The Nervous System: Autonomic Nervous System
General Actions of the Autonomic Nervous System (ANS)
- Visceral Motor System: Functions involuntarily to control effectors such as glands, cardiac muscle, and smooth muscle.
- Visceral Reflexes: Automatic, unconscious responses of visceral effectors to stimuli.
- Example: A rise in blood pressure triggers a reflex to decrease heart rate.
Divisions of the ANS
Sympathetic Division
- Often termed as the "Fight-or-flight" response.
- Functions:
- Increases heart rate
- Elevates blood glucose levels
- Reduces blood flow to skin and gastrointestinal (GI) tract
Parasympathetic Division
- Often referred to as the "Rest and digest" response.
- Functions:
- Decreases heart rate
- Stimulates digestion and promotes waste elimination
- Autonomic Tone: Reflects the balance between sympathetic and parasympathetic activity.
Neural Pathways
- Preganglionic Fiber:
- Originates from the neurosoma in the brainstem or spinal cord.
- Terminates in a ganglion.
- Postganglionic Fiber:
- Contains the neurosoma in the ganglion.
- Extends to the target organ.
Comparison of the Somatic and Autonomic Nervous Systems
| Feature | Somatic | Autonomic |
|---|
| Effectors | Skeletal muscle | Glands, smooth muscle, cardiac muscle |
| Control | Usually voluntary | Usually involuntary |
| Efferent pathways | One nerve fiber from CNS to effector; no ganglia | Two nerve fibers from CNS to effector; synapse at a ganglion |
| Effect on target cells | Always excitatory | Excitatory or inhibitory |
The Sympathetic Division
- Thoracolumbar Division: Preganglionic neurosomas are located in the lateral horns of the spinal cord.
- Characterized by short preganglionic fibers and long postganglionic fibers.
- Sympathetic chain ganglia (paravertebral ganglia) extend from cervical to coccygeal levels.
The Parasympathetic Division
- Craniosacral Division: Characterized by long preganglionic fibers and short postganglionic fibers.
- Terminal ganglia are located in or near the target organs.
The Enteric Nervous System
- Functions as the nervous system of the digestive tract.
- Innervates smooth muscle and glands involved in digestion.
- Regulates:
- Motility of the esophagus, stomach, and intestines
- Secretion of digestive enzymes and gastric acid.
Comparison of the Sympathetic and Parasympathetic Divisions
| Feature | Sympathetic | Parasympathetic |
|---|
| Origin in CNS | Thoracolumbar | Craniosacral |
| Location of ganglia | Paravertebral ganglia adjacent to spinal column and prevertebral ganglia anterior to it | Terminal ganglia near or within target organs |
| Fiber lengths | Short preganglionic, long postganglionic | Long preganglionic, short postganglionic |
| Effects of system | Often widespread and general | More local and specific |
Disorders of the ANS
- Horner Syndrome: Results in unilateral pupillary constriction, sagging of the eyelid, and flushing of skin due to lesions in the sympathetic division.
- Raynaud Disease: Characterized by paleness, cyanosis, and pain in the digits when exposed to cold or stress, often linked to excessive vasoconstriction. Most common in young women.