The Autonomic Nervous System

A&P1

Autonomic Nervous System (ANS)

A self-regulating system of the nervous system that controls involuntary bodily functions, such as heart rate, digestion, and respiratory rate. It consists of the sympathetic and parasympathetic nervous systems. “Involuntary Nervous System”

ANS Subdivisions

Sympathetic division and Parasympathetic division

Sympathetic Division

Prepare and regulate the body’s actions. “Fight or flight” response. All about preparing the body for immediate action, often in response to perceived threats or physical challenges, which is why it is often referred to as the "fight or flight" system.

Parasympathetic Division

Focuses on conserving and restoring energy during restful periods. All about maintenance of functions, rest, repair, and energy conservation, earning itself the nickname “rest and digest” system

Dual Innervation of the Divisions

refers to the fact that most organs and tissues in the body receive input from both the sympathetic and parasympathetic divisions. Allows for precise regulation of organ function, with each division having antagonistic (opposite) effects on the other, which helps maintain balance (homeostasis)

Parasympathetic Functions major functions

Promotes relaxation and energy conservation by slowing the heart rate and respiratory rates and lowering blood pressure. Stimulates digestive processes such as increasing gastrointestinal tract activity to support nutrient absorption.

Sympathetic Division major functions

Major Functions: Stimulates adrenal glands to release adrenaline, amplifying the fight or flight response. Increases HR and vasoconstricts blood vessels to improve oxygen and nutrient delivery to muscles and vital organs. Bronchodilation to improve airflow through the lungs, increasing oxygen intake

Parasympathetic Division additional functions

Additional functions: Facilitates restful sleep by helping to relax HR and muscles which aids in cellular repair and regeneration. Pupillary constriction which helps focus on close-up objects. Promotes contraction of the bladder and relaxation of urinary sphincter.

Sympathetic Division additional functions

Additional functions: Activates sweat glands to help cool the body during physical exertion. Widens pupils to enhance vision. Slows digestive processes including reducing saliva production in order to redirect energy to more critical functions during stress.

Bronchodilation

Widening of the airways

Neurons

Nerve cells that transmit information from one area of the body to another. In the ANS these are referred to as:

• Preganglionic

• Postganglionic

Preganglionic Neurons

Neuron originating in the CNS

Postganglionic Neuron

Neuron that extends to the target organ (i.e., smooth muscle, cardiac muscle, glands). It is the second neuron in the ANS two-neuron chain.

Ganglion (Ganglia)

Cluster of nerve cell bodies located outside of the CNS. The site where preganglionic and postganglionic neurons synapse. Two types: sympathetic and parasympathetic.

Sympathetic Division Features

Origin: Thoracolumbar - Thoracic & lumbar region of the spinal cord

Neuron length: Short preganglionic, long postganglionic

Location of Ganglia: Close to the spinal cord

Parasympathetic Division Features

Origin: Craniosacral - Brain and sacral spinal cord

Neuron length: Long preganiglionic, short postganglionic

Location of Ganglia: In or near the visceral effector organ

ANS Neural Pathways

Unlike the one-neuron pathway in the CNS and somatic division (neuromuscular junction), the autonomic division instead uses a two-neuron chair

ANS Neural Pathway: Preganglionic Neuron

• the first neuron in the chain which originates in the CNS (either brain or spinal cord)

• It travels to the autonomic ganglion and releases the neurotransmitter acetylcholine which acts on the receptors on the postganglionic neuron

ANS Neuronal Pathway: Postganglionic Neuron

• Second neuron in the two-neuron chain

• Receives the signal from the preganglionic neuron and extends to the target effector organ (smooth muscle, cardiac muscle, glands)

• A different neurotransmitter is then released, depending on the division of the ANS:

  • Sympathetic division: releases norepinephrine (NE)

  • Parasympathetic division: releases acetylcholine (ACh)

The 2 major neurotransmitters of the ANS

• Acetylcholine

• Norepinephrine

• The effect (excitatory or inhibitory) is dependent on which type of receptor the neurotransmitter binds to on the postganglionic neuron

Acetylcholine

• released by cholinergic neurons at:

  • all preganglionic neurons

  • all parasympathetic postganglionic neurons

Norepinephrine

• released by adrenergic neurons

  • almost all sympathetic postganglionic neurons except those at the sweat glands

Cholinergic Neurons and Receptors

release acetylcholine (ACh), but the effect (excitatory or inhibitory) depends on which type of receptor the Ach binds to

• Nicotinic receptor

• Muscarinic receptor

Nicotinic Receptor

Found in ganglia (pre and postganglionic neurons synapse, in hormone-producing cells of adrenal medulla, and in the sarcolemma of skeletal muscle cells at the neuromuscular junction (somatic division)

• Effect is almost always excitatory, binding opens ion channels, and depolarizes the cell, increasing the likelihood an action potential will be generated.

Muscarinic Receptor

Found in the effector cells (target organs like the heart, lungs, etc.), in the parasympathetic system.

• Effect can be excitatory or inhibitory

• Example: Binding of ACh to cardiac muscle cells slows heart rate, whereas binding to intestinal smooth muscle cells increases motility.

Adrenergic Neurons

release norepinephrine (NE) or epinephrine (adrenaline), and the effect depends on the receptor type that NE or epinephrine binds to

• Alpha receptors (a1, a2)

• Beta receptors (β1, β2, β3)

Alpha receptors a1

Found primarily in smooth muscle of blood vessels, the eye (pupils) and other stretch organs like the bladder.

• generally excitatory, causing vasoconstriction, increased blood pressure and pupillary constriction

Alpha receptor a2

Present on presynaptic neurons

• generally inhibitory - preventing the release of norepinephrine, reducing sympathetic activity.

Beta receptor β1

Mainly found in the heart, especially in the sinoatrial node, atria and ventricles

• Primarily excitatory - Increasing HR, contractility and conduction speed in the heart

Beta receptor β2

Primarily in smooth muscles of the lungs, blood vessels and uterus

• Mostly inhibitory - relaxation of smooth muscles, leading bronchodilation and vasodilation

Beta receptor β3

Primarily found in adipose tissue and the bladder.

• Both excitatory and inhibitory - promoting lipolysis is excitatory in nature but also causes relaxation of bladder muscle which is considered inhibitory.

Lipolysis - Breakdown of fat into fatty acids

Autonomic Reflexes

quick, automatic reactions to certain stimuli, working to keep the body’s systems balanced and functioning properly.

Examples of autonomic reflexes include:

• Baroreceptor Reflex

• Pupillary Light Reflex

Baroreceptor Reflex

• Helps to control blood pressure by adjusting heart rate and blood vessel diameter in response to changes in blood pressure (high or low).

Pupillary Light Reflex

When bright light hits your eyes, your pupils automatically shrink (constrict). This is controlled by the parasympathetic nervous system to protect the retina from damage. If the light gets dimmer, the pupils dilate (expand) to let in more light.

Autonomic Dysreflexia

a condition where the sympathetic nervous system becomes overactive, causing dangerously high blood pressure.

• happens mainly in people with spinal cord injuries, especially at the neck or upper spine.

• in this condition, the body responds too strongly to stimuli (like a full bladder or tight clothing) because the normal parasympathetic control can’t balance things out properly.

• medical emergency that needs to be treated quickly

Visceral Sensory Neuron

a sensory neuron that carries information from the internal organs (viscera) to the central nervous system (CNS)