05 Autonomic Nervous System

what kind of response is associated with the ANS?

fight or flight & rest and digest

• emotional responses

• controls cardiac and smooth muscle tissue

• involuntary responses that regulate many of the internal organs to achieve homeostasis

divisions of the ANS

Sympathetic – “fight-or-flight”

Parasympathetic – “rest and digest”

what is the thoracolumbar system

simply another name for the sympathetic nervous system

• It gets this name because the nerve fibers initiate the sympathetic "fight-or-flight" response originate from the thoracic and upper lumbar spinal cord

Where is the "central neuron" of the sympathetic system located?

In the lateral horn of the spinal cord, specifically in the thoracic and upper lumbar regions.

How do central neurons project to the ganglia in the sympathetic system?

They project to the ganglia adjacent to the vertebral column through the ventral spinal roots.

What are the "Sympathetic chain ganglia"?

The majority of the sympathetic system ganglia (axons) that run alongside the vertebral column. There are typically 23 on each side of the body.

What is the breakdown of the 23 sympathetic chain ganglia per side by region?

• Cervical region: 3

• Thoracic region: 12

• Lumbar region: 4

• Sacral region: 4

The cervical region (neck) has sympathetic ganglia, but the central neurons originate in the thoracic/lumbar spine. How are they connected?

They are connected to the spinal cord (SC) indirectly via ascending and descending connections through bridges within the sympathetic chain.

What are the two types of nerve fibers (rami communicantes) that connect the sympathetic chain to the spinal nerve, and are they myelinated or unmyelinated?

• White rami communicantes: Myelinated fibers.

• Gray rami communicantes: Unmyelinated axons

In the sympathetic nervous system, what is the specific term for the central neuron's axon, and what is it called after it synapses?

• Preganglionic fiber: The axon from the central neuron (short & myelinated).

• Postganglionic fiber: The axon of the ganglionic neuron. represents output of a

  ganglion that directly influences and organ; (long & unmyelinated)

Describe the sympathetic pathway when the target effector is at the same level as the spinal segment.

The nerve projects to the chain ganglion at the same level as the target organ. The preganglionic fiber synapses with the ganglionic neuron, and the postganglionic fiber (via gray rami) projects to the target.

• Example: SN T1 synapses with the T1 chain ganglion to innervate the trachea

Describe the sympathetic pathway when the target effector is located superior or inferior to the spinal segment where the preganglionic fibers emerge.

The preganglionic fiber travels up or down the sympathetic chain. The synapse occurs at a chain ganglion superior or inferior to the central neuron's location.

• Example: SN T1 innervation of the eye. The signal tracks up the chain to the superior cervical ganglion (paravertebral ganglia) to synapse with the postganglionic neuron.

Where do central neurons terminate if they do not synapse in the chain ganglia?

They terminate on collateral ganglia (also called prevertebral ganglia) via the greater or lesser splanchnic nerves.

Which organs do the collateral/prevertebral ganglia primarily control, and what are the three specific ganglia named in the text?

• Target: Organs in the abdomen (part of the Enteric Nervous System).

• Ganglia: Celiac, superior mesenteric, and inferior mesenteric ganglion.

Give the example of the pathway involving the greater splanchnic nerve and the stomach

The greater splanchnic nerve at T5 synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves innervating the stomach.

How do the sympathetic preganglionic fibers that project to the adrenal medulla differ from typical sympathetic fibers regarding ganglia?

heir axons do not terminate in a ganglion. They project directly to the adrenal medulla.

What specific cells in the adrenal medulla are contacted by preganglionic sympathetic fibers, and what do they do?

• They contact chromaffin cells.

• These cells release signaling molecules directly into the bloodstream.

What are the two main anatomical components (location of preganglionic neurons) of the Craniosacral system in the parasympathetic division?

1. Nuclei of the brain stem.

2. Lateral horn of the sacral spinal cord (SC)

How does the general layout of connections/circuits in the parasympathetic division compare to the sympathetic division, and where are the preganglionic fibers located?

• The layout is similar to the sympathetic division.

• Preganglionic fibers are located in cranial nerves (cranial region) and spinal nerves (sacral region).

Where are the terminal (intramural) ganglia located in the parasympathetic system, and what is the defining characteristic of the postganglionic fibers regarding distance?

• Location: Near or found within the target organs.

• Postganglionic Fibers: They project from the terminal ganglia to the target organ over a short distance (unlike the long postganglionic fibers of the sympathetic system).

What is the Edinger-Westphal nucleus? (Include its location, system, and associated nerve).

• It is the cranial component of the parasympathetic system in the midbrain.

• It is part of the oculomotor complex.

• Its axons travel with CN III fibers.

Trace the specific parasympathetic pathway involving the Edinger-Westphal nucleus and the eye.

1. Preganglionic PS fibers of CN III travel to the ciliary ganglion.

2. Postganglionic PS fibers project to the smooth muscles of the iris.

What is the function and nerve pathway of the Salivary nuclei (located in the upper medulla)?

• Nerve pathway: Axons project through CN VII and CN IX to ganglia controlling salivary glands.

• Function: Controls salivary glands. (Note: PS fibers in CN VII also activate a ganglion influencing tear production).

Where are the neurons located that project through CN X, and which areas do they innervate?

• Location: Dorsal nucleus of vagus nerve and nucleus ambiguous.

• Innervation: Terminal ganglia of the thoracic and abdominal cavities (e.g., mesenteric plexus).

What are the synapses of the ANS, what neurotransmitter is released, and what are the two receptor types?

1. Cholinergic - acetylcholine is released

• Nicotinic receptor: Ligand-gated cation channel.

• Muscarinic receptor: G-protein coupled receptor.

1. Adrenergic - norepinephrine is released

• Receptor Type: G-protein coupled

• Subtypes: α-adrenergic receptors (3 types) and β-adrenergic receptors (2 types).

what are the 2 types of Autonomic system signaling molecules

1. Preganglionic

2. Postganglionic

PREGANGLIONIC Signaling: What is the neurotransmitter and receptor for BOTH Sympathetic and Parasympathetic divisions?

• Neurotransmitter: Acetylcholine (ACh)

• Receptor: Nicotinic receptor
  (Note: This is true for both divisions)

SYMPATHETIC POSTGANGLIONIC Signaling: What are the standard neurotransmitters/receptors, and what are the two major EXCEPTIONS?

• Standard: Norepinephrine → α- or β-adrenergic receptors.

• Exceptions (Release Acetylcholine → Muscarinic receptor):

  1. Sweat glands.

  2. Blood vessels associated with skeletal muscles only.

PARASYMPATHETIC POSTGANGLIONIC Signaling: What is the neurotransmitter and receptor?

• Neurotransmitter: Acetylcholine (ACh)

• Receptor: Muscarinic receptor

AUTONOMIC VARICOSITY: What is it, and what is it compared to?

• Definition: Chains of swellings along the length of an autonomic postganglionic fiber.

• Compared to: Synaptic end bulbs in Neuromuscular Junctions (NMJs).

NEUROEFFECTOR JUNCTIONS: What are they, and what are they compared to?

• Definition: Connections between a postganglionic fiber and target effectors.

• Compared to: Synapses in Neuromuscular Junctions (NMJs).

NEUROMODULATORS: What does this term refer to?

It refers to neurotransmitters.

BASIC CIRCUIT: What is the basic circuit of a reflex called?

A reflex arc.

VISCERAL/AUTONOMIC REFLEXES: How does the efferent (outgoing) branch differ from a typical reflex?

It uses a two-step pathway (preganglionic then postganglionic) unlike a single-step somatic motor pathway.

• Step 1: Preganglionic fiber emerges from the lateral horn neuron (spinal cord) or cranial nucleus neuron (brain stem) and goes to a ganglion.

• Step 2: Postganglionic fiber projects to the target effector.

AFFERENT BRANCH COMPARISON: Is the afferent (incoming) branch different between the Autonomic Nervous System and Somatic Nervous System?

No, it is the same. Cell bodies in sensory ganglia project to the CNS to initiate the reflex.

AFFERENT BRANCH (Visceral vs. Somatic): How does the afferent branch of the visceral reflex differ from the somatic nervous system?

It features non-conscious perception of internal senses associated with viscera (e.g., baroreceptors on the walls of the aorta and carotid sinuses).

REFERRED PAIN: What happens when strong visceral sensations reach conscious perception?

They are often felt in unexpected places (this phenomenon is known as referred pain).

LONG REFLEX: Define a long reflex in the autonomic nervous system.

Afferent branches that enter the spinal cord (SC) or brain and involve efferent branches. (Requires CNS integration).

SHORT REFLEX: Define a short reflex in the autonomic nervous system.

A reflex that is completely peripheral; it involves the local integration of sensory input with motor output.

NO CNS INVOLVEMENT: Under what specific condition does a reflex occur without any CNS involvement?

When a sensory neuron projects directly to the ganglionic neuron, causing activation of the effector target (e.g., enteric nervous system).

AUTONOMIC TONE: What does "autonomic tone" refer to?

The resting state of the autonomic system.

HOMEOSTASIS (Dual Innervation): How is homeostasis maintained at the target effector?

It is due to competing Sympathetic (SNS) and Parasympathetic (PSNS) dual innervation at the target effector via chemical signaling.

FOREBRAIN STRUCTURES: What is their general role in central control?

They coordinate the balance required for homeostasis through integration.

HYPOTHALAMUS: What is its main function?

It is the control center for both autonomic and endocrine function (e.g., pupillary light reflex).

PUPILLARY LIGHT REFLEX PATHWAY: Trace the full pathway from the retina to the response.

1. Light enters retina →

2. Optic nerve →

3. Hypothalamus (sympathetic response due to low light OR parasympathetic response due to bright light) →

4. Dorsal longitudinal fasciculus and medial forebrain bundle →

5. Edinger-Westphal nucleus of CN III / lateral horns of thoracic spinal cord →

6. Response

AMYGDALA: Where is it located and what is its function?

• Location: A group of nuclei in the medial region of the temporal lobe.

• Function: Part of the limbic lobe involved in emotional responses and has structures contributing to memory. It influences the hypothalamus' activity level based on emotional state.

AMYGDALA PATHWAY (Example): Trace the pathway for the fight-or-flight response initiated by the amygdala.

Amygdala (via medial forebrain bundle) → Hypothalamus → Fight or flight response and release of stress hormones from the adrenals.

MEDULLA (Cardiovascular Center): What does the cardiovascular center in the medulla control, and what triggers its activation?

• Controls: Smooth and cardiac muscle of the Cardiovascular System (CVS).

• Activation: A change in blood pressure activates the medulla for coordination of the ANS.

CARDIAC ACCELERATOR NERVES: What are they, and what is their function?

Preganglionic sympathetic fibers responsible for increasing heart rate.

VASOMOTOR NERVES: What are they, and what is their function?

Preganglionic sympathetic fibers responsible for constricting blood vessels (BVs).

SOLITARY NUCLEUS: What is its sensory input, and what is its output?

• Input: Receives sensory input about BP and cardiac function from CN IX and CN X.

• Output: Activates sympathetic stimulation of the heart and BVs through the upper thoracic lateral horn.

DORSAL MOTOR NUCLEUS OF THE VAGUS NERVE: What is its function?

It is the motor nucleus for the parasympathetic functions of the vagus nerve:

• Decreasing heart rate

• Relaxing bronchial tubes

• Activating ENS function

NUCLEUS AMBIGUOUS: What does it contribute to, and what are its targets?

• Contribution: Contributes to parasympathetic output of the vagus nerve.

• Targets: Muscles in the pharynx and larynx (for swallowing and speech) and tone of the heart.