Autonomic Pathways

Introduction – Autonomic Pathways

The autonomic nervous system (ANS) is the part of the nervous system that controls involuntary functions of the body, such as heart rate, breathing, digestion, and sweating. It is divided into two branches, the sympathetic nervous system and the parasympathetic nervous system, which work together to maintain a balance in the body’s functions.

The sympathetic nervous system prepares the body for action in response to a perceived threat or stress. It increases heart rate, blood pressure, and breathing rate, and decreases digestion and urinary function. The sympathetic nerves originate in the thoracic and lumbar regions of the spinal cord and are responsible for the “fight or flight” response.

Noradrenergic nerve terminal

• Norepinephrine production 
  • adrenergic neurons transport tyrosine by a transport protein to synthesize norepinephrine
    • adrenergic and dopaminergic neurons use the same pathway
      • adrenergic neurons modify NE to Epi
      • dopaminergic neurons terminates at dopamine
  • tyrosine → dopa → dopamine → norepinephrine → epinephrine
  • the rate limiting step is the conversion of tyrosine to dopa by tyrosine hydroxylase
    • can be inhibited by metyrosine 
• NE pathway feedback
  • NE: negative feedback by acting on presynaptic α₂ autoreceptors to inhibit further NE release
  • ACh: acts on M₂ receptors also to inhibit further NE release
  • angiotensin II: activates NE release
• Pathway Inhibitors
  • reserpine: inhibits the vesicular monoamine transporter (VMAT) preventing the storage of neurotransmitters
  • guanethidine: replaces the NE inside the neurotransmitter vesicles and depletes the vesicle of NE
• cocaine and tricyclic antidepressants: inhibits the NET (reuptake)

Cholinergic Nerve Terminal

• Acetylcholine (ACh) production
  • synthesized from choline and acetyl-CoA by choline acetyltransferase
    • acetyl-CoA is made in the mitochondria
    • choline is transported into the nerve by choline transporter (CHT)
• Pathway Inhibitors
  • hemicholinium: blocks the uptake of choline by inhibiting CHT 
  • vesamicol: inhibits vesicular acetylcholine transporter (VAT) preventing the storage of acetylcholine
  • botulinum toxin: cleaves the SNAP proteins thereby preventing the presynaptic vesicles from docking at the terminal and releasing acetylcholine 

Function

The ANS also plays a role in many other bodily functions, including:

1. Temperature regulation: The ANS helps to regulate body temperature by controlling sweating and blood flow to the skin.
2. Sexual function: The ANS is involved in sexual arousal and orgasm.
3. Cardiovascular function: The ANS helps to regulate blood pressure and heart rate.
4. Respiratory function: The ANS controls breathing rate and depth.
5. Urinary function: The ANS controls bladder and urinary function.
6. Gastrointestinal function: The ANS controls digestion, absorption, and elimination of food.

Overall, the autonomic nerve pathways are essential for maintaining homeostasis and ensuring that the body’s internal environment remains stable and functional.

Studies

The autonomic nervous system (ANS) is a complex network of nerve pathways that control many of the body’s involuntary functions. Studying the ANS can provide insights into how the body responds to different stimuli and how it maintains homeostasis.

There are several approaches to studying the autonomic pathways, including:

1. Electroencephalography (EEG): This technique involves recording electrical activity in the brain and can be used to study the effects of different stimuli on autonomic function.
2. Heart rate variability (HRV): HRV is a measure of the variation in time between heartbeats and can be used to assess the balance between the sympathetic and parasympathetic nervous systems.
3. Blood pressure monitoring: Blood pressure can be measured non-invasively using a sphygmomanometer or more invasively using a catheter. Changes in blood pressure can provide information about the activity of the autonomic nervous system.
4. Neuroimaging: Techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) can be used to study the neural pathways involved in autonomic function.
5. Pharmacological studies: Drugs that affect the ANS can be used to study the role of different neurotransmitters and receptors in autonomic function.
6. Animal studies: Animal models can be used to study the autonomic nervous system and its role in various physiological processes.

Overall, studying the autonomic pathways can provide insights into how the body regulates its internal environment and adapts to different stressors. These insights can have implications for the treatment of various medical conditions, including cardiovascular disease, hypertension, and diabetes.

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