Neural Anatomy of the ANS

Neurofiber anatomy contains…

• Pre-ganglionic and post-ganglionic fibers
• Neurotransmitter vesicles
• Junctional Spaces
• Receptors

Neural Anatomy of the Parasympathetic Fiber

This is a simple reminder of things you should have learned in physiology already, hopefully.

Preganglionic fiber releases ACh —> Ganglion (Cell Bodies) —> Nicotinic receptors on the Postganglionic fiber cause the release of more ACh —-> muscarinic receptors on the motor plate get stimulated.

Electrical activity goes down the preganglionic fiber. At the end of this fiber are neural vesicles which contain acetycholine. The ACh will attach to the receptors on the post ganglionic fibers called nicotinic receptors.

We will be focusing on the post-ganglionic fiber activity for this discussion. The post ganglionic fiber transmits the electrical activity to the end which releases ACh in the junctional space, again. These will attach to the receptors on the motor plate (the muscle) called the muscarinic receptors. So we’re going to talk about what happens when muscarinic receptors are stimulated.

If it stimulates the muscarinic receptors of the heart, it will slow down the heart. Why? Because this is a parasympathetic event.

Can we make the heart rate very low for a long period of time? No, because the body produces an enzyme that destroys ACh, called acetylcholinesterase (Achase) in the same junctional space. So the body has to continually keep producing more ACh.

Can we block the muscarinic receptor? Yes. The anticholinergic drugs block these receptors.

Neural Anatomy of the Sympathetic Fiber

Now we switch gears to the sympathetic. In the sympathetic, everything up to the post ganglionic fiber is the same.

Postganglionic fiber releases norepinephrine —-> adrenergic receptors on the motor plate get stimulated.

What changes here is that instead of ACh being released in the junctional space, it’s norepinephrine and the receptors that norepinephrine attaches to are the adrenergic receptors. These include the alpha-1, beta-1 and beta-2 adrenergic receptors.

If it’s attaching to beta-1, the heart speeds up. But will it speed it up indefinitely? No, because the primary mechanism is reuptake and the secondary is MAO which will destroy the norepinephrine.

Can we block the adrenergic receptors? Yes. Beta blockers and alpha blockers block these receptors.

Recap

Parasympathetic fiber

• The post-ganglionic neurotransmitter is ACh.
• The receptors on the post ganglionic fiber are muscarinic.
• ACh is destroyed by AChase.
• The muscarinic receptor is blocked by anticholinergics.
• Drugs that mimic ACh are called parasympathomimetics.

Sympathetic

• The post-ganglionic neurotransmitter is norepinephrine (a catecholamine)
• The adrenergic receptors include
  • Alpha adrenergic
  • Beta 1 adrenergic
  • Beta 2 adrenergic
• The activity can be stopped by reuptake and monoamine oxidase
• The adrenergic receptor is blocked by alpha/beta blockers.