Sympathetic Neurotransmitters and Receptors

Overview of the Sympathetic Nervous System

  • The sympathetic nervous system utilizes multiple neurotransmitters and receptors.

  • Unlike the parasympathetic system which primarily uses acetylcholine, sympathetic responses involve a variety of neurotransmitters, including norepinephrine and epinephrine.

Structure and Function of Sympathetic Neurons

  • The sympathetic nervous system consists of two neurons:

    • Preganglionic neuron:

    • Arises from the ventral lateral horn of the spinal cord, specifically between the levels T1 and L2.

    • Myelinated and relatively short in length.

    • Synapses in the sympathetic chain.

    • Postganglionic neuron:

    • Unmyelinated and longer, traveling to target organs after synapsing with the preganglionic neuron.

Pathways of Sympathetic Neurons

  • Preganglionic neurons may:

    • Enter the sympathetic chain and synapse at the same level.

    • Ascend or descend within the chain and synapse at a different level (e.g., synapsing at the superior cervical ganglia for innervation of the head and neck).

    • Exit the chain at various levels including sacral regions.

Synaptic Communication in the Sympathetic Nervous System

  • At the synapse between preganglionic and postganglionic neurons:

    • The preganglionic neuron releases acetylcholine (ACh).

    • Acetylcholine binds to nicotinic receptors on the postganglionic neuron, which is an ionotropic receptor.

    • Action of Nicotinic Receptors:

    • When acetylcholine binds, it opens ion channels, allowing sodium ions (Na+) to enter the postganglionic cell.

    • This influx of sodium generates an excitatory postsynaptic potential (EPSP), potentially reaching threshold to trigger an action potential.

Target Organ Interaction of Postganglionic Neurons

  • The postganglionic neurons synapse onto target tissues:

    • Typically utilize adrenergic receptors, which respond to norepinephrine or epinephrine.

    • An exception exists in the case of the eccrine sweat glands where acetylcholine binds to muscarinic receptors to cause sweating.

Types of Adrenergic Receptors

  • Alpha and Beta Adrenergic Receptors:

    • Alpha Receptors (α1, α2): Major receptors in the SNS, associated with contractile effects.

    • α1 receptors: Found on smooth muscles of blood vessels, cause vasoconstriction (e.g., skin, abdominal viscera, kidneys).

    • α2 receptors: Located on adrenergic axon terminals, inhibit norepinephrine release.

    • Beta Receptors (β1, β2, β3): Associated with relaxing effects and metabolic regulation.

    • β1 receptors: Found in the heart, increase heart rate and force of contraction, also present in kidneys to stimulate renin release.

    • β2 receptors: Found in bronchioles, relax smooth muscles to facilitate airflow, and in skeletal muscle blood vessels for vasodilation.

    • β3 receptors: Located in adipose tissue, stimulate the breakdown of triglycerides for energy during stress responses.

Physiological Implications of Sympathetic Activation

  • The sympathetic nervous system mediates the fight or flight response:

    • Increase in heart rate and blood pressure.

    • Dilation of bronchioles to improve oxygen intake.

    • Vasoconstriction of blood vessels to non-essential organs such as the skin and digestive tract, redirecting blood flow to muscles.

    • Activation of sweat glands primarily via cholinergic mechanisms (exception: eccrine sweat glands utilize ACh).

Summary of Receptor Functions and Locations

  • α1: Vasoconstriction in skin and abdominal viscera.

  • α2: Inhibition of norepinephrine release; reduce activity in the pancreas.

  • β1: Cardiac acceleration; increases renin release from kidneys.

  • β2: Bronchodilation; vasodilation in skeletal muscles.

  • β3: Lipolysis in adipose tissue.

Memorization Strategy for Sympathetic Receptors

  • The general sequence of sympathetic responses can be remembered as follows:

    • α1 (abdominal blood vessels: constriction), then β2 (bronchioles: dilation), followed by β1 (heart: increase in contractility).

    • This order emphasizes the need to redirect blood away from non-essential areas (abdominal viscera) towards essential areas (heart, lungs, muscles).

Conclusion

  • Understanding the detailed characteristics and functions of sympathetic neurotransmitters and receptors provides insight into how the body reacts during stress and facilitates effective responses to environmental changes.