Fight or rest? Understanding the Autonomic Nervous System.

Question

Describe the anatomical differences between the sympathetic and parasympathetic nervous divisions of the autonomic nervous system, including their origins, neurotransmitters, and physiological effects on target organs.

Answer 1

The autonomic nervous system controls the automatic functions in your body like heart rate, breathing, digestion, and sweating. It has two parts that work like a balance. The sympathetic nervous system gets your body ready to handle stress or emergencies. It starts in the middle of your spine and sends signals using chemicals to make your heartbeat faster, open your lungs, slow down digestion, and get more energy into your muscles. This is often called the fight or flight response.

The parasympathetic nervous system does the opposite. It helps your body calm down and focus on rest, healing, and digestion. It starts in your brain and the lower part of your spine and uses the same chemical signal throughout. When this system is active, your heart slows down, your breathing gets gentler, your stomach gets more active, and your body stores energy. Both systems are always working behind the scenes, adjusting depending on what you need in the moment.

Answer 2

The autonomic nervous system (ANS) controls involuntary body functions such as heart rate, digestion, respiration, and pupil size. It consists of two main divisions: the sympathetic (fight or flight) and the parasympathetic (rest and digest) systems. These divisions often act in opposition to maintain internal balance, or homeostasis.

Anatomical Differences

The sympathetic nervous system originates from the thoracolumbar region of the spinal cord, specifically from segments T1 to L2. Its ganglia are located near the spinal cord, either in the paravertebral (sympathetic chain) or prevertebral ganglia. Sympathetic neurons have short preganglionic fibers and long postganglionic fibers.

In contrast, the parasympathetic nervous system arises from the craniosacral regions—the brainstem (cranial nerves III, VII, IX, and X) and sacral spinal cord segments S2 to S4. Its ganglia are found near or within the target organs, resulting in long preganglionic and short postganglionic fibers.

Neurotransmitters

Both sympathetic and parasympathetic preganglionic neurons release acetylcholine (ACh), which acts on nicotinic receptors in the ganglia.

For postganglionic neurotransmitters:

1. The sympathetic division primarily uses norepinephrine (NE), acting on adrenergic receptors (alpha and beta) in target tissues. An exception is that sweat glands use ACh instead.
2. The parasympathetic division uses acetylcholine again, which acts on muscarinic receptors in the target organs.

Physiological Effects

1. On the heart, the sympathetic system increases heart rate and contractility, while the parasympathetic system decreases heart rate.
2. In the lungs, the sympathetic division causes bronchodilation, whereas the parasympathetic causes bronchoconstriction.
3. In the gastrointestinal tract, sympathetic activity slows motility and secretion, while parasympathetic activity enhances both.
4. For the eyes, the sympathetic system dilates pupils (mydriasis), while the parasympathetic constricts them (miosis).
5. In the salivary glands, the sympathetic system produces thick, viscous saliva, and the parasympathetic produces watery, enzyme-rich saliva.
6. In the bladder, the sympathetic system relaxes the detrusor muscle and contracts the internal sphincter (inhibiting urination), while the parasympathetic system does the opposite to promote urination.
7. The adrenal medulla is directly stimulated by sympathetic preganglionic fibers to release epinephrine and norepinephrine; it receives no parasympathetic input.

Answer 3

Sympathetic:

1. originate from the thoracolumbar regions of the spinal cord (T1-L3). nerves will travel through the sympathetic trunk if they need to innervate cranial and/or sacral targets
2. pre-synaptic neuron is shorter than the post-synaptic neuron, and synapses with the post-synaptic neuron in a sympathetic ganglion distant from the organ (closer to spinal cord)
3. pre-synaptic neuron releases acetylcholine as its neurotransmitter, post-synaptic neuron releases norepinephrine as its neurotransmitter
4. physiological effects are those related to "fight or flight" (non-exhaustive):
5. increased contractile force of cardiac muscle and increased heart rate to promote delivery of blood/oxygen/nutrients to vital organs such as the brain and skeletal muscle
6. increased respiratory rate and relaxation of bronchiolar smooth muscle to relax airways and promote respiration
7. peripheral vasoconstriction to divert blood away from non-essential organs (eg. GI organs) paired with vasodilation of muscle-related arteries which favors blood flow to the brain and skeletal muscle (ie. organs necessary for "fight or flight")
8. pupillary dilation via ophthalmic nerve (branch of CN V)-mediated activation of the dilator muscle to enhance vision by allowing more light into the eye
9. inhibition of urination via hypogastric nerve-mediated relaxation of the detrusor muscle and contraction of the smooth muscle within the bladder neck

Parasympathetic:

1. originate from the craniosacral regions of the CNS/spinal cord (cranio referring to the cranial nerves within the brainstem, sacral referring to S1-S3)
2. pre-synaptic neuron is longer than the post-synaptic neuron, and synapses with the post-synaptic neuron in a ganglion closer to the target organ
3. both the pre- and post-synaptic neurons will release acetylcholine as their neurotransmitter
4. physiological effects are those related to "rest and digest" which relate to normal function and tend to oppose those of the sympathetic branch (non-exhaustive):
5. decreased contractile force of cardiac muscle and increased heart rate
6. decreased respiratory rate and constriction of bronchiolar smooth muscle to return airways to homeostatic conditions
7. peripheral vasodilation to promote blood flow toward non-essential organs (eg. GI organs) and vasoconstriction of muscle-related arteries
8. pupillary constriction via ciliary nerve (stimulated by CN III)-mediated activation of the constrictor muscle
9. facilitation of urination via pelvic nerve-mediated contraction of the detrusor muscle