetic nervous system (PNS)
  • Sympathetic nervous system (SNS)
• The SNS is also known as the "efferent fibres"
• The ANS controls organs that are not under our control, such as the viscera (e.g. heart, lungs, digestive system)

Neurochemical Transmission of the ANS

• Cholinergic neurotransmission:
  • Synthesis: acetylcholine (ACh) is produced from acetyl-CoA and choline
  • Storage: ACh is stored in vesicles in the terminal buttons of cholinergic neurons
  • Release: ACh is released from the vesicles into the synapse
  • Binding: ACh binds to nicotinic receptors (NR) and muscarinic receptors (MR)
  • Terminal action: ACh activates NR and MR, leading to muscle contraction or relaxation
• Adrenergic neurotransmission:
  • Synthesis: norepinephrine (NE) is produced from tyrosine
  • Storage: NE is stored in vesicles in the terminal buttons of adrenergic neurons
  • Release: NE is released from the vesicles into the synapse
  • Binding: NE binds to α and β receptors on the postsynaptic effector cells
  • Terminal action: NE activates α and β receptors, leading to muscle contraction or relaxation

Parasympathetic Nervous System (PNS)

• Origin: midbrain, medulla oblongata, and sacral cord
• Nerve fibres: long preganglionic nerve fibres, short postganglionic nerve fibres
• Neurotransmitters:
  • ACh: released at the ganglia (NR) and effector organs (MR)
• Functions:
  • Digestion
  • Conservation of energy
  • Maintenance of organ function

Sympathetic Nervous System (SNS)

• Origin: thoracic and upper lumbar regions of the spinal cord
• Nerve fibres: short preganglionic nerve fibres, long postganglionic nerve fibres
• Neurotransmitters:
  • ACh: released at the ganglia (NR)
  • NE: released at the effector organs (α and β receptors)
• Functions:
  • Mobilizing the body's resources to respond to fear and anxiety (3Fs: fear, fight, flight response)

Comparison between Voluntary “Somatic NS” and Involuntary “ANS”

• Somatic nervous system:
  • Originates from the central nervous system (CNS)
  • Controls skeletal muscles
  • Neurotransmitter: ACh (nicotinic receptors)
• Autonomic nervous system:
  • Originates from the peripheral nervous system (PNS)
  • Controls involuntary muscles (smooth muscle, cardiac muscle)
  • Neurotransmitters: ACh (muscarinic receptors), NE (α and β receptors)

Neurotransmitters of the ANS

• Cholinergic pathways:
  • Synthesis, storage, and release of ACh
  • Inactivation by acetylcholinesterase (AChE) and pseudocholinesterase
• Adrenergic pathways:
  • Synthesis, storage, and release of NE
  • Inactivation by reuptake, monoamine oxidase (MAO), and catechol-O-methyltransferase (COMT)

Effect of ANS on Various Organs

• Eye:
  • Pupil dilation (sympathetic)
  • Pupil constriction (parasympathetic)
  • Accommodation for near vision (parasympathetic)
• Heart:
  • Increased heart rate and force (sympathetic)
  • Decreased heart rate and force (parasympathetic)
• Blood vessels:
  • Constriction (sympathetic)
  • Dilatation (parasympathetic)
• Lung:
  • Bronchodilation (sympathetic)
  • Bronchoconstriction (parasympathetic)
• Gastrointestinal tract (GIT):
  • Relaxation (sympathetic)
  • Contraction (parasympathetic)
• Urinary tract:
  • Urinary retention (sympathetic)
  • Urine flow and voiding (parasympathetic)

Note: The study notes are organized by topic, with bullet points and subtopics to help illustrate the key concepts and relationships between them.### Sympathetic and Parasympathetic Nervous System

Skin

• Pilomotor smooth muscle contraction (erects hair) is stimulated by α1 receptors
• Sweat glands are stimulated by α1 receptors, causing sweating, especially on the forehead and palms
• Parasympathetic system stimulates cholinergic sweating, which is thermoregulatory

Skeletal Muscle

• β2 receptors enhance neuromuscular transmission and cause tremors
• β2 receptors inhibit histamine release from mast cells

Metabolic Function

• α2 receptors in the pancreas decrease insulin release, leading to gluconeogenesis
• β2 receptors in the pancreas increase insulin release
• β2 receptors in the liver stimulate glycogenolysis, leading to increased glucose release
• β1,3 receptors in fat cells stimulate lipolysis, leading to increased FFA
• β2 receptors in skeletal muscle stimulate glycogenolysis, leading to increased lactic acid and contractility

Blood

• α2 receptors in the liver stimulate the release of K+, leading to initial hyperkalemia
• β2 receptors in skeletal muscle stimulate the uptake of K+, leading to subsequent hypokalemia

Nerve Terminals

• α2 receptors decrease noradrenaline release
• β2 receptors increase noradrenaline release

General ANS

• Most organs receive dual sympathetic and parasympathetic innervation
• Some organs, including the suprarenal gland, sweat glands, most blood vessels, spleen, and piloerector muscles, only receive sympathetic innervation