2-9 Autonomic Motor System 2

General Functions at Target Cells

Sympathetic Division

• Activates body for "fight or flight" by:

  • Releasing norepinephrine (NE) from postganglionic neurons.

  • Secreting epinephrine (Epi) from adrenal medulla.

• Prepares body for intense physical activity by:

  • Increasing heart rate.

  • Diverting blood to skeletal muscles.

  • Dilating airways for increased oxygenation.

• Tunes down "housekeeping" organs and constricts blood vessels to those organs.

Parasympathetic Division

• Acts antagonistically to sympathetic division:

  • Allows the body to "rest and digest."

  • Releases acetylcholine (ACh) from postganglionic neurons.

• Functions include:

  • Slowing heart rate.

  • Increasing digestive activities.

  • Increasing blood flow to the digestive tract.

Synaptic Transmission

Cholinergic Synapses

• Acetylcholine (ACh) as neurotransmitter:

  • Used by all preganglionic neurons (both sympathetic and parasympathetic).

  • Released from most parasympathetic postganglionic neurons to target cells.

Adrenergic Synapses

• Norepinephrine (NE) as neurotransmitter:

  • Released by most sympathetic postganglionic neurons to target cells.

Varicosities in Postganglionic Neurons

• Structure:

  • Axons of postganglionic neurons have swellings called varicosities.

  • Varicosities release neurotransmitter along the axon length to act on target cells instead of having a simple nerve terminal.

• Interaction:

  • Sympathetic and parasympathetic neurons innervate the same tissues but release different neurotransmitters.

  • One target cell usually receives signals from both types of neurons (sympathetic and parasympathetic).

Autonomic Neurotransmitters

Types of Neurotransmitters

• ACh: Ach Nicotinic Receptors (N. AchR) and Muscarinic Receptors (M. AchR).

• NE/Epi: Adrenergic Receptors.

• Differentiations in organ locations for parasympathetic and sympathetic nerves as follows:

  • Cranial and Sacral Parasympathetic Nerves vs. Thoracolumbar Sympathetic Nerves.

• Target effects include:

  • ACh affecting visceral effectors.

  • NE and Epi influencing circulation.

Adrenergic Receptors

• Three types of adrenergic receptors:

  • Alpha (α1, α2)

  • Beta (β1, β2)

• Receptor differences:

  • Alpha receptors are most sensitive to norepinephrine.

  • Beta receptors have varied affinities for NE and Epi depending on the receptor type.

Adrenergic Synapses and Effects

Sympathetic Stimulation Outcomes

• Increased contraction and constriction in most arterioles, leading to less blood flow to visceral organs.

• Heart rate and contraction strength increase.

• Effects based on receptors stimulated:

  • Alpha receptors causing vascular constriction.

  • Beta receptors affecting heart rate and contraction.

Adrenergic Stimulation Features

• Can be triggered by:

  • Epinephrine from the blood.

  • Norepinephrine from sympathetic nerves.

• Responses vary based on receptor type:

  • Stimulation of heart and smooth muscles of blood vessels.

  • Inhibition of bronchiolar contraction.

Cholinergic Stimulation

• ACh released from preganglionic neurons:

  • Always stimulatory to postganglionic neurons (via Nicotinic AChR as ligand-gated Na+ channels).

• Postganglionic actions:

  • Stimulatory or inhibitory depending on the receptor type (muscarinic receptors).

• Autonomic effects are often opposite between sympathetic and parasympathetic divisions.

Cholinergic Receptors

Types and Locations

• Nicotinic Receptors:

  • Located in autonomic ganglia.

  • Stimulated by ACh from preganglionic neurons.

  • Act as ligand-gated ion channels for Na+.

• Muscarinic Receptors:

  • Found in smooth and cardiac muscle, glands.

  • Respond to ACh from postganglionic neurons.

  • Can have stimulatory or inhibitory effects depending on the ion channels activated.

Organs with Dual Innervation

• Most visceral organs receive input from both divisions:

  • Opposing Effects: Heart rate; sympathetic increases, parasympathetic decreases.

  • Digestive Functions: Parasympathetic increases, sympathetic decreases.

  • Bronchioles: Parasympathetic constricts, sympathetic dilates.

  • Blood vessels respond variably depending on location.

• Complementary Effects:

  • Example: Salivary gland secretion with oppositional influence on salivary quality.

Comparative Overview of Autonomic Nervous System

Sympathetic vs. Parasympathetic

• Sympathetic:

  • Originates in the thoracic to lumbar segments.

  • Uses norepinephrine at target synapse.

  • Effects often widespread due to varicosities.

• Parasympathetic:

  • Springs from brain and sacral segments.

  • Employs ACh for neurotransmission.

  • Usually localized effects to specific organs.

Neurotransmitter Inactivation

• Sympathetic:

  • Uptake and diffusion of norepinephrine.

• Parasympathetic:

  • Enzymatic breakdown of ACh by acetylcholinesterase (AChE).