The resting division of the nervous system refers to the separation of the nervous system into its sympathetic and parasympathetic divisions while the body is at rest. The sympathetic nervous system controls the fight-or-flight response, while the parasympathetic nervous system controls rest-and-digest and feed-and-breed responses. At rest, the parasympathetic division dominates to promote homeostatic and anabolic functions.
The autonomic nervous system is divided into sympathetic and parasympathetic divisions that work antagonistically to regulate involuntary bodily functions. The sympathetic division prepares the body for stressful or emergency situations by increasing heart rate, constricting blood vessels, dilating pupils, inhibiting digestion, and more. This fight-or-flight response diverts resources away from unnecessary functions to promote survival. On the other hand, the parasympathetic division conserves and restores bodily resources by slowing the heart, constricting pupils, stimulating digestion, and more. This promotes homeostatic and anabolic processes for growth and regeneration.
At rest, when the body is not under stress, the parasympathetic division dominates to maintain homeostasis and promote recovery. The sympathetic division is relatively inactive but primed to respond rapidly to perceived threats. This balanced interplay between the two divisions allows the body to conserve energy at rest while remaining prepared for action.
Sympathetic Division
The sympathetic division originates in the thoracic and lumbar regions of the spinal cord. It uses norepinephrine as its primary neurotransmitter. When activated, it stimulates the fight-or-flight response through broad physiological effects:
- Increases heart rate and strength of contractions
- Constricts blood vessels
- Dilates pupils
- Inhibits digestion
- Stimulates sweat glands
- Inhibits salivation and lacrimation
- Relaxes bronchial muscles
- Inhibits insulin release
- Releases glucose from energy stores
- Increases mental alertness and focus
This coordinated response prepares the body for intense physical activity and promotes survival in the face of stress or danger. Once the threat passes, the parasympathetic system reverses these effects to return the body to homeostasis.
Parasympathetic Division
The parasympathetic division originates in the brainstem and sacral spinal cord. It uses acetylcholine as its primary neurotransmitter. When active, it stimulates the rest-and-digest response:
- Decreases heart rate
- Dilates blood vessels
- Constricts pupils
- Stimulates salivation, digestion, and absorption
- contracts the bladder
- Causes erection
- Stimulates lacrimation
- Stimulates insulin release
This conserved and restored bodily resources while promoting growth, regeneration, and reproduction. The parasympathetic system reverses the sympathetic fight-or-flight response to return the body to homeostasis following stress.
Resting State
At rest, when the body is not under threat, the parasympathetic system dominates to maintain homeostasis and promote recovery from any exertion. Heart rate and respiration slow, pupils constrict, digestion resumes, and glucose storage increases. The sympathetic system remains relatively inactive but poised to respond quickly if needed.
This resting state allows the body to direct resources towards tissue growth and repair, storing energy, eliminating wastes, and other anabolic processes. These regenerative functions are vital for health but are suspended during fight-or-flight activation. The alternating cycle between sympathetic and parasympathetic states enables both survival and thriving.
Interplay Between Divisions
The sympathetic and parasympathetic divisions balance each other through antagonistic effects on target organs. For example, the sympathetic system speeds up heart rate while the parasympathetic system slows it down. This allows precise bi-directional control.
Even at rest, both systems are partially active. Their effects are integrated to establish an appropriate homeostatic setpoint for each regulated variable like heart rate or digestion. Small deviations trigger reciprocal changes in sympathetic and parasympathetic outflow to restore homeostasis.
This fine-tuned balance can be disrupted by chronic stress, which sustains sympathetic activation beyond the normal homeostatic limits. Dysregulation of the autonomic system contributes to stress-related illnesses.
Neural Pathways
Sympathetic and parasympathetic nerves follow separate central and peripheral pathways:
- Sympathetic preganglionic fibers emerge from the thoracolumbar spinal cord (T1-L2) and synapse in paravertebral ganglia.
- Parasympathetic preganglionic fibers emerge from the brainstem (cranial nerves) and sacral spinal cord (S2-S4) and synapse in terminal ganglia near or within target organs.
- Postganglionic fibers complete the journey to effector organs for both divisions.
This anatomy allows precise regional control over sympathetic and parasympathetic effects across the body.
Neurotransmitters
The sympathetic and parasympathetic divisions use different primary neurotransmitters:
- Sympathetic: Norepinephrine
- Parasympathetic: Acetylcholine
The catecholamine norepinephrine activates the fight-or-flight response, while acetylcholine slows the heart and stimulates digestion. Other neurotransmitters like epinephrine and dopamine also play roles.
Organs Innervated
Most organs receive dual sympathetic and parasympathetic innervation to enable push-pull regulation. Key examples include:
- Heart: Sympathetic – increases rate; Parasympathetic – decreases rate
- Lungs: Sympathetic – dilates bronchi; Parasympathetic – constricts bronchi
- Pupils: Sympathetic – dilates; Parasympathetic – constricts
- Digestive system: Sympathetic – inhibits; Parasympathetic – stimulates
However, some systems are primarily under sympathetic control (sweat glands, adrenal medulla) or parasympathetic control (lacrimal glands).
Autonomic Reflexes
The nervous system uses autonomic reflex arcs to rapidly regulate organ function:
- Sensory input (e.g. blood pressure change) is detected by a receptor.
- This stimulates a reflex through autonomic pathways.
- Efferent fibers carry signals to effector organs (e.g. heart).
- Organ response (e.g. altered heart rate) restores homeostasis.
No conscious control is required. For example, baroreceptors detect dropping blood pressure and trigger reflex sympathetic activation to increase vascular resistance and restore pressure.
Testing Autonomic Function
Clinicians can assess sympathetic and parasympathetic function through various tests:
- Heart rate: Measure resting heart rate and response to deep breathing, Valsalva maneuver, and postural changes.
- Blood pressure: Measure response to postural changes.
- Pupillary reflex: Assess pupil response to light.
- Thermoregulatory reflex: Observe response to cold stress.
- QSART: Evaluate sympathetic sudomotor function.
Diagnostic tests aid in detecting autonomic neuropathy or imbalance.
Disorders
Dysfunction of the sympathetic or parasympathetic systems underlies various disorders:
- Autonomic neuropathy: Nerve damage impairs regulation of heart rate, blood pressure, sweating, and digestion.
- Hyperthyroidism: Excess thyroid hormone overstimulates the sympathetic system.
- Hypoglycemia: Low blood sugar triggers excessive sympathetic discharge.
- Orthostatic hypotension: Blood pressure drops upon standing due to sympathetic failure.
- Erectile dysfunction: Impaired parasympathetic innervation affects erection.
Treatment aims to address the underlying condition contributing to autonomic imbalance.
Conclusion
The resting division of the nervous system refers to the parasympathetic dominance that occurs when the body is not under stress. The parasympathetic system conserves resources and promotes regeneration, while the sympathetic system remains relatively inactive but poised to respond to threats. This balanced interplay allows homeostatic functioning and flexibility. Disorders arise when chronic stress or disease disrupts the equilibrium between the sympathetic and parasympathetic nervous systems.