4.6 Autonomic nervous system

The autonomic nervous system (ANS) is crucial for maintaining balance in your body. It controls involuntary functions like heart rate and digestion. The ANS has two main parts: the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) systems.

These systems work together to keep your body in check. The sympathetic system revs you up for action, while the parasympathetic system helps you relax. Understanding how they affect different organs is key to grasping how your body responds to stress and rest.

Sympathetic vs Parasympathetic Divisions

Overview of the Autonomic Nervous System (ANS)

• The autonomic nervous system (ANS) is a division of the peripheral nervous system that controls involuntary functions of the body (heart rate, digestion, respiration)
• The ANS is divided into two main branches: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS)
• The SNS and PNS often have opposing effects on the same organ systems, allowing for precise regulation of bodily functions to maintain homeostasis

Sympathetic Nervous System (SNS) Functions

• The SNS is responsible for the "fight or flight" response, which prepares the body for stressful or emergency situations
• Increases heart rate, blood pressure, and blood glucose levels
• Decreases digestion and other non-essential functions
• Examples of SNS activation: increased heart rate during exercise, pupil dilation in low light conditions

Parasympathetic Nervous System (PNS) Functions

• The PNS is responsible for the "rest and digest" response, which promotes relaxation, digestion, and energy conservation
• Decreases heart rate, blood pressure, and respiration
• Increases digestion and other restorative functions
• Examples of PNS activation: decreased heart rate during sleep, increased salivation and peristalsis during digestion

Autonomic Nervous System Functions

Effects on the Cardiovascular System

• SNS increases heart rate and contractility, while PNS decreases heart rate and contractility
• SNS causes vasoconstriction in most blood vessels, while PNS causes vasodilation in certain blood vessels
• Examples: SNS increases heart rate during exercise, PNS decreases heart rate during rest

Effects on the Respiratory System

• SNS increases the rate and depth of breathing, while PNS decreases the rate and depth of breathing
• Examples: SNS increases breathing rate during physical activity, PNS decreases breathing rate during relaxation

Effects on the Digestive System

• SNS decreases motility and secretion, while PNS increases motility and secretion to promote digestion and absorption of nutrients
• Examples: PNS stimulates peristalsis and secretion of digestive enzymes after a meal, SNS inhibits digestion during stress

Effects on the Urinary System

• SNS promotes the retention of urine by contracting the internal urethral sphincter
• PNS promotes the release of urine by relaxing the internal urethral sphincter and contracting the detrusor muscle of the bladder
• Examples: SNS activation during stress can cause urinary retention, PNS activation during bladder filling leads to urination

Effects on the Reproductive System

• SNS is involved in sexual arousal and orgasm
• PNS is involved in the relaxation phase following orgasm
• Examples: SNS increases blood flow to reproductive organs during sexual arousal, PNS promotes relaxation and recovery after orgasm

Autonomic Reflexes for Homeostasis

Overview of Autonomic Reflexes

• Autonomic reflexes are involuntary, automatic responses to stimuli that help maintain homeostasis in the body
• These reflexes are mediated by the ANS and involve the integration of sensory information, central processing in the brain or spinal cord, and efferent output to effector organs

Baroreceptor Reflex

• Helps maintain blood pressure by adjusting heart rate and vascular tone in response to changes in blood pressure
• Baroreceptors in the carotid sinus and aortic arch detect changes in blood pressure
• Example: When blood pressure increases, baroreceptors send signals to the brain, which activates the PNS to decrease heart rate and blood pressure

Chemoreceptor Reflex

• Helps maintain blood pH and oxygen levels by adjusting respiration in response to changes in blood chemistry
• Chemoreceptors in the carotid and aortic bodies detect changes in blood pH and oxygen levels
• Example: When blood pH decreases or oxygen levels drop, chemoreceptors send signals to the brain, which activates the SNS to increase breathing rate and depth

Pupillary Light Reflex

• Helps regulate the amount of light entering the eye by constricting the pupil in response to bright light and dilating the pupil in response to dim light
• Photoreceptors in the retina detect changes in light intensity
• Example: When exposed to bright light, photoreceptors send signals to the brain, which activates the PNS to constrict the pupil and reduce the amount of light entering the eye

Other Autonomic Reflexes

• Cough reflex, sneeze reflex, and gag reflex help protect the respiratory and digestive systems from potential irritants or obstructions
• Examples: Cough reflex is triggered by irritants in the airways, sneeze reflex is triggered by irritants in the nasal passages, gag reflex is triggered by stimulation of the back of the throat

Autonomic Neurotransmission: Neurotransmitters and Receptors

Primary Neurotransmitters in the ANS

• Acetylcholine (ACh) and norepinephrine (NE) are the primary neurotransmitters involved in autonomic neurotransmission
• In the SNS, preganglionic neurons release ACh, which binds to nicotinic receptors on postganglionic neurons
• Postganglionic neurons primarily release NE, which binds to adrenergic receptors (α1, α2, β1, β2) on target organs
  • Exceptions include postganglionic sympathetic neurons that innervate sweat glands and some blood vessels, which release ACh instead of NE
• In the PNS, both preganglionic and postganglionic neurons release ACh
• ACh binds to nicotinic receptors on postganglionic neurons and muscarinic receptors (M1, M2, M3) on target organs

Adrenergic Receptors

• Adrenergic receptors are classified as either α (alpha) or β (beta) receptors, with subtypes within each class (α1, α2, β1, β2)
• The specific effects of NE on target organs depend on the type of adrenergic receptor expressed
• Examples: α1 receptors mediate vasoconstriction, β1 receptors mediate increased heart rate and contractility, β2 receptors mediate bronchodilation and vasodilation in skeletal muscle

Muscarinic Receptors

• Muscarinic receptors are classified into five subtypes (M1-M5), with M2 and M3 being the most important for autonomic functions
• The specific effects of ACh on target organs depend on the type of muscarinic receptor expressed
• Examples: M2 receptors mediate decreased heart rate and contractility, M3 receptors mediate increased secretion and motility in the digestive system