Neurotransmitters in Autonomic Nervous System

Autonomic Nervous System

• The autonomic nervous system (ANS) is responsible for involuntary functions
• It has two divisions: sympathetic and parasympathetic
• The purpose of this chapter is to examine the structure and function of the ANS, compare and contrast the somatic and ANS, examine ANS neurotransmitters and compare and contrast the sympathetic and parasympathetic nervous systems
• The somatic nervous system includes sensory and motor neurons, it's involved in sensory perceptions and voluntary movements
• Motor neurons innervate skeletal muscles either excitatory or inhibitory
• This system is voluntary
• The autonomic nervous system receives input from sensory receptors located in organs, blood vessels, and muscles
• The axon of a single, myelinated somatic motor neuron extends from the central nervous system to the skeletal muscle fiber it innervates
• The ANS usually operates without conscious control, with centers in the hypothalamus and brain stem regulating reflexes
• Interoceptors are sensory receptors located in blood vessels, visceral organs, and muscles that monitor conditions within the internal environment
• Examples of interoceptors include chemoreceptors, monitoring blood CO2 levels and mechanoreceptors that detect stretch in organ or blood vessel walls
• Autonomic motor neurons regulate visceral activities by either increasing or decreasing activities in effector tissues
• Autonomic responses aren't consciously altered and serve as the basis for polygraph tests
• Yoga and biofeedback techniques can regulate autonomic function

Comparison of Somatic and Autonomic Nervous Systems

• Most autonomic motor pathways consist of two motor neurons in series.
• A preganglionic neuron has its cell body in the central nervous system and the axon extends to an autonomic ganglion.
• A postganglionic neuron has its unmyelinated axon extending from the ganglion to the effector.
• Most body organs have dual ANS innervation, receiving impulses from both sympathetic and parasympathetic neurons.
• Sympathetic stimulation often leads to a "fight-or-flight" response, increasing alertness and metabolism
• Parasympathetic stimulation promotes "rest-and-digest" activities, slowing down most body functions
• There are two types of autonomic ganglia, sympathetic and parasympathetic

Anatomy of Autonomic Motor Pathways

• Each division of the autonomic nervous system (ANS) has two motor neurons, preganglionic and postganglionic
• Preganglionic neurons have their cell bodies in the brain stem or spinal cord, and axons innervate autonomic ganglia
• Postganglionic neurons have their cell bodies within the ganglia, and axons innervate the effectors (target organs)
• Sympathetic division neurons have cell bodies in the lateral horns of the gray matter, in T1-L2 spinal cord segments
• Parasympathetic neurons have cell bodies in nuclei of cranial nerves III, VII, IX, X and lateral gray matter of S2-S4 spinal cord segments.
• 80% of parasympathetic flow travels through the vagus nerve.
• Four ways sympathetic preganglionic neurons can connect with postganglionic neurons
• Major autonomic plexuses in abdomen and pelvis: celiac, superior mesenteric, inferior mesenteric, renal, and hypogastric plexuses; named after the artery through which they are distributed.

ANS Neurotransmitters and Receptors

• Autonomic neurons are classified as cholinergic or adrenergic based on the neurotransmitter they release
• Cholinergic neurons release acetylcholine (ACh), which acts on either nicotinic or muscarinic receptors.
• Adrenergic neurons release norepinephrine (noradrenaline), which acts on alpha and beta receptors
• Nicotinic receptors are excitatory and found at synapses between pre- and postganglionic neurons and at the neuromuscular junction.
• Muscarinic receptors are excitatory or inhibitory, depending on the target tissue and are activated by ACh.
• Alpha and beta adrenergic receptors can be excitatory or inhibitory, their effects are varied.

Physiology of the ANS

• Autonomic tone is the balance between sympathetic and parasympathetic activity and it's regulated by the hypothalamus
• Sympathetic stimulation leads to secretion of norepinephrine, an increase in heart rate and strength, blood vessel constriction of non-essential organs, and dilation of vessels in essential organs.
• Parasympathetic stimulation promotes rest and digest/ relaxation. A mnemonic for responses associated with parasympathetic activity is SLUDD (Salivation, Lacrimation, Urination, Digestion, Defecation).

Integration and Control of Autonomic Functions

• Functions like heart rate and blood vessel diameter are controlled by autonomic reflexes involving a receptor, sensory neuron, integrating center, motor neuron, and effector.

Stress Response

• The stress response involves the activation of the sympathetic nervous system, a complex cascade of hormone releases, and long-term responses for adaptation

Comparison of Sympathetic and Parasympathetic

• Sympathetic is "fight-or-flight", and parasympathetic is "rest-and-digest".
• Key differences such as the length of axons, axon divergence, presence of white and gray communicating rami; the neurotransmitters released at different synapses

Raynaud Phenomenon

• Excessive sympathetic stimulation of blood vessels in the extremities, causing vasoconstriction and ischemia, resulting in numbness and color changes (e.g., in the fingers).