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Autonomic Nervous System Physiology Study notes, Study notes of Human Physiology

Cardiff UniversityHuman Physiology

Functional Anatomy and Physiology of the Autonomic Nervous System (ANS)

Typology: Study notes

2025/2026

Available from 05/17/2026

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Functional Anatomy and Physiology of the Autonomic Nervous System (ANS)
Understanding Neurotransmitters and Receptors in the ANS -
- Neurotransmitters -
Acetylcholine (ACh) and
norepinephrine (NE) are the primary
neurotransmitters in the ANS.
-Acetylcholine
(ACh)- plays a crucial role
in the central nervous system
(CNS) and the peripheral
nervous system (PNS).
-It is
synthesized from choline and acetyl coenzyme A (acetyl-CoA) by the
enzyme choline acetyltransferase (ChAT).
- Location - ACh is released at cholinergic synapses, where it acts
as a chemical messenger to transmit signals across the synaptic cleft.
Functions in CNS - learning, memory, and arousal. It acts as a
neurotransmitter in pathways related to attention, cognition, and sleep-
wake cycles.
Functions in PNS - the primary neurotransmitter of the
parasympathetic nervous system (PNS).
Location -It is released by preganglionic neurons of the PNS and
by postganglionic neurons that innervate target organs.
-ACh binds to cholinergic receptors, including nicotinic
receptors found on postganglionic neurons and muscarinic
receptors located on effector organs such as smooth muscle,
cardiac muscle, and glands.
- Activation of these receptors mediates various
physiological responses, including stimulation of smooth
muscle contraction, cardiac inhibition, and glandular
secretion.
-Norepinephrine (NE) - neurotransmitter and hormone that belongs to the
class of catecholamines. It is synthesized from dopamine by the enzyme
dopamine beta-hydroxylase within the synaptic vesicles of noradrenergic neurons.
Location - released by postganglionic neurons of the sympathetic
nervous system (SNS) at adrenergic synapses.
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Functional Anatomy and Physiology of the Autonomic Nervous System (ANS) Understanding Neurotransmitters and Receptors in the ANS -

  • Neurotransmitters - Acetylcholine (ACh) and norepinephrine (NE) are the primary neurotransmitters in the ANS.
  • Acetylcholine (ACh) - plays a crucial role in the central nervous system (CNS) and the peripheral nervous system (PNS). -It is synthesized from choline and acetyl coenzyme A (acetyl-CoA) by the enzyme choline acetyltransferase (ChAT).
  • Location - ACh is released at cholinergic synapses, where it acts as a chemical messenger to transmit signals across the synaptic cleft. Functions in CNS - learning, memory, and arousal. It acts as a neurotransmitter in pathways related to attention, cognition, and sleep- wake cycles. Functions in PNS - the primary neurotransmitter of the parasympathetic nervous system (PNS). Location - It is released by preganglionic neurons of the PNS and by postganglionic neurons that innervate target organs. -ACh binds to cholinergic receptors, including nicotinic receptors found on postganglionic neurons and muscarinic receptors located on effector organs such as smooth muscle, cardiac muscle, and glands.
  • Activation of these receptors mediates various physiological responses, including stimulation of smooth muscle contraction, cardiac inhibition, and glandular secretion.
  • Norepinephrine (NE) - neurotransmitter and hormone that belongs to the class of catecholamines. It is synthesized from dopamine by the enzyme dopamine beta-hydroxylase within the synaptic vesicles of noradrenergic neurons. Location - released by postganglionic neurons of the sympathetic nervous system (SNS) at adrenergic synapses.

-It acts as a chemical messenger to transmit signals across synapses and elicit physiological responses in target tissues. -NE binds to adrenergic receptors, including alpha and beta adrenergic receptors, located on effector organs such as smooth muscle, cardiac muscle, and glands. -The effects of NE depend on the type of adrenergic receptor activated and its location. Function - regulation of blood pressure, modulation of heart rate, and regulation of the stress response.

  • Receptors - Cholinergic receptors respond to ACh, including nicotinic and muscarinic receptors. Adrenergic receptors respond to NE, including alpha and beta adrenergic receptors. Functional and Anatomical Differences Between Sympathetic and Parasympathetic ANS
  • Sympathetic : Originates from thoracolumbar region (T1-L2), responsible for "fight or flight" responses, short preganglionic fibers and long postganglionic fibers. Effects of Activation of Sympathetic Autonomic Nervous System (ANS) - Arterial Pressure Increase -
  • Activation of the sympathetic ANS leads to vasoconstriction, which increases arterial pressure by narrowing blood vessels and increasing peripheral resistance. Blood Flow to Muscles -
  • Sympathetic activation redirects blood flow to muscles by dilating blood vessels in skeletal muscle beds, facilitating increased oxygen and nutrient delivery during physical activity. Muscle Strength -

- Pre-Ganglionic Neurons - - Soma located in the lateral horn of the spinal cord. - Fibers travel via the ventral root. - Myelinated B fibers. - Exit the spinal cord via white communicating rami. - Three types of connections: 1. Synapse in the same ganglion they entered. 2. Pass up or down to a different ganglion.

  1. Exit via sympathetic nerve to synapse in a peripheral ganglion. Sympathetic Connections and Projections -
  • Efferents and Afferents -
  • Post-Ganglionic Neurons -
  • Unmyelinated C fibers.
  • Exit via gray communicating rami.
  • Afferents travel via paravertebral ganglia, white rami, and the dorsal root. Key Points
  • Sympathetic efferents originate from pre-ganglionic neurons in the lateral horn of the spinal cord.
  • These neurons synapse in either the same ganglion, another ganglion, or peripheral ganglia.
  • Post-ganglionic neurons transmit signals to target tissues, traveling via unmyelinated C fibers.
  • Sympathetic ganglia, both para-vertebral and pre-vertebral, play crucial roles in coordinating sympathetic responses throughout the body.
  • Parasympathetic : Originates from craniosacral region (cranial nerves III, VII, IX, X and sacral nerves S2-S4), responsible for "rest and digest" responses, long preganglionic fibers and short postganglionic fibers. Parasympathetic Nervous System (PNS) Anatomy and Control - 1. Cranial Parasympathetic -
  • Pre-ganglionic soma in supraspinal nuclei - Efferents form Cranial Nerves (CN)
  • Consists of CN III, VII, IX, X.
  • Visceral Control - Vagus nerve (CN X) regulates various visceral functions.

2. Sacral Parasympathetic - - Pre-ganglionic soma in Spinal Cord (SC) - Innervates bladder, anus, and genitalia. Parasympathetic Preganglionic Neurons -

  • Located in the supraspinal nuclei of the brainstem. Cranial Nerve Ganglia and Innervation - - Ganglion/Cranial Nerve/Input from Nucleus/Innervates -
  • Ciliary/III/Edinger- Westphal/Eye (pupil and ciliary muscles).

Pterygopalatine/VII/Superior salivatory/Lacrimal and nasal glands.

Submandibular/VII/Superior salivatory/Salivary glands.

  • Optic/IX/Inferior salivatory/Parotid salivary gland. Control of Parasympathetic System -
  • Hypothalamus - Controls basal body functions and modulates the Autonomic Nervous System (ANS). Brain Stem Transection -
  • Above the medulla: Basal control of arterial pressure but not by the hypothalamus.
  • Below the medulla: Arterial pressure falls to less than one-half normal. CNS modulates ANS. **Enteric Nervous System (ENS) of the GI Tract
  • Myenteric Nerve Plexus** - Controls motility. - Submucosal (Meissner’s) Nerve Plexus - Regulates fluid/ion balance.
  • Intestinal glands primarily controlled by the enteric NS, less so by the ANS.
  • PNS stimulates enteric NS, enhancing motility and secretion.
  • SNS doesn't affect enteric NS directly but strong stimulation inhibits it, leading to increased sphincter tone and decreased motility/secretion.
  • Optic (CN II) - Sensory nerve for vision, originates from the retina of the eye.
  • Oculomotor (CN III) - Motor nerve for eye movements, innervates most extraocular muscles except the superior oblique and lateral rectus.
  • Trochlear (CN IV) - Motor nerve for eye movements, innervates the superior oblique muscle.
  • Trigeminal (CN V) - Mixed nerve, sensory for the face and motor for mastication (chewing).
  • Abducens (CN VI) - Motor nerve for eye movements, innervates the lateral rectus muscle.
  • Facial (CN VII) - Mixed nerve, sensory for taste (anterior two-thirds of the tongue) and motor for facial expression.
  • Vestibulocochlear (CN VIII) -Sensory nerve for hearing and balance, originates from the inner ear.
  • Glossopharyngeal (CN IX) - Mixed nerve, sensory for taste (posterior one-third of the tongue) and motor for swallowing.
  • Vagus (CN X) - Mixed nerve, innervates multiple organs in the thorax and abdomen, including the heart, lungs, and digestive tract.
  • Accessory (CN XI) - Motor nerve for head and shoulder movements, innervates the sternocleidomastoid and trapezius muscles.
  • Hypoglossal (CN XII) - Motor nerve for tongue movements, innervates intrinsic and extrinsic muscles of the tongue. 5. Clinical Relevance -
  • Cranial nerve dysfunction can result in various neurological deficits, affecting sensory, motor, or autonomic functions in the head and neck region.
  • Assessment of cranial nerves is an essential component of neurological examinations and aids in diagnosing conditions such as strokes, tumors, and neuropathies. .

Homeostasis is the main function of the Autonomic Nervous System Autonomic vs Somatic Efferent Pathways - Autonomic Efferent Pathways -

  • Control - Involuntary control of visceral functions and regulation of internal organs.
  • Divisions -Divided into sympathetic and parasympathetic divisions.
  • Neurotransmitters - Utilize both acetylcholine (ACh) and norepinephrine (NE) as neurotransmitters. Two-Neuron Pathway -
  • Preganglionic neuron - Originates in the central nervous system (CNS) and synapses with a postganglionic neuron in autonomic ganglia.
  • Postganglionic neuron - Extends from autonomic ganglia to target organs.
  • Ganglia Location - Preganglionic cell bodies located in the CNS, while postganglionic cell bodies located in autonomic ganglia outside the CNS.
  • Effectors - Innervate smooth muscle, cardiac muscle, and glands.
  • Function - Regulates functions such as heart rate, digestion, respiration, and glandular secretion. Somatic Efferent Pathways -
  • Decrease Heart Rate (HR). -Sympathetic (T1-T4) -
  • SAN (Sinoatrial Node), AVN (Atrioventricular Node), and Myocardial Tissues:*
  • Increase HR.
  • Blood Vessels of Upper Body (Head, Neck, and Thoracic Region) - Sympathetic (T5-L2) - - Blood Vessels of Lower Body (Abdomen, Pelvis, Lower Limbs). Parasympathetic Regulation of the Heart -
  • Afferents - Baroreceptors (Glossopharyngeal nerve).
  • Preganglionic Neurons -
  • Dorsal Motor Nucleus (Vagus nerve) and Nucleus Ambiguous (Glossopharyngeal nerve).
  • Postganglionic Neurons -
  • Activate muscarinic acetylcholine receptors (mAChRs) in SAN (Sinoatrial Node), leading to hyperpolarization and lower HR.
  • Parasympathetic nerves can stimulate nitric oxide release, resulting in vasodilation. Sympathetic Regulation of the Heart -
  • Preganglionic Neurons (Intermediolateral Column) -
  • T1-T4 Sympathetic Preganglionic Neurons (SPNs) synapse in sympathetic chain.
  • T5-L2 SPNs synapse in celiac, superior, and inferior mesenteric ganglia.
  • Postganglionic Neurons -
  • Release Norepinephrine (NE) to activate:
  • β1 Adrenergic Receptors (AR) in SAN and AVN: Increase depolarization and speed of action potential propagation, increasing HR.
  • β1 AR in myocardial tissue: Increase contractility via increased intracellular calcium release.
  • α1 AR in smooth muscles of the splanchnic bed and lower limbs. Synaptic Connections, Neurotransmitter, and Receptors in the ANS -
  • Synaptic connections - Synapses occur between preganglionic and postganglionic neurons, as well as between postganglionic neurons and target organs. - Neurotransmitters and receptors - ACh is used by preganglionic neurons of both divisions and by postganglionic neurons of the parasympathetic division. NE is released by postganglionic neurons of the sympathetic division. Molecular and Physiological Differences Between Cholinergic and Adrenergic Systems -
  • Cholinergic - Uses ACh as a neurotransmitter, activates both nicotinic and muscarinic receptors.
  • Adrenergic - Uses NE as a neurotransmitter, activates alpha and beta adrenergic receptors. Determining the Effects of a Compound on the ANS - - Sympathomimetic - Mimics the effects of sympathetic activation, such as increased heart rate and blood pressure.
  • Parasympathomimetic - Mimics the effects of parasympathetic activation, such as decreased heart rate and increased digestion. Physiological Effects of Exercise on the ANS -
  • Exercise activates the sympathetic nervous system, increasing heart rate, blood pressure, and blood flow to muscles.
  • Parasympathetic activity decreases during exercise to allow for increased sympathetic activity.
  • Increased Sympathetic Activity - During exercise, sympathetic nervous system activity increases to facilitate the body's response to physical exertion.
  • Parasympathetic Withdrawal - Parasympathetic activity decreases during exercise to allow for increased sympathetic dominance, promoting physiological changes necessary for physical activity.
  • Enhanced Cardiovascular Function -Exercise stimulates sympathetic activity, leading to increased heart rate, cardiac output, and vasoconstriction in non-essential vascular beds to redirect blood flow to active muscles.
  • Improved Respiratory Function - Sympathetic activation during exercise enhances bronchodilation and increases respiratory rate and tidal volume to meet the increased oxygen demand of working muscles.
  • Blood Pressure Regulation -Exercise-induced sympathetic activation can lead to transient increases in blood pressure to support increased cardiac output and vasodilation in active muscles. Physiological Mechanisms of Sympatholytic Effects of Exercise -
  • Regular exercise can lead to adaptations that reduce sympathetic nervous system activity at rest, such as decreased resting heart rate and blood pressure.
  • Post-Exercise Hypotension -Following exercise, sympathetic activity gradually decreases, leading to a period of reduced sympathetic tone known as post-exercise hypotension. This can result in a temporary decrease in blood pressure after exercise.