Neurotransmitters and Receptors of the Autonomic System

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Autonomic Nervous System Neurotransmitters and Receptors

• There are two types of autonomic neurons: cholinergic or adrenergic.

• Receptors for neurotransmitters are integral plasma proteins what are located in the plasma membrane of the postsynaptic neuron or effector cell.

Cholinergic Neurons and Receptors

• They release Ach.

• They are all the sympathetic and parasympathetic neurons, sympathetic postganglionic neurons that innervate most sweat glands, and all parasympathetic postganglionic neurons.

• ACh from synaptic clefts in presynaptic neurons > binds specific cholinergic receptors in postsynaptic neurons.

• There are two types of cholinergic receptors: nicotinic receptors and muscarinic receptors.

• ACh causes depolarization in tissues with nicotinic receptors.

• ACh causes depolarization or hyperpolarization in tissues with muscarinic receptors.

• ACh is deactivated by acetylcholinesterase (AChE).

Adrenergic Neurons and Receptors

• Adrenergic neurons release norepinephrine (NE), also known as noradrenalin.

• NE in synaptic vesicles > diffusion across synaptic cleft > bind to adrenergic receptors.

• Adrenergic receptors bind both hormones norepinephrine and epinephrine.

• There are two types of receptors: alpha and beta.

• They are further classified as alpha1 (excitation), alpha2 (inhibition), beta1 (excitation), beta2 (inhibition), and beta3 (present on brown fat; activation causes heat produciton).

• NE action is terminated either when it is reabsorbed back into the releasing axon, or by enzymatic inactivation by catechol-O-methyltransferase (COMT) also known as monoamine oxidase (MAO).

• NE creates long lasting effects because its reuptake is slower than that of ACh.

Agonists and Antagonists

• Agonists activate receptors, mimicking the effect of a natural neurotransmitter or hormone.

• Antagonists deactivate receptors.

Physiologic effects of the Autonomic Nervous System

• Sympathetic and parasympathetic systems are in opposition to each other; mediated by the hypothalamus.

• A few structures receive only sympathetic innervation.

Sympathetic Responses

• It dominates during mental or physical stress.

• 4E's : exercise, excitement, emergency, and embarrassment.

• Activates the fight-or-fight response.

• Effects are longer lasting than parasympathetic system.

Parasympathetic Responses

• It dominates during periods of relaxation.

• Activates the rest-and-digest response.

• SLUDD: salivation, lacrimation, urination, digestion, and defecation.

• Also causes 3D: decrease in heart rate, diameter of airways, and diameter of pupils.

Control of Autonomic Functions

• Receptor: located at the distal end of a sensory neuron.

• Sensory neuron: conducts impulses from receptors to CNS.

• Integrating center: located in the hypothalamus and brain stem (thus, we are not aware of their responses), these centers relay signals from sensory neurons to motor neurons. There are no integration centers for urination and defecation.

• Motor neurons: relay signals from integrating centers to effectors.

• Effector: they are the smooth muscle, cardiac muscle, or glands.

Further Topics on Autonomic Nervous System:

1. Introduction to the Autonomic Nervous System
2. Control of the Autonomic Nervous System
3. Divisions the Autonomic Nervous System
4. Notes on Sympathetic Nervous System
5. Notes on Paraympathetic Nervous System
6. Neurons of the Autonomic Nervous System
7. Neurotransmitters and Receptors of the Autonomic Nervous System
8. FAQ on Autonomic System

Additional Reading:

Basic Neurology

1. Peripheral Nervous System
2. Central Nervous System
3. The Ventricular System
4. The Spinal Cord
5. The Brain Stem
6. The Cerebellum
7. Visual Pathways
8. Diencephalon
9. Basal Ganglia
10. Cerebral Cortex
11. Sleep Disorders
12. Autonomic Nervous System
13. Cranial Nerves and Parasympathetic Ganglia
14. Cells of the Nervous System
15. Cerebrospinal fluid
16. Additional short notes on Cerebrum
17. Functions and Diseases of Cerebrum
18. Subcortical Grey Matter
19. Notes on The Spinal Cord
20. Regulation of Heart Rate by Autonomic Nervous System
21. Action Potentials, Axon Conduction, and Neuromuscular Junction
22. Types of Seizures
23. What is a Cough Reflex?
24. Notes on Congenital Prosopagnosia
25. Findings in Parkinson's Disease
26. Types of Heat Strokes
27. Types of Strokes
28. What is Benign Intracranial Hypertension?
29. What is Cauda Equina Syndrome?
30. Cranial Nerve Locations in Brain Stem

Neurology Videos

1. Video of Neurology Examination in a Clinical Setting

Medical Images

Useful Medical Images & Diagrams (link opens in a new window)

Related Topics

1. Nervous System Disorders
2. Histology of Nervous Tissue
3. Cranial Nerve Reflexes
4. Motor System Examination

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