2-9 Autonomic Motor System 2

General Functions at Target Cells

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.

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.

Acetylcholine (ACh) as neurotransmitter:
- Used by all preganglionic neurons (both sympathetic and parasympathetic).
- Released from most parasympathetic postganglionic neurons to target cells.

Norepinephrine (NE) as neurotransmitter:
- Released by most sympathetic postganglionic neurons to target cells.

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).

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.

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.

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.

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.

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.

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.

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.

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.