Autonomic and Somatic Nervous Systems

Autonomic Nervous System

• The autonomic nervous system (ANS) regulates processes subconsciously.
• It controls smooth muscle, cardiac muscle, and glands.
• The somatic nervous system (SNS) controls skeletal muscle.
• SNS processes are consciously perceived or controlled.

Comparison of Somatic and Autonomic Nervous Systems

• ANS involves two neurons, a preganglionic and a postganglionic neuron.
• SNS involves one neuron.
• ANS responses are often involuntary, while SNS responses are voluntary.
• ANS neurotransmitters can excite or inhibit effector cells (e.g. smooth muscles, cardiac muscles, and glands).
• SNS neurotransmitters always excite their effector cells (e.g., skeletal muscles).

Divisions of the Autonomic Nervous System

• The ANS has two divisions: parasympathetic and sympathetic.

Parasympathetic Division

• The parasympathetic division is associated with homeostasis, conservation of energy and replenishment of nutrient stores.
• It is also called the craniosacral division because the preganglionic neurons are in the brainstem and sacral region.
• The synapse in the parasympathetic division is either found in the terminal ganglia close to the target organ or intramural ganglia in the wall of the target organ.

Sympathetic Division

• The sympathetic division is activated during emergencies ("fight-or-flight").
• It prepares the body for action by increasing alertness and metabolic activity.
• It is also called the thoracolumbar division because the preganglionic neurons are in the thoracic and lumbar regions of the spinal cord.
• Preganglionic axons typically travel with somatic motor neurons and exit the spinal cord via the anterior roots.
• They synapse either in the sympathetic chain or in ganglia located close to the vertebral column.

Neurotransmitters

• All preganglionic axons release acetylcholine (ACh).
• Most postganglionic sympathetic axons release norepinephrine (NE).
• Postganglionic parasympathetic axons release acetylcholine (ACh).
• The effect of released neurotransmitters depends on the receptor type.

CNS Control of Autonomic Function

• The CNS, specifically the cerebrum, hypothalamus, brainstem, and spinal cord, influence autonomic function.
• The hypothalamus plays a crucial role in integrating and controlling autonomic functions, including emotional responses.

Autonomic Plexuses

• Autonomic plexuses are networks of nerves that contain both sympathetic and parasympathetic fibers.
• They help regulate organs and tissues by coordinating innervation from different sources.

White and Gray Rami

• White rami are branches that carry sympathetic preganglionic neurons from the spinal nerve to the sympathetic trunk (located in the lateral column).
• Gray rami are branches that carry postganglionic sympathetic neurons back to the spinal nerve, innervating structures in the body wall.

Overview of Sympathetic Pathways

• There are three main sympathetic pathways: the spinal nerve pathway, the post-ganglionic sympathetic pathway, and the splanchnic nerve pathway.
• The latter involves a preganglionic axon traveling with splanchnic nerves to synapse at a prevertebral ganglion.
• There is also the adrenal medulla pathway, where the preganglionic axon synapses directly with cells in the adrenal medulla, releasing norepinephrine into the blood stream.

Comparison of Parasympathetic and Sympathetic Divisions

• The parasympathetic division is responsible for rest and digestion, while the sympathetic division is activated for "fight or flight" situations.
• The pathways, neurotransmitters, and ganglia associated with each division differ significantly, allowing for localized, specific control of target organs.