Motor Division of the Peripheral Nervous System: Autonomic System, Pathways, and Neurotransmitters

Motor Division of the Peripheral Nervous System

Autonomic System

The autonomic nervous system (ANS) is responsible for involuntary control of bodily functions, regulating internal organs, blood vessels, and glands. It is divided into the parasympathetic and sympathetic divisions, each with distinct roles and neurotransmitters.

• Parasympathetic: "Rest or digest" functions; promotes routine maintenance activities and conserves energy.

• Sympathetic: "Fight or flight" response; prepares the body for stressful or energetic activity. Dominant during exercise.

Key Effects and Neurotransmitters:

• Norepinephrine: Increases heart rate, increases force of ventricular contraction.

• Epinephrine: Vasodilation of coronary and skeletal muscle blood vessels; vasoconstriction of peripheral blood vessels; increases release of glucose from liver; relaxes gastrointestinal tract smooth muscle.

Regulation:

• ANS activity is mediated through the hypothalamus.

• Homeostasis: The internal environment is maintained by negative feedback systems.

• Hypothalamic-pituitary hormone pathway: Increases calcium via bone and kidney regulation.

Nervous System Overview

The nervous system integrates sensory input and coordinates responses to maintain homeostasis and regulate behavior.

• Sensory input: Information from somatic and visceral receptors is relayed to the medulla oblongata, pons, and hypothalamus.

• Medulla: Regulates blood pressure and water balance.

• Hypothalamus: Creates autonomic, endocrine, and behavioral responses (e.g., hunger, thirst, temperature regulation).

• Cerebrum: Involved in higher-order functions and can influence emotions that affect autonomic functions.

Anatomy of Sympathetic and Parasympathetic Pathways

Both sympathetic and parasympathetic divisions use a two-neuron chain: a preganglionic neuron (originates in the CNS) and a postganglionic neuron (located in the peripheral ganglion).

• Preganglionic neuron: First neuron, originates in the CNS, synapses with the postganglionic neuron in an autonomic ganglion outside the CNS.

• Postganglionic neuron: Second neuron, innervates the target tissue.

Sympathetic Pathways

• Originate in the thoracic and lumbar spinal cord.

• Ganglia are located in a chain close to the spinal column (sympathetic chain ganglia) or along the descending aorta.

• Short preganglionic neurons, long postganglionic neurons.

Parasympathetic Pathways

• Originate in the brainstem (cranial nerves) and sacral region of the spinal cord.

• Ganglia are near or within target organs.

• Long preganglionic neurons, short postganglionic neurons.

• Vagus nerve (cranial nerve X): Carries about 75% of all parasympathetic fibers, innervating head, neck, and internal organs.

Divergence and Neurotransmitters

• One preganglionic neuron may synapse with 8-9 postganglionic neurons, allowing for widespread effects.

• All autonomic preganglionic neurons release acetylcholine (ACh) onto nicotinic receptors.

• Most postganglionic sympathetic neurons release norepinephrine onto adrenergic receptors.

• Most postganglionic parasympathetic neurons release acetylcholine onto muscarinic receptors.

Review: Nervous System Neurocrines and Receptors

Neurocrines include neurotransmitters, neuromodulators, and neurohormones. Neurotransmitters are classified by structure into seven classes:

• 1. Acetylcholine

• 2. Amines

• 3. Amino acids

• 4. Peptides

• 5. Purines

• 6. Gases

• 7. Lipids

Catecholamines

• Formed from tyrosine (an amino acid).

• Include norepinephrine, epinephrine, and dopamine.

Cholinergic Neurons and Receptors

• Secrete acetylcholine.

• Receptors that bind acetylcholine:

  • Nicotinic receptors: Found in skeletal muscle and CNS; chemically gated ion channels.

  • Muscarinic receptors: Found in cardiac and smooth muscle, exocrine and endocrine glands, and CNS; G protein-coupled receptors.

Adrenergic Neurons and Receptors

• Secrete norepinephrine or epinephrine.

• Receptors that bind norepinephrine or epinephrine:

  • Alpha and beta receptors: Found in cardiac and smooth muscle, exocrine and endocrine glands, and CNS; G protein-coupled receptors.

Amino Acid Neurotransmitters

• Include glutamate, GABA, glycine, and aspartate.

• Play major roles in CNS signaling.

Summary Table: Autonomic Neurotransmitters and Receptors

Division	Preganglionic Neurotransmitter	Postganglionic Neurotransmitter	Target Receptor
Sympathetic	Acetylcholine (ACh)	Norepinephrine (NE)	Adrenergic (alpha, beta)
Parasympathetic	Acetylcholine (ACh)	Acetylcholine (ACh)	Muscarinic cholinergic

Additional info:

• Some autonomic pathways use dopamine as a neurotransmitter, especially in the CNS.

• Neurotransmitter release and receptor binding are essential for synaptic transmission and the regulation of target organ function.