Ganglionic pathways (Important)

General Structure: Most autonomic pathways (both parasympathetic and sympathetic) involve a two-neuron sequence.
- Pre-ganglionic Neuron: First neuron in the sequence.
  - Location: Cell body originates from the CNS, specifically the brain stem or sacral region of the spinal cord for the parasympathetic system.
  - Synapses with the second neuron in a ganglion.
- Ganglionic Neuron: Second neuron in the sequence.
  - Location: Cell body located in a peripheral ganglion.
  - Sends axon to innervate target tissues such as muscles or glands.

Preganglionic Neuron: Neuron that synapses before the ganglion in the autonomic pathway.
Ganglionic Neuron: Neuron that synapses at the ganglion in the autonomic pathway.
Ganglion: Collection of neuron cell bodies in the peripheral nervous system.

Division Origin: Cranial-sacral division, originating from brain stem and sacral spinal cord.
Synapses:
- Pre-ganglionic neuron synapses in ganglia, known as:
  - Intramural ganglia: Located within the walls of the target organ.
  - Terminal ganglia: Located just outside the target organ.

Division Origin: Thoracolumbar division, with cell bodies located in spinal cord segments T1 to L2.
Structural Differences:
- Ganglia Location: Ganglia are close to the spinal cord, leading to short preganglionic neurons and long postganglionic neurons.
- Axons:
  - Preganglionic axons are myelinated.
  - Postganglionic axons are unmyelinated.
  - Pre-ganglionic axon branches to contact multiple ganglion neurons, resulting in a broad sympathetic response.

Description: Chains of sympathetic ganglia running parallel to the spinal cord known as sympathetic trunks.
Resemblance: Often likened to a "string of pearls," where the pearls are the ganglia and the strings are the axons connecting them.

Unique Feature: Innervation occurs through a single neuron pathway from spinal cord to adrenal medulla without synapsing at a ganglion.
Pathway: Preganglionic axon extends to the adrenal gland, stimulating secretion of catecholamines (norepinephrine and epinephrine).

Typical Neurotransmitter for Sympathetic Neurons: Norepinephrine, which provokes fight-or-flight responses.
Role of Adrenal Medulla: Releases norepinephrine and epinephrine into bloodstream to prolong sympathetic effects.

Prolonged Action: Sympathetic stimulation initiates responses that may last beyond the immediate stimuli.
- Example: Heart rate remains elevated post-stress due to lingering catecholamines in circulation.

Cholinergic Neurons: Neurons that release acetylcholine (ACh). All preganglionic neurons in both divisions of the ANS are cholinergic.
Adrenergic Neurons: Neurons that release norepinephrine. Majority occurs in the sympathetic division, specifically postganglionic neurons.

Nicotinic Receptors: Located on all autonomic postganglionic cell bodies, always generating stimulatory effects through excitatory postsynaptic potentials.
Muscarinic Receptors: Found on target organs innervated by parasympathetic neurons; can provoke either stimulatory or inhibitory effects depending on the specific receptor type and organ.

Categories: Alpha and Beta adrenergic receptors.
- Alpha Receptors: Generally stimulatory, predominantly found in blood vessels.
- Beta Receptors: Can be stimulatory or inhibitory depending on body area. Example: Stimulatory on heart (increased rate) and inhibitory on smooth muscle (e.g., bronchodilation).

Definition: Most organs are innervated by both sympathetic and parasympathetic fibers, allowing for collaborative control.
Types of Interactions:
- Antagonistic: Sympathetic and parasympathetic have opposite effects (e.g., heart rate changes).
- Cooperative: Both divisions contribute to complementary functions (e.g., arousal and response during sexual activity).

Sympathetic Input: Often involved in “fight or flight,” increasing heart rate, blood pressure, and energy mobilization.
Parasympathetic Input: Associated with “rest and digest,” promoting bodily maintenance functions.