Lecture 8 — Introduction to the Autonomic Nervous System
Overview of the nervous system
• The central nervous system (CNS) comprises the brain and spinal cord. The peripheral nervous system (PNS) includes afferent sensory pathways toward the CNS and efferent motor pathways away from it.
• The peripheral efferent system includes the somatic nervous system (skeletal muscle) and the autonomic nervous system (smooth muscle, cardiac muscle and exocrine glands).
• The autonomic nervous system (ANS) controls many involuntary functions and helps maintain homeostasis. Its two major divisions are sympathetic and parasympathetic.
Somatic versus autonomic motor pathways
• A somatic motor neuron travels from the CNS directly to skeletal muscle.
• An autonomic pathway generally uses two neurons: a preganglionic neuron from the CNS and a postganglionic neuron from an autonomic ganglion to the visceral effector.
• Sensory input can trigger both somatic and autonomic reflex responses.
Parasympathetic pathway
• Preganglionic parasympathetic neurons release acetylcholine (ACh), which activates nicotinic receptors on postganglionic neurons.
• Parasympathetic ganglia are located in or near target organs. Postganglionic neurons release ACh onto muscarinic receptors on the target organ.
• Pathway in words: CNS to preganglionic neuron to ganglion near organ to postganglionic neuron to effector organ.
Sympathetic pathway
• Preganglionic sympathetic neurons are generally short and synapse in ganglia near the spinal cord. They release ACh onto nicotinic receptors.
• Preganglionic fibers can branch to many postganglionic neurons, helping produce widespread responses.
• Most sympathetic postganglionic neurons release norepinephrine onto adrenergic receptors at their target organs.
• Important exception in the lecture: sympathetic innervation of sweat glands uses postganglionic ACh.
Adrenal medulla
• Preganglionic sympathetic ACh stimulates nicotinic receptors on adrenal medullary chromaffin cells, which release catecholamines such as epinephrine and norepinephrine.
Cholinergic and adrenergic receptors
• Cholinoceptors respond to ACh. Muscarinic receptors mediate many parasympathetic effects in visceral organs; nicotinic receptors are present in autonomic ganglia, adrenal medulla, and at skeletal neuromuscular junctions.
• Adrenergic receptors respond to norepinephrine and epinephrine. The lecture distinguishes these from cholinergic receptor types.
Pathway comparison
• Parasympathetic: preganglionic ACh to nicotinic receptor; postganglionic ACh to muscarinic receptor.
• Typical sympathetic: preganglionic ACh to nicotinic receptor; postganglionic norepinephrine to adrenergic receptor.
• Sympathetic sweat-gland exception: postganglionic ACh rather than norepinephrine.
Prepared for Dear Study. Review alongside the original university PDF for figures and slide-specific details.
