Homeostatic Imbalances of the ANS

Overview: Homeostatic imbalances in the autonomic nervous system (ANS) can lead to various disorders affecting smooth muscle activity, particularly in blood vessels. Conditions such as autonomic dysreflexia, hypertension, and Raynaud’s disease illustrate the consequences of dysfunctional autonomic regulation.

Autonomic Dysreflexia:

A life-threatening condition characterized by uncontrolled sympathetic activation.

Common in individuals with quadriplegia or spinal cord injuries above T6.

Triggers include stimulation of pain receptors and visceral contractions (e.g., bowel stimulation).

Hypertension:

Often results from an overactive sympathetic vasoconstrictor response due to stress.

Can lead to chronic high blood pressure and associated cardiovascular issues.

Raynaud’s Disease:

Condition where fingers and toes become pale and cyanotic due to exaggerated vasoconstriction.

Provoked by cold exposure or emotional stress.

Severe cases may result in ischemia and gangrene due to lack of blood supply.

Sympathetic Nervous System

Overview: The sympathetic nervous system is a division of the autonomic nervous system responsible for the body's "fight or flight" response. It prepares the body to respond to stress by increasing heart rate, redirecting blood flow, and releasing energy stores.

Preganglionic Neurons:

Originate in the lateral gray horns of spinal segments T1-L2.

Release acetylcholine (ACh) to stimulate ganglionic neurons.

Postganglionic Neurons:

Synapse with preganglionic neurons at various locations:

In the ganglion it first reaches.

Ascend or descend to higher or lower ganglia (sympathetic chains).

At prevertebral ganglia.

May pass through sympathetic ganglia to adrenal medulla without synapsing.

Most release norepinephrine (NE) at neuroeffector junctions.

Ganglia Locations:

Ganglia are located near the spinal cord (sympathetic chain ganglia) or close to target organs (prevertebral ganglia).

Physiological Effects:

Triggers the "fight or flight" response.

Increases heart rate and blood pressure.

Dilates airways and pupils.

Redirects blood flow from non-essential functions to muscles and brain.

Mobilizes energy stores through the release of glucose and fatty acids.

Parasympathetic Nervous System

Overview: The parasympathetic nervous system is one of the two divisions of the autonomic nervous system, responsible for promoting "rest and repose" functions. It primarily conserves energy and facilitates bodily maintenance activities through its long preganglionic and short postganglionic neurons.

Preganglionic Neurons:

Originate in the brainstem and sacral spinal cord (S2-S4).

Long fibers extend to ganglia located near or within target organs.

Postganglionic Neurons:

Short fibers that innervate target organs directly.

Release acetylcholine (ACh) at neuroeffector junctions.

Ganglia Locations:

Ganglia are situated close to or within the walls of visceral organs.

Associated with cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus).

Physiological Effects:

Promotes "rest and digest" responses, including decreased heart rate, increased digestive activity, and relaxation of sphincters.

Counteracts the sympathetic nervous system's "fight or flight" response by facilitating energy conservation and restorative processes.

Nervous System Overview

Overview: The nervous system is a complex network responsible for coordinating and processing sensory information and motor commands. It consists of the central nervous system (CNS) and peripheral nervous system (PNS), which includes both somatic and autonomic divisions, each with distinct functions and structures.

Central Nervous System (CNS):

Comprises the brain and spinal cord.

Responsible for integrating, processing, and coordinating sensory data and motor commands.

Information processing involves the integration and distribution of information within the CNS.

Peripheral Nervous System (PNS):

Includes all neural tissue outside the CNS.

Divided into two main components:

Sensory Division: Brings information to the CNS from receptors in peripheral tissues and organs.

Motor Division: Carries motor commands from the CNS to peripheral tissues and systems.

Somatic Nervous System (SNS):

Controls skeletal muscle contractions.

Somatic sensory receptors provide sensations such as position, touch, pressure, pain, and temperature.

Special sensory receptors are involved in smell, taste, vision, balance, and hearing.

Autonomic Nervous System (ANS):

Regulates involuntary functions of smooth muscle, cardiac muscle, glands, and adipose tissue.

Comprised of two divisions:

Sympathetic Division: Prepares the body for 'fight or flight' responses.

Parasympathetic Division: Promotes 'rest and digest' activities.

Involves preganglionic and postganglionic neurons that communicate using specific neurotransmitters and receptors.

Comparison of Somatic and Autonomic Nervous Systems:

Somatic nervous system controls voluntary movements; autonomic nervous system regulates involuntary actions.

Different pathways and types of effectors (skeletal vs. smooth/cardiac muscle and glands).

Distinct neurotransmitter systems and receptor types involved in their respective responses.

Neurotransmitters and Receptors

Overview: Neurotransmitters are chemical messengers that transmit signals across synapses in the nervous system. Key neurotransmitters include acetylcholine and norepinephrine, which interact with specific receptors to mediate various physiological responses in the autonomic nervous system (ANS).

Cholinergic Neurons:

Neurotransmitter: Acetylcholine (Ach)

Receptors:

Nicotinic Receptors: Found on sympathetic and parasympathetic preganglionic neurons, adrenal medulla, and neuromuscular junction.

Muscarinic Receptors: Located on target effectors innervated by parasympathetic postganglionic neurons and sweat glands.

Adrenergic Neurons:

Neurotransmitter: Norepinephrine (NA) / Noradrenaline (NE)

Function: Released as a neurotransmitter by sympathetic postganglionic neurons or as a hormone from the adrenal medulla.

Receptors:

Alpha (α) Receptors: Respond to norepinephrine.

Beta (β) Receptors: Also respond to norepinephrine; involved in various sympathetic responses.

Comparison of Nervous Systems:

Autonomic vs. Somatic Nervous System:

Autonomic controls involuntary functions (e.g., heart rate, digestion), while somatic controls voluntary movements.

Preganglionic and Postganglionic Neurons:

Preganglionic neurons release acetylcholine, while postganglionic neurons may release either acetylcholine (parasympathetic) or norepinephrine (sympathetic).

Physiological Components of ANS:

Sympathetic Division: Prepares the body for 'fight or flight' responses, primarily using norepinephrine.

Parasympathetic Division: Promotes 'rest and digest' activities, primarily using acetylcholine.

Autonomic Nervous System (ANS)

Overview: The Autonomic Nervous System (ANS) regulates involuntary bodily functions and operates largely under subconscious control. It consists of two main divisions, the sympathetic and parasympathetic systems, which work together to maintain homeostasis by controlling visceral organs.

Sympathetic Division:

Originates from thoracolumbar region (T1-L2).

Preganglionic neurons release acetylcholine (ACh), stimulating ganglionic neurons.

Most postganglionic fibers release norepinephrine (NE) at neuroeffector junctions.

Responsible for "fight or flight" responses, increasing heart rate, and redirecting blood flow to muscles.

Parasympathetic Division:

Originates from craniosacral region (brainstem and S2-S4).

Preganglionic fibers release ACh, stimulating ganglionic neurons located near or in target organs.

All postganglionic fibers release ACh at neuroeffector junctions.

Responsible for "rest and repose" responses, promoting digestion and energy conservation.

Neurotransmitters:

Acetylcholine (ACh): Used by both preganglionic neurons in sympathetic and parasympathetic divisions; also released by all postganglionic fibers in the parasympathetic division.

Norepinephrine (NE): Primarily released by most postganglionic fibers in the sympathetic division.

Receptors:

Adrenergic receptors: Respond to norepinephrine and epinephrine in the sympathetic division.

Cholinergic receptors: Respond to acetylcholine in both divisions, with subtypes including nicotinic and muscarinic receptors.

Motor Pathways:

Two-neuron pathway:

Preganglionic Neurons: Cell bodies located in CNS (brain or spinal cord); myelinated axons extend to autonomic ganglia.

Postganglionic Neurons: Located entirely outside the CNS in PNS; unmyelinated axons terminate in visceral effectors (e.g., cardiac muscle, smooth muscle, glands).

Comparison with Somatic Nervous System:

ANS controls involuntary actions while the somatic nervous system governs voluntary movements.

ANS utilizes a two-neuron pathway, whereas the somatic system uses a single motor neuron to skeletal muscles.